A hurricane is a type of tropical cyclone, which is the general term for all circulating weather systems over tropical waters. In the Northern Hemisphere these storms circulate in a counterclockwise rotation. Tropical cyclones are classified as follows:
Tropical Depression - Maximum sustained winds less than 39 mph
Tropical Storm - Maximum sustained winds from 39 to 73 mph
Hurricane - Maximum sustained winds of 74 mph or greater (See Saffir-Simpson Scale).
Hurricanes are products of the tropical ocean and atmosphere. Powered by heat from the sea, they are steered by the easterly trade winds and the temperate westerlies as well as by their own ferocious energy. Around their core, winds grow with great velocity, generating violent seas. Moving ashore, they sweep the ocean inward while spawning tornadoes and producing torrential rains.
The Atlantic Hurricane Season begins June 1 and ends November 30 each year. The peak hurricane threat for the U.S. exists from mid-August to late October. Developing hurricanes gather heat and energy through contact with warm ocean waters. The addition of moisture by evaporation from the sea surface powers them like giant heat engines.
The process by which a disturbance forms and subsequently strengthens into a hurricane depends on at least three conditions. Warm waters and moisture are mentioned above. The third condition is a wind pattern near the ocean surface that spirals air inward. Bands of thunderstorms form, allowing the air to warm further and rise higher into the atmosphere. If the winds at these higher levels are relatively light, this structure can remain intact and allow for additional strengthening.
The center, or eye, of a hurricane is relatively calm. The most violent activity takes place in the area immediately around the eye, called the eyewall. At the top of the eyewall (about 50,000 feet), most of the air is propelled outward, increasing the air's upward motion. Some of the air, however, moves inward and sinks into the eye, creating a cloud free area.
Storm Surge and Storm Tide
Storm surge is a large dome of water, often 50 - 100 miles wide, that sweeps across the coastline near where the hurricane makes landfall. The surge of high water topped by waves is devastating. Storm tide is the combination of storm surge and the normal astronomical tide. If the storm arrives at the same time as high tide, the water height will be even greater.
A 15 foot surge added to the normal 2 foot tide creates a storm tide of 17 feet. This mound of water, topped by battering waves, moves ashore along the coastline. The combination of storm surge, battering waves, and high winds is deadly.
Hurricane force winds of 74 mph or greater, can destroy poorly constructed buildings and mobile homes. Debris, such as signs, roofing material, siding and small items left outside, become flying missiles. Winds often stay above hurricane strength well inland. It is important to prepare your home from the effects of hurricane force winds.
Widespread torrential rains often in excess of six inches can produce deadly and destructive floods. Normal drainage systems along the coast may be full from the storm surge, causing flooding in areas not normally prone to floods. Flooding may also be a major threat to areas well inland. In 1979, Tropical Storm Claudette brought 45 inches of rain to an area near Alvin, Texas, contributing to more than $600 million in damage.
Flooding is the number one weather-related killer in the United States and cause more damage nationwide than any other natural disaster. Flash flooding occurs within six hours of a rain event. Flooding is a longer term event and may last a week or more. Inland flooding due to hurricanes can be extensive as rainfall from a hurricane sometimes can be measured in tens of inches.
Nearly half of all flash flood fatalities are auto related. Just two feet of water will carry away most automobiles. Never drive through flooded roadways. If your vehicle stalls, leave it immediately and seek higher ground. Rapidly rising water may engulf the vehicles and its occupants and sweep them away. Be especially careful when driving at night.
Do you have flood insurance? Many people don't realize that their homeowner's policies do not cover losses due to flooding. If you are in a flood zone, you need flood insurance.
If you are in a flood zone and a flood warning has been issued, evacuate immediately. If you have time, turn off all utilities at the main switch, open basement windows to equalize water pressure on the foundations and wills and move all valuables to a higher floor if possible, but only if you have time. If you're caught in the house by suddenly rising waters, move to the second floor and/or the roof. Take warm clothing and a flashlight and radio with you. Do not try to swim to safety. Wait for help. Rescue teams will be looking for you.
