How to use: Calculate heat index (how hot it feels) when humidity combines with air temperature. Enter Temperature (must be >80°F or >27°C for accurate results) and select unit. Enter Relative Humidity percentage (0-100%). Results show "feels like" temperature accounting for humidity's effect on evaporative cooling. Formula uses NWS heat index equation. Danger levels: Heat Index 80-90°F = fatigue possible, 90-105°F = heat cramps/exhaustion possible, 105-130°F = heat exhaustion likely, >130°F = heat stroke imminent. High humidity prevents sweat evaporation, reducing body cooling. Essential for summer safety, outdoor work planning, and heat advisories. Always seek shade and hydration when heat index exceeds 90°F.