Definition
An instrument used to measure the relative humidity of the atmosphere. It uses a bundle of human hair, which lengthens as humidity rises and shortens as humidity falls. The change in length is mechanically linked to a pointer on a calibrated dial that displays relative humidity as a percentage.
Plain English
A simple device that tells you how much moisture is in the air by using a strand of hair. Hair stretches slightly when the air is damp and shrinks when the air is dry, and that tiny movement is shown on a dial.
Context Anchor
Seen in aviation weather and basic weather-instrument discussions, especially when learning how humidity can be measured.
Derivation
Hygrometer comes from the Greek hygros, meaning 'wet' or 'moist,' and metron, meaning 'measure.' So a hygrometer is literally a 'wetness measurer,' and the 'hair' part tells you what does the sensing.
Why Pilots Care
Humidity readings help calculate density altitude and assess risks such as carburetor icing or reduced visibility.
Intuition Check
A hair hygrometer does not measure whether hair feels wet. It uses the small length change of hair to indicate how much moisture is in the air.
Example Sentence 1
The weather station's hair hygrometer showed 85 percent relative humidity, suggesting fog was likely overnight.
Example Sentence 2
Before calculating density altitude the pilot checked the hair hygrometer reading from the weather station.