Your body maintains a thin envelope of warmed air against your skin, and that envelope is most of your insulation. Wind strips it away and replaces it with fresh cold air continuously, so heat leaves your body far faster than still air would allow.
Wind chill expresses that accelerated loss as an equivalent still-air temperature. A wind chill of 17° means exposed skin loses heat at roughly the rate it would in calm 17° air, while a thermometer still honestly reads 30°.
How fast frostbite happens
These apply to exposed skin, face, ears, fingers. It's the fastest-moving weather risk there is, and the reason wind chill warnings exist as their own category.
What wind chill does not affect
Objects. Your car's engine, your pipes, and your plants cool only to the actual air temperature, wind makes them reach it faster but no colder. Water will not freeze at an air temperature of 35° no matter what the wind chill says. This is one of the most persistent weather misunderstandings.
Frostbite is localized tissue freezing: numbness, waxy or grayish skin, firm to the touch. Rewarm gradually with body heat or warm, not hot, water. Never rub.
Hypothermia is whole-body core cooling: shivering that eventually stops, confusion, slurred speech, drowsiness. It's an emergency. Wet clothing and wind make it possible at temperatures well above freezing, even in the 50s.
Frost, freeze, and the growing season
- Frost advisory: temperatures near 32-36° with conditions favoring frost formation. Sensitive plants at risk.
- Freeze warning: widespread temperatures at or below 32° during the growing season. Cover plants, protect pipes.
- Hard freeze: generally 28° or below for several hours. Ends the growing season and kills unprotected vegetation.
Frost can form when the official air temperature reads a few degrees above freezing, because that reading comes from five feet up while the ground surface radiates heat and runs colder. Clear, calm nights are when it happens, the same conditions that produce radiation fog.