119
edits
Changes
From CUGC Wiki
→The International Standard Atmosphere
=== The International Standard Atmosphere ===
The '''International Standard Atmosphere (ISA)''' is a '''model''' for the atmosphere widely used in aviation. It is established based on extensive observations. It is not meant to be exact as the atmospheric conditions can vary actively (especially at low altitudes, knows as weather). Also, the temperature in the atmosphere at low levels is subject to seasonality as we well understand. Despite these factors, the ISA is a good model and perhaps the most acceptable one to be used if some aviation equipment is to be designed.
The modelling approach of the ISA is to divide the atmosphere into several layers, within each layer the static temperature is assumed to vary linearly. If \(T\) is known, it is then possible to use the theories as described before to solve for the density and the static pressure simultaneously.
The ISA can come in a tabulated format with properties of the atmospheric air at discrete altitudes, or in an analytical format in which the mathematical solutions of the thermodynamic properties are presented as functions. These functions are piecewise, because the temperature function is so.
For aeroplanes that fly at high altitudes (especially above the '''tropopause'''), often an air data computer is carried on board, in which this model is coded for reference. For low level flying such as gliding, because the tropopause is not reached, the temperature is a linear function of altitude. Therefore, after some mathematical calculations, an explicit function mapping static pressure to altitude can be derived. This function is then implemented mechanically into an altimeter.
For the purpose of further discussions, it is important to understand that, for typical gliding altitudes:
*Temperature decreases linearly with altitude
*Static pressure decreases with altitude (according to a power law, though for gliding purposes this can be assumed to be linear)
*Density decreases with altitude
== Instrumentation ==
