Environmental Engineering Reference
In-Depth Information
TORNADOES
human impact
Tornado! The very word brings alarm in areas such as the Mid-West and Mississippi
valley of the United States. It conjures up the vision of a darkening sky, the appearance of
a pale cloud, the familiar and frightening tornado funnel. The funnel may descend from
the cloud base, getting larger and darker, until it eventually touches the ground,
accompanied by a tremendous roaring wind. Debris is caught in the funnel, and as it
moves across the countryside it leaves complete devastation in its wake.
The tornado is normally narrow, about 0·5 km wide, and seldom does it move more
than 20 km. But exceptions do occur, with some being up to 1 km wide and travelling
500 km. How fast the wind blows within the funnel we cannot tell; no recorder has
survived its passage. From damage evidence, speeds of over 400 km hr
−1
are believed to
occur.
Tornadoes are found in many parts of the world, even Britain, but they achieve their
greatest strength and frequency over the continental plains of the United States. The
reason for this concentration is the frequent juxtaposition of layers of air with great
contrasts in air temperature and moisture. Warm, moist air ahead of a cold front may be
drawn in from the Gulf of Mexico. Behind it, cold, dry air may be sweeping southwards
from the Canadian Arctic. Such a situation is ideal for the development of the
cumulonimbus clouds needed to spawn tornadoes.
As with hurricanes, the precise mechanism by which a funnel forms is not understood.
It is probable that tornadoes are produced by thermal and mechanical effects acting in the
cloud. But why some clouds generate tornadoes and others do not is a mystery.
Nevertheless, favourable conditions are recognized and tornado warnings are issued by
the local US weather services.
Over the sea, similar funnels are termed waterspouts. As convection over the sea tends
to be less intense than over land, the waterspout is much weaker than the tornado but may
cause some damage to small boats, or to light buildings if it makes landfall.
KEY POINTS
1
Air masses are a feature of the atmospheric circulation. They develop in anticyclonic
areas where air movement is less rapid and acquire the characteristics of the
underlying surface. As they move away from their source area they transport these
thermal characteristics. So we can refer to tropical maritime air masses or tropical
continental ones.
2
Within the main circulation flows of the westerlies and the easterlies we can find
disturbances which give rise to more unsettled weather. These have a variety of forms
and names. In temperate latitudes the cyclone, depression or low and its associated
fronts are the main types of disturbance which bring rain. In tropical latitudes the
tropical cyclone is the best known feature but there are other less severe disturbances
such as easterly waves.
