Daily differences between heat index (the "feels like" temperature) and air temperature in 2011 in five US cities. The heat index is a popular metric for heat exposure in environmental health research because it combines measurements of both air temperature and moisture (humidity, for example). However, the algorithm used to calculate heat index varies across studies and is rarely the same as that used by the US National Weather Service. I am currently investigating use of this metric in environmental health research, and I created a package in R, "weathermetrics", that allows researchers to calculate heat index using the National Weather Service algorithm.