Clear Sky Clock
Published on Oct 16, 2007 at 5:57 pm.
No Comments.
Filed under observing.
Before going outside to set up telescopes, you may want to know if you have any realistic chance to observe anything. That is particularly true for amateur astronomers who drive for a couple hours or so to get to dark sky observing sites. It is also useful for those of us who are trying to determine whether or not to set up telescope for out students. There are a number of aids to help you make that determination. One of the easiest, but least accurate, is often television or radio forecasts. Often I find that the forecast is “clear” when they really mean “not raining.” The “clear” is often far from clear in terms of being able to set up telescopes and observe anything.
Better than the general television or news coverage is the National Weather Service web site. But, that, too, lacks all of the detail that you really need. It is good for a general impression, or when the answer is very obvious: very clear or very cloudy. But, the problem comes with changing weather. The forecast is for the whole night. It may say that today is cloudy and raining, tonight is partly cloudy with a chance of showers, and tomorrow is clear. So, you infer that it is clearing overnight, perhaps. But, when does it clear? Is is early, late, or when? Do the clouds gradually become less, or does it clear off quickly as a front passes? Is there a chance of rain early and then clear, or might there be scattered showers until dawn? These are questions that the astronomer wants to know.
A much better site is Accuweather. There, you can get hour-by-hour forecast, which gives you a better feel for what the weather will do. I have found that the forecast is reasonably accurate. So, that is what I use to make most of my decisions. Accuweather now even has an astronomy feature and even an astronomy blog written by Lisa Wieser. So, this is a very good place to go for some information. You can find the times of sunrise/sunset and moonrise/moonset. But, sometimes you want even detailed forecasts of sky conditions. Accuweather’s site still lacks some of the details that you might want if the night is borderline. It still simply calls the sky clear, partly cloudy, cloudy, etc. But, not all degrees of clear are equally clear. Sometimes if you are wanting to look at very dim objects which are very difficult to see, you want to know just how clear is meant by clear.
That is where another tool comes in that I have found. It is called the Clear Sky Clock. At the top of this post, I have placed a graphic produced by the Clear Sky Clock for Fort Worth. It is not as easy to look at and read quickly as the data at Accuweather, but it is a bit more informative. Cloud cover is rated not by simply partly cloudy or partly clear, but by a forecast of the percentage of cloud cover. Now, I am not really convinced that such a forecast can be so exact, so I don’t know that this is any improvement over the Accuweather hourly forecast, and it is tougher to read. But, more importantly for astronomy, it has a forecast for transparency and seeing. Both are rated on five point scales, and that is the detailed forecast that you really want for astronomy. There is also a forecast of the sky darkness. This accounts for moonlight, twilight, known light pollution, etc. The Clear Sky Clock also has a graphic for relative humidity and wind speed, both of which can be found on Accuweather’s hourly forecast. Unfortunately, the Clear Sky Clock does not have Accuweather’s RealFeel, which clues you in on how best to dress for the observing session. Also, the Clear Sky Clock at present only covers North America, so observes elsewhere will have to use other resources.
If anyone has any other favorite sources of weather information that you like to use to determine whether or not to observe, feel free to share them in the comments.
-Astroprof
