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Hurricanes

Highlights

Hurricanes are nature's most powerful storms, bred out of heat, moisture and intense low pressure. Serving as a heat-release valve in the Earth's atmosphere, hurricanes can be as expansive as 600 miles in diameter, build to a height of more than 50,000 feet into the sky and pack sustained winds of up to 185 mph. In the Northern Hemisphere, they always swirl counterclockwise. To give the public a reading on their power and potential for destruction, hurricanes are classified into five categories under the Saffir-Simpson Scale. The scale is based on maximum sustained winds: Category 1: 74 to 95 mph; Category 2: 96-110 mph; Category 3: 111-130 mph; Category 4: 131-155 mph; Category 5: More than 155 mph.

When a hurricane is a Category 3, 4 or 5 it is considered a major - or intense - hurricane. Since 1925, hurricanes have caused almost $5 billion in damage per year on average in the United States. Major hurricanes account for more than 80 percent of the damage and strike the United States every 1.3 years on average.

Storm surge, a steady rise of sea waters above tide levels, is the most deadly aspect of hurricanes, as it can result in severe flooding and battering waves. Inland flooding also has proven to be a prolific killer. Yet, many people die in the aftermath of storms from downed electrical wires, water-covered ditches or falling objects such as trees.

Some of the most deadly U.S. hurricanes include:

-- The Galveston Hurricane of 1900 hit Galveston, Texas on Sept. 8, 1900 as a Category 4 system with an estimated death toll between 8,000 and 12,000;

-- The Great Florida Hurricane of 1928 hit near West Palm Beach, Fla., on Sept. 16, 1928 as a Category 4 with an estimated death toll of about 2,500, most of those around Lake Okeechobee;

-- And Hurricane Katrina hit southeast Louisiana on Aug. 29, 2005 as a Category 3 and was estimated to kill more than 1,800 in New Orleans and along the Gulf Coast.

Florida is the most hurricane vulnerable state in the nation. Since 1900, 67 hurricanes have struck the state, 29 of them major. Texas is closest behind with 40 hurricanes, 17 major.

Scientists say hurricane activity rises and falls based on a natural cycle of warm waters ebbing and flowing into the tropical regions where hurricanes frequently are spawned. These cycles, or eras, can last 20 to 30 years. However, in recent years, a growing number of scientists say global warming has increased the number of major hurricanes each year, as well as increased their intensity.

The storm seasons spanning 1995-2005 comprised the most active 10 consecutive hurricane years on record. Hurricanes are born out of low-pressure areas called tropical waves. In the Atlantic basin, these waves are frequently created by the heat of the North African desert. The waves feed off warm ocean waters. Thunderstorms develop and a low-pressure center forms.

When the thunderstorm activity forms a closed circulation around a low-pressure center, the system is deemed to be a tropical depression. When the sustained winds reach 39 mph, the system becomes a tropical storm. When those winds reach 74 mph, it becomes a hurricane.

The strongest winds of a hurricane are usually found near the eye wall, or the circle of clouds around the center. The eye can be 10 to 30 miles in diameter. Generally, the stronger the hurricane, the smaller the eye.

While hurricanes most commonly form between June and November, the most powerful hurricanes are spawned near the Cape Verde Islands between mid August and the end of September. Officially, the hurricane season begins June 1 and ends November 30. « Show less

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