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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.)

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    $\begingroup$ The closer you are to the equator, the more of the sky you can see. Cold air is drier than warm air. Cold air at 20% relative humidity is drier that warm air at 20% relative humidity. $\endgroup$
    – user21
    Jun 14, 2015 at 2:08
  • $\begingroup$ Dust, pollen and persistent high level cirrus haze are problems. $\endgroup$
    – Wayfaring Stranger
    Jun 14, 2015 at 11:05
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    $\begingroup$ Incidentally, the Tibetan plateau is an excellent observatory site. LCOGT is planning to set up a node there. $\endgroup$
    – Warrick
    Jun 16, 2015 at 11:24

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• 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).

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  • $\begingroup$ "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." Well if it was me working there, or heck, if it was me paying multi-millions for the observatory, I would not want to put it on or near any volcano that could erupt. This includes dormant volcanoes. $\endgroup$
    – DrZ214
    Jun 17, 2015 at 22:34
  • $\begingroup$ Kilauea, the volcano that shares the Big Island with Mauna Kea and Mauna Loa, is not dormant. $\endgroup$
    – Pete Becker
    Jun 18, 2015 at 23:01
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    $\begingroup$ Mauna Loa is not dormant, either (most recent eruption: 1984). Mauna Kea, on the other hand, hasn't erupted since about 2500 BC, so it's probably a bit less of a risk. (The Canary Islands telescopes are also on islands with historical volcanic activity.) $\endgroup$
    – Peter Erwin
    Jun 19, 2015 at 15:51

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