63. The storm crosses the ocean.—Figure 62 shows that the storm has gone out over the sea east of Newfoundland. It blows the ocean into waves that rock and roll the ships at sea.
We can trace the path (Fig. 62) that the cyclone has followed from the Pacific Ocean, across the valleys of the Columbia, the Mis sissippi, and the St. Lawrence rivers, and out into the Atlantic Ocean.
These storms sometimes cross the Atlantic and pass over England and France, Germany and Russia, and on into Asia. Meanwhile, another storm and yet another are following along behind.
64. When a cyclone passes.—When a low pressure area is approaching, the wind is sucked in from the south and east and the weather becomes warmer. When we are in the center of an area of low pressure the winds vary, and the warm moist air of the cyclonic area, becoming cool as it rises, cannot hold as much moisture, and either rain or snow falls. When the cyclone has passed on, the cold wind blows from the northwest or west, and the temperature falls. In a few days another storm comes. Thus every few days in sum mer the needed rain comes to the cornfields and cottonfields. Now you see why all the eastern part of North Americagets good rains, and why the ocean between New England and old England is so often stormy.
65. The prevailing westerlies.—Why do the cyclones travel eastward? It is because the United States is in the cool northern zone, where the prevailing wind blows from the west towards the east. Naturally the cyclonic storms, which are just big, twisting eddies, are carried along by the more powerful stream of the prevailing wind, just as a little whirling eddy is carried along in a flowing stream of water.
66. Air pressure and the b aromet e r.—We wonder what it is that starts the cyclone. Let us try an experiment. Light a short piece of candle and set it up in a lamp chimney or even in a paper tube. The candle heats the air, which becomes lighter, rises, and draws other air in to take its place.
You can see these move ments of the air by hold ing a smok ing piece of paper near the bottom of the lamp chimney.
Some times the air becomes light over a part of the earth's surface, just as it does over the candle.
The weight of the air is measured by an instrument called aba rometer. If the air is heavy, the barometer is said to be " high " ; if the air is light, the barometer is said to be "low". On the average, at sea level the air presses about fifteen pounds to the square inch.
67. The high ba rometer and the low barometer.—If the ba rometer is high, the pressure of the air may be fifteen and a half pounds per square inch, or 2232 pounds to the square foot. If the barometer is low, the pressure of the air may even be 100 or 150 pounds less per square foot. That is a great pressure difference. Now if the barometer in one region, let us say in Kansas, becomes low while the barometer is high on all sides, there is less pressure on the farms of Kansas than there is on the farms in Dakota or Louisiana. It is like the experi ment with the lamp chimney. The heavy air around the lamp chimney rushed in to take the place of the lighter air. That is just what happens in our cyclone (Fig. 66). The heavy air in Dakota and Louisiana flows toward the lighter air in the center of the "low" in Kansas, and the heavy air pushes up the light air, just as it does in the lamp chim ney.
68. Starting the cyclone. cyclonic storm begins at the place where a center of air rises, becomes lighter than the air surrounding it. As it rises, the heavy air pushes it up and rushes in from all directions to take its place, as the air did in our lamp-chimney experi ment (Sec. 66). The air, rushing toward this center, twists around, much as water twists when it runs out through the hole at the bottom of a basin or of a bath tub, or through a round hole in the bottom of a tomato can.
These great twisting cyclones, several hun dred miles across, are pushed along toward the east by the pre vailing westerly wind. How long did it take those in Fig. 65 to go 1000 miles? 69. The cold wave. —The cyclone, with its warm weather and its rain, is not the cause of all of the rough weather we get. The cold spells called cold waves are quite anoth e r thing.