AIR MASSES AND FRONTS

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Warm Fronts When a warm front approaches a mountain range, the upper section of the frontal surface is above the effects of the mountain range and does not come under its influence (view A of fig. 4-45). As the lower portion of the frontal surface approaches the range, the underlying cold wedge is cut off, forming a more or less stationary front on the windward side of the range. The inclination of the frontal surface above the range decreases and becomes more horizontal near the mountain surfaces, but the frontal surface maintains its original slope at higher altitudes (view B of fig. 4-45). While the stationary front on the windward side of the range may be accompanied by prolonged precipitation, the absence of ascending air on the leeward side of the range causes little or no precipitation. The warm air descending the leeward side of the range causes the cloud system to dissipate and the warm front to travel as an upper front. 4-48 Frontogenesis (the formation of a new front or the regeneration of an old front) may occur in the pressure-trough area that accompanies the front. The frontal surface then gradually forms downward as the frontal system moves away from the mountain and extends to the earth’s surface again (views C and D of fig. 4-45). The effect of the mountain range on a warm front is to widen and prolong the precipitation on the windward side of the range, while on the leeward side the precipitation band is narrowed and weakened, or is nonexistent. Occluded Fronts Mountain ranges have much the same effect on occluded fronts as they do on warm and cold fronts. Cold type occlusions behave as cold fronts, and warm type occlusions behave as warm fronts. The occlusion process is accelerated when a frontal wave approaches a mountain range. The warm front is retarded; but the cold front continues at its normal movement, quickly overtaking the warm front (views A and B of fig. 4-46). A B AG5f0446 Figure 4-46.—Acceleration of the occlusion process and development of a frontal wave cyclone. C
When a cold front associated with an occluded frontal system passes a mountain range, the cold front may develop a bulge or wave. In the case of an occlusion, a new and separate low may form at the peak of the warm sector as the occluded front is retarded by a mountain range (view C of fig. 4-46). The low develops on the peak of the wave because of induced low pressure that results when air descends on the leeward side of the mountain and warms adiabatically. The development of a new low on a frontal wave and ultimate separation from its original cyclone is a fairly common occurrence. This can occur over open oceans but occurs more frequently along the west coast of mountainous continents and along the west coast of Japan. The typical stages of this type of frontal modification are shown in figure 4-47. Orographic features play a great role in certain preferred areas of this phenomena, but over the ocean some other factors -10 -15 -30 A B C D 4-49 must be operative. In some cases, a rapidly moving wave overtakes the slow moving occlusion and may be the triggering mechanism for this cyclogenesis. Whatever the exact nature of its causes, this type of cyclogenesis proceeds with great rapidity. Initially, the old occlusion in view A of figure 4-47 either moves against a mountain range or is overtaken by another cyclone. The occlusion then undergoes frontolysis (view B of fig. 4-47). The new occlusion forms immediately and soon overshadows its predecessor in both area and intensity (view C of fig. 4-47). However, the cold occlusion, having greater vertical extension, exerts a certain control on the movement of the new center, which at first follows the periphery of the old center. Later, the two centers pivot cyclonically (view D of fig. 4-47) about a point somewhere on the axis joining them until the old center has filled and loses its separate identity. Figure 4-47.—Stages in the development of a secondary wave cyclone. -30 -15 -15 AG5f0447
Page 1 and 2: Temperature, in the form of heating
Page 3 and 4: Characteristics of Air Masses The c
Page 5 and 6: WARM WARM AIR MASS MODIFICATION AIR
Page 7 and 8: THE PROCESS HOW IT HAPPENS RESULTS
Page 9 and 10: On the leeward side of the Great La
Page 11 and 12: 140 O 30 150 O mP 140 O mT 40 140 O
Page 13 and 14: FOG OR LOW STRATUS WARM OCEAN COLD
Page 15 and 16: 40 O 90 O 90 O Maritime Tropical (m
Page 17 and 18: 4-17 A B WINTER AIR MASSES CLOUDS C
Page 19 and 20: Maritime Tropical (mT) Air in Winte
Page 21 and 22: toward a warmer air mass. The coole
Page 23 and 24: Unstable Waves The unstable wave is
Page 25 and 26: temperatures and warm tropical air
Page 27 and 28: 75 60 45 30 15 0 15 30 45 60 180 15
Page 29 and 30: COLD FRONT COLD AIR From this it ca
Page 31 and 32: frontal intensity, is defined as th
Page 33 and 34: a rather broad zone of altostratus
Page 35 and 36: miles ahead of the front. These clo
Page 37 and 38: AG5f0435 Figure 4-35.—Typical iso
Page 39 and 40: Clearing usually occurs after the p
Page 41 and 42: eporting surface-pressure rises is
Page 43 and 44: A COOL AIR FLOW STRATOCUMULUS DISTA
Page 45 and 46: through it from the warm air mass a
Page 47: flow down the leeward side of the r

AIR MASSES AND FRONTS

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