While a few early-season hurricanes form in the Gulf of Mexico or Western Caribbean, the majority of the big storms form in the Atlantic from June to November each year. They usually begin as a cluster of clouds, called a tropical wave, moving off the northwest coast of Africa. These systems are swept westward across the North Atlantic on the trade winds, just above the equatorial doldrums, or Intertropical Convergence Zone (ITCZ).
The processes by which hurricanes form and subsequently strengthen depend on at least three conditions: warm surface waters, high humidity, and the ability to concentrate heat in a vertical column. Obviously, summer in the tropics brings the necessary warm water and humidity. The latter factor, however, depends on winds at all levels of the surrounding atmosphere to be essentially from the same direction and at the same speed. Dangerous storms begin to form when air is warmed by contact with the water and is moistened by evaporation from it. The need for these conditions explains why hurricanes form only in summer and early fall months when the waters are warm. However, even when the water temperature and surrounding humidity are favorable for hurricane formation, adverse winds in the upper atmosphere often prevent them from developing. This is why El Nino years in the Pacific, which often spew strong west winds into the Caribbean, have lower incidence of big storms. We all know that warm air rises. Because of the earth's rotation, called the Coriolis effect, all fluids moving vertically begin to spiralclockwise in the northern hemisphere and counter-clockwise in the southern half of the planet. As this warm, moist air begins its twist, it is drawn in toward the center of the system. And the closer it gets to the center, the faster it moves, drawing more air into its base as it spirals upward. The strong winds created by the moving air produce turbulent seas, and huge amounts of spray become suspended in the air. This suspended spray increases the rate of evaporation, creating more humidity so that the storm begins to feed on its own energy. Given ideal conditions, a tropical wave can pass through the stages of tropical depression (winds under 35 mph), to tropical storm (winds 35 to 73 mph), and into a hurricane (winds over 74 mph) in a matter of hours.
The eye of the hurricane is relatively calm. The most violent activity takes place in the area immediately adjacent to the eye, an area called the eyewall. As the spiraling air rises and cools, moisture condenses into tiny droplets around the eye wall, forming clouds and bands of rain that weathermen refer to as feeder bands. The condensation process releases even more latent heat, causing the air to rise even higher. That in turn results in more condensation. The final result is a column of rapidly rising air that produces an intense low-pressure area near the storm center.
At the top of the eyewall most of the air is propelled outward in an anticyclonic flow. Some of the air, however, moves inward and sinks into the eye. This air is warmed rapidly by compression; and as it warms, its moisture holding capacity increases and the air-dries out. In fact, the eye becomes nearly free of clouds in intense hurricanes. At the middle to upper levels of the storm, the temperature inside the eye is much warmer than outside. This temperature difference creates a large pressure differential across the eyewall, contributing to the violence of the storm.
I had the opportunity to leave my shelter and experience the eye of Hurricane Hugo. I went from experiencing a violent storm to a perfectly calm, clear nightI could even see the stars. This tranquil event did not last long as I was blown away by an immediate change from calm to hurricane force winds. What made it even worse was that the winds were now from the opposite direction.
What You Can Expect High wind, rain in quantities that often produce floods, storm surge, and tornadoes are the significant effects of a hurricane that will cause sailors grief.
- High Winds Pieces of building, awnings, trashcans, tanks, roofing, and anything that can be set flying in the strong winds may effect serious damage to your home or boat. It is estimated that a sheet of plywood carried by a minimal hurricane can strike with 450 pounds of force. The palm tree at the right is an example of what this force means.
- Rains and Floods In 1992 Hurricane Andrew was a relatively dry storm dropping only five to seven inches of rain when it moved across Florida at 18 mph. Slower moving storms dump much more water and can take hours to move through. In 1950, a September hurricane dumped 38.7 inches on Yankeetown, FL, in 24 hours. In 1998, Mitch dumped as much as two feet of rain over parts of Central America, causing devastating floods and mudslides that killed thousands. Even if the small ditch or canal in your backyard doesn't flood from the hurricane itself, debris often clogs culverts and floodgates, causing rain water to back up. If a storm drops 10 inches of rain, it could take up to two weeks to remove ankle-deep water. Snakes and other wildlife, as well as mosquitoes, become a real issue.
- Storm Surge Hurricane storm surge is an increase in the ocean's level, and can come ashore up to five hours before the eye of the storm crosses the coast. It is a great dome of water, often 50 miles wide, which sweeps across the coastline near the area where the eye of the hurricane makes landfall. The dome is a wall of water blown in front of the wind and is partially lifted by the low pressure, and it tends to increase in height as it enters shallow coastal waters. The surge can be more than 18 feet high and many miles wide. Opal's 15-foot surge stretched 120 miles in 1995. It crushed waterfront homes, cut new channels through islands, and moved whole dunes. The surge acts like a giant bulldozer, sweeping everything in its path. Camille's storm surge in 1969 reached 26 feet; one Mississippi-coast apartment complex disappeared except for its slab, as did all but one of the two dozen residents who decided to stay.
