Definition
An instrument used to measure the moisture content of the air. It consists of two thermometers mounted side by side: one with a dry bulb and one with a wet bulb covered by a moistened wick. Air is moved across both bulbs, and the difference between the two temperature readings is used to determine relative humidity and dew point.
Plain English
A weather instrument that figures out how moist the air is by comparing two thermometers — one dry and one with a wet cloth around its tip. The wet one cools as the water evaporates, and the gap between the two readings tells you how humid the air is.
Context Anchor
Seen in aviation weather discussions, weather observing, and explanations of humidity, dew point, fog, and cloud formation.
Derivation
From the Greek psychros, meaning 'cold,' and meter, meaning 'measure.' The name reflects how the instrument works: the wet bulb cools as water evaporates, and that cooling is what gets measured.
Why Pilots Care
Accurate humidity data affects density altitude calculations, aircraft performance, and decisions about icing or fog potential.
Grounding Statement
If the wet thermometer reads much cooler than the dry one, the air is fairly dry; if the two readings are close, the air is already holding a lot of moisture.
Example Sentence 1
The weather observer used a psychrometer to determine the relative humidity before issuing the morning report.
Example Sentence 2
A sling psychrometer was swung in the open air to obtain current humidity values at the airfield.