Cold waves are caused by an area of heavy air or high ba rometer, which is, as you know, the opposite of the cy clone or "low." In Sec. 67 we learned that a high barometer may show the air to be more than one hundred pounds heavier to the square foot than is the air in a place where there is a low barometer. When this is so, the heavy air flows out in all directions from the high barometer, just as water does when you pour it into the middle of a flat dish. As this air comes down from above it has but little water, and, therefore, it makes clear weather as well as cool weather.
Fig. 63 is the same as Fig. 60, except we see also in Fig. 63 the high barometer, or cold wave, which brings clear weather to follow the low barometer, or warm rainy time. On this map, the cyclone is central in Kentucky and the cold wave is central in Montana, with a northwest wind blowing from the cool high toward the warm low or cyclone. Fig. 64 is the same as Fig. 61, with the cold wave added. We see that the cyclone has gone on to the St. Lawrence Valley, and the cold wave has reached the Cotton Belt. The north wind and the northwest wind are now blowing clear and cool where two days before the weather was warm and rainy.
70. The weather procession.—It is these two barometer brothers, the high and the low, that give us our weather,—warm with rain when the barometer is low, and cool and clear when the barometer is high. This pair follow each other across the United States in a procession that never stops, for as soon as a low goes off to sea, or dies out, a high comes along after it. The high it its turn, passes off to sea or dies out, and another low follows it. Thus, on and on, on and on they go, never stopping, for hun dreds and thousands and millions of years. The United States Weather Bureau at Washington prints a map every day show ing the weather as it is at eight o'clock in the morning. Such a map is shown at Fig. 156. Perhaps you can get some of these maps for your school, if you write to the Weather Bureau at Washington, D. C., for them. When you write you might also ask where the nearest weather bureau is.
71. The thundershower.
—To understand how much the cyclones help the Prairie Corn and Small Grain Belt, we need to know one thing more about the rain. In winter these cyclones usu ally cause it to rain or snow all day. In the summer
time, however, a cyclone often gives us hot, muggy weather, with sunshine most of the day but with a thun dershower in the afternoon.
In summer the cyclones advance eastward more slowly than in winter, because the westerly wind is weaker in summer. The air near the earth becomes very warm, because the surface of the earth is heated by the sun. The heated air expands and gets light so that a few cubic miles of it are pushed up by the heavier, cooler air, very much as a piece of wood floats in a tub when water is poured in. As this air goes up, the pressure on it grows less, with the result that it ex pands. Expanding cools it again. To see that air is cooled by expanding, let a little of the compressed air out of an automobile tire or bicycle tire, and notice how cool the escaping air feels. Since cool air will hold less moisture than warm air, big white clouds form in the heated air that rises on a summer day, just as little clouds form over the spout of the boiling teakettle. Sometimes the water falls as rain; hence the thunder shower.
The lightning of a thunderstorm is the electricity jumping between cloirds or be tween the clouds and the earth. You can make some electric sparks by rubbing a rubber comb y it against a woolen cloth on a MOIR cold day. Sometimes if you stroke a cat's fur in cold weather you can see sparks of electricity and hear them snap. The sparks that you see are really very small flashes of lightning.
One big summer cyclone may cause fifty or even a hundred thunderstorms to occur in a single afternoon. They may be scattered over three or four states, each one may be only two or three miles wide, but it may wet a strip many miles long because the wind blows it forward. The storm pours down rain as it goes. Often one farm will get a soaking rain, while a farm a mile away will get only a sprinkle.
72. The tornado is a very small cyclone blowing with terrible force. Sometimes when the thunderclouds form in the level country of the central part of the United States, the thundercloud rises very quickly, and the air rushing in to take its place is set whirling in a small area, like the water when it goes out of the bottom of a washbasin or out through a hole in the bottom of a can. These small whirling storms are called tornadoes; they are the most terrible storms of all.