[Ref Pilot’s Handbook of Aeronautical Knowledge, FAA-H-8083-25B Page 12-2, The Pilot’s Manual PM2 Ground School Page 317-318]

What’s atmosphere?

The atmosphere is a blanket of air made up of a mixture of gases that surrounds the Earth.


Two main functions of atmosphere?

  1. absorbs energy (heat) from the sun
  2. protects life on Earth from high energy radiation and the frigid vacuum of space


Composition of the atmosphere

In any given volume of air, nitrogen accounts for 78% of the gases that comprise the atmosphere, while oxygen makes up 21 %. Argon, carbon dioxide (CO2) , and traces of other gases make up the remaining one percent.

These mixture of gases (the atmosphere) surrounding the surface of the earth up to 350 miles (304 NM, 563km) and four (4) layers have been identified based on the thermal characteristics, chemical composition, movement, and density.

Gas composition of atmosphere


(4) Four layers of the atmosphere

  1. Troposphere     (up to 23,000ft over poles and 65,000ft over equator ; -57ºC)
  2. Stratosphere     (up to 160,000ft)
  3. Mesosphere
  4. Thermosphere



Thickness (top) of troposphere varies based on latitude and by season – it is lowest over the poles and highest at the equator and it is lower in winter and higher in summer.

The top of troposphere is at range of 6 to 20 km – it is about 7 km (3.8NM,  23,000ft) over the northern and southern poles and 20 km (10.8NM, 65,600 ft) over equatorial regions.

Inside the troposphere, the average temperature lapse rate is about -2 °C every 1,000 feet of altitude gain. The pressure lapse rate is about -1 inch per 1,000 feet of altitude gain (-34 mb per 1,000 feet of altitude gain).



Tropopause is the boundary between top of the troposphere and the stratosphere, which traps moisture and the associated weather
in the troposphere.

What’s tropopause?
Ans:  boundary between top of the troposphere and the stratosphere;

What’s the significance (function) of the tropopause?

  1. traps moisture and the associated weather in the troposphere
  2. the location of the tropopause is important because it is commonly associated with the location of the jet stream and possible clear air turbulence (CAT)



Stratosphere extends from the tropopause to a height of about 160,000 ft (26.3 NM,  50km).


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# Weather (WX)

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