Environmental Engineering Reference
In-Depth Information
tornadoes (Figure 7.14). It goes under a variety of names: 'hurricane' in the Caribbean
and the United States, 'typhoon' in the Pacific and 'cyclone' in the Bay of Bengal. To
qualify as a hurricane, the storm must contain winds reaching over sixty-three knots (32
m s
−1
). Less intense storms are called tropical cyclones or tropical storms.
If we look at the parts of the globe affected by these cyclones, it is apparent that they
develop only over the warmer parts of the oceans (Figure 7.15). In each hemisphere it is
during the summer and autumn seasons that cyclones are most likely to strike.
Despite the danger and damage of hurricanes, we know surprisingly little about their
origins, except that they all form over the tropical seas where temperatures are above 27°
C and that they do not form within about 5° latitude of the equator. Once developed, they
move towards the west within the trade winds, gradually increasing in intensity. Before
dying out the storm usually begins to swing polewards. A few manage to maintain their
identity but they gradually decay and acquire the characteristics of a mid-latitude
depression. Many September storms and floods in north-west Europe can be traced back
to Caribbean hurricanes.
Once started, the development of the hurricane is fairly predictable in terms of flow in
the middle levels of the atmosphere and regional temperature patterns. But what starts it
off? In order for the cyclonic wind circulation to develop, we must have air converging,
which requires some form of initial disturbance. It must also be sufficiently far away
from the equator, at least 5°, to allow the Coriolis force to divert the inflowing air flow
and initiate rotation. We do find a variety of small disturbances within the tropics where
vertical movements and rotation can be started. As they are small in size and found over
the seas, their location is difficult to determine. Once the air begins
Plate 7.5
Hurricane Hugo about to hit the south-east coast of
the United States, 21 September 1989. The tight spiral of
