Wind chill is a calculated value based off of air temperature and wind rate. It is an estimation of the "apparent air temperature" felt by one's skin. Moving air over something cools it faster (think of a fan on a hot summer day or blowing over a hot cup of soup) so it is important to pay close attention to the wind chill on cold winter days. Even if is 0ºF, a strong enough wind can cause a wind chill of -33ºF, which can cause frostbite in just ten minutes. The best advice is to use common sense when going outside anytime in winter: dress appropriately and know your limits. If you feel too cold, go inside. Wind chill ratings are provided by most weather services during winter or can seen on this chart, courteous of the NOAA.