There are a number of different ways to gauge a hurricane season. One of these is the Accumulated Cyclone Energy index, which is widely referred to as ACE.
It expresses the combined intensity and duration of individual cyclones and provides a measure of activity for an entire hurricane season. For a single storm, it is calculated by summing the squares of the maximum sustained wind speeds measured every six hours while they are at least tropical storm strength. This number is then divided by 10,000 to make it more user-friendly. Overall, the stronger and longer-lived a storm is, the higher its ACE value.
The ACE for a season is the sum of the ACE values from individual storms that occurred that year. NOAA considers a season with an ACE of 111 or higher to be above average, while an ACE of 66 or lower is regarded as below average.
The Atlantic hurricane season runs from June 1 through November 30.
(Note: Year to date the Atlantic Basin has had 13 storms with a combined ACE of 202 and there are still two months left in the 2017 hurricane season.)