Definition
The temperature read from a thermometer whose bulb is wrapped in a wet wick and exposed to a moving airstream. As water evaporates from the wick, it cools the bulb, and the lower the surrounding air's humidity, the more evaporation occurs and the lower the reading. When the air is fully saturated, no evaporation takes place and the wet-bulb reading equals the dry-bulb (regular air) temperature.
Plain English
It's the temperature you get when you cover a thermometer's bulb with a wet cloth and blow air across it. Drier air makes more water evaporate, which cools the bulb more, so the reading drops further below the actual air temperature.
Context Anchor
Seen in aviation weather study, especially when discussing moisture in the air, cloud formation, fog, precipitation, and winter weather.
Derivation
Named literally for how it's measured: the thermometer bulb is kept wet, in contrast to the dry-bulb thermometer that reads ordinary air temperature. The pairing of the two readings is the basis of the old sling psychrometer used by weather observers.
Why Pilots Care
Higher wet-bulb temperatures indicate greater humidity, which reduces air density and therefore decreases engine, propeller, and wing performance, raising required takeoff distances and reducing climb rates.
Analogy
It's the same effect as stepping out of a swimming pool on a dry day and feeling cold even though the air is warm -- evaporation pulls heat out of the water on your skin. The drier the air, the colder you feel.
Grounding Statement
If air is dry, a wet surface cools quickly as water evaporates; if air is already moist, the wet surface cools very little.
Intuition Check
Wet-bulb temperature is not simply the temperature of wet air or rainy air. It is the temperature caused by evaporation cooling a wet thermometer or wet surface.
Example Sentence 1
The briefer noted that the wet-bulb temperature was below freezing, so any precipitation aloft would likely fall as freezing rain.
Example Sentence 2
During preflight planning the pilot compared the dry-bulb and wet-bulb temperatures to determine whether high humidity would extend the required runway length.