# Accumulated cyclone energy

Accumulated cyclone energy (ACE) is a measure used by various agencies including the National Oceanic and Atmospheric Administration (NOAA) and the India Meteorological Department to express the activity of individual tropical cyclones and entire tropical cyclone seasons. It uses an approximation of the wind energy used by a tropical system over its lifetime and is calculated every six hours. The ACE of a season is the sum of the ACEs for each storm and takes into account the number, strength, and duration of all the tropical storms in the season. The highest ACE calculated for a single storm is 82, for Hurricane/Typhoon Ioke in 2006.[1]

## Calculation

The ACE of a season is calculated by summing the squares of the estimated maximum sustained velocity of every active tropical storm (wind speed 35 knots [65 km/h, 40 mph] or higher), at six-hour intervals. Since the calculation is sensitive to the starting point of the six-hour intervals, the convention is to use 00:00, 06:00, 12:00, and 18:00 UTC. If any storms of a season happen to cross years, the storm's ACE counts for the previous year.[2] The numbers are usually divided by 10,000 to make them more manageable. One unit of ACE equals 104 kn2, and for use as an index the unit is assumed. Thus:

${\displaystyle {\text{ACE}}=10^{-4}\sum v_{\max }^{2}}$

where vmax is estimated sustained wind speed in knots.

Kinetic energy is proportional to the square of velocity, and by adding together the energy per some interval of time, the accumulated energy is found. As the duration of a storm increases, more values are summed and the ACE also increases such that longer-duration storms may accumulate a larger ACE than more-powerful storms of lesser duration. Although ACE is a value proportional to the energy of the system, it is not a direct calculation of energy (the mass of the moved air and therefore the size of the storm would show up in a real energy calculation).

A related quantity is hurricane destruction potential (HDP), which is ACE but only calculated for the time where the system is a hurricane.[3]

## Atlantic basin ACE

50
100
150
200
250
300
1950
1960
1970
1980
1990
2000
2010

### Categories

Atlantic basin cyclone intensity by Accumulated cyclone energy, timeseries 1850-2014

A season's ACE is used by NOAA and others to categorize the hurricane season into 4 groups by its activity.[4] Measured over the period 1951–2000 for the Atlantic basin, the median annual index was 87.5 and the mean annual index was 93.2. The NOAA categorization system divides seasons into:

• Above-normal season: An ACE value above 111 (120% of the 1981–2010 median), provided at least two of the following three parameters are met: number of tropical storms: 13, hurricanes: 7, and major hurricanes: 3.
• Near-normal season: neither above-normal nor below normal and the next number of tropical storms: 12, hurricanes: 6, and major hurricanes: 2.
• Below-normal season: An ACE value below 66 (71.4% of the 1981–2010 median), or none of the following three parameters are exceeded: number of tropical storms: 9, hurricanes: 4, and major hurricanes: 1.

According to the NOAA categorization system for the Atlantic, the most recent above-normal season is the 2019 season, the most recent near-normal season is the 2014 season, and the most recent below normal season is the 2015 season.

#### Hyperactivity

The term hyperactive is used by Goldenberg et al. (2001)[5] based on a different weighting algorithm[6], which places more weight on major hurricanes, but typically equating to an ACE of about 153 (175% of the 1951–2000 median) or more.

### 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. This single storm 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.[7]

Since 1950, the highest ACE of a tropical storm was Tropical Storm Laura in 1971, which attained an ACE of 8.6. The highest ACE of a Category 1 hurricane was Hurricane Nadine in 2012, which attained an ACE of 26.3. The lowest ACE of a tropical storm were tropical storms Chris (2000), Philippe (2017), Imelda (2019) and Olga (2019), all of which were tropical storms for only six hours and had an ACE of just 0.1. 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; both of which had an ACE of just 1.5. The lowest ACE of a major hurricane (Category 3 or higher), was Hurricane Gerda in 1969, with an ACE of 5.3.[8] The only years since 1950 to feature two storms with an ACE index of over 40 points have been 1966, 2003, and 2004, and the only year to feature three storms is 2017.[9]

The following table shows those storms in the Atlantic basin from 1950–2019 that have attained over 40 points of ACE.[8]

