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.
Thanks for the update on Irma, Chris. Frightening to know that this hurricane compares with Katrina and Andrew.
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