Here’s what makes powerful Irma a truly rare storm

Irma is one of the strongest hurricanes ever to move across the Atlantic basin.

It’s 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 are both major hurricanes it’s tempting to make comparisons. Some of these are valid; others not so much.

A close-up image of Irma Wednesday morning as it passed by the Caribbean island of St. Martin.

Irma really has entered a class of its own: It’s a storm quite rare in the annals of intensity.

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. Now imagine that type of storm with 185 mph winds, and you have Irma. By comparison, winds when Harvey made landfall gusted to 132 mph, which are exponentially less intense and a full hurricane category lower.

Hurricane season peaks in September.

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

Because Harvey was a slow-moving hurricane, it dropped a record amount of rainfall. However, the 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’s moving through (outside of the western Caribbean and Gulf of Mexico).

Harvey brought over 4 feet of rain to parts of Texas.

Irma has already set many records as of this writing. Perhaps there are more to come.

The winds of 185 mph 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 mph, was stronger.

When 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 this area.

Irma hitting St. Martin on Wednesday morning.

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, really capture just how unique Irma really is.

Accumulated Cyclone Energy is one way to measure a storm’s intensity.

We have now had 11 named storms this hurricane season. Katia became a tropical storm Wednesday morning, becoming the 11th.

Only in 1933, 1936, 1995, 2005, 2011, and 2012 have we reached this number of storms so early in the season.

Hurricane season comes to a close Nov. 30. We have a lot of potential for more storms this year.

Only time will tell if there are more major hurricanes to track in the coming weeks or the atmosphere decides to take a break after so much activity.