I don't know why I did not think of this earlier, but in an earlier life, I designed air conditioning systems for commercial projects. One of the key factors to consider for a commercial building's "load" is the solar heat gained by the structure. To calculate this, ASHRAE (the American Society of Heating, Refrigeration and Air Conditioning Engineers) used data from the U. S. Air Force. The USAF, it turns out, keeps excellent logs of weather data for a number of major points in the US, and records, among other things, cloud cover (as a percent of the sky) every three hours (both day and night). These files are archived somewhere (that's where ASHRAE obtained them), and I think they might make good references for estimating sky cover at various points around the southwest. Trouble is, I can't remember where that data was stored. I know it is stored on-line (I quickly browsed the Air Force Weather Agency's web page, but did not find links to it), but don't recall where. If anyone in this group knows what I am talking about or knows the link to the USAF data, please let me know. Richard Harshaw Cave Creek, AZ "If the world should blow itself up, the last audible voice would be that of an expert saying it can't be done." -- Peter Ustinov