What Is Accumulated Cyclone Energy?
ACE is calculated by summing the squares of a tropical cyclone's maximum sustained wind speed (in knots) at six-hour intervals throughout its lifetime — but only while it is at tropical storm intensity or stronger (≥ 34 knots / 39 mph).
Formula: ACE = Σ (V_max²) × 10⁻⁴
In plain language: a slow-moving, long-lived major hurricane contributes enormously to ACE. A fast-moving tropical storm that dissipates in 36 hours contributes very little.
Why Named Storm Count Is Not Enough
Consider two hypothetical seasons: Season A has 20 named storms, all tropical storms lasting 3 days each. Season B has 12 named storms, half of which are major hurricanes lasting 10 days each. Season A has more storms. Season B has far more ACE — and far more potential for death and destruction. Named storm count tells you how many times the naming list was used. ACE tells you how dangerous the season actually was.
Historical ACE Benchmarks
| Season | ACE | Notable |
|---|---|---|
| 2020 | 180.4 | Most active on record |
| 2005 | 167.5 | Katrina, Rita, Wilma |
| 2010 | 165.6 | Very active, few US landfalls |
| 2013 | 36.1 | Extremely quiet season |
| 30-yr avg (1991–2020) | ~123 | NOAA baseline |
A season with ACE above 150 is considered extremely active. Below 70 is well below average.
How Forecasters Use ACE
Colorado State University (CSU), NOAA, and Tropical Storm Risk (TSR) all issue ACE forecasts alongside named storm and hurricane count predictions. ACE is highly sensitive to large-scale climate patterns — La Niña seasons consistently produce higher ACE than El Niño seasons.
What ACE Doesn't Measure
ACE does not directly measure how many storms made landfall, where storms tracked, or economic/human casualties. A high-ACE season with storms that stay over open water causes less damage than a lower-ACE season where a single major storm makes a catastrophic landfall.
Follow 2026 season ACE in real time on our Season Outlook page. Read the month-by-month season guide to understand when ACE typically accumulates.