Accumulated cyclone energy

Accumulated cyclone energy (ACE) is a metric used to compare overall activity of tropical cyclones, utilizing the available records of windspeeds at six-hour intervals to synthesize storm duration and strength into a single index value. The ACE index may refer to a single storm or to groups of storms such as those within a particular month, a full season or combined seasons. It is calculated by summing the square of tropical cyclones' maximum sustained winds, as recorded every six hours, but only for windspeeds of at least tropical storm strength (≥ 34 kn; 63 km/h; 39 mph); the resulting figure is divided by 10,000 to place it on a more manageable scale.

The calculation originated as the Hurricane Destruction Potential (HDP) index, which sums the squares of tropical cyclones' maximum sustained winds while at hurricane strength, at least 64 knots (≥ 119 km/h; 74 mph) at six-hour recorded intervals across an entire season. The HDP index was later modified to further include tropical storms, that is, all wind speeds of at least 34 knots (≥ 63 km/h; 39 mph), to become the accumulated cyclone energy index.

The highest ACE calculated for a single tropical cyclone on record worldwide is 85.26, set by Hurricane Ioke in 2006.

History The ACE index is an offshoot of Hurricane Destruction Potential (HDP), an index created in 1988 by William Gray and his associates at Colorado State University who argued the destructiveness of a hurricane's wind and storm surge is better related to the square of the maximum wind speed () than simply to the maximum wind speed (). The HDP index is calculated by squaring the estimated maximum sustained wind speeds for tropical cyclones while at hurricane strength, that is, wind speeds of at least 64 knots (≥ 119 km/h; 74 mph). The squared windspeeds from six-hourly recorded intervals are then summed across an entire season. This scale was subsequently modified in 1999 by the United States National Oceanic and Atmospheric Administration (NOAA) to include not only hurricanes but also tropical storms, that is, all cyclones while windspeeds are at least 34 knots (≥ 63 km/h; 39 mph). Since the calculation was more broadly adjusted by NOAA, the index has been used in a number of different ways such as to compare individual storms, and by various agencies and researchers including the Australian Bureau of Meteorology and the India Meteorological Department. The purposes of the ACE index include to categorize how active tropical cyclone seasons were as well as to identify possible long-term trends in a certain area such as the Lesser Antilles.

Calculation Accumulated cyclone energy is calculated by summing the squares of the estimated maximum sustained velocity of tropical and subtropical cyclones when wind speeds are at least tropical storm strength (≥ 34 kn; 63 km/h; 39 mph) at recorded six-hour intervals (00:00, 06:00, 12:00, and 18:00 UTC). The sums are usually divided by 10,000 to make them more manageable. One unit of ACE equals and for use as an index the unit is assumed. Thus:

(for ≥ 34 kn),

where is estimated sustained wind speed in knots at six-hour intervals.

Atlantic Ocean

Within the Atlantic Ocean, the United States National Oceanic and Atmospheric Administration and others use the ACE index of a season to classify the season into one of three categories. These three categories are above-normal, near-normal, and below-normal, and are worked out using an approximate tercile partitioning of seasons based on the ACE index over the 70 years between 1951 and 2020. Embedded within the above-normal category is a fourth category, extremely active, which captures seasons of extreme activity. The median value of the ACE index from 1951 to 2020 is 96.7 × 104 kt2.

+ Classification criteria Category ACE Index [kt2] % of 1951–2020median Extremely active > 159.6 > 165% Above-normal > 126.1 > 130% Near-normal 73.0–126.1 75%–130% Below-normal < 73.0 < 75%Reference:

+ Top 10 Atlantic hurricane seasons Season TS HU MH ACE 1933 20 11 6 258.57 2005 28 15 7 245.3 1893 12 10 5 231.15 1926 11 8 6 229.56 1995 19 11 5 227.10 2004 15 9 6 226.88 2017 17 10 6 224.88 1950 16 11 6 211.28 1961 12 8 5 188.9 1998 14 10 3 181.76

Individual storms in the Atlantic The highest ever ACE estimated for a single storm in the Atlantic is 73.6, for the San Ciriaco hurricane in 1899, a Category 4 hurricane which lasted for four weeks and had an ACE higher than many whole Atlantic storm seasons. Other Atlantic storms with high ACEs include Hurricane Ivan in 2004, with an ACE of 70.4; Hurricane Irma in 2017, with an ACE of 64.9; the Great Charleston Hurricane in 1893, with an ACE of 63.5; Hurricane Isabel in 2003, with an ACE of 63.3; and the 1932 Cuba hurricane, with an ACE of 59.8.

Since 1950, the highest ACE of a tropical storm was Tropical Storm Philippe in 2023, which attained an ACE of 9.4. The highest ACE of a Category 1 hurricane was Hurricane Nadine in 2012, which attained an ACE of 26.3. The record for lowest ACE of a tropical storm is jointly held by Tropical Storm Chris in 2000 and Tropical Storm Philippe in 2017, both of which were tropical storms for only six hours and had an ACE of just 0.1225. The lowest ACE of any hurricane was 2005's Hurricane Cindy, which was only a hurricane for six hours, and 2007's Hurricane Lorenzo, which was a hurricane for twelve hours; Cindy had an ACE of just 1.5175 and Lorenzo had a lower ACE of only 1.475. The lowest ACE of a major hurricane (Category 3 or higher) was Hurricane Gerda in 1969, with an ACE of 5.3. The following table shows those storms in the Atlantic basin from 1851–2021 that have attained over 50 points of ACE.

