sudden evaporation. But evaporation requires heat, and this heat is
taken from our bodies, and a chill results.

Proper ventilation would eliminate much of the physical danger of
social events; fresh, dry air should be constantly admitted to crowded
rooms in order to replace the air saturated by the breath and
perspiration of the occupants.

29. Weather Forecasts. 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;