Eye of the Storm

Eye of the Storm

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).

Saffir–Simpson Hurricane Scale

The Saffir-Simpson scale was developed in the 1970s by Herbert Saffir and National Hurricane Center director Robert Simpson. The scale rates a hurricane’s intensity based on wind speed, and helps estimate damage and flooding from a hurricane landfall.

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. 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 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. Advances in computer forecasting, along with emergency evacuations, held the storm surge death toll to fewer than a dozen people in the United States from 1970 through the 2003 hurricane season.

During 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. 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.