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 2024-08-30.

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% 9% 3 8% 5% 5% 5% 4 5% 7% 32%	95781
	Night	18% 11% 4 8% 5% 4 4 3 5% 6% 31%	93277
Transparency	Day	51% 5% 12% 22% 10% 1	95500
	Night	54% 4 11% 19% 10% 1	93112
Seeing	Day	22% 12% 28% 23% 11% 3	14347
	Night	24% 9% 29% 27% 9% 1	23256

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	17% 16% 5% 13% 6% 6% 4 3 4 4 23%	2517
	11th to 20th	Day	15% 14% 5% 10% 6% 5% 4 3 5% 7% 25%	2520
	21st to end	Day	16% 16% 4 10% 5% 5% 5% 4 5% 6% 24%	2772
February	1st to 10th	Day	16% 16% 4 10% 6% 6% 5% 3 3 6% 25%	1652
	11th to 20th	Day	15% 12% 4 12% 6% 5% 5% 4 5% 5% 27%	1776
	21st to end	Day	17% 17% 4 8% 6% 5% 4 3 4 5% 27%	1570
March	1st to 10th	Day	18% 12% 4 10% 6% 5% 5% 3 4 5% 26%	3116
	11th to 20th	Day	13% 16% 5% 11% 6% 5% 4 3 5% 6% 26%	3269
	21st to end	Day	15% 9% 3 9% 5% 4 4 4 4 5% 37%	4166
April	1st to 10th	Day	22% 11% 3 8% 4 5% 4 3 4 5% 30%	2280
	11th to 20th	Day	20% 9% 3 6% 4 4 4 2 5% 6% 37%	2254
	21st to end	Day	21% 10% 3 9% 4 4 4 3 5% 6% 32%	2448
May	1st to 10th	Day	19% 10% 3 8% 5% 5% 5% 4 5% 8% 29%	4327
	11th to 20th	Day	16% 9% 3 8% 5% 5% 5% 4 6% 8% 31%	4398
	21st to end	Day	14% 7% 2 7% 4 4 6% 5% 6% 10% 36%	4837
June	1st to 10th	Day	20% 7% 3 6% 5% 5% 5% 4 6% 8% 30%	2660
	11th to 20th	Day	14% 6% 2 7% 5% 4 4 4 6% 9% 39%	2660
	21st to end	Day	20% 8% 3 7% 5% 4 5% 4 6% 7% 32%	2660
July	1st to 10th	Day	11% 7% 3 7% 5% 5% 6% 4 7% 9% 36%	4440
	11th to 20th	Day	12% 5% 2 6% 5% 5% 5% 5% 7% 9% 37%	4440
	21st to end	Day	11% 6% 2 5% 4 4 4 4 6% 8% 44%	4814
August	1st to 10th	Day	13% 5% 3 7% 5% 5% 5% 5% 6% 9% 36%	2646
	11th to 20th	Day	10% 5% 3 6% 5% 5% 6% 4 7% 9% 41%	2415
	21st to end	Day	12% 6% 2 6% 3 5% 5% 4 6% 8% 42%	2486
September	1st to 10th	Day	11% 5% 2 6% 5% 4 5% 5% 6% 8% 42%	2160
	11th to 20th	Day	14% 6% 2 4 3 2 3 3 5% 8% 48%	2071
	21st to end	Day	15% 6% 2 6% 4 4 5% 4 6% 8% 40%	1980
October	1st to 10th	Day	20% 6% 3 7% 6% 5% 5% 4 5% 7% 33%	1877
	11th to 20th	Day	17% 9% 2 8% 7% 6% 5% 4 7% 7% 28%	1746
	21st to end	Day	26% 10% 3 7% 4 4 4 3 5% 5% 29%	1782
November	1st to 10th	Day	18% 10% 3 7% 4 4 4 4 5% 7% 32%	1671
	11th to 20th	Day	18% 11% 4 7% 5% 5% 6% 5% 5% 6% 28%	1653
	21st to end	Day	22% 12% 4 7% 5% 5% 5% 4 5% 7% 24%	1520
December	1st to 10th	Day	24% 16% 6% 9% 6% 5% 4 3 4 4 19%	1520
	11th to 20th	Day	22% 15% 4 8% 4 3 4 3 6% 6% 25%	1906
	21st to end	Day	27% 15% 5% 10% 6% 5% 3 3 4 3 19%	2772
January	1st to 10th	Night	17% 17% 6% 10% 6% 5% 5% 4 4 4 23%	4429
	11th to 20th	Night	12% 14% 5% 13% 7% 7% 5% 4 6% 6% 21%	4440
	21st to end	Night	18% 17% 5% 10% 7% 5% 4 3 4 4 24%	4884
February	1st to 10th	Night	15% 16% 6% 12% 7% 5% 5% 4 5% 5% 21%	2527
	11th to 20th	Night	15% 12% 4 10% 6% 6% 5% 3 5% 6% 29%	2288
