Accumulated Cyclone Energy (ACE) is an index forecasters use to measure the energy output of tropical storms and hurricanes, as well as the overall intensity of hurricane seasons as a whole.

A strong, long-lived storm will generate a much higher ACE value than a weak storm that only sticks around for a day or two. Major hurricanes often produce very high ACE values, as much as 50 or higher, while disheveled tropical storms can produce an ACE value as low as 0.2.

Forecasters add together the ACE values for individual tropical storms and hurricanes to judge the overall intensity of an ocean basin's tropical cyclone season.

A season that generates 20 named storms may seem impressive, but if there were lots of weak tropical storms with only a few hurricanes in the mix, the season's overall ACE value will reflect the lackluster strength of that year's storms.

ACE is calculated by summing together the squared value of a storm's maximum sustained winds (in knots) every six hours for as long as the system remains a tropical storm or hurricane. This value is divided by 10,000 and rounded to the nearest tenth to arrive at a final ACE value.

Example: Tropical Storm Tartuffe lasts for 24 hours with 35-knot winds. The storm's ACE calculation would look like this:

35² + 35² + 35² + 35² = 4,900

4,900 ÷ 10,000 = 0.5, which is the storm's ACE value.

The average ACE across the Atlantic basin between 1991 and 2020 was 122.5. The season with the highest ACE was 1933 with a value of 258.6, followed by the hurricane seasons of 2005 (245.3) and 1893 (231.1).

Given that 1991–2020 was a relatively active period, experts use the average ACE between 1951 and 2020 (96.7) to quantify the activity of an Atlantic hurricane season.

The U.S. Climate Prediction Center uses the following values to call a season above- or below-normal.

The storm with the highest ACE value in the Atlantic Ocean was the Hurricane of 1899, which generated an ACE of 73.6. The second-highest was an ACE of 70.4 generated by Hurricane Ivan in 2004. The third-highest was 2017's Hurricane Irma, which produced an ACE value of 64.9.

The tropical cyclone with the highest ACE value ever recorded globally was 2023's Cyclone Freddy. The storm reportedly racked up an ACE value of 87.0 after surviving 36 days and travelling nearly 8,000 miles across the Indian Ocean.