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Hurricanes & Tropical Storms flood Coastlines and cause billions in damage |
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Main Articles -
Casualty by Natural
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Written by Administrator
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June 04, 2007 |
What is a Hurricane? 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)
- Hurricane - wind speeds greater than 64 knots (74 mph / 119 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
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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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Last Updated ( July 16, 2009 )
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