What is wind chill, and is it a real temperature?

Your body constantly warms a thin boundary layer of air against your skin. Wind blows that layer away and replaces it with cold air, forcing you to keep reheating, so heat leaves your body faster. Wind chill quantifies that as an equivalent temperature: the calm-air temperature that would drain heat at the same rate. A thermometer does not read the wind chill, and your car's engine block does not cool below the actual air temperature no matter how hard the wind blows, because wind chill only describes objects warmer than the air that are actively producing heat.

The practical value is frostbite timing, which is why the National Weather Service rebuilt the formula in 2001 using human trials and a model of a face rather than the older estimate based on how fast water froze in a cylinder. At minus 15 wind chill, exposed skin can freeze in about half an hour; near minus 40, it can happen in ten minutes or less. That makes the number a safety instruction rather than trivia. The equivalent measure in summer is the heat index, which combines temperature with humidity to describe how badly your sweat is failing to evaporate, and it is the reason a humid 95 degrees is more dangerous than a dry 105.