Wind Chill & Heat Index Calculator

The Wind Chill Index (also called the wind chill factor) estimates how cold the air feels on exposed skin due to the combined effect of low temperature and wind. Wind accelerates heat loss from the body through convection, making it feel significantly colder than the actual thermometer reading. The index was developed by scientists in Canada and the United States and is now used by weather services worldwide to warn about frostbite risks. It applies when temperatures are at or below 50°F (10°C) and wind speeds exceed 3 mph (4.8 km/h).

The Heat Index (HI) measures how hot it feels when relative humidity is combined with high air temperature. When humidity is high, sweat evaporates more slowly, reducing the body's natural cooling mechanism. This makes it feel much hotter than the actual temperature. The heat index is based on the Rothfusz regression equation developed by the U.S. National Weather Service. It is most accurate for temperatures above 80°F (27°C) and relative humidity above 40%.

Both the Wind Chill and Heat Index are scientifically validated models based on human physiology and heat transfer physics. The Wind Chill index assumes a healthy adult walking at 3 mph with appropriate winter clothing. The Heat Index assumes a person in the shade with light clothing. Individual perception may vary based on factors like sun exposure, clothing, activity level, age, and health conditions. These indices provide a standardized estimate widely used by meteorologists and health agencies.

The modern Wind Chill formula (adopted in 2001) in °F and mph is:
WC = 35.74 + 0.6215×T − 35.75×V0.16 + 0.4275×T×V0.16
Where T = air temperature in °F and V = wind speed in mph.

In °C and km/h:
WC = 13.12 + 0.6215×T − 11.37×V0.16 + 0.3965×T×V0.16
Where T = air temperature in °C and V = wind speed in km/h. This formula replaced the older Siple-Passel index for improved accuracy.

The Heat Index uses the Rothfusz multiple regression equation:
HI = −42.379 + 2.04901523×T + 10.14333127×R − 0.22475541×T×R − 0.00683783×T² − 0.05481717×R² + 0.00122874×T²×R + 0.00085282×T×R² − 0.00000199×T²×R²
Where T = air temperature in °F and R = relative humidity percentage. Adjustments are applied for extreme humidity conditions (below 13% or above 85%) when temperatures are between 80°F and 112°F.

Frostbite risk increases dramatically as wind chill drops:
• Above −5°F (−20°C): Low risk — normal winter precautions sufficient
• −5°F to −20°F (−20°C to −29°C): Moderate risk — frostbite possible in 30 minutes
• −20°F to −40°F (−29°C to −40°C): High risk — frostbite in 10–30 minutes
• −40°F to −60°F (−40°C to −51°C): Very high risk — frostbite in 5–10 minutes
• Below −60°F (−51°C): Extreme risk — frostbite in under 5 minutes. Exposed skin should be completely covered.

The National Weather Service categorizes Heat Index risks as:
• 80–90°F (27–32°C): Caution — fatigue possible with prolonged exposure
• 90–103°F (32–39°C): Extreme Caution — heat cramps and heat exhaustion possible
• 103–124°F (39–51°C): Danger — heat cramps or exhaustion likely; heatstroke possible
• Above 125°F (51°C): Extreme Danger — heatstroke highly likely. Outdoor activity should be avoided.

Your body constantly radiates heat, creating a thin boundary layer of warm air around your skin. When wind blows, it strips away this insulating layer, forcing your body to work harder to maintain its core temperature. This process — called forced convection — accelerates heat loss. The stronger the wind, the faster heat is carried away, making it feel significantly colder. This is why a 30°F day with 20 mph winds can feel like 17°F.

The human body cools itself primarily through evaporative cooling — sweat evaporating from the skin removes excess heat. When relative humidity is high, the air is already saturated with moisture, drastically slowing evaporation. Your sweat stays on your skin without providing cooling relief, and your body temperature rises. At 90°F with 70% humidity, the heat index can reach 105°F because your body's natural air conditioning system is effectively disabled.

Actual temperature is what a thermometer measures in the shade — the kinetic energy of air molecules. "Feels Like" temperature (also called apparent temperature) incorporates the effects of wind, humidity, and sometimes solar radiation to estimate human perception. On a cold, windy day, the feels-like temperature may be 10–30°F lower than the actual reading. On a hot, humid day, it can be 10–20°F higher. This is why weather forecasts emphasize the "feels like" value — it better reflects how you should dress and prepare.