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WX 203l

Survey of

Meteorology

Laboratory

Course

2

Lab 1: Air Motions ; Outside Measurements with Instruments

1. Introduction

Wind is the horizontal motion of air relative to Earth’s surface. Air movement, or wind, occurs

over a range of scales, from large-scale systems influenced by the transfer of heat from the

equator to the poles, to small eddies or whirls that occur around obstacles or mountain barriers.

The movement of air results from small and large-scale temperature differences as well as from

Earth’s rotation. Temperature differences cause pressure differences. Pressure gradient forces

wind to blow from areas of higher pressure to areas of lower pressure. Wind direction is defined

as the direction from which the wind is blowing.

Question 1

“North wind” refers to which of the following? Select the best answer.

a. A wind blowing to the north

b. A wind blowing to the south

c. A wind originating in polar regions

Question 2

What are westerly winds? Select the best answer.

a. Winds blowing from west to east

b. Winds blowing from east to west

c. Trade winds

Activity 1

Please access the satellite image and animation provided free of charge by the NSF-funded

classzone webpage:

http://www.classzone.com/books/earth_science/terc/content/visualizations/es1905/es1905page01

.cfm?chapter_no=19

Air flows from areas of higher pressure to areas of lower pressure. Based on this fact, the

predicted wind direction for the area on the left side of this satellite image would be from the

southeast.

http://www.classzone.com/books/earth_science/terc/content/visualizations/es1905/es1905page01.cfm?chapter_no=19


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2. The Coriolis effect

The combination of the Coriolis force with the pressure gradients resulting from poleward heat

transfer lead to dominant winds within different latitudes of each hemisphere. These dominant

winds are:

Northeast trade winds between the equator and 30° latitude,

Westerly winds between about 30° and 60° latitude, and

Polar easterlies between about 60° latitude and the pole.

Question 3

Which of the following general winds affect a large part of the North American landmass? Select

the best answer.

a. Southeast trade winds

b. Northeast trade winds

c. Westerlies

d. Easterlies

Activity 2

Click the image to see the animation. Use the movie controls to step through or replay the movie:


.cfm?chapter_no=19

The Coriolis effect influences wind by deflecting its path to the right in the Northern

Hemisphere. The sequence of weather satellite images shows that the actual wind direction is

from the southwest. The satellite images show atmospheric motion over the northern Pacific

Ocean for a 36-hour period. The animation shows the Coriolis Effect in the context of everyday

weather patterns. The predicted wind aloft should be from the southeast but because of the

Coriolis Effect the wind is actually southwest. The animation of atmospheric motion over the

northern Pacific Ocean for a 36-hour period can be paused and rewound to stress important

points.

3. Land and sea breezes

Activity 3

Unequal heating of air over land and water results in breezes near shorelines. This animation

depicts the diurnal change in wind patterns along the coast. During the day, land heats more



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rapidly than the water, air rises, and a cool breeze blows in from the water. The pattern reverses

at night. The strength of the animation is that temperature values can be compared throughout the

day and then linked to wind patterns. The animation can be paused and rewound to stress

important points. Examine the changing temperature of the land throughout the 24 hours

represented in the animation:


.cfm?chapter_no=visualization

4. Local winds

Local winds are caused by small-scale differences in air temperature and pressure and affect the

lowest part of the atmosphere. Terrain strongly influences local winds. Local winds can often be

more important to fire behavior than the winds produced by large-scale pressure patterns. In

many areas, especially in complex terrain, local winds are the dominant daily winds.

Diurnal mountain winds:

Diurnal mountain winds develop over complex topography of all scales, from small hills to large

mountains. Topographic influences generally cause these winds to reverse direction twice per

day.

The mountain wind system includes:

slope winds,

along-valley winds,

cross-valley winds, and

mountain-plain winds.

As a rule, wind flows upslope, upvalley, and plain-to-mountain during the day, though there are

some exceptions. At night, downslope, downvalley, and mountain-to-plain winds tend to

dominate. Diurnal mountain winds are produced by horizontal temperatures differences that

develop daily in complex terrain. Surface heating will cause the air nearer the slope to be

warmed more than air farther from the surface. On flat ground, by contrast, there is no change in

distance from the surface heat source as one moves horizontally, so these terrain-induced

differences do not occur. During the day, the sheath of warm air near the slope serves as a natural

chimney. As the warm air rises, it results in an upward flow of air, causing an upslope wind. The

layer of warm air along the slope is turbulent and buoyant, increasing in depth as it progresses up

the slope. The process continues during the daytime as long as the slope is receiving solar

radiation.

Question 4

http://www.classzone.com/books/earth_science/terc/content/visualizations/es1903/es1903page01.cfm?chapter_no=visualization

http://www.classzone.com/books/earth_science/terc/content/visualizations/es1903/es1903page01.cfm?chapter_no=visualization

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You are working a wildland fire 20 miles from the Pacific coast in southern California. Strong

winds have been blowing from the east but are expected to die down later this afternoon. It is

now 3 p.m. What should you expect next? Select all that apply.

a. Calmer conditions will provide optimal firefighting advantage.

b. Onshore flow could push the fire back to the east.

c. Easterly winds could strengthen again overnight. d. None of the above

5. Local wind measurements with Kestrel instruments

Activity 4

This assignment assesses how well students are able to record and synthesize basic

meteorological data. This will require the use of a smart phone or other electronic device to

record your measurements, and a hand-held Kestrel sensor provided by the lab instructor.

Anemometer Kestrel is a pocket size electronic device used for measurement of wind speed,

temperature, wind chill, dew point temperature and relative humidity.

Procedure:

a. Today you will do your sampling and data collection with Kestrel sensors working in

groups. Every group of five students will elect a field manager who will communicate

with the lab instructor, ask questions and record and present the measurements from the

team.

b. Each team member should learn how to operate the Kestrel hand-held instruments. Read

instruction manual for your equipment while you wait for your turn. Make sure you are

familiar with the units for each type of measurements. Kestrel technical specifications are

provided in Table 1 below.

c. Each team selects an outdoor location for their measurements. Note that the siting is very

important. Make sure your measurements are not obstructed or influenced by tall trees

and buildings, heat sources or vents. Therefore, you need to be located at least 20-30 feet

away from objects that could cause errors in your measurements.

d. Each team should record five measurements (one per team member) of wind speed, wind

direction, temperature and humidity.

e. The team leader should record the measurements and write the averages and the standard

deviation for each type of measurement: wind, temperature, and humidity together with

the location (siting) on the board back in the classroom. At the end of the class, the

difference between the recorded measurements will be discussed. Which type of

measurements are more consistent and which vary the most (temperature, wind or

humidity)?

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Table 1. Kestrel technical data.

MEASUREMENT OF WIND SPEED

Scope of measurement 0,3...40 m/s

Measurement uncertainty (for a component in

a device axis) +/- 3% of a reading or +/- 0,1 m/s

Underrating indications for wind tilted from a

device’s axis

-1% for 5° angle

-2% for 10° angle

-3% for 15° angle

Calibration drift <2% after 100 hours of work

at 7 m/s speed

Measurement resolution

0,1 kt, m/s, km/h, mph

1 FPM up to 1999 FPM

10 FPM for 2000 FPM

1 Beaufort 0...12

Speed measurement result

reading

current average from the last 3 seconds

average an average from the moment of switching on the

power supply

maximum 3-second wind gusts

from the moment of switching on the power

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supply

Choice of result units kt (knots), m/s, km/h, mph (miles per hour), FPM (foot per

minute), Beaufort degrees

AIR TEMPERATURE MEASUREMENT

Scope of measurement -15...+50 °C

Measurement uncertainty +/- 1 °C

Measurement resolution 0,1 °C

Temperature measurement result reading

current

wind chill

Choice of result units degrees C, F

HUMIDITY MEASUREMENT

Measurement uncertainty +/- 3 %

Scope of measurement 0,1 %

Calibration drift +/- 2 % after 24 months

Response time 1 min.

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Lab 2: The Atmosphere – Pressure, Temperature and Density

1. Ideal Gas Law

Pressure, temperature and density are the three state variables of the atmosphere. Pressure (P)

equals the force per unit area exerted by collisions between molecules. The faster the molecules

are moving on average, the more momentum is transferred as a result of each molecular

collision. Thus, pressure is proportional to the average kinetic energy of the molecules in a gas,

which depends on the temperature (T). A higher temperature means that the molecules are

moving faster on average.

Moreover, when more molecules are present in the atmosphere, more collisions will take place

per unit area, and the greater the pressure becomes. The pressure is therefore proportional to the

number of molecules per unit volume, which is proportional to the mass density of the air (D).

Thus, pressure in the atmosphere is mathematically proportional to the product of D and T. This

relationship between these three state variables is called the Ideal Gas Law. It governs how these

variables are interrelated, with the assumption that air molecules behave like an ideal gas (which

is a good assumption under normal atmospheric conditions). The Ideal Gas Law may be

mathematically expressed as

P = RDT,

where R is just the gas constant for dry air.

In this portion of this lab, we will not be solving the Ideal Gas Law. Rather, we will be

experimenting with the relationship between pressure, temperature and density. Since R is a

constant, then it can be stated that pressure is proportional to density times temperature, or

𝑃 ∝ 𝐷𝑇.

In order to better understand this relationship, download and run the Molecular Workbench

provided by free of charge by the NSF-funded Concord Consortium website. The Internet

address is http://mw.concord.org/modeler/MW.jnlp. You will need to have Java installed in order

to execute this visualization tool that models molecular motion and helps us gain insight into the

Ideal Gas Law.

Load the Gas Laws module by typing http://mw2.concord.org/public/part2/gaslaws/index.cml

into the address bar on the Molecular Workbench.

Complete the What Is Pressure exercise and answer the following:

a) Increasing the number of molecules inside the balloon causes the pressure to (increase /

decrease) _______________________.

http://mw.concord.org/modeler/MW.jnlp

http://mw2.concord.org/public/part2/gaslaws/index.cml

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b) What are two other ways to change the pressure inside the balloon without adding or

taking away molecules?

________________________________________________________________________

________________________________________________________________________

Complete the Boyle’s Law exercise and answer the following:

c) Which of the four graphs accurately represents the relationship between pressure and

volume (check one)? 1____ 2____ 3____ 4_____

d) If the temperature stays the same, pressure (increases / decreases) _________________

when volume increases.

e) If the temperature stays the same, pressure (increases / decreases) _________________

when density increases.

Complete the Charles’ Law exercise and answer the following:

f) At a given air pressure, volume (increases / decreases) ______________________ when

temperature increases.

g) At a given air pressure, density (increases / decreases) ______________________ when

temperature increases.

h) Therefore, (warm / cold) ________________ air rises in the atmosphere because it is less

dense than cold air.

2. Vertical Structure of the Atmosphere

Under the influence of gravity, the atmosphere is compressed toward the surface of the earth and

a balance is normally achieved between gravity and air pressure. This force balance between

gravity and air pressure is called hydrostatic balance. As illustrated in the diagram below, the air

pressure in a volume of air must equal the total weight of the column of air above it per unit area.