Storm Year Peak classification ACE Duration
Hurricane Ivan 2004
Category 5 hurricane
70.4 23 days
Hurricane Irma 2017
Category 5 hurricane
64.9 13 days
Hurricane Isabel 2003
Category 5 hurricane
63.3 14 days
Hurricane Donna 1960
Category 4 hurricane
57.6 16 days
Hurricane Carrie 1957
Category 4 hurricane
55.8 21 days
Hurricane Inez 1966
Category 4 hurricane
54.6 21 days
Hurricane Luis 1995
Category 4 hurricane
53.5 16 days
Hurricane Allen 1980
Category 5 hurricane
52.3 12 days
Hurricane Esther 1961
Category 5 hurricane
52.2 18 days
Hurricane Matthew 2016
Category 5 hurricane
50.9 12 days
Hurricane Flora 1963
Category 4 hurricane
49.4 16 days
Hurricane Edouard 1996
Category 4 hurricane
49.3 14 days
Hurricane Beulah 1967
Category 5 hurricane
47.9 17 days
Hurricane Dorian 2019
Category 5 hurricane
47.8 15 days
Hurricane Dog 1950
Category 4 hurricane
47.5 13 days
Hurricane Betsy 1965
Category 4 hurricane
47.0 18 days
Hurricane Frances 2004
Category 4 hurricane
45.9 15 days
Hurricane Faith 1966
Category 3 hurricane
45.4 17 days
Hurricane Maria 2017
Category 5 hurricane
44.8 14 days
Hurricane Ginger 1971
Category 2 hurricane
44.2 28 days
Hurricane David 1979
Category 5 hurricane
44.0 12 days
Hurricane Jose 2017
Category 4 hurricane
43.3 17 days
Hurricane Fabian 2003
Category 4 hurricane
43.2 14 days
Hurricane Hugo 1989
Category 5 hurricane
42.7 12 days
Hurricane Gert 1999
Category 4 hurricane
42.3 12 days
Hurricane Igor 2010
Category 4 hurricane
41.9 14 days

### Atlantic hurricane seasons, 1851–2019

Due to the scarcity and imprecision of early offshore measurements, ACE data for the Atlantic hurricane season is less reliable prior to the modern satellite era,[citation needed] but NOAA has analyzed the best available information dating back to 1851. The 1933 Atlantic hurricane season is considered the highest ACE on record with a total of 259.[10] For the current season or the season that just ended, the ACE is preliminary based on National Hurricane Center bulletins, which may later be revised.

