The Wind Chill is a number meteorologists use to tell you how cold it actually feels outside in the winter.
Your skin is covered by a thin layer of warm, insulating air. When the wind blows across your exposed skin, it removes that warm layer of air, and replaces it with colder air, making you feel colder. Wind Chill applies only to warm-blooded creatures.
While pipes and thermometers may cool down faster when exposed to wind, they will never drop below the actual air temperature.
The formula for calculating Wind Chill:
Wind Chill (°F) = 35.74 + 0.6215T - 35.75(V^0.16) + 0.4275T(V^0.16)
where V is the wind speed in miles per hour and T is the actual air temperature in °F.
Since that is a little complex, here is a chart we use: