Hurricane Ian

By Paul Homewood

A life-threatening hurricane has made landfall on Florida’s south-western coast, bringing heavy rain, high winds and catastrophic storm surges.

Hurricane Ian has sustained wind speeds up to 250km/h (155mph), just shy of the threshold for category five storms – the strongest classification.

Millions of Floridians with homes in Ian’s path are under evacuation orders.

Those who remained are facing one of the most dangerous storms to hit the US in decades.

More than one million Florida residents have lost power, after Ian knocked out power in all of Cuba.

https://www.bbc.co.uk/news/world-us-canada-63052558

According to the National Hurricane Center, Ian made landfall near Fort Myers with sustained winds of 150 mph and central pressure of 940 MB:

https://www.nhc.noaa.gov/archive/2022/al09/al092022.update.09281909.shtml?

There is no doubt that it was a particularly catastrophic storm, but were wind speeds really 150 mph?

The first clue is that central pressure of 940 MB at landfall. The lowest pressure prior to landfall was 937 MB. As the chart below shows, it is unheard of for US landfall hurricanes of 940 MB to produce 150 mph (130 Kts) winds:

https://www.aoml.noaa.gov/hrd/hurdat/All_U.S._Hurricanes.html

Typically 940 MB storms have wind speeds of about 115 Kts.

The Galveston hurricane of 1915 had the same 940 MB pressure, but winds were estimated at 115 Kts. Hazel in 1954 was also 940 MB and had winds of 115 Kts too.

Two of of the most intense hurricanes to hit the US had much lower pressure than Ian, but were estimated at the time to have no stronger winds than Ian this week – Indianola in 1886 was measured at 925 MB, with estimated wind speeds of 130 Kts, the same as Ian, while the Great Miami hurricane in 1926 was 929 MB with wind speeds of 125 Kts.

Curiously the only other hurricane to buck this is Laura in 2020, with a central pressure of 939 MB and 130 Kt winds. Other than Laura, 130 Kt and over winds have not appeared in any storm with more than 934 MB pressure.

[I should point here that all of the numbers for central pressure and wind speed are at the time of landfall.]

It is significant that until the 1940s, wind speeds were always estimated from the central pressure, as there was no other way of measuring them, until satellites and hurricane hunter aircraft came along.

To be blunt, 130 Kt wind speeds are not consistent with 940 MB pressure.

And this is not the only evidence of inconsistency. Estimates of wind speeds from satellite data are largely based on the Digital Dvorak system, which measures temperatures within the cyclone.

According to the satellites, Ian only peaked at around 120 Kts at landfall. Indeed it briefly peaked higher on the 27th, when winds were estimated at 110 Kts. The red line represents the actual measurements, while the green line is the published number, in this case peaking at 135 Kts just prior to landfall:

http://rammb-data.cira.colostate.edu/tc_realtime/storm.asp?storm_identifier=al092022

In terms of central pressure, Ian is the 22nd most intense landfalling hurricane since 1851, suggesting it was in now way unusual:

https://www.aoml.noaa.gov/hrd/hurdat/All_U.S._Hurricanes.html

Finally, Ian was the first major hurricane to hit the US this year, indeed the first hurricane to do so. The historical data shows clearly that there are no adverse trends in US hurricanes: