Hurricanes are one of the most dangerous natural hazards to people and the environment. They are essential features of the Earth’s atmosphere, as they transfer heat and energy between the equator and the cooler regions towards the poles.

What is a hurricane?

A hurricane is a large rotating storm (60 to 1,000 miles in diameter) that forms over warm ocean waters. Hurricanes start life as a cluster of strong thunderstorms moving across the ocean, called a "tropical disturbance" or "tropical wave." Atmospheric conditions must be just right to turn a tropical wave into a hurricane — less than 5 percent of all tropical waves turn into a full-blown hurricane.

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 History of Hurricanes

Scientists have only been studying hurricanes only for about 100 years. But there is evidence of hurricanes occurring long in the past. For example, geologists (scientists who study the earth) believe that layers of sediment in a lake in Alabama was brought there by a hurricane in the Gulf of Mexico as long as 3,000 years ago! There is also evidence in Florida of hurricanes more than 1,000 years ago.

One of the first human records of hurricanes appears in Mayan hieroglyphics. The Mayans also practiced a kind of mitigation and risk reduction by building their major settlements away from the hurricane-prone coastline. In fact, it is the Mayan word "Hurakan" that became our word "hurricane." Hurakan was the name of one of their gods, who, they believed, blew his breath across the water and brought forth dry land. Later, Carib Indians gave the name "Hurican" to one of their gods of evil.

Many storms left important marks on history. In 1565, a hurricane scattered a French fleet of war ships and allowed the Spanish to capture a French fort in what is now Florida. In 1609, a fleet of ships carrying settlers from England to Virginia was struck by a hurricane. Some of the ships were damaged and part of the fleet grounded on Bermuda, an island nation in the Atlantic. These passengers became the first people to live on Bermuda. In 1640, a hurricane partially destroyed a large Dutch fleet that was poised to attack Cuba.

There were a number of particularly severe hurricanes as the U.S. went from the 1800s to the 1900s. Hurricanes hit Louisiana, South Carolina and Georgia in 1893 and killed as many as 4,000 people. In 1900, a famous Texas hurricane killed more than 8,000 people and was a Category 4 storm.

As forecasting improved communities were no longer surprised by hurricanes and could take measures to evacuate ahead of the storm. While destruction still continues, the number of deaths in hurricanes had dropped significantly.

How do hurricanes form?

There are various trigger mechanisms required to transform frequent storms into rarer hurricanes. These trigger mechanisms depend on several conditions being ‘right’ at the same time. The most influential factors are:

• a source of very warm, moist air - derived from tropical oceans with surface temperatures greater than 26C,
• sufficient spin or twist from the rotating earth - this is related to latitude

As the warm sea heats the air above it, a current of very warm moist air rises up quickly, creating a centre of low pressure at the surface. Trade winds rush in towards this low pressure and the inward spiralling winds whirl upwards releasing heat and moisture before descending.

The rotation of the Earth causes the rising column to twist, gradually taking on the form of a cylinder whirling around an eye of relatively still air, free from clouds. The rising air cools and produces towering cumulus and cumulonimbus clouds.

Further aloft at 6 miles the cloud tops are carried outwards to give thick layer clouds due to the outward spiralling winds leaving the hurricane core.

Anatomy of a Hurricane

Hurricanes have an "eye" of calm winds and low pressure, surrounded by an "eyewall" of intense thunderstorms with high winds and heavy rain. "Spiral bands" of intense thunderstorms spiral into the eyewall of the hurricane from the outer parts of the storm. "Tropical cyclone" is the generic name for a hurricane. Tropical cyclones are called "typhoons" when they occur in the western Pacific Ocean, and "cyclones" in the Indian and south Pacific Oceans.

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.

1. Exhaust
Hot air drawn into the atmosphere.

2. Storm Clouds
Spiral in upper atmosphere.

3. Eye
Cool air descends into the 20-mile wide eye, creating a small center of calm weather.

4. Eye Wall
Storm's fiercest winds.

5. Spiraling Winds
In the hurricane's lower realms, air flows in towards the center and whirls upward. These counterclockwise winds gain speed as they approach the eye, like a whirlpool. The narrower the eye, the stronger the wind.

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 6 months supply of electrical energy for the whole of the USA!

Hurricanes typically last from 2 to 14 days. They tend to move from east to west, at speeds between 10 and 30 mph. Their intensity is ranked on a scale of 1 to 5 called the "Saffir-Simpson scale." The Saffir-Simpson scale measures three types of activity: wind speed, air pressure, and storm surge (a large dome of water, often 50 to 100 miles wide, that sweeps across the coastline near where a hurricane makes landfall).

