Studying heat waves with the extended heat index

Description

The existing heat index, which maps temperature and humidity to a "feels like" temperature, is undefined for the most severe heat waves on Earth today. This poses an obstacle to identifying and studying the meteorological processes that most directly impact people through thermal stress. Here, we extend the heat index to all combinations of temperature and humidity. This reveals that NOAA's approximation to the heat index has major errors, underreporting the heat index during past heat waves by as much as 20 degrees Fahrenheit. With the extended heat index, we now have a metric by which to identify the most severe heat waves at the SGP site and the upcoming SE US site.