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PPL » Meteorology » Atmospheric Pressure and Density

8.10 Atmospheric Pressure and Density

8.10.2 Define ‘atmospheric pressure’.

Atmospheric Pressure

  • The atmospheric pressure is a measure of the weight (force) of the air above the surface of the earth in a unit area. 
  • at sea level has a mean value of 1013.25 hectopascals (roughly 14.7 psi).


8.10.4 State the pressure units used in New Zealand aviation.

  • Pascal – Pa.

  • When measuring the pressure at the earth’s surface we use the unit hectopascal (hPa). 

  • 101,300 Pa = 1013 hPa.  (100 Pa = 1 hPa)


8.10.6 State the significance of air pressure to aviation.

The changes of air pressure day to day can;

  • affect aircraft and engine performance.
    • Higher pressure = better performance
    • Lower pressure = poorer performance
  • affect the weather, changes in 
    • precipitation,
    • visibility,
    • wind
    • temperature
    • stability of air
  • affect aircraft instruments
    • altimeter readings


8.10.8 Define ‘pressure lapse rate’.

Change of Pressure with Height

  • The Pressure Lapse Rate is the rate at which pressure changes with height in the Atmosphere. 
  • As height is gained, the amount (weight) of air above decreases and the pressure therefore decreases. 
  • In a warm column of air,
    • particles are far apart, 
    • column of air will have greater height,
    • fall off of pressure will be slower,
    • i.e. 1 hPa/40 feet.
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