about 3000 of them side by side in one inch! Yet, small as
they are, when there are many of them they become distinctly visible.
We see them floating around us sometimes and call them fog or mist.
And when there are millions of them floating in the air high above us,
we call them a cloud.

The reason clouds form so high in the air is this: When air or any gas
expands, it cools. Do you remember Experiment 31, where you let the
gas from a tank expand into a wet test tube and it became so cold that
the water on the test tube froze? Well, it is much the same way with
rising air. When air rises, there is less air above it to keep it
compressed; so it expands and cools. Then the water vapor in it
condenses into droplets of water, and these form a cloud.

Each droplet forms a gathering place for more condensing water vapor,
and therefore grows. When the droplets of water in a cloud are very
close together, some may be jostled against one another by the wind.
And when they touch each other, they stick together, forming a larger
drop. When a drop grows large enough it begins to fall through the
cloud, gathering up the small droplets as it goes. By the time it gets
out of the cloud it has grown to a full-sized raindrop, and falls to
earth. The complete story of rain, then, is this:

HOW RAIN IS CAUSED. The surface of the oceans and lakes is warmed by
the sun. The water evaporates, turning to invisible water vapor. This
water vapor mingles with the air. After a while the air is caught in a
rising current and swept up high, carrying the water vapor with it. As
the air rises, there is less air above it to press down on it; so
it expands. When air expands it cools, and the water vapor which is
mingled with it likewise cools. When the water vapor gets cool enough
it condenses, changing to myriads of extremely small drops of water.
These make a cloud.

A wind comes along; that is, the air in which the cloud is floating
moves. The wind carries the cloud along with it. More rising air, full
of evaporated water from the ocean, joins the cloud and cools, and
the water forms into more tiny droplets. The droplets get so close
together that they shut out the sun's light from the earth, and people
say that the sky is darkening.

Meanwhile some of the droplets begin to touch each other and to stick
together. Little by little the drops grow bigger by joining together.
Pretty soon they get so big and heavy that they can no longer float
high