When these two forces are not balanced, such as within a thunderstorm or mountain wave,

upward or downward accelerations occur in the atmosphere.

Assuming that the atmosphere is in approximate hydrostatic balance, when we measure the air

pressure, we are in essence weighing the column of air above us per unit area. Pressure therefore

decreases with altitude and decreases with depth in the atmosphere. At sea level, the standard or

average pressure is 14.7 lbs./in2 = 1013 hPa (also known as millibars) = 29.92” Hg.

Pressure

Gravity

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On display in the Meteorology Lab is a mercury (Hg) barometer, the industry standard pressure

instrument. In a mercury barometer, the weight per unit area of the column of mercury equalizes

with the weight per unit area of the atmosphere. Under standard conditions, the mercury column

averages 29.92” at sea level and decreases about 1” for every 1,000’ of altitude in the atmosphere

(A graph of altitude vs. standard altitude is shown below.) As you might expect, air density also

decreases with height, dropping by about half about every 20,000’.

Figure 1. Variation of pressure with height. The two points represent the pressure and altitude at

the top and bottom of an arbitrary layer of the atmosphere.

Pressure can also be measured using aneroid or electronic pressure sensors. Altimeters actually

measure pressure, which is in turn converted to altitude based on an assumption of standard

atmospheric conditions and hydrostatic balance. Non-standard pressure can be corrected by

(p1, z1)

(p2, z2)

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simply calibrating the altimeter to the current observed sea-level pressure at a given location (the

altimeter setting). Non-standard pressure cannot be corrected in an altimeter. Therefore, when

flying from high to low (pressure or temperature) look out below, because your indicated altitude

will be higher than your true altitude.

Temperature is inversely related to air density, which affects how quickly the pressure decreases

with height. This is governed by the hypsometric equation below, which can be used to reduce

station pressure to mean sea level (MSL) or to estimate the thickness of a layer of air for a given

air temperature (the warmer the air, the thicker the layer will be).

)(ln2

112 equationchypsometri

p

pHzz

)( heightscaleg

TRH

In these equations, z2 - z1 is the altitude thickness of a layer of air with an average air temperature

of T in Kelvins. p1 is the pressure at the bottom of the layer of air and p2 is the temperature at the

top of the layer as shown in Fig. 1. R is the gas constant for dry air 287.05 J K-1

kg-1

, and g is the

acceleration of gravity (9.81 m s-2

).

Altimeter Exercise

In this exercise, we are going to measure the altitude of the Academic Complex. This will require

the use of an android phone or a hand-held Kestrel sensor provided by the lab instructor, along

with a scientific calculator that calculates the natural log (ln). A free Android app to use for this

exercise is called “Androsensor.” Follow the procedure below.

Procedure:

a) Obtain the temperature of the outside air either using the Kestrel sensor, rooftop

temperature sensor, or the latest METAR from the Prescott airport available online

(KPRC). Convert this temperature to Kelvins. This value is T.

T = ___________________ K

b) Calculate the scale height H in meters using the temperature in Kelvins you obtained in

the previous step (H = 287.05 × T ÷ 9.81).

H = ___________________ m

c) Measure the pressure as close to the ground and as accurately as possible using a Kestrel

or Android phone. This value is p1. Use whatever pressure unit you’d like.

p1 = __________________

d) Now, measure the pressure again at the level of the roof. This value is p2. Use the same

pressure unit as the previous step.

p2 = ___________________

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e) Finally, obtain the height of the building as follows:

z2 - z1 = H × ln (p1 ÷ p2)

z2 - z1 = ___________________ m

The accuracy of this height estimate will depend on the accuracy of the temperature and pressure

readings that you obtain. You might want to try this same exercise next time you go hiking to

measure your altitude change.

Reducing Pressure to Sea Level

In this exercise, we are going to reduce station pressure to mean sea level (MSL).

Procedure:

a) Estimate the temperature of the air between the surface and MSL by taking the average of

the current METAR temperature and the METAR temperature 12 hours ago. (Your lab

assistant can help you get it). Convert this temperature to Kelvins. This value is T.

T = ___________________ K

b) Calculate the scale height H in meters using the temperature in Kelvins you obtained in

the previous step (H = 287.05 × T ÷ 9.81).

H = ___________________ m

c) Obtain the pressure at a location where you know the precise altitude (MSL). This value

is p2 (use your desired pressure unit).

p2 = ___________________

d) Write the altitude from the previous step in meters.

z2 = ___________________ m

e) Finally, obtain the reduced MSL pressure (p1) using the exp() or ex function on a

scientific calculator using the expression below. The pressure units you obtain for p1 will

be the same ones you used for p2. (WARNING: This calculation will probably not be

accurate enough to use as your altimeter setting for flight.)

p1 = p2 × exp (z2 ÷ H)

p1 = __________________

Layers of the Atmosphere

Unlike pressure and density, temperature does not vary monotonically with altitude. Rather,

temperature increases with height in some layers and decreases in others. The change of

temperature with height depends on the altitudes where energy from the sun and the earth is

being absorbed by the atmosphere. For example, in the troposphere, the standard MSL

temperature is 15°C and the temperature decreases on average 2°C per 1,000’ altitude distance

from the surface of the earth increases. But in the stratosphere and thermosphere, the temperature

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increases with height due to the absorption of UV and x-ray/gamma-ray radiation respectively. In

the figure below (Fig. 2), label the four layers of the atmosphere (the troposphere, stratosphere,

mesosphere, and thermosphere). Separate the layers using a horizontal line between each layer

and label these lines the tropopause, stratopause and mesopause.

Figure 2. Standard temperature (°C) with altitude in the atmosphere. Your task will be to label

the four layers of the atmosphere and separate them with three horizontal lines labeled as the

tropopause, stratopause and mesopause.

50,000

100,000

150,000

300,000

250,000

200,000

30

40

50

100

90

80

70

60

20

10

-100 100 0

Alt

itu

de (k

m

MSL

)

Alt

itu

de

(f

t M

SL)

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Lab 3: Radiation Measurements -- Spectrometer Lab.

1. Introduction

Radiation measurements observe changes in radiation flux or energy in the Earth-atmosphere

system. Some of the Sun's energy is reflected by clouds, aerosols, and the Earth itself. The rest is

absorbed and re-emitted by land, water, clouds, and the atmosphere. The planet as a whole

warms when the global net radiation balance is positive and cools when it's negative.

Figure 1. Earth’s radiation balance. Incoming versus outgoing radiation. Courtesy of JeffreyT.

Kiel from NASA Goddard Space Flight Center.

Satellites can measure the radiation budget particularly well at the top of Earth's atmosphere and

monitor it over time, which is critical for detecting long-term climate tendencies. The current

challenge is to be able to quantify how each of the atmospheric, land, and ocean systems are

responding to net increases and decreases in the Earth's overall radiation budget. This requires

more information and improved observing systems.

http://www.cmdl.noaa.gov/gallery/dutton_talk_2005/Slide5?full=1

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Activity 1

Purpose and Objectives:

Plot out, understand and discuss the daily cycle of net radiation at certain location.

Recall that net radiation is equal to the energy coming into the earth (from the sun) and energy

going out of the earth (as infrared radiation and heat from the earth’s surface). The following

table shows the daily cycle of net radiation from June 15, 2005 at a field site in Northern

Manitoba, Canada at 55°N.

Negative numbers indicate an energy deficit, positive numbers an energy surplus.

Table 1. Net radiation data for June 15, 2005, Northern Manitoba, Canada

==================================================================

Time Net Radiation (W m-2) Time Net Radiation (W m-2)

Midnight -50 Noon 729

1 am -52 1 pm 676

2 am -56 2 pm 641

3 am -49 3 pm 522

4 am -39 4 pm 442

5 am 14 5 pm 208

6 am 130 6 pm 102

7 am 151 7 pm 24

8 am 375 8 pm -28

9 am 500 9 pm -70

10 am 577 10 pm -69

11 am 641 11 pm -66

===================================================================

Use the data from the table to plot out the daily cycle of net radiation at this location. Connect

your points with a smooth line.

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a. What happens to the earth’s temperature when it experiences an energy deficit (negative net

radiation)?

b. What happens to the earth’s temperature when it experiences an energy surplus (positive net

radiation)?

c. Using your graph from the previous page, estimate the time at which the energy deficit turned

to surplus.

d. Using your graph from the previous page, estimate the time at which the energy surplus turned

to deficit.

e. Estimate the time when the minimum temperature was experienced at the site, keeping in mind

how net radiation affects the earth’s temperature.

f. This data was taken from a day in the middle of June at a location in the subarctic Northern

Hemisphere (55°N). Explain two ways in which the data would be different if you looked at a

day from mid-December, both in terms of the total net radiation and the timing of the net

radiation curve.

Activity 2

Purpose and Objectives:

a. Gain an understanding of the wavelengths of visible light, and other parts of the

electromagnetic spectrum.

b. Learn the difference between active and passive remote sensing techniques.

c. Create reflectance spectra for various objects, both indoors and outside.

d. Compare your reflectance spectra to published reflectance spectra (search online sources).

Materials

a. ALTA reflectance spectrometer.

b. ALTA reflectance spectrometer manual.

c. Computer with online access.

d. Standard white card for photography, or white cardboard or heavy paper (high brightness,

at least 88).

e. Several leaves of different colors, including a very green leaf (geranium leaf works the

best), brown (dead) leaf, yellow leaf.

f. Several colors of printer paper, including neon shades if possible.

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Procedure for taking a reflectance spectrum:

1. First, turn on the ALTA spectrometer and record the dark voltage when no lamps are

illuminated. ___________ = Dark voltage

2. Look at the green leaf. What do you expect the reflectance spectrum for this leaf might

look like, check online (the internet)? Sketch a graph of it here:

3. Use the green leaf and record the reflectance at each of the ALTA wavelengths on the

Worksheet for Calculating Reflectance. Graph your results (Graph Template 1 from the

manual). Compare your results to the reference spectrum for spinach from the manual.

4. What anomalies do you observe? List them here:

5. The ALTA’s different LEDs are of different brightness, and the light sensor is most

sensitive to red and infrared light, and least sensitive to violet light. Does this help explain the

anomalies? Explain.

6. You’ll need to standardize your data. Use the “Standard” white card, which reflects

about 85% of ALL light that hits it. Record the spectrometer readings for each of the ALTA

wavelengths. Now, you can standardize the results from the green leaf, and calculate the percent

reflectance for each wavelength. Use the Worksheet for Calculating Reflectance.

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(Display voltage for sample – dark voltage)

% reflectance = ----------------------------------------------------------- * 100

(Display voltage for standard – dark voltage)

7. Collect data and graph reflectance spectra for the green leaf. Compare your reflectance

spectra for leaves with the reference spectrum for spinach. Discuss and explain any differences.

