A still 20°F morning and a blustery 20°F morning are not the same morning. The difference is wind.
What wind chill actually measures
Your body keeps itself warm by holding a thin film of heated air right against your skin — a kind of invisible jacket. In still air it sits there and insulates you. The moment wind picks up, it scrapes that warm film away and replaces it with cold air, again and again, so your body sheds heat far faster than the thermometer alone would suggest.
Wind chill puts a number on that. It folds air temperature and wind speed into a single "feels-like" figure that reflects how quickly your skin loses heat. It isn't the real air temperature — it's a measure of the rate of heat loss, expressed as the temperature that would feel the same in calm air.
Quick read: at 5°F with a 30 mph wind, it feels like roughly −19°F. The air never got that cold — the wind just hauled your body heat away as if it had.
The formula behind it
Today's standard is the 2001 NWS / Environment Canada index, built from wind-tunnel trials measuring how fast volunteers' faces actually cooled. It takes temperature in Fahrenheit and wind speed in mph:
WC = 35.74 + 0.6215·T − 35.75·V0.16 + 0.4275·T·V0.16
The calculator runs this exactly, converting other wind units to mph first. One important boundary: the formula is only valid at or below 50°F and for wind above 3 mph. Warmer or calmer than that, wind doesn't meaningfully chill the body, so the number stops being useful.
Why "bright sun" changes the answer
The formula assumes a clear night — no sun at all. In reality, direct sunshine warms your skin and clothing even when the air stays frigid, and can make conditions feel 10 to 18°F warmer than the calculated value. That's what the sun toggle reflects: flip it on for a bright, exposed day, and the felt temperature rises to match.
The myth worth clearing up: pipes and cars
This one causes real confusion. Wind chill only applies to living things that produce their own heat. Wind makes a pipe, a car radiator, or a bottle of water reach the air temperature faster — but it can never push them below it. If the air is 10°F, that pipe bottoms out at 10°F no matter how hard the wind blows. A pipe only freezes when the actual air temperature is below 32°F. So when you hear a scary feels-like number, your water lines are reacting to the real temperature, not the wind.
Frostbite and hypothermia: not the same thing
Frostbite is local: skin and the tissue beneath it literally freeze, usually starting at the extremities — fingers, toes, nose, ears. It needs the true temperature near the skin to be below freezing, and the wind decides how fast it happens. Hypothermia is whole-body: your core temperature drops dangerously, and it can set in even above freezing if you're wet, underdressed, or exposed long enough. Wind raises the risk of both by speeding heat loss, which is why a wind layer matters as much as a warm one.
From freezing cans to wind tunnels
The idea dates to Antarctic explorer Paul Siple, who coined the term in 1939 and, with Charles Passel, measured how fast water froze in cans under different winds. That early index overstated the bite of the wind because cans aren't skin. The 2001 revision replaced it using human facial-cooling trials, producing the more realistic figures — typically 10 to 15°F warmer at strong winds — that weather services use today.