From Glossary of Meteorology
A rule proposed by Toussaint for the linear decrease of temperature with height in an atmosphere for which the temperature at mean sea level is 15°C.
It is given by
where t is the temperature in degrees Celsius and z is the geometric height in meters above mean sea level. Toussaint's formula is used to determine temperature below 11 000 m in the ICAO Standard Atmosphere.
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