Natural Hazards: Causes and Effects - Disaster Management Center ...

The total number of dead was estimated at 1,400, with over 4,000 injured. The total number
affected was 1.2 million (23 percent of total population). The total damaged was estimated at
$830 million. 3
Geographical Distribution
Tropical cyclones are known around the world by various names: hurricanes in the Atlantic and
Caribbean, typhoons in the West Pacific, baguios in the Philippines, cordonazos in Mexico,
tainos in Haiti (see Table 5.1). A tropical cyclone is essentially a rotating storm in the tropical
oceans. It is conventionally defined as a circular storm with rotating wind speeds in excess of
64 knots (32 meters per second). The life span of a tropical cyclone is, on average, about six to
nine days until it enters land or recurves into temperate latitudes, but this may vary from a few
hours to as much as three to four weeks. Tropical cyclones form in the oceans between 5 to 30
degrees north and south of the equator. They are found in all oceans of the world, with the
probable exception of the South Atlantic and the South Pacific east of 140 deg. W longitude
(See Fig. 5.1). 4
No two tropical cyclones follow the same track; some recurve, some do not; some loop; some
slow to a standstill and some will accelerate. The movement of a tropical cyclone is generally
12 knots or less.
It is important to be aware of the regional names given in the above table so that, for example,
what is described as a severe cyclone in the Bay of Bengal will be understood as essentially the
same phenomenon as that which is called a hurricane when it occurs in the north Atlantic.
Natural Preconditions for Disaster Occurrence
Cyclones are born in the hot, humid late-summer environment of the tropics (June to August in
the Caribbean, November to April in the South Pacific). As the sun warms the oceans,
evaporation and conduction transfer heat to the atmosphere so rapidly that air and water
temperatures seldom differ by more than 1 degree F. The water vapor generated by such
evaporation is the fuel that drives a tropical storm, because as the vapor condenses into clouds
and precipitation it pumps enormous amounts of heat into the cyclone. The fuel supply is
controlled by the evaporation rate—which explains why cyclones cannot develop when the
ocean temperature is below about 24 degrees Centigrade (76 degrees F).
The frequent products of this mix of heat and moisture are several thunderstorms that can
become the seedling for a tropical cyclone—but it must be nurtured further. The trigger for most
Atlantic hurricanes is an easterly wave, a westward-migrating low-pressure center that may
have begun as an African thunderstorm. Typhoons in the Pacific and Indian oceans, and a few
hurricanes in the Atlantic, emerge from waves in the equatorial trough, the calm, cloudy
doldrums that separate the trade winds of the two hemispheres. 5
To develop and mature into a tropical storm, storm seedlings must overcome many obstacles.
In fact only about nine of the more than 1000 seedlings tracked each year in the Atlantic will
evolve into gale-force tropical storms or full-fledged cyclones.
The sole difference between harmless thunderstorms and a dangerous cyclone is the rotation
that organizes weather systems. This spin, which meteorologists call vorticity, is ever-present in
temperate latitudes, where the Coriolis effect of the earth’s rotation is pronounced. But in the
tropics the weak Coriolis effect must be augmented by the wind itself. (The Coriolis effect is the