Heat Index Calculator

Heat index, or 'apparent temperature,' is what the temperature feels like to the human body when humidity is factored in. It's calculated using the NOAA/NWS Rothfusz regression equation, which combines air temperature and relative humidity. At 90°F with 65% humidity, the heat index is about 103°F - significantly hotter than the actual air temperature.

Your body cools itself through sweat evaporation. High humidity slows this evaporation, making it harder for your body to cool down. At 95°F with 90% humidity, the heat index reaches 127°F because sweat barely evaporates. Conversely, dry heat (low humidity) feels cooler because sweat evaporates efficiently.

NOAA defines five heat danger levels by heat index: Normal (<80°F) - minimal risk; Caution (80-90°F) - fatigue possible; Extreme Caution (90-103°F) - heat cramps/exhaustion possible; Danger (103-124°F) - heat cramps/exhaustion likely, heatstroke possible; Extreme Danger (>124°F) - heatstroke highly likely.

Dew point is the temperature at which air becomes saturated and water vapor condenses into dew. A dew point above 65°F feels muggy, and above 70°F is oppressive. Unlike relative humidity, dew point is an absolute measure of moisture - a 60°F dew point feels equally humid whether it's 70°F or 100°F outside.

Wet bulb temperature is the lowest temperature achievable through evaporative cooling alone (like a wet cloth in front of a fan). It's critical for HVAC design, outdoor worker safety, and climate research. A wet bulb temperature of 95°F (35°C) is the human survivability limit - the body cannot cool itself regardless of shade or hydration.

Take precautions when heat index exceeds 90°F. Symptoms to watch: Caution zone - fatigue, headache; Extreme Caution - muscle cramps, heavy sweating; Danger - nausea, dizziness, rapid pulse; Extreme Danger - confusion, hot/dry skin, unconsciousness. Call 911 immediately for heatstroke symptoms.