By Paul Homewood
Before the days of satellites and hurricane hunter aircraft, the only way to estimate sustained wind speeds in the middle of hurricanes was to use minimum pressure.
Helene, you will recall had pressure of 938mb, and, supposedly, sustained winds of 140 mph. I have compared these windspeeds with all of the other two US hurricanes with 938mb at landfall, plus Rita and Harvey (937mb) and Laura (939mb)
- Unnamed in 1898 – 132 mph
- Helene (by coincidence!) in 1958 – 126 mph
- Rita in 2005 – 115 mph
- Harvey in 2017 – 132 mph
- Laura in 2020 – 150 mph
In graph form:
https://www.aoml.noaa.gov/hrd/hurdat/All_U.S._Hurricanes.html
.
The graph shows very clearly how windspeeds have been inflated in the last few years.
Why, for example, are the 1898 and 1958 hurricanes rated at 132 mph and 126 mph, when Helene with the same pressure is estimated at 140 mph? Harvey had even lower pressure but winds were only 132 mph.
In contrast, Laura, we are told, had winds of 150 mph, with a higher pressure than Harvey.
Minimum pressure does not tell us everything, of course. A small hurricane tends to have higher windspeeds than a large one with the same pressure, because it is more tightly wound. Helene however was a large storm, so if anything this should have lowered windspeeds.
Clearly it is impossible to compare accurately recent storms with those of the past. We now have satellites which can track hurricanes 24/7 and aircraft that can now fly into the middle of the strongest hurricanes, with an array of equipment which did not even exist thirty years ago.
Without wishing to belabour the point, two other hurricanes made landfall at 140 mph in the US; both had lower pressure:
- 1900 Galveston – 936mb
- 1989 Hugo – 934 mb
Galveston is the deadliest in US history, killing up to 12,000, and remains one of the most infamous disasters there. Hugo, which hit South Carolina, was the costliest hurricane in US history at the time. The NHC report on Hugo noted that the 140 mph wind speeds were based on pressure of 934mb and wind speeds at flight level.
https://web.archive.org/web/20180911204403/https://www.weather.gov/media/ilm/climate/Hugo/NHC_report_Hugo.pdf
It is also worth noting that aircraft do not directly measure wind speeds. In Helene’s case, they were estimated from windspeeds at flight level. But hurricane hunters have other methods, such as dropsondes and SFMR, which often give different conclusions. Yet there is no mention of these in the NHC report. Did they come up with lower speeds?
https://www.nhc.noaa.gov/archive/2017/al09/al092017.discus.023.shtml?
You will note that flight level winds were 136 kt, yet Hugo’s were 140 kt. Yet both are declared as 120 kt at ground level. Clearly Helene’s wind speeds have been overestimated compared to Hugo’s. If Helene’s are genuinely accurate, hurricanes prior to the last few years were all much more powerful than the official record shows.
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September 30, 2024 at 06:34AM
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