HurricaneHurricane Katrina devastates New Orleans, Gulf Coast
Recent events: Catastrophic hurricanes since 2000
The components and causes of a hurricane
The effects of hurricanes
The human factor
Hurricanes in the United States
For More Information
When Katrina struck the coasts of Louisiana and Mississippi on August 20, 2005, it reminded us that hurricanes certainly deserve the title "greatest storms on Earth." No other storm system has the power of a hurricane. To get a sense of the power contained in a hurricane, consider the following: 1) Just 1 percent of the energy in an average hurricane could supply the entire United States with electric power for one year; and 2) the energy unleashed by a hurricane in one day is equal to the energy of four hundred 20-megaton hydrogen bombs.
Hurricanes that strike populated areas are among the deadliest and most destructive of all natural disasters. A single strong hurricane can kill hundreds or thousands of people. Fortunately, the number of deaths caused by hurricanes has declined greatly in recent years (particularly in the United States) due to the development of early detection systems. Historically, hurricanes have been directly responsible for an average of 162 deaths per year. However, this includes the Galveston, Texas, hurricane of 1900 in which 6,000 people lost their lives. Since 1965, hurricanes have accounted for about 20 to 30 deaths per year in the United States.
Population growth and the construction of vacation and retirement homes in hurricane-prone areas raise the risk of increased losses from property damage. However, the cost of property damage from hurricanes has not shown a steady increase. According to information from the National Oceanic and Atmospheric Administration (NOAA) the increase is very slight in inflation-adjusted dollars from 1971 to 2000. In 1992, there was a loss of nearly forty billion in inflation-adjusted dollars. If this spike is not included, the cost of property damage would show a slight decrease. On the other hand there were huge property losses in 2004 and 2005. When these numbers are included, property damage losses from 1971 to 2006 show a slight but definite upward trend. However, it is not a steady increase, since some years are high and some are low.
The word "hurricane" is used to refer to tropical cyclones that form in the northern Atlantic Ocean or in the eastern Pacific Ocean off the coasts of Mexico and Central America. A tropical cyclone is any rotating weather system that forms over tropical waters, which are within the area of 23.5° north latitude and 23.5° south latitude. Hurricanes that occur in the western North Pacific and China Sea region are called "typhoons" (pronounced TIE-foons). The word "typhoon" may come from taaîfung, which is Cantonese for "great wind." It is also possible that the word comes from the ancient Greek word tuphon, which means whirlwind. Hurricanes that form over the Indian Ocean are called "cyclones" (pronounced SIGH-clones).
Hurricane Katrina devastates New Orleans, Gulf Coast
Hurricane Katrina shocked the world after hitting the U.S. Gulf Coast on August 29, 2005. Katrina was the third most powerful hurricane ever to make landfall, or come across a land mass, in the United States. Until this disaster, recent advances in early detection systems had helped to decrease the disastrous effects of hurricanes. From 1965 to 2005, deaths from hurricanes in the United States were around twenty to thirty people per year. In contrast, Katrina alone claimed over eighteen hundred lives and caused up to $100 billion in damage. This storm, which displaced at least a half-million people, will be remembered as one of the deadliest and costliest natural disasters in U.S. history.
Katrina first made landfall on August 25 in Florida as a Category 1 storm according to the Saffir-Simpson intensity scale. The storm caused some damage and loss of life across the state before exiting land and beginning a path through the Gulf of Mexico toward Louisiana. In the warm waters of the gulf, Katrina intensified to a Category 5 storm. The Gulf Coast braced itself for the oncoming hurricane.
Katrina lost momentum as it moved toward the Gulf Coast. The storm gradually reduced to a Category 3 on its path toward landfall. However, since the storm was a Category 4 just before the eye of the storm made landfall, the area experienced sustained winds of more than 130 miles (209 kilometers) per hour. Katrina made landfall near New Orleans, Louisiana, setting off a disastrous chain of events.
Katrina hit Louisiana just after 6 am on a Monday. Within hours, low-lying parts of New Orleans and surrounding parishes stood under water of up to 10 feet (3 meters) deep. The city of New Orleans, which lies below sea level, is protected from the waters of Lake Ponchartrain and the Mississippi River by a 350-mile-long (563-kilometer-long) series of levees. Soon after the storm hit, three of these protective levees failed, and
WORDS TO KNOW
- air mass:
- a large body of air with approximately the same temperature and humidity throughout.
- air pressure:
- barometric pressure; the force exerted on a surface due to the weight of air.
- an instrument that measures wind speed.
- cirrostratus cloud:
- a thin layer of high-altitude clouds that cover most of the sky, but are semitransparent.
- cirrus cloud:
- a wispy, high-level cloud.
- cold front:
- the leading edge of a moving cold air mass.
- the process by which water changes from a gas to a liquid.
- where things come together, such as the trade winds blowing from the north and south near the equator.
- Coriolis effect:
- the deflection of a moving object such as an air mass due to the rotation of Earth.
- cumulonimbus cloud:
- a tall, vertically developed cloud reaching to the top of the troposphere or above and capable of producing heavy rain, high winds, and lightning.
- cumulus cloud:
- fluffy, white, mid-level cloud.
- a weather system characterized by air that flows inward and circulates around a low pressure area.
- an area of clear sky and warm, dry, descending air at the center of a hurricane.
- eye wall:
- a vertical area of thick clouds, intense rain, and strong winds marking the outer boundary of the eye.
- inundation by water of normally dry land.
- the boundary between two air masses of different temperatures.
- a tropical cyclone that forms in the northern Atlantic Ocean or in the eastern Pacific Ocean.
- hurricane warning:
- hurricane landfall is imminent.
- hurricane watch:
- hurricane landfall is possible.
- Intertropical Convergence Zone:
- the area of rising air near Earth's equator where the trade winds converge.
- inversion, atmospheric:
- a stable reversal of the normal pattern of atmospheric temperature, formed when a warm air mass sits over a cold air mass near the surface.
- jet stream:
- a fast-flowing, relatively narrow air stream found at an altitude of approximately 36,000 feet (11,000 meters).
- the point on a coast the center of a hurricane first crosses.
- latent heat:
- the heat that must be removed from a quantity of water vapor to cause it to turn into a liquid, or that must be added to a quantity of liquid water to cause it to turn into a vapor; called latent because the temperature of the quantity of water or water vapor does not change.
water poured in, filling the city like a bathtub. By the end of the first day, entire neighborhoods were completely underwater. At the disaster's height, 80 percent of the city was submerged.
Calls for evacuation of New Orleans began on Friday and Saturday, a few days before the storm hit the Gulf Coast. However, thousands of people were still in New Orleans when the storm hit. Many people had no access to transportation—by some estimates, one-third of New Orleans' population did not own a car. Some were elderly or in poor health, some had small children, and some simply did not want to leave their homes. Around the time the storm hit, citizens who remained in New Orleans were advised to head for the Louisiana Superdome or the convention center, which were being used as temporary shelters for evacuees. Many found refuge in these shelters, but thousands were left behind in homes, hospitals, jails, and the streets.
The rest of the world watched in horror as tragedy unfolded. Parts of the city remained submerged for weeks. Fires raged as the floodwaters became increasingly polluted with gasoline, landfill contents, industrial waste, snakes, and rats. Many people were stranded in the hardest-hit areas, which were mostly poor neighborhoods. Rescue workers used helicopters and boats to navigate the floodwaters in order to rescue people from trees and rooftops. Hundreds waited on highway overpasses to be rescued as the water rose. The region lacked electricity and drinkable water. Civil unrest mounted as military and police forces tried to keep the peace.
Though much of the public focus on Hurricane Katrina was on the New Orleans tragedy, the storm pulverized a long stretch of the Gulf Coast region, including Florida, Mississippi, Tennessee, and Alabama. Storm surges, or abnormal sea rises, occurred as far away as Mobile, Alabama. Hundreds of people died, and thousands upon thousands lost their homes and livelihoods.
Weather extremes: The Galveston disaster
Before Hurricane Katrina, the worst hurricane disaster and the worst weather disaster in the United States occurred on September 8, 1900, in Galveston, Texas. It was the worst storm of the twentieth century in terms of lost lives. Initial estimates were that more than six thousand people died when a storm surge 20 feet (6 meters) in height swept across the low-lying barrier island. Modern estimates put the death toll closer to seventy-two hundred.
Galveston's residents had received warning from the U.S. Weather Bureau on September 6 that a tropical storm was detected near Cuba. However, they did not pay much notice. After all, other tropical storms had come their way but had inflicted only minor damage. Galveston residents had no way of knowing the power of the storm that was about to strike them.
It was not until the morning of September 8 that the higher-than-usual tides and strong winds hinted at the severity of the approaching storm. However, rather than evacuating the island, most residents merely took shelter in brick houses at higher elevations. The problem with that strategy was the highest point on the island, at that time, was only 9 feet (about 3 meters) above sea level. That meant that no place on the island was safe from the storm's rushing waters.
