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The atmosphere

The gaseous envelope surrounding the Earth is called the atmosphere. There is no defined
upper limit to the atmosphere, but most aviation activity takes place within the first 60,000 ft.
(18,000 m) and therefore we need not study the constitution of the atmosphere above that

Gas composition

The gases found in the atmosphere are in the following proportions (by volume)

ï‚· Nitrogen            78%

ï‚· Oxygen              21%

ï‚· Other gases (trace gases) 1% (e.g. argon, carbon

                      dioxide, helium etc.)

These proportions do not change with altitude.

Oxygen is essential for the sustenance of life and the combustion of materials. In the context of
aviation, oxygen is required for the combustion of fuel, a deficiency of this gas resulting in
incomplete burning and reduced engine efficiency.

Water vapour is present in the atmosphere in varying proportions, and is responsible for the

weather around the earth, which in turn affects aircraft operations and performance.

Additionally, the presence of water vapour may cause icing of the airframe or engine which may
impair an aircraft’s performance.

Regions of the atmosphere
The atmosphere is divided into a number of layers:

The troposphere – The troposphere is the level that contains the ‘weather system’ consisting
of various air masses, winds and clouds. Due to this the troposphere is rather unstable
compared to the stratosphere.

In the troposphere the temperature drops at a rate of approximately 2ºC per 1,000 ft (6.5ºC per
1,000 m) of altitude; although so-called ‘inversions’ may occur from time to time resulting in the


The tropopause – This is the upper limit of the troposphere where temperature stops
decreasing with an increase in altitude. The tropopause is therefore the upper limit of significant
weather, the first point of lowest temperature, and additionally it is the region for maximum wind

The height of the tropopause varies with latitude (due to centrifugal force), season of the year,
and prevailing weather conditions with the result that it is usually higher in low latitudes, in
summer and in fine weather.

Issue 2 – April 2016                       1-10                                 Total Training Support Ltd
                      Module 8.1 Physics of the Atmosphere                                 © Copyright 2016
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