weather

The sky at night is not black. Dark yes, but certainly not black. I now have enough data from dark sky sites in central and western Victoria and the Ice In Space Astro Camp at Lostock in NSW to offer some comments on what makes the sky bright and how far you have to travel to get a 'dark sky'.

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Summary

Almost all free weather websites and smartphone apps use data from the United States GFS global weather model as it is the only one openly published for all to use. But for cloud forecasts in the Australian region, there are two significantly better (although less user friendly) options you should also be looking at. All up then, here are my three recommended sources for cloud forecasts in the Australian region: 1) Synthetic Satellite Forecast Loop from the Australian Bureau of Meteorology ACCESS model: Australian Region Forecast Loop. 2) European Model forecast via the Norwegian site yr.no (the only site I know of that publishes European Model data: e.g. Heathcote, Victoria. Search for your nearest largest town, then choose 'Hour by Hour' and 'Detailed' to see the full cloud breakdown for high/medium/low cloud. 3) US Model via Skippy Sky: This is the best display of GFS data and the most user friendly of these three options. I would place greater weight on the previous two models though, particularly for short-term forecasts.

This set of images compares the actual infrared satellite image for midday Saturday 13th September 2008 with forecasts made on the same day and up to 5 days ahead. The forecasts made up to 2-3 days ahead are quite accurate, so that a reasonable estimate of the astronomy prospects for a Saturday night could be made on the Thursday beforehand. While the general trends are still correct in the 5 day forecast, the details for a particular location become much less accurate.

Edit Oct 2009: Since I originally wrote this article, SkippySky has become available. It uses the same US GFS weather model data so the discussion about accuracy is still relevant, but SkippySky is a much more convenient way to access the cloud forecast data, with great maps for all of Australia among other places.

Check out Cloud Forecasts for Astronomers for the story behind some of this and how to generate your own cloud forecast maps using US GFS model data.

Also see Cloud Forecast Accuracy for comparisons of the forecast charts with actual satellite images.