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SHEAR LINE

A shear line is a narrow zone across which there is an abrupt change in the horizontal wind component. As was stated earlier, the primary characteristic of these lines is the low-level horizontal wind discontinuity. The discontinuity results from strong subsidence (downward vertical motion) associated with an intensifying high-- pressure system poleward of the polar front. As the tail end of the polar front advances into the tropics, the subsidence causes frontolysis. In lower levels of the troposphere, strong easterly winds develop poleward of the line and weaker easterly winds equatorward of the line. Figure 9-3-1


Figure 9-3-1.—Shear line resulting when the polar high invades the tropics and reinforces the subtropical high (also note induced trough).







Figure 9-3-2.—A shear line in the wind field.

  illustrates shear line formation. In streamline analysis, the area where the polar front becomes a shear line is denoted by a neutral point. See figure 9-3-2.

Because of the subsidence on the poleward side of shear lines, the clouds along these lines exhibit lower tops than those of the frontal zones. Isolated convective buildups occur along the lines, but the predominant clouds are of the lower varieties, with tops 10,000 to 15,000 feet or less. The three most common clouds found along shear lines are altostratus, high stratocumulus, and nimbostratus. Rainy periods are common along with scattered in-stability showers, reduced risibilities, and low ceilings. The seas poleward of a shear line can build rapidly, because of the increased wind speed and relatively small change in wind direction. With the passage of a shear line, the sudden increase in wind speed is known as the "surge of the trades." Over North Africa in winter, polar outbreaks are not accompanied by great amounts of precipitation (until reaching the equatorial zone), but strong surface winds poleward of the front and/or shear line raise widespread dust, called ‘‘Harmattan haze," in West Africa.

Learning Objective: Define easterly wave and identify the weather associated with the three wave classifications.

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