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. William
Gray of the Colorado State University, in that the ACE index also includes systems while
at tropical storm strength. This index is also highly correlated (0.94) with the Net
Tropical Cyclone (NTC) activity index used by Dr. Gray.

The ACE index, combined with the seasonal total
number of named storms, hurricanes, and major hurricanes, allows on to idenfity 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**:

The 1950-2000 *mean value* of the ACE index is
93.7 and the *median value* is 86.0

*Above-normal (very active) season*: An ACE
index value exceeding 103, which is 120% of the median and 110% of the mean, combined with
above-average values of at least two of the following three parameters: seasonal number of
tropical storms, hurricanes, and major hurricanes.

*Near-normal season*: An ACE index value of
65-103, which is 76% - 120% of the median and 70% - 110% of the mean, or an ACE value
slightly exceeding 120% of the median but with less than two of the above three parameters
being above the long-term average.

*Below-normal (relatively quiet) season*:
An ACE index value below 65, which is 76% of the median
and 70% of the 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.