Outside air temperature

Most performance and flight planning graphs and tables use either degrees Celsius or Fahrenheit or both. The Kelvin scale, however, is used for Mach number calculations. For example, the speed of sound in dry air is

${\displaystyle {c}=38.945{\sqrt {K}},}$

where:

${\displaystyle c}$ is the speed of sound in knots,

${\displaystyle K}$ is the outside air temperature in kelvins.

Outside air temperature can be obtained from the aviation meteorological services, on the ATIS or measured by a probe on the aircraft. When measured by the airplane's probe in flight, it may have to be corrected for adiabatic (ram effect) rise and friction,[3] particularly in high performance aircraft. Therefore, the outside air temperature is usually calculated from the total air temperature.