A hurricane is a large rotating storm centred around an area of very low pressure, with strong winds blowing at an average speed in excess of 72 miles per hour. The whole storm system may be five to six miles high and 300 to 400 miles wide. It moves forward like an immense spinning top, at speeds of up to 30 m.p.h.
• A “hurricane” is the most severe category of the meteorological phenomenon known as the “tropical cyclone.”
• Tropical cyclones are low pressure systems that haave thunderstorm activity and rotate counterclockwise. A tropical cyclone that has winds of 38 mph (33 kt) or less is called a tropical depression. When the tropical cyclone’s winds reach 39-73 mph (34-63 kt), it is called a tropical storm. When the winds exceed 74 mph (64 kt), the storm is considered to be a hurricane.
• The Saffir-Simpson Hurricane Scale defines hurricane strength by categories. A Category 1 storm is the weakest hurricane (winds 74-95 mph or 64-82 kt); a Category 5 hurricane is the strongest (winds greater than 155 mph orr 135 kt).
The category of the storm does not necessarily relate directly to the damage it will inflict. Lower category storms (and even tropical storms) can cause substantial damage depending on what other weather features they interact with, where they st

trike, and how slow they move.

A hurricane forms in the southern Atlantic Ocean, Caribbean Sea, Gulf of Mexico or in the eastern Pacific Ocean. Hurricanes need warm tropical oceans, moisture and light winds above them. If the right conditions last long enough, a hurricane can produce violent winds, incredible waves, torrential rains and floods.
Hurricanes rotate in a counterclockwise direction around an “eye.” Hurricanes have winds at least 74 miles per hour. There are on average six Atlantic hurricanes each year; over a 3-year period, approximately five hurricanes strike the United States coastline from Texas to Maine.

There is nothing like them in the atmosphere. Born in warm tropical waters, these spiraling masses require a complex combination of atmospheric processes to grow, maature, and then die. They are not the largest storm systems in our atmosphere or the most violent, but they combine these qualities as no other phenomenon does.
In the Atlantic Basin, they are called hurricanes, a term that echoes colonial Spanish and Caribbean Indian words for evil spirits and big winds. These awesome storms have been a deadly problem for residents and sailors ever since the early days of colonization. Today, hurricane damage costs billions.

C a u s e s

o f h u r r i c a n e s

Hurricanes can develop when the sun’s rays heat tropical waters to at least 82 degrees Fahrenheit. This causes the air to grow warmer and rise. Water rises with it as vapor, and the heat causes the air to rise faster and faster. In about 12 hours, the heated air will begin to circle counterclockwise, forming stronger and stronger winds that whirl with increasing speed. Air that is motionless is calm. When it moves slowly, it is a light breeze. If it moves slightly faster, it becomes a fresh breeze, and even faster is a high wind. A wind becomes a hurricane when it reaches speeds of more than 74 miles (120 km) per hour. It is unknown what top hurricane speeds are, because instruments have always been destroyed by high winds or flying debris. Some estimate that hurricane winds do not surpass 250 miles (400 km) per hour, while others say they can reach 600 miles (960 km) per hour.
The moist, hot winds of a hurricane swirl upward around a calm column of low-pressure air known as the eye. Some hurricanes can develop more than one eye, but the largest is always the main eye and me

easures an average of 15 miles across. Inside the eye there are no storm clouds, almost no wind, and calm conditions. However, beneath this tranquil center is wild, stormy chaos.
The rising winds rotate in increasingly tight spirals around the eye, rising faster and faster. Cooler air is also sucked in, heated, and sent twisting up the eye. When the hot air rises high enough, it cools and forms water droplets that cluster together as clouds and drop rain. The rain is blown around by fierce winds.
Hurricanes were once thought to be shaped like doughnuts, circling around the eye. However, satellite images have shown them more to resemble pinwheels, with thicker clouds near the center and long, thin, trailing ends.
The atmospheric pressure is lowest closest to the eye, making the winds there spin faster. At the storm’s outer edges, they blow more slowly. A hurricane moves west or northwest at an average of ten miles an hour, with a wind radius as large as 100 miles. As it moves, it churns up the sea beneath the eye, sucking ocean water up and creating huge ocean waves. The raised water is carried along by the storm to the shore.
The Birth of
f a Tropical Cyclone