The hurricane's assault on Galveston lasted for several hours, starting at about 4 pm. Winds gusting to 100 miles (160 kilometers) per hour were recorded before the island's anemometer, an instrument that measures wind speed, was destroyed. A 20-foot (6-meter) storm surge crashed onto shore and entirely submerged the island. The storm leveled almost every structure within three blocks of the south shore and severely damaged many others. As buildings collapsed, the wreckage was carried inland to smash into other buildings. Many people were thrown into the water, where high winds and surging water battered them with the debris. To add to the misery of the survivors, 10 inches (25 centimeters) of rain fell during the night. Once the storm had passed and flood waters subsided, very few structures were left standing in the town.
Since that time, Galveston has been rebuilt and extensive measures have been taken to prevent future disasters. The residents have constructed a seawall facing the Gulf of Mexico that is 17 feet (5 meters) tall and 3 miles (5 kilometers) long. They also brought in sand and raised the elevation of the island, at some points as high as 17 feet (5 meters) above sea level.
The tragedy of Hurricane Katrina reminded the United States and the world of the complicated social and political aspects of natural disasters. Residents of the hardest-hit areas in the Gulf Coast region were among the nation's poorest people. Two-thirds of New Orleans residents were African American, a fact that caused a number of leaders to speculate that the government's botched relief efforts reflected a lack of concern for minorities.
Hurricane Katrina is not the only recent hurricane to have caused substantial destruction and economic loss in the United States. The years since the turn of the twenty-first century have been very active in the formation of hurricanes. With the exception of Hurricane Katrina, most of these storms did not cause extensive loss of life. However, they have been extremely costly. Hurricanes that have made landfall in the United States since 2000 have caused more than $148 billion in damage.
Hurricane Isabel was the only Category 5 storm of the 2003 Atlantic hurricane season. It made landfall as a Category 2 storm on September 18, 2003, just south of Cape Hatteras, North Carolina. It produced storm surges as high as 10 feet (3 meters) above normal tide levels and spun off a number of tornadoes, violently rotating winds capable of great destruction, in the days following its landfall.
Isabel caused damage from South Carolina all the way to Ontario, Canada. The official damage estimate was $3.37 billion. As with many hurricanes, the majority of that damage resulted from flooding, which affected more than sixty million people in the mid-Atlantic region. The flooding also caused extensive crop damage in several states. All told, fifty people died: sixteen as a direct result of the storm and thirty-four indirectly.
Hurricanes Charley, Frances, Ivan, and Jeanne
In 2004, four separate hurricanes bombarded the state of Florida over a six week period, causing more than $30 billion in damage and claiming more than sixty lives in Florida alone. Hurricane Charley was the first to hit, a Category 4 storm that made landfall in southwest Florida on August 14. It was the strongest hurricane to hit the United States since Hurricane Andrew in 1992. Because Charley was predicted to make landfall further north, near Tampa Bay, many residents in the affected parts of Florida were caught off guard. The storm claimed ten lives and caused $15 billion in losses, making it the third-costliest hurricane in U.S. history.
Just weeks later, on September 4 and 5, Hurricane Frances battered both the east and west coasts of Florida. After initially making landfall on the eastern seaboard near West Palm Beach, Frances traveled across the Florida peninsula and into the Gulf of Mexico. It once again hit Florida, this time in the panhandle in the northwest portion of the state. Frances caused forty-two deaths in the United States and about $9 billion in losses.
Hurricane Ivan was the strongest storm of the 2004 Atlantic hurricane season. After causing major damage in Grenada, Jamaica, and the Cayman Islands, Ivan made landfall as a Category 3 storm in Alabama on September 16. On September 20 it looped back across the Florida peninsula, doing most of its damage in the state's panhandle near the Alabama border. Ivan killed sixty-four people in the Caribbean and fifty-seven people in the United States, including fourteen in Florida, and caused $13 billion in damage.
Less than two weeks later, on September 25, Hurricane Jeanne made landfall on Florida's east coast, just two miles north of Frances. Where Frances traveled west after making landfall, Jeanne turned north and headed up the coastline to Georgia. Jeanne killed five people and caused about $6.9 billion in damage in the United States, but the worst of its damage came in Haiti, where more than three thousand people died in flooding and mud slides caused by the storm.
The 2005 Atlantic hurricane season
Charley, Frances, Ivan, and Jeanne combined to make the 2004 Atlantic hurricane season one of the deadliest and costliest ever, with losses of more than three thousand lives and $42 billion. The 2005 Atlantic hurricane season was even more severe. It was the most active hurricane season to date. Tropical storms, which are storms that have sustained winds of 39 to 73 miles (63 to 117 kilometers) per hour, are given names from a pre-determined list. During the 2005 Atlantic hurricane season, there were twenty-seven named tropical storms. This number included fourteen hurricanes and seven major hurricanes.
Three of the six most powerful hurricanes ever recorded in the Atlantic basin formed in 2005, including Wilma, the most powerful in history. In addition to Hurricane Katrina, the most destructive hurricane in U.S. history, hurricanes Dennis, Rita, and Wilma all caused significant damage. All told, damage from the 2005 Atlantic hurricane season topped $100 billion.
Hurricane Dennis made landfall as a Category 3 storm in the Florida panhandle in July, less than a year after Hurricane Ivan struck the same area. Dennis claimed eighty-nine lives, including fifteen in the United States, and did more than $2.2 billion in damage.
Hurricane Rita, a Category 5 storm, was the first hurricane to make landfall in the United States after Hurricane Katrina, hitting near the Texas-Louisiana border on September 24. Rita had been projected to make landfall near Houston, Texas, prompting nearly three million residents of the greater Houston area to evacuate. The evacuation created an enormous gridlock, with the trip from Houston to Dallas, normally three or four hours, taking between twenty-four and thirty-six hours to complete. After it made landfall, Rita caused $10 billion in damage. Though only seven direct deaths were recorded, 113 people died from indirect causes, many in the evacuations that preceded the storm.
With the formation of Hurricane Wilma in October, the 2005 Atlantic hurricane season surpassed the 1933 season as the most active in history. After setting the record as the most powerful hurricane in Atlantic basin history, Wilma devastated the Yucatán peninsula in Mexico and then traveled east and made landfall in southwest Florida on October 24. Wilma caused more than $12 billion in damage and killed thirty-five people in Florida.
The year 2005 saw so many hurricanes that, for the first time since the practice of naming hurricanes began in 1950, all of the allotted names were used, and the later storms of the season were named using letters of the Greek alphabet. Tropical Storm Zeta, the sixth storm to be named with a Greek letter, was the final storm of the 2005 season.
A hurricane is the most intense form of tropical cyclone, which is any rotating weather system that forms over tropical waters. Winds blow inwards toward, and rotate around, an area of low pressure. Cyclones rotate counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. To qualify as a hurricane, a storm must have a well-defined pattern of rotating winds with maximum sustained winds greater than 74 miles (119 kilometers) per hour. Sustained winds are winds that blow continuously for at least one minute.
A hurricane is made up of a series of tightly coiled bands of thunderstorm clouds. These bands spiral around an almost totally calm region, called the eye, at the center of the hurricane. A hurricane may have within it hundreds of strong thunderstorms, which have the very tall cumulonimbus clouds producing rain and thunder. The diameter of an average hurricane is about 350 miles (560 kilometers) while the diameter of the largest hurricanes approaches 900 miles (1500 kilometers).
Less intense forms of tropical cyclones are referred to as tropical storms, tropical depressions, or tropical disturbances. A tropical storm is similar to a hurricane in that it has organized bands of rotating strong thunderstorms, yet its maximum sustained winds are only 39 to 73 miles (63 to 117 kilometers) per hour. A tropical depression, the weakest form of tropical cyclone, consists of rotating bands of clouds and thunderstorms with maximum sustained winds of 38 miles (61 kilometers) per hour or less. A tropical disturbance is a cluster of thunderstorms that is beginning to demonstrate a cyclonic circulation pattern. Tropical disturbances frequently occur over tropical waters. Only a small percentage of these disturbances, however, become hurricanes.
Is global warming producing more hurricanes?
Atmospheric scientists have determined that Earth's temperature is increasing. This phenomenon is called global warming. Since one of the effects of global warming is to heat up tropical ocean water, many scientists expect hurricanes to become more frequent. Warmer water also contains more energy for hurricanes, so global warming may give rise to storms more powerful than ever witnessed before. The world's hurricane breeding regions are expected to expand as well, as a larger area of water warms to 80°F (27°C)—the minimum water temperature necessary for hurricane formation.
Recently, hurricanes have been forming more frequently and with greater ferocity. Hurricanes' duration and strength have increased by about 50 percent over the last thirty years. The 2005 Atlantic hurricane season was the most active season on record. In the eleven years between 1995 and 2005, twenty-nine Atlantic hurricanes were so destructive and costly that their names were permanently retired from the National Hurricane Center's list of available storm names. In the thirty-three years preceding, only twenty-eight storms had been retired.
