NOAA Clear/Cloudy Climate Maps

National Climatic Data Center's (NCDC) Climate Maps Of The United States
"Lower 48" from 1960 thru 1990.    (mean number of clear/cloudy days)

---------------------------------- Clear Sky PDF maps ------------------------------
Annual
January February March April May June July August September October November December

---------------------------------- Cloudy Sky PDF maps ---------------------------
Annual
January February March April May June July August September October November December

For more info, visit:    NOAA website