What is a hurricane? | How do hurricanes form? | How are hurricanes classified? | Which regions have hurricanes? | When do hurricanes occur, and who forecasts them?

  What is a hurricane?   [Top]

  A hurricane is a non-frontal, low-pressure system over tropical or sub-tropical waters with organized convection (thunderstorm activity) and definite cyclonic surface wind circulation. Hurricanes with maximum surface winds of less than 39 mph are first called tropical despressions. Once the depression reaches a maximum wind speed of at least 40 mph, they are called tropical storms and are assigned names. Once the winds reach 74 mph, the storm is formally called a hurricane, typhoon, or a tropical cyclone depending on what part of the world the storm occurs in. The word "hurricane" comes from the name "Hurican," the Caribbean god of evil.

Hurricanes are defined by the following characteristics:

• They are tropical, meaning that they are generated in tropical areas of the ocean near the Equator.
• They are cyclonic, meaning that their winds swirl around a central eye. Wind direction is counterclockwise (west to east) in the Northern Hemisphere and clockwise (east to west) in the Southern Hemisphere.
• They are low-pressure systems. The eye of a hurricane is always a low-pressure area. The lowest barometric pressures ever recorded have occurred inside hurricanes.
• The winds swirling around the center of the storm have a sustained speed of at least 74 mph.

  How do hurricanes form?   [Top]

  In order for a hurricane to form, certain enviornmental conditions must be met:

1. Warm ocean waters at least 80 degrees F throughout a sufficient depth (around 150 ft.). Warm waters are necessary to fuel the hurricane.
   
2. An atmosphere which cools fast enough with a height that is potentially unstable. It is the thunderstorm activity which allows the heat stored in the ocean waters to be released for hurricane development.
   
3. Moist layers near the mid-troposphere. Dry mid levels are not conducive for allowing the continuing development of widespread thunderstorm activity.
   
4. A minimum distance of at least 300 miles from the equator. For hurricane development to occur, there is a requirement for amounts of the Coriolis force to provide for near gradient wind balance to occur. Without the Coriolis force, the low pressure of the disturbance cannot be maintained.
   
5. A pre-existing near-surface disturbance with sufficient vorticity and convergence. Hurricanes cannot be generated spontaneously. To develop, they require a weakly organized system with sizable spin and low level inflow.
   
6. Low values (less than 23 mph) of vertical wind shear between the surface and the upper troposphere. Vertical wind shear is the magnitude of wind change with height. Large values of vertical wind shear disrupt the forming hurricane and can prevent genesis, or, if a hurricane has already formed, large vertical wind shear can weaken or destroy the hurricane by interfering with the organization of deep convection around the cyclone center.
   
   

  Most Atlantic hurricanes begin off the west coast of Africa, starting as thunderstorms that move out over the warm, tropical ocean waters. It can take anywhere from hours to several days for a thunderstorm to develop into a hurricane. Although the whole process of hurricane formation is not entirely understood, three events must happen for hurricanes to form:

• A continuing evaporation-condensation cycle of warm, humid ocean air. Warm, moist air from the ocean surface begins to rise. As this warm air rises, its water vapor condenses to form storm clouds and drops of rain. The condensation releases heat called latent heat. This latent heat warms the cool air which causes it to rise. This rising air is replaced by more warm, humid air from the ocean below. This cycle continues, drawing more warm air into the developing storm and continuously moving heat from the surface to the atmosphere. This exchange of heat from the surface creates a pattern of wind that circulates around a center of the hurricane.
• A difference in air pressure (pressure gradient) between the surface and high altitude. High-pressure air in the upper atmosphere (above 30,000 ft) over the storm's center also removes heat from the rising air, fueling the air cycle and the hurricane's growth. As high-pressure air is sucked into the low-pressure center of the storm, wind speeds increase.
• Wind patterns characterized by converging winds at the surface and strong winds at higher altitudes. Converging winds are winds moving in different directions that run into each other. Converging winds at the surface collide and push warm, moist air upward. This rising air reinforces the air that is already rising from the surface, so the circulation and wind speeds of the storm increase. In the meantime, strong winds blowing at constant speeds at higher altitudes (up to 30,000 ft) help to remove the rising hot air from the storm's center, maintaining a continual movement of warm air from the surface and keeping the storm organized. If the high-altitude winds do not blow at the same speed at all levels, the storm loses organization and weakens. Once the wind blows at an inconsistant level for enough time, the hurricane weakens enough to end. To view the wind patterns, click here.

