How about that heat index?

The modern-day “heat index” is a much more inclusive measure of summer temperatures, accounting for humidity and its pronounced effect on the human body when combined with hot weather. Developed in the late 1970s, the heat index or “feels-like temperature” quantifies the relationship between humidity, temperature and the efficiency of our body’s natural cooling system, perspiration.

When the external environment is warm enough to start raising our internal body temperature, we start to sweat. As perspiration dampens the skin, it begins to evaporate into the surrounding air. Because evaporation is a cooling process, this does a decent job of regulating body temperature and maintaining homeostasis.

When the air outside is loaded with moisture and relative humidity is high, perspiration becomes far less efficient at keeping the body cool. This is because higher humidity causes evaporation to take place less readily, which translates to a lower rate of evaporative cooling. This explains why “dry heat” in arid parts of the globe is often easier to manage than that of subtropical regions even at the exact same temperature.

Similar to how wind chills are referenced during cold air outbreaks, heat indices may sound a bit dramatic when mentioned in the throws of a summer heat wave: “Triple-digit heat indices ...,” “Feels like 113 degrees ...” “Dangerously hot heat index.” These headlines are not just attention-grabbers. In reality, they are scientifically based at the intersection of biology and meteorology. So much so that the National Weather Service regularly uses heat indices as criteria for issuing heat-related alerts such as advisories and excessive heat warnings, which trigger responses at the local level for the sake of public safety in the face of extreme weather conditions.