StormYearPeak classificationACEDurationHurricane Three189973.628 daysHurricane Ivan200470.423 daysHurricane Irma201764.913 daysHurricane Nine189363.520 daysHurricane Isabel200363.314 daysHurricane Fourteen193259.815 daysHurricane Donna196057.616 daysHurricane Carrie195755.821 daysHurricane Inez196654.621 daysHurricane Sam202153.814 daysHurricane Luis199553.715 daysHurricane Allen198052.312 daysHurricane Esther196152.218 daysHurricane Matthew201650.912 days

Historical ACE in recorded Atlantic hurricane history

Records of tropical storms, hurricanes, and major hurricanes before the satellite era (prior to the mid–1960s) are subject to an undercount bias, due to the difficulty of identifying storms.

Classification criteria

+ Accumulated Cyclone Energy - Atlantic Season ACE TS HU MH Classification185136.24631Below normal185273.28551Near normal185376.49842Near normal185431.00531Below normal185518.12541Below normal185648.94642Below normal185746.84430Below normal185844.79660Below normal185955.73871Below normal186062.06761Below normal186149.71860Below normal186246.03630Below normal186350.35950Below normal186426.55530Below normal186549.13730Below normal186683.65761Near normal186759.97971Below normal186834.65430Below normal186951.021071Below normal187087.811102Near normal187188.39862Near normal187265.38540Below normal187369.47532Below normal187447.05740Below normal187572.48651Below normal187656.05542Below normal187773.36831Near normal1878180.8512102Extremely active187963.63862Below normal1880131.081192Above normal188159.25740Below normal188259.4675642Below normal188366.7432Below normal188472.06441Below normal188558.3860Below normal1886166.16512104Extremely active1887181.2619112Extremely active188884.945962Near normal1889104.0425960Near normal189033.345421Below normal1891116.1051071Near normal1892115.8375950Near normal1893231.147512105Extremely active1894135.42754Above normal189568.765620Below normal1896136.0825762Above normal189754.54630Below normal1898113.23751151Near normal1899151.0251052Above normal190083.345732Near normal190198.9751360Near normal190232.65530Below normal1903102.071071Near normal190430.345640Below normal190528.3775511Below normal1906162.881163Extremely active190713.06500Below normal190895.111061Near normal190993.341264Near normal191063.9531Below normal191134.2875630Below normal191257.2625741Below normal191335.595640Below normal19142.53100Below normal1915130.095653Above normal1916144.012515105Above normal191760.6675422Below normal191839.8725641Below normal191955.04521Below normal192029.81540Below normal192186.53752Near normal192254.515531Below normal192349.31941Below normal1924100.18751152Near normal19257.2525410Below normal1926229.55751186Extremely active192756.4775841Below normal192883.475641Near normal192948.0675531Below normal193049.7725322Below normal193147.8351331Below normal1932169.66251564Extremely active1933258.5720116Extremely active193479.06751371Near normal1935106.2125853Near normal193699.7751771Near normal193765.851141Below normal193877.575942Near normal193943.6825631Below normal194067.79960Below normal194151.765643Below normal194262.4851141Below normal194394.011052Near normal1944104.45251483Near normal194563.4151152Below normal194619.6125730Below normal194788.491052Near normal194894.97751064Near normal194996.44751672Near normal1950211.282516116Extremely active1951126.3251283Above normal195269.081152Below normal195398.50751473Near normal1954110.881673Near normal1955158.171394Above normal195656.67251241Below normal195778.6625832Near normal1958109.69251273Near normal195977.10751472Near normal196072.9842Below normal1961188.89751285Extremely active196250.45740Below normal1963112.091073Near normal1964153.04251375Above normal196586.71751041Near normal1966145.21751573Above normal1967121.7051361Near normal196845.0725950Below normal196914918123Above normal197040.181472Below normal197196.52751361Near normal197235.605730Below normal197347.85841Below normal197468.1251142Below normal197576.0625963Near normal197684.17251062Near normal197725.3175651Below normal197863.21751252Below normal197992.9175962Near normal1980148.93751192Above normal1981100.32751273Near normal198231.5025621Below normal198317.4025431Below normal198484.2951351Near normal198587.98251173Near normal198635.7925640Below normal198734.36731Below normal1988102.99251253Near normal1989135.1251172Above normal199096.80251481Near normal199135.5375842Below normal199276.2225741Near normal199338.665841Below normal199432.02730Below normal1995227.102519115Extremely active1996166.18251396Extremely active 199740.9275831Below normal1998181.767514103Extremely active1999176.52751285Extremely active2000119.14251583Near normal2001110.321594Near normal200267.99251242Below normal2003176.841673Extremely active2004226.881596Extremely active2005245.328157Extremely active200678.5351052Near normal200773.8851562Near normal2008145.71751685Above normal200952.58932Below normal2010165.482519125Extremely active2011126.30251974Above normal2012132.632519102Above normal201336.121420Below normal201466.725862Below normal201562.6851142Below normal2016141.25251574Above normal2017224.877517106Extremely active2018132.58251582Above normal2019132.20251863Above normal2020180.372530147Extremely active2021145.55752174Above normal202294.42251482Near normal2023145.55652073Above normal2024161.582518115Extremely active2025130.81354Above normal202610.8800Current season

Eastern Pacific

Within the Eastern Pacific Ocean, the United States National Oceanic and Atmospheric Administration and others use the ACE index of a season to classify the season into one of three categories. These four categories are extremely active, above-, near-, and below-normal and are worked out using an approximate tercile partitioning of seasons based on the ACE index and the number of tropical storms, hurricanes, and major hurricanes over the 30 years between 1991 and 2020.

For a season to be defined as above-normal, the ACE index criterion and two or more of the other criteria given in the table below must be satisfied.

The mean value of the ACE index from 1991 to 2020 is 108.7 × 104 kt2, while the median value is 97.2 × 104 kt2.