The North Atlantic hurricane season 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) 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 August-October period, which is considered the peak of the hurricane season.
Measuring overall activity: The Accumulated Cyclone Energy (ACE) index
The term "overall activity" refers to the collective strength and duration of Atlantic tropical storms and hurricanes occurring during a given season. The measure of overall activity used by NOAA is called the Accumulated Cyclone Energy (ACE) index. The ACE index is essentially a wind energy index, defined as the sum of the squares of the estimated 6-hourly maximum sustained wind speed (knots) for all named systems while they are at least tropical storm strength. Since this index represents a continuous spectrum of both system duration and intensity, it does not suffer from the discontinuities inherent in more widely used measures of activity such as the number of tropical storms, hurricanes, or major hurricanes.
Two other measures of overall activity, developed by Dr. William Gray at the Colorado State University, are the Hurricane Destruction Potential (HDP) index and the Net Tropical Cyclone (NTC) index. These indices are correlated at approximately 0.95 with the ACE index. NOAA uses the ACE index instead of the HDP index when making and verifying their seasonal outlooks because the ACE index includes the contribution from systems while at tropical storm strength, whereas the HDP index does not. The ACE index is used instead of the NTC index because it allows one to easily quantify activity occurring in different parts of the Atlantic Basin, and because it does not suffer from the re-sampling issues inherent in the mathematical formulation of the NTC index.
NOAA uses the ACE index, in combination with the numbers of named storms, hurricanes, and major hurricanes, to categorize North Atlantic hurricane seasons as being above normal, near normal, or below normal.
NOAA definitions of above-normal, near-normal, and below-normal Atlantic hurricane seasons
The1950-2000 mean value of the ACE index is 93.7, and the median value is 86.0
Above-normal season: An ACE index value well above 103 (103 corresponds to120% of the median ACE value or 110% of the mean), or an ACE value slightly above 103 combined with at least two of the following three parameters being above the long-term average: number of tropical storms, hurricanes, and major hurricanes.
Near-normal season: An ACE index value in the range 66-103 (corresponding to 76%-120% of the median or 70%-110% of the mean), or an ACE index value slightly above 103 but with less than two of the following three parameters being above the long-term average: numbers of tropical storms, hurricanes, and major hurricanes.
Below-normal season: An ACE index value below 66, corresponding to below 76% of the median or 70% of the mean.
The 1950-2000 seasonal means and ranges of tropical storms, hurricanes and major hurricanes during above normal, near normal, below normal, and all Atlantic hurricane seasons, is summarized in the following table.