Definition
A descriptive phrase referring to the process of evaporation, in which a liquid changes into a gas (vapor) by absorbing heat energy from its surroundings without the liquid itself becoming hotter. The absorbed heat is used to break the molecular bonds holding the liquid together rather than to raise its temperature, and is known as latent heat of vaporization.
Plain English
When a liquid turns into a gas, it has to soak up heat to do so. That heat goes into the change itself, not into making the liquid warmer. So the liquid can become a gas while staying at the same temperature.
Context Anchor
Seen in aviation weather discussions about evaporation, cloud formation, humidity, and cooling effects such as carburetor icing.
Derivation
State comes from Latin words meaning a condition or way of standing. In this use, it means the physical form of a substance, such as liquid or gas, not a location or government area.
Why Pilots Care
This principle explains several things pilots deal with directly: why wet surfaces and skin feel cold as moisture evaporates, why carburetor ice can form even on warm days as fuel vaporizes, and why clouds and precipitation behave the way they do. The heat absorbed during evaporation has to come from somewhere, and that 'somewhere' is usually the surrounding air, fuel, or aircraft surface.
Analogy
Boiling water is a useful picture: once water is boiling, adding more heat mainly turns more liquid into steam instead of raising the water temperature.
Grounding Statement
Heat goes into changing the liquid into vapor, not into warming the liquid up.
Intuition Check
State does not mean a U.S. state here. It means the physical form of a substance, such as liquid or gas.
Example Sentence 1
Carburetor ice can form because fuel changing state from liquid to gas without increasing its temperature draws heat out of the surrounding air, cooling it below freezing.
Example Sentence 2
The evaporation rate increases with lower atmospheric pressure even though the liquid temperature remains unchanged.