In the Caribbean, the official hurricane season is from June 1 to November 30, but hurricanes can happen any time of the year. Hurricanes are named by the National Weather Service. Some past hurricanes affecting the Caribbean have been named: David, Gilbert, Andrew, Marilyn, Hugo and Fran.

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.

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.

Hurricanes: where and when do they occur?

Hurricanes are products of a 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 and floods.

Each year, on average, 10 tropical storms, of which six become hurricanes, develop over the Atlantic Ocean, Caribbean Sea, or Gulf of Mexico. Many of these remain over the ocean; however, about five hurricanes strike the United States coastline every three years. Of these five, two will be major hurricanes, category 3 or greater on the Saffir-Simpson Hurricane Scale.

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 30mph. An average hurricane can travel about 300 to 400 miles a day, or about 3000 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.

In the eastern Pacific, hurricanes start forming by mid-May. In the Atlantic, Caribbean, and Gulf of Mexico, hurricanes season starts  in June. For the United States, peak hurricane threat exists from mid-August to late October although the official hurricane season extends through November. Over other parts of the world, such as the western Pacific, hurricanes can occur year-round.

Typical areas where hurricanes begin their development and the common paths of storm movement.

Keep these rules of thumb in mind:

• June- do not expect much activity.
• July- a slight increase in activity.
• August- especially mid to late August- a rapid increase in activity.
• September- increased upswing in number of storms and hurricanes.
• October- trend towards less activity- though still somewhat active in earlyOctober.
• November- activity greatly diminishes.

Storm surge

The most dangerous part of a hurricane is its storm surge - literally a huge wall of water that can flood areas near the shore. Although hurricanes are usually described in terms of its wind speeds, flooding kills more people than wind.

http://hurricane.accuweather.com

Low pressure in the hurricane can act as a plunger, slightly pulling up the water level. However, the components that contribute to the greatest storm surge affect are the winds blowing to the left side of the storm and the topography of the land as the storm makes land fall. The strongest surge comes ashore just to the right of the eye, where the fierce hurricane winds are blowing toward land. Winds on the left side of the storm might actually cause the water level to run slightly lower than normal. Higher water level allows waves to strike farther inland, causing massive property damage.

Storm Tide

If the storm surge arrives at the same time as the high tide, the water height will be even greater. The storm tide is the combination of the storm surge and the normal astronomical tide. For example as hurricane moves ashore, 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 an area of the coastline as much as 100 miles wide. The combination of the storm surge, battering waves, and high winds is deadly.

Other Hurricane Effects

Tornadoes: may form especially in the spiral rainbands of a hurricane as it moves onshore. The changing wind speeds with height acts like a huge twisting mechanism, thus allowing the possibility of tornado formation.

Flooding Rain: is a very real threat from a landfalling hurricane. (Hurricane Agnes in 1972 and hurricane Floyd in 1999 caused severe flooding across inland locations long after landfall.) This is especially true if the hurricane stalls out over a region, raining itself out. This can be a serious problem for areas hundreds of miles away from where the hurricane (or tropical storm) initially came ashore. Even if you live 'inland' it is important to keep tabs on the movement of any potentially landfalling tropical cyclone.

Damaging Winds: will accompany any hurricane, no matter what category it is. A hurricane, by definition, has winds of at least 74 miles per hour. This wind speed alone is enough to cause damage to poorly constructed signage and knock over some trees and other vegetation. Obviously, the stronger the hurricane (higher winds), the more potential for wind damage exists. See Hurricane Preparation for more information about preparing for the high winds of a hurricane.

Lightning and Hail: are less frequent occurrences during hurricanes than during other severe weather events- like thunderstorms. Lightning is more frequent during a typical afternoon thunderstorm because there are more factors present that promote lightning development. The same reason generally holds true concerning hail during a hurricane. There will be some lightning during a hurricane but some of the 'flashes' will actually be electric transformers exploding or powerlines sparking; sending an eerie glow into the sky.

Death of a Hurricane

Just as many factors contribute to the birth of a hurricane, there are many reasons why a hurricane begins to decay. Wind shear can tear the hurricane apart. Moving over cooler water or drier areas can lead to weakening as well. Landfall typically shuts off the hurricane's main moisture source, and the surface circulation can be reduced by friction when it passes over land. This leads to the eye filling with cloud and the hurricane dies.Generally, a weakening hurricane or tropical cyclone can reintensify if it moves into a more favorable region or interacts with mid-latitude frontal systems.

