Some Perspective on Historic Hurricane Irma

Category 5 Hurricane Irma   (Source: WUSA9)

Hurricane Irma is one of the strongest hurricanes on record in the Atlantic Ocean Basin.  Irma was upgraded to a Category 5 storm on the Saffir-Simpson scale on Tuesday, September 5.  Irma’s highest sustained winds reached 185 mph and its lowest pressure was 914 millibars.  By comparison, standard sea level air pressure is approximately 1013 millibars.  The lower the air pressure, the stronger the hurricane is.

Irma’s maximum sustained winds of 185 mph tied it with Hurricanes Gilbert, Wilma and the unnamed 1935 Florida Keys hurricane for second highest on record in the Atlantic Ocean.  Only Hurricane Allen in 1980 had higher sustained winds (190 mph).  While the 1935 Labor Day hurricane made landfall in the Florida Keys as a Category 5, Hurricanes Allen and Wilma, fortunately, weakened before impacting the United States.  Only three Category 5 hurricanes have previously made landfall in the United States, with Camille (1969) and Andrew (1992) being the other two.  Categories on the Saffir-Simpson hurricane scale (1 – 5) are assigned by the sustained wind speed as opposed to the central air pressure in a hurricane.

According to Dr. Philip Klotzbach of Colorado State University, Hurricane Irma’s air pressure of 914 millibars is the “lowest pressure by an Atlantic hurricane outside of the western Caribbean and Gulf of Mexico on record.”  A common misconception in some news reports has been that Irma is the strongest Atlantic hurricane on record.  While Hurricane Irma’s peak sustained winds of 185 mph tied for the second highest sustained winds of any Atlantic hurricane, there have been 10 Atlantic hurricanes with a lower minimum air pressure.  It is an accurate and complete statement to say that Irma is the strongest hurricane in that part of the Atlantic Ocean Basin (excluding the Gulf of Mexico and Caribbean Sea).

There is an inverse relationship between the air pressure and sustained wind speeds around the center of a hurricane.  A good analogy would be to think of a figure skater: the faster they spin is proportional to how tightly they wrap their arms around their body.  In the same way, the tighter the circulation of a hurricane, the lower the air pressure and the faster the wind speeds are.

Sometimes a delay exists between a lower air pressure and a commensurate increase or decrease in wind speeds.  For this reason, scientists generally consider air pressure to be a more accurate measure than wind speed of how strong a hurricane is.  For example, Hurricane Katrina was a Category 5 hurricane with sustained winds of 175 mph and a minimum air pressure of 902 millibars while over the Gulf of Mexico.  However, while Katrina’s sustained winds weakened to Category 3 intensity (125 mph) when it made landfall, its air pressure was still quite low (920 mb).  Although a Category 5 with sustained winds of 165 mph, Hurricane Andrew’s air pressure was slightly higher (922 mb) than Katrina’s when it made landfall in 1992. 

The National Hurricane Center will continue to provide the latest updates on historic Hurricane Irma.