What makes a really good observatory site, besides altitude? Let's limit this to optical telescopes.

I understand that the higher you are, the less atmosphere is above you to get in the way of observations. From reading around, there seem to be a few more but I'm not sure:


Not near an active or dormant volcano (but Mauna Kea seems to disprove this?). This is kinda a bummer because a lot of tall mountains seem to be volcanic.
Somewhere with clear and/or dry weather for as much of the year as possible
Not near major light pollution like cities
Cold weather is better than hot weather? Not sure I understand that; if it's uniformly hot or cold then I don't see how it makes a difference.


Am I missing any? Or are these wrong in some way?

As a concrete example, would the Tibetian plateau make a really good observatory site? (Not considering construction or transportation costs to a remote area, of course.)
• Not near an active or dormant volcano (but Mauna Kea seems to disprove this?). This is kinda a bummer because a lot of tall mountains seem to be volcanic.


There's nothing intrinsically wrong with dormant volcanos, as Mauna Kea and the observatories in the Canary Islands demonstrate. Not sure where you would have gotten that idea.


  • Somewhere with clear and/or dry weather for as much of the year as possible


Clear weather is essential -- you can't observe when it's cloudy!

Dry weather is very good, especially for infrared observations (water vapor blocks a lot of infrared light).


  • Not near major light pollution like cities


Yes.


  • Cold weather is better than hot weather? Not sure I understand that; if it's uniformly hot or cold then I dont see how it makes a difference.


Cold climates tend to have drier air. (Antarctica is an extreme case.) Of course, being on top of a mountain means colder air, which is why Mauna Kea is good despite it's being in the tropics.


  Am I missing any? Or are these wrong in some way?


You also want stable air with good "seeing", which rules out places with turbulent air and lots of wind (and is another reason mountaintops are good: they're usually above the most turbulent layers of the atmosphere).
Quoting from this page (which has a good discussion of the general topic):


  Seeing ... requires at minimum a lack of extra turbulence at all atmospheric levels, and seems to be best satisfied in the convergence zones just outside the tropics, at latitudes about ± 30º. Also, minimal local turbulence is often associated with mountain peaks that reach into the otherwise undisturbed oceanic airflow, as on islands or coastal ranges (and given the direction of the planet's rotation, this generally favors western coast ranges).