Jet Streams And The Weather

Now, clearly, we are speaking here of average atmospheric and oceanic motions in some sense.

There are many smaller scale motions that are very important, for example, sea breezes or the flows over mountainous terrain. Two very important features of atmospheric motions that I have not mentioned are the atmospheric jet streams. Jet streams are relatively narrow, very high velocity air currents that exist high in the atmosphere.5

Before about the middle of the 20th century, little was known about the details of the motion of the atmosphere at very high altitudes. During the first half of that century, humans were taking to the air in crude (by today's standards) airplanes and hot air balloons and zeppelins (blimps). But aviation really came of age during World War II. Toward the end of that war, the American Air Force prepared for a bombing mission targeting Tokyo industrial facilities, including Nakajima's Musashino plant, where a large fraction of Japan's combat aircraft engines were manufactured. On November 24, 1944, 110 B-29s took off from Saipan carrying 277.5 tons of bombs. As they neared Japan, flying at 27,000 to 30,000 ft, the winds at that altitude began to pick up. By the time they reached the target area, flying from east to west, they were fighting 140 mph headwinds. It was so difficult to gauge the drift of the bombs and other factors that most of the bombs missed their targets, and little damage was done to the Musashino plant. Later, precision bombing fared no better. These encounters with what we now recognize as an atmospheric jet stream forced the Americans to change tactics, and low-level incendiary raids replaced high-altitude missions. It is hard to believe that this was the first encounter with a phenomenon that we take for granted every day as we watch the local weather report on television.

There are actually many jet streams, the two most important to weather and climate being the subtropical jet stream and the polar front jet stream. You have seen and heard reference to the polar front jet stream during the weather portion of your local TV newscast. It happens that when two air masses of different temperatures exist closely, the wind velocity increases strongly with altitude. (This is called the thermal wind.) Thus, when the warm air from the Hadley cell meets the relatively cool air from the Ferrel cell in subtropical latitudes, an upper atmospheric jet forms: the subtropical jet (see Figure 1.2). Similarly, when the relatively warm air of the Ferrel cell meets the cold air of the polar cell, another upper air jet forms: the polar front jet. Air in these jet streams can reach very high velocities of several hundred miles per hour, flowing from west to east. Now, the polar front jet, as its name implies, forms over the frontal region between cold, high latitude air and warmer air on the equatorward side. This jet which forms at the border between cold air and warm air is not stable. Instead, it meanders around the hemisphere in a wiggly pattern, eventually giving rise to the ever-changing weather patterns that you see when you watch the daily weather report. To get a general idea of how this happens, refer to Figure 1.5. It shows a sequence of disturbances in the upper air waves. The amplitudes of the waves or wiggles increase until they break into cold regions,

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FIGURE 1.5: Upper air and the jet stream: the propagation of disturbances. The polar jet stream is not stable. This high-speed jet of air high in the atmosphere forms a wiggly pattern. The amplitude of the wiggles increases until some regions separate from the jet and form a series of regions that rotate either clockwise or counterclockwise. The regions that rotate counterclockwise contain cold air from high latitudes, whereas the regions that rotate clockwise are warm regions.5

which rotate counterclockwise, whereas the regions between them are warm regions, which rotate clockwise. Of course, the whole regions move toward the east because they are in the regions of the westerlies. This is clearly a very simple view of how fronts and local weather patterns form, but it gives you the general idea.

The subtropical jet stream, on the other hand, occurs along a line of descending air, or air that has a generally downward motion, that creates a high-pressure region near the ground where the diverging air currents prevent the occurrence of fronts near the ground.

But motion within the Hadley cell also has some surprises. It forms a set of several cells with longitudinal motion, that is, motion more or less perpendicular to the Hadley circulation (Figure 1.6) in the west-to-east or east-to-west direction. This has been named the Walker circulation

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Jet Streams Circulatio

FIGURE 1.6: The Walker Circulation. The pattern of this atmospheric circulation is shown in the figure for a "normal" year; this is the La Niña condition. The wind pushes the ocean water westward off the west coast of South America. The surface ocean water that is pushed off the coast is replaced by cooler water from the deeper ocean. The rising air over the Brazilian rain forests produces clouds and rain, whereas the descending air over coastal Peru gives rise to a dry climate. When the circulation weakens or reverses, the prevailing winds no longer drive the water toward the west, and the cool water from the deep ocean no longer replaces the warm equatorial water off the coast of South America.6

FIGURE 1.6: The Walker Circulation. The pattern of this atmospheric circulation is shown in the figure for a "normal" year; this is the La Niña condition. The wind pushes the ocean water westward off the west coast of South America. The surface ocean water that is pushed off the coast is replaced by cooler water from the deeper ocean. The rising air over the Brazilian rain forests produces clouds and rain, whereas the descending air over coastal Peru gives rise to a dry climate. When the circulation weakens or reverses, the prevailing winds no longer drive the water toward the west, and the cool water from the deep ocean no longer replaces the warm equatorial water off the coast of South America.6

after Sir Gilbert Walker, a scientist who postulated its existence while attempting to determine the causes of (and to predict) monsoon failures in India.

Continue reading here: El Nio And La

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