This map shows the 1961-1990 average annual air temperature for the Upper Skagit Basin south of the international border. The map was creating using PRISM data from Oregon Climate Center. The map stops at the international border since no similar data are available for British Columbia.
PRISM temperature data are estimates of average total annual air temperature for selected areas based on temperature records from individual weather stations and topographic elevation. (c) For instance, estimated temperatures in the Upper Skagit are based on records from Diablo Dam and several other low altitude climate stations. Since air temperature tends to decrease as you rise in elevation, a digital map of elevation is used in conjunction with climate station data to estimate average annual temperature for the area between stations.