Definition
The ratio, expressed as a percentage, of the amount of water vapor actually present in a given volume of air to the maximum amount that air could hold at the same temperature and pressure.
Plain English
How full the air is of moisture compared to how much it could hold right now. 100% means the air is completely full and water will start to condense; 50% means the air is holding half of what it could.
Context Anchor
Seen in weather reports, maintenance conditions, and discussions of moisture, fog, corrosion, and icing risk.
Derivation
From Latin relativus (compared to something) and humidus (moist). The word 'relative' is the key — the value only makes sense in comparison to the air's current capacity, not as an absolute amount of water.
Why Pilots Care
High relative humidity increases the likelihood of carburetor icing, fog formation, and reduced engine performance in certain conditions.
Grounding Statement
Warm air can hold more water than cold air, so the same amount of moisture can be 50% relative humidity on a hot afternoon and 100% by the cool of evening — which is why dew forms overnight.
Intuition Check
Do not read “relative humidity” as simply “how wet the air feels.” It means how much moisture is in the air compared with how much that air could hold at its current temperature.
Example Sentence 1
With relative humidity near 100% and temperatures in the carburetor icing range, the pilot applied carburetor heat as a precaution.
Example Sentence 2
High relative humidity at the surface increased the chance of fog forming overnight at the destination airport.