Introduction to How Hurricanes Work hurricane season hurricanes typhoons cyclones Defining a Hurricane tropical cyclone tropical depressions

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1 How Hurricanes Work Introduction to How Hurricanes Work Every year between June 1 and November 30 (commonly called hurricane season), hurricanes threaten the eastern and gulf coasts of the United States, Mexico, Central America and the Caribbean. In other parts of the world, the same types of storms are called typhoons or cyclones. Hurricanes wreak havoc when they make landfall, and they can kill thousands of people and cause billions of dollars of property damage when they hit heavily populated areas. In this article, we'll discuss how hurricanes form and move, and look at the destruction and damage they can cause. We'll also examine how meteorologists track hurricanes. You'll be amazed at the power and impact of these storms! Defining a Hurricane According to the National Hurricane Center, "hurricane" is a name for a tropical cyclone that occurs in the Atlantic Ocean. "Tropical cyclone" is the generic term used for low-pressure systems that develop in the tropics. "Tropical cyclones with maximum sustained surface winds of less than 17 meters per second (39 mph / 62.7 kph / 34 knots) are called tropical depressions. Once the tropical cyclone reaches winds of at least 17 meters per second (m/s), it is typically called a tropical storm and assigned a name. If winds reach 33 m/s (74 mph / 119 kph / 64 kt), then it is called a "hurricane." Hurricanes are defined by the following characteristics: They are tropical, meaning that they are generated in tropical areas of the ocean near the Equator. They are cyclonic, meaning that their winds swirl around a central eye. Wind direction is counterclockwise (west to east) in the Northern Hemisphere and clockwise (east to west) in the Southern Hemisphere (more about this later). They are low-pressure systems. The eye of a hurricane is always a low-pressure area. The lowest barometric pressures ever recorded have occurred inside hurricanes. The winds swirling around the center of the storm have a sustained speed of at least 74 mph. How a Hurricane Forms Hurricanes form in tropical regions where there is warm water (at least 80 degrees Fahrenheit / 27 degrees Celsius), moist air and converging equatorial winds. Most Atlantic hurricanes begin off the west coast of Africa, starting as thunderstorms that move out over the warm, tropical ocean waters. A thunderstorm reaches hurricane status in three stages: Tropical depression - swirling clouds and rain with wind speeds of less than 38 mph (61.15 kph / 33 kt) Tropical storm - wind speeds of 39 to 73 mph (54.7 to kph / 34 to 63 kt) Hurricane - wind speeds greater than 74 mph (119 kph / 64 kt) Photo courtesy NOAA Hurricane Ivan over the Gulf Coast of the United States 2:45 p.m. EDT, September 15, 2004

2 It can take anywhere from hours to several days for a thunderstorm to develop into a hurricane. Although the whole process of hurricane formation is not entirely understood, three events must happen for hurricanes to form: A continuing evaporation-condensation cycle of warm, humid ocean air Patterns of wind characterized by converging winds at the surface and strong, uniform-speed winds at higher altitudes A difference in air pressure (pressure gradient) between the surface and high altitude Warm, moist air from the ocean surface begins to rise rapidly. As this warm air rises, its water vapor condenses to form storm clouds and droplets of rain. The condensation releases heat called latent heat of condensation. This latent heat warms the cool air aloft, thereby causing it to rise. This rising air is replaced by more warm, humid air from the ocean below. This cycle continues, drawing more warm, moist air into the developing storm and continuously moving heat from the surface to the atmosphere. This exchange of heat from the surface creates a pattern of wind that circulates around a center. This circulation is similar to that of water going down a drain. This photo is a composite of three days' views (August 23, 24 and 25, 1992) of Hurricane Andrew as it slowly moved across south Florida from east to west. "Converging winds" are winds moving in different directions that run into each other. Converging winds at the surface collide and push warm, moist air upward. This rising air reinforces the air that is already rising from the surface, so the circulation and wind speeds of the storm increase. In the meantime, strong winds blowing at uniform speeds at higher altitudes (up to 30,000 ft / 9,000 m) help to remove the rising hot air from the storm's center, maintaining a continual movement of warm air from the surface and keeping the storm organized. If the high-altitude winds do not blow at the same speed at all levels -- if wind shears are present -- the storm loses organization and weakens. High-pressure air in the upper atmosphere (above 30,000 ft / 9,000 m) over the storm's center also removes heat from the rising air, further driving the air cycle and the hurricane's growth. As high-pressure air is sucked into the low-pressure center of the storm, wind speeds increase. Parts of a Hurricane Once a hurricane forms, it has three main parts: Eye - the low-pressure, calm center of circulation Eye wall - area around the eye with the fastest, most violent winds Rain bands - bands of thunderstorms circulating outward from the eye that are part of the evaporation/condensation cycle that feeds the storm Hurricane Size Hurricanes vary widely in physical size. Some storms are very compact and have only a few trailing bands of wind and rain behind them. Other storms are looser, so the bands of wind and rain spread out over hundreds or thousands of miles. Hurricane Floyd, which hit the eastern United States in September 1999, was felt from the Caribbean islands to New England.

