Heat index algorithm used by the US National Weather Service on their online heat index calculator. While the US National Weather Service offers an online calculator to determine heat index from air temperature and moisture, it can only convert one heat index value at a time. For environmental health research, it is often necessary to calculate heat index for thousands of weather observations. With permission from members of the National Weather Service, I created an R function that implements this algorithm, which I've made available to other researchers through the "weathermetrics" package.