The NOAA Accumulated Cyclone Energy (ACE) Index is a measure of total wind energy for basin and landfalling tropical cyclone activity. The seasonal basin (eg Atlantic or Northwest Pacific) ACE index is defined as the sum of the squares of 6-hourly 1-minute maximum sustained wind speeds for all systems while they are at least tropical storm strength. We define the US ACE index as the sum of the squares (divided by 6) of hourly 1-minute maximum sustained wind speeds for all systems of at least tropical storm strength and over the US mainland. Since the ACE index reflects a combination of storm intensity and duration it is a better measure of overall activity and likely damage than the number of either basin or landfalling tropical storms or hurricanes.
About Tropical Storm Risk