They are massive, violently spiraling storms that tear across both land and water, devastating nearly everything in their path. Hurricanes, the most severe type of tropical cyclone, are products of several factors--including temperature, wind and moisture--that under the right conditions, can produce some of the worst destruction known to humankind. Although hurricanes can be unpredictable, science has been able to decipher much about how they work. In this article, find out how hurricanes work, how to prepare for them and how scientists have learned to predict their movement in order to save lives.

How do hurricanes form?

Hurricanes develop out of three basic factors: a pre-existing weather disturbance with thunderstorms; warm ocean water, with a temperature of at least 80 degrees Fahrenheit that extends about 150 feet; and light upper level winds that maintain about the same speed and direction throughout the atmosphere.

Hurricanes originate from pre-existing weather disturbances, which form off the coast of Africa every three to four days. These disturbances typically take the form of tropical waves containing areas of turbulent weather. When a disturbance enters the warm ocean water, it feeds off heat from within the ocean. Winds near the water’s surface merge with the weather disturbance, and as the air rises, the ocean’s warmth creates moisture and heat that fuel the developing storm. The moisture is condensed into drops, generating additional heat and giving more energy to the storm. Thunderstorms form, and if the wind stays light, the storm can organize and gather strength.

As it first forms, the storm is relatively unorganized, but as it strengthens, it takes on a spiral shape, and becomes what is called a tropical depression. Once the storm reaches 39 miles per hour, it is considered a tropical storm, and if winds reach at least 74 miles per hour, it is considered a hurricane. It is at this time that the storm develops what is called the eye, a cloudless area in the middle of the storm that develops as air at the center sinks, dries and creates warmth.

As hurricanes move onto land or cooler water, they are deprived of the warmth they need to maintain their structure, and begin to disintegrate. Usually, landfall robs the storm of moisture and causes it to weaken. Even as they fall apart, however, hurricanes can still be dangerous.

Tropical cyclones can also form out of the tail end of a cold front or from an upper-level low, but still need warm ocean waters in order to organize into a hurricane.

What is a hurricane?

A hurricane consists of bands of thunderstorms that move in a spiral motion towards the center, or eye, of the storm. The eye is a calm area at the center of the storm that is usually between 20 and 40 miles across. Here, the sky is clear and there is little or no wind. Encircling the eye is the eyewall, a wall of thunderstorms with the strongest winds in the storm. Both the eye and the eyewall can change structure, which can cause the wind speed to change. The eye can expand or become smaller, and two eyewalls can develop. The outer edges of the storm are called the spiral rainbands. These are bands of thunderstorms that slowly spiral counterclockwise, and can stretch a few hundred miles from the storm’s center. They can be as small as a few miles or as large as 300 miles long.

Hurricanes generally move forward between 15 and 20 miles per hour, although they can stall, producing devastating rainstorms. Others have far exceeded the typical speed, reaching more than 60 miles per hour. Their paths can vary as well, from a relatively straight course, to one that is erratic and difficult to predict. Hurricanes also produce massive waves that reach in all directions, in addition to accumulating large amounts of water. This accumulation of water is called a storm surge, and can cause sea level to raise more than 20 feet when the hurricane hits land.

When do hurricanes occur?

Hurricanes need warmth to develop, which is why they usually form during the summer and fall, when the ocean waters are warm enough to provide fuel for the storm. Ocean water cools and warms slowly, so in the spring the water is not yet warm enough to support a storm; once the oceans warm during the summer, the water stays warm throughout the fall season.

Hurricanes are most common during the early summer and late fall, with 97 percent of hurricanes occurring between June 1 and November 30, the official dates for the Atlantic hurricane season. Although most tropical storms develop during this six-month period, hurricanes can and have occurred outside this time frame. Worldwide, September is the most active month, and May is the quietest, but storm activity varies depending on the region. The following list outlines the most active periods for various regions.

• Atlantic Basin: Storm activity is highest from August to October, with 78 percent of the tropical storm days, 87 percent of the minor hurricane days and 96 percent of the major hurricane days originating during this period. Activity peaks around early to mid-September.

• Northeast Pacific Basin: This region has a longer storm season than many, extending from late May or early June until late October or early November. Storm frequency is greatest from late August or early September.

• Northwest Pacific Basin: In this region, storms occur throughout the entire year, although storms are most frequent between July and November. Activity peaks in late August or early September, and is less frequent in February and the first half of March.

• North Indian Basin: Here, tropical storms are most common between April and December, with activity peaking in both May and November. Storms are most severe between April and June and between late September and early December.

• The Southwest Indian and Australian/Southeast Indian basins: Storm seasons are similar for these two regions, extending from late November or early December until May. Peak activity occurs in mid-January and again in mid-February or early March.

• Australian/Southwest Pacific Basin: In this region, activity begins in late October or early November to May, peaking in late February or early March.

