evaporation of moisture into the atmosphere keeps the
atmosphere more or less full of water vapor; but the atmosphere can
hold only a definite amount of vapor at a given temperature, and as
soon as it contains the maximum amount for that temperature, further
evaporation ceases. If clothes are hung out on a damp, murky day they
do not dry, because the air contains all the moisture it can hold, and
the moisture in the clothes has no chance to evaporate. When the air
contains all the moisture it can hold, it is said to be saturated, and
if a slight fall in temperature occurs when the air is saturated,
condensation immediately begins in the form of rain, snow, or fog. If,
however, the air is not saturated, a fall in temperature may occur
without producing precipitation. The temperature at which air is
saturated and condensation begins is called the _dew point_.

28. How Chills are Caused. The discomfort we feel in an overcrowded
room is partly due to an excess of moisture in the air, resulting from
the breathing and perspiration of many persons. The air soon becomes
saturated with vapor and cannot take away the perspiration from our
bodies, and our clothing becomes moist and our skin tender. When we
leave the crowded "tea" or lecture and pass into the colder, drier,
outside air, clothes and skin give up their load of moisture through
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, an