It’s not really that cold out, is it?
Are you asking the thermometer? To a chemist or a physicist, temperature is really just a measure of how fast the molecules that make up air are moving, how much energy they have.
To those of us more interested in what the thermometer says outside, temperature has more to do with the rate at which we exchange heat with the environment. At the same temperature, a metal spoon will feel hotter than a wooden one; The metal spoon, being metal and thus more conductive exchanges heat with us at a faster rate, and so feels hotter.
Cold works the same way. The faster we lose heat, the colder it feels, even if the reading on the thermometer hasn’t gone down any.
Windchill, then, has to do with the way wind changes the rate at which we exchange heat with the air around us, specifically the rate at which we lose heat.
It makes an obvious sort of sense. The more wind, the more particles of air move by us, the more opportunities for particles of air to get a little warmer and us to get a little colder. But it’s actually worse. Wind will strip away that nice little layer of air you’ve already exchanged some heat with. It’s slightly warmer (which means its taking slightly less of your heat) and keeping all that really cold air from touching your skin. Insulating. Wind strips away that insulating layer of air.
Windchill, while ostensibly a measure of how cold it feels, is really a measure of heat loss. At it turns out, your body cares far more about how cold it feels than how cold the thermometer reads. While your skin temperature isn’t going to drop below ambient temperature, your body will perceive things as colder than they are, and respond accordingly. Frostbite? Hypothermia? The symptoms of those are the result of the body responding to how cold it feels.
Thirty degrees and windy can’t actually drop your skin’s temperature below thirty, but it’ll feel colder, and that is enough to increase the risk of cold related injury such as frostbite. While the equations to calculate windchill vary a bit, windchill warnings are serious business.
It’s not really that cold out, is it? Not if you ask the thermometer. If you’re asking me, however…
Trego's Mountain Ear 