The Formation and Behavior of Hurricanes

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Hurricanes, in part because of the large area they cover, are one of themost dangerous and damaging of all natural disasters. According to NationalCenter for Atmospheric Research numbers, each year hundreds hurricanescause an average of more than $5 billion in property damage and more than 20deaths in the United States alone, arguably one of the best prepared countriesin the world. If not for the advance warnings provided by modern forecastingtechniques, the death tolls would be dramatically higher.

While our understanding of hurricanes is far from perfect, scientists dounderstand many of the factors that cause hurricanes to form, strengthen andultimately collapse as they run out of energy.

What defines a hurricane?

A hurricane is a tropical storm with winds of 74 mph or more, according to LouisianaHomeland Security and Emergency Preparedness. The winds circulate aroundthe storm's center or eye, traveling counterclockwise in the northernhemisphere and clockwise in the southern hemisphere. Hurricanes can includedangerous winds, extremely heavy rains, storm surges, and may even spawntornadoes.

What conditions are necessary for the formation and growth of ahurricane?

Hurricanes draw their energy from the warm waters of the tropical oceans,requiring surface water temperatures of 81 degrees or higher, says the Universityof Illinois. They form over oceans because moisture in the air is alsorequired. Moist air holds more energy in the form of latent heat. Tropicalwinds also play a role. Low wind shear allows the storms to build vertically,concentrating the energy of latent heat from the atmosphere over a smallerarea, increasing the intensity of the storm and likelihood of it developinginto a hurricane. Hurricanes form in the tropics along either side of theequator but not right on it, because they also require help from the Earth's Coriolisforce to produce their initial spin or rotation.

What is the difference between a hurricane, a typhoon and a tropicalcyclone?

These three terms all refer to the same type of storm. The only realdifference is the location, says the University of NorthCarolina. In North America, we use the term hurricane, but the same kindsof storms are called typhoons in the northern-west Pacific Ocean and tropicalcyclones in the Indian Ocean and southern-west Pacific Ocean. Tropical cycloneis also a generic term for tropical depressions with circulating winds whetheror not they reach hurricane strength.

What are the growth stages of a hurricane?

The conditions required for a hurricane may start out as a series ofthunderstorms called a tropical depression that have a central region of lowpressure and begin to develop some cohesive rotation. As the associated windsincrease in speed to 39 mph to 74 mph, the rotation of the storm becomes moreorganized and more circular. At this point, it will be called a tropical stormand receive an official name. As a tropical storm strengthens further adistinctive eye forms in the center of the swirling winds. Minimum sustainedwind speeds of 74 miles per hour or greater make the storm a hurricane.

How does a hurricane get stronger?

Hurricanes are a self-reinforcing system. They are fueled by warm, moist airflowing toward a central region of low pressure. The incoming air is pushedvertically near the storm's center. As it rises, the air cools and can nolonger hold as much moisture. The moisture in the air condenses out in the formof precipitation. As it does so, heat held by the water vapor is released intothe surrounding air. As the air is heated, it expands, pushing air away fromthe storm center at the upper level. This reduces the amount of air above thecenter of the storm (measuring in the mass or weight of the air), which lowersthe air pressure at the lower levels of the hurricane's center. Reduced airpressure here draws even more air toward the center at the lower level and thehurricane's strengthening cycle repeats continuously as long as the air beingdrawn into the system has enough "latent heat" trapped in water vaporto keep the cycle going.

Why do hurricanes seem to aim for the east coast of the United States?

Hurricanes that form along the east or west sides of North America aregenerally pushed in a northeasterly direction due to the prevailing directionof the tropical trade winds in that location. As they move up the east coast,they get pushed along by the Gulf Stream which tends to push them back out overthe North Atlantic.

Why do hurricanes weaken or dissipate after they make landfall?

Hurricanes require a constant influx of warm, very moist air to sustainthemselves. As they move north into cooler waters or the eye moves over land,there simply isn't enough moisture or latent heat to allow the hurricane tostrengthen. As its latent heat is released, it runs out of energy and thesystem weakens.

Do hurricanes cause tornadoes?

Yes. The strong winds circling in the hurricane's eye wall can produce veryintense tornadoes either within the eye wall itself or just outside it.Hurricane-spawned tornadoes can occur with little warning and can be verydangerous even in areas that are not accustomed to tornado activity.

What is a storm surge?

A storm surge occurs when the winds of a hurricane are blowing toward theshoreline from the ocean. They literally push the surface water toward theyland where it piles up, getting deeper and deeper as long as the winds continuein the same direction. The storm surge can be compounded by incoming tidesresulting in flooding far inland.

Brad Sylvester spent more than 18 years working in the consumerelectronics industry before becoming a full-time freelance writer. He has a keeninterest in science and the environment, and possesses a nearly insatiablecuriosity about almost everything. You can follow him on Twitter @back2n8ure.


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