The jet stream flows in an irregular path, not straight around the world at one latitude. An experiment can be conducted to demonstrate this effect on a small scale, using a rotating pan, ice, water, and a heating element. The pan is filled with water of a moderate temperature. A container of ice water is placed at the center of the pan, and the heating element is placed under the periphery of the pan, all the way around (Fig. 2-5). This sets up a "battle" between cold water and hot water. Sawdust, or some other fine substance that will float, is sprinkled on the surface of the water, facilitating observation of the water circulation. The pan is then spun around.
Although the pan is circular, and not spherical like the earth, the forces imposed on the water are similar to the forces that occur in the atmosphere of our planet. The center of the pan does not move but simply rotates, as does the north or south pole of the earth. The outer edge of the pan has considerable tangential speed, as does the equator. The water in the pan develops a miniature
"jet stream" somewhere between the center and the periphery, and this stream flows in the direction of rotation, as can be seen by watching the sawdust. If the pan is rotated fast enough, the stream develops irregularities that resemble the meanders of the jet stream in the real atmosphere.
If the temperature of the heating element is increased, simulating the conditions of summer, the stream becomes smaller, moving in toward the center of the pan. If the heat is turned down and extra ice is placed in the container at the center, the stream moves toward the periphery of the pan, just as the real jet stream moves toward the equator in the winter.
The jet stream flows in an irregular way because of inherent instability of the system. A bend in the jet stream is called cyclonic if it turns toward the left (counterclockwise) in the northern hemisphere or toward the right (clockwise) in the southern hemisphere. A bend is termed anticyclonic if it turns toward the right in the northern hemisphere, or toward the left in the southern hemisphere. When the polar air mass pushes toward the equator, a cyclonic bend is produced in the jet stream; when the tropical air mass advances on the pole, an anticyclonic bend is produced.
Cold low-pressure systems on the polar side of a cyclonic trough in the jet stream tend to be stable and persistent. The same is true of warm high-pressure systems that exist on the equatorial side of an anticyclonic ridge. Troughs and ridges can form over the oceans or over the continents. Persistent ridges tend to develop over the oceans during winter and over land in the summer. Troughs tend to form over the continents in the winter. Continental ridges bring warm, dry, fair weather, and sometimes they produce heat waves. Troughs generate stormy weather.
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