p, there
is less air above it to push down on it; so it expands. This makes it
cool, and the water vapor in it begins to condense and form snow or
rain. Therefore the side of mountain ranges against which the wind
usually blows, almost always has plenty of rainfall.

It is different on the farther side of the mountain range. For here
the air is sinking. As it sinks it is being compressed. And as it is
compressed it is heated. If you hold your finger over the mouth of a
bicycle pump and compress the air in the pump by pushing down on the
handle, you will find that the pump is decidedly warmed. When the air,
sinking down on the farther side of the mountain range, is heated,
the water vapor in it is not at all likely to condense. Therefore rain
seldom falls on the side of the mountains which is turned away from
the prevailing winds.

HOW DEW AND FROST ARE FORMED. The heat of the earth radiates out into
the air and on out into space. At night, when the earth loses its heat
this way and does not receive heat from the sun, it becomes cooler.
When the air, carrying its water vapor, touches the cool leaves and
flowers, the water vapor is condensed by the coolness and forms drops
of dew upon them. Or, if the night is colder, the droplets freeze as
they form, and in the morning we see the grass and shrubs all covered
with frost.

THE CAUSE OF FOGS. When warm air is cooled while it is down around us,
the water vapor in it condenses into myriads of droplets that float
in the air and make it foggy. The air may be cooled by blowing in from
the warm lake or ocean in the early morning, for at night the land
cools more rapidly than the water does. This accounts for the early
morning fogs in many cities that are on the coasts.

Likewise when the wind has been blowing over a warm ocean current,
the surface of the warm water evaporates and fills the air with water
vapor. Then when this air passes over a cold current, the cold current
cools the air so much that the moisture in it condenses and forms fog.
That is why there are fog banks, dangerous to navigation, in parts of
the ocean, particularly off Labrador.

WHY YOU CAN SEE YOUR BREATH ON COLD DAYS. You really make a little fog
when you breathe on a cold morning. The air in your lungs is warm. The
moisture in the lungs evaporates into this warm air, and you breathe
it out. If the outside air is cold, your breath is cooled; so some of
the water vapor in it condenses into very small