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
Measuring overall activity
The term "overall activity" reflects a combination of intensity and duration
of Atlantic tropical storms and hurricanes during a given season. One such measure of
seasonal activity is the Accumulated Cyclone Energy (ACE) index, which is calculated as
the sum of the squares of the estimated 6-hourly maximum sustained wind speed (in units of
knots) for all systems while they are at least tropical storm strength. This index 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 from the number of
tropical storms or hurricanes alone. This index is a slight modification of the Hurricane
Destruction Potential (HDP) index used by Dr. Gray, in that the ACE index also includes
systems while at tropical storm strength. This index is also highly correlated with the
Net Tropical Cyclone (NTC) activity index used by Dr. Gray.
The ACE index, combined with the numbers of named storms, hurricanes, and major
hurricanes, provides a method by which one can identify North Atlantic hurricane seasons
as being above normal, normal, or below normal.
Above-normal, normal, and below-normal Atlantic hurricane seasons are defined as
Please note that the long-term (1950-2000) mean value of the ACE index is 93.7.
Also, the long-term (1950-2000) median value of the ACE index is 86.0
Above-normal (very active) season: A value of the ACE index greater than 120% of
the long-term median value (which corresponds to 110% of the long-term mean), combined
with at least two of the following three parameters being above the long-term average:
number of tropical storms, hurricanes, and major hurricanes.
Normal season: A value of the ACE index between approximately 76% and 120%
of the long-term median value (which corresponds to between 70% and 110% of the long-term
mean), or an ACE value slightly exceeding 120% of the median (110% of the long-term mean)
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 (relatively quiet) season: A value of the ACE index below
approximately 76% of the long-term median value (which is below approximately 70% of the
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.