It is not yet clear whether this increase in hurricane activity is attributable to global warming. Historically, hurricane activity has been cyclical, with several decades of high activity followed by several decades of fewer storms. For instance, the 1920s–30s and 1950s–60s were very active periods, while the 1970s–80s were relatively calm. Whether the frequent hurricanes of the 1990s and early twenty-first century are part of a long-term trend toward more hurricanes or simply another cycle of high activity is an ongoing scientific debate. While there is a near consensus in the scientific community that one of the effects of global warming will be more frequent and powerful hurricanes, some scientists caution against attributing any particular hurricane or weather phenomena to global climate change.
A hurricane starts out as a tropical disturbance and passes through the stages of tropical depression and tropical storm on its way to maturity. A hurricane will continue to grow as long as there is a fresh supply of warm, humid air. Once the hurricane crosses over colder waters or land, its supply of warm, humid air is cut off and it weakens. A typical hurricane lasts about thirteen days. A hurricane may pass back through those weaker stages, in reverse order, as it dissipates.
Over the ocean, a hurricane generates waves 50 feet (15 meters) or greater in height. When a hurricane reaches land, it may pound the shore with a wall of water up to 20 feet (6 meters) high. This wall can produce severe flooding across 100 miles (160 kilometers) of coastline. Hurricanes also bring fierce winds and intense downpours of rain. It is not unusual for coastal and inland communities to receive 6 to 12 inches (15 to 30 centimeters) of rain when a hurricane comes onshore. For its onshore finale, a dissipating hurricane may spin off numerous tornadoes.
What is the structure of a hurricane?
A hurricane consists of spiraling bands of clouds called rain bands, around a calm, low-pressure center, the eye. The rain bands, which are tightly coiled around the eye, produce heavy rains and forceful winds. Surrounding the outer edge of the rain bands is a region of wispy, high-level cirrus or cirrostratus clouds.
The hurricane's eye has an average diameter of 12 to 40 miles (20 to 65 kilometers). Within the eye, winds are light and skies are partly cloudy or clear. The reason that clouds break up in the eye of a storm is that air sinks in that region. Air warms as it falls and the water droplets within it evaporate.
The lowest pressure of the storm exists in the eye. Typically, pressure there dips to 950 millibars (28 inches, or 71 cm), although measurements as low as 900 millibars (26.5 inches, or 67.3 cm) have been recorded. By way of comparison, the average pressure at sea level is 1013.25 millibars (29.92 inches, or 76.00 cm).
It may take an hour or more for the eye of the hurricane to pass over an area. The calm weather associated with the eye sometimes fools the residents of that area into thinking the storm is over when, in fact, the heavy winds and rain will soon resume.
The region immediately surrounding the eye, called the eye wall, is the strongest part of the storm. The eye wall is a loop of thunderstorm clouds that produce torrential rains and forceful winds. The closer one gets to the center of the storm without actually entering the eye, the faster the winds blow. Within a radius of 6 to 60 miles (10 to 100 kilometers) of the eye, winds may reach speeds of 100 to 180 miles (160 to 300 kilometers) per hour.
Weather report: Comparing hurricanes and extratropical storms
Hurricanes are tropical cyclones. An extratropical cyclone is a large-scale storm that forms in the middle latitudes. Extratropical cyclones are produced by interactions between contrasting warm and cold fronts.
Tropical and extratropical storms have different mechanisms of energy supply. An extratropical cyclone gets its energy from temperature contrasts between the air masses found on either side of a front, while a hurricane gets energy from warm ocean waters and the latent heat released as the moist surface air rises.
Both extratropical cyclones and hurricanes are centered around an area of low pressure, a hurricane has a much steeper pressure gradient. The air pressure at the center of a hurricane is typically much lower than it is at the center of an extratropical cyclone.
Other differences between the two types of cyclones include:
- The diameter of the typical extratropical cyclone is three times the size of the diameter of the typical hurricane.
- Hurricanes weaken with height while extratropical cyclones intensify with height.
- Air sinks in the center of a hurricane while it rises in the center of an extratropical cyclone.
- Air at the center of a hurricane is warmer than the surrounding air while the air at the center of an extratropical cyclone is colder than the surrounding air.
- The winds of a hurricane are strongest at the surface while the winds of an extratropical cyclone are strongest at upper levels, in the jet stream.
Despite all these differences, it is possible for hurricanes to transform into extratropical cyclones. This happens when a hurricane moves across a front and draws in air of different temperatures. Then the hurricane, which was weakening as it traveled over land, intensifies as it becomes linked with a low-pressure area aloft.
An example of a hurricane that became linked with an extratropical storm is Hurricane Agnes of 1972. After combining with a low-pressure system in the Northeast, Agnes produced heavy rains and extensive flooding. Harrisburg, Pennsylvania, for instance, received 12.5 inches (32 centimeters) of rain in twenty-four hours. The rising waters in central Pennsylvania forced the evacuation of more than 250,000 people. Agnes caused a total of 122 deaths and $6.4 billion in property damage. The floods were responsible for around half of the deaths and about two-thirds of the property damage.
The winds are a result of the pressure gradient, which is the difference in air pressure between high- and low-pressure areas relative to their distance apart. The pressure gradient between the edge of the storm and the eye drives the winds of a hurricane. The closer in to the eye, the steeper the pressure gradient becomes and the faster the winds blow. At points farther away from the eye of the storm, the pressure gradient becomes more gradual and the winds become weaker.
The most violent part of the hurricane is the side of the eye wall in which the wind blows the same direction that the storm is progressing. In that region, the hurricane's winds combine with the winds that are steering the hurricane, to create the storm's fastest winds.
The cyclonic wind circulation of the hurricane weakens with height, starting at about 9,800 feet (3,000 meters). The airflow actually reverses direction at heights greater than approximately 50,000 feet (15,000 meters). That means that while the eye of the storm and the surface beneath it are intensely low pressure areas, a high pressure area exists above.
The fact that a hurricane has a ceiling below the top of the troposphere, the lowermost portion of Earth's atmosphere, makes it possible to fly above a hurricane in an aircraft and take aerial photographs of the entire system. It is not possible to do this in a middle-latitude thunderstorm since the mature cumulonimbus clouds in that system, the clouds that produce the lightning, extend to the top of the troposphere and sometimes beyond.
Where do hurricanes form?
Hurricanes are tropical phenomena. The tropics are defined as the region of Earth bounded by 23.5° latitude, north and south. The tropics receive the most direct sunlight of anywhere on the planet, making that region the world's warmest.
Interesting facts: "Hurricane Huron"
In September 1996 an extratropical cyclone formed over the Great Lakes. It formed from what meteorologists call a cut-off low-pressure area (a low-pressure area that has become separated from other weather systems and may remain stationary). This was a particularly intense low. It started out as a typical "coldcore" extratropical cyclone, but over time it evolved into a "warm-core" system, similar to a hurricane, with an eye and spiral bands of gusty rain squalls.
The storm likely formed because the low pressure area stalled over Lake Huron. At the time, the lake waters were at their maximum seasonal high temperature, 70°F (21°C). This would be too low under normal conditions for tropical storm development. But over time, the water warmed the air enough that the storm developed its warm core. From a satellite, it strongly resembled a tropical storm, and researchers at Pennsylvania State University dubbed it "Hurricane Huron." This incident is thought to be the only such storm ever documented over the Great Lakes.
The heating of Earth's surface leads to the daily formation of cumulus clouds and afternoon thunderstorms. While these thunderstorms are generally not severe individually, they sometimes become organized in lines called tropical squall clusters or squall lines, which are severe. These are similar to squall lines of thunderstorms that form over land in the middle latitudes.
Hurricanes generally form only within specific areas in the tropics, between 5° and 20° north and south, although they occasionally form as far north and south as 30°. At higher latitudes, the water is too cold for hurricanes to form.
The reason hurricanes won't form between 5° north and 5° south has to do with the lack of Coriolis effect at and near the equator. The Coriolis effect is the gradual change in direction of global winds due to the rotation of Earth. The Coriolis effect is necessary for the formation of rotating tropical storms. When the trade winds, which are prevailing winds that blow northeast north of the equator and from the southeast south of the equator, meet at the equator the Coriolis effect is canceled out.
Hurricane-breeding areas are found in several clusters over the warmest regions of the world's tropical oceans. The six primary hurricane-breeding regions are:
- The eastern North Atlantic Ocean, west of Africa and westward to the Caribbean Sea and the Gulf of Mexico.
- The eastern Pacific Ocean, west of Mexico.
- The western portion of the North Pacific Ocean, east of China.
- The South Indian Ocean, east of Madagascar.
- The North Indian Ocean and the waters surrounding India—the Bay of Bengal and the Arabian Sea.
- The portions of the Pacific and Indian Oceans adjacent to northern and western Australia.
The South Atlantic and the eastern portion of the South Pacific Ocean on either side of South America, where water temperatures are cooler, are notably free of hurricanes. The map in this section shows the primary hurricane breeding regions.
When do hurricanes form?
