1. CHAPTER 3 ATMOSPHERIC PRESSURE

2. 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 = lbs or tons
Weight per square mile = 29,506,498 tons
Total weight of atmosphere = 5,783,000,000,000,000 tons

3. METAR KMWH 121952Z 00000KT 1/4SM FG OVC001 02/02 A3055 RMK AO2
SLP361 T
TAF AMD KMWH Z 1220/ KT 1 1/2SM BR OVC004
FM VRB03KT 1SM BR OVC003
FM KT 3SM BR OVC004
FM VRB03KT 1SM BR OVC001
KMWH Z 25004KT 10SM BKN095 M01/M01 A3052 RMK AO2 SLP350 T
KMWH Z 1200/ KT P6SM BKN130 BKN200
FM KT P6SM SCT008 BKN070
FM KT 5SM BR BKN008
FM KT 2SM BR OVC004
FM KT 3SM BR OVC006

4. KEPH 120353Z AUTO 01010KT 10SM SCT090 00/M02 A3050 RMK AO2 SLP344 T00001017
KEAT Z AUTO 00000KT 10SM CLR 02/01 A3051 RMK AO2 SLP345 T
KEAT Z 1201/ KT P6SM VCFG SKC
FM KT 6SM BR SCT005 BKN200
FM VRB03KT 3SM BR BKN005
FM KT 1/2SM FG OVC001
KGEG Z 30003KT 1/2SM R21/4500VP6000FT BR SCT001 BKN002 OVC004 01/01 A3050 RMK AO2 SFC VIS 1 1/2 SLP348 T
KGEG Z 1200/ KT 2SM BR OVC005
TEMPO 1200/1203 1/2SM FG OVC002
FM VRB04KT 1/2SM FG OVC002
FM VRB02KT 1/2SM -SN FZFG OVC001
FM VRB03KT 2SM -DZ BR OVC002

5. 14.7 lbs. per square inch

6. Volume of Earth’s Atmosphere
Given: 99% is contained within 31 miles of the surface
Total volume = 6,076,000,000 cubic miles

7. 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

8. BAROMETERS Any instrument that measures pressure is called a barometer
Aneroid Barometers work similar to Altimeter

9. Aneroid barometer

10. Mercury Barometer

11. 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)

12. BAROMETERS ELEVATION

13. BAROMETERS ELEVATION

14. 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.

15. MERCURY BAROMETERS
Standard sea level pressure = inches of mercury or hectopascals (=millibars)
pressure = force per unit area

16. 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

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

19. Lines of equal pressure are then connected called isobars.

20. 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

21. 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

22. 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.

23. Other PRESSURE Definitions
Col = the neutral are between two highs and two lows (like a mountain pass on a map

24. 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

25. 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.

26. ALTIMETRY Absolute altitude - the height above the surface (AGL)
True altitude - actual altitude above sea level

27. 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

28. INDICATED ALTITUDE Temperature affects indicated altitude
Cold temperature correction charts

30. 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

31. DENSITY ALTITUDE

32. ICAO cold temperature error table

33. 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 and only pressure altitudes are flown

34. Chapter #3 What 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!

35. 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

36. 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

37. 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

38. 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

39. CHAPTER 4 WIND

40. 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.

41. 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.

42. CONVECTION (24)

43. 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

44. 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.

45. 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.

46. 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

47. 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

48. CORIOLIS FORCE

49. 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

50. SURFACE FRICTION

51. SURFACE FRICTION
Into a low on the surface out of a High

52. 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)

53. 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.
DAYTIME W
NIGHTIME C C

54. 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.

55. 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.

56. CHINOOK WIND
Moist and Dry are cool at different lapse rates. Is a katabatic wind. Chapter 6 more

57. 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

58. LAND AND SEA BREEZES

59. WIND SHEAR
It Can Happen
Any altitude
Any direction
Any gradient

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

61. 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

62. WIND SHEAR
Be careful with low level temperature inversions. Wind just above the inversion may be strong.

63. 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.