The relative humidity usually varies greatly through the day, being generally lowest when the temperature is highest, and vice versa. It is an element of much practical interest, because it is one of the main factors in determining the drying power of the air, the other important one being wind. The air feels dry when evaporation proceeds rapidly from our skin, either on account of low relative humidity, brisk air movement, or both. People are hardly conscious of high relative humidity except when, in hot weather, it retards the evaporation of perspiration, and the latter collects in liquid form on the skin.
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Relative humidity does not owe its importance in human affairs solely to its physiological effects, for it plays a prominent part in numerous industries—textile, metallurgical, chemical, leather, food, and all those employing drying processes. In the spinning of cotton and wool, for example, the humidity of the workroom greatly affects the weight of the material, the size of the yarn, and the length and flexibility of the fibers. Humidity must likewise be taken into account in such diverse industries as manufacturing candy, bread, high explosives and photographic films, drying macaroni and tobacco, and operating blast furnaces. There are engineers who specialize in the business of installing “humidifying” and “dehumidifying” systems in workshops, and also, for hygienic purposes, in schoolhouses and other public buildings.
The absolute humidity, the relative humidity and the dew point (the temperature to which the air must be brought to start condensation of its moisture) are all determined by means of instruments called hygrometers. The hair hygrometer depends for its action upon the fact that a hair, freed from oil, not only absorbs moisture from the atmosphere, but elongates when damp and contracts when dry. The instrument, which includes a single human hair or a bundle of such hairs, is so designed that these changes move an index over a graduated scale. This and other types of hygrometer can be arranged to record their own readings continuously, constituting a hygrograph.
The form of hygrometer most commonly met with at meteorological stations is called a psychrometer. This usually consists of a pair of mercurial thermometers, one of which, known as the “wet-bulb thermometer,” has its bulb wrapped in thin muslin. The other, called the “dry-bulb,” is an ordinary thermometer. The muslin is moistened, either just before making a reading, or continuously with a wick. In the former case the thermometer is generally whirled several times before the reading is taken. Unless the air is saturated, the wet bulb is cooled by evaporation, and the difference between the readings of the two instruments enables the observer, with the aid of suitable tables, to obtain the absolute and relative humidity and the dew point. The most accurate results are obtained from the aspiration psychrometer, of Assmann, in which air is drawn past the bulb of the thermometer by a small fan, driven by clockwork.
Deposits of liquid and frozen water from the atmosphere, in their various forms, are known collectively as “precipitation,” and in the aggregate they constitute a feature of the weather hardly less important than temperature. Indeed an average rainstorm or snowstorm is a more obtrusive event than any other equally common manifestation of the weather; while an excess of precipitation or a prolonged lack of it, constituting a drought, may be as serious in its consequences as a “hot wave” or a “freeze.”
Precipitation—familiarly called “rainfall”—is much more extensively measured than any other meteorological element, for there are, throughout the world, a vast number of places at which this is the only feature of the weather that is regularly observed. In Europe alone there are about 19,000 “rainfall stations.” Rainfall is measured in depth; viz., in inches or millimeters. A moderate shower of several hours’ duration will yield an inch or two of rain, while in extreme cases several inches may fall in an hour. Snow is sometimes measured as such—i. e., the actual depth that falls, or, more commonly, the amount lying on the ground from day to day—but in order that records of snowfall may be combined with those of rainfall for the purpose of determining the total precipitation, the snowfall must be reduced to its “water equivalent,” either by melting the snow before measurement or by estimating this equivalent or by weighing the snow caught in a receiver of known area and computing the corresponding depth of water.
There are many kinds of rain gauge. As a rule the gauge has a funnel-shaped receiver with a small opening through which the water flows into the lower part of the gauge; loss of the accumulated water by evaporation is thus checked. There is usually some device for magnifying the depth of rainfall in order to facilitate measurement. In American gauges the rain flows into an inner tube having one-tenth the horizontal area of the receiver, and its depth is thus magnified ten times. A measurement is made by thrusting a graduated wooden stick to the bottom of the tube and noting the height to which the stick is wetted.
