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Extra dwellings is one part. Another is any new "anti-astro" infrastructure, that may come with expansion, such as floodlighting of sport fields and mega-stores such as Bunnings and shopping centers. This is the real killer.
"Seasonal" - yes, but more specifically everyday changes in moisture content in the atmosphere, and particulates like from dust and fires, and combined with your local geography. High humidity/particulate content means poor transparency as star light doesn't get through, and it also means that any ground-based lighting is also reflected back to us on the ground. A sea breeze is typically bad news as it is moisture laden. Transparency can alter dramatically from one night to the next, and hence the severity of light pollution - more transparent the sky, the less the glare we see of light pollution.
The geographical component means the propensity for humidity/fog to pool in your local area. Plains especially are poor for clear skies as mist and fog has nowhere to go, so it just sits where you are. Valleys - mist and fog settles into them. In the case of valleys, you don't want to set up in the valley, but on the ridge tops as that is where things are clear. The good thing about this altitude difference is the pooled mist/fog down in the valley also helps trap some the light pollution that is coming from underneath it!
Careful site selection will mean you not only improve sky transparency, but the weather patterns have been taken into account, along with local infrastructure, geography, microclimate, local land use (agricultural land (bad news) vs native forest (much better)), and the such, which also means less mist and less dew (even eliminate dew from the equation), and even improved seeing. You can be just two hours away from the Big Smoke, but if you've chosen your site carefully, you can actually have a better astro-location than a site that is four hours away from the City Lights.
Alex.
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