Observatoire de l'Agence Spatiale Canadienne
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Observatoire de l'Agence Spatiale Canadienne clear sky chart from 2005-11-06 to 2026-06-01.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	16% 10% 3 8% 5% 5% 5% 4 5% 7% 31%	132155
	Night	17% 12% 4 9% 5% 5% 4 3 5% 6% 29%	131014
Transparency	Day	52% 5% 12% 21% 10% 1	135612
	Night	56% 4 10% 19% 10% 1	135644
Seeing	Day	22% 13% 29% 23% 11% 3	16511
	Night	24% 10% 29% 27% 9% 1	25526

By Month

Percentage Cloud Cover NE (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	15% 17% 6% 15% 6% 5% 4 3 4 4 22%	4661
	11th to 20th	Day	13% 16% 6% 14% 7% 6% 5% 3 4 6% 22%	4650
	21st to end	Day	11% 13% 5% 11% 8% 8% 6% 4 5% 5% 24%	5124
February	1st to 10th	Day	15% 15% 4 10% 6% 6% 5% 3 4 5% 26%	1826
	11th to 20th	Day	14% 12% 5% 12% 6% 5% 5% 4 5% 6% 26%	1969
	21st to end	Day	16% 16% 4 9% 6% 5% 4 3 5% 5% 27%	1730
March	1st to 10th	Day	17% 12% 5% 11% 6% 5% 5% 3 4 6% 25%	4576
	11th to 20th	Day	13% 14% 4 10% 5% 4 4 3 5% 7% 31%	4788
	21st to end	Day	18% 11% 4 10% 5% 4 4 3 4 5% 33%	6102
April	1st to 10th	Day	22% 11% 3 9% 5% 5% 4 3 5% 5% 30%	2520
	11th to 20th	Day	21% 10% 3 7% 4 4 4 2 4 6% 36%	2494
	21st to end	Day	20% 10% 2 8% 4 4 4 4 5% 6% 32%	2706
May	1st to 10th	Day	25% 11% 4 9% 5% 4 5% 4 5% 7% 22%	6342
	11th to 20th	Day	19% 9% 3 8% 5% 5% 4 4 6% 8% 30%	6458
	21st to end	Day	20% 9% 3 7% 4 4 5% 4 5% 8% 31%	7103
June	1st to 10th	Day	20% 7% 3 6% 5% 5% 5% 4 6% 7% 31%	2800
	11th to 20th	Day	14% 6% 2 7% 5% 4 4 4 7% 9% 40%	2800
	21st to end	Day	20% 8% 3 7% 5% 5% 5% 4 6% 7% 32%	2800
July	1st to 10th	Day	12% 7% 3 8% 6% 6% 6% 5% 7% 9% 33%	6360
	11th to 20th	Day	11% 5% 2 6% 5% 5% 5% 4 7% 9% 41%	6360
	21st to end	Day	11% 6% 2 5% 4 4 4 4 6% 9% 45%	6926
August	1st to 10th	Day	13% 6% 3 8% 5% 6% 5% 5% 7% 9% 33%	4563
	11th to 20th	Day	13% 6% 3 7% 5% 5% 5% 3 7% 9% 38%	4127
	21st to end	Day	15% 6% 3 7% 4 4 5% 4 6% 8% 40%	4312
September	1st to 10th	Day	11% 5% 2 6% 5% 4 5% 5% 7% 8% 44%	2400
	11th to 20th	Day	13% 6% 2 4 4 3 3 3 6% 8% 49%	2301
	21st to end	Day	16% 6% 2 6% 4 5% 4 4 6% 8% 41%	2200
October	1st to 10th	Day	18% 6% 3 7% 5% 5% 5% 4 5% 7% 35%	2085
	11th to 20th	Day	17% 9% 2 8% 6% 5% 5% 4 6% 7% 30%	1940
	21st to end	Day	25% 9% 3 7% 4 4 4 3 5% 5% 30%	1980
November	1st to 10th	Day	18% 11% 4 8% 5% 5% 4 4 5% 7% 31%	1851
	11th to 20th	Day	17% 11% 4 7% 5% 5% 6% 5% 5% 6% 29%	1827
	21st to end	Day	23% 13% 4 7% 5% 5% 4 4 5% 6% 23%	1680
December	1st to 10th	Day	19% 20% 6% 11% 6% 4 4 3 4 4 18%	2680
	11th to 20th	Day	19% 14% 5% 9% 5% 4 5% 4 6% 5% 25%	3066
	21st to end	Day	23% 12% 4 11% 6% 5% 5% 5% 6% 4 18%	4048
January	1st to 10th	Night	13% 19% 7% 14% 6% 4 4 3 3 4 21%	8234
	11th to 20th	Night	12% 16% 7% 17% 7% 6% 5% 3 5% 5% 16%	8235
	21st to end	Night	12% 13% 5% 12% 9% 6% 5% 3 4 5% 25%	9057
