The Temperature Humidity Index (THI) has been used for over 30 years to alert livestock producers and haulers about conditions that threaten animal well being. Much of the relevant bioclimatic work to-date has been done using daily means or maxima. This paper presents results from analysis of THI frequency and magnitude derived from hourly data for over 30 stations in the central United States. Separate analyses are performed for THI threshholds of 75, 79, and 84. As expected, there is a pronounced diurnal cycle in THI, but some summer nights still exhibit high enough THI values that animals might not benefit from nighttime cooling. July is typically the month with a maximum frequency of elevated THI values; however, the heat threat can persist into August, especially during summers with above normal temperatures. Temporal analysis suggests considerable interannual variation and spatial analysis indicates that there is a pronounced gradient from higher values in the southeast to low THI values in the west and north.