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 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.
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.
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 up, there is less air above it to push down on it; so it expands. This makes it cool, and the water vapor in it begins to condense and form snow or rain. Therefore the side of mountain ranges against which the wind usually blows, almost always has plenty of rainfall.
It is different on the farther side of the mountain range. For here the air is sinking. As it sinks it is being compressed. And as it is compressed it is heated. If you hold your finger over the mouth of a bicycle pump and compress the air in the pump by pushing down on the handle, you will find that the pump is decidedly warmed. When the air, sinking down on the farther side of the mountain range, is heated, the water vapor in it is not at all likely to condense. Therefore rain seldom falls on the side of the mountains which is turned away from the prevailing winds.