Equation 2.28 is the basis of a set of atmospheric representations called standard atmospheres. In fact, there is no accepted standard. Tables of atmospheric properties displayed in many aerodynamics textbooks usually are based on the Air Force ARDC = Air Research and Development Command model atmosphere published in 1959. This model, briefly described herein, is based on radiosonde balloon observations of the temperature distribution in the atmosphere.
Understand that the values predicted by this approximate model represent only an average set of properties. No diurnal or seasonal changes and no geographical effects are reflected in this model. The definition of the model is shown in Fig. 2.8. The temperature distribution through the atmosphere is represented by a set of straight lines. Vertical lines represent “isothermal” layers such as the stratosphere in which temperature changes are negligible. Other layers, such as in the troposphere, are characterized by the rate at which the temperature changes with altitude in an assumed linear manner. The rate of change in a given “gradient” layer is called the lapse rate, denoted by symbols an in the figure. What is significant is that this set of lines defines the temperature, T = T(h), from which all further information can be found by means of the ideal gas equation (Eq. 2.1) and the hydrostatics equation (Eq. 2.28).