Hygrometry is that branch of physics which deals with the moisture contained in the atmosphere. It involves a knowledge of the principles of heat, and is generally studied with the questions of evaporation and condensation. The greatest amount, of water vapour that can be contained in a cubic foot of air will depend upon the temperature. For every temperature there is a definite maximum pressure producible by the water-vapour, this pressure increasing with the temperature. Thus, as the air gets hotter, it becomes able to contain further quantities of vapour without deposition.
It is not usual for the air to contain as much vapour as it can hold under its given conditions of temperature. The fraction of this maximum that is actually present is called the relative humidity. This evidently increases as the temperature falls, without change of the absolute quantity of vapour per cubic foot, until a temperature is reached when the air cannot become cooler without condensation of vapour taking place. At this critical temperature, called the dew-point, the air is saturated with water-vapour, and its relative humidity is unity. [Dew.] The dew-point cannot be higher than the actual temperature of the air without danger of condensation occurring with great violence. Such a state of things is rare; it is only possible when the atmosphere is very pure and free from dust or other nuclei that act as centres of formation of water-drops. Instruments for determining the dew-point are simple in construction. They are arranged to cool down the air by application of cold water or by evaporation of some such volatile liquid as ether. In this way the air is brought to a temperature below the dewpoint, and deposition of dew results. With a sensitive thermometer the exact temperature of deposition is obtained. Hygrometers working in the above way are direct-reading. The the wet and dry bule best are due to Regnault and hygrometer. to Dines. A pair of thermometers, the bulb of one of which is kept continually moist by being surrounded with a moist wick, constitute the Wet and Dry Bulb Hygrometer. The wet bulb thermometer records a lower temperature than the other, unless the air is saturated, on account of continual evaporation from its surface. From the two observed temperatures the dew-point may be calculated. Hygroscopes merely exhibit the presence of moisture in the air. They generally consist of some substance capable of readily absorbing moisture whose elastic properties are modified thereby and rendered easily visible. The type is illustrated by Saussure's hair hygrometer, in which a length of hair varies with the degree of moisture of the surrounding air.