The annual hurricane season, for any given location, is during the months when ocean temperatures are highest. This period lags behind the year's warmest months on land, since it takes the oceans longer than the land to both warm up and cool down.
For the Northern Hemisphere, hurricane season is roughly June through November. In the Southern Hemisphere, hurricanes occur most frequently between December and May. The exception to this rule is the western portion of the North Pacific Ocean, where hurricanes form year-round.
Within the long hurricane season, there are peak months for hurricane formation that vary with location. For instance, the maximum number of hurricanes in the North Atlantic—hurricanes that threaten the U. S. eastern seaboard—occur in August and September.
How do hurricanes form?
The first step in the formation of a hurricane is the development of a cluster of thunderstorms, called a tropical disturbance. Tropical disturbances develop regularly over tropical waters. Only a small percentage of these disturbances, however, evolve into hurricanes. According to a study conducted by examining satellite photos, only 50 of the 608 tropical disturbances detected in a six year period over the Atlantic Ocean grew into tropical storms, and only about half of those developed into hurricanes.
The reason that hurricanes are not more common is that hurricane formation requires a very specific set of atmospheric conditions. Some of the factors necessary for hurricane formation are similar to those necessary for thunderstorm formation. These include a warm, moist air mass and an unstable atmosphere. In the case of a hurricane, the air is warmed and made moist by ocean water that is at least 80°F (27°C). Since ocean water is stirred up by storms, this water must be warm to a depth of about 200 feet (61 meters).
Hurricane Linda sets a record
Hurricane Linda, which hit in September 1997, was the strongest hurricane ever record in the eastern Pacific. Linda was so powerful that meteorologists proposed adding a new category, Category 6, to the Saffir-Simpson Scale. The scale is used to measure destructive potential; it presently goes up to 5. Linda packed sustained winds of 185 miles (298 kilometers) per hour with gusts greater than 200 miles (322 kilometers) per hour. By way of comparison, Hurricane Andrew, which did extensive damage to Florida in 1992, had maximum sustained winds of 140 miles (225 kilometers) per hour.
Linda's record-setting winds were measured on September 12, when the hurricane was still about 500 miles (800 kilometers) south of Mexico's Baja California Peninsula. Luckily, the hurricane took a northwesterly route and remained at sea. The hurricane was about 1,000 miles (1,600 kilometers) west of Baja's southern tip when it broke up on September 18. Linda's only effects on land were huge waves that pounded the Baja Peninsula, the west coast of mainland Mexico, and coastal Southern California.
Hurricane formation also requires that the air, from the surface up to about 18,000 feet (5,500 meters), be extremely humid. The higher the temperature of the ocean water, the more water evaporates. That, in turn, raises the humidity of the surface air. The warm surface air cools as it rises and the moisture within it condenses, or turns into water. When moisture condenses, it releases latent heat, the heat removed from a quantity of water vapor to cause it to turn into a liquid, and that heat provides additional energy to the storm system.
Another necessary ingredient for the formation of a hurricane is surface winds that are converging, or blowing toward a common point. Where winds converge at the surface, air rises. A number of events can trigger surface air convergence. One is that a front, a leading edge of an air mass of warm or cold air, moves into the tropics from the middle latitudes. By the time the front reaches the tropics, the temperatures on either side of the front have equalized. However, the front still has an associated low pressure area aloft. The winds at the surface converge to a point beneath that low pressure area and rise.
Another source of convergence of surface winds is the meeting of the trade winds from the north and the south, along the intertropical convergence zone, or ITCZ. Convergence at the ITCZ promotes hurricane development only during the of winter and summer when the ITCZ is located furthest from the equator. At other times of year, when the sun is directly over the equator, the ITCZ also sits very close to the equator. At those times the ITCZ does not experience the Coriolis effect. Once the ITCZ shifts to a point 4 or 5 degrees from the equator, however, the Coriolis effect is apparent.
A third possible source of converging winds is an easterly trade wind that flows in a wavelike fashion from Africa. As the trade wind proceeds westward, a low pressure trough may flow over the tropical waters, triggering surface winds to converge beneath it.
Hurricane formation also requires ideal conditions in the winds aloft. Namely, the winds at all altitudes need to be light and blowing in approximately the same direction and speed, so as not to scatter the moisture and dissipate the developing storm. Hurricane formation is most likely to occur when the upper air is cold, a factor that contributes to the low pressure.
Most often, there is some condition in the winds aloft that prohibits hurricane development. For instance, in some latitudes, particularly between 20 and 30 degrees, the upper-level air sinks. It warms as it falls, creating an inversion, or reversal of the normal temperature pattern, that prevents the upward development of thunderstorms.
Another condition that works against hurricane formation is a relatively dry middle layer of air. The amount of latent heat necessary to generate a hurricane requires condensation to occur at all heights throughout the system. Moisture will not condense until the relative humidity of the air, which is the amount of water vapor in an air mass for a particular temperature, has reached 100 percent.
How do hurricanes unfold?
Let's assume that the following atmospheric conditions exist: warm, humid surface air; cold air and low pressure aloft; converging surface winds; and light winds and high humidity at all altitudes. The stage has been set for the development of a hurricane.
The most widely accepted model of how a hurricane forms is called the Organized Convection Theory. According to this theory, the first step in the development of a hurricane is the formation of large thunderstorm clouds. That proceeds as follows: As warm air rises to the region of low pressure above, condensation occurs. Latent heat released during condensation warms the air at greater and greater heights. Eventually, the cold layer of upper air also becomes warm. As a result, its air pressure increases.
When the low pressure region aloft becomes transformed into a high pressure region, the thunderstorm ceases to develop upward. Rather, at that height air diverges outward, away from the high pressure area at the center of the cloud. The air flows to the edge of the cloud and then sinks back to the surface.
As the same time that the pressure aloft rises, the pressure at the surface falls. It is the formation of the surface low, at the center of the storm, that is the most essential factor in hurricane development. The winds begin to circulate in a counterclockwise pattern (in the Northern Hemisphere) around the center of, but not directly into, this surface low. At the center of the storm, the air sinks directly from the high pressure area above to the low pressure, below.
Extreme weather: Sea Islands storm
On the night of August 27, 1893, a fierce hurricane laid waste to the South Carolina Sea Islands off the coast of Beaufort, South Carolina. Some two thousand (some sources claim as many as five thousand) islanders perished in the storm. Virtually all survivors on the islands were left homeless and penniless.
The primary reason the hurricane took so many lives was not that it was an unusually large storm, but because the islanders had no advance warning and no way to escape. The National Weather Service, which had issued a hurricane warning to mainlanders, could not get word to the islanders for lack of telegraph or telephone links.
Most of the island's thirty thousand residents were former slaves. Prior to the Civil War (1861–1865), the slaves had worked on plantations on the islands. Near the start of the war, the plantation owners had fled, leaving behind their former slaves. The plantations were then divided, and the newly freed slaves were given small pieces of land. In the years since the end of slavery, the islanders had built thriving communities.
The hurricane generated tides nearly 20 feet (6.1 meters) above average sea level and winds of 120 miles (193 kilometers) per hour. After the storm passed, survivors found a grisly scene of death and destruction. Bodies littered the beaches and marshes. The destruction of buildings and crops was near total. Most sources of fresh water had been contaminated with sea water. Virtually all animal life on the island had died.
News of the islanders' plight reached the mainland some two days after the hurricane. It took nearly a month for a large-scale assistance campaign to begin. Until that time, island residents survived mainly by eating berries. The American Red Cross coordinated the relief effort under the direction of its founder, Clara Barton. Arriving in the area on September 30, Barton organized deliveries of food, clothing, medicine, and supplies to the islands. Many African American Civil War veterans also volunteered their services in the relief effort. By the following July, reconstruction of the Sea Islands communities was nearly complete.
The winds increase in speed the closer they get to the center. In the process, they generate large ocean waves. These waves create friction on the wind, interrupting the air flow and causing the air to converge. Where the air converges, it rises, carrying moisture and warmth upward to form new thunderstorms.
WORDS TO KNOW
- Organized Convection Theory:
- a widely accepted model of hurricane formation.
- pressure gradient:
- the difference in air pressure between a high and low pressure area relative to the distance separating them.
- rain band:
- a band of heavy thunderstorms forming a tightly coiled spiral around the center of a tropical storm.
- relative humidity:
- the amount of water vapor in an air mass relative to the amount of water in a saturated air mass of the same temperature.
- Saffir-Simpson Hurricane Damage Potential Scale:
- a scale devised by Herbert Saffir and Robert Simpson intended to be used to predict a hurricane's destructive potential.
- squall line:
- a moving band of strong thunderstorms.
- storm surge:
- an abnormal rise of the sea over and above normal tides due to strong winds and low pressure accompanying a storm or hurricane.
- a rainstorm accompanied by thunder and lightning and produced from a cumulonimbus cloud.
- a violently rotating column of air that reaches the ground and is attached to a cumulonimbus cloud; it is nearly always observable as a "funnel cloud."
