Altitude and dryness are only two factors. The other major one is the relative calmness of the atmosphere. The air is stable and calm because the "ground" is

Altitude and dryness are only two factors. The other major one is the relative calmness of the atmosphere. The air is stable and calm because the "ground" is flat for many miles around at Mauna Kea, and the mountain protrudes extra high above the local "ground". Before light pollution really kicked into gear, Mt. Wilson and Mt. Palomar were also pretty good because the ground is flat upwind: The Pacific Ocean. Sites in the middle of mountain ranges aren't necessarily any good - you have to see what's upwind of the site. The high plains near Denver, CO are no good because the Rockies are upwind, for example. The technical term in astronomy circles is "seeing" if you want to learn more.