Measuring the Impacts of the Atlantic Hurricane Season

Hurricane Maria, September 2017  (Source: NOAA)

The Atlantic Ocean sees an annual average of 12 tropical storms, of which 6 become hurricanes and that includes 3 major hurricanes.  A “major” hurricane is a Category 3 or greater on the Saffir-Simpson Hurricane Wind Scale, with sustained winds of at least 111 mph.  Hurricane season in the Atlantic Ocean Basin runs from June 1 through November 30.

The number and intensity of tropical storms and hurricanes varies from one season to the next.  Consequently, scientists also use something called “Accumulated Cyclone Energy” or “ACE.”  NOAA more formally measures ACE on the “ACE Index.”  That’s a measure of how active and intense the hurricane season is based on the overall “wind speed and duration of each tropical cyclone.”  An average amount of ACE during an Atlantic hurricane season is 95.4 units (10⁴ kt²⁾.  ACE is independent of whether or not a particular hurricane season is a destructive one.

Four of 10 ten most active Atlantic hurricane seasons based on ACE have occurred since 1995.  The busiest hurricane season on record based on total number of tropical storms and hurricanes was in 2005 (28 tropical storms, 15 hurricanes with 7 “major” hurricanes) with an ACE Index score of 250.  Meanwhile, the busiest Atlantic hurricane season on the ACE Index occurred in 1933 with a score of 259 according to NOAA.  A total of 20 tropical storms, 11 hurricanes with 6 “major” hurricanes developed in 1933.  

However, the 2005 Atlantic hurricane season had been the costliest hurricane season for the United States with hurricanes Katrina, Rita and Wilma.  It was surpassed last season which featured hurricanes Harvey, Irma and Maria.  

A hurricane season doesn’t have to active to be destructive or deadly.  For example, by total number of storms and on the ACE Index, the 1983 and 1992 hurricane seasons were both quieter than average.  In fact, the 1983 Atlantic hurricane season had an overall ACE Index of 17 that was the lowest value since 1925 (7).  However, Hurricane Alicia caused widespread damage in Texas in 1983.  Meanwhile, Hurricane Andrew developed in 1992 and remains the last of three Category 5 hurricanes on record to make landfall in the United States.  Like Alicia nine years earlier, Andrew was the only major hurricane of the 1992 season.

There have been several Atlantic hurricane seasons with multiple tropical storms and a few minimal hurricanes, but no “major” ones.  The most recent was in 2013 when a total of 14 tropical storms and 2 hurricanes developed.  The total amount of ACE in the Atlantic for 2013 was only 36.  This illustrates why the ACE Index is such a useful tool that provides a more detailed measure of how active a particular hurricane season is beyond the total number of tropical storms and hurricanes.  However, one particularly intense or long-lasting hurricane could skew the amount of ACE for an entire season, so it’s important to look at both the seasonal amount of ACE and the total number of storms.