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Question

Does the term wind chill only apply to humans, animals, etc. or does it also apply to material things like automobiles, tractors, etc.?
Asked by: Brad A. Rowe

Answer

In still air, as internal energy is conducted from a warmer body to the surrounding air, the air near the object is warmed. That increased air temperature reduces the temperature difference between the body and air, slowing the rate of heat loss. When the air surrounding a warm body is moving, the warmed air is replaced more quickly by cool air, keeping the temperature difference, and rate of energy transfer, higher.

It is the flow of energy to/from the skin that determines our sense of hot and cold. If the skin is cold, room temperature air feels warm. If the skin is warm, the same room temperature feels cool.

Wind chill temperature is based on a combination of air temperature and wind speed. As air temperature decreases and wind speed increases, the air feels colder because the temperature difference between skin and air is maximized as any warmed air is quickly replaced by cold air. That larger temperature difference increases heat flow, and our sense of coldness. Wind chill is calibrated to the human body's sense of coldness due to that effect.

Heat always flows from higher to lower temperature, so the temperature of human skin will not drop below that of the surrounding air, regardless of wind. Likewise, inanimate objects (like automobiles) will not drop below the temperature of surrounding air. So while a wind will reduce the time it takes for a warm object to cool down to air temperature, it can't result in a final temperature below that of the air itself. If air temperature is -10 degrees and the wind-chill temperature is -25 degrees, for example, automobiles and tractors only need antifreeze protection down to -10 degrees.
Answered by: Paul Walorski, B.A. Physics, Part-time Physics Instructor
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