- trade winds:
- an area near the equator of prevailing winds that blow from the northeast north of the equator and the southeast south of the equator.
- tropical cyclone:
- any rotating weather system that forms over tropical waters.
- tropical depression:
- a storm with rotating bands of clouds and thunderstorms and maximum sustained winds of less than 38 miles (61 kilometers) per hour.
- tropical disturbance:
- a cluster of thunderstorms that is beginning to demonstrate a cyclonic circulation pattern.
- tropical storm:
- a storm with organized rotating bands of strong thunderstorms and maximum sustained winds between 39 to 73 miles (63 to 117 kilometers) per hour.
- tropical wave:
- an elongated area of low air pressure, oriented north to south, causing areas of cloudiness and thunderstorms.
- the region of Earth between 23.5° north latitude and 23.5° south latitude.
- the lowest atmospheric layer, where clouds exist and virtually all weather occurs.
- tropical cyclone that form in the China Sea or in the western North Pacific Ocean.
- warm front:
- the leading edge of a moving warm air mass.
- an area at middle latitudes of prevailing winds that blow toward the northeast in the Northern Hemisphere.
The rising air increases the air pressure aloft. As a result, the surface air pressure becomes even lower. A chain reaction is set in motion: new thunderstorms are formed, the upper-level high pressure area becomes higher, and the surface low pressure area becomes lower. Each process drives the other as the storm grows into a mature hurricane.
A hurricane continues to grow as long as a fresh supply of warm, humid air is available. Once the hurricane crosses over colder waters or land, its source of energy is cut off, and it begins to dissipate.
As long as more air flows out from the top of the storm center than flows into the storm from the surface, the hurricane continues to intensify. The point at which that trend reverses the hurricane begins to die.
The level of air flow is a indication of air pressure. A rapid outflow at the top of the hurricane indicates that pressure is high at upper levels and, consequently, low at the surface. However, when the outflow slows, the pressures at the top and bottom of the storm center begin to equalize. In the absence of a strong surface low, the winds weaken. Then there is nothing to support the coiled organization of thunderstorms. The system unwinds and individual thunderstorms dissipate.
How long do hurricanes last?
The life cycle of a typical hurricane is about thirteen days. It starts out on day one as a cluster of thunderstorms, a condition that meteorologists call a tropical disturbance. By day three, the thunderstorms have become organized into bands that swirl about a low-pressure center. The system, at this point, is called a tropical depression.
The surface air pressure drops, the winds intensify, and the storm continues to grow. By day five, winds are blowing faster than 39 miles (63 kilometers) per hour. At this stage, the system is upgraded to a tropical storm. By day seven winds are blowing faster than 74 miles (118 kilometers) per hour. The storm is now classified as a hurricane.
For the next few days, as the hurricane moves across the warm water, it maintains its strength and integrity. About day twelve it crosses onto land and weakens. By day thirteen it has dissipated.
How do hurricanes move across the world?
Hurricanes that form over the northern Pacific and northern Atlantic oceans are guided to the west-northwest by easterly trade winds. These winds blow a hurricane along at about 10 miles (16 kilometers) per hour. Once a hurricane encounters the Azores-Bermuda High, the semipermanent high pressure system in the eastern Atlantic, it is directed to the northwest, through the Caribbean and toward the East Coast of the United States.
If the hurricane travels so far northward that it reaches the middle latitudes, it will be steered to the northeast by the westerlies, which are prevailing winds in the middle latitudes that blow towards the northeast in the Northern hemisphere. The westerly winds blow the hurricane along at about 55 miles (88 kilometers) per hour, which is significantly faster than the trade winds. This faster movement at higher latitudes makes it more challenging to get advance warning to communities in the hurricane's path.
While the trade winds, Azores-Bermuda High, and westerlies establish a general hurricane route, the specific path taken by a hurricane is much more difficult to predict. The specific path depends on the structure of the hurricane and how it interacts with its environment. Exactly how these factors influence hurricane movement is not well understood. The path also depends on the size and location of the Azores-Bermuda High, which varies over the course of the year.
Hurricane Katrina formed over the southeastern Bahamas as Tropical Depression Twelve on August 23, 2005. It formed from a combination of a tropical wave (a north-south trending low-pressure area) and the remnants of Tropical Depression Ten. The system strengthened to a tropical storm on August 24 and was given the name Katrina. The tropical storm headed towards Florida and became a hurricane only two hours before its first landfall between Hallandale Beach and Aventura, Florida, on the morning of August 25.
Like most storms, Katrina weakened as it passed over land, but quickly regained hurricane status after entering the warm waters of the Gulf of Mexico. On August 27, the storm reached Category 3 intensity on the Saffir-Simpson Hurricane Scale, becoming the third major hurricane of the 2005 season. Katrina continued to intensify and reached Category 5 status on the morning of August 28. It attained its maximum intensity at 1 pm Central Daylight Time on August 28, with maximum sustained winds of 175 miles (280 kilometers) per hour and a minimum central pressure of 902 millibars. The pressure measurement made Katrina the fourth most intense Atlantic hurricane on record at the time and the most intense storm ever recorded in the Gulf of Mexico. However, it held these records for only a short time. Both records were broken by Hurricane Rita a few weeks later.
Katrina weakened in intensity as it approached land but remained a dangerous storm. It made its second landfall on the Louisiana coast southeast of New Orleans at 6:10 am CDT on August 29 as a Category 3 hurricane with sustained winds of 125 miles (205 kilometers) per hour. At landfall, hurricane-force winds extended outward 120 miles (190 kilometers) from the center and the storm's central pressure was 920 millibars After moving across the Mississippi Delta and over Breton Sound, Katrina made its third landfall near the Louisiana-Mississippi border with sustained winds of 120 miles (195 kilometers) per hour—still a Category 3 storm.
Katrina maintained its status as a hurricane until it moved over 150 miles (240 kilometers) inland. It was finally downgraded to a tropical storm as it passed near Jackson, Mississippi, and to a tropical depression near Clarksville, Tennessee. The remnants of Katrina were still recognizable as it entered the Great Lakes region on August 31. It was then absorbed by a frontal boundary and became an extratropical cyclone. This storm then moved into Ontario and Quebec where it caused additional flooding and isolated several small villages.
Some hurricanes follow a smooth course, while others move erratically, shifting direction suddenly and inexplicably. For instance, some hurricanes that seem certain to spare the U.S. Atlantic Coast will turn to the west and crash onto shore. On the other hand, coastal communities may brace for a hurricane's onslaught only to be spared at the last minute as the storm turns back to sea.
Hurricanes that form over the eastern Pacific, off the west coast of Mexico, generally move to the west or northwest and travel over the ocean until they dissipate. That is the reason we hear so little about those hurricanes. Occasionally, however, a Pacific hurricane will turn to the north or northeast and strike the west coast of Mexico. A handful of such hurricanes have devastated Mexican coastal communities over the past several decades. The remnants of those hurricanes have brought heavy rains and flooding to the U.S. Southwest and West Coast.
One would expect Hawaii to suffer from a large number of hurricanes, given that Hawaii lies in the path of hurricanes formed off the west coast of Mexico. But that is not the case. As it turns out, Hawaii is located far enough to the west that by the time most hurricanes reach it, they have been significantly weakened.
There have been exceptions to this rule, however. About once a decade, Hawaii is struck by a major hurricane. A notable hurricane in recent history is Hurricane Iniki, which hit the island of Kauai on September 11, 1992. Iniki caused $1.8 billion worth of damage and seven deaths.
The hurricanes that directly affect the United States are typically those formed over the tropical North Atlantic, the Caribbean, or the Gulf of Mexico. On average, six hurricanes form over those waters during each hurricane season, two or three of which strike the U.S. Atlantic or Gulf coasts. The primary factor that determines whether a hurricane hits the United States is the location at which the hurricane moves to the northeast. If a hurricane is still over the ocean when it changes course from northwest to northeast, it will miss the coast. However, some hurricanes move over land before they change direction.
The U.S. West Coast is rarely struck by hurricanes, because storms that form in the eastern Pacific are normally blocked by cold ocean currents. In September of 1932 a hurricane moved up the Gulf of California, producing gusty winds and heavy rainfall in the Arizona desert. In September of 1939, a tropical storm slammed into San Diego with winds of 52 miles (84 kilometers) per hour. In September 1976, a hurricane gusted to 76 miles (122 kilometers) per hour at Yuma, Arizona.
When a hurricane moves onto land, it is capable of causing tremendous damage. A hurricane's winds are assumed by most people to be its most destructive element, but this is often not the case. The winds do cause a great deal of damage, but floods caused by ocean swells and torrential rains cause the most hurricane damage. Floodwaters, both in coastal and inland areas, account for about 90 percent of hurricane fatalities. Another destructive force of hurricanes is the tornadoes that hurricanes spawn in their final stages.
