What is a Hurricane? An intense tropical weather system with a well defined circulation and maximum sustained winds of 74 mph (64 knots) or higher. (NOAA) Hurricanes occur throughout the Atlantic and Pacific Oceans and go by several different names, depending on the region in which they occur. In the Atlantic and Eastern Pacific, they are simply "hurricanes". In the South Pacific, Australians For simplified purposes, all refer to them as "cyclones", and in other Pacific storms of this nature, which regions, they are known as "typhoons." spin around a center or an eye, are often referred to generically as "tropical cyclones” Ingredients for a Hurricane As powerful as hurricanes are, • WARM OCEAN WATER (Sea surface they are very delicate weather temperatures must be 26.5°C (81°F) systems. It takes an almost exact set of weather conditions to This is a sea surface temperature map for the northern hemisphere create and sustain a hurricane. summer. The yellow, orange, and red colors show water temperatures warm enough to sustain hurricanes (> 26.5°C). http://ww2010.atmos.uiuc.edu/(Gl)/wwhlpr/hurricane_ocean.rxml 1 Ingredients for a Hurricane (cont.) Ingredients for a Hurricane (cont.) • A PRE-EXISTING AREA OF DISTURBED • LIGHT WINDS IN THE UPPER WEATHER (Tropical Wave) ATMOSPHERE (LOW VERTICAL WIND SHEAR) USA TODAY All hurricanes form from a cluster of When these waves being to organize themselves into a thunderstorms called a tropical wave. In swirl, they become tropical the Atlantic, most tropical waves originate depressions. These off the coast of Africa or in the Gulf of depressions carry top sustained winds, wind Mexico. lasting more than one minute, of 35 miles per hour. Less than 10 % of tropical weather disturbances grow into tropical storms because it is relatively rare for several factors to be just right for their development. http://ksks.essortment.com/hurricaneformat_rmem.htm http://ksks.essortment.com/hurricaneformat_rmem.htm Tropical depressions can grow into tropical storms in a matter or hours or even days, depending on the atmospheric conditions surrounding the depression. In the Pacific, these waves originate The tropical depression is deemed a tropical storm off the coast of Central America and when its maximum sustained winds exceed 40 mph. Mexico. However, these waves can With many tropical storms, we begin to see an eye develop in all the oceans of the wall in development as well as a near-definite center of circulation, the eye. When the tropical cyclone tropics. reaches tropical storm status, it is given a name. http://ksks.essortment.com/hurricaneformat_rmem.htm http://ksks.essortment.com/hurricaneformat_rmem.htm 2 With favorable conditions: warm water, open sky and away from land, the tropical storm will grow into a hurricane. It gains this status when its maximum sustained winds exceed 74 mph. At this wind velocity, it is also deemed a category one hurricane, a minimal hurricane. At this stage, the eye will become a bit more defined and an eye wall, a wall of thunderstorms on the westward side of the eye, begins to develop. Air spirals inward – from high pressure to low pressure http://ksks.essortment.com/hurricaneformat_rmem.htm How Hurricanes Form • http://us.cnn.com/SPECIALS/2004/hurrican Parts of the storm es/interactive/forms/frameset.exclude.html A storm's elements 1. Exhaust Hot air drawn into the atmosphere. 2. Storm clouds Spiral in upper atmosphere. Eye Wall 3. Eye Cool air descends into the 20-mile wide eye, creating a small center of calm Immediately outside of the weather. eye is the eye wall region, an area of vigorous 4. Eye wall Storm's fiercest winds. tall/deep clouds, heavy rainfall, and the strongest 5. Spiraling winds observed winds. In the hurricane's lower realms, air flows in towards the center and whirls upward. These counterclockwise winds gain speed as they approach the eye, like a whirlpool. The narrower the eye, the Palm Beach Post stronger the wind. http://www.comet.ucar.edu 3 Eye Spiral Bands The center of a hurricane is known as the eye. It is a Localized areas of tall/deep clouds, heavy rain, and high 20-65 km (12-40 mile) wide region of relatively clear winds, known as spiral bands, may extend a few hundred and calm conditions kilometers outward from the brought about by descending air. center of a hurricane. http://www.comet.ucar.edu http://www.comet.ucar.edu Winds: Low-level Inflow and High-level Outflow Hurricane intensity is ranked according to the Saffir-Simpson Scale. At the surface, the air spirals inward in a counterclockwise Category Wind Speed Storm Surge Damage (cyclonic) circulation. The km/hr mi/hr m ft circulation becomes weaker 119-154 74-95 1-2 4-5 with height, eventually turning 1 Minimal into clockwise (anticyclonic) 2 155-178 96-110 2-3 6-8 Moderate outflow near the top of the 3 179-210 111-130 3-4 9-12 Extensive storm. 