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Delaware Valley Amateur Astronomers

French Creek State Gamelands
Clear Sky Chart
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1. Explain color and details when you mouse-over:
2. Click on a block to show full forecast map
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Last updated 2017-09-26 00:32:12. No Image below? Read this. Not showing todays data? Clear your cache.
A site used by the Delaware Valley Amateur Astronomers.
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Page Contents

What is it?

How do I read it

How to read Sky conditions:

How to read Ground conditions:

How do I see the full maps?

How can I put the Chart on my web page?

How do I make sure I keep getting it?

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What is it?

It's the astronomer's forecast. At a glance, it shows when it will be cloudy or clear for up to the next two days. It's a prediction of when French Creek State Gamelands, PA, will have good weather for astronomical observing.

The data comes from a forecast model developed by Allan Rahill of the Canadian Meteorological Center. CMC's numerical weather forecasts are unique because they are specifically designed for astronomers. But they have 763 forecast maps. It can be a chore to find the one you want.

So, I (Attilla Danko) wrote a script to generate the images like the one above which summarizes CMC's forecast images just for French Creek State Gamelands and the surroundings out to about 10 miles.

There are charts for 5422 locations.

How do I read it?

Summary: In the rows labeled "sky conditions", find a column of blue blocks. You can probably observe then.

Details: Read the image from left to right. Each column represents a different hour. The colors of the blocks are the colors from CMC's forecast maps for that hour. The two numbers at the top of a column is the time. A digit 1 on top of a 3 means 13:00 or 1pm. It's local time, in 24hr format. (Local time for French Creek State Gamelands is -4.0 hours from GMT.)


The line labeled darkness is not a weather forecast. It shows when the sky will be dark, assuming no light pollution and a clear sky. Black is a dark sky. Deep blue shows interference from moonlight. Light blue is full moon. Turquoise is twilight. Yellow is dusk and white is daylight. For those who prefer numbers, the scale is also calibrated. The numbers are the visual limiting magnitude at the zenith. (The brightness of the faintest star a standard observer can see straight up.) Mouse over a darkness block for details.

It is based on Ben Sugerman's Limiting Magnitude calculations page. It takes into account the sun's and moon's position, moon phase, solar cycle and contains a scattering model of the atmosphere. It doesn't consider light pollution, dust, clouds, snow cover or the observer's visual acuity. So your actual limiting magnitude will often be different.

How do I see the full maps?

To see CMC's full map for a particular hour, click on a colored block. The CMC map your browser will load will be the map closest to the hour you picked. The time on the CMC map might look odd because it's in GMT, while the blocks on the chart are in local time.

It's worth checking a few of the full maps before committing to a long drive out to an observing site.

May I put the Chart on my web page?

If your website does not make money from ads and does not charge admission, then yes. Please use this html:

<a href=http://www.cleardarksky.com/c/FrnchCrkPAkey.html> <img src="http://www.cleardarksky.com/c/FrnchCrkPAcsk.gif?c=2246445"></a>

Or, if you would prefer a simplified thumbnail:

<a href=http://www.cleardarksky.com/c/FrnchCrkPAkey.html> <img src="http://www.cleardarksky.com/c/FrnchCrkPAcs0.gif?1"></a>

But please don't copy other html or text from this page.

Hey, this is cool. How do I make sure I keep getting it?

Just keep using it. I intend to keep updating this image for as long as CMC is willing to generate the underlying maps. But there are ways that you can help:

If you find this clear sky chart, or CMC maps linked by the colored blocks, useful please send Allan Rahill of the CMC an email (and feel free to copy me). Allan needs to show his boss that his astronomy forecasts are actually being used.

You can also help keep clear sky charts free for everyone by being a sponsor. Please feel free to tell sponsors that they're cool.

Clear Sky Chart Home -- Danko's Homepage

The techno-geek section

Page updated by gt72 at 2017-09-26 00:32:12 which is -4.0 hours from UTC.
Update rank: 208. Thanks to anyone thinking of sponsoring this page.

Using coordinates: lat-long 40.1750 -75.79694; cloud2: (481, 130); trans2: (553, 331); wind2: (553, 332); hum2: (553, 332); temp2: (553, 332); seeing2: (553, 331)

Copyright 2017 A. Danko. Page updated 2017-09-26 04:32:12UT on server3.