8. Collect data and graph reflectance spectra for the paper samples that are provided.

Discuss and explain the differences between these.

9. Go outside to collect data for reflectance of the sky, concrete, rock, tree, metal and glass.

Discuss and explain any differences among these.

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Lab 4: Wind and Pressure

1. Introduction

In Lab 2, the vertical pressure pattern was explained. Standard pressure at sea level is about

1013hPa and always decreases with increasing altitude. In approximately one mile in the

vertical, pressure normally decreases by about 15%, and in two miles in the vertical, the pressure

decrease is about 30%. Assuming that the sea level pressure is 1000hPa, estimate the pressure at

Prescott, AZ ________________________ and Leadville, CO ________________________.

Horizontal variations in pressure are much smaller. In two miles (from campus to the flight

line), the horizontal pressure difference is barely detectable except by very sensitive instruments.

However, it is small horizontal pressure variations that produce horizontal air motions called

wind.

Horizontal pressure changes are called pressure gradients and give rise to what is called the

pressure gradient force (PGF). A typical pressure gradient would be on the order of 4hPa over a

distance of 100 miles, or less than 0.5% change. As the pressure gradient increases (greater

change over the same distance), the PGF increases, and consequently the wind speed increases.

Weather maps are intended to depict the large-scale weather patterns that are associated with

either clear, sunny skies or cloudy, rainy weather. Because of the pressure dependency on

altitude, the surface pressure in Prescott is always less than standard sea-level pressure, and the

surface pressure in Leadville is even lower still. Without some adjustment, there would always

be low pressure centers over Prescott and Leadville, and the weather map would look a lot like a

topographic map but showing elevations in units of pressure. To eliminate variations in surface

pressure due to altitude, a fictitious column or mass of air is added to a station’s measured

pressure to show what that station’s pressure would be if it was located at sea level.

2. Surface station model

This lab introduces the techniques used in plotting and analyzing weather maps. The data are

plotted around a station circle in a standardized format.

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The wind is envisioned as blowing down the shaft of the arrow toward the station circle and thus

would be oriented differently depending on the actual wind direction. The depicted wind

direction is from the north northwest or 350º. The depicted wind speed is 15 knots.

Short barb = 5kts

Long barb = 10kts

Flag = 50kts

The surface air temperature is always plotted to the upper left of the station circle in units of ºF.

The surface dew point temperature is always plotted to the lower left of the station circle in units

of ºF. Sea-level pressure (SLP) is plotted to the upper right of the station circle to the nearest

tenth of a hectopascal (hPa) which is equivalent to millibars (mb), a unit which is still quite often

used. However, to save space, the entire numerical value of the SLP is not normally plotted.

Only the tens, units, and tenths digits are plotted. The plotted value must be preceded by a 9 or

10 for the complete value. The depicted value is 1010.7hPa. Generally, if the plotted value is

greater than 500, it is preceded by a 9, and if the plotted value is less than 500, it is preceded by a

10. Thus,a a plotted value of 986 would equal 998.6hPa, and a plotted value of 243 would equal

1024.3hPa.

An explanation of the weather symbols, sky cover, and pressure trend can be found at

http://www.hpc.ncep.noaa.gov/html/stationplot.shtml.

http://www.hpc.ncep.noaa.gov/html/stationplot.shtml

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Decode the information from each of the following station models:

Temperature__________ Temperature __________

Dew point temperature__________ Dew point temperature__________

Sea level pressure__________ Sea level pressure__________

Wind direction__________ Wind direction__________

Wind speed__________ Wind speed__________

Temperature__________ Temperature __________

Dew point temperature__________ Dew point temperature__________

Sea level pressure__________ Sea level pressure__________

Wind direction__________ Wind direction__________

Wind speed__________ Wind speed__________

39

18

075

71

54

972

-8

-12

386

106

41

998

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3. Surface map analysis

The two maps above show the same data at the same valid time. The bottom map has the

addition of isobars, lines of constant pressure, which help to illustrate the organization and

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pattern of the raw data. In general, isolines or isopleths are lines of constant value, and are often

used to analyze pressure (isobars), temperature (isotherms), and other meteorological data fields.

Isopleths can be thought of in different ways. They can be considered as boundaries separating

higher values from lower values. On the diagram below, draw isopleths separating the 6s and 7s.

The value of those isopleths can be thought of as 6.5.

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7

7 7 7 7 6 6 7 7 7 7 6 6 6 7 7 7

6 6 6 6 6 6 6 6 6 6 6 6 6 7 7 7

6 6 6 6 6 6 6 6 6 6 6 6 6 7 7 7

6 6 7 7 7 7 7 7 7 6 6 6 6 7 7 7

7 7 7 7 7 7 7 7 7 6 6 6 6 7 7 7

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 6

7 7 7 7 7 7 7 7 7 7 7 7 7 6 6 6

In addition to separating areas of different values, isopleths also represent places of equal value

along the line itself. On the diagram below, draw isopleths for the values of 2, 4, 6, and 8.

3 3 3 4 5 5 5 5 5 5 5 5 6 7 7 7 7 7 7

3 3 4 5 5 4 4 5 5 5 5 6 7 7 7 7 7 7 8

3 4 5 5 4 3 3 4 5 5 5 6 7 7 7 7 7 8 9

4 5 5 4 3 3 3 3 4 5 5 6 7 7 7 7 8 9 9

5 5 4 3 2 2 3 3 4 5 5 6 7 7 7 7 8 9 9

5 4 3 2 1 1 2 3 4 5 5 5 6 7 7 7 7 8 9

5 4 3 2 1 1 2 3 4 5 5 5 5 6 7 7 8 9 9

5 5 4 3 2 2 3 4 5 5 5 5 6 7 7 8 9 9 9

5 5 5 4 3 3 4 5 5 5 6 7 7 7 8 9 9 9 9

5 5 5 5 4 4 5 5 5 5 5 6 7 7 7 7 8 9 9

5 5 5 5 5 5 5 5 5 5 5 5 5 6 7 7 7 7 8

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Isopleths can also be considered as level curves as seen on a topographic map which shows

height above mean sea level.

4. Rules for isopleths

The following guidelines or rules should be considered when drawing isopleths:

a. An isopleth never crosses itself. If such a crossing did occur it would change the

orientation of the isopleth or how it acts as a boundary separating higher values from

lower values.

b. Isopleths never cross each other otherwise there would be two values for the same

variable at the point of intersection. The atmosphere is single-valued at a given time and

point in space.

c. Isopleths must be closed curves except at the edge of the map. Refer back to the sample

weather maps or the second sample exercise.

d. Isopleths should curve smoothly and gradually.

e. Adjacent isopleths should show similar patterns.

f. Isopleths should reflect the gradient, i.e., an isobar with a value of 1004hPa should be

positioned closer to a station with a reported value of 1003hPa than to a station with a

reported value of 1007hPa.

5. Surface analysis

Analyze the maps on the next two pages. On the first map draw isobars at 4hPa intervals from

1012hPa to 1036hPa. Those are the standard isobars and the standard isobar interval. The

http://en.wikipedia.org/wiki/File:Topographic_map_example.png

25

1016hPa isobar is already drawn to assist in drawing the additional isobars. On the second map,

draw isotherms at 10º intervals from -10ºF to +80ºF. The 50ºF isotherm is already drawn.

28

Lab 5: Radiosonde Launch; Introduction to SkewT-logP Chart

1. Introduction

Radiosondes are released at 0000 and 1200 UTC each day by the National Weather Service

(NWS) field offices, and provide a vertical profile or "sounding" of the atmosphere. Vertical

profiles are also provided by aircraft and satellite measurements. Thermodynamic diagrams (e.g.,

Skew-Temperature-LogPressure diagrams help forecasters to determine the relative humidity,

stability, vertical wind shear, severe weather, and flash flood potential of the atmosphere in the

vicinity of the sounding.

Most forecast centers have programs to calculate and display key forecast parameters from

thermodynamic diagrams. Critical information from the thermodynamic profiles includes:

a. Stability – Is the troposphere stable or unstable? Where are the stable or unstable layers?

b. Temperature – How close is the environmental temperature profile to the moist or dry

adiabatic profile?

c. Dewpoint – How close is it to the temperature profile? Where are the dry or moist layers?

d. Lifting Condensation Level (LCL) – How high?

e. Level of Free Convection (LFC) – How high?

f. CAPE (Convective Available Potential Energy) and Convective Inhibition (CIN) – Need

to know amounts and vertical distribution of these measures of stability; they are proportional to

the maximum updraft and downdraft speeds, respectively.

Activity 1

Today you will participate in a weather balloon/radiosonde launch from the roof of AC-1.

Procedure:

a. Issue a NOTAM: Contact the Flight Data Position of FSS (Lockheed Martin in Prescott

Valley) at the phone number: 1-877-487-6867 or if that doesn’t answer: (928) 583-6126

between 24 and 48 hours prior to the launch. This is in compliance with FAR Part 101.

Provide the following information:

(1) The balloon identification. (None)

(2) The estimated date and time of launching, amended as necessary to remain within

plus or minus 30 minutes.

(3) The location of the launching site. (Launch location: Lat=34°36’55.5”,

Lon=112°27’4.7”; Drake 158 radial, 12 mi.)

(4) The cruising altitude. (No cruising altitude: Termination altitude about 70,000’.)

29

(5) The forecast trajectory and estimated time to cruising altitude or 60,000 feet standard

pressure altitude, whichever is lower. (Time: about 45 – 60 minutes; Trajectory: Depends

on the wind profile.)

(6) The length and diameter of the balloon, length of the suspension device, weight of the

payload, and length of the trailing antenna. (Balloon diameter about 6’, payload about

0.25 lbs. or 250 g.)

(7) The duration of flight. (About two hours).

b. Contact Prescott Tower at 928-445-2160 and Center Control at 505-856-4535 to ensure

that the NOTAM has been received (at least 20 minutes before the launch).

c. Inflate Balloon:

Open a balloon (small cardboard box) and place it inside the balloon launcher

Using a foot-long piece of string, tie the nozzle of the balloon around the cone-shaped

helium dispenser at the base of the launcher

Connect the helium hose on the side of the launcher to the gray hose from the helium

tank (it should snap into place).

Make sure that the green shroud over the top of the launcher is tightly secured before

inflating

Inflate balloon until it completely fills the launcher (Note: Always turn on the black

nozzle before turning on the main nozzle; Turn off the main nozzle before turning off the

black nozzle. This will avoid the buildup of pressure in the pressure gauge.)

If helium tank runs out, make sure you change to a different tank.

Fold balloon nozzle in half and tie off very tightly using a foot-long piece of string.

