Other Facts about Heat
When the air is near the saturation point, the weather is oppressive and is said to be very humid. For comfort and health, the air should be about two thirds saturated. The presence of some water vapor in the air is absolutely necessary to animal and plant life. In desert regions where vapor is scarce the air is so dry that throat trouble accompanied by disagreeable tickling is prevalent; fallen leaves become so dry that they crumble to dust; plants lose their freshness and beauty.
The likelihood of rain or frost is often determined by temperature and humidity. If the air is near saturation and the temperature is falling, it is safe to predict bad weather, because the fall of temperature will probably cause rapid condensation, and hence rain. If, however, the air is not near the saturation point, a fall in temperature will not necessarily produce bad weather.
The measurement of humidity is of far wider importance than the mere forecasting of local weather conditions. The close relation between humidity and health has led many institutions, such as hospitals, schools, and factories, to regulate the humidity of the atmosphere as carefully as they do the temperature. Too great humidity is enervating, and not conducive to either mental or physical exertion; on the other hand, too dry air is equally harmful. In summer the humidity conditions cannot be well regulated, but in winter, when houses are artificially heated, the humidity of a room can be increased by placing pans of water near the registers or on radiators.