full text of the classic FAA guide
THE STANDARD ATMOSPHERE
Continual fluctuations of temperature and pressure in our restless atmosphere create some problems for engineers and meteorologists who require a fixed standard of reference. To arrive at a standard, they averaged conditions throughout the atmosphere for all latitudes, seasons, and altitudes. The result is a STANDARD ATMOSPHERE with specified sea-level temperature and pressure and specific rates of change of temperature and pressure with height. It is the standard for calibrating the pressure altimeter and developing aircraft performance data. We refer to it often throughout this book.