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I had an interesting question land in my email box the other day from a Katy viewer:
Frank — just out of curiosity, if a hurricane’s winds are measured at say 120 mph at ground level at landfall, how high up the hurricane “wall” is that windspeed still registering at 120 mph, or does it diminish or increase in speed at increasing heights? Or does windspeed in fact vary considerably throughout the hurricane? Jim
Actually, Jim, the answer is in the last part of your question as every storm is different, hence, wind speeds can vary from top to bottom, bottom to top, and somewhere in between!
The National Hurricane Center, of course, recognizes the importance of knowing just how strong the wind speeds are at the ground level not just because that’s where everything is built, but wind drives storm surge which is also at the bottom of the storm. In general, wind speeds are about 90% lower from top (10,000 feet) to bottom (your feet), but that can vary considerably depending on the storm.
For instance, the NHC looked at Hurricane Mitch (1998) which at hurricane hunter flight level had winds no higher than 150mph. A standard ‘reduction’ factor would assume surface winds were 135mph. However, at the surface, dropsondes indicated that surface winds reached 165mph. Mitch clearly weakened from the top down.
Hurricane Bonnie that same year (1998) measured 115mph winds at flight level but top surface winds measured only 98-104mph, thus storms can come in weaker the lower you go.
Probably the more useful measurement is how winds can increase from the surface to just 30 stories up. Consider how many high rises we have built and continue to build in Houston. A prime example of higher winds the higher you go was during Hurricane Georges (1998). You can see (in yellow) the surface winds were 110mph which is right at the top of a Category 2 storm, while just 300 feet higher (30 stories up), the surface winds (in blue) were closer to 145mph, a strong Cat 4!
Keep in mind, our high-rises here in the Houston area go to 40 and 50 stories! You can see plenty of damage photos to high-rises in Miami after Hurricane Wilma -- the higher you go, the more windows that are gone!
The table below shows how wind speeds generally increase the higher you go.
You can have a look at the 2000 study from the National Hurricane Center. Thanks for the question, Jim and, while not perfect, hopefully this gave you an answer!
Frank
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