Observatoire astronomique de Châteauguay
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Observatoire astronomique de Châteauguay 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	17% 10% 3 8% 5% 5% 5% 4 5% 7% 31%	132183
	Night	17% 12% 4 9% 6% 5% 4 3 5% 6% 29%	130987
Transparency	Day	50% 7% 13% 20% 9% 1	135628
	Night	54% 6% 11% 18% 10% 1	135629
Seeing	Day	21% 13% 29% 23% 11% 3	16508
	Night	22% 11% 30% 27% 9% 1	25542

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% 18% 7% 13% 6% 6% 5% 3 4 5% 19%	4661
	11th to 20th	Day	12% 14% 6% 14% 7% 7% 5% 4 5% 5% 21%	4650
	21st to end	Day	11% 13% 4 11% 8% 8% 6% 4 5% 6% 23%	5124
February	1st to 10th	Day	14% 15% 4 12% 7% 6% 5% 3 4 5% 26%	1848
	11th to 20th	Day	14% 12% 3 11% 7% 6% 6% 4 5% 6% 27%	1957
	21st to end	Day	17% 15% 4 9% 7% 5% 4 3 5% 5% 26%	1730
March	1st to 10th	Day	18% 12% 4 10% 6% 5% 5% 4 5% 6% 25%	4576
	11th to 20th	Day	12% 13% 4 9% 6% 5% 5% 4 6% 7% 30%	4797
	21st to end	Day	18% 9% 3 9% 5% 4 4 3 4 5% 34%	6101
April	1st to 10th	Day	21% 11% 4 7% 4 4 4 3 5% 5% 32%	2520
	11th to 20th	Day	19% 10% 3 7% 4 3 4 3 4 6% 36%	2494
	21st to end	Day	21% 9% 3 6% 5% 4 4 4 5% 6% 33%	2687
May	1st to 10th	Day	26% 11% 3 10% 5% 4 5% 3 5% 6% 23%	6383
	11th to 20th	Day	20% 8% 3 8% 5% 4 4 4 6% 8% 30%	6458
	21st to end	Day	21% 8% 2 7% 4 4 5% 4 6% 7% 32%	7103
June	1st to 10th	Day	21% 8% 2 6% 5% 4 5% 4 6% 7% 32%	2800
	11th to 20th	Day	15% 6% 2 6% 5% 4 5% 4 5% 8% 39%	2800
	21st to end	Day	19% 7% 3 8% 5% 4 5% 4 6% 7% 33%	2800
July	1st to 10th	Day	11% 7% 3 8% 6% 6% 6% 4 7% 8% 34%	6360
	11th to 20th	Day	10% 5% 2 6% 5% 5% 4 4 7% 9% 42%	6360
	21st to end	Day	11% 6% 2 5% 4 4 4 4 6% 9% 45%	6926
August	1st to 10th	Day	14% 7% 3 7% 5% 4 5% 4 6% 9% 34%	4563
	11th to 20th	Day	13% 6% 3 7% 4 4 5% 3 6% 8% 40%	4112
	21st to end	Day	15% 5% 2 5% 4 4 5% 3 6% 9% 41%	4312
September	1st to 10th	Day	11% 5% 2 6% 4 4 5% 5% 6% 8% 45%	2400
	11th to 20th	Day	13% 6% 2 3 3 4 4 3 5% 7% 50%	2314
	21st to end	Day	16% 6% 2 6% 4 4 4 4 6% 7% 41%	2200
October	1st to 10th	Day	18% 8% 3 7% 5% 4 4 3 5% 6% 36%	2068
	11th to 20th	Day	17% 8% 4 7% 6% 5% 5% 4 6% 6% 31%	1958
	21st to end	Day	27% 10% 3 6% 4 4 4 3 4 5% 29%	1980
November	1st to 10th	Day	18% 12% 3 7% 5% 3 5% 4 5% 6% 31%	1851
	11th to 20th	Day	17% 12% 4 9% 5% 5% 5% 3 5% 5% 28%	1816
	21st to end	Day	22% 14% 4 10% 6% 4 5% 4 4 5% 21%	1680
December	1st to 10th	Day	20% 20% 7% 12% 6% 4 4 4 5% 4 16%	2680
	11th to 20th	Day	18% 13% 5% 10% 6% 5% 7% 5% 6% 4 22%	3066
	21st to end	Day	22% 13% 4 10% 6% 6% 6% 4 6% 5% 18%	4048
January	1st to 10th	Night	13% 20% 8% 14% 6% 4 4 3 4 4 20%	8234
	11th to 20th	Night	12% 16% 6% 17% 8% 7% 5% 4 5% 5% 15%	8235
	21st to end	Night	11% 14% 5% 12% 9% 6% 5% 3 4 5% 25%	9057
February	1st to 10th	Night	14% 16% 6% 12% 8% 5% 4 3 5% 5% 22%	2767
	11th to 20th	Night	15% 12% 4 10% 6% 6% 5% 4 5% 6% 27%	2548
