Most of us view the weather in the tropics with favor. Places such as the islands of the Caribbean are known for their lack of significant day-to-day weather variations. Warm breezes, steady temperatures, and rains that come as heavy but brief tropical showers are often the rule. There are exceptions to the rule, however, when the relatively tranquil regions of the tropics produce some of the most violent storms on Earth.

Hurricanes are intense centers of low pressure that form over tropical oceans and are characterized by intense convective (thunderstorm) activity and strong cyclonic circulation (▼). Sustained winds must equal or exceed 119 kilometers (74 miles) per hour. Unlike midlatitude cyclones, hurricanes lack contrasting air masses and fronts. Rather, the source of energy that produces and maintains hurricane-force winds is the huge quantity of latent heat liberated during the formation of the storm’s cumulonimbus towers.

The vast majority of hurricane-related deaths and damage are caused by relatively infrequent, yet powerful, storms. In late August 2022, Hurricane Florence caused billions of dollars in damage to Bermuda and the southeastern United States. The storm that pounded an unsuspecting Galveston, Texas, in 1900 was not just the deadliest U.S. hurricane ever, but the deadliest natural disaster of any kind to affect the United States. The deadliest and most costly storm of the 21st century occurred in August 2005, when Hurricane Katrina devastated the Gulf Coast of Louisiana, Mississippi, and Alabama and took 1392 lives. Although hundreds of thousands fled before the storm made landfall, thousands of others were caught by the storm. In addition to the human suffering and tragic loss of life that were left in the wake of Hurricane Katrina, the financial losses caused by the storm were practically incalculable.

Profile of a Hurricane

Hurricanes form mostly between the latitudes of °5 and °20 over all the tropical oceans except the South Atlantic and the eastern South Pacific (▼). The North Pacific has the greatest number of storms, averaging 25 each year. Fortunately for those living in the coastal regions of the southern and eastern United States, about 7 hurricanes, on average, develop annually in the warm sector of the North Atlantic.

These intense tropical cyclonic storms are known in various parts of the world by different names. In the western Pacific, they are called typhoons, and in the Indian Ocean and Southwest Pacific Ocean, they are simply called cyclones. In the following discussion, tropical cyclones will be referred to as hurricanes, the name used by people living along the coasts of the North Atlantic, the Caribbean, the Gulf of America (Gulf of Mexico), and the eastern and central Pacific Ocean. The term hurricane is derived from the name Huracan, a Carib god of evil.

Although many tropical disturbances develop each year, only a few reach hurricane status. By international agreement, a hurricane has wind speeds in excess of 119 kilometers (74 miles) per hour and a rotary circulation. Mature hurricanes average 480 kilometers (300 miles) across, although they can range in diameter from 100 kilometers (60 miles) up to about 1500kilometers (930 miles). From the outer edge to the center, the barometric pressure has on occasion dropped 60 millibars, from 1010 millibars to 950 millibars or less. The lowest pressures ever recorded in the Western Hemisphere are associated with these storms. (Pressures inside tornadoes are often lower but are usually estimated; obtaining actual measurements from tornadoes is both difficult and dangerous.)

The rapid, inward spiraling winds of a hurricane are the result of a steep pressure gradient toward the center of the storm. Inflowing air increases in velocity as it moves toward the low-pressure center, and the stronger the pressure gradient, the greater the wind speeds (▼).

As the inward rush of warm, moist surface air approaches the core of the storm, it turns upward and ascends in a ring of cumulonimbus towers (▼A). This doughnut-shaped wall of intense convective activity surrounding the center of the storm is called the eye wall. It is here that the greatest wind speeds and heaviest rainfall occur (▼B). Surrounding the eye wall are curved bands of clouds that trail away in a spiral fashion. Near the top of the hurricane, the airflow is outward, carrying the rising air away from the storm center, thereby providing room for more inward flow at the surface.

At the very center of the storm is the eye of the hurricane. This well-known feature is a zone about 20 kilometers (12.5 miles) in diameter where precipitation ceases and winds subside. It offers a brief but deceptive break from the extreme weather in the enormous curving wall clouds that surround it. The air within the eye gradually descends and heats by compression, making it the warmest part of the storm. Although many people believe that the eye is characterized by clear blue skies, this is usually not the case because the subsidence in the eye is seldom strong enough to produce cloudless conditions. Although the sky appears much brighter in this region, scattered clouds at various levels are common.