3 Photo courtesy NASA/GSFC Hurricane Floyd was a Category 3 storm that brought intense rains and record flooding to the eastern United States and Canada. Nearly 90 percent of the fatalities associated with this storm were drownings due to inland flooding. Hurricane Categories Once a hurricane forms, it is rated on the Saffir-Simpson Hurricane Scale. There are five categories in this rating system. Saffir-Simpson Hurricane Scale Category Wind Speed to 95 mph (119 to 153 kph) 96 to 110 mph (155 to 177 kph) 111 to 130 mph (178.6 to 209 kph) 131 to 154 mph (210 to kph) >155 mph (> kph) Effects Storm surge 4 to 5 ft (1.2 to 1.5 m) above normal Some flooding Little or no structural damage Storm surge 6 to 8 ft (1.8 to 2.4 m) above normal Trees down Roof damage (shingles ripped off) Storm surge 9 to 12 ft (2.7 to 3.7 m) above normal Structural damage in houses Mobile homes destroyed Severe flooding Storm surge 13 to 18 ft (4 to 5.5 m) above normal Severe flooding inland Some roofs ripped off Major structural damage Storm surge at least 18 ft (5.5 m) above normal Severe flooding further inland Serious damage to most wooden structures Hurricanes in categories 3, 4 and 5 can cause widespread damage, from severe inland flooding to the loss of life, property, agriculture and livestock. In the next section, we'll look at how this damage occurs. Weather Alerts According to The Weather Channel Online, there are four weather alerts for tropical storms and hurricanes. Depending on where you are located in proximity to the storm, you may find yourself under one of these alerts: Tropical-storm watch Tropical-storm warning Hurricane watch Hurricane warning

