INTRODUCTION TO METEOROLOGY PART TWO

SC 208

DECEMBER 2, 2014

JOHN BUSH

ATMOSPHERIC SCIENCES

• Meteorology

– Short term weather systems in time spans of hours, days, weeks or months

– Emphasis is on forecasting near/medium term weather

• Climatology

– Frequency and trends of weather systems over years and even millenia

– Emphasis on understanding and forecasting changes in long term weather patterns

SOME FUNDAMENTAL SCIENCES

• Thermodynamics

• Fluid mechanics

• Computer modeling

• Instrumentation

• Chaos theory: “even detailed atmospheric modeling cannot, in general, make precise long-term weather predictions”

WHAT MIGHT YOU WANT TO GET FROM A METEOROLOGY COURSE?

• Interpret media weather forecasts

• Develop feeling for the way forecasts are made

• Understand your very local weather

• Understand why and how major events happen

• Understand origin of particular weather phenomena-lightning and thunder, clouds and fog—”May Gray” and “June Gloom”-- Santa Anas, spectacular sunsets

• Satisfy simple curiosity

METEOROLOGY: AN INTRODUCTION TO THE WEATHER PROFESSOR ROBERT G FOVELL

• Lecture 1: Nature abhors extremes

• Lecture 2: Temperature, pressure, and density

• Lecture 3: Atmosphere—composition and origin

• Lecture 4: Radiation and the greenhouse effect

• Lecture 5: Sphericity, conduction, and convection

• Lecture 6: Sea breezes and Santa Anas

• Lecture 7: An introduction to atmospheric moisture

• Lecture 8: Bringing air to saturation

• Lecture 9: Clouds, stability and buoyancy Part 1

• Lecture 10: Clouds, stability and buoyancy Part 2

• Lecture 11: Whence and whither the wind Part 1

• Lecture 12: Whence and whither the wind Part 2

SOME FUNDAMENTAL CONCEPTS

• Temperature

• Pressure

• Density

• Buoyancy

• Perfect Gas Law

• Atmospheric composition

• Structure of the atmosphere

• Solar radiation

• Heat transfer

TEMPERATURE SCALES Celsius °C Fahrenheit °F Absolute °K T °K= t°C + 273.2

Some benchmark temperatures

•Boiling Water (sea level) 100 °C 212 °F

•Hot day 38 °C 100 °F

•Body temperature 37 °C 98.6 °F

•Warm day 30 °C 86 °F

•Indoors 20 °C 68 °F

•Cool day 10 °C 50 °F

•Freezing (impure water) 0 °C 32 °F

•Freezing (pure water) -40 °C -40 °F (Supercooled water)

Conversion

•Exactly t°F = 9/5 t°C + 32

•Approximately t °F ~ 2 t °C +28

UNITS OF PRESSURE

PRESSURE

Force per unit area Newton / meter2 = 1 Pascal

Hectopascal (hPa) = 100 Pascals = 1 millibar (mb)

Pound (force) / in2 (psi)= 69 hPa

Inch of mercury (inHg) = 33.9 hPa

Standard atmospheric pressure at sea level 1013.25 hPa , mb

14.7 lb/in2

29.9 inHg

ISOBARIC CHART

AUGUST 4, 2014

September 23, 2014 October 2, 2014

DECEMBER 2,2014

Los Angeles Times

DENSITY AND BUOYANCY • Density: Mass per unit volume

– 1.2754 Kg/m3 dry air 1,013 mb 0°C

– 1.2041 Kg/m3 dry air 1,013 mb 20°C

• Buoyancy

– Archimedes' principle: the upward force exerted on a body immersed in a fluid, is equal to the weight of the fluid that the body displaces = density(1) times volume.

