Iain,
I've written a piece on identifying the optimal site for astronomy purposes. It talks about topography, geography, and what to avoid as much as possible.
A big open grassy field in a valley may sound promising, but it is the worst place possible. Turf, and especially agricultural land, expels a lot of moisture during the evening, leading to an endless battle with dew. And the valley itself means that if fog forms, guess where it settles? Gain access to a rocky ridge top above the valley and not only are you above the fog line, but you avoid just about 100% of any dew! That is why a few of us loved Katoomba Airfield - dew was never a problem, and being on top of a tall sandstone plateau fog never visited us, even though Katoomba itself was fog-bound. We lost access to the Airfield when Rod, the Airfield manager, was killed last February
, and we've been looking for another good site since.
Selecting the best site for Astronomy purposes
Of course, as with all things astro, having access to any site is a matter of compromise. If the ideal site isn't possible, then knowing what to avoid as much as possible will allow one to make the best choice possible under the circumstances.