4 A tropical-storm watch is issued when sustained winds from 39 to 73 mph (54.7 to kph / 34 to 63 kt) are possible in your area within 36 hours. A tropical-storm warning indicates that these conditions are likely in your area within 24 hours. A hurricane watch is issued when hurricane conditions (sustained winds greater than 74 mph / 119 kph / 64 kt) are possible in your area within 36 hours. A hurricane warning is issued when these conditions are likely in your area within 24 hours. Hurricane Damages The damage caused by a hurricane results from a number of aspects of the storm. Hurricanes bring with them huge amounts of rain. A big hurricane can dump dozens of inches of rain in just a day or two, much of it inland. That amount of rain can create inland flooding that can totally devastate a large area around the hurricane's center. High sustained winds cause structural damage. These winds can also roll cars, blow over trees and erode beaches (both by blowing sand and by blowing the waves into the beach). The prevailing winds of a hurricane push a wall of water, called a storm surge, in front of it. If the storm surge happens to synchronize with a high tide, it causes beach erosion and significant inland flooding. Hurricane winds often spawn tornadoes, which are smaller, more intense cyclonic storms that cause additional damage. The extent of damage depends on a few things: The category of the hurricane Whether the storm comes ashore head-on or just grazes the coastline Whether the right or left side of the hurricane strikes a given area The right side of a hurricane packs more punch because the wind speed and the hurricane speed-of-motion are complimentary there. On the left side, the hurricane's speed of motion subtracts from the wind speed. This combination of winds, rain and flooding can level a coastal town and cause significant damage to cities far from the coast. In 1996, Hurricane Fran swept 150 miles (241 km) inland to hit Raleigh, N.C. Tens of thousands of homes were damaged or destroyed, millions of trees fell, power was out for weeks in some areas and the total damage was measured in the billions of dollars. Tracking a Hurricane Hurricanes in the Northern Hemisphere rotate counterclockwise (west to east) and move through the ocean clockwise (east to west). In the Southern Hemisphere, hurricanes rotate clockwise (east to west) and move counterclockwise (west to east). These motions, known as the Coriolis effect, are caused by the Earth's rotation. To monitor and track the development and movement of a hurricane, we rely on remote sensing by satellites, as well as data gathered by the Hurricane Hunters. The Hurricane Hunters are members of the 53rd Weather Reconnaissance Squadron/403rd Wing, based at Keesler Air Force Base in Biloxi, Mississippi. Since 1944, the U.S. Department of Defense (which oversees the U.S. military) has been the only organization to fly into tropical storms and hurricanes. Since 1965, the Hurricane Hunters team has used the C-130 Hercules, a very sturdy turboprop plane. The only difference between this plane and the cargo version is the specialized, highly sensitive weather equipment installed on the WC-130. The team can cover up to five storm missions per day, anywhere from the mid- Atlantic to Hawaii. Weather satellites use different sensors to gather different types of information about hurricanes: Visible - clouds, circulation patterns Radar / Doppler radar - rain, wind speeds, precipitation amounts Infrared - temperature differences, cloud heights The Hurricane Hunters gather information about wind speeds, rainfall and barometric pressures within the storm. The information is relayed back to the National Hurricane Center in Miami, FL, where it is interpreted and distributed to national and local news media. The National Hurricane Center predicts the hurricane's movement and intensity using various weather models and issues hurricane watches and warnings to areas in the storm's path. Our modern system (tracking, early detection, warnings) has greatly reduced the loss of life during a hurricane.

5 Hurricane Names To better track hurricanes, weather officials decided to name them. The names are chosen by the World Meteorological Organization. According to the National Oceanic & Atmospheric Administration (NOAA): "For several hundred years, hurricanes in the West Indies were often named after the particular saint s day on which the hurricane occurred. For example, 'Hurricane San Felipe' struck Puerto Rico on September 13, Another storm struck Puerto Rico on the same day in 1928, and this storm was named 'Hurricane San Felipe the Second.'" Until World War II, hurricanes were given only masculine names. In the early 1950s, weather services began naming storms alphabetically and with only feminine names. By the late 1970s, this practice was replaced with alternating masculine and feminine names. The first hurricane of the season is given a name starting with the letter A, the second with the letter B and so on. According to NOAA, "the name lists... have an international flavor because hurricanes affect other nations and are tracked by the public and weather services of many countries." Hurricanes in the Pacific Ocean are assigned a different set of names than Atlantic storms. For example, the first hurricane of the 2001 hurricane season was a Pacific Ocean storm near Acapulco, Mexico, named Adolf. The first Atlantic storm of the 2001 season was named Allison. A list of names through 2011 is available from the National Hurricane Center. According to the NOAA: "Whenever a hurricane has had a major impact, any country affected by the storm can request that the name of the hurricane be 'retired' by agreement of the World Meteorological Organization (WMO). Retiring a name actually means that it cannot be reused for at least 10 years, to facilitate historic references, legal actions, insurance claim activities, etc. and avoid public confusion with another storm of the same name." Photographer: Vladislav Gurfinkel Agency: Dreamstime.com Hurricanes in the Northern Hemisphere rotate counterclockwise. In the Southern Hemisphere, hurricanes rotate clockwise.