Aqueous Vapor

About 0.001 per cent of the world's total water supply is in the form of aqueous vapor (water vapor) in the atmosphere. The amount of atmospheric water vapor varies according to the temperature, because warm air can hold more vapor than can cold air.

Absolute humidity is a measure of the actual amount of water vapor in a given volume of air (usually expressed in terms of grams of water vapor per cubic meter of air). Relative humidity, on the other hand, is a measure of the amount of water vapor in a given volume of air expressed as a per­centage of the amount that volume of air could hold at that temperature if it were saturated. Air that is saturated - with a relative humidity of 100 per cent - is said to be at dew point, because any cooling causes condensation.

Aqueous Vapor comes from the evaporation of water in the sea, lakes and moist ground; most plants and animals also give off water vapor as a natural by-product of their metabolic processes. The water vapor is transported upwards by atmospheric turbulence, another consequence of the heating of the ground by solar radiation.

As the rising air cools, its capacity to hold water vapor diminishes until it reaches dew point. The water vapor then condenses around minute par­ticles in the air to form tiny water droplets that are so light that they remain suspended in the atmo­sphere. In condensing, heat is released from the vapor; the movement of water vapor through the atmosphere is, therefore, one of the ways in which heat is redistributed between the hot tro­pics, where evaporation is greatest, and the cooler, temperate regions to the north and south, where the condensation may occur.