After a flood, call your insurance agent. Have your policy and list of possessions handy to simplify the adjuster's work. When it is safe to return home, be sure your house is not in danger of collapsing before entering. Watch for live electrical wires and don't turn on any electrically operated light or appliance until an electrician has checked your system. Don't strike a match or use a flame as escaping gas could cause an explosion. Open windows and doors to let air circulate. Take photos to record the damage. Throw out perishable foods, hose down appliances and furniture, even if they have been destroyed. You need to keep these for the adjuster's inspection. Pump basement gradually (one third each day to prevent further structural damage). Shovel out mud while it is still wet. Have your water tested before using (see Sewage Contamination). Make any temporary repairs necessary to stop further losses from the elements and prevent looting.
Hurricanes also produce tornadoes, which add to the storm's destructive power. These tornadoes most often occur in the thunderstorms embedded in rain bands well away from the center of the hurricane. They can, however, also occur near the eyewall.
Nature's Most Violent Storms
Although tornadoes occur in many parts of the world, these destructive forces of nature are found most frequently in the U.S. east of the Rocky Mountains during the spring and summer months. In an average year, 800 tornadoes are reported nationwide, resulting in 80 deaths and over 1,500 injuries.
A tornado is defined as a violently rotating column of air extending from a thunderstorm to the ground. The most violent are capable of tremendous destruction with wind speeds of 250 mph or more. Damage Paths can be in excess of one mile wide and 50 miles long.
What Causes Tornadoes?
Thunderstorms develop in warm, moist air in advance of eastward-moving cold fronts. These thunderstorms often produce large hail, strong winds, and tornadoes. Occasionally, large outbreaks of tornadoes occur along strong frontal systems that form in the Central States and move east. Several states may be affected by numerous severe thunderstorms and tornadoes.
Tornadoes occasionally accompany tropical storms and hurricanes that move over land. They are most common to the right and ahead of the path of the storm center as it comes ashore. Before thunderstorms develop, a change in wind direction and an increase in wind speed with increasing height creates an invisible, horizontal spinning effect in the lower atmosphere.
Rising air within the thunderstorm updraft then tilts the rotating air from horizontal to vertical. An area of rotation, 2-6 miles wide, now extends through much of the storm. Most strong and violent tornadoes form within this area of strong rotation.
Tornadoes take many shapes and sizes:
Weak Tornadoes - Account for 69% of all tornadoes and less than 5% of the deaths. Their lifetime is usually less than 10 minutes with winds less than 110 mph.
Strong Tornadoes - Account for 29% of all tornadoes and nearly 29% of the deaths. They last 20 minutes or longer with winds of 110-205 mph.
Violent Tornadoes - Account for only 2% of all tornadoes and 70% of the deaths. They can last over one hour with winds greater than 205 mph.
Waterspouts - Weak tornadoes that form over warm water and are most common along the Gulf Coast. Waterspouts occasionally move inland becoming tornadoes and causing damage and injuries.
Know The Terms!
Tornado Watch - Issued when conditions are favorable for tornadoes to develop. You should monitor local radio or TV stations to stay informed and to know immediately if a Tornado Warning is issued.
Tornado Warning - Issued when a tornado has been sighted in the area. Take shelter immediately! New radar technology known as DOPPLER has the ability to detect wind directions that may indicate a tornado and a tornado warning may be issued before one is actually sighted by the public.
NOTE: In this area, tornadoes form quickly and seldom last very long. Warning for these types of tornadoes is often not possible. Remain alert for signs of an approaching tornado. Many people have stated that a tornado "sounds like a train". Tornadoes are usually not detected until they have picked up visible dust and debris.
What To Do If A Warning Is Issued
In a building, move to a pre-determined shelter such as a basement. If underground shelter is not available, move to an interior room or hallway on the lowest floor and get under a sturdy piece of furniture.Stay away from windows.Get under something sturdy like a heavy table. If you can, cover yourself with a blanket or sleeping bag.In a high rise building, use the stairs to go to the designated shelter area or an interior room on the lowest floor possible.Get out of automobiles. Do not try to outrun a tornado in your car.If caught outside, lie flat in a nearby ditch or depression in the ground. Mobile homes, even if tied down, offer little protection from tornadoes and should be abandoned.
Also see the Fujita Scale (used to classify the extent of a storm's damage and power).