Hurricane,

an intense tropical cyclone with maximum sustained wind speeds exceeding 119 km (74 mi) per hour. The term "hurricane" is applied to such storms in only certain regions of the world: the North Atlantic Ocean, the northeastern Pacific Ocean (east of the international date line), and the South Pacific Ocean east of 160° E longitude. Powerful storms of this type are called typhoons in the northwestern Pacific Ocean, and severe tropical cyclones in the South Pacific Ocean west of 160° E as well as in the southeastern Indian Ocean; they are referred to as severe cyclonic storms in the North Indian Ocean and tropical cyclones in the southwestern Indian Ocean.

Most hurricanes originate within the doldrums, a narrow equatorial belt characterized by intermittent calms, light variable breezes, and frequent squalls, and lying between the northeast and southeast trade winds. Since the doldrums of the Atlantic are situated largely to the north of the equator, hurricanes do not occur in the South Atlantic Ocean. The Pacific doldrums extend north and south of the equator; thus hurricanes do occur in both the South and the North Pacific.

Hurricanes generally arise from collections of storms and clouds known as tropical disturbances. Under certain conditions the disturbance may begin a cyclonic rotation. These conditions include sufficiently warm ocean water, enough atmospheric moisture, minimal wind shear, and a location at least 500 km (300 mi) from the equator--in areas too close to the equator, the Coriolis force is not strong enough to trigger cyclonic rotation. The rotating disturbance is known as a tropical depression. If the depression intensifies to the extent that the sustained speed of the surface winds exceeds 61 km (38 mi) per hour, it becomes a "tropical storm," and is commonly given a name by meteorologists. Storms that grow so strong that wind speeds surpass 119 km (74 mi) per hour qualify as hurricanes.

Hurricanes consist of high-speed winds blowing circularly around a low-pressure center, known as the eye of the storm. The low-pressure center develops when the warm, saturated air prevalent in the doldrums is underrun and forced upward by denser, cooler air. From the edge of the storm toward its center, the atmospheric pressure drops sharply and the wind speed rises. The winds attain maximum force close to the point of lowest pressure (about 965 millibars, or 28.5 in of mercury, for a moderate-strength hurricane). The diameter of the area affected by winds of destructive force may exceed 240 km (150 mi). Gale winds prevail over a larger area, averaging more than 480 km (300 mi) in diameter.

The destructiveness of a hurricane is determined not only by its wind speed but also by the magnitude of the associated storm surge (rise in sea level) and flooding. The Saffir-Simpson Hurricane Scale, created in the 1970s by the American engineer Herbert S. Saffir (1917-    ) and Robert H. Simpson (1912-    ), a former director of the National Hurricane Center (NHC), rates a hurricane's intensity on a scale of 1 to 5. It is used by the NHC to estimate potential property damage and flooding along the coast after a hurricane landfall. The strongest (and rarest) hurricanes, those in Category 5, have winds that exceed 249 km. (155 mi) per hour. Within a hurricane's eye, which averages around 24 km (15 mi) in diameter, the winds stop and the clouds lift, but the seas remain very violent.

Saffir-Simpson Hurricane Scale
Category Central Barometric Pressure 1 Wind Speed 2 Damage Storm Surge 3
1 more than or at 28.94 in 74-95 mph Minimal 4-5 ft
2 28.50-28.91 in 96-110 mph Moderate 6-8 ft
3 27.91-28.47 in 111-130 mph Extensive 9-12 ft
4 27.17-27.88 in 131-155 mph Extreme 13-18 ft
5 less than 27.17 in more than 155 mph Catastrophic more than 18 ft

1 in inches of mercury

2 in miles per hour

3 height above normal tide

Hurricanes generally move in a path resembling the curve of a parabola. In the northern hemisphere the storms usually travel first in a northwesterly direction and in the higher latitudes turn toward the northeast. In the southern hemisphere the usual path of the hurricane is initially to the southwest and subsequently to the southeast. Hurricanes travel at varying rates. In the lower latitudes the rate ranges from 8 to 32 km/hr (5 to 20 mph), and in the higher latitudes it may increase to as much as 80 km/hr (50 mph). Those areas in which the hurricane winds blow in the same direction as the general movement of the storm are subjected to the maximum destructive violence of the hurricane. Hurricanes weaken as they pass over land or cooler water.