4 211-250 131-155 4-6 13-18 Extreme 5 >250 >155 >6 >18 Catastrophic http://www.comet.ucar.edu The Atlantic hurricane season is from June 1 to November 30. The Eastern Pacific hurricane season is from May 15 to November 30. Hurricane risk for the United States 4 Storms are given names so that it is easier to talk about them and refer to them. The old system was based on If the Category 1 hurricane is in open water and other using coordinates, which was very confusing because it is conditions are favorable, it will develop within a matter of not unusual for there to be two or even three hurricanes developing in an ocean. short days into full-blown hurricane with winds exceeding 2002 2003 2004 2005 2006 2007 100 mph. Arthur Ana Alex Arlene Alberto Andrea Bertha Bill Bonnie Bret Beryl Barry Cristobal Claudette Charley Cindy Chris Chantal Dolly Danny Danielle Dennis Debby Dean Edouard Erika Earl Emily Ernesto Erin Fay Fabian Frances Franklin Florence Felix Gustav Grace Gaston Gert Gordon Gabrielle Hanna Henri Hermine Harvey Helene Humberto Isidore Isabel Ivan Irene Isaac Ingrid Josephine Juan Jeanne Jose Joyce Jerry Kyle Kate Karl Katrina Kirk Karen Lili Larry Lisa Lee Leslie Lorenzo Marco Mindy Matthew Maria Michael Melissa Nana Nicholas Nicole Nate Nadine Noel Omar Odette Otto Ophelia Oscar Olga Paloma Peter Paula Philippe Patty Pablo Rene Rose Richard Rita Rafael Rebekah Sally Sam Shary Stan Sandy Sebastien Teddy Teresa Tomas Tammy Tony Tanya Vicky Victor Virginie Vince Valerie Van Wilfred Wanda Walter Wilma William Wen Major hurricanes with winds above 130 mph are rare. They have strong eye walls, which are intense and violent cloud Hurricanes bands that contain the highest winds of the storm. produce damaging surface winds, heavy rains, and storm surges. Damage-Causing Winds Rainfall While high winds cause Rainfall is difficult to predict at significant structural and times because while a hurricane may spend nearly a week environmental damage, developing out in the ocean, it storm surges are frequently may make landfall and not dump the most devastating element much rain at all. of a hurricane. 5 Shallow-water Coastline Storm Surges If the coastline is shallow, A storm surge is a rise in sea level along a water cannot flow away from coastline caused by the combination of a the mound and the mound hurricane's surface winds and the physical grows. If the coastline is deep, water can disperse and geography of a coastline. Surface winds the mound may grow slowly Deep-water Coastline above the ocean's surface push water toward or disperse depending on the hurricane's eye, creating a mound of hurricane strength. An water. The mound of water is then influenced example of a shallow-water by the slope of the coastline as the hurricane coastline is the Gulf Coast while an example of a deep- approaches land. Each of the animations above depicts surge water coastline is found in dynamics in action. As you watch the storm- surge animations, notice the effect that the New England. physical geography of each coastline has on storm surge. Also note the waves on top of the ocean's surface. Wind, waves, and sea-level rise all contribute to storm-surge damage. http://www.comet.ucar.edu http://www.comet.ucar.edu Predicting Worst Storm-surge Damage The greatest storm-surge damage is to the right of a hurricane's eye (as you face Tropical cyclones throughout the world, the shoreline) in the Northern in general, travel from East to West. Hemisphere. It occurs to the right of the eye because winds, ocean waves, and sea-level rise are all moving in an onshore direction. In contrast, to the left of the eye, ocean waves and sea-level rise are moving in an onshore direction, but the winds are blowing in an offshore direction. The offshore direction of the winds moderates the effect of the storm surge. http://www.comet.ucar.edu However, if it makes landfall on a small island or small cluster of islands, the opposite could occur. Because of the warmer and shallower water Once a hurricane makes landfall on a surrounding most small islands, the hurricane may continent or large island, its development intensify greatly. This is often the case with Atlantic usually stops and the storm weakens. When hurricanes that strike the Bahamas. These hurricanes they strike a large land mass, the hurricane often intensify slightly when they are over the warm will fizzle into a cluster of thunderstorms or water of the Bahama islands, which are very close to South Florida. rain showers. http://ksks.essortment.com/hurricaneformat_rmem.htm http://ksks.essortment.com/hurricaneformat_rmem.htm 6 How To Track A Hurricane The National Hurricane Center, which is part of the National Tracking Hurricanes Using Satellite Imagery Weather Service and the National Oceanic and Atmospheric Administration, tracks tropical storms and hurricanes.