If ice water is poured into a glass, a mist will form on the outside
of the glass. This is because the water vapor in the air becomes
chilled by contact with the glass and condenses. Often leaves and
grass and sidewalks are so cold that the water vapor in the atmosphere
condenses on them, and we say a heavy dew has formed. If the
temperature of the air falls to the freezing point while the dew is
forming, the vapor is frozen and frost is seen instead of dew.

The daily 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