he months go by, winter freezes the few pools that remain. No snow
falls. Living creatures die by the tens of thousands. But the winter
is less cold than usual, because there is now so much water vapor in
the air that it acts like a great blanket holding in the earth's heat.

With spring no showers come. The dead trees send forth no buds. No
birds herald the coming of warm weather. The continents of the world
have become vast, uninhabitable deserts. People have all moved to the
shores of the ocean, where their chemists are extracting salt from the
water in order to give them something to drink. By using this saltless
water they can irrigate the land near the oceans and grow some food to
live on. Each continent is encircled by a strip of irrigated land and
densely populated cities close to the water's edge.

It is many years before the oceans disappear. But in time they too
are transformed into water vapor, and no more life as we know it is
possible in the world. The earth has become a great rocky and sandy
ball, whirling through space, lifeless and utterly dry.

That which prevents this from really happening is very simple: In the
world as it is, water vapor condenses and changes to drops of water
whenever it gets cool enough.

HOW WATER VAPOR GETS INTO THE AIR. The water vapor gets into the air
by evaporation. When we say that water evaporates, we mean that it
changes into water vapor. As you already know, it is heat that makes
water evaporate; that is why you hang wet clothes in the sun or by the
fire to dry: you want to change the water in them to water vapor. The
sun does not suck up the water from the ocean, as some people say; but
it warms the water and turns part of it to vapor.

What happens down among the molecules when water evaporates is this:
The heat makes the molecules dance around faster and faster; then the
ones with the swiftest motion near the top shoot off into the air. The
molecules that have shot off into the air make up the water vapor.

The water vapor is entirely invisible. No matter how much of it there
is, you cannot see it. The weather is just as clear when there is a
great deal of water vapor in the air as when there is very little, as
long as none of the vapor condenses.

HOW CLOUDS ARE FORMED. But when water vapor condenses, it forms into
extremely small drops of real water. Each of these drops is so small
that it is usually impossible to see one; they are so tiny that you
could lay