What makes a hurricane season active?

 

Hurricane Wilma, Category 5, October 2005 (Source: NOAA)

Meteorologists at Colorado State University and the University of Pennsylvania have already issued their forecasts calling for a very active 2024 Atlantic hurricane season. How do meteorologists compare one active hurricane season to another?

Three of the five busiest Atlantic hurricane seasons, basely solely on the number of named storms, have occurred within the last four years. A total of 30 tropical storms developed in 2020, surpassing the previous record of 28 from 2005. Seven major hurricanes, Category 3 or higher on the Saffir-Simpson Hurricane Wind Scale (111/+ mph sustained winds), developed in both 2020 and 2005. An "average" Atlantic hurricane season has 14 named storms, of which 7 becomes hurricanes with 3 major hurricanes, according to NOAA.

However, despite the fact that the 2020 and 2005 seasons each had a similar number of tropical storms and hurricanes (20, 14, 7 vs. 28, 15, 7), there were major differences in severity. The overall number of named storms doesn't take into account how strong or long-lived a particular storm is. 

That's why the metric ACE (Accumulated Cyclone Energy) was developed in the 1990s by research scientists at Colorado State University. It ranks how busy a hurricane season is not only by the overall number of tropical storms and hurricanes, but by their duration and intensity as well.

That's significant because a hurricane season can have a high number of named storms, but many of them could be weak and/or short-lived. For example, the 2020 hurricane season that featured 30 named storms, saw seven tropical storms develop before the first hurricane (Hanna) on July 25. The first major hurricane of 2020 didn't develop until Hurricane Laura, the 12th named storm of the season, reached Category 3 status on August 26. Given the higher number of weaker storms, the 2020 season finished with a cumulative ACE total of 179.8.

By comparison, the 2005 Atlantic hurricane season that had the same number of major hurricanes as in 2020 (seven), had a much higher ACE score of 245.3. The key difference being 2005 had longer-lasting and more intense storms than 2020. Four Category 5 hurricanes developed in the Atlantic in 2005 alone - the record for any season - compared to none in 2020.

When looking at the Top 10 list for busiest Atlantic hurricane season based on highest number of tropical storms and hurricanes, only three also rank in the Top 10 for busiest season based on their ACE score. While eight of the Atlantic's 10 busiest hurricane seasons based on number of named storms have occurred since 1995, only five on the ACE list have occurred since then. 

The only season that ranks among the Top 10 for most named Atlantic storms and their ACE score is 2005. Despite having more named storms than other Atlantic season on record, the 2020 season is only the 12th most active season based on its ACE score.