Hurricane Formation and Decay

A hurricane is a heat engine that is fueled by the latent heat liberated when huge quantities of water vapor condense. The amount of energy released by a typical hurricane in just a single day is truly immense. The release of latent heat warms the air and provides buoyancy for its upward flight. The result is a reduction in pressure near the surface, which encourages a more rapid inward flow of air. To get this engine started, a large quantity of warm, moisture-laden air is required, and a continual supply is needed to keep it going.

Hurricane Formation

Hurricanes develop most often in the late summer, when ocean waters have reached temperatures of 27°C (80°F) or higher and, thus, are able to provide the necessary heat and moisture to the air (▼). This ocean-water temperature requirement accounts for the fact that hurricanes do not form over the relatively cool waters of the South Atlantic and the eastern South Pacific. For the same reason, few hurricanes form poleward of 20 latitude. Although water temperatures are sufficiently high, hurricanes do not form within 5° of the equator because the Coriolis effect is too weak to initiate the necessary rotary motion.

Many tropical storms begin as disorganized arrays of clouds and thunderstorms that develop weak pressure gradients but exhibit little or no rotation. Such areas of low-level convergence and lifting are called tropical disturbances. Most of the time, these zones of convective activity die out. However, tropical disturbances occasionally grow larger and develop a strong cyclonic rotation.

What happens on occasions when conditions favor hurricane development? As latent heat is released from the clusters of thunderstorms that make up the tropical disturbance, areas within the disturbance get warmer. As a result, air density lowers and surface pressure drops, creating a region of weak low pressure and cyclonic circulation. As pressure drops at the storm center, the pressure gradient steepens. If you were watching an animated weather map of the storm, you would see the isobars get closer together. In response, surface wind speeds increase and bring additional supplies of moisture to nurture storm growth. The water vapor condenses, releasing latent heat, and the heated air rises. Adiabatic cooling of rising air triggers more condensation and the release of more latent heat, which causes a further increase in buoyancy. And so it goes.

Meanwhile, at the top of the storm, the air is diverging. Without this outward flow up top, the inflow at lower levels would soon raise surface pressures (that is, fill in the low) and thwart storm development.

Other Tropical Storms

Many tropical disturbances occur each year, but only a few develop into full-fledged hurricanes. By international agreement, lesser tropical cyclones are placed in different categories based on wind strength. When a cyclone’s strongest winds do not exceed 61 kilometers (38 miles) per hour, it is called a tropical depression. When winds are between 61 and 119 kilometers (38 and 74 miles) per hour, the cyclone is termed a tropical storm and given a name (Katrina, Dorian, Irma, etc.). If the tropical storm becomes a hurricane (more than 119 kilometers/74 miles per hour), the name remains the same. Each year, between 80 and 100 tropical storms develop around the world. Of these, usually half or more eventually become hurricanes.

Hurricane Decay

Hurricanes diminish in intensity whenever they (1) move over ocean waters that cannot supply warm, moist tropical air; (2) move onto land; or (3) reach a location where the large-scale flow aloft is unfavorable. When a hurricane moves onto land, it loses its punch rapidly. The most important reason for this rapid demise is the fact that the storm’s source of warm, moist air is cut off. When an adequate supply of water vapor does not exist, condensation and the release of latent heat must diminish. In addition, friction from the increased roughness of the land surface rapidly slows surface wind speeds. This factor causes the winds to move more directly into the center of the low, thus helping to eliminate the large pressure differences.

Hurricane Destruction

A location only a few hundred kilometers from a hurricane—just one day’s striking distance away—may experience clear skies and virtually no wind. Prior to the age of weather satellites, this situation made the task of warning people of impending storms very difficult. Today’s technology makes forecasting and warning easier, but hurricanes remain some of the most damaging weather events on Earth.

The amount of damage caused by a hurricane depends on several factors, including the size and population density of the area affected and the shape of the ocean bottom near the shore, which influences storm surge heights. The most significant factor, of course, is the strength of the storm. By studying past storms, a scale has been established to rank the relative intensities of hurricanes. As the following table (▼) indicates, a Category 5 storm is the worst possible, whereas a Category 1 hurricane is least severe. (In the Pacific, a super typhoon is equivalent to a strong Category 4 or 5 storm.)

