Geoscience 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 0ยท5 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 and above 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. In Britain
many tornadoes develop near cold fronts with similar, but less intense, conditions.
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.
Occasionally disturbances arise to upset this quiet
regime. On a dramatic scale there is the tropical cyclone,
which is discussed in the next section, but on a smaller
scale there is the easterly wave. As its name implies, this
represents weather-forming systems related to wave-like
structures in the easterly flow of air. They reach their
maximum intensity at about the 700 hPa level.
The wave does not necessarily move at the same speed
as the easterly flow, and it may even exceed the average
wind speed. Preceding the wave, convectional cloud dies
down, owing to surface divergence and subsidence of
the air, while the wind backs towards the north-east
(
Figure 7.15
).
As the main axis of the wave approaches,
convergence becomes dominant, causing ascent of the air,
cloud formation and precipitation just ahead of the low-
pressure trough. The wind suddenly veers as the wave
passes, to be followed fairly quickly by the clearance of the
cloud and a return to undisturbed trade wind flow.
The passage of the wave is not dramatic, therefore, but
in areas where the weather hardly changes it does at least
provide a little variety. Moreover, in areas such as the
Caribbean, where the waves are frequent, they are
responsible for a significant proportion of the annual
precipitation.
provide visible evidence of the continuous evaporation from
the warm tropical seas.
Photo: Peter Smithson
