CHAPTER 3 ATMOSPHERIC

PRESSURE

Weight of Earth’s Atmosphere

• Given: Weight per square inch = 14.7 lbs• Given: Earth’s surface area = 196,000,000

sq miles (statute)• Weight per square foot = 2116.8 lbs or

1.0584 tons• Weight per square mile = 29,506,498 tons• Total weight of atmosphere =

5,783,000,000,000,000 tons

• METAR KMWH 121952Z 00000KT 1/4SM FG OVC001 02/02 A3055 RMK AO2

• SLP361 T00170017•  • TAF AMD KMWH 121935Z 1220/1318 05004KT 1 1/2SM BR

OVC004• FM122100 VRB03KT 1SM BR OVC003• FM130900 33004KT 3SM BR OVC004• FM131300 VRB03KT 1SM BR OVC001•  • KMWH 120352Z 25004KT 10SM BKN095 M01/M01 A3052

RMK AO2 SLP350 T10061011•  • KMWH 112323Z 1200/1224 35005KT P6SM BKN130

BKN200 • FM120300 26005KT P6SM SCT008 BKN070 • FM120800 28003KT 5SM BR BKN008 • FM121400 36004KT 2SM BR OVC004 • FM122000 34004KT 3SM BR OVC006

• KEPH 120353Z AUTO 01010KT 10SM SCT090 00/M02 A3050 RMK AO2 SLP344 T00001017

• KEAT 120355Z AUTO 00000KT 10SM CLR 02/01 A3051 RMK AO2 SLP345 T00220006

•  • KEAT 120106Z 1201/1224 02004KT P6SM VCFG SKC • FM120600 28004KT 6SM BR SCT005 BKN200 • FM121000 VRB03KT 3SM BR BKN005 • FM121500 00000KT 1/2SM FG OVC001• •  • KGEG 120353Z 30003KT 1/2SM R21/4500VP6000FT BR SCT001

BKN002 OVC004 01/01 A3050 RMK AO2 SFC VIS 1 1/2 SLP348 T00060006

•  • KGEG 120029Z 1200/1224 00000KT 2SM BR OVC005 • TEMPO 1200/1203 1/2SM FG OVC002 • FM120300 VRB04KT 1/2SM FG OVC002 • FM121100 VRB02KT 1/2SM -SN FZFG OVC001 • FM121900 VRB03KT 2SM -DZ BR OVC002

14.7 lbs. per square inch

Volume of Earth’s Atmosphere

• Given: 99% is contained within 31 miles of the surface

• Total volume = 6,076,000,000 cubic miles

ATMOSPHERIC PRESSURE

• Evangelista Toricelli who was a student of Galileo invented the barometer in 1643.

• There are 2 types of Barometers: Mercurial and Aneroid (without liquid) Wafer type.

• Baro =Greek for weight• Aneroid = not wet

BAROMETERS• Any instrument that measures pressure is

called a barometer

• Aneroid Barometers work similar to Altimeter

Aneroid barometer

Mercury Barometer

BAROMETERS• Mercury Barometers need to be

corrected before any of the pressure readings can be used for maps.

• Elevation must be corrected (set for sea level)

• Temperature (corrected to 0 degrees C)

• Acceleration of gravity (45 degree latitude)

BAROMETERS ELEVATION

BAROMETERS ELEVATION

MERCURY BAROMETERS• Atmospheric pressure forces

mercury from the open dish upward into the evacuated glass tube. The height of the mercury column is a measure of atmospheric pressure.

MERCURY BAROMETERS• Standard sea level pressure =

29.92 inches of mercury or 1013.25 hectopascals (=millibars)

• pressure = force per unit area

PRESSURE VARIATION

• Pressure Varies with

• Altitude - Pressure drops at an average of 1 inch/ 1000’ as we go up in the atmosphere

• Also with: Temperature

• Stations then take the local pressure and plot it on maps to follow the pressure patterns.

• Lines of equal pressure are then connected called isobars.

LOW PRESSURE

• Low = center of pressure surrounded on all sides by higher pressure also called a cyclone. Cyclonic

• rotates counterclockwise • area of rising air• usually clouds present• bad weather

HIGH PRESSURE• High = a center of pressure

surrounded on all sides by lower pressure also called an Anticyclone. Anticyclonic

• rotates clockwise• area of descending air• usually no clouds• good weather

Other PRESSURE Definitions

• Trough - an elongated area of low pressure with the lowest pressure along a line marking maximum cyclonic curvature.

• Ridge - an elongated area of high pressure with the highest pressure along a line marking maximum anticyclonic curvature.

Other PRESSURE Definitions• Col = the neutral are between two

highs and two lows (like a mountain pass on a map

Surface/Upper Air Maps

• We will discuss more in detail latter on.

• You can find many different kinds of weather maps for different pressure analysis.

• 250, 500, 700 etc…

• These charts can be very useful in determining the weather at specific altitudes

• Example 700mb chart is approximately 10,000 ft MSL

ALTIMETRY• The Altimeter is basically an

aneroid barometer (measures height)

• Indicated altitude - read off a correctly set altimeter

• Pressure altitude - altitude of the 29.92” line or read off altimeter when set to 29.92

• Density altitude - pressure altitude corrected for nonstandard temp.

