Temperature alone does not tell how hot the air feels. Humidity slows the evaporation of sweat, which is the body’s main cooling mechanism. The heat index combines air temperature and relative humidity into a single value that estimates the perceived temperature.

A heat index of 40 °C feels significantly more oppressive than the same temperature in dry air. Values above certain thresholds trigger heat advisories and warnings because the risk of heat exhaustion and heat stroke rises sharply. The index is most relevant during warm months when humidity is also elevated.

Charts and formulas convert the two measurements into the index value. Weather services publish the heat index alongside the actual temperature in forecasts. Outdoor workers, athletes, and vulnerable populations are advised to take extra precautions when the index is high.

• Combines temperature and humidity
• Estimates how hot it feels
• Used for heat safety warnings
• Most relevant in warm, moist conditions