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 2025-02-06.

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% 32%	108462
	Night	18% 12% 4 9% 5% 4 4 3 5% 6% 29%	109792
Transparency	Day	49% 8% 13% 21% 9% 1	111955
	Night	54% 7% 12% 18% 10% 1	114463
Seeing	Day	20% 13% 29% 23% 11% 3	14794
	Night	22% 10% 30% 27% 9% 1	23908

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% 19% 7% 12% 6% 6% 4 3 4 4 19%	3597
	11th to 20th	Day	13% 15% 7% 13% 6% 6% 5% 4 5% 5% 22%	3570
	21st to end	Day	13% 15% 5% 11% 6% 6% 5% 4 5% 6% 25%	3960
February	1st to 10th	Day	15% 16% 4 12% 7% 6% 4 2 4 5% 25%	1715
	11th to 20th	Day	14% 13% 3 11% 7% 5% 6% 4 5% 5% 27%	1765
	21st to end	Day	18% 15% 4 8% 6% 4 4 4 5% 5% 27%	1570
March	1st to 10th	Day	21% 12% 4 9% 5% 5% 5% 3 5% 5% 26%	3116
	11th to 20th	Day	14% 16% 4 11% 6% 5% 4 3 5% 5% 26%	3278
	21st to end	Day	15% 8% 3 9% 4 4 4 3 5% 6% 39%	4165
April	1st to 10th	Day	21% 11% 3 7% 5% 4 4 3 5% 5% 32%	2280
	11th to 20th	Day	19% 9% 3 7% 5% 3 4 4 4 6% 37%	2254
	21st to end	Day	22% 10% 3 6% 5% 4 4 4 4 6% 33%	2431
May	1st to 10th	Day	21% 9% 3 9% 5% 5% 5% 4 5% 6% 29%	4353
	11th to 20th	Day	18% 8% 3 8% 5% 4 4 4 6% 8% 33%	4398
	21st to end	Day	14% 6% 2 7% 5% 4 5% 4 7% 9% 36%	4837
June	1st to 10th	Day	22% 7% 2 5% 5% 4 5% 4 6% 8% 31%	2660
	11th to 20th	Day	15% 6% 2 6% 5% 4 5% 4 5% 8% 39%	2660
	21st to end	Day	20% 7% 3 8% 5% 4 5% 4 5% 7% 33%	2660
July	1st to 10th	Day	11% 6% 2 7% 5% 5% 5% 4 6% 9% 38%	4440
	11th to 20th	Day	12% 6% 2 6% 5% 5% 5% 5% 8% 9% 39%	4440
	21st to end	Day	12% 6% 2 5% 4 4 5% 4 6% 8% 44%	4814
August	1st to 10th	Day	14% 8% 3 8% 5% 4 5% 4 6% 9% 33%	4423
	11th to 20th	Day	13% 6% 3 7% 4 4 5% 3 6% 8% 40%	3984
	21st to end	Day	15% 5% 2 5% 4 4 5% 3 6% 9% 41%	4180
September	1st to 10th	Day	11% 5% 2 6% 4 4 5% 5% 6% 8% 44%	2280
	11th to 20th	Day	14% 6% 2 3 3 4 4 3 5% 7% 49%	2199
	21st to end	Day	16% 6% 2 6% 4 4 4 4 6% 7% 40%	2090
October	1st to 10th	Day	19% 8% 3 7% 6% 4 5% 3 5% 6% 34%	1965
	11th to 20th	Day	18% 8% 4 7% 6% 5% 5% 4 6% 6% 31%	1860
	21st to end	Day	27% 10% 3 6% 4 4 4 3 4 6% 30%	1881
November	1st to 10th	Day	18% 12% 3 7% 5% 3 5% 4 5% 6% 32%	1761
	11th to 20th	Day	17% 12% 4 9% 5% 5% 5% 3 6% 6% 29%	1730
	21st to end	Day	22% 14% 4 10% 6% 4 5% 4 5% 6% 22%	1600
December	1st to 10th	Day	20% 20% 7% 12% 6% 4 4 4 5% 4 16%	2600
	11th to 20th	Day	18% 13% 5% 10% 6% 5% 7% 5% 6% 4 22%	2986
	21st to end	Day	23% 13% 4 10% 6% 6% 5% 4 6% 5% 19%	3960
January	1st to 10th	Night	14% 22% 8% 14% 6% 4 4 3 4 4 18%	6349
	11th to 20th	Night	10% 16% 7% 16% 8% 7% 5% 4 5% 6% 16%	6315
	21st to end	Night	14% 17% 6% 12% 7% 5% 5% 3 4 5% 24%	6996
February	1st to 10th	Night	15% 16% 6% 12% 8% 5% 4 3 5% 5% 21%	2571
	11th to 20th	Night	15% 12% 4 10% 6% 6% 5% 4 5% 6% 28%	2300
	21st to end	Night	17% 13% 4 10% 5% 5% 3 3 4 4 32%	1884