Tie the top of a parachute to the balloon nozzle using a foot-long piece of string.

d. Activate sounding computer and radiosonde:

Turn on the gray SPS220 box to the right of the sounding computer

Turn on the gray ground-check box to the left of the sounding computer

Open a radiosonde package (keep packaging just in case the sounding fails)

Carefully place the radiosonde inside the ground check apparatus, with the instrument

probe closed in the dry chamber (make sure not to clamp down on the probe)

Connect the cable to the base of the radiosonde ("UP" side needs to be up)

30

Follow directions on the ground check instrument, using all default responses (Just push

"Select" until it asks you no more questions). Note the frequency.

e. Open DigiCORAIII program (on desktop) and follow DigiCORAIII instructions in manila

folder.

f. Connect the battery to the radiosonde before launch.

g. When instructed, launch the radiosonde as follows:

Carry the balloon launcher to the down-wind side of the building (as far away from walls

as possible)

Carefully remove the radiosonde from the ground check apparatus, plug the battery into

the radiosonde and attach it to the instrument.

Being careful not to damage the instrument probe, attach the plastic hook of the

radiosonde spool to the slit in the bottom of the parachute.

Remove the black plastic stopper from the spool to ensure that the string will unravel as

the radiosonde ascends.

While someone holds the balloon nozzle and radiosonde, other person opens the shroud

over the launcher.

Make sure balloon does not touch any rough surface, and release the balloon.

h. Call local Air Traffic Control at 1-877-487-6867 IMMEDIATELY after the launch to notify

that the balloon is entering their airspace.

i. Call Center Control at 505-856-4535 IMMEDIATELY after the launch to notify that the

balloon is entering their airspace.

j. Clean up:

Put all equipment away and throw away all trash (but keep radiosonde package and

radiosonde if possible if launch failed)

Turn off the ground check apparatus and SPS220 box by the sounding computer

Turn off lights, lock all doors.

k. Your lab instructor will e-mail you later during the day (after the termination of the weather

balloon flight) the .jpg file of the plotted radiosonde skewT-logP graphic.

31

Activity 2

This will require the use of a skewT-logP chart, provided by your lab instructor.

1. Make sure you have learned and understood the meaning of all different lines in the provided

skewT- logP meteorological chart. You may ask your lab instructor to explain and show you

examples.

3. The following sounding was obtained in central Alberta, Canada at a time when thunderstorms

were developing:

______________________________________________________________________________

Pressure (mb) Temperature (°C) Dew Point (°C)

920 (surface) 23.5 14.5

850 17.0 12.5

800 12.5 10.8

770 10.0 6.0

745 10.0 -1.5

660 2.0 -10.0

555 -10.0 -13.0

525 -12.0 -19.0

500 -13.0 -18.0

400 -24.5 -30.5

300 -39.5 -

215 -55.0 -

190 -53.0 -

180 -49.0 -

125 -53.0 -

______________________________________________________________________________

Plot the temperature and Dew point temperature profiles. Discuss the atmosphere of this example

2. Suppose a parcel of air from 920 mb ascends adiabatically. At what pressure would

condensation be reached, i.e. determine graphically the Lifting Condensation Level (LCL)?

32

Lab 6: Moisture and Clouds

1. Introduction

As the old joke says, “Without water, meteorology would be a very dry subject.” This lab will

look at water or moisture in the atmosphere and the visible expression of that water in the form

of clouds. Water is one of the very few substances that can exist in the atmosphere in all three

states: solid, liquid, and gas. It can even exist in all three states simultaneously in the same small

volume of air as snowflakes, liquid cloud droplets, and gaseous water vapor for example. The

transition from one state to another is called a change of state or phase change and involves the

absorption or release of energy. Text books typically give the normal composition of the

atmosphere as 78% nitrogen, 21% oxygen, and 1% all other gases most of which is argon.

Gaseous water or water vapor is a variable component of the atmosphere and can vary in

concentration from near 0% in extremely cold regions to up to 4% in very warm, steamy, tropical

areas thus changing the relative concentrations of the gases in the atmosphere.

Because of the relationship of temperature to the concentration of water vapor in the atmosphere,

the common expression is that warm air can hold more water vapor than cold air. Thermometers

measure the air temperature, and thus the capacity of the air to hold water vapor. Other

instruments are used to measure the actual amount of water vapor in the atmosphere. The

measure of the actual amount of water vapor in the atmosphere is commonly reported in

meteorological observations as the dew point temperature or simply the dew point. The dew

point represents the temperature to which the air would have to be cooled for saturation to occur.

By converting the temperature and dew point to units of mass of water vapor – grams of water

vapor per kilogram of dry air – the humidity, or more specifically the relative humidity, can be

calculated. The mass measurement of water vapor is called the mixing ratio where the

temperature or capacity of the air to hold water vapor is the saturation mixing ratio and the dew

point or actual amount of water vapor is the actual mixing ratio. Relative humidity (RH) is

expressed as a percent as follows:

actual mixing ratio

RH= 100%saturation mixing ratio

2. Calculate relative humidity

Table 1 presents saturation mixing ratio (sat MR) as a function of temperature. (There is also a

dependence on pressure as implied in the table title. The pressure dependence will be ignored

for the purposes of this lab.) Note that there is not a linear relationship between temperature and

saturation mixing ratio. Refer to the table and fill in the blanks below for sat MR for the

temperatures given. Then calculate the difference in sat MR for the two pairs of temperatures.

33

T = 2ºC Sat MR ________ T = 28ºC Sat MR ________

T = 0ºC Sat MR ________ T = 26ºC Sat MR ________

Difference ________ Difference ________

Table 1. Saturation mixing ratio at sea-level pressure as a function of temperature.

Temperature (ºC) Saturation

mixing ratio

(g/kg)

Temperature (ºC) Saturation

mixing ratio

(g/kg)

-10 1.794 16 11.560

-8 2.009 18 13.162

-6 2.450 20 14.956

-4 2.852 22 16.963

-2 3.313 24 19.210

0 3.819 26 21.734

2 4.439 28 24.557

4 5.120 30 27.694

6 5.894 32 31.213

8 6.771 34 35.134

10 7.762 36 39.502

12 8.882 38 44.381

14 10.140 40 49.815

Fill in the blanks below for the temperature/dew point pairs given and calculate the RH for each

pair. The actual mixing ratio is simply the saturation mixing ratio for the given dew point.

Temperature (ºC) Dew point (ºC) Sat MR Actual MR RH

14 4

14 12

24 4

24 12

24 20

34 4

34 12

34 20

3. Measure relative humidity

It normally requires some complex instrumentation and mathematics to measure the dew point

and then calculate the mixing ratio as in Table 1. Once that is accomplished, it is simple to

34

calculate the relative humidity. However, there is a rather simple looking apparatus called the

sling psychrometer that can be employed to determine relative humidity. This apparatus consists

of two thermometers attached to a frame and connected by a chain or bearing to a handle. One

of the thermometers is designated the dry bulb thermometer and the second thermometer has a

gauze wick covering the bulb and is designated the wet bulb thermometer. To operate, the gauze

wick is moistened with water, thus the name wet bulb. The thermometers are then whirled by

grasping the handle. The whirling action provides ventilation to enhance the evaporation of

water from the wick of the wet bulb.

Fig. 1. Sling psychrometer.

Evaporation of water from the wick requires absorption of heat of vaporization which the water

obtains from the bulb of the thermometer thus cooling the wet bulb thermometer. It takes several

minutes for the wet bulb to reach equilibrium with the atmosphere. Once the wet bulb

temperature has stopped dropping, the dry bulb and wet bulb temperatures can be recorded. The

difference between the dry bulb and wet bulb temperatures is called the wet bulb depression.

Once again, using some complex mathematics, a psychrometric table can be prepared that gives

the relative humidity as a function of the dry bulb temperature and the wet bulb depression

(Table 2).

Use the sling psychrometer to determine the relative humidity indoors and at two sites around

campus that you think will have different relative humidities. Fill in the table below by

recording the values you observe and using Table 2 to determine the relative humidities for each

location.

35

Location Dry bulb (ºC) Wet bulb (ºC) Wet bulb

depression (ºC)

Relative

humidity

Indoors

Outdoors #1

Outdoors #2

Explain any observed differences between the three locations. Are there any sources of error that

could have influenced your measurements?

Table 2. Psychrometric table showing relative humidity in per cent as a function of dry bulb

temperature (ºC) and wet bulb depression (ºC).

36

4. Cloud identification

Clouds form when invisible or gaseous water vapor condenses and becomes visible in the form

of liquid droplets or solid ice crystals. The most common cloud producing process is to cool the

atmosphere until a high enough relative humidity is achieved for condensation to occur. Relative

humidity in the range of 75% to 100% (saturation) is required for clouds to form. Upward

vertical motion is the most common mechanism that produces the required cooling.

Vertical motion can occur over several scales of motion to produce different cloud types. The

basic cloud types are shown in Fig. 2. Early meteorologists used Latin words to name the cloud

formations. With weak vertical motion over a large area, stratus or layered clouds develop. If

the stratus extends through a deep enough layer, precipitation can develop and the cloud is then

called nimbostratus (layered, precipitation-producing). Strong vertical motion on the order of

meters per second (ms-1

) produces vertically oriented clouds called cumulus or heaped clouds.

With sufficient moisture and concentrated vertical motion, cumulus clouds can develop into

cumulonimbus (heaped, precipitation-producing) or thunderstorms, also commonly abbreviated

as CB. With somewhat weaker and more widespread vertical motion, stratocumulus can develop

and cover a large portion of the sky. The variation in brightness or color from white to gray is

the clue to the stronger vertical motion in stratocumulus when compared to stratus.

Fig. 2. Common cloud types.

37

Clouds are further classified by the height of the base of the cloud above ground level (AGL).

With cloud bases above 6500 ft (2000 m) AGL, the prefix “alto” is added for altostratus and

altocumulus clouds. The prefix “cirro” is added to clouds with bases above 20,000 ft AGL to

form cirrostratus and cirrocumulus. High clouds without stratiform or cumuliform

characteristics are simply cirrus.

The activity for this section can be completed while outdoors with the sling psychrometer if

clouds are present, but due to the many days with clear skies, it may have to be completed at a

later date.

Observe the sky and sketch the cloud formations that you observe and label the sketch with the

generic cloud names. Also, estimate the bases of the clouds. Obtain the current metar

observation for Prescott Airport (KPRC) and compare the cloud heights in the observation with

your estimated heights.

KPRC _______________________________________________________________

38

Lab 7: Satellite and Radar Interpretation

1. Introduction

In the previous labs you have learned about moisture, clouds and cloud identification. In this lab,

we learn how to observe clouds and precipitation using remote sensing imagery. The types of

imagery that we will examine are satellite and radar. The objective is to gain some basic imagery

interpretation skills that could be useful in a variety of fields.

2. Satellite Imagery

Geostationary weather satellites are located about 22,000 miles above the earth’s equator. At this

distance, the orbital rate of the satellite is exactly the same as the earth’s rotational rate. This

keeps the satellite directly above a fixed location at the equator. There are two Geostationary

Orbiting Environmental Satellites (GOES) for North America. GOES-W scans the Western half

of North America as well as the northeastern Pacific Ocean, while GOES-E scans the Eastern

half of North America and the northwestern Atlantic basin. The satellites each have an imager, a

device which produces visible, infrared and water vapor images every 15 minutes. Since the

satellites are geostationary, time-lapse movie loops of the images can be created to illustrate

trends in the development or dissipation of weather systems in the atmosphere. These loops also

help interpret the cloud patterns and wind direction at various altitudes.

