Definition
A temperature scale that begins at absolute zero, the theoretical point at which all molecular motion stops. One Kelvin is the same size as one degree Celsius, so to convert from Celsius to Kelvin, add 273. Water freezes at 273 K and boils at 373 K. Kelvin values are written without a degree symbol.
Plain English
A temperature scale that starts counting from the coldest possible temperature in the universe instead of from the freezing point of water. Each step up the scale is the same size as a Celsius degree, so 0 °C equals 273 K.
Context Anchor
Seen in aviation weather, engine, and performance discussions when a formula needs temperature measured from absolute zero.
Derivation
Named after Lord Kelvin (William Thomson), the British physicist who proposed an absolute temperature scale in 1848. Knowing it is a person's name explains why it has no degree symbol — Kelvin is treated as a unit, like meter or volt, not as a 'degree of something.'
Why Pilots Care
Many physics and engineering formulas — including the gas laws that govern engine performance and atmospheric behavior — only work correctly when temperature is in Kelvin. Plugging in Celsius or Fahrenheit produces wrong answers because those scales can go negative, and the math assumes you are measuring from absolute zero.
Analogy
Think of Kelvin like a ruler that starts at the true end of the object. Celsius is useful for everyday weather, but Kelvin starts from the true zero point for heat energy.
Grounding Statement
Picture a thermometer that can never show a negative number, because zero is the coldest anything can ever be.
Intuition Check
Kelvin temperature is not just another way to write Celsius. It uses the same size steps as Celsius, but it starts at absolute zero, so the number is different.
Example Sentence 1
Example Sentence 2
Thermodynamic tables in the aircraft manual list combustion temperatures in Kelvin to keep all values positive.