How busy was that hurricane season?

 

2025 Atlantic hurricane season as of October 8 (Source: NOAA)

The total number of tropical storms and hurricanes isn’t always an accurate measure of how busy a hurricane season has been. That’s because a season can have a high number of weak and/or short-lived tropical storms that can inflate the overall number of tropical storms and hurricanes. 

Scientists at Colorado State University have developed a measure that takes this into account. Known as the ACE score (“Accumulated Cyclone Energy”), it takes such factors as storm intensity and duration into account. A longer lasting hurricane, for example, will have a higher ACE score than a short-lived tropical storm.

An “average” Atlantic hurricane season features a total of 14 tropical storms, of which 7 become hurricanes with 3 major hurricanes, according to NOAA. That’s for the 30-year period of 1991-2020. The average ACE score for the same time period is 122.3 knots squared (kn²).

The 10 busiest Atlantic hurricane seasons based on the overall number of named storms is not the same as the 10 seasons with the highest ACE score. That’s a significant illustration of the difference between a season having fewer, more intense and/or longer-lasting hurricanes as opposed to a higher number of less intense, short-lived tropical storms.

For example, the 2017 Atlantic hurricane season had a higher than average number of systems with 17 tropical storms, 10 hurricanes and 6 major hurricanes and finished with an ACE Score of 224.9 kn². The 2020 season had almost double the  number of tropical storms (30), with 14 hurricanes and 7 major hurricanes, but finished with an appreciably lower ACE score of 179.8 kn² due to having more, weaker systems.

Having these two ways to accurately measure how active a given hurricane season is has become especially important in recent decades that have featured an overall uptick in tropical activity. In fact, the 30-year period of 1995 – 2024 has been considerably busier in the tropical Atlantic than the previous 30-year period. While eight of the 10 seasons with the highest number of Atlantic tropical storms and hurricanes have occurred since 1995, only five with the highest ACE score have occurred in the same time period. 

Only three seasons (2005, 1995 and 1933) appear in both top 10 lists for highest number of tropical storms and hurricanes, as well as highest seasonal ACE score. The 2025 Atlantic hurricane season isn’t anywhere near either top 10 list with a total of only 10 named tropical storms, 4 hurricanes and 3 major hurricanes so far this season.

Busiest Hurricane Seasons
By ACE Score (kn2)
Source: Colorado State University		Tropical Storms	Hurricanes	Major Hurricanes
258.3	1933	20	11	6
245.3	2005	28	15	7
231.1	1893	12	10	5
229.6	1926	11	8	6
227.1	1995	19	11	5
226.9	2004	15	9	6
224.9	2017	17	10	6
211.3	1950	16	11	6
188.9	1961	12	8	5
181.8	1998	14	10	3

 

By named storms
	ACE Score (kn2)
2020	179.8	30	14	7
2005	245.3	28	15	7
1933	258.6	20	11	6
2012	132.6	19	10	2
2011	126.3	19	7	4
2010	165.5	19	12	5
1995	227.1	19	11	5
1887	181.3	19	11	2
2024	161.5	18	11	5
2019	129.8	18	6	3