astronomer’s forecast


What is it?

It’s the astronomer’s forecast. 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 the Computer Deconstruction Laboratory in Wall Township, New Jersey will have good weather for astronomical observing.

The data comes from a forecast model developed by Allan Rahill of the Canadian Meteorological Center. CMC’s numerical weather forecasts are unique because they are specifically designed for astronomers. But they have 763 forecast maps. It can be a chore to find the one you want.

Attilla Danko wrote a script to generate the images like the one above which summarizes CMC’s forecast images just for Computer Deconstruction Laboratory and the surroundings out to about 10 miles.

Graphics, text, and back-end programming courtesy of Attilla Danko and

How do I read it?

Summary: In the rows labeled “sky conditions”, find a column of blue blocks. You can probably observe then.

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 Computer Deconstruction Laboratory is -4.0 hours from GMT.)

Cloud Cover





CMC’s text page explaining this forecast is here.


Overcast Cloud 75% Cloud 50% Cloud 25% Clear Sky

This is not a CMC forecast. It’s data form the “european” forecast model ECMWF. It provides a cloud cover forecast. It’s here for comparison with the CMC cloud cover forecast. The ECMWF model covers a larger area and forecasts further into the future. New data becomes available around 17:00 and 05:00 local time. Which is different than when the CMC forecasts update. This isnt an astronomy-specific forecast. So like many civil forecasts “clear sky” might still mean “some cirrus”.

Accuracy Summary: The ECMWF forecast is more accurate than the CMC cloud forecast when predicting mostly clear, but less accurate when predicting mostly cloudy. But ECMWF forecasts out for 72 hours instead of 44 for CMC. The far future ECMWF cloud forecasts are still usable.

Accuracy details: Averaged over North America for the last 30 days: when the forecast is predicting less than 12 hours into the future, mostly-clear forecasts (cloud<25%) have been right 85% of the time. Mostly-cloudy forecasts (cloud>75%) have been right 71% of the time. When the forecast is predicting more than 60 hours into the future, the mostly-clear accuracy is 81% and the mostly-cloudy is to 71%.

Data from the Norwegian Meteorological Institute, also known as MET Norway.

The ECMWF full forecast for near the Computer Deconstruction Laboratory is here.

Too cloudy to forecast Poor Below Average Average Above average Transparent

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. It is somewhat independent of the cloud cover forecast in that there can be isolated clouds in a transparent air mass, and poor transparency can occur when there is very little cloud.


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




Bad seeing can occur during perfectly clear weather. Often good seeing occurs during poor transparency. It’s because seeing is not very related to the water vapor content of the air.

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.




-4 -3 -2 -1 0 1.0 2.0 3.0 3.5 4.0 4.5 5.0 5.2 5.4 5.6 5.8 6.0


It is based on Ben Sugerman’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.






This forecasts ground-level relative humidity.

Humidity variations can indicate the likelihood of optics and eyepieces dewing.




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 interpret the colors.





How do I see the full maps?

To see CMC’s full map for a particular hour, click on a colored block. The CMC map your browser will load will be the map closest to the hour you picked. The time on the CMC map might look odd because it’s in GMT, while the blocks on the chart are in local time.


Graphics, text, and back-end programming copyright 2019  Attilla Danko and