Hyperactive
Above Normal
Near Normal
Below normal
Accumulated Cyclone Energy
Season ACE TS HU MH Classification
1851 36.24 6 3 1 Below normal
1852 73.28 5 5 1 Near normal
1853 76.49 8 4 2 Near normal
1854 31.00 5 3 1 Below normal
1855 18.12 5 4 1 Below normal
1856 48.94 6 4 2 Below normal
1857 46.84 4 3 0 Below normal
1858 44.79 6 6 0 Below normal
1859 55.73 8 7 1 Below normal
1860 62.06 7 6 1 Below normal
1861 49.71 8 6 0 Below normal
1862 46.03 6 3 0 Below normal
1863 50.35 9 5 0 Below normal
1864 26.55 5 3 0 Below normal
1865 49.13 7 3 0 Below normal
1866 83.65 7 6 1 Near normal
1867 59.97 9 7 1 Below normal
1868 34.65 4 3 0 Below normal
1869 51.02 10 7 1 Below normal
1870 87.8 11 10 2 Near normal
1871 88.39 8 6 2 Near normal
1872 65.38 5 4 0 Below normal
1873 69.47 5 3 2 Near normal
1874 47.05 7 4 0 Below normal
1875 72.48 6 5 1 Near normal
1876 56.05 5 4 2 Below normal
1877 73.36 8 3 1 Below normal
1878 180.85 12 10 2 Hyperactive
1879 63.63 8 6 2 Below normal
1880 131.08 11 9 2 Near normal
1881 59.25 7 4 0 Below normal
1882 59.4675 6 4 2 Below normal
1883 66.7 4 3 2 Near normal
1884 72.06 4 4 1 Below normal
1885 58.3 8 6 0 Below normal
1886 166.165 12 10 4 Hyperactive
1887 181.26 19 11 2 Hyperactive
1888 84.945 9 6 2 Near normal
1889 104.0425 9 6 0 Near normal
1890 33.345 4 2 1 Below normal
1891 116.105 10 7 1 Near normal
1892 115.8375 9 5 0 Near normal
1893 231.1475 12 10 5 Hyperactive
1894 135.42 7 5 4 Near normal
1895 68.765 6 2 0 Below normal
1896 136.0825 7 6 2 Near normal
1897 54.54 6 3 0 Below normal
1898 113.2375 11 5 1 Near normal
1899 151.025 10 5 2 Near normal
1900 83.345 7 3 2 Near normal
1901 98.975 13 6 0 Near normal
1902 32.65 5 3 0 Below normal
1903 102.07 10 7 1 Near normal
1904 30.345 6 4 0 Below normal
1905 28.3775 5 1 1 Below normal
1906 162.88 11 6 3 Near normal
1907 13.06 5 0 0 Below normal
1908 95.11 10 6 1 Near normal
1909 93.34 12 6 4 Near normal
1910 63.9 5 3 1 Below normal
1911 34.2875 6 3 0 Below normal
1912 57.2625 7 4 1 Below normal
1913 35.595 6 4 0 Below normal
1914 2.53 1 0 0 Below normal
1915 130.095 6 5 3 Near normal
1916 144.0125 15 10 5 Above normal
1917 60.6675 4 2 2 Below normal
1918 39.8725 6 4 1 Below normal
1919 55.04 5 2 1 Below normal
1920 29.81 5 4 0 Below normal
1921 86.53 7 5 2 Near normal
1922 54.515 5 3 1 Below normal
1923 49.31 9 4 1 Below normal
1924 100.1875 11 5 2 Near normal
1925 7.2525 4 1 0 Below normal
1926 229.5575 11 8 6 Hyperactive
1927 56.4775 8 4 1 Below normal
1928 83.475 6 4 1 Below normal
1929 48.0675 5 3 1 Below normal
1930 49.7725 3 2 2 Below normal
1931 47.835 13 3 1 Below normal
1932 169.6625 15 6 4 Hyperactive
1933 258.57 20 11 6 Hyperactive
1934 79.0675 13 7 1 Near normal
1935 106.2125 8 5 3 Near normal
1936 99.775 17 7 1 Near normal
1937 65.85 11 4 1 Near normal
1938 77.575 9 4 2 Near normal
1939 43.6825 6 3 1 Below normal
1940 67.79 9 6 0 Near normal
1941 51.765 6 4 3 Below normal
1942 62.485 11 4 1 Below normal
1943 94.01 10 5 2 Near normal
1944 104.4525 14 8 3 Near normal
1945 63.415 11 5 2 Below normal
1946 19.6125 7 3 0 Below normal
1947 88.49 10 5 2 Near normal
1948 94.9775 10 6 4 Near normal
1949 96.4475 16 7 2 Near normal
1950 211.2825 16 11 6 Hyperactive
1951 126.325 12 8 3 Above normal
1952 69.08 11 5 2 Near normal
1953 98.5075 14 7 3 Near normal
1954 110.88 16 7 3 Near normal
1955 158.17 13 9 4 Hyperactive
1956 56.6725 12 4 1 Below normal
1957 78.6625 8 3 2 Near normal
1958 109.6925 12 7 3 Near normal
1959 77.1075 14 7 2 Near normal
1960 72.9 8 4 2 Near normal
1961 205.395 12 8 5 Hyperactive
1962 35.5675 7 4 0 Below normal
1963 117.9325 10 7 3 Near normal
1964 169.7675 13 7 5 Near normal
1965 84.33 10 4 1 Below normal
1966 145.2175 11 7 3 Above normal
1967 121.705 8 6 1 Near normal
1968 45.0725 8 5 0 Below normal
1969 165.7375 18 12 5 Hyperactive
1970 40.18 10 5 2 Below normal
1971 96.5275 13 6 1 Near normal
1972 35.605 7 3 0 Below normal
1973 47.85 8 4 1 Below normal
1974 68.125 11 4 2 Below normal
1975 76.0625 9 6 3 Near normal
1976 84.1725 10 6 2 Near normal
1977 25.3175 6 5 1 Below normal
1978 63.2175 12 5 2 Below normal
1979 92.9175 9 6 2 Near normal
1980 148.9375 11 9 2 Near normal
1981 100.3275 12 7 3 Near normal
1982 31.5025 6 2 1 Below normal
1983 17.4025 4 3 1 Below normal
1984 84.295 13 5 1 Near normal
1985 87.9825 11 7 3 Near normal
1986 35.7925 6 4 0 Below normal
1987 34.36 7 3 1 Below normal
1988 102.9925 12 5 3 Near normal
1989 135.125 11 7 2 Near normal
1990 96.8025 14 8 1 Near normal
1991 35.5375 8 4 2 Below normal
1992 76.2225 7 4 1 Below normal
1993 38.665 8 4 1 Below normal
1994 32.02 7 3 0 Below normal
1995 227.1025 19 11 5 Hyperactive
1996 166.1825 13 9 6 Hyperactive
1997 40.9275 8 3 1 Below normal
1998 181.7675 14 10 3 Hyperactive
1999 176.5275 12 8 5 Hyperactive
2000 119.1425 15 8 3 Above normal
2001 110.32 15 9 4 Near normal
2002 67.9925 12 4 2 Near normal
2003 176.84 16 7 3 Hyperactive
2004 226.88 15 9 6 Hyperactive
2005 250.1275 28 15 7 Hyperactive
2006 78.535 10 5 2 Near normal
2007 73.885 15 6 2 Near normal
2008 145.7175 16 8 5 Above normal
2009 52.58 9 3 2 Below normal
2010 165.4825 19 12 5 Hyperactive
2011 126.3025 19 7 4 Above normal
2012 132.6325 19 10 2 Above normal
2013 36.12 14 2 0 Below normal
2014 66.725 8 6 2 Near normal
2015 62.685 11 4 2 Below normal
2016 141.2525 15 7 4 Above normal
2017 224.8775 17 10 6 Hyperactive
2018 132.5825 15 8 2 Above normal
2019 130.425 18 6 3 Above normal

