THE ATMOSPHERE

Our atmosphere is made up of four regions which contain all the gases that protect us from the dangers of outer space. These four regions are the Troposphere, Stratosphere, Mesosphere and the Thermosphere.

Between the regions are boundaries; the Tropopause, Stratopause and Mesopause.

The Troposphere

The troposphere is where almost all weather occurs. Commercial jets cruise in the stratosphere to avoid most of this weather, except for deep convective systems breaking through the tropopause, such as Cumulonimbus (CB) tops.

Our atmosphere extends further out into space at the equator than at the poles due to higher surface temperatures in the tropics. As temperature decreases with height in the troposphere the temperature of the tropopause is lowest at the equator and highest at the poles.

From the tropopause temperature is constant with increasing altitude. This affects performance and fuel calculations. Also, it indicates the presence of jet streams and associated high wind and turbulence. Hence, the location of the tropopause is of interest to flight crew.

The Stratosphere

The stratosphere extends from approximately 11km to about 50km amsl. Here, temperature increases with height, up to almost 0ºC at its top. This shift is due to the precense of small quantities of ozone.

Ozone is a molecule consisting of three atoms of oxygen. The ‘ozone layer’ is concentrated in the lower part of the stratosphere and protects us by reducing the amount of damaging ultraviolet light reaching earth’s surface.

The Mesosphere

The mesosphere extends from the stratosphere, approxiately 50km to 90km above earth. Here, temperatures decrease to -100ºC. Thin clouds comprised of ice crystals, noctilucent clouds, sometimes occur at approximately 80km amsl.

The Thermosphere

The highest layer extending from 90km to 10,000km, merging into space, is the thermosphere. Also known as the ionosphere (lower part) and exosphere (upper part). Here, temperature rises with height up to 1000ºC.

The tropopause occurs at approximately 20,000 feet over the poles and approximately 60,000 feet above the equator.

The International Standard Atmosphere, ‘ISA’, assumes 36,000 feet as the average height of the tropopause.