be a shower of rain when he has predicted fair weather. Or
sometimes the air that has been rising to the west, and which has made
him predict bad weather, may stop rising; the storm may be over before
it reaches his station. Then his prediction of bad weather is wrong.
Or sometimes the storm unexpectedly changes its path. There are many
ways in which a weather prophecy may go wrong; and then we blame the
weather man. We are likely to remember the times that his prophecy is
mistaken and to forget the many, many times when it is right.

[Illustration: FIG. 156. An aneroid barometer is more convenient than
one made with mercury. The walls are forced in or spring back out
according to the pressure of the air. This movement of the walls
forces the hand around.]

HOW SNOW IS FORMED. The difference between the ways in which snow and
rain are formed is very slight. In both cases water evaporates and its
vapor mingles with the warm air. The warm air rises and expands. It
cools as it expands, and when it gets cool enough the water vapor
begins to condense. _But_ if the air as it expands becomes _very_
cold, so cold that the droplets of water freeze as they form and
gather together to make delicate crystals of ice, snow is formed.
The ice crystals found in snow are always six-sided or six-pointed,
because, probably, the water or ice molecules pull from six directions
and therefore gather each other together along the six lines of this
pull. At any rate, the tiny crystals of frozen water are formed and
come floating down to the ground; and we call them _snowflakes_. After
the snow melts it goes through the same cycle as the rain, most of it
finally getting back to the ocean through rivers, and there, in time,
being evaporated once more.

[Illustration: FIG. 157. Different forms of snowflakes. Each snowflake
is a collection of small ice crystals.]

Hail is rain that happens to be caught in a powerful current of rising
air as it forms, and is carried up so high that it freezes in the
cold, expanding air into little balls of ice, or hail stones, which
fall to the ground before they have time to melt.

WHY ONE SIDE OF A MOUNTAIN RANGE USUALLY HAS RAINFALL. When air that
is moving along reaches a mountain range, it either would have to
stop, or rise and go over the mountain. The pressure of the air behind
it, moving in the same direction, keeps it from stopping, and so it
has to go up the slopes and over the range. But as it goes u