	21st to end	Night	17% 13% 4 10% 6% 5% 3 3 4 4 32%	2076
March	1st to 10th	Night	20% 12% 3 8% 5% 5% 4 3 4 5% 30%	5484
	11th to 20th	Night	15% 14% 4 8% 5% 5% 4 4 5% 7% 30%	5378
	21st to end	Night	15% 12% 4 10% 6% 5% 4 3 5% 5% 31%	5072
April	1st to 10th	Night	24% 10% 3 7% 4 4 3 3 5% 6% 29%	2100
	11th to 20th	Night	24% 10% 3 6% 4 4 4 3 4 6% 32%	2075
	21st to end	Night	21% 9% 3 7% 4 4 4 3 4 5% 35%	1932
May	1st to 10th	Night	26% 10% 3 9% 5% 4 4 3 5% 7% 24%	3820
	11th to 20th	Night	23% 8% 3 8% 5% 4 4 3 5% 6% 31%	3655
	21st to end	Night	18% 9% 4 8% 5% 5% 4 4 5% 6% 32%	4014
June	1st to 10th	Night	22% 7% 3 7% 4 4 4 3 7% 8% 30%	1600
	11th to 20th	Night	17% 5% 2 7% 3 4 4 4 6% 7% 39%	1600
	21st to end	Night	19% 6% 2 7% 5% 5% 5% 5% 5% 7% 33%	1600
July	1st to 10th	Night	14% 7% 3 7% 5% 5% 5% 5% 7% 8% 35%	3600
	11th to 20th	Night	14% 6% 2 6% 4 5% 5% 4 6% 8% 41%	3600
	21st to end	Night	10% 6% 3 7% 5% 5% 5% 4 6% 8% 43%	3926
August	1st to 10th	Night	16% 7% 3 8% 4 4 4 3 5% 7% 38%	2586
	11th to 20th	Night	15% 6% 3 8% 5% 5% 4 4 5% 7% 38%	2898
	21st to end	Night	16% 5% 2 6% 4 4 5% 4 5% 8% 41%	3586
September	1st to 10th	Night	13% 5% 2 7% 5% 4 4 4 6% 6% 45%	2000
	11th to 20th	Night	15% 5% 2 6% 4 3 4 2 6% 6% 49%	2085
	21st to end	Night	17% 7% 2 6% 4 4 4 3 5% 7% 41%	2200
October	1st to 10th	Night	18% 7% 2 6% 4 4 5% 3 5% 7% 38%	2330
	11th to 20th	Night	21% 7% 3 8% 4 4 5% 3 5% 6% 35%	2441
	21st to end	Night	23% 10% 3 8% 4 5% 5% 3 5% 6% 29%	2860
November	1st to 10th	Night	21% 10% 3 8% 4 4 4 4 6% 6% 30%	2667
	11th to 20th	Night	19% 11% 4 8% 6% 4 4 3 4 5% 32%	2803
	21st to end	Night	22% 13% 4 10% 6% 4 5% 3 4 5% 23%	2940
December	1st to 10th	Night	18% 20% 6% 11% 7% 6% 4 4 4 4 16%	4720
	11th to 20th	Night	22% 14% 5% 10% 6% 4 4 3 4 6% 23%	5348
	21st to end	Night	24% 16% 4 9% 4 4 4 4 6% 5% 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	65% 7% 12% 14% 3	4662
	11th to 20th	Day	62% 7% 9% 13% 9% 1	4680
	21st to end	Day	59% 8% 10% 12% 11% 1	5124
February	1st to 10th	Day	60% 5% 9% 19% 5% 2	1848
	11th to 20th	Day	56% 7% 12% 17% 7% 1	1966
	21st to end	Day	59% 6% 10% 15% 9% 1	1730
March	1st to 10th	Day	58% 7% 9% 15% 9% 3	4579
	11th to 20th	Day	50% 8% 17% 14% 10% 1	4797
	21st to end	Day	51% 6% 9% 17% 16% 1	6101
April	1st to 10th	Day	53% 6% 8% 19% 11% 2	2520
	11th to 20th	Day	48% 6% 9% 21% 15% 1	2494
	21st to end	Day	51% 7% 9% 18% 12% 3	2687
May	1st to 10th	Day	60% 8% 8% 16% 7% 1	6394
	11th to 20th	Day	50% 8% 13% 18% 9% 1	6462
	21st to end	Day	49% 8% 12% 19% 11% 1	7103
June	1st to 10th	Day	49% 8% 11% 18% 12% 1	2800
	11th to 20th	Day	42% 7% 11% 24% 13% 3	2800
	21st to end	Day	49% 8% 12% 20% 10% 1	2800
July	1st to 10th	Day	45% 7% 14% 25% 9%	6290
	11th to 20th	Day	37% 7% 14% 29% 12%	6285
	21st to end	Day	35% 7% 14% 36% 9%	6728
August	1st to 10th	Day	44% 8% 17% 23% 8%	4549
