On October 29, 2012, Hurricane Sandy pounded the entire east coast of the United States. The storm had formed over the Caribbean Sea seven days earlier. It tore across Jamaica and Cuba before moving into the Atlantic Ocean. As it churned north along the eastern seaboard, the hurricane merged with a western low-pressure system. Sandy was now more than 1,100 miles (1,800 kilometers) wide—the largest Atlantic hurricane on record. But instead of blowing out to sea, the storm turned inland and smashed into New Jersey. It lashed coastal communities with torrential rain and winds of 110 miles (177 kilometers) per hour. Waves from the storm surge reached as high as two-story buildings. Farther west, up to 3 feet (1 meter) of snow fell in parts of the Appalachian Mountains. Ultimately, the storm affected 24 U.S. states before sputtering out over northern Canada. Hurricane Sandy was one of the most crippling storms in U.S. history, second only to Hurricane Katrina in 2005.
Like most violent storms, a hurricane is a type of cyclone. A cyclone forms when two air masses meet, their air currents moving in opposite directions. The encounter produces a gigantic vortex of wind spiraling toward an area of low air pressure at its center. In movement, the cyclone is like a top, spinning as it advances.
Tropical cyclones develop over the warm seas of the tropics or subtropics. Unlike cyclones farther from the equator, the air masses that meet to form a tropical cyclone are all warm and moist. Meteorologists call this maritime tropical air.
Tropical cyclones in the North Atlantic Ocean, Caribbean Sea, and Gulf of Mexico are called hurricanes. Hurricanes also occur in the Pacific Ocean off the coasts of Mexico and Central America. Similar storms in the northwestern Pacific Ocean are known as typhoons. They affect Japan, China, Taiwan, and the Philippines. Tropical cyclones also occur in the South Pacific, along the coasts of the Indian Ocean, and in the Bay of Bengal and the Arabian Sea. Satellite observations reveal that typhoons occasionally make landfall on the east coast of South America.HURRICANES VS. TORNADOES
Hurricanes are the most violent storms on Earth. Tornadoes may take the record for wind speed, with swirling gusts that reach 300 miles (500 kilometers) per hour. But hurricanes are monsters by comparison.
While tornadoes seldom exceed 1/4 mile (0.4 kilometer) in diameter, the diameter of a hurricane can reach 500 miles (800 kilometers). Within this area is a huge cyclone of swirling wind, with the strongest winds circulating around the core of the storm. These are the true hurricane-force winds, exceeding 73 miles (117 kilometers) per hour in speed. Hurricane-force winds can extend outward from the storm's center for 75 miles (120 kilometers) in all directions. Swirling around these winds is a wider area of gale-force winds, which exceed 39 miles (63 kilometers) per hour.
A hurricane is also longer-lived than a tornado. A typical hurricane moves west across the Atlantic into the Caribbean Sea, then travels north along the U.S. coastline and off through Canada's Maritime Provinces. The entire journey can last for days or even weeks.LIFE CYCLE OF A HURRICANE
Hurricanes always form in areas of moist, warm, turbulent air, which are common over tropical and subtropical seas. Three types of disturbances have been identified as most likely to give rise to hurricanes. The most common is the tropical wave, a zone of low pressure that moves from east to west over tropical seas. A second type of hurricane "seedling" develops along the intertropical convergence zone (the doldrums), a region of low pressure in the tropics that moves from one side of the equator to the other with the seasons. Finally, hurricanes occasionally arise from midlatitude weather disturbances that move toward the equator over subtropical waters.
But only a few of these turbulent areas will become hurricanes: a hurricane will develop only when its upper-level winds push away the air at the top of the turbulence. This creates a central area of low air pressure—the eye—and the winds begin to rotate around it. Hurricanes can spin at 100 miles (160 kilometers) per hour or more, with winds blowing stronger near the water surface and weaker at the top of the storm. Simultaneously, hurricanes advance quite slowly—at about 10 to 15 miles (16 to 24 kilometers) per hour.
All hurricanes draw energy from the warm surface of the tropical seas. Warm water vapor rises up into the storm and condenses into a cloud. The condensation releases energy in the form of heat. As long as a hurricane remains over or near warm water, it will receive this energy. The storm will weaken if it moves over land or cold water, or if its upper-level winds no longer push away the air at the top. It loses its central eye as well as its warm-water energy source.
