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Click here to Order your Radar Equipment Online 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
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 |
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