Clear Sky Clocks
by JC Willette

Reprinted from the http://cleardarksky.com/csk/ © 2002 with permission from Atilla Danko.
For more information, contact Antilla Danko.

This has been a rough month for observing. Our star parties for the month were clouded out and were cancelled due to weather. I received several calls from members and folks interested asking if it was still on or not. Unfortunately the clouds made it impossible to observe anything.

As a test of our information dissemination procedure, I sent out emails to the membership and prospective members that the events were cancelled. I also posted it on our website. Normally you will hear me tell others that I do not cancel events so far in the future and some of you have heard me tell you that even if there are clouds in the sky, I still go out and most of the time the clouds will disappear after sunset. After a serious look at our Clear Sky Clocks provided by Antilla Danko and Alan Rahill, I could predict with high accuracy that the skies would be cloudy and it would be useless for us to even try to go and observe. I could see that all 38 hours of the clock were cloudy for our window and beyond, I therefore canceled the event. I ended up doing this for all the star parties in September.

So what do we do on the days that our star parties have been cancelled? I thought about that and decided to give you a better look at what our friends have provided to us through the Clear Sky Clocks for our observation sites. These folks have put the Clear Sky Clock together for astronomers to use for forecasting observing nights.



What is it?

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 ASN LV observing sites, will have good weather for astronomical observing.

The forecast data for the above image comes from those very cool guys at the. Canadian Meteorological Center. They run three computer weather simulations every twelve hours. The output looks just like satellite pictures, but the dates are from the future. To see them, click on the 'cloud', 'tran' or 'seeing' blocks.

CMC's numerical weather forecasts are unique because they are specifically designed for astronomers. But they have 68 forecast images. It can be a chore to find the one you want.
So, Attilla Danko wrote a script to generate the images like the one for our observing site which summarizes CMC's 68 forecast images. If you are traveling about the country you can check on any of the clocks for 721 other locations.

If you find this image, or the underlying forecast maps, useful please email Allan Rahill. He needs to show his bosses at CMC that his astronomy forecasts are actually being used. Feel free to copy Antilla Danko.

How do I read it?

Summary: Find a blue block in the first row. There probably won’t be any clouds in the sky then.

Details: Read the image from left to right. Each column represents a different hour. The first two colored blocks in the columns are the colors from CMC's forecast maps for the observing site, for that hour. The two numbers at the bottom of a column is the local time, in 24hr format, of that hour.

The image shows one hour resolution. But currently CMCs forecasts only every 3 hours. CMC is planning true hourly resolution for summer 2002. In the meantime, this clock shows an interpolated forecasts for the hours between CMC forecasts. Blocks are CMC's actual forecast. The circles are interpolated.
The line, labeled cloud is visible-light cloud forecast. It forecasts percentage cloud cover. Dark blue is clear. Lighter shades of blue are increasing cloudiness and white is overcast. This forecast may miss low cloud and afternoon thunderstorms.

The line, labeled tran, is the transparency forecast. Here 'transparency' means just what astronomers mean by the word: the total transparency of the atmosphere from ground to space. It's calculated from the total amount of water vapor in the air. Dark blue means excellent transparency befitting Arizona. Light blue is better than average and pale blue is worse than average. White means that there is at least some broken cloud. Look at the cloud forecast for the same time to see how much cloud there will be. The transparency forecast seems to be somewhat pessimistic.

The line, labeled seeing, is the astronomical seeing forecast. This is an experimental forecast. Excellent seeing means at high magnification you will see fine detail on planets and stars will show diffraction rings. In bad seeing, planets might look like they are under a layer of rippling water and show little detail at any magnification, but the view of galaxies will probably be undiminished. Bad seeing is caused by turbulence combined with temperature differences in the atmosphere. This forecast attempts to predict turbulence and temperature differences that affect seeing for all altitudes.

The excellent-to-bad seeing scale is calibrated for instruments in the 11 to 14 inch range. There are some more details in CMC's seeing forecast page. There are gaps in the line of seeing blocks because CMC's seeing model does not consider daytime heating, so the forecast is only available for the night.

Note also that you may observe worse seeing though your telescope than what a perfect seeing forecast would predict. That is because tube currents and ground seeing mimic true atmospheric seeing. You may also observe better seeing then predicted here when observing with an instrument smaller than 11 inches.

You can help improve the seeing forecast by submitting observations to the Astronomical Seeing Observations program.

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. Mouse over a darkness block for details. The colors represent the limiting visual magnitude at the zenith. The legend row at the bottom shows the magnitude that each color represents, from mag 6, for a dark sky, to mag -4 for daylight. It is based on Ben Sugarman's Limiting Magnitude calculations page. It takes into account the sun and moon 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 map?

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 its in GMT, while the blocks on the clock are in local time.

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

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

Just reload this page. I'm prepared to keep updating this image every hour for as long as CMC is willing to generate the underlying maps.

But, please note the future of CMC's underlying maps is not certain. If you find this image, or CMC maps this image links to, useful please send Alan Rahill of the CMC an email and feel free to copy Antilla Danko. Allan needs to show his boss that his astronomy forecasts are actually being used.

Jon and I have personally used these Clear Sky Clocks and have found them to be very accurate! I would personally like to thank Jon for his mastery in pinpointing exact locations on the earth so that Antilla could create our observing site Clear Sky Clocks.

Please email Antilla at danko@cleardarksky.com and his friend Alan at Allan.Rahill@ec.gc.ca and let them know how much we appreciate their skills and time to provide us with this accurate weather forecast for observing.

You may also take a look at the CMC at http://www.cmc.ec.gc.ca/
cmc/htmls/mainpage.html to get a better understanding of the information provided by the Clear Sky Clocks. It would be a shame if all this hard work falls by the wayside.

Until later - keep your eye on the sky!