Definition
A handheld weather instrument used to measure relative humidity. It consists of two thermometers mounted side by side on a frame with a handle. One thermometer (the wet bulb) has its bulb covered by a wick moistened with water; the other (the dry bulb) reads ambient air temperature. The instrument is whirled through the air for a short period, causing water on the wet bulb to evaporate and cool that thermometer. The temperature difference between the two thermometers is then read against a chart or table to determine relative humidity and dew point.
Plain English
A simple weather tool with two thermometers — one dry, one with a wet cloth on its tip — that you spin through the air. The wet one cools as the water evaporates, and the difference between the two readings tells you how much moisture is in the air.
Context Anchor
Seen in basic weather observation, preflight weather study, and discussions of humidity, dew point, fog, and cloud formation.
Derivation
Psychrometer comes from the Greek psychros meaning 'cold' and metron meaning 'measure' — literally a 'cold-measurer,' because it works by measuring the cooling caused by evaporation. 'Sling' refers to the way it's used: swung or slung through the air by hand.
Why Pilots Care
Accurate humidity data improves density altitude estimates that directly affect takeoff performance and climb capability.
Grounding Statement
When you swing a sling psychrometer, the wet thermometer cools as water evaporates, and that cooling difference tells you how much moisture is in the air.
Intuition Check
Do not read “sling” as cargo or a strap used to carry something. Here it means the instrument is swung through the air to make air flow over the thermometers.
Example Sentence 1
Before the morning briefing, the weather observer used a sling psychrometer to check the dew point and confirm fog was likely to form.
Example Sentence 2
Updated sling psychrometer readings were used to recalculate density altitude before the afternoon departures.