This has been the most active month for hurricanes on record

On Wednesday morning, Hurricane Lee became the fifth major hurricane of the 2017 season in the Atlantic. It doesn’t pose any threat to the United States, happily, but it serves as a reminder of how unusual this month has been.

Our analysis of data from the National Oceanic and Atmospheric Administration shows that on only one day this month — Sept. 15 — was there no active hurricane for any part of the day. On 12 days, there was only one hurricane. On 10, two. On four, three — including Sept. 8, when Irma, Jose and Katia were whirling in the Atlantic as in the photo above.

(The other storm that hit Category 3 or higher was Harvey, which dissipated in August.)

How unusual is all of this activity? Very.

The NOAA offers data on recorded storms back to 1851, allowing us to compare this month to months past. It’s important to note that our ability to detect hurricanes is much, much better today than it was 150 years ago, given our access to satellites and airplanes. With that caveat in mind, we can compare September 2017 to months past by tallying the number of hurricane-days — the number of days in a month in which one hurricane was active for part of the day. Sept. 8, then, is three hurricane-days: One day, three hurricanes.

Using that metric, we see that no month has seen more hurricane-days overall. The second-closest months were August 1893 and September 1926, in which there were 38 hurricane-days. This month there have been 44, by our count — with three days to go and two hurricanes still out there.

Looking only at the number of days with major hurricanes — storms registering as a Category 3 or higher — this month ties September 2004, in which there were also 22 such days.

That was the year that Hurricanes Charley, Frances, Ivan and Jeanne all hit Florida, with the latter three all making landfall in September.

(Notice on the chart above the one dim block at the top of the 2016 column. That was Hurricane Alex, which formed in January — the first since 1955.)

As the Weather Underground noted on Tuesday, there’s another way in which September 2017 has been unusual. Looking at a measure called accumulated cyclone energy (ACE), which tallies the top sustained wind speed of a storm, it cited Colorado State University’s Phil Klotzbach: “[T]he Atlantic’s ACE total for September thus far was 155.4. …

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