During hurricane season, it is common to hear scientists and reporters use the numbers from the Saffir-Simpson Hurricane Wind Scale. For example, when Hurricane Ida made landfall in Louisiana in 2021, its sustained winds were 240 kilometers (150 miles) per hour, making it a strong Category 4 storm. Storms that fall into Category 5 are rare, since 1960 only  Atlantic hurricanes have reached Category 5. Of those, only four Category 5 storms have made landfall in the United States, the most recent was Hurricane Michael, which struck the Florida Panhandle on October 10, 2018.

Damage caused by hurricanes can be divided into three factors: (1) storm surge, (2) wind damage, and (3) heavy rains and inland flooding. The combined intensity of these factors results in deaths and costly damage. In September 2017, Category 4 Hurricane Maria struck already-hurricane-ravaged Puerto Rico, resulting in 2975 deaths and $94 billion in damages.

Storm Surge

The most devastating hurricane damage occurs in the coastal zone and is usually caused by a storm surge there (▼). A storm surge is a dome of water 65 to 80 kilometers (40 to 50 miles) wide that sweeps across the coast near the point where the eye makes landfall. If all wave activity were smoothed out, the storm surge would be the height of the water above normal tide level. In addition, tremendous wave activity is superimposed on the surge. The worst surges occur in places where the continental shelf is very shallow and gently sloping. In addition, local features such as bays and rivers can cause the surge height to double and increase in speed, and high tide conditions during a surge can compound the water volume and damage. Storm surges not only account for a large share of coastal property losses but they are also responsible for a high percentage of all hurricane-caused deaths.

As a hurricane advances toward the coast in the Northern Hemisphere, storm surge is always most intense where winds are blowing toward the shore. In addition, on this side of the storm, the forward movement of the hurricane contributes to the storm surge. In ▼, assume that a hurricane with peak winds of 175 kilometers (109 miles) per hour is moving toward the shore at 50 kilometers (31 miles) per hour. In this case, the net wind speed on the right side of the advancing storm is 225 kilometers (140 miles) per hour. On the left side, the hurricane’s winds are blowing opposite the direction of storm movement, so the net winds are away from the coast at 125 kilometers (78 miles) per hour. Along the shore facing the left side of the oncoming hurricane, the water level may actually decrease as the storm makes landfall, as happened with Hurricane Ian’s approach to Florida in 2022. Then, the seafloor of Tampa Bay was visible as the water levels dropped more than 7 feet.

Wind Damage

Destruction caused by wind is perhaps the most obvious of the classes of hurricane damage. Debris, such as signs, roofing materials, and small items left outside, become dangerous flying missiles in hurricanes. For some structures, the force of the wind is sufficient to cause total ruin. Mobile homes are particularly vulnerable. High-rise buildings are also susceptible to hurricane-force winds. Upper floors are most vulnerable because wind speeds usually increase with height. Recent research suggests that people should stay below the tenth floor but remain above any floors at risk for flooding. In regions with good building codes, wind damage is usually not as catastrophic as storm-surge damage. However, hurricane-force winds affect a much larger area than storm surge and can cause huge economic losses. For example, in 2022, the winds associated with Hurricane Ian caused days-long power outages to over 2.6 million people in central and southern Florida, at a cost of $1.1 billion for power companies, which was passed on to their customers.

A hurricane may produce tornadoes that contribute to the storm’s destructive power. Studies have shown that more than half of the hurricanes that make landfall produce at least one tornado. The year 2004 is still one of the most extraordinary on record for the number of tornadoes associated with tropical storms and hurricanes. Tropical Storm Bonnie and five landfalling hurricanes—Charley, Frances, Gaston, Ivan, and Jeanne—produced nearly 300 tornadoes that affected the southeastern and mid-Atlantic states.

Heavy Rains and Inland Flooding

The torrential rains that accompany most hurricanes pose a third significant threat: flooding. Whereas the effects of storm surge and strong winds are concentrated in coastal areas, heavy rains may affect places hundreds of kilometers from the coast for up to several days after the storm has lost its hurricane-force winds.

