Clear Sky Chart

Astronomy-Weather Map
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Map provided by the Canadian Meteorological Center and is Crown copyright, product of Environment Canada.
Allan Rahill, of CMC, developed the forecast.

Cursor shows location of the Orson Pratt Observatory Clear Sky Chart.
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Previous ForecastTime
Local Time 13:00 19Z
Cloud Cover
This Forecast Time
Local Time 14:00 20Z
Cloud Cover
Next Forecast Time
Local Time 15:00 21Z
Cloud Cover
What does Z mean?

The times on the maps are in Zulu time. Also casually called GMT or UT. A conversion table to local times is here. But the colored blocks, labelled in local time, will always point to the correct maps in Zulu time on this page.

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Why check these maps?

The colored blocks on the clear sky charts show the forecast from these maps for the location of the chart. But the colored bock doesnt show you how nearby very different weather could be. Look at these maps to see if a colored block on the chart corresponds to a tiny clear hole surrounded by clouds or a large clear area. The larger the area of the same color under the cross hair, the higher the accuracy of the forecast. .

Why isnt there a single forecast map for observing conditions?

The observation of fine detail seen in telescopes on planets or the moon requires good seeing but can still be good during poor transparency. Visiblity of galaxies and nebulae in telescopes, or the milky way with the un-aided eye, requires good transparency but isn't much affected by seeing. All astronomical observations require a low percentage of cloud cover or clear skies. Humidity, Wind, and Temperature won't determine if you can observe, but they might affect how comfortable you are or how much your telescope will shake or dew up. Many factors affecting observing means many maps.

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Copyright 2023 Attilla Danko

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