March	1st to 10th	Night	21% 11% 3 7% 5% 4 4 3 4 5% 33%	3724
	11th to 20th	Night	19% 18% 4 8% 5% 4 3 3 4 4 27%	3678
	21st to end	Night	13% 11% 4 8% 5% 4 4 3 5% 4 38%	3466
April	1st to 10th	Night	24% 10% 3 7% 5% 5% 3 3 6% 6% 30%	1900
	11th to 20th	Night	23% 10% 2 6% 4 4 4 3 4 6% 33%	1875
	21st to end	Night	22% 9% 3 7% 4 4 4 3 4 5% 35%	1748
May	1st to 10th	Night	22% 10% 2 8% 4 4 4 3 5% 7% 30%	2615
	11th to 20th	Night	23% 8% 3 6% 4 4 4 3 5% 5% 36%	2495
	21st to end	Night	16% 8% 3 7% 4 5% 4 4 5% 8% 37%	2738
June	1st to 10th	Night	22% 7% 3 6% 4 4 4 3 7% 8% 31%	1520
	11th to 20th	Night	17% 5% 2 7% 3 4 4 4 6% 7% 40%	1520
	21st to end	Night	19% 6% 3 7% 5% 4 5% 5% 5% 7% 33%	1520
July	1st to 10th	Night	15% 7% 3 7% 6% 5% 5% 4 6% 7% 35%	2520
	11th to 20th	Night	14% 6% 2 5% 4 5% 5% 4 6% 8% 41%	2520
	21st to end	Night	12% 6% 3 7% 5% 4 4 4 6% 8% 42%	2739
August	1st to 10th	Night	17% 7% 3 8% 5% 4 4 3 5% 7% 38%	2506
	11th to 20th	Night	15% 7% 3 8% 5% 5% 5% 4 5% 7% 37%	2807
	21st to end	Night	16% 5% 2 6% 4 4 5% 4 5% 8% 41%	3476
September	1st to 10th	Night	13% 5% 2 7% 5% 4 4 4 6% 5% 45%	1900
	11th to 20th	Night	15% 5% 2 6% 3 3 4 2 6% 6% 49%	1981
	21st to end	Night	17% 6% 2 6% 5% 4 4 3 5% 7% 41%	2090
October	1st to 10th	Night	19% 7% 2 7% 4 4 5% 3 5% 7% 37%	2213
	11th to 20th	Night	22% 7% 3 8% 4 4 5% 3 5% 6% 34%	2319
	21st to end	Night	23% 11% 3 8% 4 5% 5% 2 5% 6% 29%	2717
November	1st to 10th	Night	20% 10% 3 8% 5% 4 4 4 6% 6% 30%	2537
	11th to 20th	Night	19% 10% 4 8% 6% 4 4 3 4 5% 32%	2669
	21st to end	Night	22% 13% 4 10% 5% 4 5% 3 4 5% 23%	2800
December	1st to 10th	Night	18% 20% 6% 12% 6% 6% 4 4 4 4 16%	4580
	11th to 20th	Night	22% 14% 5% 10% 5% 4 3 3 4 6% 23%	5208
	21st to end	Night	24% 16% 4 9% 4 4 4 4 6% 5% 20%	6996

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% 8% 11% 14% 2	3598
	11th to 20th	Day	61% 7% 11% 14% 6% 1	3600
	21st to end	Day	57% 9% 11% 13% 9% 1	3960
February	1st to 10th	Day	61% 6% 9% 18% 4 2	1715
	11th to 20th	Day	56% 7% 12% 17% 7% 1	1774
	21st to end	Day	58% 6% 11% 15% 9% 1	1570
March	1st to 10th	Day	58% 8% 9% 18% 6% 1	3119
	11th to 20th	Day	54% 9% 13% 13% 10% 1	3278
	21st to end	Day	44% 7% 11% 20% 16% 2	4165
April	1st to 10th	Day	53% 7% 8% 19% 11% 2	2280
	11th to 20th	Day	47% 7% 9% 22% 14% 2	2254
	21st to end	Day	51% 7% 10% 17% 12% 3	2431
May	1st to 10th	Day	51% 10% 9% 20% 9% 1	4367
	11th to 20th	Day	47% 10% 15% 18% 9% 1	4402
	21st to end	Day	40% 11% 13% 23% 12% 1	4837
June	1st to 10th	Day	49% 9% 11% 18% 11% 1	2660
	11th to 20th	Day	42% 7% 11% 24% 13% 3	2660
	21st to end	Day	50% 8% 12% 20% 9% 1	2660
July	1st to 10th	Day	41% 7% 14% 27% 10% 1	4370
	11th to 20th	Day	38% 8% 17% 30% 7%	4365
	21st to end	Day	37% 8% 14% 32% 10%	4661
August	1st to 10th	Day	45% 9% 16% 22% 8%	4409
	11th to 20th	Day	40% 7% 19% 27% 7%	4026
