Atlantic hurricane

A bar chart of the activity of Atlantic hurricane seasons from 1945 to 2005.

Atlantic hurricane refers to a tropical cyclone that forms in the Atlantic Ocean north of the equator, usually in the Northern Hemisphere summer or autumn.

When applied to hurricanes, "Atlantic" generally refers to the entire "Atlantic basin," which includes the North Atlantic Ocean, Caribbean Sea and the Gulf of Mexico.

Almost all Atlantic tropical storms and hurricanes form in the months of June through November. The Atlantic hurricane season officially runs from June 1 to November 30. The U.S. National Hurricane Center monitors the basin and issues reports, watches and warnings about tropical weather systems for the United States. Other countries around the basin track and issue warnings for tropical weather in their territories.

On average, 10.1 named storms occur each season, with an average of 5.9 becoming hurricanes and 2.5 becoming major hurricanes (Category 3 or greater). The climatological peak of activity is around September 10 each season.

Steering factors

Tropical cyclones are steered by the surrounding flow throughout the depth of the troposphere (the atmosphere from the surface to about eight miles (12 km) high. Neil Frank, former director of the U.S. National Hurricane Center, used the analogy of a leaf carried along in a stream to describe the way atmospheric flow affects the path of a hurricane across the ocean. Specifically, air flow around high pressure systems and toward low pressure areas influence hurricane tracks.

In the tropical latitudes, tropical storms and hurricanes generally move westward with a slight tend toward the north pole, under the influence of a high pressure axis, the subtropical ridge that extends east-west to the north of the tropics. South of the subtropical ridge, easterly winds (blowing from east to west) prevail. If the subtropical ridge is weakened by a trough in the jet stream or other conditions, however, a tropical cyclone may turn poleward and then recurve, or curve back toward the east. Poleward (north) of the subtropical ridge, westerly winds prevail and generally steer tropical cyclones that reach northern latitudes toward the east. The westerly winds (westerlies) also steer extratropical cyclones with their cold and warm fronts from west to east.

Modified after HRD FAQ E6, by Chris Landsea. This NOAA content is in the public domain.

Climatology

Probability of a tropical cyclone of tropical storm or hurricane strength at a specific date, expressed as systems per 100 years

Tropical storms and hurricanes by month, for the period 1944-2005
(North Atlantic region)
Month	Total	Average
January–April	4	0.1
May	8	0.1
June	35	0.6
July	58	0.9
August	173	2.8
September	224	3.6
October	114	1.8
November	33	0.5
December	7	0.1
Source: NOAA + additions for 2001-05

Forecasting by climatology is very unreliable for use in any specific instances, but serves to characterize the general properties of an average season.

Most storms form in warm waters near the equator and the Intertropical convergence zone from tropical waves and other disturbances. Like all basins, few tropical systems form less than 10 degrees from the equator, as the Coriolis force is usually too weak to initiate sufficient rotation. Storms most often form in the warm waters of the Gulf of Mexico and the Caribbean Sea, and can form as far east as Cape Verde, producing strong and long-lasting Cape Verde-type hurricanes. Systems may also form, or retain or gain strength, over the Gulf Stream off the coast of the eastern United States.

When a tropical system forms in a sufficiently southerly position, it initially travels west and slightly north while strengthening in warm water. After being approached by a trough moving west to east, most storms typically recurve to the northeast, following the trough. The turn happens at approximately the latitude of the Florida peninsula (25°N to 30°N). The storm weakens as it encounters cooler water or land, sometimes transitioning into an extratropical storm.

Although most storms are found to the south and west, near the Gulf of Mexico and the Caribbean, occasionally storms will form further north and east from disturbances other than tropical waves.

There is a strong correlation between Atlantic hurricane activity and the presence of an El Niño or La Niña in the Pacific Ocean. El Niño events increase the wind shear over the Atlantic, producing a less-favorable environment for formation and decreasing tropical activity in the Atlantic basin. Conversely, La Niña causes an increase in activity due to a decrease in wind shear. http://www.aoml.noaa.gov/hrd/Landsea/elnino/

June

Typical locations and tracks of tropical systems in June. Blue is likely, green more likely, and orange most likely.

