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

force caused by the earth’s rotation that deflects a moving body to the right in the Northern
Hemisphere and to the left in the Southern Hemisphere.)
When two wind currents move side by side, the faster current tends to curl around the slower
one. If the faster current is on the right (viewed from upwind), the curl is to the left, yielding
positive vorticity in the Northern Hemisphere because it adds to the counterclockwise Coriolis
effect; a right-hand curl creates negative vorticity. A curving wind also possesses vorticity—
positive for a left-hand turn, negative for a right turn.
When positive vorticity becomes strong enough to spin a storm seedling, it starts a chain
reaction. The thunderstorms, not revitalized by a steady influx of warm, moist air, organize
around a deepening low-pressure center, called a tropical depression. This dramatically
increases the likelihood of cyclone formation; fully 70 percent of these depressions develop into
cyclones. (See Fig. 5.2.)
The depression becomes a tropical storm when its winds reach gale force, 62 kilometers per
hour (40 miles per hour). The storm often already has as much total energy as a cyclone, but
its winds are widely distributed and hence much slower; the ring of maximum wind may be as
much as 320 kilometers (200 miles) across. The final step to cyclone status merely
concentrates this energy. As pressure falls at the storm center, the ring of maximum wind
contracts dramatically, until it is perhaps 50 kilometers (30 miles) in diameter. Outside this
circle the velocity drops rapidly. 6
Disaster Event
When the cyclone-force winds move onto land the storm becomes the potential disaster. The
lower the atmospheric pressure in the center of the storm, the more violent the action of wind,
storm surge, and waves is likely to be. The storms can be classified by intensity, as is shown
below.
The Beaufort scale (Table 5.2) is used to estimate the velocity of the wind by observing the
effects of rising winds on the ocean surface and a variety of familiar objects.
Another standard relating cyclone intensity to damage potential has been developed by the
National Hurricane Center in the United States. It has been adapted from the Saffir/Simpson
hurricane scale. This descriptive scale, over a range of categories 1 through 5, is shown in the
following Table 5.3.
Flooding
High winds are, of course, only a part of the problems that are brought by the storm.
Devastating floods from extremely heavy rainfall often accompany tropical cyclones. Flash
floods of great volume and short duration may result from the cyclone’s rain, especially in hilly or
mountainous terrain. Runoff from the intense rainfall accumulates quickly in restricted valleys
and flows rapidly downstream, often as a large “wave.” Flood flows frequently contain large
concentrations of sediment and debris.
The damage generated by these floods is increased where they cause mudslides that either
cover or undercut roads, erode agricultural soil and contribute to long-term serious
environmental degradation.
Tidal floods can also be caused by the combination of waves generated by cyclone winds and
flood runoff resulting from the heavy rains that accompany cyclones. These floods may extend
over large distances along a coastline. Their duration is usually short, being dependent upon
the elevation of the tide, which rises and falls twice daily. 7