The hurricane season for the North Atlantic basin
(including the Caribbean Sea and Gulf of Mexico) 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, categories 1 and 2 on the Saffir-Simpson scale) 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 period August-October,
which is considered the peak of the hurricane season. In an average season the United
States experiences 1-2 landfalling hurricanes. The Caribbean Islands experience one
hurricane every two years in non_active seasons, and 1-2 hurricanes in active years.
Measuring overall activity: One overall measure of seasonal activity is the
Accumulated Cyclone Energy (ACE) index, which is calculated by taking the sum of the
squares of the estimated 6-hourly maximum sustained wind speed (in units of m/s) for all
systems while they are at least tropical storm strength. The ACE index implicitly accounts
for the number of tropical storms and hurricanes, as well as their duration, giving more
weight to stronger systems. Thus, it 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 purely by 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 and colleagues at the Colorado State University, in that the ACE index
also includes systems while at tropical storm intensity. The ACE index, combined with
independent numbers of named storms, etc. provides a method by which one can partition
between active, near-average, and inactive hurricane years. We define active, near-average
and inactive hurricane years as follows:
Active (above-average) year: A value of the ACE index above 110% of normal,
combined with at least two of the following three parameters being above the long-term
average: number of tropical storms, hurricanes, and intense hurricanes.
Near-average year: A value of the ACE index typically between approximately 70%
and 110% of normal, or an ACE value slightly exceeding 110% of normal but with less than
two of the following three parameters being above the long-term average: numbers of
tropical storms, hurricanes, and intense hurricanes.
Inactive (below-average) year: A value of the ACE index below approximately 70%
of normal.
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.