Heat Index vs Air Temperature: Humidity and Feels-Like Heat

Air temperature alone misses how humidity blocks sweat evaporation — your primary cooling mechanism. Heat index combines dry-bulb temperature and relative humidity into a single feels-like value describing heat stress on the human body in shade. Direct sun can add 10–15°F on top.

Evaporation and cooling

Sweat evaporating from skin removes heat efficiently in dry air. When relative humidity is high, sweat lingers, evaporation slows, and skin feels clammy while core temperature rises. The Heat Index Calculator maps temperature and humidity pairs to NWS-style risk categories.

When heat index diverges most from air temp

• Hot + humid (Southeast US, tropical coasts): index far above dry bulb
• Hot + dry (desert): air temp feels close to actual — evaporation works
• Moderate temp + extreme humidity: still uncomfortable for exercise
• Nighttime humidity: heat index drops but recovery sleep may stay poor without AC

Heat index caution levels

80–90°F index: fatigue possible with prolonged exertion. 90–105°F: heat cramps and exhaustion risk — hydrate, rest in shade. Above 105°F: heat stroke possible — limit outdoor work, check vulnerable people without cooling.

Heat index vs wet bulb and UV

Wet-bulb temperature measures cooling when water evaporates freely — critical for physiological limits in extreme heat research. UV index measures sunburn radiation, not humidity comfort. Use heat index for sticky summer afternoons; add sun exposure separately when planning outdoor labour.

Frequently asked questions