Heat Index & Wind Chill Calculator
Enter the temperature and either humidity or wind speed to see what it actually feels like outside. Get the official NWS danger level and recommended safety actions. Switch between summer heat index and winter wind chill modes. Nothing uploaded.
Mode
Apparent Temperature (Heat Index)
Apparent Temperature (Wind Chill)
Learn more: Heat index, wind chill, and apparent temperature
How heat index is calculated and why humidity matters more than you think
Heat index is the "feels like" temperature when humidity is factored in. The NWS Rothfusz regression formula uses actual air temperature and relative humidity to estimate how hot it feels to the human body. At 85F (29C) with 60% humidity, the heat index is about 89F (32C). At 85F with 95% humidity, it jumps to 110F (43C) - because sweat can't evaporate efficiently in humid air, and your body can't cool itself. The higher the humidity, the worse the heat stress at the same air temperature. Wet-bulb temperature (the lowest temp achievable through evaporative cooling) is even more critical: above 35C wet-bulb, the human body cannot cool itself even at rest, making conditions life-threatening.
Wind chill and frostbite risk: cold weather apparent temperature
Wind chill is the opposite problem: moving air accelerates heat loss from exposed skin. The North American JAG/TI formula (current NWS standard) combines air temperature and wind speed to estimate apparent temperature. At 0F (-18C) with 20 mph wind, wind chill is -24F (-31C). At 0F with 40 mph wind, it's -45F (-43C). Wind chill affects only exposed skin - properly covered skin stays warm. The NWS frostbite chart maps wind chill to frostbite risk time: above -27C (about -15F), frostbite takes 30+ minutes; below -35C, it's 10 minutes; below -48C (-55F), it's just 5 minutes. Wind chill does not affect core body temperature or hypothermia risk directly - it only predicts frostbite on exposed areas.
FAQ
Why is the heat index different from air temperature?
The heat index accounts for humidity, which prevents sweat evaporation. In dry air (low humidity), sweat evaporates efficiently and cools the body well, so the heat index is close to air temperature. In humid air, sweat cannot evaporate, so it stays on the skin and the body struggles to cool itself. The heat index measures this combined stress using the NWS regression formula.
What is wet-bulb temperature and why is 35 C critical?
Wet-bulb temperature is the lowest temperature achievable by evaporative cooling (sweating). It combines air temperature, humidity, and wind. At 35C (95F) wet-bulb or above, the human body cannot cool itself through sweating, even in perfect conditions and at rest. Prolonged exposure above this threshold causes heat stroke and death. This calculator shows wet-bulb as a warning when heat conditions approach dangerous levels.
Does wind chill cause hypothermia faster than still-air cold?
Wind chill does not speed up hypothermia (core body temperature drop) directly - it only predicts frostbite on exposed skin. However, if cold wind forces you indoors or restricts your movement, you stay in the cold longer, which can hasten hypothermia. The real threat is frostbite on the face, ears, and hands. Keep these areas covered in high wind chill conditions.