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The hurricane season for the North Atlantic basin (including the Caribbean Sea and Gulf of Mexico) officially runs from 1 June to 30 November. During this period the average number of systems reaching tropical storm status (maximum sustained winds between 39-73 mph), hurricane status (maximum sustained winds of at least 74 mph, categories 1 and 2 on the Saffir-Simpson scale) and major (or intense) hurricane status (maximum sustained winds exceeding 110 mph, categories 3-5 on the Saffir-Simpson scale) are ten, six, and two, respectively. However, the vast majority of tropical storm and hurricane activity typically occurs during the period August-October, which is considered the peak of the hurricane season. In an average season the United States experiences 1-2 landfalling hurricanes. The Caribbean Islands experience one hurricane every two years in non_active seasons, and 1-2 hurricanes in active years.

Measuring overall activity: One overall measure of seasonal activity is the Accumulated Cyclone Energy (ACE) index, which is calculated by taking the sum of the squares of the estimated 6-hourly maximum sustained wind speed (in units of m/s) for all systems while they are at least tropical storm strength. The ACE index implicitly accounts for the number of tropical storms and hurricanes, as well as their duration, giving more weight to stronger systems. Thus, it is a robust measure of overall activity and does not suffer the shortcomings inherent in attempting to determine the relative activity of a particular season purely by the number of tropical storms or hurricanes alone. This index is a slight modification of the Hurricane Destruction Potential (HDP) index used By Dr. William Gray and colleagues at the Colorado State University, in that the ACE index also includes systems while at tropical storm intensity. The ACE index, combined with independent numbers of named storms, etc. provides a method by which one can partition between active, near-average, and inactive hurricane years. We define active, near-average and inactive hurricane years as follows:

Active (above-average) year: A value of the ACE index above 110% of normal, combined with at least two of the following three parameters being above the long-term average: number of tropical storms, hurricanes, and intense hurricanes.

Near-average year: A value of the ACE index typically between approximately 70% and 110% of normal, or an ACE value slightly exceeding 110% of normal but with less than two of the following three parameters being above the long-term average: numbers of tropical storms, hurricanes, and intense hurricanes.

Inactive (below-average) year: A value of the ACE index below approximately 70% of normal.

The mean number of tropical storms, hurricanes and major hurricanes in these years, as well as the expected range of activity, based on data since 1950, is summarized in the following table.

Season  Type Mean # of Tropical Storms Range of Tropical Storms Mean # of Hurricanes Range of Hurricanes Mean # of Major Hurricanes Range of Major Hurricanes
Active 12.1 10 to 19 8.1 6 to 12 3.5 2 to 7
Near-average 9.5 6 to 14 5.4 4 to 8 1.9 1 to 3
Inactive 6.9 4 to 9 3.6 2 to 5 0.9 0 to 2
Overall 9.7 4 to 19 5.9 2 to 12 2.2 0 to 7

Seasonal mean and range of landfalling hurricanes in the Continental United States and Caribbean Islands.

Season Type Continental United States Caribbean Islands
  Mean        Range Mean       Range
Active 2.3             0-4 2.3           0-6
Near average 1.8             0-7 0.9           0-3
Inactive 0.9             0-2 only 1 every 3 years


Active years feature an average of 12 tropical storms, with a range of 9 to 19. At least 11 tropical storms have been observed in 94% of active years.

On average 8 tropical systems become hurricanes in active years, with a range of 6 to 12. At least 7 hurricanes have been observed in 70% of active years.

On average 3-4 tropical systems become major hurricanes in active years, with a range of 2 to 7. At least 3 major hurricanes have been observed in 82% of active years.

The combination of at least 11 tropical storms, of which 7 become hurricanes and 3 of which become major hurricanes has occurred in 70% of all active years. This combination has occurred only once in the last half-century outside of an active year.

In active years the United States and the Caribbean Islands each experience an average of 2-3 hurricane strikes. For the Caribbean Islands, this frequency of hurricane landfall is much larger than that which is observed in inactive years (only one landfalling hurricane in every 3 inactive years).