Hurricane Irma has entered a class all its own

It's tempting to make comparisons between Hurricanes Harvey and Irma. Some of these are valid, others not so much.

Irma is one of the strongest hurricanes to ever move across the Atlantic basin. –NASA via AP

Irma is one of the strongest hurricanes to ever move across the Atlantic basin. It is coming on the heels of Harvey, the first category 4 storm to reach the United States mainland since Charley hit Florida in 2004. Since Harvey and Irma were both major hurricanes, it’s tempting to make comparisons. Some of these are valid, others not so much.


Irma really has entered a class of its own, with an intensity that is quite rare. Harvey brought record rain because of unusually weak upper-level winds, which kept the storm from moving very much, but the storm itself was a rather typical major hurricane. Major hurricanes are not unusual, they just don’t happen every year.


All hurricanes are basically rotating clusters of thunderstorms around a central eye. Think of the worst thunderstorm you’ve ever experienced, and now imagine that type of storm with 185 mile-per-hour winds, and you have Irma. When Harvey made landfall, its winds gusted to 132 miles per hour, a full hurricane category lower.

We still don’t know if Irma will come on shore in the United States or what category it might be when it does, but it has the potential to be worse than Harvey in terms of wind and storm surge. Irma’s rainfall, however, will not come close to Harvey, no matter how strong it gets; it is just moving too fast.

Harvey dropped a record amount of rainfall, but its winds, while strong, didn’t set any records. Harvey’s minimum atmospheric pressure of 938 millibars was typically low for a hurricane, but Irma’s low of 914 millibars put it in the top 10 lowest in the Atlantic and actually the lowest in the area of the Atlantic it is moving through (outside of the western Caribbean and Gulf of Mexico).

Irma has already set many records as of Wednesday morning; perhaps there are more to come. The winds of 185 miles per hour ties for the second-strongest winds of all time in Atlantic hurricanes. The Florida Keys storm of 1935, Gilbert in 1988, and Wilma in 2005 also had this level of wind. Only Allen in 1980, with winds of 190 miles per hour, was stronger.


When the Irma went through the chain of islands known as the Leewards on Wednesday morning with these strong winds, it became the strongest known hurricane to ever pass through the area.

Meteorologists like to use something called Accumulated Cyclone Energy to measure the strength of a storm. It also helps scientists keep track of whether overall storms are increasing in their strength. Irma has accumulated more ACE energy than the previous eight named storms this season, including Harvey.

Irma generated more ACE energy in a 24-hour period than any other hurricane on record in the tropical Atlantic, breaking the 1980 record set by Allen. I think these ACE statistics, more than any other data, really capture just how unique Irma really is.

We have now had 11 named storms this hurricane season. (Katia became a tropical storm Wednesday morning.) Only 1933,1936, 1995, 2005, 2011, and 2012 have reached this number of storms so early in the season. Hurricane season comes to a close Nov. 30. We have a lot more potential for more storms this year. Only time will tell if there are more major hurricanes to track in the coming weeks or if the atmosphere decides to take a break after so much activity.


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