In the temperate zone, where westerly winds predominate, pressure patterns move in an east-erly direction. In the tropics, however, weather usually moves in the opposite direction. Normally, a moist layer, 5,000 to 8,000 feet deep exists in this region. During unfavorable weather, this layer deepens to more than 12,000 feet. Convergence occurs in opposing trade wind streams, northward flowing air, and areas of cyclonic curvature. The presence of a deep, moist layer and convergent winds account for the weather in equatorial and tropical regions.

Equatorial and tropical weather, including tropical cyclones, waves in the easterlies, the intertropical convergence zone, and other tropical weather zones, are discussed in unit 9.

In the winter, the most favorable conditions for vigorous frontal activity are concentrated along the east coasts of North America and Asia. These conditions are associated with polar front activity. Cold air masses from continental sources meet warm, moist air from over the oceans. The warm ocean currents along these coasts greatly accentuate the frontal activity. The great temperature difference of the air masses, caused by the contrasting characteristics and proximity of their sources and the moisture that feeds into the air from the warm ocean currents, accounts for the intensity and persistence of these frontal zones off the east coasts in the winter. Modifica-tion of the air masses as they sweep eastward across the ocean leads to modified frontal activity on the west coasts. Refer back to figures 4-2-6 and 4-2-7 for the location of the following frontal zones:

1. Polar fronts in the Atlantic. In the Atlantic, in winter, polar fronts are found situated in various locations between the West Indies and the Great Lakes area. Intensity is at a maximum when the fronts coincide with the coastline. Waves, with cold and warm fronts, form along the polar front and move northeastward along the front. Like all cyclonic waves, they develop low-pressure centers along the frontal trough. They may grow into severe disturbances and go through the usual stages of development: formation, growth  occlusion, and dissipation. These cyclonic waves occur in families. Each family of waves is associated with a southward surge, or outbreak, of cold polar air. The polar front commonly extends approximately through the Great Lakes area. As the polar air advances, it pushes the front southward. The outbreak occurs, and polar air, joining the trade winds, spills equatorward.

There is no regular time interval for these large outbreaks of polar air, but the average period is about 5 1/2 days between them. Under average conditions, there are from three to six cyclonic waves on the polar front between each outbreak of polar air. The first of these usually travels along the front that lies farthest to the north. As the polar air accumulates north of the front, the front is pushed southward, and the last wave therefore follows a path that starts farther south than the path followed by the first wave. These families of polar front cyclones appear most frequently over the North Atlantic and North Pacific in the winter.

During the summer months, the polar fronts of the Atlantic recede to a location near the Great Lakes region, with the average summer storm track extending from the St. Lawrence Valley, across Newfoundland, and on toward Iceland. Polar outbreaks, with their accompanying fam-ily groupings of cyclones, are very irregular in summer and often do not exist at all. Frontal activity is more vigorous in the winter than in the summer because the polar and tropical air masses have greater temperature contrasts in the winter, and polar highs reach maximum development in the winter. Both of these factors increase the speed of winds flowing into fronts. Over oceans of mid-dle latitudes, a third factor helps to make winter fronts more vigorous than summer fronts. In the winter, continental air becomes very unstable when it moves over the comparatively warm ocean surface; in the summer, it remains relatively stable over the comparatively cool ocean. Sum-mer frontal activity (in middle latitudes) is therefore weak over oceans as well as over land. The high moisture content of maritime air causes much cloudiness, but this moisture adds little energy to frontal activity in the relatively stable summer air.

2. The polar fronts in the Pacific. These fronts are similar to those of the Atlantic, except that in the winter there are usually two fronts at once. When one high dominates the subtropical Pacific in the winter season, the Pacific polar front forms near the Asiatic coast. This front gets its energy from the temperature contrast between cold northerly monsoon winds and the tropical maritime air masses, and from the warm, moist Kuroshio current. In moving along this polar front of the Asiatic North Pacific in winter, storms occlude before reaching the Aleutian islands or the Gulf of Alaska. Because of its steady cyclonic circulation, the Aleutian low becomes a focal center, or a gathering point, for cyclones. The occluded fronts move around its southern side like wheel spokes. This frontal movement is limited to the southern side of the Aleutian low because mountains and the North American winter high-pressure center prevent fronts from passing northward through Alaska without considerable modification.

In the winter the cyclones reach the Aleutians and the Gulf of Alaska. Here, Arctic air from the north meets the relatively warmer maritime air from the south. The Pacific arctic front of winter is found in this region. Although many occluded storms dissipate in the Gulf of Alaska, others strongly regenerate with waves developing on what were once occluded fronts.

When the Pacific subtropical high divides into two cells or segments (as it does 50 percent of the time in the winter and 25 percent of the time in the summer), a front forms in the vicinity of Hawaii. Along this front, storms develop and move northeastward. These storms, called Kona storms, have strong southwest winds and bring heavy rains to the islands. Those storms that succeed in moving beyond the realm of the northeast trade winds, which stunt them, may develop quite vigorously and advance to the North American coast, generally occluding against the mountains. When this second polar front exists, two systems of cyclonic disturbances move across the Pacific. Because of their greater sources of energy, however, storms that originate over the Kuroshio current and move toward the Aleutians are almost always more severe. In the Atlantic, a second polar front, similar in nature and source to the second polar front of the Pacific, sometimesthough rarely-develops.

During the summer months, the Pacific polar front lies to the north of Kamchatka and the Aleu-tians and shows no rhythmic polar outbreaks.

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