ALTIMETRY• Absolute altitude - the height above the

surface (AGL)

• True altitude - actual altitude above sea level

TEMPERATURE• Causes an airmass to expand or

contract

• This however does not necessarily effect pressure with a given volume of air

• therefore the pressure line will be higher when warmer

• the pressure line will be lower when colder

INDICATED ALTITUDE• Temperature affects indicated altitude• Cold temperature correction charts

DENSITY ALTITUDE

• High Density altitude refers to height not density. Gives:

• reduced power

• reduced thrust

• reduced lift

• Use the same airspeeds but ground speed is higher

DENSITY ALTITUDE

ICAO cold temperature error table

• http://www2.faa.gov/airports_airtraffic/air_traffic/publications/ATpubs/AIM/Chap7/aim0702.html

PRESSURE CHANGES IN FLIGHT (read Pages 18-19)

• When flying from High to Low “Look out below”

• When flying from Low to High “High in the sky”

• Above 18,000 feet the altimeter is set to 29.92 and only pressure altitudes are flown

Chapter #3What causes a L or H pressure? 1

• Temperature

• In a closed container more temp = more pressure

• You might think that the higher the temp the higher the pressure

• But No!

What causes a L or H pressure? 1• Usually the highest pressures are

found in cold regions

• Why?

• Because of Density

• Usually the higher density offsets the lack of movement of the molecules

What causes a L or H pressure? 2• Convergence• movement of air aloft is not always at

the same speed• where it slows down it piles up into a

High pressure• the piling up of air is called

convergence

What causes a L or H pressure? 3

• Divergence

• opposite of convergence

• the upper level wind speeds up and stretches the air out creating a Low pressure

• usually good wx under an upper level divergence

What causes a L or H pressure? 4• Thermal tides

• At an average altitude of 60 mi (thermosphere) changes of over 500ºC

• the rapid warming and cooling of upper air causes great density oscillations

• shows up as small pressure changes at the surface because of the high altitude

CHAPTER 4 WIND

WIND

• Differences in temperature create differences in pressure. These pressure differences drive a complex system of winds in a never ending attempt to reach equilibrium. Wind also is a transportation device for water vapor and cloud condensation nuclei.

CONVECTION

• Warm air rises

• Cold air sinks

• With convection, warm air rises cools then sinks. Uneven surface heating.

• The wind sets up an advection process whereby the cool air is blown along the ground until it is warmed then it rises again and repeats the process.

CONVECTION (24)

PRESSURE GRADIENT

• Pressure gradient = difference in pressure / distance

• Sets up a flow from high to low

• The closer the isobars, the stronger the pressure gradient force and the stronger the wind

PRESSURE GRADIENT

• Think of a Topographical map. If you’re a ball on the top of a steep mountain (high pressure system) and you roll off into the low lying are below (low pressure system) the steeper the gradient the faster the wind.

CORIOLIS FORCE• This force describes the apparent force

due to the rotation of the earth

• All free moving objects such as ocean currents, artillery projectiles, air molecules and aircraft seem to deflect from a straight line path because the earth rotates under them.

On this non-rotating platform the ball travels in a straight line from one guy to another

On this counter-clockwise rotating platform the ball seems to veer to the right from the perspective of the persons on the platform

CORIOLIS FORCE• Flow would normally be 90º to isobars

except for Coriolis Force

• Causes a deflection of winds to the right in the Northern Hemisphere

• To the left in the Southern Hemisphere

• The deflection turns the winds parallel to the isobars at altitude

• Near the ground, the deflection depends on surface friction

CORIOLIS FORCE

CORIOLIS FORCE• Surface friction slows the wind allowing the

pressure gradient force to over power Coriolis

• Over land 45º to the isobars

• Over water 10º to the isobars

• The magnitude varies with the speed of the wind and the latitude

• As speed increases Coriolis increases

• As latitude nears the poles, Coriolis increases

SURFACE FRICTION

SURFACE FRICTION

• Into a low on the surface out of a High

GLOBAL WIND CIRCULATION PATTERNS

• 30º Latitude subtropical westerlies

• 60º Latitude polar easterlies

• Intertropical convergence zone (ITCZ) - The boundary zone separating the northeast trade winds of the Northern Hemisphere from the southeast trade winds of the Southern Hemisphere (p28)

MOUNTAIN AND VALLEY WINDS

• The slope warms during the day warming the air causing it to rise.

• The slope cools at night cooling the air causing it to sink.

DAYTIMEC

W

C

NIGHTIME

KATABATIC WIND

• Any wind blowing down an incline.

• A perfect example is when the Columbia basin gets snow, causing cold air to form near the surface creating an artificial High

• This pressure gradient then causes a wind in the Columbia gorge down by Portland.

• Even though the air warms through adiabatic compression it is not enough to offset the temp differential.

CHINOOK WIND• The Chinook is a warm dry wind that descends

downslope

• Temperature sometimes raises sharply (36ºF)

• Air blowing up the windward side is cooled by adiabatic expansion

• This causes a loss of moisture and gain in heat (latent heat of fusion)

• The leeward side then sees warm dry air through adiabatic compression.

CHINOOK WIND• Moist and Dry are cool at different lapse

rates. Is a katabatic wind. Chapter 6 more

LAND AND SEA BREEZES

• Day - sea breeze (from sea to land)

• Warm land, cool water

• Night - land breeze (from land to sea)

• Cool land, warm water

LAND AND SEA BREEZES

WIND SHEAR

• It Can Happen

• Any altitude

• Any direction

• Any gradient

WIND SHEAR

• Two fluids moving in opposite direction create friction and eddies along a common shallow mixing zone referred to as the shear zone.

WIND SHEAR• Tailwind shearing to a calm or

headwind component

• initially the airspeed increases, the aircraft pitches up, and the altitude increases.

• Headwind shearing to a tailwind - initially airspeed decreases, aircraft pitches down, and altitude decreases

WIND SHEAR• Be careful with low level

temperature inversions. Wind just above the inversion may be strong.

WIND SHEAR• If climbing or landing a few knots

from the normal stall speed going through the shear zone can induce a stall.

• Check your winds a loft FD forcast.