As hurricanes move north, they draw cooler, drier air into their winds. A system with wind speeds below 74 miles (119 kilometers) per hour is classified by meteorologists as a tropical storm. When sustained winds are less than 39 miles (63 kilometers) per hour, the storm is classified as a tropical depression.HURRICANE STRUCTURE
Hurricanes tend to be more symmetrical in the tropics and subtropics and more asymmetrical, even elliptical, in midlatitudes. This asymmetry in midlatitudes is apparent in both the shape of the eye and that of the entire storm.
When viewed from above via satellite, a mature hurricane looks roughly like a doughnut. The small hole in the center is the eye. Immediately surrounding this eye is a bright ring of clouds. Beyond this ring is what appears to be a swirling mass of clouds. Actually, the hurricane is made up of a series of separate squall bands. Each is an area of violently gusting winds associated with heavy rain and thunderstorm activity. Because the clouds of each squall band merge across the top, the hurricane appears as an unbroken swirl of clouds when viewed by satellite from above.
The banded structure of a hurricane is best seen on color-coded, infrared (heat) satellite images. Bands whose cloud tops reach higher into the atmosphere emit less heat, and thus show up as distinctly colored regions on the infrared image. Radar can also identify a hurricane's spiral-band structure—as well as the location of its eye. Unfortunately, radar is effective only after the hurricane has moved within close range.
In general, the winds within the squall bands grow stronger the closer they are to the eye. The heaviest activity is in the eye wall, the broad band immediately surrounding the eye. This area is the bright ring of a cloud so often seen in satellite images.
Contrary to popular belief, the eye of the hurricane is neither perfectly calm nor cloud-free. Breezes within the eye can be gusty, often blowing between 10 and 25 miles (16 and 40 kilometers) per hour in various directions. Sometimes the eye of a hurricane disappears from satellite view, hidden by low clouds. This often happens as the storm moves into middle latitudes. A few hurricanes even produce light rain or drizzle within the eye. Indeed, the eye of the storm can be called "calm" only when compared to the violent weather that surrounds it.
Another prominent feature seen on weather-satellite pictures is what meteorologists refer to as feeder bands—elongated zones of clouds, showers, and thundershower activity. Feeder bands extend from outside the hurricane into the outer edges of the storm, where they become the so-called outer-spiral squall band. A hurricane may have one or several feeder bands. As long as a feeder band is over warm water, it will continue to feed energy into the storm.HURRICANE FACTS AND FIGURES
Hurricane Seasons. Hurricanes and other tropical cyclones form over tropical and subtropical oceans where water temperatures exceed 78° F (26° C). Hurricane "seasons" are determined largely by regional water temperatures. In general, hurricane and typhoon seasons peak in late summer and early fall, when both ocean and air temperatures are at their warmest. There are exceptions, however. Pacific typhoons have occurred throughout the year, and hurricanes have hit the east coast of North America and northern South America in every month of the year except February.
Tropical cyclones in the Bay of Bengal and the Arabian Sea follow their own unique pattern. There the major cyclone season precedes the Indian summer monsoon (April and May), with a second, milder season during the postmonsoon (September and October).Vital Statistics. The life span of a hurricane (or typhoon) may vary from a day or two to as long as four or five weeks. In recorded history, Atlantic hurricanes have ranged in size from 40 to 600 miles (65 to 950 kilometers) in diameter; an average diameter measures approximately 300 to 500 miles (500 to 800 kilometers). Hurricane Sandy in 2012 set a new record as the largest Atlantic hurricane—1,100 miles (1,800 kilometers) in diameter. The height of thunderstorm clouds in the spiral squall bands may exceed 45,000 feet (13,700 meters) in altitude. The diameter of a hurricane's eye is typically about 20 miles (32 kilometers). The largest ever measured was the 40- by 80-mile (64- by 128-kilometer) elliptical eye of 1955's Hurricane Diane, when the storm was centered over Chesapeake Bay.
The average forward speed of a hurricane is approximately 15 miles (24 kilometers) per hour. But there is a wide range—from nearly stationary to, on rare occasions, faster than 40 miles (64 kilometers) per hour. Such record-breaking speeds typically occur as the storm reaches higher latitudes, just before it begins to pull cool air into its vortex.
