Atmosphere

The properties of the atmosphere are very important to those of us who operate aircraft with fixed or rotary wings, (i.e. aeroplanes or helicopters), and powered by air breathing engines. The performance (efficiency) of the wing and engine is dependent on various parameters (such as pressure, density and temperature), of the atmosphere at the operating altitude.
In general terms the atmosphere is made up of four concentric gaseous layers extending up to some 2,500,000 ft:

• Troposphere extends from the sea level up to the tropopause. The tropopause is not at a constant height, but varies from 20,000 ft to about 60,000 ft depending on latitude. At higher latitudes (poles) the tropopause is lower.

• Stratosphere extends from the tropopause up to about 120,000 ft. The main characteristic of the stratosphere is constant temperature (isothermal), with the increasing altitude. The upper level of the stratosphere is the normally accepted limit of conventional air breathing engines.

• Mesosphere extends up to 300,000 ft. The upper level of the mesosphere represents the design altitude of the X 15 high altitude rocket powered research aircraft.

• Thermosphere extends out to about 400 500 km. The thermosphere is the outer layer of the atmosphere, where temperature increases significantly.
The above layers of the atmosphere are the main accepted method of describing various divisions of the gaseous envelope that surrounds the earth, however from time to time scientists define other 'spheres' within the four listed above to specify a particular phenomenon or characteristic. The ionosphere is an important example:

• Ionosphere extends from about 200,000 ft to 1,250,000 ft and defines the zone within the mesosphere/thermosphere which contains significant ionization (presence of charged particles). The varying intensity of the ionosphere affects the passage of electromagnetic transmissions, both natural and man made.