Hurricane seasons seem to vary in intensity, occurring in cycles alternating between high storm activity and significantly quieter periods. For example, between 1944 and 1969, there was an average of 2.7 major hurricanes each year, but between 1970 and 1994, there was only an average of about 1.5 major hurricanes a year. In 1995, intensity began to increase again, with an average of 3.55 hurricanes every year between 1995 and 2003. The cause for these cycles is not known, but it is believed they may be due to long-term temperature changes in the Atlantic Ocean.

How are hurricanes monitored?

Thanks to technological advances, meteorologists can track and predict hurricane movement more accurately than ever before. When the hurricane first forms and is still hundreds of miles from land, direct measurements can be difficult to obtain. By using satellites, ships and buoys, however, meteorologists can indirectly measure the storm and monitor it when it is still over water. As the hurricane approaches land, more direct measurements can be obtained, using tools like reconnaissance aircraft. Once the hurricane is within 200 miles of the coast, radar becomes the primary tool used for obtaining information. Meteorologists also use computer models to predict the storm’s intensity and movement, but because so much atmospheric data is needed, the models can produce inaccurate forecasts. If not enough observational data was obtained while the storm was still over water, for example, the result of the computer model may differ from the storm’s actual movement.

The National Hurricane Center uses this data to determine when and where to issue storm watches. A hurricane watch is issued when the storm is predicted to hit the area in 36 hours or less. Once the storm is about 24 hours away, a hurricane warning is issued. While more sophisticated technology has allowed meteorologists to more accurately forecast hurricanes, predicting the storm’s movement can still be difficult. Hurricanes are complex storms, and their behavior is not completely understood. Once a tropical storm forms, it must be constantly monitored in order to keep up with the many changes that can occur within the storm.

Forecasting hurricanes is a joint effort between the National Oceanic and Atmospheric Administration’s Tropical Prediction Center and local Weather Forecast Offices. The Tropical Prediction Center consists of the National Hurricane Center, the Tropical Analysis and Forecast Branch and the Technical Support Branch. Local offices work with the national center during a storm to help predict the hurricane’s movement and intensity.

How destructive is a hurricane?

Hurricanes bring with them torrential rains, high winds, storm surge (a large dome of water), flooding and tornadoes. While a storm’s intensity can indicate how destructive it may be, ultimately the level of damage depends on when and where the storm hits. If a hurricane strikes a heavily populated area, for example, property damage and loss of life will likely be greater than in less populated areas.

The storm surge poses the greatest threat to coastal areas. This large mound of water, accumulated as the storm develops, can be anywhere from 50 to 100 miles wide, and 15 feet or more at the peak. If it hits the coastline, it can cause massive destruction. Also threatening are the hurricane’s high winds, which precede the eye of the storm. The winds can be destructive in themselves, but they also carry debris that can cause injury and property damage. Rainfall and flooding also accompany a hurricane, and are particularly devastating in mountainous regions, where they can cause flash floods and mudslides. Rainfall usually ranges between 6 and 12 inches. Finally, tornadoes often develop at the outer edges of the storm as it hits land, causing enormous destruction. These tornadoes can develop days after the storm strikes land, and are usually more common in more intense hurricanes.

In addition to the direct impact of these hazards, there are indirect consequences as well. Chemical spills could be caused by the storm’s destruction, there could be car accidents and fires may be started by people using candles or other flammable materials when electricity is lost. The clean-up phase brings risks of its own, including accidents and heart attacks as people struggle to repair the damage done.

How can I prepare for a hurricane?

Ideally, hurricane preparedness should start well before hurricane season begins. Residents should find out if they are in an area prone to hurricanes, and if they live where flooding is a risk. If they do live in a danger area, they should put together an emergency plan and decide where to go if forced to evacuate. In addition, they should make sure their homes are secure, and that they have covers for windows and doors. A survival kit is also important, complete with a two-week supply of essential items like food, water and medication.

If a hurricane watch or warning is issued, residents in the threatened area should make sure their emergency kits are complete, find a safe place for important papers, secure their doors and windows and make sure they have enough cash, food and water and fuel for vehicles. If a power outage occurs, banks, stores and service stations will likely close down, leaving residents without access to basic items they need to survive. If forced to evacuate, they should also tell friends and family where they’ll be. Before they leave, they should also shut off the water and gas and turn off their house’s main circuit breaker. Once the hurricane strikes, residents should stay inside, away from doors and windows. They should also find an area in the building’s interior, such as a closet or bathroom, that is more secure. They should remain inside until radio reports confirm the storm has passed, because even if winds seem to have stopped, it may mean that the eye of the storm is passing over and the winds will begin again after the eye has moved on.

In Conclusion

Hurricanes have the potential to cause catastrophic damage and loss of life. In the past, hurricanes sometimes killed thousands of people, because there was no way to predict or track them. However, today’s technology provides advance warning for people in the path of the storm, allowing them to take shelter. And as scientists develop new methods for forecasting hurricane movement, we gain a greater understanding of how the storms work and how we can protect ourselves from their destruction.