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These facts also account for the formation of dew, which is caused by the reduction of the temperature and the deposition of the moisture which the warmer atmosphere of the day had held in suspension. Dews will hence be usually most abundant when cool nights succeed warm days, and on a clear night than when the skies are obscured by clouds, because a cloudless sky is usually much colder than a beclouded one. It is also essential for the copious formation of dew, that the ground or other substance on which it is deposited should be much cooler than the superincumbent air; for if the ground be warm it will impart its temperature to the air near its surface and dew will not be formed.