Amount of water vapour in the air. One of the most variable characteristics of the atmosphere, humidity is an important factor in climate and weather: it regulates air temperature by absorbing thermal radiation both from the Sun and the Earth; it is directly proportional to the latent energy available for the generation of storms; and it is the ultimate source of all forms of condensation and precipitation. Humidity varies because the water-holding capacity of air is determined by temperature. When a volume of air at a given temperature holds the maximum amount of water vapour possible, the air is said to be saturated. Relative humidity is the water-vapour content of the air relative to its content at saturation. Saturated air has a relative humidity of 100%; near the Earth the relative humidity rarely falls below 30%.

This entry comes from Encyclopædia Britannica Concise.
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