chapter 3 atmospheric pressure. weight of earth’s atmosphere given: weight per square inch = 14.7...
TRANSCRIPT
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CHAPTER 3 ATMOSPHERIC
PRESSURE
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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
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• 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
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• 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
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14.7 lbs. per square inch
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Volume of Earth’s Atmosphere
• Given: 99% is contained within 31 miles of the surface
• Total volume = 6,076,000,000 cubic miles
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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
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BAROMETERS• Any instrument that measures pressure is
called a barometer
• Aneroid Barometers work similar to Altimeter
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Aneroid barometer
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Mercury Barometer
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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)
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BAROMETERS ELEVATION
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BAROMETERS ELEVATION
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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.
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MERCURY BAROMETERS• Standard sea level pressure =
29.92 inches of mercury or 1013.25 hectopascals (=millibars)
• pressure = force per unit area
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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
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• Stations then take the local pressure and plot it on maps to follow the pressure patterns.
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• Lines of equal pressure are then connected called isobars.
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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
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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
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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.
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Other PRESSURE Definitions• Col = the neutral are between two
highs and two lows (like a mountain pass on a map
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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
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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.
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ALTIMETRY• Absolute altitude - the height above the
surface (AGL)
• True altitude - actual altitude above sea level
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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
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INDICATED ALTITUDE• Temperature affects indicated altitude• Cold temperature correction charts
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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
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DENSITY ALTITUDE
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ICAO cold temperature error table
• http://www2.faa.gov/airports_airtraffic/air_traffic/publications/ATpubs/AIM/Chap7/aim0702.html
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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
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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!
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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
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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
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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
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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
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CHAPTER 4 WIND
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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.
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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.
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CONVECTION (24)
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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
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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.
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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.
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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
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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
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CORIOLIS FORCE
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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
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SURFACE FRICTION
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SURFACE FRICTION
• Into a low on the surface out of a High
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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)
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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
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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.
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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.
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CHINOOK WIND• Moist and Dry are cool at different lapse
rates. Is a katabatic wind. Chapter 6 more
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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
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LAND AND SEA BREEZES
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WIND SHEAR
• It Can Happen
• Any altitude
• Any direction
• Any gradient
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WIND SHEAR
• Two fluids moving in opposite direction create friction and eddies along a common shallow mixing zone referred to as the shear zone.
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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
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WIND SHEAR• Be careful with low level
temperature inversions. Wind just above the inversion may be strong.
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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.