	21st to end	Day	39% 7% 18% 21% 15% 1	4180
September	1st to 10th	Day	36% 9% 15% 28% 12%	2280
	11th to 20th	Day	34% 7% 14% 31% 14% 1	2199
	21st to end	Day	40% 8% 15% 26% 10% 1	2090
October	1st to 10th	Day	45% 6% 15% 26% 8%	3264
	11th to 20th	Day	43% 7% 19% 25% 6%	3063
	21st to end	Day	45% 5% 20% 22% 8%	3081
November	1st to 10th	Day	50% 8% 14% 20% 8% 1	1761
	11th to 20th	Day	54% 7% 11% 16% 11% 1	1730
	21st to end	Day	61% 10% 12% 13% 4	1600
December	1st to 10th	Day	70% 7% 9% 12% 2	2600
	11th to 20th	Day	60% 8% 11% 14% 8%	2986
	21st to end	Day	64% 8% 10% 12% 6%	3960
January	1st to 10th	Night	69% 7% 10% 11% 3	6349
	11th to 20th	Night	65% 9% 11% 11% 4	6360
	21st to end	Night	61% 6% 10% 11% 11%	6996
February	1st to 10th	Night	64% 5% 10% 15% 5% 1	2571
	11th to 20th	Night	55% 7% 13% 17% 8%	2302
	21st to end	Night	56% 5% 9% 17% 12% 1	1884
March	1st to 10th	Night	53% 7% 9% 20% 9% 3	3734
	11th to 20th	Night	58% 7% 9% 14% 10% 1	3679
	21st to end	Night	46% 7% 9% 22% 14% 3	3466
April	1st to 10th	Night	54% 6% 9% 17% 13% 2	1900
	11th to 20th	Night	52% 6% 9% 19% 12% 2	1875
	21st to end	Night	51% 6% 8% 17% 13% 4	1748
May	1st to 10th	Night	52% 8% 11% 17% 9% 3	2621
	11th to 20th	Night	47% 9% 12% 19% 11% 2	2496
	21st to end	Night	44% 7% 11% 19% 18% 1	2739
June	1st to 10th	Night	49% 8% 12% 19% 11% 1	1520
	11th to 20th	Night	42% 7% 14% 21% 14% 3	1520
	21st to end	Night	49% 8% 13% 18% 12% 1	1520
July	1st to 10th	Night	46% 7% 14% 20% 13%	2480
	11th to 20th	Night	38% 8% 18% 27% 9%	2472
	21st to end	Night	39% 8% 18% 23% 13%	2651
August	1st to 10th	Night	46% 7% 13% 25% 8%	2496
	11th to 20th	Night	44% 8% 18% 21% 8%	2831
	21st to end	Night	40% 7% 15% 22% 14% 1	3476
September	1st to 10th	Night	38% 8% 12% 26% 16% 1	1900
	11th to 20th	Night	37% 6% 14% 31% 13% 1	1981
	21st to end	Night	42% 7% 12% 24% 15%	2090
October	1st to 10th	Night	44% 7% 14% 25% 9%	3694
	11th to 20th	Night	44% 6% 13% 28% 9%	3855
	21st to end	Night	48% 5% 16% 18% 11% 1	4467
November	1st to 10th	Night	53% 6% 12% 19% 8% 1	2537
	11th to 20th	Night	54% 6% 12% 18% 10% 1	2669
	21st to end	Night	61% 7% 11% 15% 5%	2800
December	1st to 10th	Night	71% 6% 11% 10% 3	4580
	11th to 20th	Night	62% 5% 9% 13% 10%	5208
	21st to end	Night	63% 5% 10% 13% 9%	6996

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	27% 19% 28% 18% 8% 1	200
	11th to 20th	Day	27% 18% 34% 18% 3 1	200
	21st to end	Day	21% 15% 35% 21% 6% 1	220
February	1st to 10th	Day	28% 22% 37% 12% 2	265
	11th to 20th	Day	22% 18% 39% 18% 2 1	373
	21st to end	Day	27% 17% 29% 25% 2	382
March	1st to 10th	Day	25% 15% 34% 20% 7%	460
	11th to 20th	Day	18% 22% 35% 18% 6% 1	460
	21st to end	Day	28% 15% 32% 16% 9% 1	506
April	1st to 10th	Day	27% 18% 28% 18% 8% 1	460
	11th to 20th	Day	27% 19% 27% 16% 9% 3	452
	21st to end	Day	25% 17% 28% 19% 9% 2	460
