The North Atlantic hurricane season officially runs from 1 June to 30 November. The vast majority of tropical storm and hurricane activity typically occurs during the August-October (ASO) peak of the hurricane season. An average hurricane season features approximately 11 named storms (maximum sustained surface winds between 39-73 mph), with six of those becoming hurricanes (maximum sustained surface winds of at least 74 mph) and two becoming major hurricanes (maximum sustained surface winds exceeding 111 mph, categories 3-5 on the Saffir-Simpson hurricane scale).
Measuring total seasonal activity: The Accumulated Cyclone Energy (ACE) index
The phrase "total seasonal activity" refers to the collective intensity and duration of Atlantic named 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 maximum sustained surface wind speed (knots) measured every six hours for all named systems while they are at least tropical storm strength.
NOAA uses the ACE index, combined 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’s Atlantic hurricane season classifications
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 named 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 no more than one of the following three parameters being above the long-term average: numbers of named storms, hurricanes, and major hurricanes.
Below-normal season: An ACE index value below 66 x 104 kt2, corresponding to less than 75% of the median or 71% of the mean.
The 1951-2000 base period is used in the above classifications, where the mean value of the ACE index is (93.2 x 104 kt2), and the median value is (87.5 x 104 kt2).
The following table shows the 1950-2005 seasonal means and ranges for named 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 28
||6 to 15
||2 to 8
||6 to 14
||4 to 8
||1 to 3
||4 to 9
||2 to 5
||0 to 2
||4 to 28
||2 to 15
||0 to 8