In coastal areas, most of the property damage, as well as about 90 percent of deaths, are due to the hurricane's storm surge. A storm surge is a wall of water that sweeps on shore when the eye of a hurricane passes overhead. Storm surges range from 3 to 6.5 feet (1 to 2 meters) in a weak hurricane, to over 16 feet (5 meters) in a strong hurricane. A storm surge affects a stretch of coastline between 40 to 100 miles (65 to 160 kilometers) long. It levels any structure in its path. When squeezed into narrow channels, a storm surge produces flooding of inland bays and rivers.
The abnormal swelling of the ocean that produces a storm surge is caused by the combined effects of a storm's pressure gradient and high winds. In the open ocean, strong winds and high pressure around the edges of the hurricane push down on water, lowering its level. The displaced water flows toward the center of the storm, where pressure is lowest. The water at the center rises and then spirals downward to about 200 feet (60 meters) beneath the surface, where it flows outward.
When the storm moves into shallow waters, there is no place for the mound of water beneath the storm center to descend. Thus, the water there is forced to pile upward. The mound of water reaches its greatest height where it crashes onto shore. The storm surge is highest in strong hurricanes, especially where the ocean floor slopes gradually to the shore.
The largest storm tide in recent U.S. history was 25 feet (8 meters). It was caused by Hurricane Camille in August 1969, which made landfall in Pass Christian, Mississippi. The storm tide destroyed more than 5,500 homes. In addition, it damaged around 12,500 homes and 700 businesses. The area of greatest destruction was along a 60 mile (97 kilometer) stretch of coastline in Mississippi, Alabama, and Louisiana. As far as 125 miles (201 kilometers) away, the water level was 3 feet (.9 meters) higher than usual.
The largest recorded storm surges have occurred in other parts of the world. In 1737, a storm surge estimated to be 40 feet (12 meters) tall struck the Bay of Bengal, killing more than three hundred thousand people. A storm surge of 42 feet (13 meters) inundated Bathurst Bay, Australia, in 1899.
The destructive capabilities of a storm surge are the result of two factors: the density of the sea water, which is about 64 pounds per cubic foot (1030 kilograms per cubic meter); and the debris that the water sweeps along. Debris may include boats that were ripped from their moorings by the waves, pieces of destroyed buildings, trees, and sand. As the water plus this debris moves farther onshore, it batters and flattens anything in its path. A storm surge also destroys structures such as buildings, roads, and sea walls by eroding the sand and soil beneath them, causing them to buckle or collapse.
|Ten Deadliest Hurricanes to Strike the United States Since 1900|
|Florida (Lake Okeechobee)||1928||4||2,500-3,000|
|Katrina (Lousiana, Mississippi)||2005||3||1,836|
|Florida (Keys and S. Texas)||1919||4||600|
|Audrey (Louisiana and Texas)||1957||4||390|
|Louisiana (Grand Isle)||1909||3||350|
|Louisiana (New Orleans)||1915||4||275|
A storm surge may also have the secondary effect of causing flooding of inland bays and rivers. This flooding results when storm surge water is squeezed into narrow channels. Sometimes inland waters rise even higher above normal levels than coastal waters in the wake of a storm surge.
The winds of a hurricane also cause a significant amount of damage to coastal areas. Hurricane winds range between 75 and 180 miles (120 and 290 kilometers) per hour. Winds of this strength can damage buildings and homes and knock down trees and telephone poles, as well as cause beach erosion. A hurricane's winds can totally demolish lightweight structures such as mobile homes and poorly constructed buildings.
Part of wind damage is due to objects that are picked up and hurled through the air. Shingles, metal siding, road signs, and any items left outdoors become deadly missiles during a hurricane.
Most wind damage from a hurricane occurs within 125 miles (200 kilometers) of the coast. Once a hurricane travels farther inland, it generally begins to weaken. Occasionally a hurricane will retain its strength for greater distances. In 1989, for example, Hurricane Hugo ripped through Charlotte, North Carolina, with winds gusting to 100 miles (160 kilometers) per hour. Charlotte is about 175 miles (280 kilometers) inland.
Heavy rain and flooding
A hurricane's destruction is certainly not limited to coastal areas. To the contrary, for hundreds of miles inland, and for several days after the hurricane-strength winds have died down, the storm may continue to produce torrential rains and flooding. When an area receives more than 6 inches (15 centimeters) of rain, flooding is likely. Hurricanes typically drop 5 to 10 inches (13 to 25 centimeters) on the land in their path. Some hurricanes have produced more than 25 inches (63 centimeters) of rain in a period of twenty-four hours.
Inland flooding is the most destructive element of some hurricanes. Examples of this include:
- Hurricane Diane in 1955 brought rains and flooding to Pennsylvania, New York, and New England. The flooding caused nearly 200 deaths and $4.2 billion in damage.
- Hurricane Camille in 1969 brought 9.8 inches (25 centimeters) of rain to Virginia's Blue Ridge Mountains. The storm resulted in 150 deaths.
- Tropical storm Claudette in 1979 dumped 45 inches (114 centimeters) of rain outside of Alvin, Texas, resulting in over $600 million in damage.
Heavy rains due to hurricanes are not always harmful. These rains may spell relief for regions with parched soil and withering crops. In some cases, the value of saved crops in one region is greater than the value of property destroyed by the flooding in another region. In some areas of the world, particularly in the Far East, farmers are dependent on annual hurricane rains for their economic survival.
Tornadoes are another hazard of hurricanes. About one-quarter of all hurricanes that come on shore in the United States produce tornadoes. A single hurricane, on average, spawns ten tornadoes.
The thunderstorms embedded in the outer regions of a hurricane are the most likely to spawn tornadoes, although tornadoes also form in thunderstorms close to the eye wall. The greatest number of tornadoes are produced in the portion of the hurricane that is northeast of the eye.
Theodore Fujita, a tornado specialist from the University of Chicago, is the author of a recent theory that the greatest damage from hurricane-induced tornadoes is caused by small funnels called spin-up vortices. These vortices are each 9 to 30 feet (3 to 10 meters) in diameter and last only about ten seconds. The hurricane winds combine with the tornado winds, so the vortices produce winds of around 200 miles (320 kilometers) per hour.
Ranking hurricanes by strength
As with tornadoes, hurricanes are ranked according to their strength. However, there is one main difference: Tornadoes are classified after they have struck an area based on the actual damage they have created, while hurricanes are categorized before they strike land based on their potential damage to coastal areas.
Each hurricane is placed into one of five categories according to its strength, described in the Saffir-Simpson Hurricane Damage Potential Scale. This scale was developed in the early 1970s by Robert Simpson, then director of the National Hurricane Center, and Herbert Saffir, an engineer who designed Miami's hurricane-proof building code.
According to the Saffir-Simpson scale, hurricanes in Category 1 are the weakest and hurricanes in Category 5 are the strongest. The factors that determine a hurricane's strength include: air pressure at the eye of the storm; range of wind speeds; potential height of the storm surge; and the potential damage caused. The categories of potential damage are defined as follows: 1) minimal; 2) moderate; 3) extensive; 4) extreme; and 5) catastrophic. A hurricane's ranking is upgraded or downgraded as it goes through its stages of development.
Saffir-Simpson hurricane intensity scale
Most of the hurricanes that strike the United States are Category 1 or 2. Only two hurricanes ranked as Category 3 or higher strike the United States every three years, on average.
Of the 126 tropical storms or hurricanes that hit the United States between the years 1949 and 1990, only 25 were Category 3 or higher. Those twenty-five storms, however, caused three-quarters of all property damage done by tropical storms or hurricanes during that period.
Category 5 hurricanes are the rarest kind—the United States only experienced three of them during the entire twentieth century. The first was a hurricane on Labor Day, 1935 (the practice of hurricane naming did not begin until 1953), the second was Hurricane Camille in 1969, and the third was Hurricane Allen in 1980. (The hurricane that leveled Galveston, for example, would have been listed as a Category 4.)
People have always settled in hurricane-prone regions. This creates great risk for property damage when hurricanes strike. The effects of global warming, reportedly made worse by pollution, may be having an effect on hurricanes. The world—including the oceans—has been warming at a dramatic rate in recent years. Many scientists claim that as temperatures continue to rise, we can expect hurricanes to increase in number and size.
For people who live in areas affected by hurricanes, it is essential to understand and follow safety procedures. That includes making preparations before the hurricane season begins, knowing what to do when a hurricane watch or warning is issued, and how to respond once a hurricane has passed.
To prepare for hurricane season, you must understand the risks that hurricanes pose to your area, learn the evacuation routes inland, and find out where emergency shelters are located. It is also crucial to develop a safety plan for your family and keep a disaster kit on hand. A kit should include the following for each person: nonperishable food; three gallons of bottled water; one change of clothing and footwear; one blanket or sleeping bag; first aid kit; flashlight, radio, and batteries; extra set of car keys; credit card or cash; and diapers for infants.
The U.S. government's failed Katrina response
The destruction caused by Hurricane Katrina shocked the world. Almost as surprising, though, was the state and federal government's slow and awkward response to the disaster. Thousands of people were stranded for days on rooftops and highway overpasses, and in homes, hospitals, the Superdome, and the convention center. Food, water, and basic supplies were delayed for days. Many people died while a horrified nation looked on.