Tropical cyclones form over warm waters from pre-existing disturbances. These disturbances typically emerge every three or four days from the coast of Africa as “tropical waves” that consist of areas of unsettled weather. Tropical cyclones can also form from the trailing ends of cold fronts and occasionally from upper-level lows.
The process by which a tropical cyclone forms and subsequently strengthens into a hurricane depends on at least three conditions shown in the figure below
1. A pre-existing disturbance with thunderstorms
2. Warm (at least 80ºF) ocean temperatures to a depth of about 150 feet
3. Light upper level winds that do not change much in direction and speed throughout the depth of the atmosphere (low wind shear)

Heat and energy for the storm are gathered by the disturbance through contact with warm ocean waters. The winds near the ocean surface spiral into the disturbance’s low pressure area. The warm ocean waters add moisture and heat to the air which rises. As the moisture condenses into drops, more heat is released, contributing additional energy to power the storm. Bands of thunderstorms form, and the storm’s cloud tops rise higher into the atmosphere. If the winds at these high levels remain relatively light (little or no wind shear), the storm can remain intact and continue to strengthen.
The Eye
The hurricane’s center is a relatively calm, clear area usually 20-40 miles across. People in the midst of a hurricane are often amazed at how the incredibly fierce winds and rain can suddenly stop and the sky clear when the eye comes over them. Then, just as quickly, the winds and rain begin again, but this time from the opposite direction.

The dense wall of thunderstorms surrounding the eye has the strongest winds within the storm. Changes in the structure of the eye and eyewall can cause changes in the wind speed, which is an indicator of the storm’s intensity. The eye can grow or shrink in size, and double (concentric) eyewalls can form.

The Spiral Rainbands

The storm’s outer rainbands (often with hurricane or tropical storm-force winds) can extend a few hundred miles from the center. Hurricane Andrew’s (1992) rainbands reached only 100 miles out from the eye, while those in Hurricane Gilbert (1988) stretched over 500 miles. These dense bands of thunderstorms, which spiral slowly counterclockwise, range in width from a few miles to tens of miles and are 50 to 300 miles long. Sometimes the bands and the eye are obscured by higher level clouds, making it difficult for forecasters to use satellite imagery to monitor the storm.

How does the hurricane obtain its energy?
Great amounts of energy are transferred when warm water is evaporated from tropical seas. This energy is stored within the water vapour contained in moist air. As this air ascends, 90% of the stored energy is released by condensation, giving rise to the towering cumulus clouds and rain. The release of heat energy warms the air locally, causing a further decrease in pressure aloft. Consequently, air rises faster to fill this area of low pressure, and more warm, moist air is drawn off the sea, feeding further energy to the system. Thus, a self-sustaining heat engine is created.
Only as little as 3% of the heat energy may be converted mechanical energy of the circulating winds. This relatively small amount of mechanical energy equates to a power supply of 360 billion kilowatt hours per day – or six months’ supply of electrical energy for the whole of the USA!
Hurricane prediction
Other than basic knowledge of general hurricane occurrence, there are no atmospheric conditions that can be measured and combined to predict where a hurricane will develop. Therefore, we can only forecast its path once formed. A network of instruments, men and equipment at the National Hurricane Center in Miami, Florida, search out potential hurricanes in their early stages and track them through their life-cycle until they decay and die. Satellites detect hurricanes in their early stages of development and can help to provide early warning of imminent hurricanes. Reinforced aircraft, fitted with instruments, fly through and over hurricanes, and weather radar can locate storms within 200 miles of the radar station.
Hurricanes: where and when?

