The wind chill temperature was developed to give people a general idea of how much colder it feels when the wind is blowing. The stronger the wind, the colder the wind chill.
It feels colder when the wind blows because the wind will blow away the layer of warmth that your body produces near the surface of your skin. (Think of it as pulling away your natural blanket).
Note that the actual temperature does not change -- if it's 40 degrees with a wind chill of 25, water will not freeze. Wind chill only applies to humans and animals and does not affect inanimate objects.
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