Visible (VIS) imagery depicts the amount of solar radiation scattered or reflected back to space

by either the clouds or the surface. The lighter (darker) a pixel is on a visible image, the higher

(lower) the amount of sunlight being scattered back up to space at that pixel location. Dense

cloud (such as thunderstorms) and fresh, deep snow are the whitest pixels to be seen on a given

image. Visible imagery is only available during daylight hours, but one can more easily identify

cloud types stratiform (smooth-looking) or cumuliform (bumpy) clouds best in this type of image

because the pixel size is generally smaller than the other two types of imagery (and therefore

easier to see). Stratiform clouds form in stable air, cumuliform clouds form in unstable air.

Infrared (IR) imagery shows the intensity (or radiance) of infrared imagery emitted by the clouds

or the ground in every pixel on the image. Since the infrared intensity depends directly on

temperature, this type of imagery indicates the cloud top temperature (or in the absence of cloud,

the surface temperature) at each pixel. Since temperature generally decreases with height, cloud

top temperature can be related to altitude. Where the IR image is dark, the temperatures are

warm (cloud tops are near the surface of the earth). Where the image is light, temperatures are

very cold (cloud tops are high). Gray would indicate middle tropospheric clouds. Sometimes

“enhancement” colors are applied to the image, making some or all of the image in color shades

that help interpret the cloud top temperatures more easily. IR imagery is available both day and

night, but the pixel size is larger, causing the texture of the clouds to be less defined. Another

disadvantage with IR imagery is that the tops of very low clouds or fog are usually similar to the

surrounding surface temperatures, causing them to be difficult to distinguish from the ground.

39

Water Vapor (WV) imagery indicates the humidity of the middle troposphere (~18,000 –

20,000’). This imagery could NOT be used to attempt to find weather features near the surface of

the earth (such as fronts), nor should it be used to identify where the lower atmosphere is moist

and capable of producing precipitation. Rather, it should be used to identify mid to upper-

atmospheric flows such as lows, jet streams, etc. The water vapor streaks on the image are

parallel to the wind. A swirling pattern is usually a low pressure area in the upper atmosphere,

and a narrow core of dry air usually indicates a jet stream. Dark colors indicate dry air, lighter

color indicates more humid air, white patches are clouds.

First things to consider when analyzing a satellite image:

Determine which type of image you are looking at (VIS, IR, or WV) based on text on the

image and/or the appearance of the image

Look at the date/time stamp and convert from Zulu to MST by subtracting 7h

Be familiar with the geography of the region you are analyzing

Get the “big picture” of the weather pattern first, then look at details

Identifying surface features [HINT: It’s also good to use a physical map as a reference]:

Water (daytime or summer when water is cooler than land) – Darker on VIS; lighter on IR

Water (nighttime or winter when water is warmer than land) – Darker on VIS; darker on IR

Forested Mountains – Dark on VIS; slightly lighter on IR

Sandy/Rocky areas – Lighter on VIS; same temp as surroundings on IR (e.g. White Sands, NM)

Metropolitan areas – May be darker or lighter on VIS; darker on IR

Fog – White dendritic pattern in valleys, butting up against coastlines or mountain ranges; same

temp as surroundings on IR (camouflaged)

Snow – Best seen in VIS imagery, white dendritr pattern (mtns), white area with dark rivers

(plains).

Identifying cloud types: Using Fig. 2 in Lab 6 as your guide to cloud types, one can identify

cloud types using satellite images. Use the IR imagery to identify if the cloud tops are dark (low

clouds), gray (middle clouds), or white (high) clouds. High clouds also cast shadows on VIS

imagery at low sun angles. The only exception would be very thick clouds that have low cloud

bases and high tops (such as cumulonimbus). Use VIS imagery to identify if it is a thick or thin

cloud and to identify the shape and texture of the clouds. Wispy clouds are typically ice clouds.

Flat clouds are stratiform. Bumpy clouds are cumuliform.

Identifying thunderstorms: White clouds on both IR and VIS. Bumpy cloud tops on VIS. Wispy

looking high altitude anvil cloud spreads downwind from the top of the storm. For the most

severe storms, the anvils spread typically toward the northeast with the region of cold and bumpy

cloud tops on the southwest side of each thunderstorm. In light wind, anvils spread in all

directions.

40

Identifying thunderstorm systems (like a squall line): Identify a long line or cluster of

thunderstorms (sometimes covering up to several states; most common in the Plains, Southeast,

or Midwest). This will show up as a large line or circle with white bumpy tops on the VIS

imagery and white tops on the IR.

Identifying an extratropical cyclone: Find a very large comma-shaped cloud pattern covering

many states (up to 1,000 miles across) in both the IR and the VIS. The center of the spiraling

clouds is where we would place an “L” on a Weather Map indicating an extratropical syclone.

The cloud band making up the southern extension of the comma cloud is caused by the cold

front, and the cloudy area extending east of the comma cloud is associated with the warm front.

There is usually a dark streak in the water vapor imagery just behind the cyclone. This is the

upper-level jet stream.

Identifying a tropical cyclone (i.e. hurricane or typhoon): Find a large symmetric mass of

rotating clouds with spiral rainbands. It will tend to be white on the top in both IR and VIS, with

a very bumpy top. They are annular (or donut) shaped, with a dark eye usually in the middle.

Exercises: The following questions refer to the image above.

1. Which type of image is this? (VIS / IR / WV) _____

2. Which point above is located in large band of marine stratus cloud? _____

3. Which point above is within an area of fog trapped between mountains? _____

4. Which point above is within a thin cirrus cloud streak? _____

5. Which point above is located on a snow covered plain? _____

6. Which point above is located on snow-covered mountain peaks? _____

VIS

A

B

C

E

D

41

B

A

C D

E F

G

B

A

C D

E F

G

42

The following questions refer to the previous three images.

7. In order, what is the image type (WV/VIS/IR) for each of the images above?

Top: _____ Middle: _____ Bottom: _____

8. What was the date and time in Arizona when this image created? _______________MST

9. Which point is located within an area of low cumulus clouds? _____

10. Which point indicates a thunderstorm with a sheared anvil by strong winds aloft? _____

11. Which point indicates a thunderstorm with weak winds aloft? _____

12. Which point is located at the center of a cyclone? _____

13. What type of cyclone is it (extratropical or tropical)? ____________________________

14. A cold front is located at which point? _____

15. Which point is located in low stratus clouds? _____

16. Which point is located in high stratiform cloud tops (most likely nimbostratus)? _____

17. Notice that points J, K, and L are located in cloud-free areas. Order them from least to

most humid in the mid troposphere. least: _____ medium: _____ most: _____

18. Which of the three points (J, K, L) is located near the jet stream? _____

19. What is the wind direction at point L (from the NW, W, or SW)? _____

J

K

L

B

A

C D

E F

G

43

20. Which type of cyclone is shown in the

VIS image at the left? (extratropical /

tropical) _____________

21. List two differences between this

cyclone and the one identified in the

previous exercise.

_________________________________

_________________________________

22. Draw an “L at the center of the cyclone.

A

B

C

D

EF

G

44

The following questions refer to the two images above.

23. At which point do you see forested mountains? _____

24. Which point is an urban area? _____

25. Which point is low cloud or fog? _____

26. Which point is located in fair weather cumulus? _____

27. Which point is located in a thunderstorm? _____

28. Which point is a sandy/rocky desert area? _____

29. Which is warmer, the ocean or the land? ________________

30. Which is warmer, the water on the west coast or east coast of the Baja of CA? _________

A

B

C

D

EF

G

45

3. Weather Radar Imagery

In the next lab, we will learn about reflectivity, which is a measure of the number and sizes of

particles in the air. A weather radar scans the atmosphere using an antenna that transmits

microwave pulses, and then receives the pulses that are scattered back. The radar also measures

the time it takes the microwave pulses to travel to and from each location that is sampled. These

data are then processed, giving us the final reflectivity product.

The radar data are usually plotted on a PPI (plan position indicator) image, with the radar

location located in the middle of a circular pattern of pixels. The reflectivity values are plotted

using a colorful scale of values, shown at the appropriate range and direction from the radar that

they were measured. Care should be taken not to misinterpret a PPI plot as a constant altitude

plot! For each radar scan, a different tilt angle above the horizon is employed. The most common

PPI scan that is available online is the base scan, which is typically 0.5° above the horizon.

The pixel size on a PPI image increases with range from the radar because the beamwidth is

about 1° wide. Therefore, small precipitation cells tend to appear larger or more smeared out at

far range from the radar. Another thing to keep in mind is that the radar also detects bugs, birds,

mountains, and other objects or artifacts that can show up on the radar reflectivity PPI.

For the remainder of this lab, we practice interpreting radar reflectivity on a PPI plot. The most

important skill is to learn to distinguish between tall convective echoes (associated with stronger,

more turbulent updrafts) and stratiform echoes (generally less turbulent and horizontally layered

precipitation patterns). Convective echoes generally have higher reflectivity values than

stratiform echoes, but not always! The best way to distinguish convective echoes is to look for

strong horizontal pixel-to-pixel changes in the reflectivity. This is because strong convective

updrafts tend to be more localized, causing the reflectivity to change greatly with distance along

the plot. Stratiform echoes are usually identified as all echoes that were not identified as

convective. (Remember, aircraft should avoid flying through all convective echoes and any

stratiform echoes that are connected to convective ones!).

At the top of the next page is a sample PPI plot containing a squall line, with the convective

echoes outlined in white. The rest of the echoes are stratiform. The location of the radar antenna

is also shown at the center of the PPI, denoted by the star shape. Note how the pixels get larger

with increasing range from the radar antenna site.

On the top right of the next page are some samples of some severe thunderstorm signatures.

Fingers are elongated strong echoes (> 60 dBZ) that denote possible large hail. Hook echoes are

the signature of supercell thunderstorms, which commonly produce tornadoes and large hail.

Bow echoes are intense curved sections of squall lines that bend forward from the rest of the line,

indicating widespread strong to damaging wind over a large area. Also, any echo that has

reflectivity values above about 65 dBZ is most likely severe (indicating large hail).

46

Figure. Sample radar images showing a squall line (left)

and common severe thunderstorm patterns in radar

imagery.

Exercises: In the radar images below, circle all of the convective echoes. Label any echoes that

appear severe to you as either a finger, a hook, or a bow echo. PLEASE USE PENCIL!

