The atmosphere

The conditions of the atmosphere will obviously depend on such factors as the local weather conditions, which will vary from day to day, and also on whereabouts in the world the aircraft is operated. Because of these variations a number of ‘standard atmospheres’ have been defined which represent average conditions in different parts of the world. Aircraft performance is usually related to these standard conditions and suitable corrections are made for operation in the real atmosphere, which never quite coincides with the stand­ard assumptions.

The most important factors as far as the operation of the aircraft is con­cerned are the density and temperature. The density is important because of its major influence on the aerodynamic forces, and the temperature because this governs the speed of sound, a very important parameter for high speed aircraft (Chapter 5).

We will not delve into the physics of the atmosphere but will content ourselves with a simple statement as to how these properties vary with height. These variations are summarised in Fig. 7.1 which shows the conditions for

Temperature ( C)

Fig. 7.1 The Standard atmosphere

Temperature falls with increasing altitude in the troposphere and is constant (-56.5° C) in the stratosphere. Pressure and density fall with increasing altitude in both the troposphere and the stratosphere an atmosphere appropriate to temperate latitudes. This is known as the International Standard Atmosphere (ISA).