Great amounts of energy are transferred when warm water is evaporated from tropical seas. This energy is stored within the water vapour contained in moist air. As this air ascends, 90% of the stored energy is released by condensation, giving rise to the towering cumulus clouds and rain. The release of heat energy warms the air locally, causing a further decrease in pressure aloft. Consequently, air rises faster to fill this area of low pressure, and more warm, moist air is drawn off the sea, feeding further energy to the system. Thus, a self-sustaining heat engine is created.
Only as little as 3% of the heat energy may be converted mechanical energy of the circulating winds. This relatively small amount of mechanical energy equates to a power supply of 360 billion kilowatt hours per day – or six months’ supply of electrical energy for the whole of the USA!
Hurricanes form between 5° and 30° latitude and initially move westward (owing to easterly winds) and slightly towards the poles. Many hurricanes eventually drift far enough north or south to move into areas dominated by westerly winds (found in the middle latitudes). These winds tend to reverse the direction of the hurricane to an eastward path. As the hurricane moves poleward it picks up speed and may reach between 20 and 30 m.p.h. An average hurricane can travel about 300 to 400 miles a day, or about 3,000 miles before it dies out.
Hurricanes occur between July and October in the Atlantic, eastern Pacific and the western Pacific, north of the equator. South of the equator, off Australia and in the Indian Ocean, they occur between November and March

The name hurricane should only be used for those tropical storms occurring in the Atlantic. In the Pacific they are known as typhoons, in the Indian Ocean as cyclones and in Australia as willy-willies. They are given names beginning with ‘A’, ‘B’, etc., in order of occurrence, and the names are alternately male and female.

Accompanying phenomena
The most common phenomena associated with hurricanes are strong winds. Other phenomena include:
 hurricane waves – large waves of up to 15 metres high are caused by the strong winds and bring about extensive flooding;
 swells – an increase in ocean level;
 rain – the hurricane picks up about 2 billion tons of moisture per day and releases it as rain.
These phenomena can cause major destruction, especially when the hurricane’s path takes it over land. However, a path over land also causes the destruction of the hurricane itself. As it moves over land, its energy source is depleted and friction across the land surface distorts the air flow. This leads to the eye filling with cloud and the hurricane dies.
Hurricane warnings
America has the most sophisticated hurricane warning system. When there are definite indications that a storm may be growing into a hurricane, the Weather Bureau puts this vast system into action.
A hurricane warning is issued to coastal areas where winds of 74 m.p.h or greater are definitely expected to occur, or dangerously high water or high waves are predicted. The general public are usually informed via television broadcasts and through a system of flying flags by day and lanterns by night.

Thankfully, the number of people injured or killed during tropical cyclones in the United States has been declining, largely because of improvements in forecasting and emergency preparedness. Nonetheless, our risk from hurricanes is increasing. With population and development continuing to increase along coastal areas, greater numbers of people and property are vulnerable to hurricane threat. Large numbers of tourists also favor coastal locations, adding greatly to the problems of emergency managers and local decision makers during a hurricane threat.

The Richelieu Apartments before Hurricane Camille The Richelieu Apartments after Hurricane

The ingredients for a hurricane include a pre-existing weather disturbance, warm tropical oceans, moisture, and relatively light winds aloft. If the right conditions persist long enough, they can combine to produce the violent winds, incredible waves, torrential rains, and floods we associate with this phenomenon.
Each year, an average of ten tropical storms develop over the Atlantic Ocean, Caribbean Sea, and Gulf of Mexico. Many of these remain over the ocean. Six of these storms become hurricanes each year. In an average 3-year period, roughly five hurricanes strike the United States coastline, killing approximately 50 to 100 people anywhere from Texas to Maine. Of these, two are typically major hurricanes (winds greater than 110 mph).
Hurricane Size

Typical hurricanes are about 300 miles wide although they can vary considerably, as shown in the two enhanced satellite images below. Size is not necessarily an indication of hurricane intensity. Hurricane Andrew (1992), the most devastating hurricane of this century, was a relatively small hurricane.

