How big is Hurricane Irma?

The most common metric for understanding the strength of a hurricane is the Saffir-Simpson scale, which is determined by maximum wind speed. Irma broke the record for consecutive hours at Category 5 on Sept. 7.

But hurricane strength is about more than just maximum wind speed. Energy is an important metric, because it can help determine the storm’s impact, how bad it will be and how far it will extend. A very large storm with moderate winds may have more total energy than an intense but small storm.

The map above, provided by RMS, a company specializing in risk assessment, compares the energy of Irma at its peak with other historic and recent hurricanes in the Atlantic basin. Irma’s wind speed combined with its size carried an energy value of 112 terajoules, which is similar to the energy of Katrina in 2005. Hurricane Sandy, not depicted in this map, was a Category 1, but according to RMS carried 330 terajoules of energy before its landfall in New Jersey because its wind field was so big. (These numbers represent energy in a thin slice of each storm measured about 10 meters off the ground. The energy of the entire height of the storm would be much, much greater.)

Irma’s maximum sustained winds were measured at 185 mph. Only one Atlantic Ocean storm in recorded history has exceeded that level: Hurricane Allen, which slammed into Mexico and Texas in 1980, had top wind speeds of 190 mph.

About 425 miles wide, Irma is significantly larger than Hurricane Andrew, which devastated parts of South Florida in 1992. Andrew’s top sustained winds were 175 mph, as were Hurricane Katrina’s in 2005.

But while Katrina had declined to a Category 3 storm by the time it struck the Gulf Coast, Andrew strengthened to a Category 5 just before it barreled into Florida. (Exact numbers for Andrew are sketchy because the storm destroyed much of the measuring equipment in its path.)

As of Sept. 6, Irma was smaller than Katrina at its peak but much larger than Andrew, and its hurricane-force winds stretched about 100 miles across — roughly the width of Florida in many places. By Sept. 11, the storm was much weaker but so spread out that its clouds covered parts of Florida and Canada simultaneously.

As Irma lumbered toward Florida, there were two other hurricanes in the Atlantic Ocean basin. This is the first time since 2010 there have been three active hurricanes. Hurricane Katia made landfall late Sept. 8 in Mexico with winds of 75 mph. Like Irma, Hurricane Jose formed in the far eastern Atlantic and traveled west. It peaked as a Category 4 hurricane with 155 mph winds as it spun in the Caribbean.