I live in a part of the country where VMC are starting to become increasingly uncommon, which has led me to wonder - which US airport historically has the most days in which VMC prevail per year? What sources of information can be used to investigate VMC trends at a given airport?
According to NOAA here are the cities with >80% annual sunshine which I've used as a proxy for VMC. You'll find these cities are all in the desert southwest region of the US where the climate is arid.
SOURCE: http://www.ncdc.noaa.gov/oa/climate/online/ccd/pctpos.txt DATA THROUGH 2004 CITY, STATE ANNUAL % AVG POSSIBLE SUNSHINE NO. OF YEARS OF DATA YUMA, AZ 90% 42 REDDING, CA 88% 10 LAS VEGAS, NV 85% 47 PHOENIX, AZ 85% 101 TUCSON, AZ 85% 53 EL PASO, TX 84% 54
To investigate long-term trends you can access historical data at the NCDC website and analyze the data for specific weather reporting stations.