	21st to end	Night	16% 15% 3 9% 5% 4 5% 2 4 5% 31%	1884
March	1st to 10th	Night	21% 11% 3 7% 5% 4 4 3 4 5% 34%	3724
	11th to 20th	Night	18% 19% 5% 8% 4 5% 3 3 3 5% 28%	3685
	21st to end	Night	13% 10% 4 9% 6% 4 4 3 4 5% 38%	3466
April	1st to 10th	Night	23% 10% 3 7% 4 4 4 3 4 7% 30%	1900
	11th to 20th	Night	22% 8% 4 7% 4 4 4 3 4 6% 33%	1875
	21st to end	Night	21% 10% 3 7% 4 4 4 4 4 5% 35%	1731
May	1st to 10th	Night	21% 10% 3 6% 5% 5% 5% 3 6% 6% 30%	2642
	11th to 20th	Night	23% 8% 3 7% 4 4 3 3 6% 6% 34%	2495
	21st to end	Night	15% 8% 3 7% 5% 4 5% 3 6% 8% 37%	2738
June	1st to 10th	Night	21% 9% 3 7% 4 4 3 4 5% 7% 32%	1520
	11th to 20th	Night	16% 5% 3 7% 3 6% 4 5% 6% 7% 38%	1520
	21st to end	Night	18% 7% 4 8% 5% 4 6% 4 5% 7% 33%	1520
July	1st to 10th	Night	14% 8% 3 8% 5% 6% 4 5% 7% 7% 34%	2520
	11th to 20th	Night	13% 6% 3 6% 4 5% 5% 4 7% 8% 41%	2520
	21st to end	Night	11% 5% 2 6% 5% 5% 5% 4 7% 8% 42%	2739
August	1st to 10th	Night	13% 6% 2 6% 4 5% 5% 4 6% 9% 40%	1512
	11th to 20th	Night	13% 5% 2 7% 4 4 5% 4 6% 7% 41%	1682
	21st to end	Night	13% 6% 2 6% 5% 4 5% 4 5% 8% 41%	2080
September	1st to 10th	Night	12% 6% 2 8% 4 3 4 4 6% 6% 43%	1800
	11th to 20th	Night	16% 6% 2 5% 3 3 2 3 5% 7% 49%	1888
	21st to end	Night	16% 7% 2 5% 4 4 3 3 5% 8% 42%	1980
October	1st to 10th	Night	17% 7% 3 6% 5% 4 4 4 5% 6% 38%	2078
	11th to 20th	Night	21% 8% 3 8% 5% 5% 5% 3 5% 5% 32%	2213
	21st to end	Night	24% 9% 4 8% 4 5% 5% 3 4 5% 30%	2574
November	1st to 10th	Night	19% 10% 3 8% 4 5% 4 3 5% 6% 32%	2407
	11th to 20th	Night	20% 10% 3 8% 4 4 5% 3 5% 7% 31%	2524
	21st to end	Night	23% 14% 4 8% 5% 4 4 3 4 5% 26%	2660
December	1st to 10th	Night	22% 17% 6% 10% 5% 4 4 4 4 5% 19%	2660
	11th to 20th	Night	23% 15% 4 9% 6% 4 4 3 4 4 25%	3288
	21st to end	Night	27% 16% 6% 10% 4 3 3 2 4 4 21%	4884

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	66% 5% 12% 14% 3	2518
	11th to 20th	Day	58% 6% 12% 17% 7%	2520
	21st to end	Day	62% 5% 13% 15% 5%	2772
February	1st to 10th	Day	62% 4 8% 18% 6% 1	1652
	11th to 20th	Day	59% 4 12% 16% 9% 1	1785
	21st to end	Day	60% 3 11% 15% 9% 2	1570
March	1st to 10th	Day	60% 4 9% 18% 7% 1	3119
	11th to 20th	Day	59% 4 12% 13% 10% 2	3269
	21st to end	Day	48% 4 8% 21% 16% 2	4166
April	1st to 10th	Day	57% 3 8% 19% 11% 2	2280
	11th to 20th	Day	50% 3 9% 23% 14% 2	2254
	21st to end	Day	53% 4 9% 19% 12% 3	2448
May	1st to 10th	Day	52% 7% 9% 22% 9% 1	4341
	11th to 20th	Day	50% 6% 15% 19% 9% 2	4402
	21st to end	Day	43% 6% 12% 24% 13% 2	4837
June	1st to 10th	Day	51% 6% 10% 20% 12% 2	2660
	11th to 20th	Day	42% 5% 12% 24% 14% 3	2660
	21st to end	Day	51% 5% 13% 20% 10% 1	2660
July	1st to 10th	Day	43% 5% 14% 29% 9%	4370
	11th to 20th	Day	41% 5% 16% 30% 7%	4365
	21st to end	Day	38% 6% 14% 34% 9%	4661
August	1st to 10th	Day	43% 6% 17% 26% 8%	2632
	11th to 20th	Day	39% 6% 18% 26% 11%	2415
