Clear Sky Chart

Astronomy-Weather Map
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ColorMeaning
> 45C
40C to 45C
35C to 40C
30C to 35C
25C to 30C
20C to 25C
15C to 20C
10C to 15C
5C to 10C
0C to 5C
-5C to 0C
-10C to -5C
-15C to -10C
-20C to -15C
-25C to -20C
-30C to -25C
-35C to -30C
-40C to -35C
< -40C
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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 Ste-Thérèse Clear Sky Chart.
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Previous ForecastTime
Local Time 05:00 09Z
Cloud Cover
Transparency
Wind
Humidity
Temperature
This Forecast Time
Local Time 06:00 10Z
Cloud Cover
Transparency
Wind
Humidity
Temperature
Next Forecast Time
Local Time 07:00 11Z
Cloud Cover
Transparency
Wind
Humidity
Temperature
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.

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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