May	1st to 10th	Day	24% 11% 33% 21% 10% 2	463
	11th to 20th	Day	16% 12% 33% 23% 13% 3	487
	21st to end	Day	17% 13% 31% 26% 11% 2	529
June	1st to 10th	Day	23% 12% 24% 29% 10% 2	490
	11th to 20th	Day	14% 11% 28% 30% 14% 2	490
	21st to end	Day	22% 12% 29% 21% 13% 3	490
July	1st to 10th	Day	12% 8% 29% 28% 19% 4	485
	11th to 20th	Day	13% 7% 32% 29% 15% 4	484
	21st to end	Day	13% 7% 28% 32% 17% 3	532
August	1st to 10th	Day	14% 6% 24% 33% 18% 5%	486
	11th to 20th	Day	11% 8% 21% 34% 17% 9%	479
	21st to end	Day	12% 10% 25% 27% 17% 9%	539
September	1st to 10th	Day	12% 9% 27% 28% 16% 8%	490
	11th to 20th	Day	14% 9% 18% 25% 21% 12%	490
	21st to end	Day	16% 9% 28% 23% 18% 6%	490
October	1st to 10th	Day	19% 12% 30% 20% 15% 4	490
	11th to 20th	Day	20% 16% 30% 19% 13% 1	468
	21st to end	Day	33% 13% 25% 21% 7%	429
November	1st to 10th	Day	28% 15% 25% 23% 8% 1	395
	11th to 20th	Day	23% 22% 31% 17% 6% 1	330
	21st to end	Day	29% 17% 30% 20% 5% 1	200
December	1st to 10th	Day	33% 11% 24% 27% 5% 1	200
	11th to 20th	Day	25% 18% 41% 15% 3 1	200
	21st to end	Day	29% 19% 26% 19% 5% 2	220
January	1st to 10th	Night	26% 14% 34% 22% 4	800
	11th to 20th	Night	26% 11% 37% 21% 6%	800
	21st to end	Night	20% 10% 38% 26% 6% 1	880
February	1st to 10th	Night	21% 13% 36% 24% 5% 1	781
	11th to 20th	Night	20% 13% 39% 24% 4	743
	21st to end	Night	24% 11% 36% 25% 4	628
March	1st to 10th	Night	24% 13% 32% 26% 5%	760
	11th to 20th	Night	26% 13% 32% 23% 5%	760
	21st to end	Night	22% 12% 31% 27% 7% 1	836
April	1st to 10th	Night	27% 13% 30% 26% 5%	760
	11th to 20th	Night	27% 13% 30% 23% 6%	751
	21st to end	Night	25% 13% 30% 24% 8%	608
May	1st to 10th	Night	25% 8% 33% 25% 9% 1	442
	11th to 20th	Night	20% 7% 33% 31% 9% 1	374
	21st to end	Night	15% 11% 32% 30% 11%	411
June	1st to 10th	Night	24% 8% 26% 33% 9% 1	380
	11th to 20th	Night	18% 6% 29% 35% 12%	380
	21st to end	Night	18% 6% 30% 32% 14% 1	380
July	1st to 10th	Night	13% 5% 28% 37% 17%	370
	11th to 20th	Night	11% 3 26% 42% 17% 1	368
	21st to end	Night	12% 4 24% 42% 17% 1	416
August	1st to 10th	Night	14% 3 17% 48% 17% 1	378
	11th to 20th	Night	15% 3 23% 35% 22% 2	570
	21st to end	Night	16% 4 23% 36% 20% 1	836
September	1st to 10th	Night	13% 5% 26% 38% 16% 1	760
	11th to 20th	Night	17% 5% 19% 34% 22% 3	760
	21st to end	Night	19% 6% 28% 28% 17% 2	760
October	1st to 10th	Night	20% 9% 29% 27% 14% 1	760
	11th to 20th	Night	24% 9% 29% 28% 9%	760
	21st to end	Night	27% 12% 30% 27% 4	836
November	1st to 10th	Night	25% 14% 29% 24% 8%	780
	11th to 20th	Night	23% 14% 32% 21% 9% 1	800
	21st to end	Night	28% 16% 29% 20% 8%	800
December	1st to 10th	Night	31% 12% 33% 20% 4	800
	11th to 20th	Night	30% 15% 32% 16% 6% 1	800
	21st to end	Night	33% 14% 30% 18% 4 1	880

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