	21st to end	Day	40% 5% 16% 26% 13%	2486
September	1st to 10th	Day	38% 6% 15% 28% 12%	2160
	11th to 20th	Day	36% 4 12% 34% 14% 1	2071
	21st to end	Day	42% 5% 15% 27% 11% 1	1980
October	1st to 10th	Day	50% 6% 12% 22% 10%	1877
	11th to 20th	Day	54% 5% 14% 19% 7%	1746
	21st to end	Day	58% 4 11% 18% 8% 1	1782
November	1st to 10th	Day	51% 5% 13% 21% 9% 1	1671
	11th to 20th	Day	56% 6% 10% 16% 11%	1653
	21st to end	Day	60% 7% 12% 14% 6%	1520
December	1st to 10th	Day	70% 5% 9% 12% 4	1520
	11th to 20th	Day	60% 5% 11% 17% 6%	1906
	21st to end	Day	70% 4 12% 9% 5%	2772
January	1st to 10th	Night	64% 4 15% 12% 4	4429
	11th to 20th	Night	61% 7% 12% 14% 5% 1	4440
	21st to end	Night	65% 4 11% 13% 6%	4884
February	1st to 10th	Night	65% 4 9% 15% 6% 1	2527
	11th to 20th	Night	58% 3 11% 19% 9% 1	2290
	21st to end	Night	56% 3 10% 17% 13% 1	1884
March	1st to 10th	Night	54% 4 10% 20% 9% 3	3734
	11th to 20th	Night	61% 4 8% 13% 13% 2	3686
	21st to end	Night	50% 3 6% 24% 15% 3	3466
April	1st to 10th	Night	55% 4 8% 19% 13% 2	1900
	11th to 20th	Night	52% 4 7% 21% 14% 2	1875
	21st to end	Night	52% 4 7% 20% 14% 4	1731
May	1st to 10th	Night	54% 5% 11% 19% 8% 3	2648
	11th to 20th	Night	50% 5% 12% 20% 11% 2	2496
	21st to end	Night	45% 5% 9% 21% 18% 2	2739
June	1st to 10th	Night	52% 5% 8% 22% 12% 1	1520
	11th to 20th	Night	44% 6% 12% 21% 14% 3	1520
	21st to end	Night	51% 5% 11% 18% 14% 1	1520
July	1st to 10th	Night	47% 5% 14% 21% 13% 1	2480
	11th to 20th	Night	40% 5% 17% 28% 10%	2472
	21st to end	Night	40% 6% 18% 24% 12%	2651
August	1st to 10th	Night	41% 6% 16% 26% 10%	1502
	11th to 20th	Night	40% 6% 17% 25% 12%	1682
	21st to end	Night	40% 6% 13% 27% 13% 1	2080
September	1st to 10th	Night	40% 5% 12% 26% 15% 1	1800
	11th to 20th	Night	37% 4 13% 32% 13% 1	1888
	21st to end	Night	42% 6% 10% 26% 15%	1980
October	1st to 10th	Night	46% 5% 10% 25% 13% 1	2078
	11th to 20th	Night	54% 4 10% 24% 7%	2213
	21st to end	Night	58% 4 12% 16% 9% 1	2574
November	1st to 10th	Night	53% 5% 11% 22% 8% 1	2407
	11th to 20th	Night	54% 5% 11% 18% 10% 1	2524
	21st to end	Night	61% 5% 11% 17% 6%	2660
December	1st to 10th	Night	68% 5% 11% 13% 4	2660
	11th to 20th	Night	63% 4 9% 16% 7%	3288
	21st to end	Night	68% 3 10% 14% 4	4884

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% 26% 20% 8% 1	180
	11th to 20th	Day	28% 17% 32% 17% 6% 1	180
	21st to end	Day	21% 16% 31% 21% 10% 1	198
February	1st to 10th	Day	31% 17% 36% 14% 1	243
	11th to 20th	Day	23% 17% 38% 19% 3 1	384
	21st to end	Day	28% 16% 29% 24% 3	382
March	1st to 10th	Day	25% 12% 33% 22% 7%	460
	11th to 20th	Day	22% 17% 33% 20% 7% 1	460
	21st to end	Day	27% 15% 30% 20% 7% 1	506
April	1st to 10th	Day	30% 14% 28% 18% 8% 2	460
	11th to 20th	Day	30% 19% 23% 15% 10% 3	452
	21st to end	Day	29% 13% 29% 18% 10% 2	460
