Environmental Engineering Reference
In-Depth Information
slows down (Figure 7.5). The air moving away from the trough draws air from the lower
atmosphere, causing a reduction in surface pressure. Thus air is seen to converge at the
ground within the depression, rise upwards into the upper atmosphere, and there diverge
as it flows away from the trough. The relative rates of surface convergence and of upper-
air divergence control the development of the surface low. If divergence exceeds
convergence the depression intensifies as air is drawn out of the system. At that stage we
find air pressure at the ground falling. If convergence exceeds divergence the depression
fills and air pressure at the surface rises. This is what happens in the final stages of the
depression.
In the northern hemisphere the troughs and ridges of the upper westerlies tend to
favour certain locations. There is normally a ridge near the Western Cordillera of North
America and a trough near the eastern coast of the United States. This means that
depression formation is most likely in the area south and east of Newfoundland (Figure
6.13). The depressions intensify, reach their maximum intensity near Iceland, then decay.
The average position of the depressions shown on mean pressure charts is near Iceland
for this reason. It represents the most frequent track of the depressions and where, on
average, they reach their lowest pressure. Because the lows are areas of rising air, they
are almost always accompanied by extensive cloud and precip-itation. The steep pressure
gradients and rapid falls of pressure which can occur cause problems for the affected
areas in terms of gales and heavy rain.
Figure 7.6 shows the typical vertical cloud distribution and temperatures associated
with a depression in mid-lati-tudes. The details of cloud location and thickness will
depend upon the nature of the upper atmospheric divergence and the temperatures and
humidities, on the time of year and on the sources of the air. If we look at the surface
pattern of precipitation from the depression (Figure 7.7), we can see how the areas of
highest rainfall tend to be just on the northern side of the depression track, with amounts
