The North Atlantic hurricane season officially runs from 1 June to 30 November. During this period the average number of systems reaching tropical storm (maximum sustained winds between 39-73 mph), hurricane (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 total seasonal activity: The Accumulated Cyclone Energy (ACE) index
The phrase "total seasonal activity" refers to the collective intensity and duration of Atlantic tropical storms and hurricanes occurring during a given season. The measure of total seasonal activity used by NOAA is called the Accumulated Cyclone Energy (ACE) index. The ACE index is 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.
Two other measures of total seasonal 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. Click here to see categorization of seasons according to season type.
NOAA definitions of above-normal, near -normal, and below-normal Atlantic hurricane seasons
The 1951-2000 mean value of the ACE index is 93.2, and the median value is 87.5
Above-normal season: An ACE index value well above 103 x 104 kt2 (corresponding to 117% of the median ACE value or 110% of the mean), or an ACE value slightly above 103 x 104 kt2 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 x 104 kt2 (corresponding to 75%-117% of the median or 71%-110% of the mean), or an ACE index value slightly above 103 x 104 kt2 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 x 104 kt2, corresponding to below 75% of the median or 71% of the mean.
The 1950-2003 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.
||Mean # of
||Mean # of
||Mean # of Major
||Range of Major
||10 to 19
||6 to 12
||2 to 8
||6 to 14
||4 to 8
||1 to 3
||4 to 9
||2 to 5
||0 to 2
||4 to 19
||2 to 12
||0 to 8