What is Accumulated Cyclone Energy (ACE)? How it's used to measure tropical activity

How do cruise lines aim to prevent storm encounters during hurricane season?

Over the summer, Gulf Coast ports could be threatened by tropical activity and hurricanes, which could disrupt the cruise industry. If you're planning to board a cruise this summer and encounter a storm, it's important to be prepared. Ashley Kosciolek, senior cruise writer for The Points Guy, joins FOX Weather to advise you on navigating the seas this summer.

An index developed to track the power of named storms around the world is known as the Accumulated Cyclone Energy or what is often referred to as the ACE.

The index uses a complex equation involving sustained wind speeds and a cyclone's duration into a single value, which accumulates as long as the system maintains at least tropical storm strength. 

A cyclone that lasts longer and has more powerful winds will have a greater ACE value than a system that remains weak and has a short lifespan. 

For example, Tropical Storm Colin, which developed off the coast of Georgia in 2022 and was classified as a minimal tropical storm for only 18 hours and racked up a total ACE value of 0.3675.

In contrast, Hurricane Ivan in 2004 accumulated nearly 70 ACE points and became a Category 5 hurricane during its month-long trek through the Caribbean, Southeast U.S., and back into the Gulf. 

Ivan holds the record for accumulating the most ACE points of any single cyclone in the basin and even produced more energy than some entire seasons, with all storm values combined.

Hurricane Ivan track 2004

(NASA)

According to Colorado State University, the average year in the Atlantic basin produces an ACE value of 122.5, but during active seasons, this value can approach double that amount. 

At least eight years have produced a combined season ACE of at least 200, which are often referred to as hyperactive seasons.

A season that accumulates an ACE of at least 121.2 is considered to be more active than normal, and eight of the nine years following 2015 all achieved this status.

BEWARE OF THE "I" STORM: IT HAS MORE RETIREES THAN ANY LETTER USED FOR ATLANTIC HURRICANE NAMES

Notable ACE index values

The late Dr. William Gray of Colorado State University developed the Accumulated Cyclone Energy (ACE) index and publicly unveiled it in 1994. 

Since its inception, the 2005 season became a record-holder, with an ACE of over 245 units, with a few years in the early 1900s and 1800s possibly generating greater amounts. 

Storms such as Dennis, Katrina, Rita and Wilma all formed during the record-breaking year, with Wilma being responsible for 32 of the ACE points.

While Wilma was not a classic, long-track Cape Verde cyclone, its rapid intensification into the most powerful hurricane ever seen in the Caribbean allowed it to accumulate ACE at a rate of more than 2 units every six hours.

2005 hurricane season tropical activity

(FOX Weather)

According to NOAA, more than 50 people were killed by the cyclone, and damage was estimated to be more than $26 billion. 

Wilma’s ACE value was comparable to Hurricane Beryl in 2024, which produced nearly 40 units, aided by its much longer track that began in the central Atlantic and continued well inland over Texas and Louisiana.

A limitation of the ACE index, similar to the Saffir-Simpson Hurricane Wind Scale, is that it does not account for impacts such as flooding, tornadoes or other inland effects. 

Hurricane Helene in September 2024 only accumulated an ACE of 7, but its widespread rainfall and flooding caused it to become the seventh costliest U.S. hurricane on record, with damage estimated at $78.7 billion and more than 200 deaths - figures that many may not associate with such a minimal ACE count.

TOP 5 STRONGEST HURRICANES EVER RECORDED IN ATLANTIC BASIN

ACE index helps to more accurately quantify season

Many tropical weather experts consider the ACE index to be a more accurate representation of a season's activity, particularly in recent years with improved tropical cyclone surveillance, than the overall count of cyclones.

With advancements in satellites, models and observations, more cyclones are now identified annually in the Atlantic basin than in the pre-satellite era, when some storms likely went undetected. 

The increase in detections causes the National Hurricane Center to issue advisories for tropical storms that may last only a few hours to a few days, contributing to inflated cyclone averages when compared to past decades.

For instance, in 2023, 20 named storms were tracked across the Atlantic Basin, but only seven intensified into hurricanes, and the season was considered to be the least impactful in at least a decade.

2023 hurricane season map

Without advanced technology, some of the season’s storms - such as Arlene, Bret, Cindy, Gert, Emily, Jose, Katia, Rina and Sean - might never have been classified as tropical cyclones, as they lasted only a few days and had little to no impact on land.

The season ended with an ACE of approximately 145.6 units, which was only slightly above normal but not high enough to be considered hyperactive, with an average of just 7.25 ACE points per cyclone.

Under the ACE method, the season would not be considered particularly potent, but because the index is not widely recognized, 2023 will forever be remembered for producing nearly two dozen storms, many of which were weak.

Tropical cyclone frequency chart from June 1 - November 30.

(FOX Weather)