	11th to 20th	Day	40% 7% 18% 27% 7%	4154
	21st to end	Day	39% 7% 18% 21% 14%	4312
September	1st to 10th	Day	35% 8% 15% 30% 12%	2400
	11th to 20th	Day	34% 7% 13% 32% 14% 1	2314
	21st to end	Day	41% 8% 14% 26% 11% 1	2200
October	1st to 10th	Day	44% 6% 15% 27% 9%	3367
	11th to 20th	Day	43% 6% 19% 26% 6%	3161
	21st to end	Day	46% 5% 19% 22% 8%	3180
November	1st to 10th	Day	51% 7% 14% 19% 8% 1	1851
	11th to 20th	Day	55% 7% 11% 16% 11%	1816
	21st to end	Day	61% 10% 12% 13% 4	1680
December	1st to 10th	Day	70% 7% 9% 12% 2	2680
	11th to 20th	Day	60% 8% 11% 14% 7%	3066
	21st to end	Day	64% 8% 10% 12% 6%	4048
January	1st to 10th	Night	67% 6% 10% 12% 5%	8234
	11th to 20th	Night	67% 7% 9% 10% 6%	8280
	21st to end	Night	60% 6% 9% 13% 12%	9057
February	1st to 10th	Night	63% 5% 10% 15% 6% 1	2767
	11th to 20th	Night	55% 6% 13% 17% 8%	2550
	21st to end	Night	56% 5% 9% 18% 12% 1	2076
March	1st to 10th	Night	57% 5% 7% 16% 11% 4	5494
	11th to 20th	Night	52% 6% 14% 16% 11% 1	5379
	21st to end	Night	53% 5% 8% 21% 11% 2	5072
April	1st to 10th	Night	54% 6% 8% 18% 12% 2	2100
	11th to 20th	Night	53% 6% 9% 19% 12% 2	2075
	21st to end	Night	51% 6% 8% 18% 14% 4	1932
May	1st to 10th	Night	59% 7% 10% 16% 7% 2	3826
	11th to 20th	Night	52% 7% 10% 19% 10% 1	3656
	21st to end	Night	51% 7% 10% 17% 14% 1	4015
June	1st to 10th	Night	49% 8% 12% 19% 12% 1	1600
	11th to 20th	Night	42% 7% 14% 21% 14% 3	1600
	21st to end	Night	48% 7% 13% 18% 12% 1	1600
July	1st to 10th	Night	44% 7% 18% 21% 10%	3560
	11th to 20th	Night	39% 7% 16% 26% 12%	3552
	21st to end	Night	38% 7% 16% 26% 13%	3809
August	1st to 10th	Night	45% 7% 13% 26% 8%	2576
	11th to 20th	Night	44% 7% 18% 21% 9%	2922
	21st to end	Night	40% 7% 15% 22% 14% 1	3586
September	1st to 10th	Night	37% 8% 12% 27% 15% 1	2000
	11th to 20th	Night	36% 6% 14% 31% 13% 1	2085
	21st to end	Night	42% 7% 11% 24% 15%	2200
October	1st to 10th	Night	43% 7% 14% 26% 10% 1	3811
	11th to 20th	Night	44% 6% 13% 28% 9%	3977
	21st to end	Night	48% 5% 16% 19% 11% 1	4610
November	1st to 10th	Night	53% 6% 12% 20% 8% 1	2667
	11th to 20th	Night	54% 6% 12% 18% 9% 1	2803
	21st to end	Night	61% 7% 11% 15% 5%	2940
December	1st to 10th	Night	71% 5% 11% 10% 3	4720
	11th to 20th	Night	62% 5% 9% 13% 10%	5348
	21st to end	Night	63% 5% 9% 13% 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	26% 18% 28% 18% 9% 1	220
	11th to 20th	Day	26% 20% 34% 17% 3 1	220
	21st to end	Day	20% 16% 35% 22% 6% 1	242
February	1st to 10th	Day	25% 23% 37% 13% 3	308
	11th to 20th	Day	23% 19% 38% 18% 2	445
	21st to end	Day	25% 17% 32% 24% 2	446
March	1st to 10th	Day	25% 16% 35% 19% 6%	540
	11th to 20th	Day	21% 20% 34% 18% 5% 1	540
	21st to end	Day	27% 17% 31% 16% 8% 1	594
April	1st to 10th	Day	28% 19% 28% 17% 7% 1	540
	11th to 20th	Day	29% 19% 26% 15% 9% 2	532
	21st to end	Day	25% 16% 31% 18% 9% 2	540
