MYRTLE BEACH, SC (WMBF) - Our atmosphere is composed of all types of gases, particles, and molecules. Some of these only make up a very small fraction of the total mass of the Earth's atmosphere, but yet are equally important.
One of those important elements is water vapor. Even though water vapor has many uses in the Earth-atmosphere system, an important one is fuel. It acts as a catalyst to stimulate thunderstorm development. With all that fuel available in the open Atlantic waters it's pretty common to see thunderstorms develop in the summer and fall. This is how those massive swirling beasts called hurricanes originate.
Hurricanes and tropical storms have meager beginnings as simple thunderstorms, but can evolve into huge, churning tempests of the sea. Just as storm systems across the U.S. are in association with low pressure systems that rotate in a counter-clockwise manner, so do tropical storms and hurricanes, just bigger and more organized.
A cluster of thunderstorms or disturbance, under the right atmospheric conditions for a long enough time, may organize into a tropical depression. Winds near the center are constantly between 20 and 40 mph. This is the first stage of a tropical cyclone. When conditions are right, it can be as little as a day or two for the next stage in the life cycle of a tropical cyclone to occur. This is known as a Tropical Storm and the storm is then given a name. The storm has now organized itself and intensified its sustained winds between 39 and 73 mph. With its intensification, it starts to take on a more rounded shape, one that looks more like a hurricane. With favorable oceanic and atmospheric conditions the tropical storm continues to strengthen and the center of the storm starts to become more apparent, which is called the eye. Pressure within the storm drops and that is what boosts wind speeds to hurricane strength. The tropical storm reaches hurricane strength and is therefore called a hurricane when it reaches sustained winds of 74 mph.
Hurricanes are definitely the largest of storms to threaten the U.S. every year, and with its size comes many different types of hazards. The first one we will talk about is also the most dangerous, storm surge.
As a tropical cyclone approaches the land, with the storms rotating winds that can cover hundreds of miles, obviously one side of the storm will be blowing away from the land while the other side blows into the land. This side poses the greatest threat with storm surge. The storms winds help blow more water toward the shore and combined with normal tides can raise sea levels 15 feet and more on shore and cause considerable damage to shorelines. The slope of the continental shelf, or how quickly the land beneath the water drops off, can also determine how high the storm surge will be.
While storm surge is only a threat along the coastline, winds can be felt many miles inland if a storm continues in that direction. But when compared to a tornado, hurricane force winds may seem light. For example, an EF5 on the Enhanced Fujita Tornado Scale can register winds in excess of 200 mph, while the highest category hurricane is winds greater than 155 mph.
So what makes hurricane force winds so dangerous? Time is the most important factor. While tornadic winds are obviously dangerous, a tornado is only fraction of the size of a hurricane, therefore a certain point in the path of a tornado will experience these heavy winds for only a few minutes at the most. Hurricane winds cover very broad areas, therefore allowing a certain point in the path of a hurricane to experience high winds for a much longer period of time. A constant battery of 155 + mph winds can devastate entire counties when hit by a hurricane.
So what drives all of these intense winds? Pressure! Differences in pressure is what makes all wind blow, so you know to have hurricane force winds there must be an enormous difference in pressure. That's why it is safe to say the lower the pressure of a particular storm, typically the stronger! Just like tornadoes, hurricanes have a scale of intensity called the Saffir-Simpson Hurricane Scale.
The Saffir-Simpson Hurricane Scale is a 1-5 rating based on the hurricane's present intensity. This is used to give an estimate of the potential property damage and flooding expected along the coast from a hurricane landfall. Wind speed is the determining factor in the scale, as storm surge values are highly dependent on the slope of the continental shelf in the landfall region.
Category One Hurricane:
Winds 74-95 mph
Storm surge generally 4-5 ft above normal. No real damage to building structures. Damage primarily to unanchored mobile homes, shrubbery, and trees. Some damage to poorly constructed signs. Also, some coastal road flooding and minor pier damage. Hurricanes Allison of 1995 and Danny of 1997 were Category One hurricanes at peak intensity.
Category Two Hurricane:
Winds 96-110 mph
Storm surge generally 6-8 feet above normal. Some roofing material, door, and window damage of buildings. Considerable damage to shrubbery and trees with some trees blown down. Considerable damage to mobile homes, poorly constructed signs, and piers. Coastal and low-lying escape routes flood 2-4 hours before arrival of the hurricane center. Small craft in unprotected anchorages break moorings. Hurricane Bonnie of 1998 was a Category Two hurricane when it hit the North Carolina coast, while Hurricane Georges of 1998 was a Category Two Hurricane when it hit the Florida Keys and the Mississippi Gulf Coast.
