What Is Accumulated Cyclone Energy (ACE)?

Accumulated cyclone energy, or ACE, is a single number that measures how much wind energy a tropical cyclone had over its life, or how much all the cyclones of a season had together. It is built from the storm’s wind speed every six hours, so a storm that is strong for a long time scores higher than one that is strong for a day.

The formula

For each six-hourly observation of a storm, take the maximum sustained wind speed in knots. If it is at least 34 knots, the threshold for tropical storm strength, square it. Add up the squares over the life of the storm and divide by 10,000 to keep the number small. In short:

ACE = sum of (wind in knots) squared, over six-hourly points at or above 34 knots, divided by 10,000.

Winds below 34 knots add nothing, so the count starts once a system is a tropical storm and stops when it weakens or changes character. A season’s ACE is the sum of the ACE of every storm in it.

Why the wind is squared

The kinetic energy of moving air grows with the square of its speed, so doubling the wind speed quadruples the energy. Squaring makes a hurricane at 120 knots count for far more than a tropical storm at 40 knots in each six-hour block. A cyclone at 100 knots for four days beats a cyclone at 60 knots for a week. This favours intensity and duration together, which is the point of the measure.

A worked example

Take a short-lived storm with six-hourly maximum winds of 35, 45, 55, 65 and 50 knots. The squares are 1,225, 2,025, 3,025, 4,225 and 2,500. They add up to 13,000, so the ACE is 1.3. A long-lived major hurricane with twenty-four observations averaging 100 knots would have a squared sum of about 240,000 and an ACE near 24. One storm like that can outweigh the rest of a quiet season.

What ACE is good for

• Comparing seasons. A season with several strong and long-lived storms scores high even when the storm count is ordinary. A season with many brief weak storms scores low. The count of named storms alone misses this.
• Comparing basins. Each ocean basin has its own typical range, and ACE lets you place a season within it.
• Tracking a season in progress. Because ACE adds up over time, it shows how active a season has been so far.

What ACE does not measure

ACE is about wind, at sea and on land alike. It says nothing about rainfall, storm surge, size, forward speed or where the storm went, and these are what cause much of the damage. A weak, slow storm can drop far more rain than a fast hurricane. A small, intense storm that stays at sea can have a large ACE and no impact at all. So use ACE for how energetic a season was, and use landfall records and impact reports for what it did.

There are technical limits too. Wind speeds are estimates. Agencies measure over different periods, one minute or ten minutes, and this changes the numbers, so values from different agencies are not directly comparable. Older records rely on aircraft and satellite estimates that were less accurate before the satellite era. Pre-1950 storms are not always matched correctly in the best-track archive. Treat long-term comparisons with care.

How we use it

Our storm data comes from the International Best Track Archive for Climate Stewardship (IBTrACS). Each storm page states which agency’s winds are shown, and ACE is computed from the six-hourly track points at or above 34 knots, using the formula above. Season and basin totals are the sums of the storm values. Active storms show the latest advisory position and intensity with its time and a link to the official advisory, and we make no forecasts. Values shown here are unofficial and must not be used for safety decisions.

You can browse storms and seasons on the storms pages, read how storm data is processed on the methodology page, and find the archive’s licence and citation on the sources page. ACE is a companion to station records: it measures a storm’s energy, where a weather record measures a single value at a single place. And where heat waves show up as clusters of station records, a cyclone shows up as a track of wind observations.

Reading an ACE number

A single season’s ACE has no absolute meaning. Look for the same basin and a long run of years, then see where the season falls. A value well above the long-run typical range marks an active season, and one well below marks a quiet one. Then ask whether the storms that drove the number reached land, since that is the part that touches people.