14 January 2009

The Misunderstood Wind Chill

Posted by Dan Satterfield

Every time we get a big blast of cold in the Tennessee Valley, I get several requests for a wind chill chart. If I have reminded you of such a forgotten need, not to worry!  I am including one in this post.

However, if you think that the wind chill is “How cold it feels”  then let me persuade you otherwise. It really is not. What wind chill IS, is a measure of the rate of heat loss on exposed skin. It makes a number of surprising  assumptions. If you have a wind chill chart more than 10 years old, it’s wrong anyhow.

First a little history!

The term “WIND CHILL” was first coined by an Antarctic explorer named Paul Sipple in 1939. He and some others did experiments on how long it took a plastic container of warm water to freeze. They realized that the time was a function of the wind speed, and the outdoor temperature together.

To put it in plain terms, how fast something will cool off, depends on the starting temperature, the current temperature, and the wind velocity.

Wind chill is a good indicator of how long it will take for hypothermia or frost bite to occur. It IS NOT a measure of how cold it feels. Also, water will not freeze at 33 degrees, no matter how hard the wind blows! The wind chill is also a good measure of how quickly your car engine will cool off on a cold night!

The equations developed by sipple in the 1940’s were looked at a bit more rigorously in the 1990’s by Meteorologists and in 2001, NOAA and the NWS revamped the wind chill. In general, the new scale will not show as cold a wind chill as the old one did. Especially in stronger winds. If you are in another country, the wind chill may be calculated a bit differently. There is no right or wrong way. It depends on the assumptions you make!

Wind chill is actually a bit controversial. Many Meteorologists, like myself, are not real fans of it. The public generally misunderstands it, and gives it more credence than it really should have.

You should also know that the wind chill equations assume the following.

 It is night time.

Skies are clear.

Your 5 feet tall.

 If the sun is shining on your skin, that will reduce considerably the effect of the wind. I have,  somewhere,  the complete paper published in the peer reviewed journals on the new Wind Chill. If you really want it, email me, and I will try and find it!

I frequently get requests from viewers for the actual equations, not the chart. (Huntsville has a lot of rocket scientists with NASA in town!) Many times, the viewer will think that it’s as simple as adding, or multiplying the wind, and the temperature. It’s not. It is, however, just a basic Algebraic formula.

Here it is for Degrees C:

WC = 13.12 + 0.6215 TC + ( 0.3965 TC – 11.37) V 0.16

TC= Temp. in Celsius V= Wind speed in mph

(Courtesy NSDL/NOAA)

If you just want the chart..here ya go!