The North Atlantic hurricane season officially runs from 1 June to 30 November. An average hurricane season features ten tropical storms (maximum sustained winds between 39-73 mph), of which an average of six become hurricanes (maximum sustained winds of at least 74 mph) and two become major hurricanes (maximum sustained winds exceeding 110 mph, categories 3-5 on the Saffir-Simpson scale). However, the vast majority of tropical storm and hurricane activity typically occurs during the August-October 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 surface wind speed (knots) for all named systems while they are at least tropical storm strength.
NOAA uses the ACE index, combined with the numbers of tropical 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
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 following table shows the 1950-2004 seasonal means and ranges of tropical storms, hurricanes and major hurricanes during above normal, near normal, below normal, and all Atlantic hurricane seasons. This table highlights the marked differences in activity between the three season types.
||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