Put a piece of ice in a pitcher of water and allow it to stand in a warm
room. You will soon notice that little beads of perspiration collect on
the outside of the pitcher. This moisture is air being condensed.
Water vapor is part of the atmosphere. Some of it is always present in
the air. The amount of vapor that the air can hold depends upon the
temperature. When the temperature is warm, the air will hold more water.
For instance, at 100° F. a cubic foot of air will hold 19.79 grains of
vapor; at 80° F., 10.95 grains; at 50° F., 4.09 grains, and 32° F., 2.17
grains. At 32° F. is the freezing point on the Fahrenheit scale.
Air containing as much water vapor as it can hold is saturated. If the
air is suddenly cooled down, that is, if the temperature falls when the
air is saturated, air molecules are contracted, and it must give up the
water, which produces rain. The ocean and the Great Lakes are the source
from which the air gets its water. It rises into the air in the form of
vapor, that is, vapor rising from the surface of the water, and the wind
distributes it over the land. Condensation turns it into clouds, and
when it is over-saturated, or rather, when the temperature drops and the
air is unable to retain any more water, then it forms into drops of
water and falls as rain. When the clouds get into the air, below the
freezing point of the water, the drops of water are changed into ice
crystals or snow flakes.
When the ice crystals are just at the point of melting into water, due
to the rise in temperature, the snowflakes lose their form and the
result is sleet.