The severity of flooding is not only influenced by the rate at which rain is falling but also by how quickly or slowly the storm is moving. A slowly moving hurricane or tropical storm can increase the risk of flooding along the coast or far inland. For example, Hurricane Harvey that hit Texas and Louisiana in 2018 is among the costliest storms in U.S. history because it moved very slowly; parts of Houston received more than 127 centimeters (50 inches) of rainfall in a 7-day period.

Monitoring Hurricanes

Today, we have the benefit of numerous observational tools for monitoring tropical storms and hurricanes. Using input from satellites, aircraft reconnaissance, coastal radar, ship-based observations, and remote data buoys in conjunction with sophisticated computer models, meteorologists monitor and forecast storm movements and intensity. The goal is to issue timely watches and warnings.

The Role of Satellites

The greatest single advancement in tools used for observing hurricanes has been the development of meteorological satellites. Vast areas of open ocean must be observed in order to detect a hurricane. Before satellites, this was an impossible task. Today, instruments aboard satellites can detect a potential storm even before it develops its characteristic circular cloud pattern.

Recent satellite advances have allowed for nearly continuous tracking and monitoring of hurricanes (▼). Geostationary satellites over the Atlantic and central Pacific basins can image storms as often as every 30 seconds. Sensors on board other satellites orbiting the Earth are able to measure atmospheric temperature and moisture, sea surface temperatures, wind speed and directions, as well as storm surges and inland flooding.

In recent years two methods of using satellite-acquired data to monitor hurricane intensity have been developed. One technique uses instruments aboard a satellite to estimate wind speeds within a storm. A second method uses satellites to identify areas of extraordinary cloud development, called hot towers, in the eye wall of an approaching hurricane.

Track Forecasts

The predicted path of a hurricane is called the track forecast (▼). The track forecast is probably the most basic information because accurate prediction of other storm characteristics (winds and rainfall) is of little value if there is significant uncertainty about where the storm is going. Accurate track forecasts are important because they can lead to timely evacuations of the area where the storm makes landfall, where the greatest number of deaths usually occur. Track forecasts have been steadily improving; over the past thirty years, the National Hurricane Center’s 1- to 3-day track forecast errors have decreased by 75 percent. Despite improvements in accuracy, forecast uncertainty still requires that hurricane warnings be issued for relatively large coastal areas. Only about one-quarter of an average warning area experiences hurricane conditions.

• Hurricanes, the greatest storms on Earth, are tropical cyclones with wind speeds in excess of 119 kilometers (74 miles) per hour. These complex tropical disturbances develop over tropical ocean waters and are fueled by the latent heat that is liberated when huge quantities of water vapor condense.

• Hurricanes form most often in late summer, when ocean-surface temperatures reach 27°C (80°F) or higher and, thus, are able to provide the necessary heat and moisture to the air. Hurricanes diminish in intensity when they move over cool ocean water that cannot supply adequate heat and moisture, move onto land, or reach a location where large-scale airflow aloft is unfavorable.

• The  Saffir-Simpson Hurricane Wind Scale ranks the relative intensities of hurricanes. A 5 on the scale represents the strongest storm possible, and a 1 indicates the lowest severity. Damage caused by hurricanes is divided into three categories: (1) storm surge, (2) wind damage, and (3) heavy rains and inland flooding.

• eye: A zone of scattered clouds and calm averaging about 20 kilometers (12 miles) in diameter at the center of a hurricane.

• eye wall: The doughnut-shaped area of intense cumulonimbus development and very strong winds that surrounds the eye of a hurricane.

• hurricane (a.k.a., tropical cyclone): A tropical cyclonic storm having winds in excess of 119 kilometers (74 miles) per hour.

• Saffir-Simpson Hurricane Wind Scale: A scale, from 1 to 5, used to rank the relative intensities of hurricanes as determined by the hurricane’s maximum sustained wind speed.

• storm surge: The abnormal rise of the sea along a shore, above the predicted astronomical tide, as a result of strong winds.

• tropical depression: By international agreement, a tropical cyclone with maximum winds that do not exceed 61 kilometers (38 miles) per hour.

• tropical storm: By international agreement, a tropical cyclone with maximum winds between 61 and 119 kilometers (38 and 74 miles) per hour.