1. Section 02 Lesson 5/6/7 Atmospheric Pressure, Temperature and Density
4/22/2017
Section 02 Lesson 5/6/7 Atmospheric Pressure, Temperature and Density
Definition of Atmospheric Pressure
Measurement of Pressure
QFF, QFE, QNH
Pressure Variation at Surface Level
Pressure Variation with Height
Density
International Standard Atmosphere
Gas Laws

2. Atmospheric Pressure Pressure is defined as force per unit area.
4/22/2017
Atmospheric Pressure
Pressure is defined as force per unit area.
Atmospheric pressure is the force exerted at the earth’s surface by a column of air above it.
Pressure at a point acts uniformly in all directions.
Units of pressure are:
millibar (mb).
hectopascal (hPa).
1mb = 1 hPa

3. Atmospheric Pressure
4/22/2017
Atmospheric pressure is defined as the force per unit area exerted against a surface by the weight of the air column above that surface.
The pressure at point "X" increases as the weight of the air above it increases. The pressure at point "X" decreases if the weight of the air above it decreases.

4. Measurement of Atmospheric Pressure
Mercury barometer
Vacuum

5. Aneroid Barometer

6. Barograph Recorder

7. QFE
Is the prevailing atmospheric/aerodrome pressure at the airfield elevation

8. QFF
Is the local station barometric pressure adjusted to mean sea level assuming an isothermal column of air at station temperature
It is only used on surface synoptic charts
It is expressed to one decimal point.

10. QNH
Is a pressure setting which when set on an altimeter sub-scale will cause it to read the airfield elevation above mean sea level.
ISA conditions are assumed for the air column
It is always expressed in integer values.

11. Pressure Variation Pressure varies widely from day to day.
4/22/2017
Pressure Variation
Pressure varies widely from day to day.
Usual range of variation,
970 mb to 1030 mb.
Lowest recorded,
870 mb, Typhoon Tip, Oct
Highest recorded,
1084 mb, Agata, Siberia, Dec

12. Extremes of Surface Pressure Variation

13. 60

14. Horizontal Pressure Variation
4/22/2017
Horizontal Pressure Variation
Isobars
Highs/Anticyclones
Ridges
Lows/Cyclones
Troughs
Pressure Gradient
Pressure Gradient Force
Isallobars

15. 4/22/2017
Isobars
A line drawn on a weather map connecting points of equal pressure is called an isobar.
The isobars are generated from mean sea level pressure reports.
The pressure values are given in hectopascals (or millibars).

16. 4/22/2017
Surface Isobar Charts
Reporting stations are not all at Mean Sea Level.
Local Barometric Pressure is adjusted to Mean Sea Level using an Isothermal column of air.
Mean Sea Level Pressure plotted on Synoptic Charts is known as the QFF.

17. Typical Surface Pressure Patterns
4/22/2017
COL Weather over land
Winter: Poor vis; radiation fog
Summer: CB/TS in late afternoon
TROUGH - formed by the extension of a low
COL - a zone of slack pressure gradients with calm or light variable winds
RIDGE - formed by the extension of an anticyclone

18. Patterns of Pressure Distribution
Anticyclones or highs: are regions where the pressure at its centre is highest relative to its surroundings. The circulation is clockwise in the northern hemisphere and anticlockwise in the southern hemisphere.
Ridge: a region of isobars extending away from a high centre with no sharp curvature. Pressure along the line of the ridge is higher than its surroundings.

19. Patterns of Pressure Distribution
Depressions or lows (or cyclones): are regions where the pressure at its centre is lowest relative to its surroundings. The circulation is anticlockwise in the northern hemisphere and clockwise in the southern hemisphere.
Trough: is a region of isobars extending away from a low centre and may have sharp curvature. Pressure along the line of the trough is lower than its surroundings
Cols: is a region of nearly uniform pressure situated between a pair of highs and a pair of lows.

20. Pressure Variation with Height
4/22/2017
Pressure Variation with Height

21. Pressure changes aloft
Pressure decreases with height
Pressure also varies horizontally aloft.
This is due to horizontal variations of mean temperature in layers in the atmosphere.

22. Pressure changes aloft
Rate of pressure decrease with height decreases significantly nearer the tropopause
Pressure is determined by the density of air above

24. Density Density = Mass per unit volume (kg/m3)
When air is heated, density becomes less
With an increase in height, both pressure and density decrease

25. Density variations

26. Density of dry and moist air
Dry air = 78% N2 , 21% O2
Molecular weight: N2 = 28
O2 = 32
= 60
H20 = 18
Therefore moist air is less dense than dry air

27. The Relationship between Pressure, Temperature and Density
Air behaves according to the Gas Law
Pressure = Temperature x Density x Gas Constant
p = T x  x C
or leaving out the constant
p  T x 
Pressure is directly proportional to Temperature and Density

28. Gas Law Continued p   T x  = Constant If Temperature held constant:
Thus air at a higher pressure is more dense than air at a lower pressure.
If the Pressure is held constant:
T x  = Constant
Thus at a given atmospheric pressure, air that is cold is more dense than air that is warm.

29. Summary of conclusions from Gas Laws
Density of air
Increases with increasing air pressure
i.e. decreasing altitude
Increases if the temperature is decreased
Decreases if the air pressure decreases
i.e. increasing altitude
Decreases if the temperature increases.

30. Summary of conclusions from Gas Laws
Note that the controlling factor in the atmosphere is air pressure.
The rapid decrease of pressure with height overcomes the tendency of the decreasing temperature with height to increase density.

31. Pressure and Height Calculations
At sea level pressure 1013hPa and 15°C
1hPa = 27 ft height change
At 500 hPa
1hPa = 50 ft height change
Radiosonde data is used to calculate the “thickness” of the atmosphere at various locations

33. International Standard Atmosphere

34. Standard Pressure Levels