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An Amateur Look at Wintertime Automobile Thermodynamics

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An Amateur Look at Wintertime Automobile Thermodynamics

Bill Crittenden
January 1, 2014

First off, I'm not a scientist, but I think I can understand some of their work.  Except for this one little thing: the common statement that wind chill only affects people.

It's largely an issue of semantics.  One that I consider as it's a flat zero degrees Fahrenheit where I am now with a stiff wind putting that wind chill well below zero.

Wind chill relates to the rate at which heat is pulled away from a heat source.  With wind constantly pushing chilled air past your skin (the opposite of what a convection oven does to a turkey) the air will pull the heat away from your body at a faster rate, making 10 degrees above and a stiff wind feel like ten below because it pulls the heat away from your skin at a rate similar to ten below and calm air.

Were it not for the movement of air, the heat source would warm the air around it, creating a little bubble of warmth where the reduced temperature difference reduces the rate of heat loss.  This is, at different temperatures, why your car needs a fan on the radiator!

The reason people say wind chill doesn't affect anything else is because wind can't really lower the temperature of an inanimate object.  Ten degrees and calm or ten degrees with gale force winds, either way your car is going to be ten degrees inside, your battery will be ten degrees, the oil ten degrees.

It really has to do with the balance of how quickly heat is pulled away from the body and how quickly it's replenished by that body, serving as shorthand for, "its windy today, so you're going to lose fingers and toes a LOT quicker if you're not careful!"

But once you start that car, it becomes a heat source.  Heat is mostly given off by the radiator and replenished by the engine.  But add a wind, particularly the very rapid relative wind speed created by driving at highway speed, and sometimes the engine can't replenish the heat as fast as the air pulls it away from the radiator.  That's why on a particularly frigid day my temperature gauge can go down as I begin to drive out of my neighborhood, and why since the beginning of motoring people have found ways to block their radiators in winter.

In my little car, I can even feel a distinct chill on a night like tonight coming off the windows as I drive at speed.

This applies for a time after the engine shuts off, as heat buildup will be pulled away much more quickly if a wind is present to pull the warming air out from under the car and keep a steady stream of cold air in the engine compartment.

It also applies to the rate of heat loss in the passenger compartment, too, were you to become stuck in a shut-off car in the freezing cold.  In this instance, you may be in calm air but you become the heat source for the interior, the temperature loss through the windows made more rapid in higher winds, that heat loss in turn dropping the temperature of the interior of the car if it's greater than the heat the passengers give off.

Of course, once the car is cooled to actual temperature, no amount of wind will pull it any lower, just as if you were to actually freeze to death, your body won't drop to the actual wind chill temperature.

Wind chill is about the loss of heat and the rate of replenishment, and it does affect cars, just not in the same way humans are affected (but after reading through all this, the common shorthand that it doesn't is understandable, isn't it?).  Also consider that no matter how calm the air is on a cold winter morning, the air may still be rushing past your car at a steady 55 miles an hour as you drive to work.

Just something for the advanced or scientifically curious driver to think about on those long, boring rides to cubicle Hell.

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