Weather Break -- The Wind Chill Temperature

This is a transcript of the Weather Break radio show for Tuesday, January 22, 2008. The episode was written by Dr. Jon Schrage.

As a kid growing up on a farm in rural Nebraska, I remember standing out at the end of the lane in the winter, waiting for the schoolbus. As a kid, I'm sure that I didn't know anything really about "wind chill temperatures", but I'm sure that I understood that it seemed a lot colder outside on days when the wind was blowing. Today, the wind chill temperature is one of several different so-called "apparent temperatures" that meteorologists can use to describe the weather conditions. Another example of an apparent temperature that you might have heard of is the heat index.

You'll often hear TV meteorologists say that the wind chill temperature is the "feels like" temperature, and that's not a bad description of the number. For example, suppose that the temperature outside right now is 30°. If there is no wind at all, the wind chill temperature will be the same as the air temperature; it FEELS like it is 30° outside because it IS 30° outside. However, if it were windy out, when you go outside it feels much colder than that--let's say 10°. In that case, the meteorologist would say that the air temperature is 30° but the wind chill temperature is 10°--it feels like it is 10° outside.

You might think that it would difficult to very accurately describe how cold it "feels like" outside, but actually that's a pretty well-understood problem. How cold it feels has to do with how quickly your body is losing heat. In the winter, your body is always warmer than the outside air, so heat is always moving from your body to the air around your body; effectively, your warm body is trying to heat up the air around you. There's a technical term for this process; it's called the "sensible heat flux", meaning that there is a flux or transfer of heat from your body to the air around you. That rate of heat flux depends on two things: how great the difference in between is between your body and the air around you, and how fast the wind is at the surface. These two factors come together in terms of the wind chill temperature. Thinking back on our example, when the air temperature is 30° and the wind chill temperature is 10°, what the meteorologist is saying is that your body is losing heat at the same as it would if the air temperature were 10°, rather than the real temperature of 30°. Your body is losing heat faster, of course, because the wind is blowing, just like blowing on a hot spoonful of soup helps cool the soup down more quickly than if you just sip the soup right out of the bowl.

Notice that this means that the wind chill temperature cannot be greater than the air temperature. If there is no wind at all, the wind chill temperature is equal to the air temperature, and as the wind increases the difference between the air temperature and the wind chill temperature increases, too.

The physics and the thermodynamics of how fast heat is transferred between two objects of different temperature under different wind conditions are pretty complicated, but it's a well-known and well-studied problem in the atmospheric sciences. Based on this information, organizations like that National Weather Service have published tables that show the wind chill temperature for various combinations of air temperature and wind speed; we have links to these tables on our website, which is weatherbreak.creighton.edu. These are the numbers that you see on the news or hear about on the radio.

When talking about the wind chill temperature, however, it's important to understand that this number doesn't describe the actually the real temperature of any object. For example, suppose that the air temperature is 35° and the wind chill temperature is 30°--in other words, the air temperature is ABOVE freezing but the wind chill temperature is BELOW freezing. Now, suppose that you set a glass of water outside under these conditions; would the water freeze? The answer is no. The water would cool to the AIR temperature, not the wind chill, so the water wouldn't get any colder than the air temperature, which we said was 35°. Where the wind chill DOES come into play, however, is in HOW FAST the water got to that temperature. When the wind chill is low, the objects are losing heat fast due to the higher wind speeds. So even though the water will only cool to a temperature of 35°, it will cool at a rate AS IF it were cooling all the way down to the wind chill temperature of 30°.

On tomorrow's episode of Weather Break, we'll take a look back at how the wind chill temperature was initially developed on an expedition to Antarctica.