Right you are. I am the person who created the image and, not surprisingly, the clear days fall generally in the dry seasons of the different regions. I tried to find the best scenes of California when there were some crops showing (always something growing somewhere there). Right again about the East Coast. The autumn is generally clear, but the Appalachians are orange-brown. My goal was to get a clear summer across the country - not so easy.
For anyone interested, the really high resolution version of this is at 278m and is a 1.9GB TIFF image (360MB compressed) - 34500x18000 or so. The high quality jpeg is about 55MB.
Hey, thanks for the information!