Environmental Engineering Reference
In-Depth Information
Figure 7.8 Thermograph trace over a four-day period
illustrating the effects on temperature of the passage of two
mid-latitude cyclones. The fall of temperature at the cold
front was unusually large for the United Kingdom.
clouds do not follow the frontal surface as closely as one might expect.
The warm front slopes at a low gradient of about 1 in 300, which means that the first
clouds associated with the front can be seen long before the surface front is near. Cirrus
clouds are the first indicators of the approach of the front, followed by a sequence of
gradual thickening and lowering of the cloud base. Cirrostratus clouds are followed by
altostratus, then nimbostratus clouds, by which time rain will be falling. In general, the
atmosphere is fairly stable at a warm front, but some convection does occur in the middle
levels, producing areas of heavier precipitation. Figure 7.9 shows an example of the
rainfall patterns associated with a warm front and a cold front.
The slope of the cold front is much steeper, at about 1 in 50. Weather activity at the
cold front can be much more intense than at the warm front. If the warm air is unstable,
the effect of uplift at the front generates thunderstorms and even tornadoes. The line of
deep cloud may be seen on satellite photographs (Plate 7.3) as a very distinct band. The
cold air descending with the heavy rain can intensify the effect of the fall in temperature.
When the air in the warm sector between the fronts is rising, cloud development near
the fronts follows the pattern described above; this is known as an ana-front (from the
Greek word meaning 'up'). Just ahead of the cold front, and at about 1 km above the
surface, strong winds develop in the warm sector. This warm, moist flow rises over the
warm front and turns south-eastward ahead of it as it merges with the mid-tropospheric
flow (Figure 7.5). This flow has been termed the 'conveyor belt', as it conveys large
quantities of energy polewards. Convective instability may be produced between this
lower, warm, moist air and the cooler, drier air aloft, to produce the
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