Public Domain Aeronautical Software (PDAS)  

Every student of aerodynamics and flight mechanics is introduced to an atmosphere table, which allows one to determine the temperature, pressure, and density at any altitude. If you don't happen to have such a table handy, you may want to look at a sample atmosphere table.

I have four typical tables on-line:

  1. Table 1 - 0 to 86 km in steps of 2 km in SI units
  2. Table 2 - 0 to 20 km in steps of 0.5 km in SI units
  3. Table 3 - 0 to 280,000 ft in steps of 5000 ft in US units
  4. Table 4 - 0 to 65,000 ft in steps of 1000 ft in US units

In addition to these tables, there is a page of big tables from 0 to 1000km that includes additional quantities.

The equations used are those adopted 15 October 1976 by the United States Committee on Extension to the Standard Atmosphere (COESA), representing 29 U.S. scientific and engineering organizations. The values selected in 1976 are slight modifications of those adopted in 1962. The equations and parameters used are documented in a book entitled U.S. Standard Atmosphere, 1976 published by the U.S. Government Printing Office, Washington, D.C. A summary of the definition of the 1976 atmosphere is on another page.

The equations defining the 1976 Standard Atmosphere to 86 km are coded for inclusion in your own aeronautical studies. The procedure is supplied in several different programming languages: Fortran, Pascal(Delphi), C, C++, Fortran77, IDL, Python, QBasic Java and JavaScript.

The fundamental procedure is a subroutine called Atmosphere that accepts altitude as an input argument and returns non-dimensional values of temperature, pressure, and density which are ratios of the quantity at altitude to that at sea-level. The equations are taken directly from the NASA publication. Since the definition of the international standard is given in SI units, the altitude is supplied in kilometers. The standard atmosphere is defined as a set of layers and the routine determines which layer contains the specified altitude. The desired layer is found by binary search. The temperature is then computed by linear interpolation. Then the pressure is computed from the hydrostatic equations and the density follows from the perfect gas law. You may download the source code of the Fortran version or the Python 3 version of Atmosphere.

As a practical matter, almost all flight takes place in the first two layers and they are commonly referred to as the troposphere and the stratosphere. The routine Atmosphere which does the very elegant binary search through the layers will usually use the first or second. An alternate routine called SimpleAtmosphere is included in this package which is only correct to 20 km, but is much simpler because it only makes one test for altitude.

In order to check the operation of the subroutine, a program called Tables is included that computes four tables showing the atmospheric quantities at various altitudes, expressed in either SI or US units. The short versions of the tables use the SimpleAtmosphere subroutine and the long tables use Atmosphere. The tables produced by the program should agree with the four online tables. To get results at values of altitude that are intermediate to the tabulated values, one usually does interpolation. Alternately, you can use the supplied Atmosphere Calculator that gives you precise values for any altitude.

Since nearly every introductory aerodynamics textbook contains such a table, you may question the value of producing yet another one. By going through this exercise, you will have these routines in your standard computational toolbox. When you are studying a new vehicle concept or flight procedure, you can concentrate on your idea and not on validating your atmosphere calculations.

The routine Atmosphere implements the first seven layers of the atmosphere, as defined in the 1976 standard. This defines the atmosphere from sea level to 86 kilometers (282,000 ft.) altitude. For heights above 86 kilometers, see the Upper Atmosphere page.

I have written several application programs that utilize the atmosphere computing procedure. Go to the Application page for further information.

You may read the page describing the hydrostatic equations.

In addition to the standard atmosphere calculation, one often wishes to compute non-standard conditions, such as a hot day or cold day. The Non-Standard Atmosphere page gives a description of these procedures.

Public Domain Aeronautical Software (PDAS)