Hurricanes: Their Nature and Impacts on Society - Climate Science ...

CHAPTER 3
3.1 LIFE OF A HURRICANE
Tropical cyclones are relatively rare weather events, with only about 84 per
year over the entire Earth. During the period 1968-1989, the Atlantic Ocean
basin averaged 9.7 tropical cyclones annually, of which an average of 5.4
became hurricanes (Neumann 1993).
3.1.1 Birth <strong>and</strong> growth
<strong>Hurricanes</strong> typically form over warm oceans from pre-existing regions of
relatively low surface pressures with an associated cluster of thunderstorms
(Riehl 1954). Regions of relatively low <strong>and</strong> high pressure exist in the tropics
due to differences in the weight of the atmosphere over these different
~ocations. Such pressure systems are analogous to those shown as "L" <strong>and</strong>
"H" on television weather reports. These regions are formed by mechanisms
I
that include differences in the heating of the atmosphere over the l<strong>and</strong> <strong>and</strong>
locean surface, <strong>and</strong> the flow of air over surface features such as mountains.
IAnother, more complicated factor in the development of areas of relatively
lhigh <strong>and</strong> low pressure is associated with propagating atmospheric waves that
I develop due to differences in the temperature of the atmosphere <strong>and</strong>/or wind
across a region.
Air tends to move towards the center of the low pressure region (<strong>and</strong> away
from high pressure). However, in the Northern Hemisphere, the rotation of
the Earth deflects the air such that it tends to move in a counterclockwise
direction around a low pressure system (<strong>and</strong> clockwise around a high pressure
system; Figure 3.1) as it spirals toward the center of the system. This wind
deflection results from the Coriolis effect, as schematically shown in Figure
3.2. As air spirals over the warm oceans toward the center of the low pressure,
the ocean provides water vapor (through evaporation) <strong>and</strong> the warm water
surface adds heat to the lower atmosphere (Ooyama 1969; Malkus <strong>and</strong> Riehl
1960). This movement of water vapor into the thunderstorms within the low
pressure area from the outside environment is essential to tropical cyclone