Geoscience Reference
In-Depth Information
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 storms with a central pressure of
below 950 hPa have been recorded somewhere within the British Isles on at least thirty occasions. Although the
depth of low pressure was not in this extreme category, on 15/16 October 1987 explosive deepening 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-force south to south-westerly winds blew across the south-east to give record wind speeds for
many locations (
Figure 7.9
).
For most of Kent, Sussex and the coastal areas of Essex and Suffolk the highest gusts
were of a speed likely to be exceeded once in 200 years. Even in the built-up area around the London Weather Centre
a gust speed of eighty-two knots was recorded, compared with the previous maximum gust of only fifty-seven knots.
Heavy rain fell in association with the storm but, unlike the wind speeds, it was not noteworthy.
Wind speeds of this force sweeping across a densely populated and wooded area are likely to have a dramatic effect,
and this storm was no exception. More trees were lost in one night than in a decade of Dutch elm disease; parkland
areas were devastated, forests flattened and many urban trees blown down to block roads. In East Sussex it was
estimated that almost 25 per cent of the original standing volume of timber was blown down. In Brittany about 20
per cent of the whole forest area was reported to have been destroyed. In general, conifers appeared more vulnerable
to being blown down than deciduous trees, woodland trees were more vulnerable than isolated trees and individual
urban trees were more vulnerable than rural trees. Urban trees rarely fell in the direction of the nearest building,
perhaps reflecting channelling within the street. Ironically the clearance of trees from many areas has resulted in a
resurgence of ground vegetation as increased levels of light and greater nutrient availability from decaying vegetation
have changed the local ecosystem.
As well as causing devastation to trees, the storm had an impact on transport, as many trees blocked roads and rail
tracks, waves caused major problems along the coast and at sea and more than a hundred flights were cancelled
from Heathrow and Gatwick airports. Power lines are always susceptible to damage during storms, and much of
south-east England was without electricity for at least six hours between 03.00 and 09.30.
There were a number of other effects as winds damaged buildings, causing scaffolding and even pieces of building
fabric to collapse, in some cases causing fatalities. Many glasshouses were destroyed by the force of the wind.
Overall the storm proved a financial nightmare to the insurance industry as well as causing major problems for the
area affected. Another slightly less intense storm affected Britain in October 2000. The main centre of the low was
