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Saturday, August 01, 2009


Cloudy, Cloudy Sky

Screen shot from Skippy Sky

Clouds are a big problem for astronomers, amateur and otherwise. Like last night for instance, where the occultation of Sigma Scorpii was clouded out.

Many questions arise. Do I set my alarm clock for some wee hour, if the likelihood of just seeing cloud is high. Should I travel great gobs of kilometres away to have a chance to see a rare event that might be clouded out where I currently am.

You could use the satellite photos and forecasts from the Australian Bureau of Meteorology, (or your local equivalent) but the satellite images stop before night time and the BOM predicts rain, not cloud.

Screen shot from 7 timer

For some time people in Northern America have had access to a Clear Sky Clock. But the rest of us have been left in the dark, so to speak. Now we have 7timer and the Australia Specific Skippy Sky. (update, SkippySky does some of Europe, North America, New Zealand and parts of South America. Sorry, I thought it was Australia only)

Both these cloud cover forecasting sites are based on the American NCEP GFS computer weather model data, so be aware of their limitations. To quote Andrew Cool, co-author of skippy Sky.

"The GFS model has a resolution of 0.5 degrees, that is only 4 data points per square degree [area of about 30kmx30km], so one should always thinks of *trends* in the forecast rather than exactly what will happen over the clothesline in your backyard."
I've used both for a while , and found them sufficiently accurate for my purposes (eg, do I get up at an awful hour of the morning ). 7Timer has the advantage of covering just about everywhere, but uses a fairly cryptic format for specifying sky transparency. I tend only to look at the cloud cover icons, because I can understand those.

SkippySky is limited to Australia, but has the great advantage of presenting data in a weather map format. This makes the question "Do I drive 200 Km to Didjabringabeeralong to observe the Leonids/Comet/Occultation" much easier to answer.

Again, remember that these are models of modest resolution, and the weather has a habit of doing what it blasted well pleases. If you bear that in mind, these programs will make dealing with cloud a whole lot easier.

This is the fourth of a planned series of posts on looking at the sky and how to find your way around it as a beginner.

First post: The Dark Adapted Eye.
Second post: Let the Moon be Your Guide
Third Post: Seeing the Emu.

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Looks good Ian. The reference to Didjabringabeeralong made me smile.
this is the first time I am coming across a blog like yours...keep it up!
Hey Ian,
Better update your post. SkippySky is not just Australia. Not sure how long it has covered other parts of the world. Remove the 'australia' part from your web address.

I seriously thought SkippySky was Oz only. Fixed the post now. Thanks Colin.
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