Environmental Engineering Reference
In-Depth Information
FRONTS
In many depressions we would find that there is not a gradual change of temperature as
the systems pass but several sudden changes. Figure 7.8 shows the trace from a
thermograph during the passage of a cyclone. If it has been cold before the storm
approaches, temperatures may rise slightly. This is due to cloud and wind stirring up the
cold air. If it has been warm, temperatures may fall, because the sun will no longer be
shining. Suddenly the temperature starts to rise, perhaps by several degrees within a few
hours. It will then remain fairly stable until the arrival of the cold air in the rear of the
cyclone. The fall in temperature is usually more sudden than the earlier rise; a fall of up
to 10° within a few minutes is not unknown, though more frequently the frontal zone may
extend in width for 100-200 km and produce a fall in temperature of a few degrees.
The sudden change of temperature clearly indicates a change of air mass. The
separation surface or zone between air of different origins is call a
front
. Where warm air
is replacing cold air we have a warm front, and where cold air is replacing warm air we
have a cold front. The typical cloud structure along the fronts is shown in Figure 7.6. The
clouds mark the main zones of rising air produced by divergence in the upper
atmosphere. That is why the
STORMS
human impact
The temperate-latitude cyclones which form a significant feature of the westerly
circulation are very varied in their characteristics. Although the majority of them will
follow a sequence as outlined in this chapter, occasionally they deepen rapidly and
produce much more severe weather than expected. A classic area for the occurrence of
such explosive storms is the eastern coast of the United States, though storms of similar
origin and intensity also develop to the north-east of Japan and near western Europe. As
the storms develop, pressure falls of 10-20 hPa over twelve hours are not uncommon.
Central pressures may reach as low as 960 hPa, with hurricane-force winds over a
considerable area. When the storms develop in winter they may be accompanied by large
volumes of snow which wreak havoc in coastal cities from Boston to Washington. The
size, frequency and intensity of these storms as they affect the coastal areas of the north-
east United States make them potentially more dangerous and destructive than hurricanes.
The cyclogenesis takes place about 400 km downstream from a 500 hPa trough, and is
situated on the cold side of the belt of strongest westerlies where air and sea surface
temperature gradients are steep. The situation is similar in Japan, where explosive
cyclogenesis also takes place relatively frequently. Such is the impact of the American
storms that some are given names. For example, on 18-20 February 1979 there was the
President's Day storm and on 9-10 September 1978 the
Queen Elizabeth II
storm, named
because of the damage inflicted on the liner.
Storms of such intensity are rare over populated parts of north-west Europe, but on
15/16 October 1987 explosive deepening of pressure took place over the Bay of Biscay,
followed quickly by an even more rapid increase of pressure of over 20 hPa in three
hours. The centre of the low moved north-eastwards across Brittany, then tracked across
southern Britain and out into the North Sea near Norfolk, producing a steep pressure
gradient over south eastern England Driven by this strong pressure gradient hurricane
