What is a Hurricane?
A hurricane is a type of tropical cyclone, which is a generic term for a low pressure system that generally forms in the tropics. The cyclone is accompanied by thunderstorms and, in the Northern Hemisphere, a counterclockwise circulation of winds near the earth’s surface.
When are Hurricanes most likely to occur?
Different areas of the world have different times when tropical cyclones are most likely to form. These periods are called Hurricane Seasons:
- Atlantic and Caribbean: June 1 to November 30 with peak season mid-August to late October.
- Central Pacific (Hawaii): June 1 to November 30 with peak season from July to September.
- East Pacific: May 15 to November 30
- Western North Pacific: Tropical cyclones can strike year round
Different Types of Tropical Cyclones
Tropical cyclones are classified as follows:
- Tropical Depression. An organized system of clouds and thunderstorms with a defined surface circulation and maximum sustained winds* of 38 mph or less
- Tropical Storm. An organized system of strong thunderstorms with a defined surface circulation and maximum sustained winds of 39-73 mph (34-63 kt)
- Hurricane. An intense tropical weather system of strong thunderstorms with a well-defined surface circulation and maximum sustained winds of 74 mph (64 kt) or higher
Hurricanes are also categorized according to the strength of their winds using the Saffir-Simpson Hurricane Scale. A Category 1 storm has the lowest wind speeds, while a Category 5 hurricane has the strongest. These are relative terms, because lower category storms can sometimes inflict greater damage than higher category storms, depending on where they strike and the particular hazards they bring. In fact, tropical storms can also produce significant damage and loss of life, mainly due to flooding.
When the winds from a tropical cyclone reaches 39 mph (34 kts), the cyclones are given names. Years ago, an international committee developed names for Atlantic cyclones (The History of Naming Hurricanes). In 1979 a six year rotating list of Atlantic storm names was adopted — alternating between male and female hurricane names. Storm names are used to facilitate geographic referencing, for warning services, for legal issues, and to reduce confusion when two or more tropical cyclones occur at the same time. Through a vote of the World Meteorological Organization Region IV Subcommittee, Atlantic cyclone names are retired usually when hurricanes result in substantial damage or death or for other special circumstances.
Storm surge and large waves produced by hurricanes pose the greatest threat to life and property along the coast. Storm Surge is an abnormal rise of water generated by a storm’s winds. Storm surge can reach heights well over 20 feet and can span hundreds of miles of coastline.
In the northern hemisphere, the highest surge values typically occur in the right front quadrant of a hurricane coincident with onshore flow; in the southern hemisphere, the left front quadrant. More intense and larger hurricanes produce higher surge. In addition, shallower offshore waters contribute to higher storm surge inundation. Storm surge is by far the greatest threat to life and property along the immediate coast.
Storm Tide is the water level rise during a storm due to the combination of storm surge and the astronomical tide. For example, if a hurricane moves ashore at a high tide of 2 feet, a 15 foot surge would be added to the high tide, creating a storm tide of 17 feet. The combination of high winds and storm tide topped with battering waves can be deadly and cause tremendous property damage along an area of coastline hundreds of miles wide.
The destructive power of storm surge and large battering waves can result in loss of life, buildings destroyed, beach and dune erosion and road and bridge damage along the coast. Storm surge can travel several miles inland. In estuaries and bayous, salt water intrusion endangers public health and the environment.
Hurricane-force winds, 74 mph or more, can destroy buildings and mobile homes. Debris, such as signs, roofing material, siding and small items left outside become flying missiles during hurricanes. Winds can stay above hurricane strength well inland. In 2004, Hurricane Charley made landfall at Punta Gorda on the southwest Florida coast and produced major damage well inland across central Florida with gusts of more than 100 mph.
Tropical cyclones can also produce dangerous tornadoes. It is not unusual for tornado watches to be issued as a storm makes landfall and these can continue well after the storm moves inland. Listen for tornado warnings and take cover as directed.
Tropical cyclones often produce widespread, torrential rains in excess of 6 inches, which may result in deadly and destructive floods. In fact, flooding is the major threat from tropical cyclones for people living inland.
Flash flooding, defined as a rapid rise in water levels, can occur quickly due to intense rainfall. Longer term flooding on rivers and streams can persist for several days after the storm.
Rainfall amounts are not directly related to the strength of tropical cyclones but rather to the speed and size of the storm, as well as the geography of the area. Slower moving and larger storms produce more rainfall. In addition, mountainous terrain enhances rainfall from a tropical cyclone.
Even if a tropical cyclone stays well offshore, it can still be dangerous. The strong winds of a tropical cyclone can cause dangerous waves that pose a significant hazard to mariners and coastal residents and visitors. When the waves break along the coast, they can produce deadly rip currents—even at large distances from the storm.
Rip currents are channeled currents of water flowing away from shore, usually extending past the line of breaking waves that can pull even the strongest swimmers away from shore. In 2008, despite the fact that Hurricane Bertha was more than a 1,000 miles offshore, the storm resulted in rip currents that killed three people along the New Jersey coast and required 1,500 lifeguard rescues in Ocean City, Maryland, over a 1 week period. In 2009, all six deaths in the United States directly attributable to tropical cyclones occurred as the result of drowning from large waves or strong rip currents.
