Tuesday, January 27, 2015

Hurricane Force wind Gusts = Not really a thing

I've heard this term (which makes me wince each time) quite a lot on news channels describing the "Blizzard of 2015".  The following is a 2011 article by my friend,
weather guru, Jan Null:

It seems that anytime there is a wind gust over about 60 mph the airwaves and other sources,
including NWS statements, are rife with the expression “hurricane force” winds. While this
might be good for conveying that it’s windy and might be dangerous, it’s both bad meteorology
and bad physics! (And calling it a hurricane force gust doesn’t make it right either)
Let’s start with some basics. The threshold for hurricane winds is when the 1-
minute sustained winds equal or exceed 74 miles per hour. Please note the word
“sustained”! According to the NOAA Hurricane Research Division, peak 3 to 5-
second gusts are approximately 30% higher than their associated sustained winds.
This means that a 74 mph sustained wind of a minimal hurricane has gusts in the
range of 96 mph. Quite a difference.

But that’s just the wind speed. What about the amount of force from the wind onto a surface that
is perpendicular to the wind? From high school physics we remember that the force associated
with a given speed is proportional to the square of the wind speed. (For the over achievers out
there, the formula to calculate this force is: F = .00256 x V^2, where F is the force in pounds per
square foot (psf), and V is the wind velocity in mph) Consequently, the amount of force with a
74 mph gust is 14.0 psf, while the force from a 96 mph gust is 23.6 psf; or 69% higher.
The bottom line is that a gust to 74 mph is NOT even close to hurricane force!
Jan Null
Certified Consulting Meteorologist
Golden Gate Weather Services
Phone: (408) 379-7500
"Climate is what you expect,
Weather is what you get". ~ R. Heinlein

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