There are urban myths and legends involving almost any topic imaginable, and meteorology is no exception. Too often in print and electronic media, and sometimes even from fellow meteorologists, there are misstatements that make me want to throw a shoe across the room. These are three of my “favorite” meteorological pet peeves.
“Hurricane Force Winds”
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 overacheivers 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!
Coming next time … “90 degrees and 90% humidity”
Do you have a favorite weather-related pet peeve? Let me know.
Jan Null, CCM
SF Weather Examiner
jnull@ggweather.com
Comments