What Is ACE? Accumulated Cyclone Energy Explained

Accumulated Cyclone Energy

ACE measures how much energy a storm, or a whole hurricane season, produced. It combines how strong storms were and how long they lasted. One long-lived major hurricane can out-score a dozen short, weak tropical storms, so ACE says more about a season than simply counting storms.

ACE = Σ V_max² × 10⁻⁴ V_max is a storm's maximum sustained wind in knots, added up every 6 hours (00, 06, 12 and 18 UTC) while it is a tropical storm, subtropical storm, or hurricane (≥34 kt).

Example: a 100 kt hurricane holding steady for one day counts 4 times: 4 × 100² × 10⁻⁴ = 4.0 ACE. Because wind is squared, a 140 kt storm scores almost 2× as much per day as a 100 kt one.

A season's ACE is the sum over all of its storms. Depressions and extratropical stages don't count.

How NOAA classifies a season

NOAA uses these seasonal ACE ranges for both the Atlantic and the Eastern Pacific:

Classification	Season ACE
Below Normal	< 73
Near Normal	73–126
Above Normal	126–159
Extremely Active	159+

A normal Atlantic season (1991–2020 average) is about 122.5 ACE.

ACE calculator

Enter a storm's maximum sustained wind for each 6-hour period, separated by commas or spaces.

Wind speed per 6-hour period Units

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From this site's data

The busiest Atlantic season since 1991 was 2005 at 252.9 ACE, 2.1× a normal season. The quietest was 1994 at 32.0.
The biggest single Atlantic storm since 1991 was Ivan (2004) with 71.5 ACE, 58% of a whole normal season on its own.
The 2026 Atlantic season is at 9.3 ACE so far (Below Normal). See the live dashboard.
The busiest East & Central Pacific season since 1991 was 2015 at 329.6 ACE, 2.5× a normal season. The quietest was 2010 at 52.0.
The biggest single East & Central Pacific storm since 1991 was Ioke (2006) with 85.3 ACE, 65% of a whole normal season on its own.
The 2026 East & Central Pacific season is at 238.8 ACE so far (Extremely Active). See the live dashboard.

Sources

NOAA Climate Prediction Center: ACE definition and season classifications. NOAA HURDAT2: storm data used for this site's numbers (1991–present).

This site is maintained by a hurricane data enthusiast, not a meteorologist. For forecasts, watches and warnings, always use the National Hurricane Center.