The first failures in dealing with Hurricane Katrina were failures of prevention and readiness. Experts from the U.S. Army Corps of Engineers had long predicted that the New Orleans levee system would not withstand a "megahurricane." For years the Army Corps had been asking the federal government for money to bolster the levees, but they would have needed to have begun the work long before in order to prevent the Katrina breaches.
Once the storm neared land, the government urged evacuation. However, the evacuation plan left thousands behind. It is estimated that as many as one-third of the citizens of New Orleans did not own a car, and thus many of those people were left without a way to flee.
The Federal Emergency Management Agency, or FEMA, which falls under the department of Homeland Security, is the primary agency responsible for handling a disaster like Katrina. In the days following the hurricane, President George W. Bush praised FEMA, saying they were doing a "heck of a job." However, the realities of the situation in New Orleans were bleak. People were desperate and starving, and the city began to descend into a state of civil unrest. President Bush decided to survey the area by airplane, flying low over the Gulf Coast Region so he could get a first hand look at the devastation. By Friday of that week, Bush declared that government response was "not acceptable." That same week Congress called for an investigation of the botched response to the disaster. Further investigation revealed that FEMA chief Michael Brown was possibly not qualified to lead such a major disaster relief effort.
Other parts of the relief efforts were troubled as well. On Thursday after the hurricane, Homeland Security Chief Michael Chertoff claimed on National Public Radio to be unaware of the sickness, hunger, and unrest at the Superdome. State and local agencies were at times reluctant to cooperate with FEMA, and communication failures abounded. One of the major lessons of Hurricane Katrina was that federal, state, and local government entities need to improve their cooperation and preparedness for major hurricanes like Katrina.
The part of the United States that experiences the greatest number of hurricanes is the Florida Keys. Forty-three hurricanes have hit the Keys in the last one hundred years. The places where hurricanes pose the greatest threat to humans, however, are the barrier islands that exist along portions of the Atlantic Coast and Gulf Coast. In the United States, people place themselves in the path of danger by settling in those areas most vulnerable to hurricane damage—the barrier islands.
The barrier islands are long, narrow strips of sand that run parallel to a 2,000-mile-long (3,200-kilometer-long) Atlantic and Gulf coastline from New York to Texas. Examples of barrier islands include New York's Fire Island, Texas's Galveston Island, North Carolina's Outer Banks, and the Sea Islands off the coasts of South Carolina and Georgia. Barrier islands, which stand only 5 to 10 feet (1.5 to 3 meters) above sea level, bear the brunt of a hurricane's wind and waves and provide the mainland with a buffer from the storm. The islands respond to hurricane beatings by moving—the sand actually becomes redistributed by the waves.
Despite the fact that living on the islands is known to be dangerous, people continue to settle there. In fact, in recent years the islands have seen a huge increase in construction of housing, hotels, and businesses geared toward tourism. In some parts of the world, people have no choice but to live in hurricane-prone regions. Bangladesh, for instance, is a magnet for hurricanes (called cyclones in that region of the world). Its coastal region endured seven of the nine deadliest hurricanes of the twentieth century. Bangladesh is also a densely populated, desperately poor country. For residents of the coast, moving inland would mean squeezing into already overflowing urban slums. But in the United States, people happily choose to settle on hurricane-prone barrier islands. Drawn by the beauty of the ocean, they pay top dollar for beach residences and flock to the islands for their vacations.
Before 1940, only 10 percent of Atlantic and Gulf Coast barrier islands held houses or hotels. Development steadily increased from that time. Many islands today are wholly buried under concrete and buildings.
Hurricane Fran, in 1996, battered North Carolina's southeastern barrier islands. Houses, cottages, condominiums, and cars were strewn across the beach and washed out to sea. Twenty-four people died in the storm, and property damage totaled $2 billion. The following week, people began rebuilding on those islands. Barrier island development is such an attractive business proposition, in part, because U.S. taxpayers—through the National Flood Insurance Program, the Federal Emergency Management Agency's Disaster Relief Program, and the Army Corps of Engineers' shoreline stabilization projects—regularly pay for storm damage.
Another danger specific to barrier islands is that once a hurricane warning has been issued, evacuation is difficult. Since construction of evacuation routes has not kept pace with the islands' population growth, bridges connecting the islands to the mainland become choked with traffic.
Hurricane watches and warnings
The ability of weather forecasters to warn the public of potential hurricane strikes has greatly increased in recent decades. Some of the sophisticated technologies that are now used to detect and track tropical cyclones include radar, weather satellites, and weather aircraft. Although the erratic nature of hurricanes makes it impossible to predict exactly when and where they will strike, forecasters can now guess at the answers to these questions with a fair degree of certainty.
When a hurricane threatens a coastal area, hurricane watches and warnings are issued. If the hurricane is predicted to be life threatening, the residents of that area are evacuated. Hurricane watches and warnings are issued by the Tropical Prediction Center (formerly the National Hurricane Center), a branch of the National Weather Service.
A hurricane watch is issued when a hurricane is headed in the general direction of an area. It means that hurricane conditions are possible in the area. Hurricane watches are announced at least thirty-six hours, and sometimes several days, in advance.
If a hurricane is poised to strike an area within twenty-four hours, a hurricane warning is issued. It means that hurricane conditions are expected in the area. For each community within the warning area, meteorologists will also issue the probability of the hurricane's center coming within 65 miles (105 kilometers) of that community. The probability is intended to give residents an idea of the type of damage likely to occur in their area. It is also considered in the decision as to whether or not evacuation is necessary.
This distance is about three times the area that will actually be affected once a hurricane comes onshore. The reason the warning area is so large is that it is possible for a hurricane to change course at any time, making it impossible to predict the exact point at which the hurricane will make landfall.
For people who live in areas affected by hurricanes, it is crucial to know and follow certain safety procedures. These procedures include making preparations before the hurricane season begins; knowing what to do when a hurricane is in progress; and taking certain steps after the hurricane has passed.
For more information about the dangers hurricanes pose to your area and hurricane preparedness, contact your local office of the National Weather Service, American Red Cross, or Federal Emergency Management Agency.
Hurricane Andrew: The costliest natural disaster of the twentieth century
In August 1992, Hurricane Andrew struck Florida and Louisiana, causing fifty-eight deaths and $30 billion in damage. Over 200,000 homes and business were damaged or destroyed, and 160,000 people became homeless. Andrew, which made landfall as a Category 4 hurricane, was the costliest natural disaster of the twentieth century in the United States.
On August 21, Andrew, which was then classified as a tropical storm in the Atlantic, appeared to be weakening. However, it then moved over warmer waters and rapidly gained strength. On August 21, its winds were only 52 miles (84 kilometers) per hour. Two days later, its winds had increased to 140 miles (225 kilometers) per hour, and Andrew had developed into an intense hurricane.
Andrew came on shore at Homestead, on the southern tip of Florida, on August 24. With winds that peaked at about 200 miles (320 kilometers) per hour and a storm surge 16.9 feet (5.1 meters) tall—a record for Florida—Andrew devastated the town. It leveled trees, utility poles, and fifty thousand homes.
Andrew then traveled westward, over land, and into the Gulf of Mexico. While Andrew had weakened during its journey across the land, it regained strength over the Gulf's warm waters. On August 25, Andrew blew into Louisiana with winds of 138 miles (222 kilometers) per hour. There, Andrew continued its relentless destruction of property. Andrew's death toll was relatively low due to the efficiency of prediction and warning systems. Over 1 million people in Florida and 1.7 million people in Louisiana and Mississippi were evacuated from areas in the storm's path. Had Andrew occurred in the first half of the 20th century, before the development of sophisticated hurricane-detecting technology, the death toll would have certainly been much higher.
What to do when you are within a hurricane watch area:
- Stay tuned to radio or television reports of the storm's progress.
- Fill your car with gas and get cash, if needed.
- If you live in a mobile home, make sure it's securely fastened, then evacuate.
- Cover all windows and doors with shutters or plywood.
- Check your supply of nonperishable food and water.
- Gather first aid materials and medications.
- Bring lawn furniture, garbage cans, garden hoses, and other lightweight items inside.
- If you have a boat, be sure it is properly secured.
- Evacuate if you live in a mobile home or high-rise, on the coastline, on an offshore island, or near a river or flood plain.
What to do when you are within a hurricane warning area:
- Stay tuned to radio or television reports of the storm's progress.
- Finish covering your windows and doors and prepare your home for evacuation.
- Evacuate immediately upon the orders of local officials and travel inland, to the home of a friend or relative, a low-rise motel, or an emergency shelter.
- Notify someone outside of the hurricane warning area of your evacuation plans.
- If you have pets that you are unable to take with you, leave them plenty of food and water.
If you are staying at home:
- Fill the bathtub and containers with drinking water, unplug small appliances, turn off propane tanks, and turn your refrigerator to its coldest possible setting.