  How are hurricanes classified?   [Top]

  Once a hurricane forms, it is rated on the Saffir-Simpson Hurricane Scale.
There are five categories in this rating system:

Saffir-Simpson Hurricane Scale
Category	Wind Speed	Effects
1	74 to 95 mph (119 to 153 kph)	Storm surge 4 to 5 ft (1.2 to 1.5 m) above normal Some flooding Little or no structural damage
2	96 to 110 mph (155 to 177 kph)	Storm surge 6 to 8 ft (1.8 to 2.4 m) above normal Trees down Roof damage (shingles ripped off)
3	111 to 130 mph (178.6 to 209 kph)	Storm surge 9 to 12 ft (2.7 to 3.7 m) above normal Structural damage in houses Mobile homes destroyed Severe flooding
4	131 to 154 mph (210 to 247.8 kph)	Storm surge 13 to 18 ft (4 to 5.5 m) above normal Severe flooding inland Some roofs ripped off Major structural damage
5	>155 mph (> 249.4 kph)	Storm surge at least 18 ft (5.5 m) above normal Severe flooding further inland Serious damage to most wooden structures

Hurricane Names

In order to keep better track of hurricanes, weather officials decided to name them. The names are chosen by the World Meteorological Organization.
According to the National Oceanic & Atmospheric Administration (NOAA):
      "For several hundred years, hurricanes in the West Indies were often named after the particular saint's day on which the hurricane occurred.       For example, 'Hurricane San Felipe' struck Puerto Rico on September 13, 1876. Another storm struck Puerto Rico on the same day in 1928,       and this storm was named 'Hurricane San Felipe the Second.'"
Until World War II, hurricanes were given only masculine names. In the early 1950s, weather services began naming storms alphabetically and with only feminine names. By the late 1970s, this practice was replaced with alternating masculine and feminine names. The first hurricane of the season is given a name starting with the letter A, the second with the letter B and so on. Hurricanes in the Pacific Ocean are assigned a different set of names than Atlantic storms. For example, the first hurricane of the 2001 hurricane season was a Pacific Ocean storm near Acapulco, Mexico, named Adolf. The first Atlantic storm of the 2001 season would be named Allison.
According to the NOAA:
        "Whenever a hurricane has had a major impact, any country affected by the storm can request that the name of the hurricane be 'retired'       by agreement of the World Meteorological Organization (WMO). Retiring a name actually means that it cannot be reused for at least 10       years, to facilitate historic references, legal actions, insurance claim activities, etc. and avoid public confusion with another storm of the       same name."

  What regions have hurricanes?   [Top]

  There are seven tropical cyclone "basins" where storms occur on a regular basis:

1. Atlantic basin (including the North Atlantic Ocean, the Gulf of Mexico, and the Caribbean Sea)
   
2. Northeast Pacific basin (from Mexico to about the dateline)
   
3. Northwest Pacific basin (from the dateline to Asia including the South China Sea)
   
4. North Indian basin (including the Bay of Bengal and the Arabian Sea)
   
5. Southwest Indian basin (from Africa to about 100E)
   
6. Southeast Indian/Australian basin (100E to 142E)
   
7. Australian/Southwest Pacific basin (142E to about 120W)
   
   

  When do hurricanes occur, and who forecasts them?   [Top]

  Hurricane Season
June 1st-September 30th

Monitoring and Forecasting
The National Hurricane Center in Miami, Florida, USA has responsibilities for monitoring and forecasting hurricanes in the Atlantic and Northeast Pacific basin east of 140W. The Central Pacific Hurricane Center has responsibilities for the remainder of the Northeast Pacific basin to the dateline. The Northwest Pacific basin is shared in forecasting duties by China, Thailand, Korea, Japan, the Philippines, and Hong Kong. The North Indian basin hurricanes are forecasted by India, Thailand, Pakistan, Bangladesh, Burma, and Sri Lanka. Reunion Island, Madagascar, Mozambique, Mauritius, and Kenya provide forecasts for the Southwest Indian basin. Australia and Indonesia forecast hurricane activity in the Southeast Indian/Australian basin. Lastly, for the Australian/Southwest Pacific basin Australia, Papua New Guinea, Fiji, and New Zealand forecast hurricanes.