IN olden times, all weather phenomena were put down to the interaction in the atmosphere of two kinds of "vapours" exuded by the earth under the influence of the sun's heat. Thus, according to the French philosopher, Guillaume Saluste Du Baftas, who for some reason felt obliged to record all his findings in iambic verse, the sun . . .
Two sorts of vapours by his heat exhales
From floating deeps, and from the flowery dales;
The one is somewhat hot, but heavy, moist and thick,
The other light, dry, burn-
ing, pure and quick.
Modern meteorologists, however, have simplified the problem. They concentrate on a single vapour, water vapour, the amount of which in the air at any time determines what we call humidity.
Air always contains some moisture, which it picks up by evaporation. But the atmosphere is unable to collect water indefinitely. The warmer a volume of air, the more moisture it can accommodate, but if more and more moisture is added, sooner or later it reaches a stage where it can hold no more and it is said to be "saturated".
And this, in fact, provides one convenient measure of the moisture content: relative humidity is defined as the actual water content of a volume of air expressed as a percentage of the amount of moisture the air could hold at that temperature if it were saturated. For example, if the amount of water vapour present in the atmosphere is only half the maximum possible at that particular temperature, then the relative humidity is 50 per cent.
But meteorologists sometimes specify the moistness of the air in other ways that may be more convenient to the exercise in hand. The absolute humidity, for instance, is the actual amount of water vapour in a unit volume of air, normally expressed as the number of grams per cubic metre.
Another measure is the vapour pressure, defined as that proportion of the atmospheric pressure which is due to the presence of water vapour. And sometimes the dew point is quoted the temperature to which moist air be cooled in order just to reach the condition of saturation.
However, it may be described, humidity is very important for the forecaster. Dew, fog and clouds are all formed by the condensation of water vapour into tiny water droplets. In addition, humidity observations on a weather chart help in locating fronts, the boundaries between different air masses that are usually associated with cloud and rain.