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ACE: A New Way To Measure Hurricane Season Strength

November 17, 2009

By WeatherBug Meteorologist, Andrew Rosenthal

As the 2009 Atlantic Basin hurricane season draws to a close, meteorologists are in agreement that it has been a quiet season. But what determines whether a season is quiet or busy? Simple methods can be used, such as counting the number of storms that form in a season, or the number of hurricanes, but these statistics don`t always tell the whole story.

Using a count of the number of storms can miss important details about the storm`s history. For example, a straight count might not differentiate between several weak hurricanes and several strong hurricanes. Another statistic, "Storm Days," is a count of the number of days storms existed. This will describe whether storms were around often, but say nothing about the storm`s strength.

As a result, a newer, slightly more complicated statistic has been developed to provide this information. Known as Accumulated Cyclone Energy, or ACE, this is a measure of the winds that are seen over the course of a storm, and over time, a season.

ACE is computed every 6 hours by multiplying the wind speed in knots with itself. These values are then totaled over the course of a storm`s lifetime. That number is divided by 10,000 to simplify the values and keep them manageable. The total of all of these values for every storm in a year determines the seasonal ACE index. For example, a hurricane with a wind speed of 100 knots (115 mph) would produce an ACE index of 1 for that 6 hour period. This is calculated by multiplying 100 by 100, getting 10,000. This is then divided by 10,000 to get 1.

The advantage to using ACE is that it can point out the difference between a strong hurricane and weaker storm, as a strong storm will have a larger individual ACE value. In fact, some of the strongest hurricanes on record can, on their own, approach or exceed the total ACE index for a weak season.

Federal researchers also uses ACE index to determine whether a season is statistically above- or below-average. In the Atlantic Basin, a season with an ACE value of 103 or higher, is considered to be above-average, while a season totaling less than 66 is considered to be below-average. Since 1950, the average ACE is 93.2. Meanwhile, the eastern Pacific basin typically sees slightly higher ACE index values, with its average set at 130. Above- and below-average values are 135 and 86 in the eastern Pacific basin.

The highest ACE was 73.4 for a storm in 1899 that devastated Puerto Rico and the Carolinas, killed more than 3,000 people and lasted 28 days. 2004`s Hurricane Ivan challenged this record with an ACE value of 70.38. Powerful but short-lived hurricanes pale compared to these storms. Hurricane Katrina totaled an ACE of only 20.

The 2008 Atlantic Basin hurricane season, with 16 named storms, had an ACE index of 145, placing it above-average, while the 2008 Eastern Pacific Basin`s hurricane season was slightly below average at an ACE index of 83. So far, the 2009 Atlantic hurricane season has an ACE index of 50.4, placing it well below average. This would be the lowest since 1997, which saw an ACE index of 40. Meanwhile, the eastern Pacific hurricane season has an ACE index of 100, meaning if it ended today, it would be an average season.

Be sure to check your WeatherBug for more information on the tropics as the season winds down. Get the latest updates anywhere on Twitter at WeatherBug WeatherBuzz.

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