What's the Deal with Windchill?
It’s really cold and windy here today, and I was wondering about how windchill works. Is it something that only affects people, or does it affect the physics of things too? For instance, would water freeze if the temperature was 35 degrees but the windchill was down to 25? What’s the algorithm used for calculating windchill, i.e. how does windspeed relate to windchill?
As I understand it, people feel colder when it’s windy because water evaporates off our skin faster, leading to a very high rate of evaporative cooling. (Like when you blow on your soup to cool it down). This makes it seem like water would freeze if the windchill was below freezing but the actual temperature was not, but that just doesn’t seem right to me.
Can anyone offer some insight?
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Wind chill is the effect of cooling the human body as a result of air motion. In the American Meteorological Society’s Glossary of meteorology, they mention that two Antarctic explorers in the 1940’s found that changing temperatures and wind speeds affected the freezing point of 250 grams of water. Because wind chill refers to the rate of heat transfer from a warm body (human, water, dog, anything), wind chill does affect other things other than humans, as you suspected. There is a formula: T(wind chill) = 0.0817(3.71*V^0.5 + 5.81 – 0.25*V)(T-91.4) +91.4 where V is the speed of the wind in mph and T is the air temperature in F. This was developed by these explorers. What we tend to use wind chill for now is to let the public know that during certain conditions, exposure time is reduced before the effects of things like frost nip, bite and other cold related conditions will occur.
Thanks! That helps a lot.
Why don’t they use it in the summer months. Say it is going to be 95, but at the beach there will be a constant breeze of 15 to 20 mph. Would a weather person be correct if they say something like ” its 95 but with the wind it will feel like 77”.?(or whatever the formula works out to. )i have never heard them say it that way.
Well, I guess there technically could be a cooling effect. I am from Chicago, and as we know (and people who live on the coasts know), even if it is a hot, yucky day, if there is a breeze coming off the lake, it feels much better. However, because “wind chill” is defined as the the cooling of the body (heat transfer) caused by wind, this is really only effective if temperatures are significantly lower than the body temperature. I’m looking here, back in the AMS glossary, and 45 F is the technical cut-off point. There is something the we do use for hot days. You may have heard of “heat index”. This was also developed in the 1940’s and is a combination of hot temperatures and humidity to “quantify” how hot it feels. In the summer, it really isn’t important if it is 95 degrees, but a breeze makes it feel like 85. What is important is if it is 95 degrees with humidity through the roof. In the summer you don’t have to worry about hypothermia so much as heat exhaustion. ;-) (Check out Wikipedia’s entry on heat index. There looked to be some math there…)
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