Tracking and Forecasting Tropical Systems

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.

Satellite loop showing the eye of Hurricane Luis (1995)

When a hurricane gets close to the coast, it is monitored by land-based weather radars.The Doppler radars are so sensitive they can detect and track clouds and even the presence of weather fronts that have no precipitation or clouds. (Severe storms often form along such invisible boundaries.) Because the radar scans at multiple elevation angles, forecasters can "see" the full structure of a storm cell, including storm tops and the presence of intense updrafts and downdrafts. Sophisticated mathematical calculations give forecasters important information derived from the radar data, such as estimates of rainfall amounts.

How to use a Hurrine Tracking Map

Meteorologists track the movement of a hurricane using latitude and longitude - and so can you! Latitude and longitude can be used to pinpoint any position on the globe. Latitude measures the distance north or south of the equator. Longitude measures the distance east or west from Greenwich, England, which is at 0 degrees longitude.

Each horizontal line on the tracking map represents a different line of latitude. These lines are labeled along the right edge of the tracking chart. For example, the bottommost thick horizontal line is labeled as 10 degrees North latitude. Each vertical line represents a line of longitude. These lines are labeled along the bottom edge of the tracking map. For example, the rightmost longitude line is 45 degrees West longitude. In the north Atlantic Ocean, all latitudes are North latitudes (meaning they lie north of the equator), and all longitudes are West longitudes, meaning they lie west of the 0 degree Greenwich, England, meridian. Sometimes latitude and longitude numbers are given without the "N" or "W" annotations; in this case, positive latitudes are the same as North latitudes (north of the equator), and negative longitudes are West longitudes.  

How to Plot a Hurricane's Position:

If you see a hurricane's position is labeled 33.0N 64.5W, find the latitude line for 33 degrees, and the longitude line for 64.5 degrees, and place a mark on the map. That puts the storm directly over the island of Bermuda.

Predicted path of hurricane Ivan (2004)

You can print out this tracking map or download the PDF to make your own tracking maps. (You will need the Adobe Acrobat Reader) Where do you think the storms will go?

What Watches and Warnings Mean!

Your National Weather Service recommends purchasing a radio that has both a battery backup and a tone-alert feature which automatically alerts you when a watch or warning is issued.

Tropical Storm Watch: Tropical Storm conditions are possible in the specified area of the Watch, usually within 36 hours.

Tropical Storm Warning: Tropical Storm conditions are expected in the specified area of the Warning, usually within 24 hours.

Hurricane Watch: Hurricane conditions are possible in the specified area of the Watch, usually within 36 hours. During a Hurricane Watch, prepare to take immediate action to protect your family and property in case a Hurricane Warning is issued.

Hurricane Warning: Hurricane conditions are expected in the specified area of the Warning, usually within 24 hours. Complete all storm preparations and evacuate if directed by local officials.

Short Term Watches and Warnings: These provide detailed information on specific hurricane threats, such as tornadoes, floods, and high winds.

Hurricane Names

All hurricanes are given names. Why is that? To help us identify storms and track them as they move across the ocean. Remember, there can be more than one hurricane at a time and without naming them, we could get confused and which storm we're talking about.

For hundreds of year, hurricanes in the Caribbean were named after the particular saint's day on which the hurricane occurred. Since 1953, however, Atlantic tropical storms have been named from lists originated by the National Hurricane Center and now maintained and updated by an international committee of the World Meteorological Organization. The lists featured only women's names until 1979, when men's and women's names were alternated. Six lists are used in rotation. Thus, the 2001 list will be used again in 2007.

The only time a new name is added is if a hurricane is very deadly or costly. Then the name is retired and a new name is chosen. If that occurs, then at an annual meeting by the WMO committee (called primarily to discuss many other issues) the offending name is stricken from the list and another name is selected to replace it.

Atlantic Names (2004 - 2006)

For Atlantic Ocean hurricanes, the names may be French, Spanish or English, since these are the major languages bordering the Atlantic Ocean where the storm occur.

Retired Hurricane Names

Sometimes names are "retired" if a hurricane has been really big and destructive. It's like when a sports jersey or number is retired after a really great athlete leaves a sport. When a name is retired it won't be used again.

Summary

• A "hurricane" is the most severe category of the meteorological phenomenon known as the "tropical cyclone."
  
• Tropical cyclones are low pressure systems that have 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 or 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 strike, and how slow they move.