Definition
A temperature scale on which water freezes at 0° and boils at 100° at standard sea-level pressure. It is the standard temperature scale used in aviation worldwide, including in weather reports, performance charts, and altimeter setting calculations.
Plain English
A way of measuring how hot or cold something is, where 0 is the freezing point of water and 100 is the boiling point of water. Pilots use this scale almost everywhere temperature comes up in flying.
Context Anchor
Pilots see Celsius in weather information, aircraft manuals, performance charts, and training material when temperature is given.
Derivation
Named after Anders Celsius, an 18th-century Swedish astronomer who proposed this 0-to-100 water-based scale. Knowing it's a person's name explains why the C is capitalized and why the scale isn't tied to any deeper physical meaning — it's just a convention that stuck.
Why Pilots Care
Temperature readings in Celsius are required for accurate density-altitude and true-airspeed calculations and for assessing icing or engine-performance risks.
Intuition Check
Do not read the C here as a general letter or another aviation abbreviation. In this term, C identifies the Celsius temperature scale, and the temperature is normally written with the degree symbol as °C.
Example Sentence 1
The METAR reported the surface temperature as 18°C with a dewpoint of 16°C, suggesting fog was likely.
Example Sentence 2
At an OAT of -10°C the pilot calculated a higher density altitude and adjusted takeoff performance numbers accordingly.