Facts About Hurricanes

Hurricanes form under specific atmospheric conditions.  Counterclockwise winds draw heat and moisture from the tropical ocean, contributing to the formation of an intense and strong tropical cyclone.  Hurricane conditions are reached when winds are sustained at 74 mph or more.  Winds revolves around a relatively calm 20 to 30 mile-wide center, referred to as an “eye,” which may spread outward as much as 400 miles.  The storm, moving forward at about 15 mph, releases heavy rains, accelerating winds and strong ocean swells.  A hurricane may be preceded by tornadoes in the right front quadrant.  As the storm approaches land, severe wind, rain, and tidal storm surges inundate coastal areas.  Most hurricanes lose strength as they move inland, but high winds and rain will continue.

Hurricanes pose three major threats:

1. Wind.  Hurricane winds exceed 74 mph.  Such winds can rip buildings apart, uproot structures or objects, damage utility lines and threaten lives.  Wind damage can occur hundreds of miles inland.  Heavy rains in mountainous areas can cause flash flooding with little warning.
2. Heavy Rain.  Hurricanes may be “dry” or “wet.”  A dry hurricane moves quickly over land and may drop a total of 5 inches of rain or less.  These hurricanes usually do not pose much of a risk from flooding but can cause great wind damage.  Wet hurricanes can drop more than 90 inches of rain per square mile and are slow moving.  If they stall, 18 inches or more of rain could drop.
3. Storm Surge.  A large dome of water, often 50 miles wide and exceed 20 feet high, can sweep across a coastline near the area where the eye of the hurricane makes landfall. The surge, aided by the hammering effect of breaking waves acts like a giant bulldozer sweeping away everything in its path.  This build-up of water level can cause severe flooding in coastal areas, particularly when the storm surge coincides with normal high tides.  Most coastline hurricane deaths are caused by storm surges.

Hurricane Watches and Warnings

The National Weather Service issues a hurricane watch when a hurricane is expected to arrive in your area. You should remain alert for approaching storms, listening carefully to broadcast weather forecasts and alerts. Remind family members where the safest places within your home are located.  A hurricane warning is issued when a hurricane has been sighted or indicated by weather radar to arrive in your area within the next twenty-four (24) hours.

The Saffir-Simpson Hurricane Scale

The Saffir-Simpson Hurricane Scale is a 1-5 rating of the hurricane's present intensity. The rating provides an estimate of the potential property damage and flooding expected to result from a hurricane landfall. Wind speed is the determining factor in the scale, as storm surge values are highly dependent on the slope of the continental shelf in the landfall region. Note that all winds are estimated using the U.S. 1-minute average.

Category One Hurricane: 

Winds 74-95 mph (64-82 kt or 119-153 km/hr). Storm surge generally 4-5 ft above normal. Building structures will sustain minor damage. Damage primarily to unanchored mobile homes, shrubbery, and trees. Some damage to poorly constructed signs. Also, some coastal road flooding and minor pier damage.

Category Two Hurricane: 

Winds 96-110 mph (83-95 kt or 154-177 km/hr). Storm surge generally 6-8 feet above normal.  Damage to buildings will include roofing materials, doors, and windows. Considerable damage to shrubbery and trees with some trees blown down. Considerable damage to mobile homes, poorly constructed signs, and piers. Coastal and low-lying escape routes flood 2-4 hours before arrival of the hurricane center. Small craft in unprotected anchorages break moorings.

Category Three Hurricane: 

Winds 111-130 mph (96-113 kt or 178-209 km/hr). Storm surge generally 9-12 ft above normal. Some structural damage to small residences and utility buildings with a minor amount of curtainwall failures. Damage to shrubbery and trees with foliage blown off trees and large trees blown down. Mobile homes and poorly constructed signs are destroyed. Low-lying escape routes are cut by rising water 3-5 hours before arrival of the center of the hurricane. Flooding near the coast destroys smaller structures with larger structures damaged by battering from floating debris. Terrain continuously lower than 5 ft above mean sea level may be flooded inland 8 miles (13 km) or more. Evacuation of low-lying residences with several blocks of the shoreline may be required.

Category Four Hurricane: 

Winds 131-155 mph (114-135 kt or 210-249 km/hr). Storm surge generally 13-18 ft above normal. More extensive curtainwall failures with some complete roof structure failures on small residences. Shrubs, trees, and all signs are blown down. Complete destruction of mobile homes. Extensive damage to doors and windows. Low-lying escape routes may be cut by rising water 3-5 hours before arrival of the center of the hurricane. Major damage to lower floors of structures near the shore. Terrain lower than 10 ft above sea level may be flooded requiring massive evacuation of residential areas as far inland as 6 miles (10 km).

Category Five Hurricane: 

Winds greater than 155 mph (135 kt or 249 km/hr). Storm surge will be generally greater than 18 ft above normal. Complete roof failure on many residences and industrial buildings. Some complete building failures with small utility buildings blown over or away. All shrubs, trees, and signs will be blown down. Complete destruction of mobile homes. Severe and extensive window and door damage. Low-lying escape routes are cut by rising water 3-5 hours before arrival of the center of the hurricane. Major damage to lower floors of all structures located less than 15 ft above sea level and within 500 yards of the shoreline. Massive evacuation of residential areas on low ground within 5-10 miles (8-16 km) of the shoreline may be required.