ABOUT HURRICANES

Hurricane Categories

Hurricanes are evaluated in a number of ways. Storms are assigned a category based on winds, storm surge and barometric pressure, using the Saffir-Simpson scale.

Category 1 storms have winds of 74-95 miles per hour, making them the weakest of hurricanes. Even these storms can generate a storm surge of 4 or 5 feet above normal high tide.

Category 2 storms have winds of up to 110 miles per hour, and can push a storm surge of 6 to 8 feet.

Category 3 storm winds can reach 130 miles per hour. This is the cutoff for "major" hurricanes, with commensurate storm surge potential of 9 to 12 feet.

Category 4 winds can be as high as 155 miles per hour, and such a storm brings a 13 to 18 foot storm surge.

Category 5 storms, with winds greater than 155 miles per hour, are very rare. These monsters can have storm surges of over 20 feet. Only 2 such hurricanes have hit the U.S. this century - Camille in 1969 and Andrew in 1992.

Birth and Death of a Hurricane

Step 1
Tropical Wave: "bump" or disruption of normal tropical easterly flow. Associated turning of wind causes low-level convergence of air; which helps with falling pressure and enhanced showers.

Step 2 This can evolve into a Tropical Depression, which is a closed circulation of air in the low levels. This in turn increases convergence and pressure falls, and wind speeds increase in a Catch-22 effect (i.e. the stronger the wind blows the greater the convergence, the quicker the pressure falls... so the stronger the wind, etc.).

Step 3 Once sustained winds reach 39 mph in the closed circulation a Tropical Storm is named. Usually there are at least 2 closed isobars of 4 mb increments around the center. If atmospheric conditions remain correct the system will evolve into a...

Step 4 Hurricane. There is usually a difference in pressure of at least 0.60 inches of mercury between the center and surrounding pressure field, with the greatest change near the center (eyewall). It is this great difference in pressure, which sometimes can be as great as 2.95 inches of mercury, that causes the wind to be so strong.

Step 5 A mature hurricane is a well-oiled meteorological machine, but disruption of the processes that drive the storm (i.e. interaction with land or colder air feeding in) will begin to destroy the storm, and the disintegration of a hurricane can often be quick and dramatic.

Anatomy of a Hurricane

1. Outflow. The high level clouds moving clockwise out away from the hurricane at heights of over 35,000 feet. These clouds are indicative of air spreading out over the top of the storm, which is essential to its development.

2. Feeder Bands. These are squally bands of showers characterized by strong gusty winds and heavy rains. These bands become more pronounced as the storm intensifies, and are fed by the warm ocean.

3. The Eyewall. A band of clouds, strong winds and heavy rains surrounding the eye of the storm. At the eyewall, there is rapid movement of air toward the center and upward into the cloud.

4. The Eye. What goes up must come down, so with the violent rising air converging toward the storm center at the eye, sinking air develops within. This air dries out, creating the clear, calm eye. Winds are very light here since the focus of convergence and hence strong winds are in the eyewall.

The Deadly Storm Surge

Low pressure in the hurricane acts as a plunger, pulling up the water level. Higher water level allows waves to strike farther inland, causing massive property damage.