May	1st to 10th	Day	24% 13% 30% 21% 10% 2	463
	11th to 20th	Day	17% 11% 31% 25% 12% 4	487
	21st to end	Day	19% 13% 32% 23% 11% 2	529
June	1st to 10th	Day	23% 12% 26% 27% 10% 2	490
	11th to 20th	Day	16% 11% 27% 29% 14% 2	490
	21st to end	Day	23% 11% 27% 23% 14% 2	490
July	1st to 10th	Day	12% 9% 25% 31% 18% 5%	485
	11th to 20th	Day	14% 8% 28% 31% 14% 4	484
	21st to end	Day	15% 6% 27% 32% 19% 2	532
August	1st to 10th	Day	15% 5% 24% 30% 20% 5%	486
	11th to 20th	Day	13% 5% 23% 32% 18% 9%	479
	21st to end	Day	16% 9% 23% 26% 17% 10%	532
September	1st to 10th	Day	13% 5% 30% 27% 16% 9%	450
	11th to 20th	Day	17% 8% 22% 27% 16% 10%	450
	21st to end	Day	17% 9% 27% 23% 17% 7%	450
October	1st to 10th	Day	22% 11% 27% 22% 14% 4	450
	11th to 20th	Day	23% 14% 29% 21% 11% 2	423
	21st to end	Day	35% 13% 24% 22% 6%	396
November	1st to 10th	Day	27% 14% 27% 24% 8% 1	365
	11th to 20th	Day	26% 19% 31% 16% 7% 2	313
	21st to end	Day	31% 9% 36% 20% 4 1	180
December	1st to 10th	Day	36% 10% 22% 24% 7% 2	180
	11th to 20th	Day	28% 21% 36% 11% 5%	180
	21st to end	Day	30% 21% 27% 17% 4 2	198
January	1st to 10th	Night	29% 12% 31% 24% 5%	760
	11th to 20th	Night	26% 11% 34% 22% 7%	760
	21st to end	Night	22% 10% 33% 27% 7% 1	836
February	1st to 10th	Night	25% 9% 35% 26% 5% 1	760
	11th to 20th	Night	22% 13% 37% 23% 5% 1	743
	21st to end	Night	25% 11% 31% 29% 4	628
March	1st to 10th	Night	25% 10% 32% 26% 6%	760
	11th to 20th	Night	27% 11% 33% 22% 7%	760
	21st to end	Night	24% 9% 32% 27% 8% 1	836
April	1st to 10th	Night	29% 11% 28% 27% 5%	760
	11th to 20th	Night	27% 14% 28% 24% 7% 1	751
	21st to end	Night	28% 11% 28% 25% 8%	591
May	1st to 10th	Night	26% 7% 34% 25% 7% 2	456
	11th to 20th	Night	22% 7% 32% 30% 8% 1	374
	21st to end	Night	19% 9% 33% 30% 9%	411
June	1st to 10th	Night	24% 6% 27% 31% 11% 1	380
	11th to 20th	Night	18% 5% 29% 34% 13%	380
	21st to end	Night	19% 5% 29% 32% 13% 1	380
July	1st to 10th	Night	14% 6% 27% 37% 16%	370
	11th to 20th	Night	13% 2 25% 45% 14% 2	368
	21st to end	Night	13% 3 24% 44% 16% 1	416
August	1st to 10th	Night	16% 3 16% 46% 18% 2	378
	11th to 20th	Night	14% 4 21% 36% 23% 2	558
	21st to end	Night	16% 4 21% 37% 20% 2	832
September	1st to 10th	Night	15% 4 25% 38% 16% 1	720
	11th to 20th	Night	18% 6% 20% 35% 19% 3	720
	21st to end	Night	20% 7% 26% 30% 15% 3	720
October	1st to 10th	Night	21% 9% 29% 28% 13% 1	720
	11th to 20th	Night	27% 8% 29% 27% 9%	720
	21st to end	Night	29% 11% 30% 25% 5%	792
November	1st to 10th	Night	25% 12% 32% 22% 8% 1	740
	11th to 20th	Night	27% 13% 30% 21% 9% 1	760
	21st to end	Night	30% 15% 27% 20% 8% 1	760
December	1st to 10th	Night	31% 13% 29% 22% 5%	760
	11th to 20th	Night	31% 13% 31% 18% 6% 1	760
	21st to end	Night	33% 14% 30% 17% 5%	836

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