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This is the Clear Sky Clock provided for Happy Ray Manor Observatory by Attilla Danko. Please email Attilla if you are interested in a Clear Sky Clock for your location.
Click below for more details about the Clear Sky Clock (CSC). How do I read the forecast? | Cloud Cover? | Transparency? | Seeing? | Darkness? | Wind? | Humidity? | Temperature? | Return to Top
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 Happy Ray Manor Observatory is -6.0 hours from GMT.)
The line, labeled Transparency, forecasts the transparency of the air. 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. Above average transparency is necessary for good observation of low contrast objects like galaxies and nebulae. However, open clusters and planetary nebulae are quite observable in below average transparency. Large globulars and planets can be observed in poor transparency. A forecast color of white formally means that CMC didn't compute the transparency forecast because the cloud cover was over 30%. So it may be possible to observe during a white transparency forecast, but the real transparency is usually yucky. Look at the cloud forecast for the same time to see how much cloud there will be. CMC's text page explaining this forecast is here.
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. Seeing is forecast for 3 hour blocks, so triples of seeing blocks will show the same color. A white block on the seeing line means that there was too much cloud (>80% cover) to calculate it. 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.
It is based on Ben Sugarman'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.
But dewing is not simply correlated to relative humidity. Dewing tends to happen when the sky is clear, the temperature is dropping and there isn't much wind. Being on a hilltop or in a small valley can make the difference between no dew and dripping telescopes. Unfortunately, the humidity forecast does not have the spatial resolution to know about small hills and valleys. Also, a forecast of 95% humidity under an opaque sky is a pretty good clue that it's time to cover the telescopes. Since there are many different levels in this forecast, with similar looking colors, it's best to activate the "explain colors when you mouse over" to interperet the colors.
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Updated 04-13-2006 |
