The atmosphere is build-up of a number of layers (refer to figure 1.1) which extend up to about 2.500.000 ft. In total there are four concentric layers, namely:
- Troposphere (extending from MSL till about 11km)
- Stratosphere (extending from 11 km till about 50km)
- Mesosphere (extending from 50km till about 80km)
- Thermosphere (extending from 80km till about 200km)
The troposphere extends from Mean Sea Level (MSL) up to the tropopause. Its depth varies due to centrifugal reaction of the earth's rotation. This means that the troposphere is deepest point at the equator and at its shallowest at the poles with the altitude varying between 20.000ft and 60.000ft. In the International Standard Atmosphere the troposphere is defined to be at 36,090ft (FL360). The troposphere more or less defines the level at which most turbojet aircraft operate at its most efficient and represents the upper limit at which the vertical development of cloud formation develops. The troposphere is charaterized by a decrease in temperature with an increase in altitude.
The stratosphere extends from the tropopause up to the stratopause at approximately 160.000ft. The main charateristic of the stratosphere in its lower levels is a constant temperature with increasing altitude. Therefore it is said to be isothermal up to 80.000ft after which temperature gently increases again until the stratosphere is reached. The ISA decribes the lowest temperature to be at -56.6 °C but it could be down to -80°C above the tropics. Flying at these levels require a ramjet, pulsejet or rocket powered plane.
The mesosphere extends from the stratopause up to the mesopause at about 280.000ft. Flying between these levels of the atmosphere represent the altitude at which rocket powered aircraft like the X-15 fly at. The mesosphere is charaterized by a rapid decrease in temperature with increasing altitude.
The thermosphere finally extends up to 200 to 400nm and represents a level of the atmosphere which is not accesible to normal flight.
Above the thermosphere lay several more atmospheric layers but are not of importance when discussing civil aviation aerodynamics.
|Figure 1.1 - Basic build-up of the atmosphere