and forms clouds and rain. Now air rises only
when there is comparatively little pressure from above. Therefore,
before and during a storm there is not so much pressure on the mercury
of the barometer and the barometer is low.

Clear weather, on the other hand, is often the result of air being
compressed, for compressing air warms it. When air is being warmed,
the water vapor in it will not condense; so the air remains clear. But
when the air is being compressed, it presses hard on the mercury of
the barometer; the pressure is high, and the mercury in the barometer
rises high. Therefore when the mercury in the barometer is rising, the
weather is usually clear.

[Illustration: FIG. 154. Finding the pressure of the air by measuring
the height of the mercury in the tube.]

These two statements are true only in a very general way, however. If
weather forecasters had only their own barometers to go by, they would
not be of much value; for one thing, they could not tell us that a
storm was coming much before it reached us. But there are weather
stations all over the civilized world, and they keep in touch with
each other by telegraph. It is known that storms travel from west to
east in our part of the world. If one weather man reports a storm at
his station, and tells how his barometer stands, the weather men to
the east of him know that the storm is coming their way. From several
such reports the weather men to the east can tell how fast the storm
is traveling and exactly which way it is going. Then they can tell
when it will reach their station and can make the correct prediction.

[Illustration: FIG. 155. The kind of mercury barometer that you buy.]

Weather men do not have to wait for an actual storm to be reported.
If the reports from the west show that the air is rising as it swirls
along--that is, if the barometer readings in the west are low--they
know that this low-pressure air is approaching them. And they know
that low pressure usually means air that is rising and cooling
and therefore likely to drop its moisture. In the same way, if the
barometers to the west show high pressure, the eastern weather men
know that the air that is blowing toward them is being compressed and
warmed, and is therefore not at all likely to drop its moisture; so
they predict fair weather.

The weather man is not ever certain of his forecasts, however.
Sometimes the air will begin to rise just before it gets to him. Then
there may