Bow echo

49

Lab 8: Precipitation Processes

1. Introduction

In this lab, we learn the microphysics of clouds and how different types of precipitation form in

the atmosphere in order to correctly anticipate which type of precipitation you will experience at

the ground or in flight. Most clouds do not precipitate. This is because clouds are made up of

very small liquid droplets or ice crystals that have formed around extremely small particulates

(called cloud condensation nuclei) in the atmosphere. The typical cloud droplet is approximately

10 µm in diameter (that’s ten millionths of a meter)! Because of wind drag and their small size,

cloud droplets fall only about 1 cm per second through the air. Therefore cloud droplets rarely

reach the ground.

In order for a cloud to precipitate, much larger particles of liquid or ice must be produced.

Raindrops typically fall through air at about 5 m/s, which is fast enough to exit the cloud and

reach the ground before evaporating. Raindrops are at least 1 mm in diameter, or 100 times the

diameter of a cloud drop, which is 1003 or 1,000,000 times the volume. Thus, the moisture from

one million cloud drops must somehow combine in order to produce one precipitation particle!

The growth of precipitation particles therefore takes time, and it requires dense cloud to be

present for at least 15 minutes.

There are two basic ways that precipitation particles form. One process is called (1) collision-

coalescence which occurs as larger cloud drops fall through an updraft faster than smaller ones,

colliding and sticking together. With time, these bigger cloud drops grow larger, fall faster and

faster and eventually grow to raindrop size. This precipitation type is more prevalent in warmer c

marine clouds whose tops are warmer than -10 °C (commonly producing drizzle or small

raindrops), or in the strong, turbulent updrafts of cumulonimbus clouds where many collisions

take place. In the next lab, you will learn how to identify convective precipitation on a radar

image, which forms through coalescence of particles in stronger updrafts (hence these updrafts

should be avoided in flight).

The second process is called the (2) ice crystal process or Bergeron process. This process is how

snowflakes form, and it generally occurs in moist air where the temperatures are between about

-20 and -10 °C. At these temperatures, clouds contain both frozen cloud particles and

supercooled droplets (unfrozen water droplets). Because ice crystals have stronger inter-

molecular bonds than liquid cloud droplets, the water droplets tend to have a higher saturation

vapor pressure than ice particles, and the invisible water vapor therefore diffuses (migrates)

towards the ice crystals. Thus, the crystals grow into snowflakes/ while the water droplets shrink.

Once large enough, the snowflakes fall toward the ground. Depending on the precise temperature

and humidity that the snowflakes grow, a variety of snowflake sizes and shapes are possible to

form as shown in the figure below. Supercooled liquid droplets can also collide with and freeze

50

onto ice particles, and in strong convective updrafts they can form white pellets of snow that

look like Styrofoam (snow pellets) or white balls of ice (graupel) or even hailstones.

Precipitation from the ice crystal process generally forms in stable air and weaker vertical

motion, creating stratiform or layered clouds and precipitation. In the next lab, you learned how

to identify stratiform echoes, which are generally less turbulent and safer for flight (except in

dissipating thunderstorms, freezing temperatures, or certain types of turbulence).

Figure: Various types of snow particles.

2. Top-Down Method

In this portion of lab, we will use the top-down method to anticipate which winter precipitation

type(s) will form for a given temperature and dew point profile in the atmosphere. There are

three steps to the top-down method:

A. Identify snow-producing layer. On a day when wintery weather is expected, usually

before a warm front passes, find the layer in the vertical temperature profile between -20

51

and -10 °C. If the air in this layer is moist enough, i.e. the dew point spread (temperature

minus the dew point temperature) is less than about 5°C in this layer, then snowflakes are

likely to form efficiently. If the air in this layer is dry, then precipitation will be unlikely

except for light rain or drizzle from low-altitude clouds.

B. Identify elevated melting layer. If a layer exists in the atmosphere where the maximum

temperature is about 1 to 3 °C, then the ice particles falling through that layer will start to

melt. If the maximum temperature is greater than 3 °C, then the snowflakes will

completely melt into raindrops.

C. Look for near-surface freezing layer. If melted precipitation falls through a layer that is

below 0 °C, it will either freeze in the air (forming ice pellets/sleet) or freeze on impact

with the ground (forming freezing rain). If the particles falling into this subfreezing layer

are only partially melted, then they will quickly freeze into sleet. On the other hand,

raindrops will likely not refreeze into sleet unless the temperature is about -6°C or less

over a layer at least 2,000’ deep. NOTE: If sleet reaches the ground, then pilots know that

freezing rain and severe aircraft icing conditions are occurring at a higher altitude.

Figure 2. Photos of (a) sleet and (b) freezing rain at the earth’s surface (NOAA).

Using the above three steps, one can anticipate the precipitation type fairly well if current

temperature/dew point profiles are available. A nice series of animations is available on the

website http://www.atmo.arizona.edu/~mullen/atmo170A1/animations/51_Sleet/51.html. It

should be noted, however, that the precipitation type can easily change with time as the

temperature or dew point profile changes. Moreover, the evaporation of rain into drier air near

the surface cools the air, causing rain to change to snow when the surface wet bulb temperature

is below 1 °C. The melting and evaporation in the elevated melting layer also cools that layer,

sometimes causing freezing rain to change to sleet or sleet to change to snow.

The following examples illustrate how to apply the top-down method. In these examples,

temperature is the solid profile and dew point is dashed. We assume that there is rising motion in

the air and precipitation is forming.

a) b)

http://www.atmo.arizona.edu/~mullen/atmo170A1/animations/51_Sleet/51.html

52

In the exercises on the next two pages, analyze the temperature and dew point profiles and

identify the precipitation type that you expect to form (snow, rain, drizzle, freezing rain, freezing

drizzle, or sleet).

Pre

ssu

re (

hP

a)

Hei

ght

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00

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f fe

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850

700

500

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10

15

20 In this example, the difference between

the temperature and the dew point in the

-20 to -10 °C layer is about 1.5 °C at the

bottom and 3.5 °C at the top. Snow will

therefore form efficiently in this layer.

At the bottom of the profile, we have a

6,000’ deep melting layer where the

temperature rises to a maximum of 3.5 °C,

so we should expect the snowflakes to

melt completely. Since the temperature

does not drop below 0°C again at the

surface, then we expect…RAIN.

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the temperature and the dew point in the

-20 to -10 °C layer is about 15 °C,

meaning that the air is fairly dry. Snow is

not expected to form in this layer.

Here, we have an elevated warm layer

capable of melting ice completely (4 °C),

yet no snowflakes to melt! Since there is

lifting in the atmosphere and this layer is

very moist, we anticipate that drizzle will

form from collision/coalescence.

Near the surface, there is a shallow cold

layer about 1,000’ deep with the min.

temperature just -2°C. The drops will

probably not freeze until reaching the

ground…FREEZING DRIZZLE.

53

Pre

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Precipitation Type: _________________________ Precipitation Type: _________________________

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55

Based on what you have learned, answer the following exploratory questions:

1. What are the differences between sleet and graupel in terms of how they look and how

they form?

________________________________________________________________________

__________________________________________________

2. What is the difference between drizzle and rain?

_____________________________________________________________

3. Name two ways rain at the surface could change to snow with time.

_____________________________________________________________

_____________________________________________________________

4. Freezing rain and sleet are most commonly observed when a warm front is approaching.

(True / False) ________________

5. How many freezing levels (i.e. altitudes where the temperature equals 0 °C) must be

present in the atmosphere for freezing rain or ice pellets to occur at the surface?

__________________________________________________

6. Could an aircraft experience icing from freezing rain conditions during flight when

freezing rain is not observed at the ground? ____________________.

Why or why not? _______________________________________________

_____________________________________________________________

7. If a pilot encountered freezing rain during flight, what precautions should he/she take to

escape? (HINT: What could be done to get the plane into warmer temperatures > 0°C

quickly and melt the ice from the plane?)

________________________________________________________________________

__________________________________________________

8. If a pilot encountered sleet during flight, what precautions should be taken to avoid flying

into freezing rain where severe icing could occur?

________________________________________________________________________

__________________________________________________

9. If freezing rain accumulated up to 1” thick on the surface of the earth and then

temperatures remained below freezing for several weeks afterwards, what negative

effects do you think this would have on the general public?

________________________________________________________________________

__________________________________________________

3. Precipitation Measurement

Being able to accurately measure the amount of precipitation reaching the earth is very

important for a variety of applications. Snow depth is important for skiing, transportation,

and for understanding how much snow pack is available for spring runoff. Knowing the

56

liquid water equivalent of precipitation is also important for flood forecasting, water resource

planning, agriculture, and many other applications.

However, precipitation is a difficult quantity to measure. The amount of rain or snow

measured at the surface can vary dramatically with horizontal distance along the surface of

the earth, especially from convective precipitation events. We therefore have far too few

automatic rain gauges in the world to accurately map out the rainfall. These measurements

are prone to large errors or instrument failures, especially on breezy or windy days when the

airflow around a rain gauge prevents the proper catchment of the precipitation particles.

Ground based and satellite borne weather radars provide millions of precipitation samples at

frequent time intervals around the world and help to provide more detailed mapping of

precipitation. Nevertheless, weather radar measurements pose their own unique set of

challenges for precipitation estimation. Since radar is based on scattering of microwave light

by precipitation particles, we must make some assumptions about how microwave light

interacts with precipitation. We assume the precipitation particles are all spherical, which is

not always true. We assume the particles are all liquid because ice is much less reflective

than liquid drops. We assume that all particles are falling at a certain fall rate, not blowing

horizontally, and not growing or evaporating as they fall toward the ground from the heights

in the atmosphere where the radar is scanning. All of these are not good assumptions, but

radar measurements help us fill in the gaps across the earth where we don’t have rain gauge

data.

Furthermore, the actual way that a radar measures precipitation also poses a significant

challenge in rainfall estimation. A radar measures the total amount of microwave light

scattered back to the antenna from a given location and converts this measurement to a

reflectivity value. The reflectivity (Z) is the raindrop diameter (mm) to the sixth power

summed for all the raindrops per unit volume (m3). It is difficult to relate this reflectivity

(dependent on the drop diameter to the sixth power) to the rainfall rate at the ground, which

depends on the drop diameter to the fourth power.

Rinehart (1995) suggested that reflectivity can be easily measured using a piece of filter

paper coated with blue dye and left out in the rain for a certain exposure time and capturing

raindrop stains. Then, using a template (provided) to convert the drop stains to droplet

diameter (in mm), one can sum the drop diameters to the sixth power (D6) over all the drops

and then divide by the volume of air above the filter paper from which the drops are falling

during the exposure time. (NOTE: One should neglect the splats around the edges of the

drops to avoid exaggerating the drop sizes.)

57

Below are two filter paper scans that were obtained during different rain events in Prescott.

Use the template provided by the lab monitor and the table below each sample to calculate

the reflectivity (Z) by calculating the sum of D6 then dividing the by the volume (V) of air

above the paper. To convert to the standard decibels of reflectivity units (dBZ), use the

equation dBZ=10 log10Z.

