NCKAS - Clear Sky Clock

North Central Kansas Astronomical Society

Eagle Road - One of the North Central Kansas Astronomical Society's Dark-Sky Observation Sites

Kansas State Clear Sky Clock for the Manhattan area

Click here, or on the image above for the Clear Dark Sky's web site where this image is loaded from.

Click here for our North Central Kansas Light Pollution and Kansas Light Pollution overlays.

How do I read it?

Summary: Find a blue block in the first row. There probably wont be any clouds in the sky then.

Details: Read the image from left to right. Each column represents a different hour. The first three colored blocks in the columns are the colors from CMC's forecast maps for Kansas State University, for that hour. The two numbers at the top of a column is the local time, in 24hr format, of that hour. (Local time for Kansas State University is -5.0 hours from GMT.)

The line, labeled Cloud Cover 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. CMC's text page explaining this forecast is here.

The line, labeled Transparency, 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 20% cloud cover and transparency was not calculated. 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. CMC's text page explaining the this forecast is here.

The line, labeled Trans+Smog is an experimental version of the transparency forecast with a smog included. Only clear sky clocks located near major cities will show a difference between the transparency and transpareny+smog forecats. It's more likely to see difference because the smog forecast model is run only once a day, around 08:30T. That means that in the afternoons, the transparency forecast will be much younger and therefore more accurate than the transparency+smog forecast. That's because the transparency forecast gets updated again around 12UT.

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 is 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. Seeing is forecast for 3 hour blocks, so triples of seeing blocks will show the same color.

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's and moon's position, moonphase, 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.