Hurricane Danny (left) in 1997 and Hurricane Fran in 1996 show the variability in hurricane size
Do not focus on the location and track of the center, because the hurricane’s destructive winds and rains cover a wide swath. Hurricane-force winds can extend outward to about 25 miles from the storm center of a small hurricane and to more than 150 miles for a large one. The area over which tropical storm-force winds occur is even greater, ranging as far out as almost 300 miles from the eye of a large hurricane.

Hurricane Circulation and Movement

In the northern hemisphere, hurricane winds circulate around the center in a counter-clockwise fashion. This means that the wind direction at your location depends on where the hurricane’s eye is. A boat on the northern edge of the orange area in Hurricane Fran (right) would experience winds from the east, while a boat on the southern edge would have westerly winds.

A hurricane’s speed and path depend on complex interactions between the storm with its own internal circulations and the earth’s atmosphere. The air in which the hurricane is embedded is a constantly moving and changing “river” of air. Other features in that flow, such as high and low pressure systems, can greatly alter the speed and the path of the hurricane. In turn, it can modify the environment around the storm.

Typically, a hurricane’s forward speed averages around 15-20 mph. However, some hurricanes stall, often causing devastatingly heavy rain. Others can accelerate to more than 60 mph. Hurricane Hazel (1954) hit North Carolina on the morning of 15 October; fourteen hours later it reached Toronto, Canada where it caused 80 deaths.
The Right Side of the Storm
As a general rule of thumb, the hurricane’s right side (relative to the direction it is travelling) is the most dangerous part of the storm because of the additive effect of the hurricane wind speed and speed of the larger atmospheric flow (the steering winds). The increased winds on the right side increase the storm surge described in the Hazards section of this module. Tornadoes are also more common here.
Looking at the figure above, pretend you are standing behind the hurricane with your back to the steering flow. In this case, the right side is the eastern section of the hurricane. (If it were travelling east to west, the right side would be the north section.) The winds around the hurricane’s eye are moving in a counterclockwise fashion. At Point A, the hurricane winds are nearly in line with the steering wind, adding to the strength of the winds. For example, if the steering currents are 30 mph and the average hurricane winds are 100 mph, the wind speed would be 130 mph at Point A. On the other hand, the winds at Point B are moving opposite those of the steering wind and therefore slow to 70 mph (100 – 30 mph). Incidentally, NHC forecasts take this effect into account in their official wind estimates.
P r e p a r a t i o n