image

GOES8/NOAA

Hurricane Floyd

Since 1943, U.S. military aircraft (and in later years civilian aircraft as well) have been flying into hurricanes to measure wind velocities and directions, the location and size of the eye, the pressures within the storms, and their thermal structure. A coordinated system of tracking hurricanes was developed in the mid-1950s, and periodic improvements have been made over the years. Radar, sea-based recording devices, geosynchronous weather satellites (since 1966), and other devices supply data to the NHC in Florida, which follows each storm virtually from the beginning. Improved systems of prediction and communication have been able to help minimize loss of life in hurricanes, with the aim of ensuring that the U.S. record for fatalities in a hurricane--according to most estimates, at least 8000 killed by a hurricane and accompanying storm surge that hit Galveston, Tex., in 1900--remains unbroken.

Atlantic hurricanes' potential for inflicting tragic losses, however, has remained evident in recent decades, highlighted by several storms, ranked in Category 5 at their peak, that wrought catastrophic destruction. Gilbert, the most intense (as indicated by lowness of central air pressure) hurricane to hit the western hemisphere in the 20th century, devastated Jamaica and parts of Mexico in 1988, killing more than 300 people. Four years later, Andrew, hitting southern Florida particularly hard, caused more than 50 deaths and over $25 billion in damage--at the time a U.S. record. In 1998, Hurricane Mitch inflicted heavy rains and flooding on Central America, leaving more than 10,000 people dead and an estimated 2 million homeless. In 2005, Hurricane Katrina killed more than 1800 Americans, as its winds and devastating storm surge ravaged New Orleans and other coastal areas of Louisiana, Mississippi, and Alabama, causing damage estimated at about $125 billion.

Katrina was the most disastrous storm in what turned out to be a record-setting year in several regards. The Atlantic hurricane season officially runs from June through November; historically speaking, nearly all hurricanes occur in this period. The 2005 season, however, stretched into January 2006 and produced 15 hurricanes, the most ever recorded in the Atlantic region. Its 27 named storms (including tropical storms) was also a new mark; since only 21 names had been reserved, authorities used the names of letters in the Greek alphabet for the season's final storms, concluding with Tropical Storm Zeta, which formed near the end of December and lasted into early January. In addition, 2005 saw four Category 5 storms, the most on record for an Atlantic hurricane season. Katrina was one; the other three were Dennis, Rita, and Wilma.

Hurricane Wilma, which developed in the northwest Caribbean Sea, established a new mark for most intense Atlantic hurricane, with a minimum central pressure estimated as at least as low as 882 millibars (26.0 in of mercury). Increased intensity causes a hurricane's eye to shrink, and at Wilma's peak strength, its eye had a diameter of only 2 nautical miles (less than 4 km), making it the smallest eye known to the NHC. Also unprecedented for an Atlantic hurricane was the speed at which Wilma intensified: it progressed from a tropical storm to a Category 5 hurricane in just 24 hours. Particularly hard hit by Wilma were the Yucatán Peninsula and southern Florida; damage in the U.S. as estimated at more than $20 billion. Wilma, together with Stan, a relatively weak storm that hit Mexico and Central America a couple of weeks earlier, illustrated the fact that the devastation wrought by a hurricane depends not just on the storm's intensity. The extremely strong Wilma caused considerable damage but produced a death toll estimated at only a few dozen. Stan at its peak was just a Category 1 storm, but it brought torrential rains, resulting in flooding and landslides--and a death toll that may have been as high as 2000.

The highest death tolls associated with cyclonic storms generally occur in the northern Indian Ocean region, where low-lying Bangladesh is especially vulnerable to storm surges. An estimated 300,000- 500,000 people perished as a result of a 1970 cyclone in Bangladesh (then known as East Pakistan), and a 1991 storm killed more than 138,000 people.