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Storms at Sea The scenes from hurricane disaster areas are shocking. Homes are destroyed. Towns and cities are in disarray. Floods often displace people. How do these powerful storms, with pouring rain, flooding, and winds at speeds of more than 250 kilometers per hour, get started? Scientists have spent a great deal of time and energy answering this question. The more scientists know, the better they can predict when and where a hurricane might hit. Hurricane research often focuses on how to How Do Hurricanes Begin? predict and prepare for hurricanes. Hurricanes are huge tropical storms with high winds. Hurricanes form over warm water in tropical areas. In these areas, the Sun’s energy produces warmer temperatures due to Earth’s tilt. During late summer, these tropical areas reach their warmest temperatures. The air over the ocean in these tropical areas is also quite warm. In fact, hurricane conditions require the water to be at or above 26.5 °C. The warm temperatures also cause evaporation. Thus, the air above the ocean becomes extremely humid. Convection currents in the air form when warm air rises and cooler air moves in underneath. This cooler air, now close to the warm water, gains thermal energy from the water and, in turn, rises. As the warmed air rises, it cools and falls, rushing in to fill the space left by the rising warmer air. This pattern sets up a continual cycle of heating and cooling, as the air rises and falls. Eventually, the convection-powered weather system takes on a spinning motion. In the Northern Hemisphere, the motion is counterclockwise due to the rotation of Earth on its axis. This phenomenon is known as the Coriolis effect. (Similar storms in the Southern Hemisphere, known as cyclones, spin clockwise.) The Coriolis effect causes air to rotate around a central area in which the air pressure is lower than Discovery Education Science The Coriolis effect causes storms to rotate counterclockwise in the Northern Hemisphere. © Discovery Communications, LLC Storms at Sea its surroundings. Continuing to gain energy from the warm ocean, wind speeds increase. When winds reach 61 kilometers per hour, the system is called a tropical depression. If wind speed increases to 63 kilometers per hour, the system becomes a tropical storm. At 119 kilometers per hour, the storm is considered a hurricane. Hurricanes can have diameters as large as 2,100 kilometers. Hurricane Motion With its counterclockwise spin and high wind speeds, a hurricane develops the familiar When Hurricane Hugo hit the continental United States in 1989, it was the strongest spiraling appearance. The low-pressure area hurricane to strike in 20 years. that forms in the center of the storm is known as the eye. The eye of a hurricane can span up to 48 kilometers. When the eye is calm, it is bordered by the eyewall, an area where air moves upward at a very fast rate, carrying large amounts of moisture into the storm. Due to the increased moisture, the eyewall often appears to be a wall of clouds. The eyewall is a turbulent area that contains the most destructive power of the storm. Hurricanes travel across the ocean. Sometimes the hurricane path approaches islands and coastal areas, bringing danger to people and animals. Hurricanes move by currents in the atmosphere known as steering winds. For forecasting purposes, it is better when the hurricane is carried by strong steering winds because this makes the hurricane path easier to predict. Weak steering winds cause a more unpredictable path. Hurricane Francis, 2004, approaches the coast of Florida. Naming and Classifying Hurricanes Once a tropical depression becomes a tropical storm, it is named. Naming storms helps make communication clear among researchers as well as to the public. During hurricane season, there may be more than one tropical storm. The World Meteorological Organization (WMO) is responsible for naming tropical storms. Each year, the WMO uses an alphabetized list of Discovery Education Science © Discovery Communications, LLC Storms at Sea names. The first tropical storm of the year gets an “A” name, the second gets a “B” name, the third gets a “C” name, and so forth throughout the season. The following year, the WMO begins with a new list. There are 6 years’ worth of lists prepared at any given time. These name lists are used on a rotating basis. If a hurricane is very destructive, however, the WMO will not reuse the name. Hurricanes are classified by wind speeds. The Saffir-Simpson Hurricane The Saffir-Simpson scale classifies Wind Scale (SSHWS) categorizes hurricanes by wind speed. hurricanes into five different levels. A category 1 hurricane is the least destructive. A category 5 can cause catastrophic damage to homes, trees, and buildings. Discovery Education Science © Discovery Communications, LLC