How Do Hurricanes Form?

Tropical storm and hurricane prediction probably never will be an exact science, but the reasons for storm formation are well understood by the scientific community. There are several elements that—when combined at the "right" amount of time and under the "right" conditions—will create a hurricane, according to the National Oceanic and Atmospheric Association (NOAA).
The beginning of life for any hurricane is a pre-existing tropical disturbance—an area of low atmospheric pressure in the air over the tropical Atlantic Ocean near Carribean islands, such as Bermuda and the Bahamas.

The warmth and moisture of the ocean during late summer and early fall months (when ocean waters reach their highest temperatures) energizes the pre-storm conditions and leads to thunderstorms.
If thunderstorms persist and winds pick up to 40 miles per hour, the tropical disturbance officially becomes a tropical storm. At this point, the National Hurricane Center names the storm, working from a pre-determined list of names that is recycled every six years. Meteorologists all over the country know to keep a close eye on the now-named storm, although many tropical storms weaken and die before becoming hurricanes.
'Heat Engine': The Energy Behind a Hurricane
Tropical storms that continue to intensify will keep pulling in warm and humid air from the lower atmosphere while spitting out cooler, drier air into the upper atmosphere. According to Chris Landsea of NOAA's hurricane research division, at this point in its development, the storm system operates like a huge "heat engine."
"The 'heat engine' gets its energy from warm, humid air over the tropical ocean and releases this heat through the condensation of water vapor," said Landsea. This energy release is what drives the powerful winds of a hurricane.
The force of the release is tremendous—the amount of heat energy released by an average hurricane is equivalent to the amount of electric energy produced by the U.S. in an entire year. A small portion of the energy released actually warms what has become the inner core of the storm. As the temperature of the air in the inner core rises, its pressure drops, increasing the speed and intensity of the winds swirling around it. These stronger winds bring more warm, moist air to the clouds surrounding the inner core of the storm further fueling its energy. When the swirling winds reach a speed of 74 miles per hour or more, the tropical storm becomes a hurricane.
Hurricane Ratings
Once a storm officially becomes a hurricane, it receives an intensity rating based on its wind speed and potential to cause damage. The rating system that is used by the National Weather Service is called the Saffir-Simpson scale. As a hurricane develops, its intensity rating often changes. In 1985, for example, Hurricane Opal grew from a Category One into a Category Four hurricane in just 18 hours.
Category One hurricanes have wind speeds between 74 and 95 miles per hour and are expected to cause minimal damage to buildings and homes. Trees, shrubbery and mobile homes tend to bear the brunt of the damage caused by Category One hurricanes.

Hurricane Bonnie, which hit the North Carolina coast in 1998, was a Category Two hurricane and caused both flooding of low-lying areas and considerable damage to trees.
With wind speeds reaching 100 miles per hour, this is typical of a Category Two storm. Category Three and Four hurricanes are characterized by even stronger winds and much more damage to homes, buildings and trees.
The most intense classification of a storm is the Category Five hurricane. A Category Five storm will have sustained winds of 155 miles per hour or more and is capable of extensive damage. Hurricane Gilbert of 1988 was the strongest Category Five hurricane ever, destroying Jamaica with wind speeds upwards of 180 miles per hour.
Anatomy of a Storm

The inner core of the hurricane is known as the eye of the storm—a calm, often clear-skied patch where winds are lightest and pressure is lowest. Surrounding the paradoxically calm region of the hurricane is a ring of clouds called the eyewall. The eyewall clouds are thunderstorm clouds, and it is in this region of the hurricane where the heaviest rains and winds originate. During Hurricane Camille, which pummeled the Gulf Coast in 1968, winds in the eyewall reached speeds in excess of 200 miles per hour. The outermost ring of the hurricane is made up of bands of heavy rains that swirl inward toward the storm's center, called spiral rainbands.
Weather Report
While the clouds and rainbands are forming in the sky above, the weather on the land below the hurricane turns nasty. The spiraling winds that accompany hurricanes can extend even further beyond the eye of the storm. Typically, hurricanes are about 300 miles wide, so they can affect fairly large areas at one time.
Unfortunately for those in its path, a hurricane's speed of travel is hard to predict and varies greatly from storm to storm. Weather experts have calculated that hurricanes move forward at an average speed of 15-20 miles per hour, but a big storm also has the potential to linger over one area for a while, causing torrential rains, or move so quickly that there is no time to prepare for its arrival.
Eventually, a hurricane's energy begins to dissipate and the storm weakens. Weather experts have identified several factors that contribute to a hurricane's demise, including the storm's movement over cooler water or drier areas. Even when a hurricane appears to have blown over, however, it can potentially reintensify if it hits weather conditions that are favorable for its development.
El Niño Effect
El Niño is a weather phenomenon that causes equatorial Pacific Ocean temperatures to be warmer than usual. El Niño periods occur in cycles, and the most recent El Niño event we've experienced was in 1997-98. During this time, there were fewer Atlantic hurricanes than the average number during hurricane season. Weather experts say that warm El Nño events are characterized by decrease in the number of tropical storms and hurricanes in the Atlantic, Gulf of Mexico and the Caribbean Sea because of the increased wind shear. The wind shear associated with El Niño essentially cuts a storm off during its development, by hindering the formation of a vertical ring of clouds. The shear actually shaves off some of the clouds, which creates a slant in the cloudwall. When a storm is slanted, the heat energy released from condensation is spread out over a larger area and doesn't necessarily feed back into the storm system to strengthen it.

A La Niña period brings weather conditions opposite to those that are associated with El Niño. La Niña brings cooler equatorial Pacific Ocean temperatures and decreased wind shear in the tropical Atlantic region-increasing the chances that a tropical disturbance will in fact develop into a hurricane.
Hurricane Mitch, which caused devastating damage to Nicaragua and Honduras in October 1998, occurred during a La Niña period. However, since Hurricane Mitch followed a lot of media hype about El Niño, said NOAA's Landsea, many people erroneously assumed that El Niño was responsible for Mitch.
"This was definitely not the case," said Landsea. "The big El Niño of 1997-98 caused a reduction in Atlantic hurricanes, with only three forming in 1997. But the El Niño finished by the spring of 1998, and was quickly replaced by La Niña by the time Hurricane Mitch occurred. We have more or less been in that same La Niña-and it has boosted the Atlantic hurricane activity both in 1998 and last year-though it is currently fading."
Preparing for the Storm
No matter how intense a hurricane season turns out to be, it is always good to be prepared for a big storm. Whether facing a hurricane watch or warning, the Red Cross recommends that the public get and stay prepared, which includes developing an evacuation plan.
When a hurricane watch is issued (meaning that hurricane conditions are possible within 36 hours), people in affected areas should tune in to local radio or TV stations for up-to-date storm information. Consider bringing any lawn furniture capable of being picked up by the wind inside and preparing houses for high winds. While many people think that taping windows shut is sufficient to prevent windows from breaking, the Red Cross recommends using something stronger, such as hurricane shutters or plywood.
When a hurricane warning is issued (meaning that hurricane conditions are expected in the specified area, usually within 24 hours), the Red Cross advises being prepared to evacuate to a shelter if local officials indicate to do so.
If the storm hits and it is not necessary to leave your home, the Red Cross advises staying away from windows and continuing to pay attention to the development of the storm. As previously mentioned, blue skies and calm winds do not necessarily mean that the hurricane is over, just that the eye of the storm is moving over the area. Once the eye passes over, the storm will pick up again with winds blowing from the opposite direction.