– The magnitude of the net force is the difference between the upward force and the weight weight of the object (density(2) times volume)

– Therefore if the density of the body is less than the density of the medium it rises until the densities become equal

COMPOSITION OF THE ATMOSPHERE

• Dry air: major components – Nitrogen N2 78%

– Oxygen O2 21%

– Argon Ar 1%

• Moist air contains water vapor: H2O

• Moist air is less dense than dry air

• The capacity of an air mass to hold water vapor depends on its temperature

• The amount of water in an air mass depends on its history

STRUCTURE OF THE ATMOSPHERE

• Standard Atmosphere Layer Base altitude Lapse rate* Temperature Pressure

km °C/km °C mb

• Troposphere 0 -6.5 +15 1013

• Tropopause 11 0 -56 226

• Stratosphere 20 +1.0 -44 55

*The lapse rate measures the rate of change of temperature with altitude—this value is for dry air

IDEAL GAS LAW • Relates pressure , volume and temperature of gases

(approximately)

– P=pressure, V=volume, T=temperature (°K)

N~mass, R and R’ are constants, ρ=density

PV = NRT or P = ρ R’T or ρ = P/R’T

• If you increase the pressure of a parcel of air it becomes denser

• If you increase the temperature of a parcel of air it becomes less dense

• If a parcel of air is less dense than its surroundings it will rise (thermally direct circulation) if not forced down (thermally indirect circulation)

• If you increase the mass in a parcel of air the PV product must increase

SOLAR RADIATION

• Solar radiation is the principal energy source for the atmosphere

• Solar radiation is scattered, reflected, or absorbed and converted to other forms of energy

– Thermal--heats atmosphere from the bottom

– Mechanical—winds & waves

– Potential—evaporation & precipitation

• Solar energy does not heat the earth uniformly—THIS IS THE PRIME REASON WE EXPERIENCE WEATHER

• The intensity of solar radiation on a given area varies in a regular way with the latitude and the seasons

CONDUCTION & CONVECTION

• Thermal energy is that part of the internal energy of an object that is responsible for the object’s temperature -- its unit of measure is the joule

• Thermal energy transfers (when possible) from high temperature to low temperature objects—called “heat transfer”

• Three processes make thermal energy transfer to and from the atmosphere possible

– Radiation/absorption

– Conduction

– Convection

SOME PROPERTIES OF MATERIALS RELATING TO CONDUCTIVE HEAT TRANSFER

• Thermal conductivity: joules/meter sec °K – Air 1 atm 27°C 0.03

– Water 27°C 0.6

– Dry sand 0.25-1.4

• Heat capacity: joules/kg °K – Air 1 atm 27°C 1000

– Water 27°C 4180

– Dry sand 830

WINDS

• Air masses in motion—require forces to change their motions

• What causes them?—Pressure gradient force (PGF)

• What determines their speed? PGF, topography …

• What determines their direction? Complex

– Direction of the pressure gradient

– Long distance high altitude winds: Coriolis Effect

CORIOLIS EFFECT

• Arises due to the fact that the earth is a rigid rotating near-spherical body

• Causes north-south winds in the northern hemisphere to appear to curve to the right

• A good reference is “The Coriolis Effect: A (Fairly) Simple Explanation”

Google Coriolis Effect

SEA BREEZE MODEL

LECTURE 12

• Whence and whither the winds Part II

QUESTIONS/TOPICS?

• Geostrophic winds

• Buys-Ballots law

• Geostrophic balance

• Effects of friction

• Why don’t geostrophic winds blow across isobars?

• Gradient wind balance

• What does he mean by “unstable” air?

• Importance of curving isobars

• How does spin induce low pressure?

NEXT CLASS

• Review of moisture and clouds

• Lecture 14 Fronts and extratropical cyclones

• Lecture 15 Middle troposphere—troughs and ridges

PRESSURE GRADIENT FORCE

PGF is proportional to the change in pressure between two points divided by the distance between them

GEOSTROPHIC BALANCE

• Most useful to explain direction and strength of winds—especially winds aloft

• Applies to winds that blow in a straight line

• A geostrophic wind blows with low pressure to its left

• Explains why cyclonic

winds blow counter-

clockwise in Northern

hemisphere

CENTRIPETAL FORCE

Centripetal force opposes PGF

Centripetal force aligns with PGF

BALANCE OF FORCES If the forces are balanced there is no net force to change the path or speed of

the wind

• Geostrophic balance: PGF with Coriolis

PGF + CF = 0

• Gradient balance: Geostrophic balance with centripetal force

PGF + CF + CENTF = 0

• Guldberg-Mohn balance: Gradient balance with frictional force

PGF + CF + CENTF + FF = 0

SURFACE WINDS INDUCE VERTICAL WINDS