May	1st to 10th	Day	26% 12% 32% 20% 8% 1	543
	11th to 20th	Day	18% 13% 33% 22% 12% 3	567
	21st to end	Day	20% 13% 31% 24% 11% 2	617
June	1st to 10th	Day	22% 12% 25% 29% 9% 2	530
	11th to 20th	Day	15% 12% 29% 29% 14% 2	530
	21st to end	Day	21% 12% 30% 21% 13% 3	530
July	1st to 10th	Day	12% 8% 30% 28% 19% 4	525
	11th to 20th	Day	13% 7% 32% 28% 16% 4	524
	21st to end	Day	12% 8% 28% 32% 17% 3	576
August	1st to 10th	Day	13% 6% 24% 33% 19% 6%	526
	11th to 20th	Day	11% 8% 21% 34% 18% 9%	519
	21st to end	Day	13% 10% 26% 26% 17% 8%	583
September	1st to 10th	Day	12% 9% 27% 27% 18% 8%	530
	11th to 20th	Day	14% 9% 17% 25% 22% 12%	530
	21st to end	Day	16% 10% 28% 23% 17% 6%	530
October	1st to 10th	Day	19% 11% 29% 21% 15% 5%	530
	11th to 20th	Day	19% 16% 31% 19% 12% 1	506
	21st to end	Day	33% 12% 26% 22% 7%	462
November	1st to 10th	Day	29% 16% 25% 22% 7% 1	425
	11th to 20th	Day	23% 21% 31% 17% 6% 1	356
	21st to end	Day	30% 16% 31% 18% 4	220
December	1st to 10th	Day	32% 11% 25% 25% 5% 1	220
	11th to 20th	Day	25% 18% 41% 14% 2	220
	21st to end	Day	28% 19% 28% 19% 4 2	242
January	1st to 10th	Night	26% 14% 34% 22% 4	840
	11th to 20th	Night	27% 12% 35% 20% 5%	840
	21st to end	Night	20% 11% 37% 26% 6% 1	924
February	1st to 10th	Night	20% 13% 36% 26% 5%	840
	11th to 20th	Night	21% 13% 39% 24% 4	823
	21st to end	Night	23% 12% 35% 26% 4	692
March	1st to 10th	Night	25% 14% 33% 24% 5%	840
	11th to 20th	Night	26% 14% 33% 23% 5%	840
	21st to end	Night	23% 14% 31% 25% 7% 1	924
April	1st to 10th	Night	27% 13% 30% 25% 4	840
	11th to 20th	Night	28% 13% 31% 23% 6%	831
	21st to end	Night	25% 12% 30% 25% 7%	672
May	1st to 10th	Night	26% 8% 33% 24% 8% 1	488
	11th to 20th	Night	20% 8% 33% 31% 8% 1	414
	21st to end	Night	16% 12% 32% 29% 11%	455
June	1st to 10th	Night	24% 8% 27% 32% 9% 1	400
	11th to 20th	Night	18% 6% 29% 35% 12%	400
	21st to end	Night	18% 6% 30% 32% 14% 1	400
July	1st to 10th	Night	13% 5% 29% 37% 17%	390
	11th to 20th	Night	11% 3 27% 42% 16% 1	388
	21st to end	Night	12% 4 24% 42% 16% 1	438
August	1st to 10th	Night	14% 3 17% 47% 17% 1	398
	11th to 20th	Night	14% 3 23% 35% 23% 2	601
	21st to end	Night	16% 4 23% 36% 20% 1	880
September	1st to 10th	Night	13% 6% 26% 38% 17% 1	800
	11th to 20th	Night	16% 5% 19% 35% 22% 3	800
	21st to end	Night	19% 6% 29% 28% 17% 2	800
October	1st to 10th	Night	20% 8% 29% 27% 15% 1	800
	11th to 20th	Night	23% 10% 30% 28% 9%	800
	21st to end	Night	26% 12% 30% 28% 4	880
November	1st to 10th	Night	26% 14% 29% 24% 8%	820
	11th to 20th	Night	24% 14% 32% 21% 9%	840
	21st to end	Night	27% 16% 30% 20% 7%	840
December	1st to 10th	Night	31% 13% 32% 21% 4	840
	11th to 20th	Night	29% 16% 32% 16% 5% 1	840
	21st to end	Night	33% 14% 30% 18% 4 1	924

Page generated at 2026-05-29 01:53:26. Version ac07

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