- Tornadoes Tornadoes are a real and special danger and many come embedded in the storm after you think all is clear. In 1998, Hurricane Gilbert's eye passed well south of the border in Mexico but spawned 41 tornadoes in Texas. Several hit San Antonio, about 350 miles from landfall, and caused $35 million in damage. The gusty winds, in excess of 90 mph that I experienced with Tropical Storm David in Charleston, SC, were damaging, but the multiple tornadoes are what really created havoc with downed trees and loss of power for three days. Cells, and sometimes entire rows, of tornadoes are spawned along the edges of the spiraling feeder bands. Even though Category 4 Hurricane Andrew had steady winds of 140 mph, it was demonstrated that the embedded tornadoes often exceeded 200 mph.
Will you be struck by a hurricane? Many researchers think we are entering a period of increased hurricane intensity, more like the period from 1940 through 1969 when monster storms swept ashore with greater frequency. Anyone who lived through Hurricane Andrew or other recent storms might disagree, but the experts say that for the past quarter-century we have gotten off easy. The last Category 5, or "catastrophic," hurricane was Camille, which struck the Gulf Coast in 1969 with winds over 200 mph and a storm surge 24 feet high. Since then, the experts say that hurricane activity has been mild. Having gone through Hurricane Hugo in 1989, I am not sure that "mild" is the proper term. I saw boats stacked up like cordwood, my office in Charleston was awash in almost 5 feet of water, and cars left behind floated awayhundreds of homes were destroyed and thousands damaged.
William Gray, the Colorado State University hurricane guru, and other noted meteorologists annually issue their respective predictions for the impending storm season. These forecasts are often misread by the general public. While the forecasts do attempt to predict the number of storms and their intensity that will occur during a particular hurricane season, they make no attempt to predict when those storms will occur, where they will be, or even whether any storm will strike land at all.
When considering the possibilities that you will be hit, it's important that you take hurricane statistics into considerationactually calculating your odds of being at ground zero by considering the number and percentage of storms that have landed in your vicinity over the past 100 years. Obviously the more storms in any given year, the more likely it is that you might be affected. There were 65 named storms between 1995 and 1999, and 41 of these actually reached hurricane strength at 75 mph. The four years between 95 and 99 were the most active on recordin 99 five major hurricanes evolved giving it a record for hurricanes.
The quietest storm season of last century was 1992 with only one significant storm that year. But residents in South Florida will never forget that stormit was Andrew. So, just because a quiet season is predicted, don't assume that you're in the clear.
The best advice is to plan for a catastrophic storm every year. You may end up buying supplies and bracing your home and boat for nothing, but then again, you may not be wasting your time. Last year I put my hurricane shutters up and took them down three times, but we were only grazed by Floyd, who caused no substantial damage. The people of North Carolina, though, weren't so lucky and were inundated with catastrophic floods from those three stormsyou just never know.
Rating a Hurricane
The Saffir-Simpson Hurricane Scale is a 1 to 5 scale based on a hurricane's intensity. It is used to give an estimate of the potential property damage and flooding expected in an inland area due to a hurricane landfall. Wind speed is the determining factor in the scale, since storm surge values are highly dependent on the slope of the continental shelf in the landfall region. Note that all winds speeds are quoted using the US one-minute average.
|ONE||74 to 95 mph||No real damage to building structures. Damage primarily to unanchored mobile homes, shrubbery, and trees. Also, some coastal road flooding and minor pier damage. |
|TWO ||96 to110 mph||Some roofing material, door, and window damage to buildings. Considerable damage to vegetation, mobile homes, and piers. Coastal and low-lying escape routes flood two to four hours before arrival of center. Small craft in unprotected anchorages break moorings.|
|THREE||111 to 130 mph||Some structural damage to small residences and utility buildings with a minor amount of curtainwall (vertical exterior building walls) failures. Mobile homes are destroyed. Flooding near the coast destroys smaller structures with larger structures damaged by floating debris. Terrain continuously lower than five feet absolute sea level (ASL) may be flooded inland eight miles or more.|
|FOUR||131 to 155 mph||More extensive curtainwall failures with some complete roof structure failure on small residences. Major erosion of beach areas. Major damage to lower floors of structures near the shore. Terrain continuously lower than 10 feet ASL may be flooded requiring massive evacuation of residential areas inland as far as six miles.|
|FIVE||Greater than 155 mph||Complete roof failure on many residences and industrial buildings. Some complete building failures with small utility buildings blown over or away. Major damage to lower floors of all structures located less than 15 feet ASL and within 500 yards of the shoreline. Massive evacuation of residential areas on low ground within five to ten miles of the shoreline may be required.|