Definition
An instrument used to measure the amount of water vapor in the air, usually expressed as relative humidity (a percentage) or as dew point.
Plain English
A device that tells you how much moisture is in the air.
Context Anchor
Seen in weather discussions, maintenance areas, hangars, and some aircraft or airport weather equipment.
Derivation
From the Greek hygros meaning 'wet' or 'moist,' plus metron meaning 'measure.' So literally: a moisture-measurer. Knowing the root makes it easier to recognize related terms like hygroscopic.
Why Pilots Care
Humidity directly reduces air density and therefore affects takeoff distance, climb rate, and engine performance.
Example Sentence 1
The weather station's hygrometer showed a relative humidity of 92 percent, which matched the fog forming over the runway.
Example Sentence 2
Weather services use a hygrometer to report dew point and relative humidity for flight planning.