February	1st to 10th	Night	14% 15% 6% 12% 7% 5% 4 4 5% 5% 22%	2793
	11th to 20th	Night	14% 13% 5% 11% 6% 5% 5% 3 4 5% 29%	2535
	21st to end	Night	15% 15% 4 9% 5% 4 5% 2 4 5% 31%	2076
March	1st to 10th	Night	21% 13% 3 8% 5% 4 4 3 4 4 30%	5484
	11th to 20th	Night	15% 15% 4 7% 4 5% 5% 4 5% 7% 30%	5385
	21st to end	Night	16% 12% 4 10% 6% 5% 4 4 5% 4 31%	5072
April	1st to 10th	Night	23% 10% 3 7% 4 4 4 3 4 7% 30%	2100
	11th to 20th	Night	22% 8% 4 7% 4 4 4 3 4 6% 33%	2075
	21st to end	Night	21% 10% 3 7% 4 4 4 4 4 5% 35%	1913
May	1st to 10th	Night	24% 11% 4 8% 5% 4 4 3 5% 6% 25%	3862
	11th to 20th	Night	24% 8% 3 8% 4 4 4 3 6% 6% 30%	3655
	21st to end	Night	17% 10% 3 8% 5% 4 4 3 5% 7% 31%	4014
June	1st to 10th	Night	21% 9% 2 7% 4 4 4 4 5% 7% 32%	1600
	11th to 20th	Night	16% 5% 3 7% 4 6% 4 5% 6% 7% 38%	1600
	21st to end	Night	18% 6% 4 7% 5% 4 6% 4 6% 7% 34%	1600
July	1st to 10th	Night	13% 7% 2 8% 5% 6% 5% 5% 7% 9% 34%	3600
	11th to 20th	Night	14% 6% 3 6% 4 5% 5% 4 6% 7% 40%	3600
	21st to end	Night	9% 5% 2 7% 5% 5% 5% 4 7% 8% 43%	3926
August	1st to 10th	Night	16% 9% 3 7% 4 5% 5% 3 5% 6% 37%	2586
	11th to 20th	Night	15% 7% 2 8% 4 5% 5% 4 6% 7% 38%	2884
	21st to end	Night	16% 6% 2 7% 5% 4 4 4 6% 7% 39%	3586
September	1st to 10th	Night	12% 6% 2 7% 4 4 4 4 6% 6% 44%	2000
	11th to 20th	Night	14% 6% 2 5% 3 3 3 3 5% 7% 50%	2098
	21st to end	Night	16% 7% 3 6% 4 4 3 3 5% 7% 42%	2200
October	1st to 10th	Night	17% 7% 3 6% 5% 4 4 4 5% 6% 40%	2310
	11th to 20th	Night	20% 8% 3 7% 5% 4 5% 3 5% 6% 34%	2459
	21st to end	Night	23% 8% 4 8% 4 5% 5% 4 4 5% 30%	2860
November	1st to 10th	Night	20% 10% 3 7% 4 5% 4 3 5% 6% 31%	2667
	11th to 20th	Night	19% 11% 3 8% 5% 4 5% 3 5% 6% 32%	2790
	21st to end	Night	22% 14% 4 8% 6% 5% 4 3 4 5% 25%	2940
December	1st to 10th	Night	17% 19% 6% 12% 7% 5% 5% 4 4 5% 17%	4720
	11th to 20th	Night	23% 13% 4 9% 5% 4 4 3 5% 6% 24%	5348
	21st to end	Night	24% 16% 5% 9% 4 3 4 3 7% 6% 20%	7150

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	67% 4 10% 16% 3	4662
	11th to 20th	Day	64% 5% 7% 13% 10% 1	4680
	21st to end	Day	62% 5% 9% 14% 11%	5124
February	1st to 10th	Day	61% 4 8% 19% 6% 2	1826
	11th to 20th	Day	59% 3 11% 17% 8% 1	1978
	21st to end	Day	60% 3 10% 16% 9% 1	1730
March	1st to 10th	Day	61% 4 8% 14% 10% 3	4579
	11th to 20th	Day	53% 4 16% 15% 10% 2	4788
	21st to end	Day	54% 4 7% 18% 16% 1	6102
April	1st to 10th	Day	57% 3 8% 19% 11% 2	2520
	11th to 20th	Day	51% 3 8% 22% 14% 1	2494
	21st to end	Day	53% 4 9% 20% 12% 3	2706
May	1st to 10th	Day	61% 5% 8% 18% 7% 1	6353
	11th to 20th	Day	52% 6% 14% 19% 9% 2	6462
	21st to end	Day	51% 5% 11% 21% 11% 1	7103
June	1st to 10th	Day	51% 5% 10% 19% 12% 2	2800
	11th to 20th	Day	42% 5% 12% 25% 14% 3	2800
	21st to end	Day	51% 5% 13% 20% 10% 1	2800
July	1st to 10th	Day	47% 5% 14% 26% 8%	6290
	11th to 20th	Day	39% 5% 14% 31% 11%	6285
	21st to end	Day	35% 5% 14% 37% 8%	6728
