Hurricane

I

Introduction

Hurricane, name given to violent storms that originate over the tropical or subtropical waters of the Atlantic Ocean, Caribbean Sea, Gulf of Mexico, or North Pacific Ocean east of the International Date Line. Such storms over the North Pacific west of the International Date Line are called typhoons; those elsewhere are known as tropical cyclones, which is the general name for all such storms including hurricanes and typhoons. These storms can cause great damage to property and loss of human life due to high winds, flooding, and large waves crashing against shorelines. The deadliest natural disaster in United States history was caused by a hurricane that struck Galveston, Texas, in 1900. The costliest and most destructive natural disaster in U.S. history was caused by the storm surge and winds created by Hurricane Katrina along the Louisiana, Mississippi, and Alabama coasts. The hurricane’s storm surge burst some levees protecting New Orleans, flooding the city and forcing a complete evacuation. See also Tropical Storm; Cyclone.

II

How Hurricanes Form

Tropical cyclones form and grow over warm ocean water, drawing their energy from latent heat. Latent heat is the energy released when water vapor in rising hot, humid air condenses into clouds and rain. As warmed air rises, more air flows into the area where the air is rising, creating wind. The Earth’s rotation causes the wind to follow a curved path over the ocean (the Coriolis effect), which helps give tropical cyclones their circular appearance.

Hurricanes and tropical cyclones form, maintain their strength, and grow only when they are over ocean water that is approximately 27°C (80°F). Such warmth causes large amounts of water to evaporate, making the air very humid. This warm water requirement accounts for the existence of tropical cyclone seasons, which occur generally during a hemisphere’s summer and autumn. Because water is slow to warm up and cool down, oceans do not become warm enough for tropical cyclones to occur in the spring.

Oceans can become warm enough in the summer for hurricanes to develop, and the oceans also retain summer heat through the fall. As a result, the hurricane season in the Atlantic Basin, which comprises the Atlantic Ocean, Caribbean Sea, and the Gulf of Mexico, runs from June 1 through November 30. At least 25 out-of-season storms, however, have occurred from 1887 through 2003, and 9 of these strengthened into hurricanes for at least a few hours.

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Hurricanes weaken and die out when cut off from warm, humid air as they move over cooler water or land but can remain dangerous as they weaken. Hurricanes and other tropical cyclones begin as disorganized clusters of showers and thunderstorms. When one of these clusters becomes organized with its winds making a complete circle around a center, it is called a tropical depression.

When a depression’s sustained winds reach 63 km/h (39 mph) or more, it becomes a tropical storm and is given a name. By definition, a tropical storm becomes a hurricane when winds reach 119 km/h (74 mph) or more.

For a tropical depression to grow into a hurricane, winds from just above the surface of the ocean to more than 12,000 m (40,000 ft) in altitude must be blowing from roughly the same direction and at the same speed. Winds that blow in opposite directions create wind shear—different wind speeds or direction at upper and lower altitudes—that can prevent a storm from growing.

III

Characteristics of Hurricanes

A hurricane consists of bands of thunderstorms that spiral toward the low-pressure center, or “eye” of the storm. Winds also spiral in toward the center, speeding up as they approach the eye. Large thunderstorms create an “eye wall” around the center where winds are the strongest. Winds in the eye itself are nearly calm, and the sky is often clear. Air pressures in the eye at the surface range from around 982 hectopascals (29 inches of mercury) in a weak hurricane to lower than 914 hectopascals (27 inches of mercury) in the strongest storms. (Hectopascals are the metric unit of air pressure and are the same as millibars, a term used by many weather forecasters in the United States. Hectopascals is the preferred term in scientific journals and is being used more often in public forecasts in nations that use the metric system.)

In a large, strong storm, hurricane-force winds may be felt over an area with a diameter of more than 100 km (60 m). The diameter of the area affected by gale winds and torrential rain can extend another 200 km (120 m) or more outward from the eye of the storm. The diameter of the eye may be less than 16 km (10 m) in a strong hurricane to more than 48 km (30 m) in a weak storm. The smaller the diameter of the eye, the stronger the hurricane winds will be. A hurricane’s strength is rated from Category 1, which has winds of at least 119 km/h (74 mph), to Category 5, which has winds of more than 249 km/h (155 mph). These categories, known as the Saffir-Simpson hurricane scale, were developed in the 1970s.

In the tropics, hurricanes move generally east to west, steered by global-scale winds. Hurricanes, typhoons, and cyclones usually “recurve” in the direction of either the South Pole in the Southern Hemisphere or the North Pole in the Northern Hemisphere. Eventually the storms move toward the east in the middle latitudes, but not all storms recurve. Hurricanes travel at varying rates. In the lower latitudes the rate usually ranges from 8 to 32 km/h (5 to 20 mph), and in the higher latitudes it may increase to as much as 80 km/h (50 mph).

In addition to generating large waves that travel out in all directions, hurricane winds pile up water. This piling up of water is known as a storm surge, and it can raise the sea level more than 6 m (20 ft) when the storm hits land.

IV

Hurricane Devastation

The deadliest natural disaster in United States history was the 1900 Galveston, Texas, hurricane, which killed an estimated 8,000 people. The storm surge accounted for most of the deaths. The costliest natural disaster in U.S. history was caused by the storm surge created by Hurricane Katrina in 2005. The hurricane’s storm surge burst levees protecting New Orleans, Louisiana, flooding the city and forcing a complete evacuation. The worst tropical storm disaster since the 20th century began was a 1970 cyclone that struck East Pakistan (now Bangladesh) when a storm surge killed an estimated 300,000 people.

Since the last third of the 20th century, floods and landslides from heavy rain were the leading cause of hurricane and tropical storm deaths. In October 1998 Hurricane Mitch’s torrential rain caused floods and landslides that killed more than 9,000 people with another 9,000 missing and presumed dead in Central America, according to the U.S. National Hurricane Center. Although the hurricane death toll steadily declined in the United States during the 20th century and at the start of the 21st century, the costs of damage soared as coastal populations grew and the value of property outstripped population growth. Before Hurricane Katrina, the costliest U.S. natural disaster was Hurricane Andrew, which hit the Miami, Florida, metropolitan area in 1992, causing $26.5 billion in damages, including both insured and uninsured losses. Some estimates of Hurricane Katrina’s damages ran as high as $125 billion. In addition, federal relief efforts were expected to cost tens of billions of dollars.