Accumulated cyclone energy: how busy seasons are measured
Accumulated cyclone energy, or ACE, measures a season by adding up how strong its storms were and for how long. By this site's calculation from the best track, the busiest Atlantic season on record is 1933, with an ACE of 258.6, just ahead of 2005 at 247.9. The busiest eastern and central Pacific season is 2015, at 329.6.
How ACE is calculated
Every six hours, at 00, 06, 12 and 18 UTC, take the storm's maximum sustained wind in knots, square it, and divide by 10,000, but only while the storm is a tropical storm or hurricane with at least 34-knot winds. Add those values over the storm's life for the storm's ACE, and over all storms for the season's. Squaring the wind gives strong storms far more weight: one six-hour step at 140 knots adds 1.96, while a step at 40 knots adds 0.16.
That weighting is the point. A season with many short, weak storms can have a high count of named storms and a modest ACE, while a season with a few long-lived major hurricanes can have few names and a huge ACE. Published ACE figures may differ slightly from the values here, because some include subtropical stages or round differently; this site's method is set out on how to read the record.
The ten highest Atlantic seasons
| Season | ACE | Hurricanes | Major |
|---|---|---|---|
| 1933 | 258.6 | 11 | 6 |
| 2005 | 247.9 | 15 | 7 |
| 1893 | 231.2 | 10 | 5 |
| 1926 | 229.6 | 8 | 6 |
| 1995 | 227.4 | 11 | 5 |
| 2017 | 224.9 | 10 | 6 |
| 2004 | 224.7 | 9 | 6 |
| 1950 | 211.3 | 11 | 6 |
| 1961 | 188.9 | 8 | 5 |
| 1887 | 181.3 | 11 | 2 |
The 1933 season reached the top with 11 hurricanes, against 15 in 2005. The Atlantic list mixes old and new seasons, which is less obvious than it looks: an early season needed long-tracked storms that ships and coasts could follow, so the energy it shows is the part that was seen. The quietest Atlantic season since 1966 is 1983, at 16.8; the middle value for that period is 103.
Counting storms versus counting energy
Two Atlantic seasons share the highest storm count in the file, 31: 2005 and 2020. The later one even had more storms that reached tropical-storm strength, 30 against 28. Yet 2020's ACE of 178 ranks 13th among Atlantic seasons, while 2005 ranks second. Per storm, 2020 averaged 5.7 units of energy and 2005 8.0.
The reason is the squared wind. Since 1966, major hurricanes make up 16% of all Atlantic storms in the file but supply 60% of their accumulated cyclone energy. In the eastern and central Pacific the same group is 23% of storms and 64% of the energy. A handful of strong, long-lived hurricanes decide where a season lands in the ACE table.
The decade view
Decade averages smooth out single years. The quiet 1970s and 1980s stand out, followed by a step up from the mid-1990s; the 2020s so far average 144, the highest of any decade in the file. Early decades sit lower partly because of missed storms, so the long-term comparison is not like for like.
The Pacific swings more from decade to decade. Among the five full decades from 1970 to 2019, its seasons averaged the most energy in the 1990s, 180, and the least in the 2000s, 101, a drop of almost half in ten years. The Atlantic's own quiet spell came earlier, in the 1970s and 1980s.
Single storms
A few storms carry whole seasons. Storm 3 of 1899 has the largest storm ACE in the Atlantic file, 73.6, followed by Ivan (2004) at 70.4 and Irma (2017) at 64.9. Ivan alone supplied 31% of the 2004 total. By contrast Katrina (2005), for all its strength, made up 8% of 2005's. In the Pacific, Ioke (2006) leads both files with 85.3, built over a track that crossed the central Pacific into the western Pacific.
Pacific against Atlantic
The two basins can be compared season by season from 1971, when both files are well observed. Over the 55 seasons from 1971 to 2025 the eastern and central Pacific averaged an ACE of 136 and the Atlantic 104, and the Pacific came out ahead in 30 of them. The balance has shifted: from 2000 on the Pacific led in only 10 of 26 seasons.
The widest Pacific lead came in 2015, when the Pacific produced 330 against the Atlantic's 62. The widest Atlantic lead came in 2004: 225 against 71. Busy Pacific years and quiet Atlantic years often coincide, which the pairing makes easy to see on the two season charts on the seasons page.
The Pacific list
| Season | ACE | Hurricanes | Major |
|---|---|---|---|
| 2015 | 329.6 | 17 | 11 |
| 2018 | 318.5 | 13 | 10 |
| 1992 | 294.7 | 16 | 10 |
| 1997 | 276.5 | 11 | 9 |
| 1990 | 250.4 | 16 | 6 |
| 2014 | 223.8 | 16 | 9 |
| 1993 | 220.3 | 11 | 9 |
| 2006 | 208.0 | 11 | 6 |
| 1978 | 207.1 | 14 | 7 |
| 1983 | 206.2 | 12 | 8 |
Every season page shows its ACE against all others in its basin, and every storm page shows how its own ACE compares with every storm in its file.
Sources
NOAA National Hurricane Center, HURDAT2 Atlantic 1851–2025 and Northeast and North Central Pacific 1949–2025. Landsea, C. W. and J. L. Franklin, 2013, Monthly Weather Review 141, 3576–3592. Counts, ranks and totals worked out on this site; see how to read the record.