- In case of strong winds, close all outside and inside doors and go into a small interior room or hallway on the first floor, away from windows and doors. If possible, crouch beneath a sturdy piece of furniture.
What to do after the hurricane:
- Stay tuned to the radio or television for information.
- Don't return home until your area has been declared safe.
- Don't attempt to drive around a barricade; if you encounter one, turn around and take a different route.
- Don't drive on roads or bridges in flooded areas or on washed out roads.
- Inspect your gas, water, and electrical lines for damage before using.
- Be sure that your tap water is not contaminated before drinking or cooking with it.
- Make as few calls as possible so you don't tie up phone lines.
Before 1950, hurricanes were identified primarily by their latitude and longitude. This practice became confusing as hurricanes moved about, especially if there was more than one hurricane at the same time on the same ocean.
In 1950, meteorologists began the practice of assigning names to all hurricanes and tropical storms that formed in the western North Atlantic, Caribbean, and Gulf of Mexico. They began naming eastern Pacific storms in 1959. From 1950–53, names were taken from the international radio codes words that corresponded with letters of the alphabet. For instance, the first three letters—"a," "b," and "c"—had the names Able, Baker, and Charlie.
In 1953, meteorologists began giving the storms female names. The names were assigned in alphabetical order, starting with the "As" for each new season. Since 1978 in the eastern Pacific, and 1979 in the northern Atlantic, male names, as well as names in French and Spanish, have also been used.
Names are now assigned in advance for six-year cycles. The names are submitted by countries that lie in the path of hurricanes and must be approved by the Region 4 Hurricane Committee of the World Meteorological Organization, which is made up of representatives of countries affected by hurricanes.
After the six-year cycle has ended, the names are reused, except for names that are retired. The names of hurricanes that cause extensive damage or loss of life, such as Gilbert, Isabel, and Andrew, are removed from the list for at least ten years. Katrina was retired from the list after the 2005 season and was replaced by Katia. A total of five names—Dennis, Katrina, Rita, Stan, and Wilma—retired in 2005 breaks the previous single-season record of four retired names, reached in 1955, 1995, and 2004.
The list for the tropical North Atlantic normally includes twenty-one names. The letters Q, U, X, Y, and Z are not used. In a typical year, twenty-one names are more than enough. But 2005 set a record for the
|Names for Hurricanes Through the Year 2011|
|Eastern Pacific Hurricane Names|
|North Atlantic Hurricane Names|
number of named storms. The letters V and W were used for the first time (for Hurricanes Vince and Wilma). Even those were not enough. When the names in a list are exhausted, the World Meteorological Organization specifies that letters of the Greek alphabet be used. For the 2005 season, six letters were necessary. The twenty-eighth and last named storm of 2005 was Tropical Storm Zeta, which formed on December 29, well past the normal end of the hurricane season.
Presently, each hurricane-producing region of the world (except the northern Indian Ocean, where cyclones are not named) has its own lists of names, drawn up years in advance. Each storm is automatically assigned the next name on the alphabetical list.
The ability of weather forecasters to warn the public of potential hurricane danger has greatly increased in recent years. Forecasters are presently able to predict, within 200 to 250 miles (320 to 400 kilometers), where a storm will be three days in the future. Forecasters at the National Hurricane Center in Miami issued their most accurate prediction yet in September 1999, when they stated that Hurricane Floyd would spare Florida and head north before striking North Carolina. As it was, Floyd missed the Florida coast by a mere 50 miles (80 kilometers).
Predictions of where a hurricane will hit land are used by public officials when calling for evacuations. Forecasters want to be as certain as possible in their predictions before recommending evacuations. If an area is evacuated only to be spared by the hurricane, residents become less likely to heed evacuation notices in the future. That attitude is perilous, since the next evacuation could be a matter of life and death. Forecasters also wish to avoid unnecessary evacuations because they are very costly. Each day of evacuation represents $1 million of lost business revenues for every mile of coastline.
Watch this: When the Levees Broke: A Requiem in Four Acts
Director Spike Lee created this wrenching 2006 HBO (Home Box Office) documentary about Hurricane Katrina and its aftermath using new footage and interviews with government official and survivors. The documentary includes very little narration. Instead, Lee lets the images and the people tell the story themselves.
Two of the most useful instruments used by hurricane researchers to detect and track tropical storms are weather aircraft and weather satellites.
Weather aircraft determine the intensity of hurricanes by probing storm clouds in upper levels of the atmosphere. They measure temperature, air pressure, and wind speed and direction. These planes have reinforced wings and bodies, in order to withstand the hail, ice, and strong winds they encounter within the clouds. The weather instruments are carried in pods beneath the plane's wings or attached to its nose cone.
In 1996, the U.S. National Oceanic and Atmospheric Association (NOAA) acquired a hurricane research jet, called the Gulfstream IV-SP. The jet can cruise right through these storms at heights of up to 45,000 feet (13,725 meters). It contains sensors that measure air pressure, temperature, humidity, and wind speed at the edges and the core of the storm. It also releases hundreds of dropwindsondes, also called dropsondes, which are instruments that transmit data on atmospheric conditions as they fall through the storm. The information collected by the jet is combined with readings taken at ground stations in order to better determine where a hurricane is headed.
Weather satellites, which circle the globe in space, provide meteorologists with pictures and other information about hurricanes and tropical storms. The first weather satellite, launched in April 1960, was TIROS 1—Television InfraRed Observation Satellite. In September 1961 weather satellites proved their value when they broadcast images of Hurricane Carla. Information from these images resulted in the nation's first widespread evacuation when 350,000 people along the Gulf Coast were removed from the path of the killer hurricane.
For most people, the words "weather satellite pictures" conjure up images of swirling clouds that are seen on television newscasts. While weather satellites do produce those photos, their function is far more extensive than that. Weather satellites determine the temperature at various atmospheric levels—from the cloud tops down to the land and oceans. They also measure humidity and wind speeds in the upper air and even track plumes of invisible water vapor and relay information from one ground station to another and pick up and transmit distress signals from vessels in the air and at sea. Today, several nations operate satellites that continuously monitor global weather.
Brinkley, Douglas. The Great Deluge: Hurricane Katrina, New Orleans, and the Mississippi Gulf Coast. New York: William Morrow, 2006.
Robinson, Andrew. Earth Shock: Hurricanes, Volcanoes, Earthquakes, Tornadoes and Other Forces of Nature. New York: W. W. Norton & Company, 2002.
Simon, Seymour. Hurricanes. New York: Harper Trophy, 2003.
Trenberth, Kevin. "Uncertainty in Hurricanes and Global Warming." Science. (17 Jun 2005): pp. 1753-1754.
"Hurricanes." FEMA for Kids. 〈http://www.fema.gov/kids/hurr.htm〉 (accessed March 9, 2007).
"Hurricanes." National Oceanic and Atmospheric Administration. 〈http://hurricanes.noaa.gov/〉 (accessed March 9, 2007).
National Hurricane Center. 〈http://www.nhc.noaa.gov/〉 (accessed March 8, 2007).
Hurricanes, called typhoons or tropical cyclones in the Far East, are intense cyclonic storms which form over warm tropical waters, and generally remain active and strong only while over the oceans. Their intensity is marked by a distinct spiraling pattern of clouds, very low atmospheric pressure at the center, and extremely strong winds blowing at speeds greater than 74 mph (120 kph) within the inner rings of clouds. Typically when hurricanes strike land and move inland, they immediately start to disintegrate, though before they do they bring widespread destruction of property and loss of life. The radius of such a storm can be 100 mi (160 km) or greater. Thunderstorms, hail, and tornados frequently are imbedded in hurricanes.
Hurricanes occur in every tropical ocean except the South Atlantic, and with greater frequency from August through October than any other time of year. The center of a hurricane is called the eye. It is an area of relative calm, few clouds and higher temperatures, and represents the center of the low pressure pattern. Hurricanes usually move from east to west near the tropics, but when they migrate poleward to the mid-latitudes they can get caught up in the general west to east flow pattern found in that region of the earth.
See also Tornado and cyclone
World Weather Watch
hur·ri·cane / ˈhəriˌkān; ˈhə-ri-/ • n. a storm with a violent wind, in particular a tropical cyclone in the Caribbean. ∎ a wind of force 12 on the Beaufort scale (equal to or exceeding 64 knots or 74 mph). ∎ fig. a violent uproar or outburst: the manager resigned in a hurricane of disagreement.
1. The name given to a tropical cyclone that develops over the North Atlantic and Caribbean. Hurricanes move westwards, then swing north, on tracks that often carry them across inhabited islands and coastal areas of Mexico and the United States.
2. A wind blowing at more than 120 km/h (75 mph), which is Force 12 on the Beaufort scale.
1. The name given to a tropical cyclone that develops over the N. Atlantic and Caribbean. Hurricanes move westward, then swing north, on tracks that often carry them across inhabited islands and coastal areas of Mexico and the USA.
2. A wind blowing at more than 120 km/h (75 mph), which is Force 12 on the Beaufort scale.