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CENTERS OF ACTION

The pressure belts of the general circulation are rarely continuous. They are broken up into detached areas of high and low pressure cells by the secondary circulation. The breaks correspond with regions showing differences in temperature from land-to water surfaces. (Turn back to figures 3-1-4 and 3-1-5. Compare the temperature dis-tribution in views A and B of figures 3-1-4 to the pressure distribution in views A and B of figure 3-1-5.) Note the gradient over the Asian Continent in January. Compare it to the warmer temperature over the ocean and coastal regions. Now look at view A of figure 3-1-5 and note the strong region of high pressure corresponding to the area. Now look at the same area in July. Note the way the temperature gradient flattens out and warms. Look at view B of figure 3-1-5 and see the low pressure area which has replaced the high pressure region of winter. These pressure cells tend to persist in a particular area and are called centers of action; that is, they are found at nearly the same location with somewhat similar intensity during the same month each year.

There is a permanent belt of relatively low pressure along the equator and another deeper belt of low pressure paralleling the coast of the Ant-arctic Continent. Permanent belts of high pres-sure largely encircle Earth, generally over the oceans in both the Northern and Southern Hem-ispheres. The number of centers of action are at a maximum at about 30 to 35 degrees from the equator.

There are also regions where the pressure is predominantly low or high at certain seasons, but not throughout the year.

In the vicinity of Iceland, pressure is low most of the time. The water surface is warmer (due to warm ocean currents) than the surface of Iceland or the icecaps of Greenland. The Icelandic low is most intense in winter, when the greatest temperature contrast occurs, but it persists with less intensity through the summer. Near Alaska, a similar situation exists with the Aleutian low. The Aleutian low is most pronounced when the neighboring areas of Alaska and Siberia are snow covered and colder than the adjacent ocean. These lows are not a continuation of one and the same cyclone. They are, however, regions of low pressure where lows frequently form or ar-rive from other regions. Here they remain sta-tionary or move sluggishly for a time, then the lows move on or die out and are replaced by others. Occasionally these regions of low pressure are invaded by traveling high-pressure systems.

Two areas of semipermanent high pressure also exist. There is a semipermanent high-pressure center over the Pacific westward of California and another over the Atlantic, near the Azores and off the coast of Africa. Pressure is also high, but less persistently so, west of the Azores to the vicinity of Bermuda. These subtropical highs are more in-tense and cover a greater area in summer than in winter. They also extend farther northward in summer. In winter, these systems move south toward the equator, following the solar equator.

The largest individual circulation cells in the Northern Hemisphere are the Asiatic high in winter and the Asiatic low in summer. In winter, the Asiatic continent is a region of strong cool-ing and therefore is dominated by a large high-- pressure cell. In summer, strong heating is present and the high-pressure cell becomes a large low-pressure cell. (See fig, 3-1-5 A and B.) This seasonal change in pressure cells gives rise to the monsoonal flow over India and Southeast Asia. Another cell that is often considered to be a center of action is the polar high. Both Arctic and Antarctic highs have considerable variations in pressure, and these regions have many traveling disturbances in summer. For example, the Greenland high (due to the Greenland icecap) is a persistent feature, but it is not a well-defined high during all seasons of the year. The Greenland high often appears to be an extension of the polar high or vice versa.

Other continental regions show seasonal varia-tions, but are generally of small size and their loca-tion is variable. Therefore, they are not considered to be centers of action.

An annual average pressure distribution chart (fig. 3-2-1) reveals several important characteristics. First, along the equator there is a belt of relatively low pressure encircling the globe with barometric pressure of about 1,012 millibars. Second, on either side of this belt of low pressure is a belt of high pressure. This high pressure area in the Northern Hemisphere lies mostly between latitudes 30° and 40°N with three well-defined centers of maximum pressure. One is over the eastern Pacific, the second over the Azores, and the third over Siberia; all are about 1,020 millibars. The belt of high pressure in the Southern Hemisphere is roughly parallel to 30°S. It, too, has three centers of maximum pressure. One is in the eastern Pacific, the second in the eastern Atlantic, and the third in the Indian Ocean; again, all are about 1,020 millibars. A third characteristic to be noted from this chart is that, beyond the belt of high pressure in either hemisphere, the pressure diminishes toward the poles. In the Southern Hemisphere, the decrease in pressure toward the South Pole is regular and very marked. The pressure decreases from an average slightly above 1,016 millibars along latitude 35°S to an average of 992 millibars along latitude 60°S. In the Northern Hemisphere, however, the decrease in pressure toward the North Pole is less regular and not as great. This is largely due to the distribution of land and water: note the extensive land masses in the Northern

Figure 3-2-1.—Average annual pressure distribution chart.

Hemisphere as compared to those of the Southern Hemisphere.

While the pressure belts that stand out on the average annual pressure distribution chart repre-sent average pressure distribution for the year, these belts are rarely continuous on any given day. They are usually broken up into detached areas of high or low pressure by the secondary circula-tion of the atmosphere. In either hemisphere, the pressure over the land during the winter season is decidedly above the annual average. During the summer season, the pressure is decidedly below the average, with extreme variations occurring such as in the case of continental Asia. Here the mean monthly pressure ranges from about 1,033 millibars during January to about 999 millibars during July. Over the northern oceans, on the other hand, conditions are reversed; the summer pressure there is somewhat higher. Thus in January the Icelandic and Aleutian lows intensify to a depth of about 999 millibars, while in July these lows fill and are almost obliterated. The polar high in winter is not a cell centered directly over the North Pole, but appears to be an extension of the Asiatic high and often appears as a wedge extending from the Asiatic continent. The cell is displaced toward the area of coldest temperatures—the Asiatic continent. In summer, this high appears as an extension of the Pacific high and is again displaced toward the area of coolest temperature, which in this case is the ex-tensive water area of the Pacific.

In winter over North America, the most significant feature is the domination by a high-- pressure cell. This cell is also due to cooling but is not as intense as the Asiatic cell. In summer, the most significant feature is the so-called heat low over the southwestern part of the continent, which is caused by extreme heating in this region.

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