## Eastern Pacific ACE

### Categories

100
200
300
400
1970
1980
1990
2000
2010

The (unofficial) categorization of seasons for this table is based mutatis mutandis on that used in the Atlantic basin:

• Above-normal season: An ACE value above 135 (117% of the median), provided at least two of the following three parameters exceed the long-term average: number of tropical storms (16), hurricanes (9), and major hurricanes (4).
• Hyperactive season: An ACE value above 194, provided that preceding criteria are met
• Near-normal season: neither above-normal nor below normal
• Below-normal season: An ACE value below 86 (75% of the median)
Observed monthly values for the PDO index, 1900–present.
Historical East Pacific Seasonal Activity, 1981–2015.

### Individual storms in the Eastern Pacific (east of 180°W)

The highest ever ACE estimated for a single storm in the Eastern or Central Pacific, while located east of the International Date Line is 62.8, for Hurricane Fico of 1978. Other Eastern Pacific storms with high ACEs include Hurricane John in 1994, with an ACE of 54.0, Hurricane Kevin in 1991, with an ACE of 52.1, and Hurricane Hector of 2018, with an ACE of 50.5.[11]

The following table shows those storms in the Eastern and Central Pacific basins from 1971–2018 that have attained over 30 points of ACE.[12]

Storm Year Peak classification ACE Duration
Hurricane Fico 1978
Category 4 hurricane
62.8 20 days
Hurricane John   1994
Category 5 hurricane
54.0 19 days
Hurricane Kevin 1991
Category 4 hurricane
52.1 17 days
Hurricane Hector   2018
Category 4 hurricane
50.5 13 days
Hurricane Tina 1992
Category 4 hurricane
47.7 22 days
Hurricane Trudy 1990
Category 4 hurricane
45.8 16 days
Hurricane Lane 2018
Category 5 hurricane
44.2 13 days
Hurricane Dora   1999
Category 4 hurricane
41.4 13 days
Hurricane Jimena 2015
Category 4 hurricane
40.0 15 days
Hurricane Guillermo 1997
Category 5 hurricane
40.0 16 days
Hurricane Norbert 1984
Category 4 hurricane
39.6 12 days
Hurricane Norman 2018
Category 4 hurricane
36.6 12 days
Hurricane Celeste 1972
Category 4 hurricane
36.3 16 days
Hurricane Sergio 2018
Category 4 hurricane
35.5 13 days
Hurricane Lester 2016
Category 4 hurricane
35.4 14 days
Hurricane Olaf 2015
Category 4 hurricane
34.6 12 days
Hurricane Jimena 1991
Category 4 hurricane
34.5 12 days
Hurricane Doreen 1973
Category 4 hurricane
34.3 16 days
Hurricane Ioke   2006
Category 5 hurricane
34.2 7 days
Hurricane Marie 1990
Category 4 hurricane
33.1 14 days
Hurricane Orlene 1992
Category 4 hurricane
32.4 12 days
Hurricane Greg 1993
Category 4 hurricane
32.3 13 days
Hurricane Hilary 2011
Category 4 hurricane
31.2 9 days

– Indicates that the storm formed in the Eastern/Central Pacific, but crossed 180°W at least once, therefore only the ACE and number of days spent in the EPAC/CPAC are included.

### Eastern Pacific hurricane seasons, 1971–2019

Accumulated Cyclone Energy is also used in the eastern and central Pacific Ocean. Data on ACE is considered reliable starting with the 1971 season. The season with the highest ACE since 1971 is the 2018 season. The 1977 season has the lowest ACE. The most recent above-normal season is the 2018 season, the most recent near-normal season is the 2019 season, and the most recent below normal season is the 2013 season.[13] The 35 year median 1971–2005 is 115 x 104 kn2 (100 in the EPAC zone east of 140°W, 13 in the CPAC zone); the mean is 130 (112 + 18).

