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 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 follows:
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.
Near-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 long-term mean).
The 1950-2000 seasonal mean and range 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.