An average of eight hurricanes develop off the east coast of North America each year. But there have been as few as one, in 1890 and 1914. A record 15 hurricanes occurred in 2005.Wind Speed. The wind speed of a hurricane can be measured directly from hurricane-reconnaissance aircraft and ground-wind instruments. It can also be measured indirectly—based on damage. The highest recorded wind speeds are about 200 miles (320 kilometers) per hour, although higher velocities have likely occurred in a few "super" hurricanes. After a small but intense hurricane devastated the Florida Keys on Labor Day 1935, engineers estimated its wind speeds at 250 miles (400 kilometers) per hour, based on a damage survey.
A hurricane's wind speed is inversely proportional to the air pressure at the storm's center. Air pressure is often measured with a mercury barometer in which mercury in a tube rises and falls according to air pressure. Meteorologists worldwide measure the height of the mercury in millibars. Average air pressure at sea level is 1013 millibars of mercury. A hurricane eye will develop only when air pressure at the center of the storm drops to 998 millibars or below. The lowest pressure ever recorded in a hurricane eye was 882 millibars in Wilma (2005). Other low measurements include 888 millibars in Gilbert (1988); 902 millibars in Katrina (2005); and 905 millibars in Camille (1969).
Storm size also plays a role in determining wind speed. In two systems with the same central pressure, the smaller storm will have the higher wind velocities. The Saffir-Simpson hurricane scale has been used since 1973 to predict the hazards of approaching hurricanes. The five categories of this wind intensity scale are based on a hurricane's average sustained wind speed in a one-minute period.HURRICANE DANGERS
Certainly hurricane winds are capable of producing widespread destruction. Damage becomes devastating when wind speeds exceed 100 miles (160 kilometers) per hour. Such winds can snap and uproot trees, hurl debris through the air, rip roofs from sturdier buildings, and flatten weaker structures. To escape from the dangerous winds, residents of hurricane-threatened areas must seek shelter in strong buildings with boarded windows.
As dangerous as a hurricane's winds are, flooding from the storm poses an even greater threat. A hurricane surge is the coastal flooding caused by a combination of air-pressure change and windswept waves. A hurricane surge is most devastating when water is blown shoreward along a gradually sloping or flat coastline. Under such conditions, a hurricane surge can reach 10 to as high as 30 feet (3 to 9.6 meters) above sea level, inundating small offshore islands and sometimes moving several miles inland. Entire populations may need to be evacuated inland for safety well in advance of the storm, before escape routes are flooded.
Hurricane flooding is worsened by the torrential rainfall that accompanies these storms. Twenty-four-hour rainfalls exceeding 20 inches (50 centimeters) have been observed in some slow-moving hurricanes. In 2009 typhoon Morakot dropped an astonishing 83 inches (211 centimeters) of rain across Taiwan. The deluge triggered landslides that buried several towns and more than 300 people under thick layers of mud. But even lesser amounts can produce flash and urban flooding in low-lying areas and along streams and tributaries. Major river flooding often follows. The danger along streams in rough terrain can be extreme.
One additional hurricane hazard must be recognized. Tornadoes frequently develop within the hurricane—mainly on the right side of the hurricane (with its front being the forward-moving edge) at about 50 to 150 miles (80 to 240 kilometers) from the hurricane center. Apparently, the stronger winds near the eye disrupt and discourage tornado formation close to the center of the storm. The record-setting hurricane in this category was Frances (2004), which spawned 123 tornadoes.HURRICANE FORECASTING
Forecasting hurricane strength and movement is challenging work. An underestimate of a hurricane's strength or an error in predicting where it will make landfall can put thousands of lives in peril.
It is easiest to predict the movements of hurricanes that have forward speeds greater than 12 miles (19 kilometers) per hour. Such storms may slightly alter their course from the predicted track, but there is rarely any drastic change. By contrast, very-slow-moving hurricanes often change course dramatically, as do some storms which demonstrate exceptionally erratic behavior.
Observations of upper-level winds over the ocean are helpful in predicting hurricane movements. Unfortunately, such data is difficult to obtain over the ocean, and is generally available only from stations on a few well-placed islands.
More and more, forecast specialists such as those at the National Weather Service Hurricane Center in Miami, Florida, issue their hurricane predictions based on computer models, coupled with the behavior of similar past storms. Such hurricane computer models are still in their infancy, although they have grown more reliable with each storm season.
Satellite observation is essential to hurricane forecasting. By keeping the seas and coasts under constant surveillance, satellites warn forecasters of any hurricane before it nears a coastline. As a result, hurricane fatalities should be few and far between—so long as people take hurricane warnings seriously and seek appropriate shelter.
Benjamin F. Abell