Category Three Hurricane:
Winds 111-130 mph
Storm surge generally 9-12 ft above normal. Some structural damage to small residences and utility buildings with a minor amount of curtainwall failures. Damage to shrubbery and trees with foliage blown off trees and large trees blown down. Mobile homes and poorly constructed signs are destroyed. Low-lying escape routes are cut by rising water 3-5 hours before arrival of the center of the hurricane. Flooding near the coast destroys smaller structures with larger structures damaged by battering from floating debris. Terrain continuously lower than 5 ft above mean sea level may be flooded inland 8 miles or more. Evacuation of low-lying residences with several blocks of the shoreline may be required. Hurricanes Roxanne of 1995 and Fran of 1996 were Category Three hurricanes at landfall on the Yucatan Peninsula of Mexico and in North Carolina, respectively.
Category Four Hurricane:
Winds 131-155 mph
Storm surge generally 13-18 ft above normal. More extensive curtainwall failures with some complete roof structure failures on small residences. Shrubs, trees, and all signs are blown down. Complete destruction of mobile homes. Extensive damage to doors and windows. Low-lying escape routes may be cut by rising water 3-5 hours before arrival of the center of the hurricane. Major damage to lower floors of structures near the shore. Terrain lower than 10 ft above sea level may be flooded requiring massive evacuation of residential areas as far inland as 6 miles. Hurricane Luis of 1995 was a Category Four hurricane while moving over the Leeward Islands. Hurricanes Felix and Opal of 1995 also reached Category Four status at peak intensity.
Category Five Hurricane:
Winds greater than 155 mph
Storm surge generally greater than 18 ft above normal. Complete roof failure on many residences and industrial buildings. Some complete building failures with small utility buildings blown over or away. All shrubs, trees, and signs blown down. Complete destruction of mobile homes. Severe and extensive window and door damage. Low-lying escape routes are cut by rising water 3-5 hours before arrival of the center of the hurricane. Major damage to lower floors of all structures located less than 15 ft above sea level and within 500 yards of the shoreline. Massive evacuation of residential areas on low ground within 5-10 miles of the shoreline may be required. Hurricane Mitch of 1998 was a Category Five hurricane at peak intensity over the western Caribbean. Hurricane Gilbert of 1988 was a Category Five hurricane at peak intensity and is one of the strongest Atlantic tropical cyclones of record.
Remember what initially fueled these monstrous storms? Water vapor. Remember water vapor is basically moisture in the gas form so you better believe there is a whole lot of moisture associated with these storms, and therefore massive amounts of rain. With that in mind, as well as the size of the storms, rain can last for days. With all that rain falling in a relatively short amount of time, flooding is a given. That is yet another danger hurricanes pose to not only our U.S. coastlines but well inland also. As mentioned early, flooding is the number one weather related killer in the U.S. and flooding associated with hurricanes and tropical storms is no different. Floods also pose great risk to homes, buildings, and crops.
Believe it or not, on top of all those hazards from hurricanes, tornadoes can also be spawned as a hurricane makes landfall. Not all hurricanes produce tornadoes but studies have shown that half of all hurricanes that make landfall in the U.S. produce at least one. These tornadoes are typically, weaker than typical tornadoes, like those produced in the Great Plains, but dangerous none the less.
With the Atlantic Ocean being so vast, along with the Gulf of Mexico and the Caribbean Sea, more than one tropical cyclone can be present at one time. To keep track of these storms scientist have developed a list of names that are rotated every six years. If a specific named storm is so deadly or costly it will be retired and another name will be added to the list. Since 1950, 48 names have been retired. Here are a few examples with what year they occurred and the area that was affected:
Andrew (1992): Bahamas, South Florida, Louisiana
Bob (1991): North Carolina & Northeast U.S
Camille (1969): Louisiana, Mississippi and Alabama
Diane (1955): Mid-Atlantic U.S. & Northeast U.S.
Floyd (1999): North Carolina, eastern seaboard
Hugo (1989): Antilles, South Carolina
Mitch (1998): Central America, Nicaragua, Honduras
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