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Please Pass the Science
by dr. scott berk

Simplistic, misleading, and somewhat condescending. . . but never boring!

1996may05: Some Cold, Hard Facts.

Now that we're finally out of the icy grip of a record-breaking winter, I feel more comfortable addressing a few cold-related questions that have come our way. William FitzHugh at the MIT Genome Center (and I might add that, having received my Ph.D. from MIT, I'm not too sure that humanity is quite ready for an "MIT genome") asks a few questions about the "wind chill factor:

If the temperature is 35 but with wind chill it's 30, will water freeze? Why is wind chill only reported in winter? Doesn't a nice breeze on a 90 degree day make it feel like 87 or so? And what's this "humiture" bullshit I've been hearing about?
Meanwhile, "Doc" of Harlsondonven, Arizona (home of the world's largest ball of cat guts), asks some more visceral questions:
You know how they demonstrate how cold it is outside [like when it was -40 in Minnesota this winter] by showing that water thrown out of a thermos freezes before hitting the ground? Well, if that's the case, how come your eyeballs don't freeze right in their sockets? Furthermore, would it be possible to urinate in very cold weather and spell you name in mid-air frozen pee-pee?
Good questions, all! And, like most questions we receive, the answers are there for the knowing. First, let's talk wind chill. The wind chill index was first proposed by Paul Siple in 1939. He and fellow Antarctic explorer P. F. Passel published a formula for calculating heat loss based on air temperature and wind speed. They arrived at this formula by watching water freeze in plastic containers. Apparently, there isn't much else to do in Antarctica, except maybe run around and around the South Pole, screaming "Look at me! Look at me! I'm running around the world!!!" And speaking of mind-numbing boredom, the adventures of Siple and Passel (and could you ask for better scientist names???) remind me of the summer I spent in a paint company R&D lab, where it was actually in my job description to watch drying paint. But I digress . . .

So, Will, by definition, a wind chill index of 30 will lead to freezing water. And since the scale was developed to predict freezing time for water, there isn't much extrapolation towards the high temperatures. Thus, while a nice breeze on a 90 degree day definitely makes it feel cooler, the Siple and Passel scale simply does not apply. In fact, there are several flaws in the wind chill index formula, especially when trying to predict how a human perceives the temperature. After all, we're not plastic containers of water (except for, perhaps, Pamela Anderson) The Siple-Passel index doesn't take into account important factors such as sunlight, humidity, activity, and clothing. If you really want to be accurate at colder temperatures, check out the "Steadman Scale," developed in 1971 for heat loss in "an average clothed mobile human" (again, Pamela Lee-Anderson may not apply). The Steadman scale hasn't caught on in the forecasting world, probably because "Wind-Chill Factor" rolls off the tongue much better than "Average Clothed Mobile Human Index."

But what about this "humiture" bullshit? Well, while wind-chill has to do with air temperature and wind speed, "humiture," or "heat index," predicts the perceived temperature based on air temperature and humidity. Why is it that people always say things like "It's not the heat, it's the humidity" (or, if you're talking about various Southern hate groups, "It's not the heat, it's the stupidity")? I'll tell you why! EVAPORATION! When water evaporates, it takes some energy with it in the process, cooling the surface from which it evaporated. This is what sweating is for; temperature regulation by evaporation. When humidity is low, this process works quite well. The sweat evaporates from your skin, cooling it. But if the air is already saturated with water vapor (high humidity), water is less apt to evaporate off your skin (actually, the water which evaporated is more likely to re-condense on your skin, heating it up, but the net effect is the same), and you don't cool off as much. Thus, the "heat index" scale tells us what we already know: 90 degrees in the desert feels more comfortable than 90 degrees in a rain forest. I have no idea how well the heat index applies to Pamela Lee. Perhaps someone should talk to David Hasselhoff.

But all of these explanations are, as they say, "academic." How can we apply these problems to every day life? Indeed, why don't our eyeballs freeze when the wind chill dips to such depths as those experienced this winter in Minnesota and the Dakotas? The answer is actually easier than you might think: Your eyeballs are almost entirely surrounded by a high-temperature bath (your body), which keeps that vitreous humor (aka "eye goo") warm and flowing. Doc's urine question deserves closer attention, however. Urine is made up of water and a host of chemical contaminants that your body is trying to get rid of. The concentration of those contaminants can dramatically effect the freezing point of the liquid. This property, un-originally dubbed "freezing point depression," is a "colligative" property of solutions. The more stuff you have dissolved in a liquid, the lower its freezing point is. This works because the individual molecules of liquid have to come together to freeze into a solid. If there's a bunch of other stuff in the solution, it's harder for the pure material to come together, much like two people trying to have a conversation in the middle of a mosh pit. For every mole (not a furry animal, but a nifty science term meaning 602,000,000,000,000,000,000,000 things) of other molecules dissolved in 1 kilogram (2.2 pounds) of water, the freezing point is reduced by 1.86 degrees Celsius (3.35 degrees Fahrenheit). This is the reason why anti-freeze works. You dissolve a bunch of ethylene glycol (curiously, also present in Dr. Pepper) in water so your radiator doesn't turn into a big popsicle in winter.

So, coming full circle to the urine question, the answer is "depending on how pure your pee is!" My advice to you, Doc, is to clear out your system with a whole lot of water before attempting this stunt. Oh, and remember, at extremely low temperatures, frostbite occurs within seconds, so remember to take adequate precautions (a penis scarf?) And be sure to write up your experiment, keeping track of all the appropriate variables. It's this kind of research that will keep America out in front (so to speak), scientifically.

So the next time you hear your forecaster call for an ungodly low wind chill, or a terribly high heat index, think of our friends Siple, Passel, and Steadman, pioneers in their field who added to the common vernacular and to weather small talk everywhere with their new temperature scales. They couldn't have done it without a lot of patience and the keen observational skills required to do SCIENCE!

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