Sample 1 Sample 2

V = 0.087 m3 V= 0.064 m

3

D (mm) # of Drops mm6 D (mm) # of Drops mm6

4 4

3.8 3.8

3.6 3.6

3.4 3.4

3.2 3.2

3 3

2.8 2.8

2.6 2.6

2.4 2.4

2.2 2.2

2 2

1.8 1.8

1.6 1.6

1.4 1.4

1.2 1.2

1 1

0.8 0.8

SUM (mm6) SUM (mm

6)

Z = SUM / V Z = SUM / V

dBZ=10 log10Z dBZ=10 log10Z

58

Based on what you have learned, answer the following exploratory questions:

1. Which sample appears wetter above? ( Sample 1 / Sample 2) ___________________

2. Which sample has the higher reflectivity? (Sample 1 / Sample 2) _________________

3. Based on your answers to numbers 1 and 2, can reflectivity be used as an indicator of how much

rain is reaching the ground? (yes / no ) ________________

4. Radar reflectivity is an exact measure of how much rainfall meets the ground. (True /

False) _______________

5. The (larger / smaller) _____________ the drops and the (more / less) _________ drops

falling through the air, the higher the reflectivity will be.

6. Number the precipitation particles below in order from 1 = least reflective to 5 = most reflective.

_____ heavy rain

_____ moderate snow

_____ small, dry hail

_____ moderate rain

_____ large, wet hail

Reference: Rinehart, R. E. 1995. Student-determined Z-R relationship for North Dakota using the filter

paper technique. Reprints: 27th Conf. on Radar Meteorology

59

Lab 9: Average Circulation and Climate

1. Introduction

Climate is average weather. Patterns of average pressure, wind, temperature and precipitation

can be used to understand the climate of various places around the world. Climate is controlled

by factors like latitude, proximity to ocean, prevailing winds and location relative to mountains.

This lab first examines average global surface and upper air flow patterns and how they vary

during the year. These will be interpreted in the light of our understanding of the general

circulation and the seasonally-varying wind patterns (monsoon) over the Asian landmass.

Next, the average surface temperature and precipitation will be described. We will try to

understand these in relation to the general circulation, latitude, proximity to oceans, mountains

and the average surface flow direction.

Finally, we will examine climographs for several locations around the world. A climograph is a

graph of average temperature and precipitation during the year. The goal of this section is to

understand the diversity of climates on our planet, ranging from humid tropical climates with

abundant precipitation, to desert climates, to moist mid-latitude climates and cold and dry polar

climates. As we will see, some locations like California have precipitation in winter and a dry

summer. Other locations, like India have summer rains and winter drought. Places like inland

Siberia are pleasantly warm in summer but bitterly cold in winter. We will seek to understand the

underlying factors that control these climates.

2. Average mean sea level pressure for December-February

Ho

H H

H

H

L L

L L

L L

H H

L

60

From the map above, answer the following questions

a. Where is average high pressure found? ___________________________________________

b. Where is average low pressure found? ___________________________________________

c. Using your knowledge of geostrophic wind, estimate prevailing low-level wind direct over

Northern Europe? ________ North Pacific between 30 and 50°N? ___________

Southern Asia? __________ Japan? ___________ Boston? ________________

3. Average mean sea level pressure for June-August

a. Where is average high pressure found? ___________________________________________

b. Where is average low pressure found? ___________________________________________

c. What is the prevailing low-level wind direction over southern Asia? ____________________

Over Japan? ___________________ Over the eastern U.S.? _________________________

d. Compare the Dec-Feb map with the Jun-Aug map.

During which season are the NH oceanic subtropical highs strongest? __________________

During which season are the NH oceanic subtropical highs furthest north? _______________

During which season are the NH mid-latitude westerlies strongest? ____________________

H H

H H H

L

L

L

L L

L

L

L L

H

L

61

4. Average 300 hPa flow

This different map projection is called a polar stereographic projection. In it we see the

300 hPa circulation looking down on the North Pole. This consists of westerlies rotating

counter-clockwise around a low centered near the pole. This is called the polar vortex.

a. In which season are the upper westerlies strongest? _________________________________

b. Lowest 300 hPa height is found in ____________ (season) over ________________ (region)

c. Describe three differences between the summer and winter 300 hPa circulation ___________

___________________________________________________________________________

The map below is the Dec-Feb average 250 hPa height & isotachs, speeds > 60 kt shaded.

d. In winter, strongest 250 hPa westerlies are found ____________________________ (region)

e. Average 250 hPa winds near Japan are around ________ kt from the _________ (direction)

H

L

L

H

H

L

62

5. Average Surface Temperature

a. Why is it cold over Tibet (north of India)? ________________________________________

b. Which coast (E or W) of the United States is warmest in summer (top panel)? ____________

Why? _____________________________________________________________________

c. Which is colder in winter – land or sea? __________________________________________

d. Which regions are coldest in winter? _____________________________________________

Why? _____________________________________________________________________

e. Which regions are hottest in summer? ____________________________________________

Why? _____________________________________________________________________

f. What is average summer T in inland Siberia (60°N,130°E) _______°C? In winter ______°C?

What are the summer and winter temperatures near Iceland _______&_______°C?

Which location (S or I) is warmer in winter? ___________ In summer? ________________

In general, which locations have a large annual temperature range? ____________________

___________________________________________________________________________

Winter

Summer

63

6. Average Precipitation

a. Where is precipitation largest in JJA? ____________________________________________

b. Where is precipitation largest in DJF? ____________________________________________

c. When is the rainy season in N. Australia? _______ In India? _______ In California? ______

d. The precipitation max near the equator is called the _________________________________

e. The winter precipitation max in the N Pacific & Atlantic is due to _____________________

f. Identify dry areas ____________________________________________________________

64

J F M A M J J A S O N D

Month

0

2

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12

14

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18

20

Pre

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UDEL, 1981-2010 1.4oN,104oE, Elev 52 ft

WSSS Singapore Changi, Singapore


Month

0

1

2

3

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6

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UDEL, 1981-2010 67.6oN,133.4oE, Elev 453 ft

UEBV Verkhoyansk, Russian Federation

7. Climate Controls - latitude

In this section we will use the climograph to look at precipitation and temperature variation

during the year for several locations. Note that scaling of the y-axis (precipitation on the left and

temperature on the right) is different for each plot.

The climographs below are for Singapore (a tropical location near the equator) and Verkhoyansk

at 67°N in inland Siberia, one of the coldest winter locations in the Northern Hemisphere.

Notice that Singapore has a very small annual temperature range < 2°F, while Verkhoyansk

ranges between -50°F in winter and a very pleasant +60°F in summer, an annual range of 110°F!

a. Why do you think Siberia has most precipitation in summer?

b. Give three reasons for the extreme winter cold in Siberia

c. Why does Singapore temperature change very little during the year?

d. Why is there potential for greater precipitation at lower latitudes?

e. What three things can you conclude about the role of latitude on climate

65


Month

0

2

4

6

8

10

12

14

16

18

20

22

24

26

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30

32

34

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38

40

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50

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UDEL, 1981-2010 47.9oN,124.6oW, Elev 177 ft

KUIL Quillayute, WA


Month

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12

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-10

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(oF

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UDEL, 1981-2010 47.7oN,117.6oW, Elev 2385 ft

KOTX Spokane, WA

8. Proximity to Ocean and Mountains

In this section we will investigate the importance of proximity to ocean on climate, and the

impact of mountains on climate. The climographs are for Quillayute, WA, a coastal location on

the west (upwind) side of the Olympic Mountains, and for Spokane, WA, an inland location

downstream from the Cascades.

Notice that Quillayute temperatures range between 38°F and 58°F, an annual range of 20°F,

while Spokane ranges between 28°F and 68°F, an annual range of 40°F. Inland Spokane is 10°F

warmer in summer and 10°F colder in winter. Quillayute precipitation ranges from 2 inches per

month in summer, to over 15 inches in winter, while Spokane receives less than 2.5 inches year

round.

a. Explain why Quillayute has a smaller annual temperature range.

b. Why do you think that Spokane has significantly less precipitation than Quillaute?

c. Why do you think the precipitation at Quillayute is maximum in early winter (November-

January)?

66

Lab 10: Air Masses, Fronts and Mid-latitude Cyclones

Introduction

Mid-latitude cyclones and their attendant fronts are responsible for much of the adverse weather

in middle latitudes, including the United States. Fronts that separate air masses of contrasting

temperature and moisture content are often the locus for severe weather. Weather observations

plotted on maps can be used to identify air masses, fronts, mid-latitude cyclones and their

movement, and help meteorologists to forecast the often rapidly changing weather patterns of the

mid-latitudes. These weather maps are most useful when the myriad of data plotted on them are

analyzed by drawing isolines, or contours, of temperature (isotherms), pressure (isobars) and

wind (streamlines) so that spatial patterns can be easily recognized.

In this lab, we will first learn how to identify different air masses from plotted observations of

wind direction, temperature and moisture. Next, by analyzing a surface chart of plotted

observations, we will make a simple forecast for a location in Illinois.

Mid-latitude cyclones generally form and intensify beneath a region of upper-level divergence

east of an upper trough. We will look at a case study of a developing mid-latitude cyclone to

identify the surface and upper air patterns that accompany cyclone development.

Finally, we will examine the climatology of mid-latitude cyclones, to identify regions of the

world that are most impacted by these storms.

1. Identification of Air Masses and Fronts

An air mass is a body of air that has fairly uniform temperature and moisture characteristics. Air

masses can be cold, warm, dry or moist. Air masses acquire the characteristics of the surfaces

over which they form. For example, an air mass originating over a tropical ocean will likely be

warm and moist. Air masses originating over oceans are moist and are classified as maritime,

while those originating over inland continents are dry and are called continental. Warm air

masses are called tropical since they originate in the tropics, while cold air masses are dubbed

polar (or arctic if they are really cold). The table below describes the various air masses.

Air Mass Source Region Characteristics

cP Canada, Arctic Very cold, dry & stable. Possible lake effect snow.

mP Gulf of Alaska or

North Atlantic

Cool, moist, unstable with showers if west coast;

cool moist, stable and possibly foggy if east coast

cT Mexico & southwest

deserts in summer

Hot and dry, high-based thunderstorms possible. Can be

windy due to vertical mixing, especially in spring

mT Gulf, tropical Pacific or

tropical Atlantic

Warm and moist. Abundant precipitation if there is a

lifting mechanism. May help thunderstorms.

67

a. From the source regions shown below, identify each air mass influencing North America

b. On the chart below, draw enough streamlines to indicate the direction of the airflow, with

warm air flow in red and cold air flow in blue, as shown. Indicate the likely air masses

involved. From the wind directions and temperatures, sketch the likely location of a cold

front and a warm front.

120 W 90 W

20 N

30 N

40 N

68

c. Analyze the chart below.

First identify a cold and a warm front. These are best located by means of wind direction and

temperature, as in the previous exercise. For example, a cold front separates cold NW winds

west of the front from warm SW winds to the east. A warm front separates cold easterlies

north of the front from warm SW winds to the south.

