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.