What does the heat index actually measure?

Your primary cooling system is evaporation: sweat absorbs heat from your skin as it turns to vapor. That process slows as the surrounding air fills with moisture, so on a humid day sweat pools rather than evaporating and you keep the heat you are trying to shed. The heat index combines air temperature and relative humidity to express how hot the conditions feel to a human body, and the combinations get dangerous quickly. Ninety degrees with 70 percent humidity carries a heat index around 105, which is in the range where heat exhaustion becomes likely with prolonged exposure and heat stroke becomes possible.

Two caveats are worth knowing. The standard heat index is calculated for shade with light wind, so direct sun can add as much as 15 degrees to the effective value, meaning the number underrepresents what an outdoor worker or athlete is experiencing. And it assumes a typical adult, while older people, young children, people on certain medications, and anyone unacclimated to heat face risk at lower thresholds. Researchers increasingly favor wet-bulb globe temperature, which incorporates sun and wind directly and is what the military and many sports bodies use to set activity limits. Heat is the deadliest weather hazard in the United States in an average year, which is the context that makes the number worth reading rather than glancing at.