Next, draw isobars every 4 mb (e.g., 1004, 1008, 1012 mb) in black. Don’t forget to ensure

that the isobars turn cyclonically (counterclockwise) at the front. Label the low pressure

center with an “L”.

Label a mT and a cP air mass. Lightly shade in grey where cloud cover exceeds 75%. Lightly

shade in green the regions receiving precipitation.

d. Assuming that the cold front is moving east, make a forecast for the point X in Illinois.

X

69

oC-40 -35 -30 -25 -20 -15 -10 -5 0 5 10

110 W 100 W 90 W 80 W 70 W 60 W 50 W25 N

30 N

35 N

40 N

45 N

50 N

55 N

98

8

10

00

10

00

10

12

1012

1012

10

24 1024

1024

10

24

1024

1024

10

36

10

36

L

H

H

L

H

H

NCEP T 700 (shd) & MSL press (hPa) for 12Z 13-Mar-1993

e. Describe the wind and weather sequence as a warm front passes. This will be the likely

forecast for Chicago.

2. Mid-latitude Cyclones

Mid-latitude cyclones develop in regions where there is a strong horizontal temperature gradient

and a source of upper-level divergence.

The map below shows sea-level pressure isobars and 700 hPa temperature (color shaded) for a

developing cyclone in March, 1993.

a. Where relative to the surface low is the cold air? __________ Why? ___________________

b. Where relative to the surface low is the warm air? _________ Why? ___________________

c. What would I expect to find at 300 hPa above the cold air (trough or ridge)? _____________

d. What would I expect to find at 300 hPa above the warm air (trough or ridge)? ____________

e. What can you say about the horizontal temperature gradient near the surface cyclone? _____

___________________________________________________________________________

70

The map below shows the 300 hPa chart for the same time. Upper-level divergence is colored

red, and convergence is blue.

f. When in relation to the upper trough is divergence? _________________________________

g. What two things does upper-level divergence do? __________________________________

h. From the previous MSL pressure chart, mark in the location of the surface low with an “L”.

Is it under upper-level divergence? ________

i. What is the direction of the 300 hPa air flow above the surface low? ____________________

j. Toward which direction do you think the surface low will move? ______________________

3. Cyclone Climatology

Mid-latitude cyclones form in preferred regions of the globe. These are generally regions of

strong thermal contrast, such as near east coasts, where cold continental air lies adjacent to the

warmer waters of the Gulf Stream and Kuroshio Current (east of Japan).

In this section, we will identify the regions where mid-latitude cyclones form and intensify, and

the regions where cyclones at any stage of development are most common. These are regions

where we would expect frequent stormy weather – strong wind, precipitation, turbulence, icing,

and reduced ceiling and visibility.

71

The map below shows regions of the NH where mid-latitude cyclones form and intensify.

a. Describe the regions where NH cyclones form and intensify

b. What factors are responsible for cyclones forming in these regions?

The map below shows regions where mid-latitude cyclones at any stage of development occur.

This is essentially produced by counting every low on 30 years of winter weather charts.

c. Given that these cyclones cause stormy conditions (IMC and high winds - adverse weather

for flight), where are the stormiest places in the Northern Hemisphere winter?

72

Lab 11: Thunderstorms and the Skew T – Log P Chart

Introduction

Thunderstorms produce heavy rain, hail, lightning, thunder, and occasionally, strong winds and

tornadoes. These dangerous phenomena develop when warm moist air is lifted in an unstable

environment. There are three requirements for thunderstorms to form: instability, abundant low-

level moisture, and a lifting mechanism (sometimes called a trigger).

1. Thunderstorm Occurrence over the U.S.

a. The map above shows the average number of thunderstorm days each year throughout the

U.S. The most frequent occurrence is in the southeastern states, with Florida having the

highest number of 'thunder' days (80 to 100+ days per year). Thinking about the requirements

for TS formation, why do you think Florida has the most TS?

b. What do you think is the lifting mechanism for TS over Florida? ______________________

c. Why so many TS over Colorado and New Mexico? _________________________________

d. Why so few TS over the western states? __________________________________________

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2. Assessing the likelihood of Thunderstorms

Thunderstorm potential is assessed by plotting sounding data on a Skew-T – log-P chart, where

observed temperature and dew point are plotted as a function of height. This is called the

environmental sounding. Next, using just the surface temperature and dew-point, a parcel curve

is drawn. If the parcel curve is to the right of the environmental temperature, thunderstorms are

likely.

a. On the blank skew T – log P chart below, plot the environmental sounding data shown in the

Table and connect points.

P 1000 900 800 700 600 500 400 300 250 200 150 100

T 20 12 5 -5 -15 -25 -40 -55 -56 -57 -55 -55

DP 10 5 0 -10 -25 -40 -50 -60

0.1 0.2 0.5 1 2 5 10 20 30MR

-20

-10

0 5 10

15

20

25

30

30

20

10

0

-10

-20

-30

-40

-60

-70

-80

T

(o C

)

260

280

300

320

340

360

(K)

1000

900

800

700

600

500

400

300

250

200

150

ALTITUDE(std atm)

Kft Km

0

2

4

6

8

10

12

14

0

10

20

30

40

74

b. Now construct a parcel curve. First lightly draw a line from the surface (1000 mb) T parallel

to the dry adiabats (the orange lines sloping to the left) up to about800 mb. Next, lightly draw

a line from the surface DP parallel to the mixing ratio lines (the green dashed lines that slope

to the right), again up to about 800 mb. Where these two lines intersect is the lifting

condensation level (LCL), which marks the base of any convective cloud. From the LCL,

continue to draw a line parallel to the saturated abiabats (the solid green curvy lines that

slope to the left) and continue this line up to 200 mb. The line you have just drawn (dry

adiabat from the surface temperature to the LCL, then saturated adiabatic up to 200 mb) is

called the parcel curve. When you are satisfied with your parcel curve, overdraw this in red.

Here, the LCL (cloud base) is found at ________ mb

c. The parcel curve represents the rate at which a parcel lifted from the surface will cool. If the

parcel is warmer than the environment, it will continue to accelerate upward while it remains

warmer than the environment. Thunderstorms generally occur when and where the parcel

curve is warmer than the environmental sounding throughout appreciable depth of the

atmosphere.

Will there be thunderstorms here? __________ Why? _______________________________

d. Commonly, the parcel curve is cooler than the environment in a shallow layer near the

surface, inhibiting thunderstorm formation. An external lifting mechanism (called a trigger)

is needed to lift the parcel past this stable layer.

Possible lifting mechanisms are _________________________________________________

At some point, the rising parcel (the red curve) becomes warmer than the environment and

will convect (rise) freely until it becomes colder than the environment again. This point is

called the level of free convection (LFC).

Here, the LFC is found at _________ mb.

The level at which the rising parcel again becomes colder than the environment is called the

equilibrium level (EL). At this point the parcel will cease to accelerate upward. The EL

generally marks the cloud top.

Here the EL (cloud top) is found at _______ mb.

e. At the end of this lab, your instructor will use a software package to plot the sounding. The

package will also print out the values of the LCL, LFC and the EL. Enter these below

LCL __________ mb; LFC __________ mb; EL __________ mb

How well did you do? ______________________

75

Lab 12: Aviation Weather Hazards

1. Introduction

This lab is intended to familiarize you with the Aviation Weather Center website in general, and

specifically the Aviation Digital Data Service (ADDS) Flight Path Tool (FPT). Consequently,

this lab must be completed using available computer resources. First, log in to:

http://aviationweather.gov/ and become familiar with the site organization. Then, in the TOOLS

pull down menu, select Flight Path Tool. First, click on the INFO tab. Read the Overview and

then select the Tutorial. After completing the Tutorial, go back to the FPT main page and click

on Launch Flight Path Tool and then Run. Then click on LOAD to run the FPT.

The FPT has the familiar map view as a default, but it also has a cross-section feature. The

default map view shows winds at 10,000ft msl. Isotachs are shaded from pale blue to dark red.

Areas shaded brown depict terrain above the msl height selected. Move the slider on the right-

hand side of the map and observe how the winds and terrain vary. Now select Temperature from

the Background Grids menu.

Are there any areas below 0°C? _________________________________ Adjust the altitude

slider to obtain a relatively small, state-size area with temperatures below 0°C.

Next select Relative Humidity. Are there any areas at that altitude with RH greater than

70% ______________________________

80% ______________________________

90% ______________________________

Areas with RH greater than 70% are generally associated with clouds (visible liquid water), and

areas with temperatures below 0°C but above -20°C indicate areas with the potential for

supercooled water drops.

Do any areas with temperatures below 0°C and RH greater than 70% overlap? ______________

The overlap areas have the potential to produce aircraft icing. Now select Icing.

Are there areas of icing potential? __________________________________

Do any of the areas of icing potential match your overlap areas of temperature and RH?

___________________________

Use both probability and severity to determine what and where the greatest intensity of icing is

indicated. _____________________________

http://aviationweather.gov/

76

Select the “airplane” tool to draw a cross-section through the icing area. Above what flight level

must you fly to completely avoid the icing potential?___________________________

Using the ICAO “airplane” tool, draw a cross-section that passes through an area of icing

between two major airports. You can use the Airports selection from the Map Overlays list to

find the ICAO identifier for the airports. How do you think that the icing forecast will affect

flights on that flight path?

_________________________________________________________________________

Now select Turbulence. Move the altitude slider up and down and observe how the turbulence

pattern varies. Is there a relationship with terrain? If so, state where and at what altitudes, you

think that terrain is a factor in producing turbulence.__________________________________

Now select Wind Barbs from the Data Overlays list. Adjust the altitude slider to flight level

32,000. What if any is the relationship between wind speed and turbulence?

__________________________________________________________________________

Using the cross-section “airplane” tool, draw a cross-section for a low level flight through an

area of turbulence associated with significant terrain features. What flight level would you have

to select to avoid moderate or greater turbulence?_____________________________________

Adjust the altitude slider to 32,000. Using the ICAO “airplane” tool, draw a cross-section that

passes through an area of turbulence between two major airports. What flight level depicts the

greatest turbulence intensity?__________________________________________________

What flight level should you choose to avoid moderate or greater turbulence?________________

Windshear is either a horizontal or vertical change in wind speed or direction. Air Force

Weather Agency Technical Note 98/002 (AFWA/TN-98/002), Meteorological Techniques (13

June 2003) presents the table below.

Table 1. Turbulence intensity for shear critical values (taken from AFWA/TN-98/002)

Turbulence Intensity

Light Moderate Severe Extreme

Horizontal Shear 25-49kt/90NM 50-89kt/90NM >90kt/90NM

Vertical Shear 3-5kt/1000ft 6-9kt/1000ft 10-15kt/1000ft >15kt/1000ft

Examine an area depicted as moderate or greater turbulence. Calculate the vertical shear in that

region._________________________________

Based on your calculation, what intensity of turbulence would you forecast for that region?

__________________________

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