Tropical
storms (called Hurricanes in
the
Atlantic, Typhoons in the Pacific and Cyclones in
the Indian Ocean)
with winds that exceed 64 knots (74 mph or 119 kph) and circulate
counter-clockwise about their centers in the Northern Hemisphere
(clockwise in the Southern Hemisphere). They originate over the North
Atlantic and North Pacific oceans near high humidity and light winds.
One well known
characteristic of a hurricane is its eye, which is
generally about 5 to 25 miles in diameter and in which are present calm
conditions, with clear skies and light winds. Hurricanes can move
rather quickly losing intensity as soon as land is encountered, but not
without having created very high tides and possibly massive destruction.
They are also
known to be the most dangerous and devastating weather
event in the world.
How are Hurricanes Formed?
Hurricanes
form in tropical regions where
there is warm water (at least 80 degrees Fahrenheit / 27 degrees
Celsius), moist air and converging equatorial winds. Most Atlantic
hurricanes begin off the west coast of Africa, starting as
thunderstorms that move out over the warm, tropical ocean waters. A thunderstorm
reaches hurricane status in three stages:
Tropical depression - swirling clouds and rain
with wind speeds of less than 33 knots (38 mph / 61.15 kph)
Tropical storm - wind speeds of 34 to 63 knots
(39 to 73 mph / 54.7 to 117.5 kph)
It can take
anywhere from hours to several days for a thunderstorm to
develop into a hurricane. Although the whole process of hurricane
formation is not entirely understood, three events must happen for
hurricanes to form:
A continuing
evaporation-condensation cycle of
warm, humid ocean air
Patterns of wind
characterized by converging
winds at the surface and strong, uniform-speed winds at higher
altitudes
A
difference in air pressure (pressure
gradient) between the surface and high altitude
Warm, Humid Ocean
Air
Warm, moist air
from the ocean surface begins to rise rapidly. As this
warm air rises, its water vapour condenses to form storm clouds and
droplets of rain. The condensation releases heat called latent heat of
condensation. This latent heat warms the cool air aloft, thereby
causing it to rise. This rising air is replaced by more warm, humid air
from the ocean below. This cycle continues, drawing more warm, moist
air into the developing storm and continuously moving heat from the
surface to the atmosphere. This exchange of heat from the surface
creates a pattern of wind that circulates around a center. This
circulation is similar to that of water going down a drain.
Patterns of Wind
"Converging
winds" are winds moving in
different directions that run into each other. Converging winds at the
surface collide and push warm, moist air upward. This rising air
reinforces the air that is already rising from the surface, so the
circulation and wind speeds of the storm increase. In the meantime,
strong winds blowing at uniform speeds at higher altitudes (up to
30,000 ft / 9,000 m) help to remove the rising hot air from the storm's
center, maintaining a continual movement of warm air from the surface
and keeping the storm organized. If the high-altitude winds do not blow
at the same speed at all levels -- if wind shears are present -- the
storm loses organization and weakens.
Pressure Gradient
High-pressure
air in the upper atmosphere
(above 30,000 ft / 9,000 m) over the storm's center also removes heat
from the rising air, further driving the air cycle and the hurricane's
growth. As high-pressure air is sucked into the low-pressure center of
the storm, wind speeds increase.
How much damage can a Hurricane cause?
The damage
caused by a hurricane results from
a number of aspects of the storm.
Hurricanes bring
with them huge amounts of
rain. A big hurricane can dump dozens of inches of rain in just a day
or two, much of it inland. That amount of rain can create inland
flooding that can totally devastate a large area around the hurricane's
center.
High sustained
winds cause structural damage.
These winds can also roll cars, blow over trees and erode beaches (both
by blowing sand and by blowing the waves into the beach).
Hurricane
winds often spawn tornadoes, which
are smaller, more intense cyclonic storms that cause additional damage.
The extent of
damage depends on a few things:
The category of the hurricane
Whether the storm comes ashore head-on or just
grazes the coastline
Whether the right or left side of the hurricane
strikes a given area
The right side of
a hurricane packs more punch because the wind speed
and the hurricane speed-of-motion are complimentary there. On the left
side, the hurricane's speed of motion subtracts from the wind speed.
This combination
of winds, rain and flooding can level a coastal town
and cause significant damage to cities far from the coast. In 1996,
Hurricane Fran swept 150 miles (241 km) inland to hit Raleigh, N.C.
Tens of thousands of homes were damaged or destroyed, millions of trees
fell, power was out for weeks in some areas and the total damage was
measured in the billions of dollars.
Once a hurricane
forms, it is rated on the Saffir-Simpson Hurricane
Scale. There are five categories in this rating system.
Saffir-Simpson
Hurricane Scale
Category
Wind Speed
Effects
1
74 to 95 mph
(119 to 153 kph)
Storm surge 4
to 5 ft (1.2 to 1.5 m) above normal - Some flooding -
Little or no structural damage
2
96 to 110 mph
(155 to 177 kph)
Storm surge 6
to 8 ft (1.8 to 2.4 m) above normal - Trees down - Roof
damage (shingles ripped off)
3
111 to 130 mph
(178.6 to 209 kph)
Storm surge 9
to 12 ft (2.7 to 3.7 m) above normal - Structural damage in
houses - Mobile homes destroyed - Severe
flooding
4
131 to 154 mph
(210 to 247.8 kph)
Storm surge 13
to 18 ft (4 to 5.5 m) above normal - Severe flooding inland
- Some roofs ripped off - Major structural damage
5
>155 mph
(> 249.4 kph)
Storm surge at
least 18 ft (5.5 m) above normal - Severe flooding further
inland - Serious damage to most wooden
structures
Hurricanes
in categories 3, 4 and 5 can cause
widespread damage, from severe inland flooding to the loss of life,
property, agriculture and livestock.
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