The heat index is a scale used to quantify how our bodies feel in different weather conditions. Combining temperature and humidity with a model of human physiology, the heat index allows us to describe the risks of extreme heat conditions more intuitively than environmental measurements alone. For example, our bodies are subjected to much greater physiological stress on a day that’s 85 degrees Fahrenheit and 70% humidity than a day that’s 85 degrees with only 20% humidity, because the evaporative cooling effect of sweating is hindered when the air flowing against skin already contains a...