1.4 Atmospheric Pressure

The air composed of several different elements in a gaseous state. The atoms and molecules of these elements move freely in the atmosphere and collide with each other with tiny force. Energy generated by these tiny forces adds up to a considerable pressure level known as force of atmospheric pressure. The force of air pressure depends on the rate at which particles collide and the force of impact. Atmospheric pressure is the total weight of the air above unit area at the point where the pressure is measured. Thus air pressure varies with location and weather, standard sea level pressure adopted by ICAO is 14.7 psi. Air density decreases exponentially with altitude, dropping by half every 18,000 ft.

The average atmospheric pressure at sea level is 1 standard atmosphere (atm) = 101.3 kPa(kilopascal) = 14.7 psi (pounds per square inch) = 29.92 Hg (inches of mercury)