<i>Clear Sky</i> <i>Chart</i>
Benjamin Hayes
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Clear Sky Chart

International Clear Sky Chart (ICSC)
It's often called the astronomer's forecast. At a glance, it shows when it will be cloudy or clear for the next few days. It provides me a prediction of when my backyard, or the nearby observatory at Bucknell University or the nearest dark site (Cherry Springs State Park) will have good weather for astronomical observing.

The data comes from a forecast model developed by Allan Rahill of the Canadian Meteorological Centre. CMC's numerical weather forecasts are unique because they are specifically designed for astronomers. But they have 1180 forecast maps. It can be a chore to find the one map that tells you if you can observe tonight.

Attilla Danko, a computer scientist by profession and astronomer by hobby, wrote a script to generate the images like the one above which summarizes CMC's forecast images for these two locations (and thousands of others) and the surrounding areas out to about 10 miles.

Click on one of the clear sky charts below to visit the main web site.
Bucknell University
Cherry Springs State Park
How to interpret these charts
There are nine rows of colored squares shown in these Clear Sky Charts. The rows are grouped into two sets, with "Sky" conditions on the top and (2) the "Ground" conditions shown on the bottom. Each row as colored squares depicting the conditions for each hour of the day for nine different parameters. The "Sky conditions" set of rows addresses five parameters: (1) Cloud Cover; (2) ECMWF Cloud; (30 Transparency; (4) Seeing; and (4) Darkness. The "Ground conditions" set of rows addresses four parameters: (1) Smoke; (2) Wind; (3) Humidity; and (4) Temperature. I typically focus my attention on the fourth line in the upper set, the row of colored squares labeled "Seeing."

The different colors correspond to the following conditions:
Stacks Image 39
You're looking for nights with squares in medium to dark blue, typically one hour after sunset and one hour before sunrise.

Excellent seeing means at high magnification you will see fine detail on planets. 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 probably 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.