August	1st to 10th	Day	45% 5% 18% 24% 8%	4549
	11th to 20th	Day	43% 5% 18% 26% 7%	4168
	21st to end	Day	43% 4 16% 22% 14%	4312
September	1st to 10th	Day	36% 6% 15% 31% 12%	2400
	11th to 20th	Day	34% 4 12% 35% 14% 1	2301
	21st to end	Day	42% 5% 14% 26% 12% 1	2200
October	1st to 10th	Day	44% 5% 14% 28% 8%	3397
	11th to 20th	Day	44% 4 18% 26% 7%	3143
	21st to end	Day	46% 4 18% 23% 8% 1	3180
November	1st to 10th	Day	53% 5% 12% 21% 8% 1	1851
	11th to 20th	Day	55% 6% 10% 18% 11%	1827
	21st to end	Day	62% 6% 12% 14% 5%	1680
December	1st to 10th	Day	71% 4 7% 15% 3	2680
	11th to 20th	Day	60% 6% 10% 16% 9%	3066
	21st to end	Day	66% 5% 12% 11% 6%	4048
January	1st to 10th	Night	68% 3 10% 13% 5%	8234
	11th to 20th	Night	69% 5% 8% 12% 6%	8280
	21st to end	Night	62% 3 8% 14% 12%	9057
February	1st to 10th	Night	64% 4 9% 16% 7% 1	2793
	11th to 20th	Night	58% 3 10% 19% 9%	2537
	21st to end	Night	56% 3 9% 18% 12% 1	2076
March	1st to 10th	Night	58% 3 7% 15% 12% 4	5494
	11th to 20th	Night	54% 5% 12% 16% 12% 1	5386
	21st to end	Night	56% 3 6% 22% 12% 2	5072
April	1st to 10th	Night	56% 3 7% 19% 13% 2	2100
	11th to 20th	Night	53% 4 7% 21% 14% 2	2075
	21st to end	Night	51% 4 7% 20% 15% 3	1913
May	1st to 10th	Night	61% 4 10% 17% 7% 2	3868
	11th to 20th	Night	54% 5% 10% 20% 10% 2	3656
	21st to end	Night	53% 5% 9% 18% 14% 1	4015
June	1st to 10th	Night	52% 5% 8% 21% 13% 1	1600
	11th to 20th	Night	44% 6% 11% 22% 14% 3	1600
	21st to end	Night	50% 5% 11% 19% 14% 1	1600
July	1st to 10th	Night	46% 5% 19% 21% 9%	3560
	11th to 20th	Night	42% 5% 15% 27% 11%	3552
	21st to end	Night	38% 5% 16% 28% 12%	3809
August	1st to 10th	Night	48% 5% 12% 27% 8%	2576
	11th to 20th	Night	46% 5% 18% 23% 8%	2909
	21st to end	Night	44% 5% 12% 23% 14% 1	3586
September	1st to 10th	Night	38% 5% 12% 27% 16% 1	2000
	11th to 20th	Night	35% 4 13% 32% 14% 1	2098
	21st to end	Night	42% 5% 10% 26% 16%	2200
October	1st to 10th	Night	44% 5% 13% 27% 11% 1	3778
	11th to 20th	Night	46% 3 11% 31% 9%	3995
	21st to end	Night	50% 4 14% 19% 12% 2	4610
November	1st to 10th	Night	53% 4 10% 22% 8% 1	2667
	11th to 20th	Night	54% 4 11% 18% 11% 1	2790
	21st to end	Night	62% 5% 11% 17% 6%	2940
December	1st to 10th	Night	72% 4 10% 11% 3	4720
	11th to 20th	Night	63% 4 9% 14% 11%	5348
	21st to end	Night	64% 4 9% 14% 9%	7150

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	31% 14% 28% 19% 8%	220
	11th to 20th	Day	28% 19% 32% 16% 5%	220
	21st to end	Day	21% 19% 31% 19% 9% 1	242
February	1st to 10th	Day	27% 18% 37% 15% 2	297
	11th to 20th	Day	22% 17% 37% 21% 3	457
	21st to end	Day	26% 16% 32% 22% 3	446
March	1st to 10th	Day	25% 15% 35% 20% 6%	540
	11th to 20th	Day	25% 16% 32% 20% 6% 1	540
	21st to end	Day	28% 16% 30% 18% 6% 1	594
April	1st to 10th	Day	31% 15% 28% 16% 7% 1	540
	11th to 20th	Day	32% 18% 24% 15% 9% 3	532
