Definition
A temperature scale on which water freezes at 0 degrees and boils at 100 degrees at standard sea-level pressure. In aviation, Celsius is the standard unit used for reporting outside air temperature, weather observations, forecasts, and performance calculations.
Plain English
A way of measuring temperature where freezing water is 0 and boiling water is 100. It is the temperature scale pilots use almost everywhere in aviation.
Context Anchor
Seen in aircraft performance charts, weather information, and standard atmosphere discussions.
Derivation
Named after Anders Celsius, the Swedish astronomer who proposed the 0-to-100 scale in 1742. Knowing the name is just a person helps avoid the assumption that 'Celsius' refers to some technical property of the air itself.
Why Pilots Care
Temperature in Celsius is required to calculate density altitude, which directly determines takeoff distance, climb rate, and engine power available.
Intuition Check
Do not treat Celsius and Fahrenheit numbers as interchangeable. For example, 30°C is a warm day, while 30°F is below freezing.
Example Sentence 1
The METAR reported the temperature as 32 degrees Celsius, so the pilot expected reduced takeoff performance from the high density altitude.
Example Sentence 2
At 35 degrees Celsius the performance chart indicated the airplane would need an extra 800 feet of runway.