Dew

Dew is water-vapour condensed into small drops on cool surfaces exposed to the air. Much curious theorising has been indulged in by older physicists as to the nature of dew. The correct principles were first advanced in 1814 by Dr. Wells. The air always holds a certain amount of water-vapour, derived from all the exposure of water on the surface of the earth. For every temperature there is a definite quantity of water-vapour that may be held in a cubic foot at a given pressure; if the temperature be at all lowered, liquid water begins to be deposited; if raised, more vapour may be introduced. At that temperature the air is said to be saturated, for under those conditions it can hold no more vapour. If the air be not saturated with moisture, by lowering its temperature sufficiently a point is reached when further abstraction of heat will cause deposition. This temperature is called the dew-point for the air under those conditions of humidity. If there be very little vapour in the air, a sensation of dryness is experienced, and we find that the dew-point is considerably below the existing temperature. At night-time certain bodies radiate heat considerably, especially when the sky is clear and when the bodies have dull surfaces. They thus become very cold, and so may cool neighbouring layers of air down to the dew-point. Dew is deposited on them, and remains till the air becomes warmer, when it may evaporate again. Deposition is facilitated by very gentle wind, which conveys freshly-laden air to the cold surfaces; but stronger wind is liable to warm the condensing bodies, and to directly assist the evaporation of any dew that has been formed.