Before a Hurricane
To prepare for a hurricane, you should take the following measures:
- Determine safe evacuation routes inland.
- Learn locations of official shelters.
- Check emergency equipment, such as flashlights, generators and battery-powered equipment such as cell phones and your NOAA Weather Radio All Hazards receiver.
- Buy food that will keep and store drinking water.
- Review your insurance policy.
- Make plans to secure your property. Permanent storm shutters offer the best protection for windows. A second option is to board up windows with 5/8” marine plywood, cut to fit and ready to install. Tape does not prevent windows from breaking.
- Install straps or additional clips to securely fasten your roof to the frame structure. This will reduce roof damage.
- Be sure trees and shrubs around your home are well trimmed.
- Clear loose and clogged rain gutters and downspouts.
- Determine how and where to secure your boat.
- If you have pets, include them in your preparedness plan. Look for pet-friendly hotels or shelters on your evacuation route.
Build An Emergency Kit
An Emergency Supplies Kit Should include:
- At least a 3-day supply of water (one gallon per person, per day)
- At least a 3-day supply of non-perishable food
- At least, one change of clothing and shoes per person
- One blanket or sleeping bag per person
- First-aid kit
- Battery-powered NWR and a portable radio
- Flashlight, extra batteries
- Extra set of car keys
- Credit card and cash
- Special items for infant, elderly or disabled family members
- Prescription and non-prescription medicines
- Pet supplies, including carrier, leash, food, tags and licenses
During a Hurricane
If a hurricane is likely in your area, you should:
- Listen to the radio or TV for information.
- Secure your home, close storm shutters, and secure outdoor objects or bring them indoors.
- Turn off utilities if instructed to do so. Otherwise, turn the refrigerator thermostat to its coldest setting and keep its doors closed.
- Turn off propane tanks.
- Avoid using the phone, except for serious emergencies.
- Moor your boat if time permits.
- Ensure a supply of water for sanitary purposes such as cleaning and flushing toilets. Fill the bathtub and other large containers with water.
You should evacuate under the following conditions:
If you are directed by local authorities to do so. Be sure to follow their instructions.
- If you live in a mobile home or temporary structure—such shelters are particularly hazardous during hurricanes no matter how well fastened to the ground.
- If you live in a high-rise building—hurricane winds are stronger at higher elevations.
- If you live on the coast, on a floodplain, near a river, or on an inland waterway.
- If you feel you are in danger.
If you are unable to evacuate, go to your safe room. If you do not have one, follow these guidelines:
- Stay indoors during the hurricane and away from windows and glass doors.
- Close all interior doors—secure and brace external doors.
- Keep curtains and blinds closed. Do not be fooled if there is a lull; it could be the eye of the storm – winds will pick up again.
- Take refuge in a small interior room, closet, or hallway on the lowest level.
- Lie on the floor under a table or another sturdy object.
After the Storm
- Keep listening to radio, TV or NOAA Weather Radio All Hazards for updated information.
- Wait until an area is declared safe before entering. If you have evacuated, be patient.
- Wait until local authorities have ensured the area is safe for reentry before you try to return home.
- Watch for closed roads. If you come upon a barricade or a flooded road, Turn Around Don’t Drown!
- Stay on firm, dry ground. Moving water only 6 inches deep can sweep you off your feet.
- Standing water may be electrically charged from power lines.
- If using a generator, avoid carbon monoxide poisoning by following the manufacturer’s instructions.
- Avoid weakened bridges and washed out roads.
- Once home, check gas, water and electrical and appliances for damage.
- Use a flashlight to inspect damage. Never use candles and other open flames indoors.
- Wear proper shoes to prevent cutting feet on sharp debris.
- Do not drink or prepare food with tap water until officials say it is safe.
- Avoid electrocution by not walking in areas with downed power poles or standing water.
Know These Key Terms
Familiarize yourself with these terms to help identify a hurricane hazard:
Tropical Depression. An organized system of clouds and thunderstorms with a defined surface circulation and maximum sustained winds of 38 MPH (33 knots) or less. Sustained winds are defined as one-minute average wind measured at about 33 ft (10 meters) above the surface.
Tropical Storm. An organized system of strong thunderstorms with a defined surface circulation and maximum sustained winds of 39–73 MPH (34–63 knots).
Hurricane. An intense tropical weather system of strong thunderstorms with a well-defined surface circulation and maximum sustained winds of 74 MPH (64 knots) or higher.
Storm Surge. A dome of water pushed onshore by hurricane and tropical storm winds. Storm surges can reach 25 feet high and be 50–1000 miles wide.
Storm Tide. A combination of storm surge and the normal tide (i.e., a 15-foot storm surge combined with a 2-foot normal high tide over the mean sea level created a 17-foot storm tide).
Hurricane/Tropical Storm Watch. Hurricane/tropical storm conditions are possible in the specified area, usually within 36 hours. Tune in to NOAA Weather Radio, commercial radio, or television for information.
Hurricane/Tropical Storm Warning. Hurricane/tropical storm conditions are expected in the specified area, usually within 24 hours.
Short Term Watches and Warnings. These warnings provide detailed information about specific hurricane threats, such as flash floods and tornadoes.
This post was originally featured on The Salvation Army’s Emergency Disaster Services website.
Photo from istockphoto.com.