# The Science Guys

### February 2002

Can water freeze if the wind chill index is 10 degrees and the temperature is 35 degrees?

Anyone experiencing winter knows that his/her comfort depends upon several factors in addition to temperature: sunshine, humidity, and wind. The total level of discomfort due to cold (and its potential dangers) is measured by the wind chill index. While wind chill takes into account the temperature and the wind speed, wind chill is actually a measure of the rate (how fast) that heat is lost.

Heat naturally flows from hot to cold. One way a person loses heat is through conduction to the surrounding air. The air molecules touching our body absorb heat from our warm skin. The rate of that heat flow depends upon the temperature difference between your skin and the air.

In still air your body warms the layer of air next to your skin and the temperature difference between your body and that air layer is lessened. Therefore the rate of heat loss drops. However if the air next to your skin is moving, cool air is constantly replacing that layer of air your body had just heated. Thus, the temperature difference between your body and the adjoining air is greater than if the air was not moving resulting in greater heat loss.

The wind chill index is related to the rate that an average, naked human body in the shade would lose heat on a still day. As the temperature decreases or the wind speed increases, the air feels colder because the temperature difference between your skin and the air is maximized. The larger the temperature difference between our skin and the air, the greater our heat loss and our sense of coldness. For example, at 40°F with a 10 mph wind, a naked body would loose heat at the same rate as if it were 28°F and no wind were present. Clothing prevents the air next to your body from moving. Air that cannot move acts as insulation and reduces the heat loss, thus keeping our body warm.

Since heat only naturally flows from a warm to a cool object, it is not possible for an object to drop below the temperature of the surrounding air, regardless of the wind. The wind will reduce the time it takes an object to cool but it cannot cool the object below the air temperature. Plain water in an automobile radiator will not freeze so long as the regular air temperature stays above 32°F, even if the wind chill drops to 10°F. Consider two identical glasses of 80°F water. Put one in a refrigerator at 35°F and place the other glass outside where the temperature is 35°F with a 25 mph wind. (Outside the wind chill factor would be 8°F.) The glass outside will cool faster, but it will not cool to less than 35°F.

To use the wind chill table, your actual temperature is the left-hand column (blue), and the wind speed is given in the top row (red). Your wind-chill temperature is the intersection of your actual temperature and the wind speed. For example, if your actual temperature is 20 degrees Fahrenheit and the wind speed is 25 mph, then the wind-chill temperature is -14 degrees Fahrenheit (see chart below). In such a case, stay inside and keep warm!