The beginning of the hurricane season is most closely related to the timing of increases in sea surface temperatures, convective instability, and other thermodynamical factors. http://hurricane.atmos.colostate.edu/forecasts/2005/nov2005/ Dr. William Gray's 2005 season summary Although this month marks the beginning of the hurricane season, the month of June generally still sees little activity, with an average of about 3 tropical systems per 5 years.

Any tropical systems usually form in the Gulf of Mexico or off the east coast of the United States.

July

Typical locations and tracks in July.

July still has relatively little activity, with about one tropical cyclone a year. Climatologically speaking, half of seasons have their first tropical storm by July 11 with a second by August 8, using data from 1944 to 1996.

Formation usually occurs in the eastern Caribbean Sea around the Lesser Antilles, in the north and east parts of the Gulf of Mexico, in the northern Bahamas, and off the coast of The Carolinas and Virginia. Storms travel west through the Caribbean and then either move more northerly and curving near the eastern coast of the U.S. or southerly and entering the Gulf of Mexico.

August

Typical locations and tracks in August.

Activity increases significantly in August, with an average of about 2.8 storms per year. About half of seasons have four named systems and one hurricane total by August 30. The average season also sees one major hurricane by September 4.

September

Typical locations and tracks in September.

The peak of the hurricane season in September corresponds to a time with low wind shear and high sea surface temperatures. The month of September sees an average of 3.6 storms a year. By September 24, the average season sees 7 named systems, of which 4 are hurricanes. In addition, two major hurricanes would be seen by September 28.

October

Typical locations and tracks in October.

The favorable conditions found during September begin to decay in October. The main reason for the decrease in activity is increasing wind shear, although sea surface temperatures are also usually cooler than in September. Activity falls off markedly, with 1.8 cyclones developing in an average season. By October 21, the average season is expected to have 9 named storms with 5 hurricanes. A third major hurricane would be expected sometime between September 28 and the end of the year for half of all seasons.

Although tropical activity is lower than in September, another small peak in activity occurs around October 20.

November

Typical locations and tracks in November.

Wind shear from westerlies increases substantially through November, generally preventing cyclone formation. On average, one storm forms during November every other year. On extremely rare occasions, a major hurricane might form, such as 155 mph category four Hurricane Lenny in 1999, which formed in mid November, and Kate, a category three which formed in late November.

December to May

Few tropical cyclones can be found in the time between seasons. In the 61 seasons between 1944 and 2005, only 8 tropical cyclones have formed in May, 7 in December, and 4 total for all four months between January and April. High wind shear and low sea surface temperatures generally preclude formation. Though a tropical cyclone has been observed in the Atlantic basin in every month in the year, no tropical cyclone has initially formed in January. However, a subtropical cyclone formed in January in the 1978 season, and both Hurricane Alice and Tropical Storm Zeta formed in December and lasted into January. (For additional information, see Off-season storms.)

Extremes

* The season in which the most tropical storms formed on record was the 2005 Atlantic hurricane season (28). That season was also the one in which the most hurricanes formed on record (15).
* The 1950 Atlantic hurricane season had the most major hurricanes on record (8).
* The least active season on record since 1965 (when data has been known to be reliable) was the 1983 Atlantic hurricane season, with one tropical storm, two hurricanes, and one major hurricane.
* The most intense hurricane on record to form in the North Atlantic basin was Hurricane Wilma (2005) (882 mbar).
* The longest-lasting hurricane was 1899 Hurricane San Ciriaco (28 days).
* The fastest-moving hurricane was the New England Hurricane of 1938 at 70 mph.
* The most tornadoes spawned by a hurricane was 123 by Hurricane Frances (2004 season).
* The strongest landfalling hurricane was the Labor Day Hurricane of 1935 (892 mbar).
* The deadliest hurricane was the Great Hurricane of 1780 (22,000 fatalities).
* The most damaging hurricane (adjusted for inflation) was Hurricane Katrina of the 2005 season which caused $75 billion in damages (2005 USD).

References

External links

*Tropical Cyclone Forecasters' Reference Guide
*NHC Tropical Cyclone Climatology
*Monthly Mean Sea Surface Temperatures

Atlantic hurricane: Encyclopedia BETA

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