Dew Point

The temperature at which air becomes saturated with moisture and condensation starts to form.

Dew point is a temperature, not a percentage — it's the point at which air, if cooled further, can no longer hold all its moisture as vapour, and that moisture starts condensing out as liquid water. Warmer, more humid air has a higher dew point than cooler, drier air.

This is the mechanism behind condensation on windows, cold pipes and steel roof structures: any surface colder than the surrounding air's dew point will have moisture condense onto it, whether or not the room's RH reading looks high. A cold store anteroom or an indoor pool room are classic examples where dew point, not just RH, drives the visible problem.

A refrigerant dehumidifier works by deliberately cooling air below its dew point on an internal coil, condensing moisture out — which is also why performance drops as ambient temperature approaches the dew point of the coil itself, at very low room temperatures.