• The best way to prepare for a hurricane is to listen to NOAA warnings. Knowing when and where a hurricane will strike will give you precious time to flee or protect yourself. Sometimes, even concrete walls of a building are not enough to ward off a hurricane’s full power. Storm winds can force water into masonry, causing them to weaken and crumble.
• Make sure your car is full of gas.
• Be aware of changing weather conditions around you, especially dark skies, rising winds, and rain.
• Put trash cans, gardening tools, barbecues, toys, outdoor furniture, and any other unconfined objects inside or attach them firmly to the ground.
• If you are near the coast, watch for incoming waves.
• If in the water or a boat, get to land immediately and find shelter from the storm.
• Make sure any water vehicles are securely fastened.
• Keep your ears open for storm warning updates.
• Fortify windows and close windows because hurricane winds can break windows. Try crisscrossing them with tape to make them more shatter-resistant.
• Take care that pets and animals are secure.
• Secure objects inside the house that could fly around.
• Check food, camping, and emergency equipment.
• Move people and valuables to the side away from the wind.
• Have fresh batteries for flashlights and portable radios.
• Store water in plastic bottles, jugs, pots, pans, and bathtubs.
• If you live in an R.V. or mobile home, leave to a designated shelter. Staying in a mobile home or beach house during is hurricane is also very risky, for they can be flattened, blown over, or swept away in floods. Those near shore are at risk from higher-than-normal waves.
• For those choosing to stay in mobile homes during the storm, try tying them down firmly with cables.
• People in low-lying areas need to seek shelter when hurricane warnings are issued. It is best to go to high ground 15 or 20 miles away from the coast, for hurricanes can bring ten-foot-high waves or higher.
• Leash pets or place them in carriers.
• Plan a quick, reliable escape route to safer high ground.
• Stay inside the house if your home is built solidly on high ground.
• Do not try to rescue people or animals during the hurricane.
• Know that the eye of the storm is the calm center and can last from minutes to half an hour. Do not go outside until you hear the all-clear signal. Many of the people who perish in hurricanes are those who venture outside to see the damage, or those who try to swim or ride high ocean waves. During 1967’s Hurricane Beulah, a 15-year-old Texan girl went surfing through the huge waves during the eye. She was drowned when the second part of the hurricane hit.
• Weather the storm inside a basement or storm cellar if you have one.
• If you do not have a basement or storm cellar, stay in the lowest story of the building. Remember to stay away from windows.
• In you are sitting in a wheelchair, set the brake and protect your head.
• If you are outside, find cover and get down.
• If you are driving, stop the car, get out, and find cover.
• Stay away from trees, buildings, windows, and power lines.
• For those living in the city, avoid downtown areas.
• Turn the refrigerator to maximum cold and open it as little as you can.
• Leave a few windows on the side away from the wind slightly open to prevent air pressing from building and blowing out walls.
• Eat the food that will perish most easily first. Don’t use any tap water unless you are sure it isn’t contaminated.
• Never stay outside to watch the storm because its fierce winds are extremely dangerous. You could be struck by flying debris or electrocuted by falling power lines. Huge waves and flood waters can sweep unwary people away.
• Check for injuries and help the wounded if necessary.
• Dress in heavy shoes and clothing.
• If you must drive (avoid it if possible), be careful of debris and collapsible roads. Also watch out for landslides.
• Avoid rivers and streams.
• Turn on your radio or TV for information regarding flood warnings, shelter locations, road conditions, emergency first aid stations, and evacuation routes.
• If you need medical help, go to a Red Cross station.
• Keep out of the way of relief crews and emergency rescue workers.
• Look out for loose electrical wires, and report them to officials.
• Fallen power lines (especially those lying in water!), weakened tree limbs, reptiles, and collapsing structures are still dangers to be cautious of.
• Report broken water mains and sewers to the utility companies.
• Do not go into an unstable-looking house.
• Even after checking for gas leaks, don’t light matches.
• Clean up any spills if possible.
• Do not use phones except for emergencies.
• Keep animals under control.
• If necessary, evacuate.

H u r r i c a n e s p r e v e n t i o n

In October 1947, weather planes tried to weaken a hurricane at sea by dropping dry ice into developing clouds. Scientists hoped to force the system to release most of its power as rain before it reached shore. The experiment didn’t work, and perhaps even caused it to turn and strike Savannah, Georgia (USA).
No more attempts were made to prevent a hurricane until 1958, when an oceanic hurricane was seeded with silver iodine crystals. The experimental results were inconclusive, but tests continue to be made. Weather planes will fly into hurricanes at up to 40,000 feet, releasing chemicals at different points and time intervals to try and reduce their power. So far, all the effects are only temporary. Treated hurricanes regain their full power and continue traveling northwest toward land.

H u r r i c a n e s i n t h e f u t u r e

Although we’ve learned a lot about hurricanes in the last five decades, it is still not entirely understood why some conditions produce hurricanes and others do not. Weather forecasters cannot always predict with absolute confidence the areas that will be hit. These storms also move erratically, a mystery that scientists continue to work on. NOAA officials continue to analyze hurricane data, and the information they learn will provide us will more accurate, earlier warnings.
The need for hurricane warnings is also increasing, as more and more people are moving into coastal regions in the Eastern and Southeastern United States. Many have never been through a hurricane before. The NOAA is trying to spread awareness about hurricane prediction and preparation. Spreading education is vital to saving lives and homes.
Hurricanes are one of the most dangerous natural hazards to people and the environment. Every year, immense damage is done by hurricanes and other similar tropical storms.
However, hurricanes are essential features of the Earth’s atmosphere, as they transfer heat and energy between the equator and the cooler regions towards the poles.

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