The data set was computed using data from the National Climatic Data Center's Surface Airways Hourly (TD-3280) database (NOAA, 2000a). The original data and this element are measured as a percentage. The mean monthly values were computed by taking the 30-year mean of the monthly means. The monthly means were computed from the daily values. The daily values were computed as the mean of relative humidity values for the 24 hourly observations. The mean annual value was computed by taking the 30-year mean of the yearly means. The yearly means were computed by averaging their 12 monthly mean values. Data extracted for Arizona and 25 mile buffer.