The Standard Atmosphere

Most people don’t need to worry too much about the temperature and pressure of the atmosphere, as long as there is some of it there to breathe.  However, pilots, airplane designers, meteorologists, and many other people need to know how the temperature and pressure typically change with altitude.  Of course, on any given day at a particular location, conditions might be different than the “typical” condition.  Still, the typical conditions are useful enough that there are several definitions of the Standard Atmosphere describing conditions clear out into what is usually considered outer space.

For ground-based applications, we only have to consider up to about 30,000 ft above sea level, and in that range, the standard atmosphere is quite simple.  For temperature, the standard atmosphere assumes that the temperature is 15 deg C at sea level and drops by 6.5 deg C per kilometer up to 11 km above sea-level.  This is around 36,000 feet.  Up to 9000 m above sea level (almost 30,000 ft) this simple expression provides the pressure of the standard atmosphere within 1%:

 These figures show the temperature and pressure as a function of elevation up to 9,000 meters.

As an example of the use of the standard atmosphere, a numerical internal combustion engine model was exercised over a range of elevations corresponding roughly to driving from Colorado Springs to the top of Pikes Peak, Colorado.  The effect on engine power for several different engine speeds is shown in this figure.