	21st to end	Day	27% 15% 30% 17% 9% 2	540
May	1st to 10th	Day	25% 14% 30% 20% 9% 2	543
	11th to 20th	Day	19% 12% 30% 25% 11% 3	567
	21st to end	Day	21% 14% 32% 22% 10% 2	617
June	1st to 10th	Day	23% 12% 27% 27% 9% 2	530
	11th to 20th	Day	17% 12% 28% 28% 14% 2	530
	21st to end	Day	22% 11% 28% 23% 13% 2	530
July	1st to 10th	Day	13% 9% 26% 31% 17% 4	525
	11th to 20th	Day	14% 8% 30% 31% 14% 4	524
	21st to end	Day	14% 7% 28% 31% 18% 2	576
August	1st to 10th	Day	13% 5% 25% 31% 21% 6%	526
	11th to 20th	Day	13% 5% 23% 32% 19% 8%	519
	21st to end	Day	16% 10% 24% 24% 17% 9%	583
September	1st to 10th	Day	12% 7% 29% 27% 16% 8%	530
	11th to 20th	Day	15% 7% 20% 29% 19% 11%	530
	21st to end	Day	18% 10% 25% 24% 17% 6%	530
October	1st to 10th	Day	20% 11% 28% 23% 14% 4	530
	11th to 20th	Day	23% 14% 30% 21% 10% 2	497
	21st to end	Day	34% 14% 23% 22% 7%	462
November	1st to 10th	Day	29% 15% 27% 22% 7%	425
	11th to 20th	Day	26% 17% 31% 18% 7% 2	367
	21st to end	Day	33% 11% 34% 17% 3 1	220
December	1st to 10th	Day	35% 10% 25% 22% 6% 2	220
	11th to 20th	Day	27% 22% 35% 11% 4	220
	21st to end	Day	28% 20% 27% 17% 5% 2	242
January	1st to 10th	Night	28% 12% 31% 24% 5%	840
	11th to 20th	Night	27% 12% 33% 22% 7%	840
	21st to end	Night	22% 10% 34% 26% 7% 1	924
February	1st to 10th	Night	24% 9% 34% 27% 5% 1	840
	11th to 20th	Night	22% 13% 37% 22% 5%	823
	21st to end	Night	24% 12% 32% 29% 4	692
March	1st to 10th	Night	26% 11% 33% 25% 6%	840
	11th to 20th	Night	26% 12% 33% 22% 6%	840
	21st to end	Night	25% 10% 31% 25% 7% 1	924
April	1st to 10th	Night	30% 12% 28% 25% 5%	840
	11th to 20th	Night	27% 14% 29% 23% 6% 1	831
	21st to end	Night	28% 11% 28% 26% 7%	653
May	1st to 10th	Night	26% 7% 34% 25% 7% 1	504
	11th to 20th	Night	22% 9% 31% 29% 8% 1	414
	21st to end	Night	19% 10% 33% 28% 9%	455
June	1st to 10th	Night	25% 6% 28% 31% 11% 1	400
	11th to 20th	Night	18% 7% 29% 34% 12%	400
	21st to end	Night	19% 6% 29% 33% 13% 1	400
July	1st to 10th	Night	13% 6% 27% 37% 16%	390
	11th to 20th	Night	13% 2 24% 45% 14% 2	388
	21st to end	Night	13% 3 24% 44% 15% 1	438
August	1st to 10th	Night	15% 3 17% 46% 18% 2	398
	11th to 20th	Night	13% 4 20% 36% 23% 2	588
	21st to end	Night	16% 5% 21% 37% 19% 2	880
September	1st to 10th	Night	15% 5% 26% 38% 16% 1	800
	11th to 20th	Night	16% 5% 18% 35% 22% 3	800
	21st to end	Night	20% 6% 27% 30% 15% 2	800
October	1st to 10th	Night	20% 8% 29% 29% 14% 1	800
	11th to 20th	Night	26% 8% 29% 28% 9%	800
	21st to end	Night	28% 11% 30% 26% 6%	880
November	1st to 10th	Night	26% 12% 31% 22% 7%	820
	11th to 20th	Night	27% 13% 29% 23% 8% 1	840
	21st to end	Night	30% 16% 28% 19% 7%	840
December	1st to 10th	Night	31% 13% 29% 22% 5%	840
	11th to 20th	Night	31% 14% 32% 17% 6% 1	840
	21st to end	Night	33% 14% 30% 17% 5%	924

Page generated at 2026-05-29 01:53:27. Version ac07

Copyright © 2026 Attilla Danko.