A climatological database of apparent temperature was developed using hourly surface observations from the past 30 years (1978 through 2007) for cities across the continental United States. Heat Index (HI) and Wind Chill (WC) were calculated hourly using documented National Weather Service equations. Apparent temperature is derived by substituting WC for temperature when the temperature was 50 °F or less, and substituting HI for temperature when the temperature is greater than 80 °F. This matches the definition used for apparent temperature by the NWS's National Digital Forecast Database (NDFD). This apparent temperature database is used to examine the climatology of extreme heat and cold using HI and WC to account for the effects of moisture and wind in maintaining a healthy body temperature.

An analysis of the apparent temperature showing the frequencies of hot and cold days as defined using a range of thresholds will be shown for specific cities. Additionally, the geographic variability of apparent temperature will be shown with frequency maps and with point comparisons between urban and suburban observation points.