Wednesday, January 10, 2018

The War on Windchill

Unforeseen Contingencies makes a foray into applied science! I don’t know about other countries, but in the United States and Canada, weather reporting in winter includes “windchill,” always discussed in dramatic and dire terms, and usually explained with “feels like.” It’s become so prevalent and so out of hand that frequently people tell me the temperature, but in fact are reporting windchill, which is not the temperature at all. And asked what “windchill” means, they mumble some vague nonsense about the wind making it colder. This is a sad state of affairs, and “we” at Unforeseen Contingencies will do our part to correct it.

What is windchill?
When I was a child, growing up in Great Falls, Montana, windchill was never mentioned in weather reports, even though Great Falls is notoriously windy.  (On those rare moments when the wind stops, people look startled and say "what's that?!")  When windchill first started being included occasionally, probably around the time I entered college, reports were careful to explain what it really meant. If an unheated object is warmer than its surrounding environment, it loses heat, by convection, until its temperature equals that of the environment. Because the object is transferring heat to the air around it, it’s surrounded by a layer of slightly warmer air, which slows the continued convection. If there’s wind, this layer of air is thinner, that is, it’s blown away, and the object loses heat faster. Windchill is meant to be a measure and predictor of how windspeed accelerates the heat loss.

Windchill was initially developed by scientists, prior to World War II, for military reasons, I think, and was calculated by examining how long it took a bulb of water starting at some temperature to freeze, given various air temperatures and windspeeds. Note that this has everything to do with physical rate of heat loss for an inanimate object. It has nothing to do with perception, “feels like,” or “wind making things colder.” Perhaps it’s useful information for someone planning on spending extended time out of doors, but it gives no important knowledge one wouldn’t have with just temperature and a wind reading.

Why reporting of windchill is bunk.
Windchill is reported as “feels like,” “perceived,” “makes it colder,” and similar nonsense. Windchill is a measure rate of heat loss, for an exposed, motionless, bulb of water, in the original calculations. Don a windproof garment – on yourself, or on the bulb of water, and the windchill changes. Walk against the wind, it changes again. Start running, and it changes again. Turn around, and run with the wind, and it changes yet again; if you time your running speed correctly, there’ll be no windchill at all. Run along on an exposed ridgeline and you’ll get the full force of the wind, and the windchill will depend on whether it’s a headwind, crosswind, or tailwind. But stay behind rows of trees or buildings, or in a protected draw, and you’ll be out of the wind, and the windchill – except for the accelerated heat loss you create by not staying in one place. I have a great difficulty accepting a measure of weather that changes depending on what clothes I wear, what direction I go, how fast, and in what terrain. It’s not a measure of weather at all.

But even stupider is “feels like,” etc. Let’s say, for example, the temperature is 30F with a 50mph. The National Weather Service (U.S.) online calculator rates this a windchill of 18F. I guarantee that (30F, 50mph) feels nothing like (18F, 0mph); I’ve experienced both on more than one occasion. They are very different, they don’t feel even vaguely similar, and I would not dress the same for them. Even goofier is the idea that “the wind makes it colder.” At (40F, 50mph), the windchill equivalent is (25F, 0mph). (Yes, I’ve experienced both.) You can wait all day for your water bottle to freeze in the former, but it will never go below 40. This seems so obvious would I feel silly pointing it out – except that I hear people make this error on occasion. Would these people think, were it sufficiently windy, they could store their ice cream outside? (“Of course not, that would be impractical. Sufficiently strong winds would blow it away.”)

What’s the alternative?
A windchill calculation does not give us information that we don’t have from temperature and windspeed. In fact, it loses information. If one only knows the windchill is 18F, does that call for light insulation and a heavy windproof garment, or fairly good insulation and no windproof? For running, skiing, snowshoeing, hiking, and other outdoor activities, these are entirely different propositions, and a big deal. It’s much more useful to simply know the actual temperature and whether it’s windy or not. Since windchill will vary with your own direction and velocity, and wind typically blows in gusts and shifts direction, any number is hokum anyway.

And so…
“We” at Unforeseen Contingencies advise that anyone venturing into windy winter weather (i) carry effective windproof outer layer, to be donned as needed, (ii) wear appropriate insulating layer(s), (iii) adjust direction of travel as needed, and especially (iv) stop citing those ridiculous windchill numbers. Join us in our War on Windchill!

Photo: Chief blogger Charles N. Steele cross country skiing at minus 17F (minus 27C) near Great Falls, MT.  Frozen Missouri River in background.

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