Hurricane Formation And Basis Of Operation

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

II

HOW HURRICANES FORM

Hurricane Opal Destruction

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

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

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

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

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

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

III

CHARACTERISTICS OF HURRICANES

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

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

Hurricane Storm Surge

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

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

IV

HURRICANE DEVASTATION

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

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

V

HOW HURRICANES ARE DETECTED AND MONITORED

Satellite Meteorology

military and civilian aircraft have been flying into hurricanes to measure wind velocities and directions, the location and size of the eye, air pressures, and temperatures in different parts of the storm. A coordinated system of tracking hurricanes was developed in the mid-1950s, and steady improvements have been made over the years. In addition to reports from aircraft, geosynchronous weather satellites (since 1966) and ocean buoys that automatically record and transmit data such as wave heights and wind speeds furnish information to the National Hurricane Center in Miami, Florida.

The National Hurricane Center is part of the U.S. National Weather Service and is the main forecast center for storms that originate over the Atlantic Ocean, the Caribbean Sea, the Gulf of Mexico, and the northeastern Pacific Ocean west to longitude 140° west. The Hawaiian Hurricane Center at the Honolulu National Weather Service office handles storms from longitude 140° west to longitude 180° west. Hurricanes rarely hit Hawaii. The centers of only two hurricanes moved ashore there from 1950 through 2003, although three others came close enough to cause wind or wave damage. Hurricane Iniki in September 1992 was by far the worst, killing six people and doing an estimated $2.3 billion in damages.

In the past, hurricanes often hit land without being detected beforehand. Today, weather satellites ensure that this never happens. As a storm begins to threaten land, forecasters call on military or civilian aircraft for detailed storm data that satellites cannot supply. When a storm comes within about 160 km (100 m) of land, weather radar images also become available. Forecasters use several computer models, which combine observational data from all around the world and mathematical equations, to make forecasts. But since forecasts from different models often disagree, they are merely tools to help humans make predictions.

VI

HOW HURRICANES ARE NAMED

The National Hurricane Center began officially naming tropical storms and hurricanes in 1950, although some forecasters had been informally naming storms since the 19th century. The World Meteorological Organization’s (WMO) Western Hemisphere Hurricane Committee selects hurricane names, using alternating men’s and women’s names in English, Spanish, and French in alphabetical order. Names of deadly storms or those that cause great damage are retired. Otherwise, names remain on six rotating, yearly lists, with each list being used again six years after its last use. WMO committees also select tropical cyclone names used elsewhere in the world.

VII

HURRICANE PREPAREDNESS

The National Hurricane Center issues a hurricane watch for areas where a hurricane could hit in about 36 hours or less. The center issues a hurricane warning when hurricane-force winds of 119 km/h (74 mph) are expected in 24 hours or less. The Hurricane Center issues watches and warnings for the United States and works closely with the weather services of other nations, which issue their own watches and warnings.

Residents of areas where hurricanes can strike, which includes the entire U.S. Atlantic and Gulf of Mexico coasts from Maine to the Mexican border, should begin preparing before the hurricane season starts. They need to learn whether they live in an area that storm surge could flood, and if so, need to decide where to go if ordered to evacuate. All homeowners should ensure they have covers to fit all windows and doors that are not impact resistant. A survival kit with a two-week supply of prescription medications, nonperishable food, and water should be prepared.

When a hurricane watch is issued, those in the affected area should make sure window covers and emergency kits are ready, and install the window covers that are most difficult to put in place. They should also fill vehicles with fuel and withdraw emergency cash before power failures close service stations and automated teller machines (ATMs). Finally, they should follow storm reports and listen for directions from local emergency management officials.

When a hurricane warning is issued, residents should quickly finish installing window and door protection panels. Those who are evacuating should be sure to take their emergency kits and important papers and notify friends and family where they are going. Evacuees should leave as soon as possible after turning off the main circuit breaker and the outside gas and water shut-off valves for their houses.

During a hurricane, everyone should stay indoors and away from doors and windows, even if they have protective covers. If debris begins striking the house, those inside should seek refuge in an interior bathroom, closet, or under a stairwell. People should not go outdoors when the wind dies down because the storm’s eye could be passing over and winds could quickly begin blowing again at full speed. Wait until radio announcements say that the hurricane has passed before going outside.

The danger is not over when a hurricane passes. In recent years in the United States, poststorm accidents have killed as many or more people than hurricane wind, storm surge, or flooding. Poststorm dangers include accidents at intersections without working traffic lights, downed power lines, fires caused by candles, falls from roofs, and injuries to those unskilled in using equipment such as chain saws.

VIII

HURRICANES, GLOBAL WARMING, AND CYCLES

Some evidence is emerging that hurricanes could be growing slightly stronger, possibly as a result of global warming. But most scientists who study hurricanes say that growing coastal populations and higher coastal property values, not global warming, account for the increasing costliness and loss of life caused by hurricanes.

Long-term records do not provide enough information to conclude whether the global total of tropical cyclones increased during the 20th century. But detailed records of Atlantic, Caribbean, and Gulf of Mexico hurricanes show that the numbers of storms increase and decrease in cycles. The cycles are most noticeable for “major” hurricanes in Categories 3, 4, and 5 with wind speeds faster than 177 km/h (110 mph). The years 1944 through 1969 were active with an average of 2.7 major hurricanes a year. A quiet period, with an average of 1.5 major hurricanes a year, began in 1970 and lasted through 1994. Another active period that began in 1995 saw 32 major hurricanes through the 2003 season, an average of 3.55 per year.

The 2005 hurricane season set records for the greatest number of tropical storms (26), the most hurricanes (14), and the most Category 5 hurricanes (3). Seven major hurricanes—that is, Category 3 or higher—occurred in 2005. For the first time since the National Hurricane Center began naming storms, letters from the Greek alphabet were used after the center exhausted its original list of 21 names. The last storm in the 2005 season, Tropical Storm Epsilon, formed just before the official end of the hurricane season on November 30 and was upgraded to a hurricane on December 2. Another notable hurricane event occurred in 2004 when Hurricane Catarina became the first recorded South Atlantic hurricane in history. The National Oceanic and Atmospheric Administration (NOAA), which oversees the National Hurricane Center, attributed the active 2005 hurricane season to a multidecadal cycle in which warmer-than-average sea-surface temperatures and low wind shear enhanced hurricane activity.

Many hurricane researchers think the cycles are related to changes in Atlantic Ocean temperatures that last decades. From the late 19th century through the 1980s about one-third of the major hurricanes that formed in the Atlantic hit the United States, which means around ten such hurricanes could have been expected to hit from 1995 through 2003. Yet for reasons atmospheric scientists do not understand, only three such hurricanes hit the United States from 1995 through 2003. That pattern changed in 2004, when three major hurricanes hit the United States. In 2005 four of the season’s seven major hurricanes—Dennis, Katrina, Rita, and Wilma—hit the United States.

Researchers who study hurricanes and climate say that the computer models used to predict global climate changes do not look at weather in the detail needed to forecast whether a warmer world would increase the number or strength of hurricanes. On the other hand, scientists have no reason to expect fewer or weaker hurricanes to form than has occurred in the past. They also have no reason to think that many storms will miss the United States as they did in the 1990s and early 2000s. This means that no matter how global climate change affects hurricanes, increased population along the coasts places more people and property in harm’s way.