"Feels like" isn't marketing — it's physiology, formalized. Wind strips heat from skin faster than still air; humidity blocks the sweat evaporation you cool with. The National Weather Service models both effects, and this calculator runs the official formulas with their honest validity ranges.
Wind chill: the cold thief
20°F air with a 20 mph wind removes heat from your skin like 4°F still air would. The 2001 NWS formula (built from human face-cooling trials) applies at ≤50°F with ≥3 mph wind. Below −18°F wind chill, NWS warns of frostbite on exposed skin within 30 minutes; below −32°F, within 10 — the risk bands the calculator displays.
Heat index: humidity's tax on cooling
90°F at 70% humidity feels like 106°F, because saturated air won't take your sweat. The Rothfusz regression applies at ≥80°F and ≥40% humidity; past a 103°F index NWS calls heat exhaustion "likely." Athletes and outdoor workers plan by this number, not the thermometer.
Where the formulas stop
Outside their ranges — mild temperatures, calm wind, dry air — the equations aren't defined, and this calculator says "feels like ≈ air temperature" rather than extrapolating nonsense. One more honest boundary: wind chill applies to warm bodies. Pipes, cars and thermometers never drop below actual air temperature, however hard the wind blows.
Companion tools
Plain unit conversion lives in the temperature converter; training in the heat intersects with the heart rate zones (heat raises HR at any pace) and hydration planning around your TDEE.
Formulas and risk thresholds: US National Weather Service.