Sky Conditions Forecast




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 MAC-Hunter Observatory, South Carolina, 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 blocks above.

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, I (Attilla Danko) wrote a script to generate the images like the one above which summarizes CMC's 68 forecast images. This one is just for MAC-Hunter Observatory and the surroundings out to about 10 miles.

There are also clocks for 513 other locations.


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 two colored blocks in the columns are the colors from CMC's forecast maps for MAC-Hunter Observatory, for that hour.  The two numbers at the bottom of a column is the local time, in 24hr format, of that hour. (Local time for MAC-Hunter Observatory is -4.0 hours from GMT.)

The image above shows one hour resolution.  But currently CMCs forecasts only every 3 hours.  CMC is planning true hourly resolution for summer 2002.  In the meantime, the Clock image above will typically show the same color for each triple of 3 hours.

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.  CMC's text page explaining this forecast is here.

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.  CMC's text page explaining the this forecast is here.

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.

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.

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 an 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.



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