Spatial pattern of a heat index for the coterminous United States and its temporal trends

This research develops a basic climatology of a heat index (National Weather Service) for the coterminous United States for the period 1961-1990. This climatology is based on hourly data contained in the Solar and Meteorological Surface Observation Network (SAMSON). The heat index was calculated for each hour for 117 stations to determine the hours equal to or above 80o F (26.7o C), 90o F (32.2o C) and 105o F (40.6o C). We then determine trend lines (slopes ) of the change in the heat index for the period of record. The average annual frequency of hours $ 80o F and 90o F were mapped for the coterminous United States and follow expected patterns based on latitude, elevation and continentality, but the patterns for heat index frequencies of $105o F are distinctly different and are centered near Phoenix, AZ and a south central region centered near Tulsa, OK. The longest runs of hours of extreme (105o F) heat index were found for Phoenix AZ, Memphis TN, and New Orleans LA. For most of the coterminous United States the trend for the heat index is positive with the most pronounced increases occurring near Tucson AZ, southern Florida and the middle Atlantic region. Much of the increase occurred during the 1980s. A region that has experienced a decrease in heat index is centered near Amarillo, TX. For much of the country an increase in temperature and/or humidity has resulted in a substantial increase in the intensity of the heat index as well as the number of extreme hours per year.