All Weather News

What is Wind Chill?

24 Nov 2013, 12:12 pm

With the recent cold blast and gusty winds, wind chill values at Mount Washington Observatory in New Hampshire dropped to a staggering -53° F. The wind gusts there at the summit are pretty impressive as well!

We’ve reached the point in the season where wind chill becomes a factor as you bundle up to head outdoors. But what exactly is wind chill? An arbitrary number? An actual temperature? Will it affect your car? Your dog? Your pool?

Wind chill refers to the “feels like” temperature outside after you factor in actual air temperature, wind speed, and how quickly the wind cools a person’s skin. We calculate wind chill with an equation that accounts for body heat loss. Here it is:

“Here is how you calculate the New Wind Chill Index:
 
New Wind Chill T(wc) = 35.74 + 0.6215T – 35.75(V0.16) + 0.4275T(V0.16)
 
where T(wc) is the Wind Chill in degrees F, V is the Wind Speed in MPH, and T is the temperature in degrees F.”   –National Weather Service
 
 

*Why does that say “New” Wind Chill? Quick History Lesson: Equivalent temperature was not universally used in North America until the 21st century. Until the 1970s, the coldest parts of Canada reported the original Wind Chill Index, a three or four digit number with units of kilocalories/hour per square meter. Each individual calibrated the scale of numbers personally, through experience. The chart also provided general guidance to comfort and hazard through threshold values of the index, such as 1400, which was the threshold for frostbite.

In November 2001 Canada, U.S. and U.K. implemented a new wind chill index developed by scientists and medical experts. It is determined by iterating a model of skin temperature under various wind speeds and temperatures using standard engineering correlations of wind speed and heat transfer rate.  –Wikipedia

The above formula from 2001 makes some assumptions: your exposed skin is roughly five feet off the ground, you’re walking directly into the wind in an open field, and you’re traveling at about 3 mph. If any of these factors are drastically different, the wind chill equation becomes drastically less accurate.

Below is a chart that simplifies it a bit:

The basic idea here is that wind chill expresses how quickly living organisms will cool off, but has little to no impact on inanimate objects. Example: If the air temperature is 40° and the wind chill is 27°, your pool won’t freeze (even though your skin is convinced it’s cold enough).

Wind DOES aid objects in cooling quickly to the surrounding air temp, but it can’t actually force them to cool below that. Example: Your pie cooling on the windowsill will chill down faster with gustier winds than on a calm day.

 So how low has the wind chill ever gotten? Here’s a nugget via mentalfloss.com: “With all of the tweaks in the formula over the years, it’s tough to say definitively, but how’s this for chilly: on July 4, 2003, a remote weather station in east Antarctica picked up a minus-94 degree day. That would be plenty frigid on its own, but the wind was also blowing at 75 miles per hour, which would be good for a wind chill of about minus-150.”

NWS Dodge City, KS has a terrific write-up on wind chill here if you’re looking for more information: http://www.crh.noaa.gov/ddc/?n=windchill

Thanks for reading, and try to stay warm out there! -Meteorologist Miranda Hilgers

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