And lasts until November 30



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Hurricanes

We are all very familiar with hurricanes and the dangers they pose for Florida. Hurricanes are large swirling masses of rain and thunderstorms which typically form in the tropics and spin counter clockwise (in the Northern Hemisphere). To be classified as a hurricane, a tropical system must have maximum sustained winds of 74 miles per hour and a closed circulation at its core.

The fuel for tropical systems is warm ocean water and moist air. That’s why the Atlantic hurricane season starts June 1st and lasts until November 30th. Typically hurricanes form south of 35 degrees north latitude, which is the latitude of North Carolina. Hurricanes form from “seedlings,” which are clumps of thunderstorms in the tropical Atlantic, the Caribbean or the Gulf of Mexico (called the Atlantic basin). Tropical systems need water of at least 80 degrees F to form. Over time, a seedling can acquire a spin and a defined central area of low pressure. Once this occurs, the National Hurricane Center calls it a tropical depression and assigns it a number. If its sustained winds reach 39 miles per hour, the system becomes a tropical storm and is given a name. Names for hurricanes and tropical storms come from a list of men’s and women’s names (in English, Spanish, and French) designated by the World Meteorological Organization (WMO). There are six lists of names. (The lists are repeated every sixth year, with the names of especially deadly and damaging hurricanes being retired and replaced by other names.)

Hurricanes are categorized by the Saffir-Simpson Scale using their maximum sustained winds.

Category 1 – Winds 74-95 mph

Category 2 – Winds 96-110 mph

Category 3 – Winds 111-130 mph

Category 4 – Winds 131-155 mph

Category 5 – Winds 156 mph or greater

The eye wall of a hurricane is a ring of intense thunderstorms that spins rapidly around the center of the storm (or eye). The strongest winds in a hurricane are located in the eye wall. But inside the eye, the winds are calm, and the sky can even be clear if the storm is well organized. The eye can range in size from only about two miles wide to more than a hundred miles wide, although the average is about 16 miles. Calm conditions inside the eye can fool people into thinking the hurricane is over. In reality, the eye is a lull in the storm, before the other side of the eye wall moves in.

In the United States, Florida averages the hurricane landfalls because the state is so far south and has such a long coastline on both the Atlantic and the Gulf. Between 1851and 2009, Florida had 37 hurricane landfalls.

Although hurricanes are much more common in tropical climates, they can strike as far north as New England and Canada. The lowest pressure ever recorded in the Atlantic basin was in Hurricane Wilma in 2005 (882 millibars when Wilma was in the southwestern Caribbean). Hurricane Camille (1969) and Hurricane Allen (1980) had estimated winds of 190 mph. During a typical season, the Atlantic basin averages 11 named storms and 6 hurricanes. Since 1995, we have been in a period of above-average hurricane activity in the Atlantic basin.

There are several dangers associated with hurricanes and tropical storms: winds, rain, storm surge flooding, and tornadoes. The winds in a strong hurricane can do major damage to buildings. In Florida, homes are now designed to resist winds of at least Category 3 strength, and in South Florida, the building code is even tougher. Another threat from tropical systems is fresh water flooding. If the storm is moving slowly, or stalled, it can produce over a foot of rain in a short period of time. This can cause major street flooding. Along the coast, one major threat from hurricanes is storm surge flooding. Storm surge is a hill of water (a rise in the ocean level) caused by high winds near the center of the hurricane. As a storm moves onshore, this water will often be pushed past beaches into populated areas near the shore. In a Category 5 hurricane, storm surge can be 25 feet high, high enough to flood out the first and second floors of high-rise buildings. That’s why emergency managers issue evacuation orders when a hurricane approaches. Tornadoes are also associated with hurricanes and even tropical storms. Because of how fast a hurricane spins, tornadoes (small, rapidly rotating columns of air) will often move very quickly and be “wrapped” in rain, making them very difficult to detect. Most tropical systems have some tornadoes embedded inside them. But the number of tornadoes in a tropical system can vary from a few to dozens.

Hurricanes are vulnerable to outside forces. They can weaken rapidly if they encounter strong winds at the cloud level surrounding the storm (called wind shear). Wind shear can tear the thunderstorms apart and disrupt the circulation of the system. A hurricane will also weaken if it encounters dry air or cool water along its path.

Since we predict the paths of tropical systems a day or more in advance, people have time to protect themselves before the storm hits. Everyone in Florida and other hurricane-prone areas should have a hurricane plan. You need to know what you need to do if a hurricane hits and where you will go if you have to evacuate. You should have a hurricane safety kit ready by June 1st of each year. This kit contains water, a first aid kit, prescription medications, important papers, batteries, a radio, and non-perishable food. A list of recommended items and forming a hurricane plan can be found here at… http://www.nhc.noaa.gov/HAW2/english/disaster_prevention.shtml

Questions:

How warm does the water temperature have to be for a hurricane to form?

How strong are the winds in a category one hurricane?

What state has the most landfalling hurricanes?

Dry air can weaken a Hurricane. T/F

Hurricane safety kits should be put together right before a hurricane hits. T/F



Links:

http://www.nhc.noaa.gov/

http://www.aoml.noaa.gov/hrd/tcfaq/tcfaqHED.html

http://www.nhc.noaa.gov/HAW2/english/disaster_prevention.shtml

http://www.nhc.noaa.gov/HAW2/english/history.shtml

http://www.wunderground.com/hurricane/hurrarchive.asp

http://en.wikipedia.org/wiki/List_of_Florida_hurricanes

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