Recent Trends in Heat Index and Wind Chill over the Continental United States

An article in Nature from Egan and Mullin (2016) concluded that 80% of the U.S. population has experienced improving weather conditions from 1974 - 2013, a period in which the globe experienced significant warming. However, the authors' use of primarily average values overlooks extreme events that might influence or alter their conclusion. This study considers the same question of recent changes in quality of weather, but in the context of changing weather extremes, which are stressful on people and society. To assess the past 40 years of weather in this context, extremes in discomfort caused by temperature, humidity, and wind were evaluated on a daily timescale. Daily values of Heat Index and Wind Chill were calculated and analyzed over the continental United States from 1976 - 2015. Results of trend line analysis show the frequency of uncomfortable and dangerous Heat Indices is increasing in most regions of the U.S. and the country as a whole, and that dangerous wind chill events are occurring much less often nationwide. Using the Q-Q plot method, variations in the highest Heat Index values from region to region were also observed, while the coldest Wind Chill values warmed across the contiguous U.S. in the most recent half of the 40-year period. Danger and discomfort from extreme heat is an increasing concern for much of the country, while extreme cold is becoming less of a concern.