# Windchill factor.

Outside in the often overlooked and Misunderstood state of Michigan, it is 18 degrees. The wind is nipping along bringing the temperature down to -11 degrees because of the windchill factor.

How or what mathematical configuration (that I’m sure is out of my grasp), determines wind chill factor?

For the Willard Scott wanna be’s out there, please keep this answer simple or I will automatically go into a coma. (IE: if the temp is X, and the wind is Y you just times/divide/add this number and --wa la…it’s the windchill factor.)

Wind Chill can be calculated as:

TWC=91.4-((91.4-T)(.478+(.301SQRT V))-.02*V))

Where ‘T’ is the actual temperature in degrees (F) and ‘V’ is the wind speed (MPH). SQRT means square root.

At wind speeds of 4 MPH or less the wind chill temperature and the actual temperature are considered to be the same.

Equation courtesy of Dr. Dewpoint:
http://www.intellicast.com/DrDewpoint/

“Drink your coffee! Remember, there are people sleeping in China.”

Dennis Matheson — dennis@mountaindiver.com
Hike, Dive, Ski, Climb — www.mountaindiver.com

And from www.weather.com, a non-mathematical definition:

The calculation of temperature that takes into consideration the effects of wind and temperature on the human body. Describes the average loss of body heat and how the temperature feels. This is not the actual air temperature.

Get someone with a cold wash cloth to stand beside you and go look at what Dr. Dewpoint says http://www.intellicast.com/DrDewpoint/Q_A/2029/

But there is also a link to a table there too.

Three way simul-post!

That would be known as a menage-a-post

And, it really wasn’t as thrilling as I had heard, either.

My wife was convinced for years that “wind chill” was a real temperature; specifically, she believed that if the radiator in your car was protected to -10 degrees F by antifreeze, and the wind chill got to 20 below, then your coolant system would freeze solid and damage the engine.

I explained to her many times that this was not true, and wind chill had more to do with the rate of heat loss from a relatively warm object (a human body) than with the actual temperature inside a sealed and relatively cold object (the radiator of a car parked overnight in Iowa), but it was pretty obvious to me that while she couldn’t find anything wrong with what I was saying she still didn’t believe it.

For all I know she still thinks this, but since it almost never gets below 40 F here she doesn’t worry about it, so it’s not an issue anymore.

Informal discussions have revealed to me that the average person is more likely to agree with her than with me.

Is there a correlative “wind chill” for the summer? I guess that would be “wind heat”, but a hot wind blowing on a hot day makes it as uncomfortable as a cold wind on a cold day.

“That was a hell of a thing.”

Well, in the summer around here, the weather guys all mention the “heat index” but I believe that is related to humidity, not wind, and I have no idea how it is calculated.

One quick web search later:
Heat Index - Wind Chill has heat index charts and lists the formula to calculate the Heat Index:
<img src = http://www.xnet.com/~jjy/heat_index_formula.gif>

Let’s try this way, then.
Heat Index Formula

Torq,

Perhaps you wife is unconvinced because you didn’t properly explain the windchill effect. It has nothing to do with the relative temperatures of the air and the body being cooled. It has to do with the evaporative effect of the air on said body. Unlike human skin, your cars radiator doesn’t (shouldn’t) sweat or otherwise give off moitsure so there’s no evaporative cooling and no windchill effect.

“Wind chill” was invented by weather guys just so that they could bring in impress you with lower, colder temperatures.

I know there’s a difference between “how cold you feel” and “what the temperature is,” but I still stick by my opinion on why it ever came up to begin with.

Same with “heat index.”

No, there are good reasons for having and reporting Wind Chill.

I do a lot of hiking and backpacking and so I pay a lot of attention to the weather. One of the more dangerous things hikers face (especially this time of year) is hypothermia. If it is wet and windy you can get dangerously cold even when the actual air temperature is fairly warm. (I’ve heard of people getting into trouble when the actual air temperature was as high as 70 degrees.) If it is cold and windy the situation is even worse. I’ve seen hikers say “well, it’s sunny and 40 degrees, so I’ll be OK”. Well, if the wind is blowing at 20 mph they won’t be OK unless they have some sort of protection from the wind. Knowing the wind chill and the differnce between it and the actual temperature helps you to plan how to dress/pack for your hike. (20 degrees with no wind and 40 degrees with a 20 degree wind chill are equally dangerous but you dress differently for them).

Heat index helps you do the same type of planning when the weather is warmer.

Anyone who spends a lot of time outside probably has similar reasons for knowing the wind chill/heat index.

“Drink your coffee! Remember, there are people sleeping in China.”

Dennis Matheson — dennis@mountaindiver.com
Hike, Dive, Ski, Climb — www.mountaindiver.com

Ah, yes, just what I need to discuss.

But let’s make it a little more work. With a real live example. Suppose its 60F outside,
the ocean temp is 55F wind is 10mph. Just how cold is it for a surfer?

Water draws heat from the body ten times faster than air. That’s a simple wet chill explanation Shir.

Phil, I’m not convinced that it’s purely evaporative cooling… it could be a factor but it’s certainly not the ONLY factor. If I’m just standing outside when it’s below freezing, I’m probably not sweating much, but it still makes a difference whether there’s a wind or not.

I think it’s more connected to thermal transfer rate:

If I’m standing outside at, say, the freezing point, but the air isn’t moving, the air near my body will be a bit warmer than the air two feet away. This gives me a sort of buffer zone between me and the colder air, because I’ve already heated the layer of air that’s actually touching my skin.

If there’s a wind, though, this layer of air is constantly being blown away, so I’m continually having to start over from ambient temperature.

Once the object, whatever it is, is cooled to ambient temperature, it doesn’t matter if there’s a wind or not. There’s no layer of warm air to blow away. If the object is wet and the air is very dry, you might still get some evaporative cooling (another diffusion controlled effect, but molecular rather than thermal this time).