transparency is almost solely a function of altitude: the higher the better. However, for visual observing, if you go too high, you'll lose visual sensitivity simply because not enough oxygen is getting to your brain. The optimum altitude range seems to be from about 1500 up to perhaps 3000 meters (5000 to 9000 feet). Below 1500m, the amount of crud increases dramatically, and above 3000m most people have at least mild effects from lack of oxygen. Visual observing from Mauna Kea without bottled oxygen is pretty crummy. Remember that astro-observing is mostly at the threshold of acuity, so even small physiological effects from altitude (or ill health etc) will have pronounced effects on your vision in these circumstances

in general, temperature falls by 5-7 degC for every 1km elevation (the lapse rate) up to the tropopause at 10-15km, so at Mt Buller in summer it gets pretty cold still!

inversion layers are those with a negative lapse rate, where temperature rises with elevation

top inversion layer of stratus cloud usually lies at elevation 500-2000m

with a humid airmass the transparency is reduced significantly. With a continental airmass from the arctic, relatively cold and dry conditions prevail, allowing the sky transparency to be at times be as good as in the semi-desertic regions. Good forecasts of such rare starry evenings will clearly be useful to the amateur astronomer.

moisture is the only element affecting sky transparency which can be both measured and forecast all across the globe. It is often the most important factor in reducing sky transparency locally.

a muggy summer day with a whitish sky is the best example of this moisture effect.

happens because the sizes of air molecules are not a lot different from the wavelengths of visible light

Rayleigh scattering also depends on altitude: higher places have less air to cause the scattering.

the amount of scattering changes as the inverse fourth-power of the wavelength: the scattering is way higher in the blue than in the red.

The main effect here is a small additional extinction right in the yellow-green.

The result is to flatten out the extinction curve in this part of the spectrum. Since the source of this is so high in the atmosphere, it is a nearly-fixed additional extinction for any site regardless of altitude.

The Canary Islands have a serious local source of dust—the Sahara. Extinction may be 0.5 and higher in summer just from high-level sand suspended over the summit where the telescopes are.

summer in south-east Australia is often plagued with bushfire smoke for weeks.

sky transparency from a given location can be measured in terms of magnitudes of star brightness lost per airmass thickness

usually these numbers apply specifically for the standard V passband. The dark-adapted eye response is somewhat to the blue of this, so we see higher extinction, but nobody measures it there, so it's easiest just to calibrate things relative to the V-band extinction. (The “dark-adapted visual” value is about 0.03 larger than the V extinction.)

extinction for points other than the zenith can be approximately predicted as the airmass increases in proportion to the secant of the zenith distance, so at 30 degrees above the horizon [60 deg zenith angle], you're looking through the equivalent of two atmospheres worth of air

Many of you may have noticed that dark patch sitting right next to the Southern Cross. The patch is known as the Coal Sack and many people think that this region is devoid of stars and totally obscured by dust. However embedded within this inky blackness are a couple of faint stars that help you to gauge the extent to which you have good transparency or dark sky on any particular night. The brightest star in the Coal Sack Nebula glows at magnitude 5.3 and if you are able to glimpse this star, the transparency of your site is quite good. The general area all around the Crux (Southern Cross) region is a good reference area for judging your limiting sky magnitude.