You can see now," he continued, "how the thunder heads of
cumulo-nimbus are beginning to show, leaden in color below, with the
white billowy tops. They're very thick, those masses of cloud, perhaps
two miles thick, and the gray rain curtain trails along behind them.
Well, Tom, what is it?" he added, turning to the boy, who was claiming
his attention.

"The wind's shifting," answered the lad.

"To the eastward? Of course. It'll be north-east in a minute or two, as
I told you. It's got to be."

"But why, sir?" asked Tom. "I don't see why a surface wind should have
to blow up against a storm."

"That," said the Forecaster, "is quite easy. If the rain is falling, it
brings down a mass of cold air with it, displacing the warm air that
lies before the advancing storm. The warm air is driven forward, but, at
the same time, the descending cold air requires warm air to replace it
in its turn, and the warm air, therefore, curves backward and flows into
the upper portion of the storm cloud, where its moisture is condensed as
rain. So, my boy, a little distance in advance of a thunderstorm there
are three currents of air, an upper current of cold air, traveling in
the same direction as the storm, and driving the cirrus clouds before
it; a current of warm air, going in the opposite direction to the storm
and pouring a torrent of warm air into the cloud; and the cold squall,
which drives out from under the thunder-cloud and which comes in violent
gusts."

"But I thought," said Fred, "that thunder and lightning came from two
clouds banging together. If most of the thunder storms travel from the
west, where does this banging come in?"

"It doesn't come in at all," the Forecaster replied; "thunder and
lightning do not result from clouds striking each other. It's not quite
so simple as that.

"The lower air is full of positive electricity just as the surface of
the earth is charged with negative electricity. As you know, boys, rain
is formed by a lot of little drops of moisture combining to form one
large drop, which, when it is heavy enough, falls to the ground. Now the
surface of every drop of moisture is charged with electricity. When
these drops come together to make one big drop, the surface of the big
drop is proportionately much smaller than the combined surfaces of all
the small drops. There isn't room enough on the surface of the big drop
to hold all the electricity that existed on the surface of the larger
number of s