How do hurricanes form?
Hurricanes are mind-boggling in their size and strength. Modern technology lets us see how big hurricanes are and track them through satellite imagery, while storm-penetrating aircraft measure their strength, in terms of wind speeds and atmospheric pressure. That technology gives us detailed warnings that people a century ago never had.
Hurricanes usually form in the tropical zones north and south of the equator, where warm waters offer ample fuel for storm formation. That's where you see convection, where warm air rises and forms clusters of thunderstorms. The Coriolis effect, created by the Earth's spinning motion, may cause this cluster of storms to rotate (counter-clockwise above the equator, or clockwise below), but the system has to be a few hundred miles from the equator for the Coriolis force to exert an effect. If the system becomes organized and strong enough, with sustained winds of 39 miles per hour, it becomes a tropical storm. At 74 miles per hour or more, it becomes a hurricane.
A hurricane can be hundreds of miles across. It's like a big engine, with upper-level winds acting as a vent, pulling the rising warm air away from the storm. Rain bands spiral around the center of the system. Air sinks in the clear center, or eye, which can be from 5 miles to more than 100 miles across.
Because the atmospheric pressure is so low in the eye, the water rises up there in a sort of mound. This contributes to the devastating storm surge when a hurricane hits land, but most of the surge comes from winds pushing the sea water ahead of the storm, raising ocean levels by several feet.
When a hurricane hits land, it tends to weaken mostly because it has lost its source of energy - the warm ocean waters. Friction might slow down the surface wind, but it also can enhance turbulence and therefore gusts, scientists have found.
The Saffir-Simpson scale is used to classify hurricanes in the United States. It was invented by engineer Herbert Saffir and former National Hurricane Center director Robert Simpson. Here's a summary:
| Saffir-Simpson hurricane scale |
|---|
| Category |
Damage |
Wind speed |
Storm surge |
Pressure |
| 1 |
Minimal |
74-95 mph
118-152 km/h
64-82 knots |
4-5 feet
(1.2-1.5 meters) |
over 980 millibars
(over 28.94 inches) |
|---|
| 2 |
Moderate |
96-110 mph
153-176 km/h
83-95 knots |
6-8 feet
(1.8-2.4 meters) |
965-980 millibars
(28.50-28.91 inches) |
|---|
| 3 |
Extensive |
111-130 mph
177-208 km/h
96-113 knots |
9-12 feet
(2.7-3.6 meters) |
945-964 millibars
(27.91-28.47 inches) |
|---|
| 4 |
Extreme |
131-155 mph
209-248 km/h
114-135 knots |
13-18 feet
(3.9-5.4 meters) |
920-944 millibars
(27.27-27.88 inches)
|
|---|
| 5 |
Catastrophic |
over 155 mph
over 248 km/h
135+ knots |
over 18 feet
(over 5.4 meters) |
under 920 millibars
(under 27.17 inches)
|
|---|
In recent years, Hurricane Andrew hit south of Miami as a Category 4 on August 24, 1992, but was upgraded to a Category 5 in 2002 after 10 years of research. Hurricane Hugo, which came ashore at Charleston, South Carolina, on September 22, 1989, was also a Category 4. Camille, which hit Mississippi on August 17, 1969, was a Category 5.
Meteorologists give hurricanes names, which are reused unless the storm is particularly destructive. The names used to be all women's names, but since 1979, men's names have been used, too.
The Atlantic hurricane season begins June 1 and ends November 30, with the peak months from August through October.
After the hurricane, dangers remain. Avoid downed power lines and be careful around debris. Turn off gas lines if you smell or hear gas, get out of the area, and report the leak to the gas company. Watch out for animals, such as snakes, that may have been moved by flood waters. Families need to take pictures of damage for insurance purposes.
