Humidity describes how much water vapor the air is holding at a given moment.

The longer version

Water vapor is invisible, but the amount of it in the air varies a great deal from place to place and hour to hour. Humidity describes how much the air holds, either as an absolute quantity or as a share of the maximum it could hold at that temperature.

Warm air can hold far more vapor than cold air, which is why the same amount of moisture feels damp on a cold morning and dry on a hot afternoon. Relative humidity climbs as air cools, and once it reaches saturation the excess vapor condenses into cloud, fog, or dew.

Where this entry sits

Clouds, wind, rain, and storms shape daily life in ways that most people barely notice. This collection explains snow, lightning, rainbows, the turning seasons, temperature, the instruments forecasters rely on, and scraps of old weather lore. The full list sits on the Weather and Climate page, where every entry is listed in order.