Relative Humidity (RH)
How much moisture the air holds compared to what it could hold at its temperature, as a percentage. The healthy indoor target is 30–50%: above ~55% sustained invites condensation, dust mites, and mold on cool surfaces; below 30% (rare here) brings static and dry skin.
The longer answer
The same air reads different RH at different temperatures, which explains winter window condensation: air that's comfortable mid-room hits the cold glass, its relative humidity soars, and water appears. Ventilation (bath fans that actually vent outside), source control, and even heating fix most of it before equipment enters the conversation.
In summer, cooling equipment dehumidifies as it runs — but only while running. Oversized AC that satisfies the thermostat in minutes skips the dehumidification part of its job, which is why right-sizing shows up in comfort, not just bills.
The Seattle angle
Puget Sound's issue is winter damp — months of moist air meeting cool surfaces in older housing. Two $15 hygrometers beat guessing about it.
Guides that use this term
Where this decision happens
Nearby in the glossary
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