Hyperactive
Above Normal
Near Normal
Below normal
Accumulated Cyclone Energy
Season ACE TS HU MH Classification
1971 139 18 12 6 Above normal
1972 136 14 8 4 Above normal
1973 114 12 7 3 Near normal
1974 90 18 11 3 Near normal
1975 112 17 9 4 Near normal
1976 121 15 9 5 Near normal
1977 22 8 4 0 Below normal
1978 207 19 14 7 Hyperactive
1979 57 10 6 4 Below normal
1980 77 14 7 3 Below normal
1981 72 15 8 1 Below normal
1982 161 23 12 5 Above normal
1983 206 21 12 8 Hyperactive
1984 193 21 13 7 Above normal
1985 192 24 13 8 Above normal
1986 107 17 9 3 Near normal
1987 132 20 10 4 Near normal
1988 127 15 7 3 Near normal
1989 110 17 9 4 Near normal
1990 245 21 16 6 Hyperactive
1991 178 14 10 5 Above normal
1992 295 27 16 10 Hyperactive
1993 201 15 11 9 Hyperactive
1994 185 20 10 5 Above normal
1995 100 10 7 3 Near normal
1996 53 9 5 2 Below normal
1997 167 19 9 7 Above normal
1998 134 13 9 6 Near normal
1999 90 9 6 2 Near normal
2000 95 19 6 2 Near normal
2001 90 15 8 2 Near normal
2002 124 15 8 6 Near normal
2003 56 16 7 0 Below normal
2004 71 12 6 3 Below normal
2005 96 15 7 2 Near normal
2006 155 19 11 6 Above normal
2007 52 11 4 1 Below normal
2008 83 17 7 2 Below normal
2009 127 20 8 5 Near normal
2010 52 8 3 2 Below normal
2011 121 11 10 6 Near normal
2012 98 17 10 5 Near normal
2013 76 20 9 1 Below normal
2014 199 22 16 9 Hyperactive
2015 287 26 16 11 Hyperactive
2016 183 22 13 6 Above normal
2017 100 18 9 4 Near normal
2018 318 23 13 10 Hyperactive
2019 97 19 7 4 Near normal

## References

1. ^ Mark Saunders; Adam Lea (2007). "Summary of 2006 NW Pacific Typhoon Season and Verification of Authors' Seasonal Forecasts" (PDF). Tropical Storm Risk. Archived from the original (PDF) on 2007-11-29. Retrieved 2013-09-01.
2. ^ Last advisory for T.S. Zeta 2005
3. ^ Bell GD, Halpert MS, Schnell RC, et al. (2000). "Climate Assessment for 1999" (PDF). Bulletin of the American Meteorological Society. 81 (6): 1328. Bibcode:2000BAMS...81.1328B. doi:10.1175/1520-0477(2000)081<1328:CAF>2.3.CO;2.
4. ^ Climate Prediction Center (May 22, 2014). "Background information: the North Atlantic Hurricane Season". United States National Oceanic and Atmospheric Administration. Retrieved June 5, 2014.
5. ^ Goldberg SB, Landsea CW, Mestas-Nuñez AM, Gray WM (July 2001). "The Recent Increase in Atlantic Hurricane Activity: Causes and Implications" (PDF). Science. 293 (5529): 474–9. Bibcode:2001Sci...293..474G. doi:10.1126/science.1060040. PMID 11463911. Supplementary material
6. ^ "Summary of 2000 Atlantic tropical cyclone activity and verification of authors' seasonal activity prediction". Archived from the original on 2006-08-22. Retrieved 2006-08-22.
7. ^ "Atlantic hurricane best track (HURDAT version 2)" (Database). United States National Hurricane Center. December 12, 2019.
8. ^ a b "Atlantic hurricanes by ACE - 1950-2012". Policlimate. Archived from the original on 10 August 2017. Retrieved 7 August 2017.
9. ^ Klotzbach, Phil; McNoldy, Brian. "The extreme 2017 hurricane season adds another remarkable record to its long list". The Washington Post. Retrieved 12 May 2018.
10. ^ "Original vs. Revised HUDRAT". NOAA. Retrieved 28 September 2017.
11. ^ "Eastern Pacific Best Track Data - (1949 - present)". National Hurricane Center. Retrieved 12 August 2018.
12. ^ Webb, Eric. "Hurricane Hector 2018 - Discussion". Twitter. Retrieved 12 August 2018.
13. ^ East North Pacific ACE (through 30 Nov. 2005)