Observatoire St-gabriel Lalemant
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Observatoire St-gabriel Lalemant clear sky chart from 2005-11-06 to 2026-10-29.

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	15% 10% 3 8% 5% 5% 5% 4 6% 7% 33%	150025
	Night	16% 12% 4 9% 5% 5% 5% 4 5% 6% 29%	142154
Transparency	Day	50% 7% 15% 19% 8% 1	153474
	Night	54% 7% 13% 17% 8% 1	146796
Seeing	Day	23% 16% 31% 20% 7% 1	18205
	Night	23% 13% 31% 25% 7% 1	26325

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	14% 18% 5% 9% 6% 5% 6% 5% 5% 5% 22%	4378
	11th to 20th	Day	10% 15% 7% 14% 7% 6% 5% 3 4 6% 23%	4650
	21st to end	Day	6% 8% 4 11% 7% 5% 6% 5% 7% 8% 33%	5124
February	1st to 10th	Day	8% 13% 5% 10% 8% 5% 6% 4 6% 6% 28%	1735
	11th to 20th	Day	12% 16% 6% 11% 6% 6% 4 3 5% 6% 25%	1948
	21st to end	Day	11% 11% 3 9% 6% 6% 6% 4 5% 6% 32%	1730
March	1st to 10th	Day	18% 15% 4 9% 5% 4 5% 3 5% 6% 26%	4576
	11th to 20th	Day	14% 15% 4 9% 5% 5% 5% 4 5% 6% 26%	4985
	21st to end	Day	16% 17% 5% 8% 4 3 3 3 4 5% 30%	5766
April	1st to 10th	Day	21% 13% 3 8% 4 3 5% 3 5% 6% 29%	2520
	11th to 20th	Day	19% 10% 3 6% 4 4 4 3 5% 6% 36%	2665
	21st to end	Day	22% 10% 3 6% 4 4 4 3 4 7% 33%	2730
May	1st to 10th	Day	20% 11% 4 8% 5% 5% 5% 4 5% 6% 29%	7653
	11th to 20th	Day	20% 8% 2 8% 5% 4 5% 4 6% 7% 32%	8221
	21st to end	Day	19% 9% 3 7% 5% 4 5% 4 6% 8% 31%	9692
June	1st to 10th	Day	19% 8% 3 6% 5% 5% 5% 4 7% 7% 32%	3150
	11th to 20th	Day	18% 7% 2 6% 4 4 5% 4 7% 7% 36%	3150
	21st to end	Day	20% 8% 3 7% 5% 5% 5% 5% 6% 7% 30%	3150
July	1st to 10th	Day	12% 6% 3 7% 5% 5% 6% 4 7% 9% 36%	8880
	11th to 20th	Day	14% 6% 3 7% 4 5% 5% 4 6% 8% 37%	8385
	21st to end	Day	10% 6% 2 6% 5% 4 5% 5% 6% 9% 41%	8993
August	1st to 10th	Day	12% 6% 2 7% 5% 5% 6% 5% 7% 9% 36%	6328
	11th to 20th	Day	14% 7% 3 8% 5% 5% 5% 4 6% 8% 35%	5953
	21st to end	Day	15% 6% 2 5% 4 4 4 4 6% 8% 42%	6532
September	1st to 10th	Day	14% 6% 2 6% 3 4 5% 4 5% 8% 44%	2491
	11th to 20th	Day	15% 5% 2 5% 3 3 3 3 4 6% 51%	2310
	21st to end	Day	17% 6% 3 5% 4 4 4 3 6% 7% 42%	2310
October	1st to 10th	Day	16% 7% 3 7% 5% 4 4 4 6% 7% 38%	2274
	11th to 20th	Day	21% 9% 3 7% 4 5% 5% 3 6% 6% 32%	1958
	21st to end	Day	24% 11% 3 7% 4 5% 4 3 5% 6% 28%	2052
November	1st to 10th	Day	19% 9% 3 9% 5% 4 4 4 5% 7% 31%	1851
	11th to 20th	Day	18% 12% 4 9% 6% 4 4 3 4 6% 28%	1769
	21st to end	Day	19% 15% 4 9% 6% 4 5% 3 6% 5% 24%	1525
December	1st to 10th	Day	16% 14% 6% 11% 7% 5% 4 3 6% 5% 23%	2350
	11th to 20th	Day	17% 11% 5% 8% 4 5% 5% 5% 6% 6% 27%	2688
	21st to end	Day	17% 13% 4 10% 5% 5% 5% 4 6% 5% 26%	3553
January	1st to 10th	Night	13% 19% 5% 12% 7% 5% 5% 4 5% 5% 20%	8516
	11th to 20th	Night	9% 18% 7% 16% 7% 6% 5% 4 5% 5% 17%	8235
	21st to end	Night	5% 9% 5% 13% 6% 6% 7% 5% 6% 7% 31%	9057
February	1st to 10th	Night	9% 12% 5% 10% 8% 6% 5% 4 5% 6% 30%	2883
	11th to 20th	Night	12% 16% 5% 11% 8% 5% 4 3 5% 4 26%	2554
	21st to end	Night	13% 14% 4 9% 6% 4 4 3 5% 6% 32%	2076
March	1st to 10th	Night	18% 14% 4 9% 6% 5% 6% 4 5% 5% 25%	5484
	11th to 20th	Night	14% 17% 4 10% 6% 5% 5% 4 5% 6% 22%	5194
	21st to end	Night	19% 18% 4 7% 4 3 4 3 4 5% 30%	5406
April	1st to 10th	Night	22% 15% 4 8% 4 3 4 3 5% 4 28%	2100
	11th to 20th	Night	22% 9% 3 7% 5% 5% 4 3 5% 6% 30%	1902
	21st to end	Night	26% 9% 3 7% 4 4 4 2 3 6% 32%	1890
May	1st to 10th	Night	22% 10% 3 7% 5% 5% 4 4 6% 6% 28%	5259
	11th to 20th	Night	24% 8% 3 7% 5% 4 4 4 5% 6% 29%	4680
	21st to end	Night	17% 7% 2 6% 4 4 5% 4 6% 8% 36%	4513
June	1st to 10th	Night	20% 9% 3 7% 6% 4 5% 4 5% 7% 30%	1470
	11th to 20th	Night	17% 7% 2 6% 4 4 4 5% 6% 6% 37%	1470
	21st to end	Night	22% 9% 2 8% 5% 5% 4 4 6% 6% 30%	1470
July	1st to 10th	Night	14% 7% 3 7% 5% 5% 5% 4 7% 9% 36%	4110
	11th to 20th	Night	17% 6% 3 7% 4 4 5% 3 7% 8% 36%	4564
	21st to end	Night	11% 7% 3 7% 4 5% 5% 4 6% 8% 42%	5084
August	1st to 10th	Night	18% 8% 3 7% 5% 4 5% 3 5% 6% 34%	3774
	11th to 20th	Night	14% 8% 3 8% 5% 5% 6% 5% 7% 8% 30%	4092
	21st to end	Night	18% 8% 3 6% 5% 4 4 4 5% 6% 39%	4578
September	1st to 10th	Night	16% 6% 2 6% 4 5% 5% 4 6% 7% 40%	2128
	11th to 20th	Night	16% 7% 3 7% 4 4 4 3 4 6% 42%	2310
	21st to end	Night	17% 7% 2 5% 4 4 4 3 4 7% 42%	2310
October	1st to 10th	Night	18% 9% 3 6% 3 4 4 4 4 6% 40%	2345
	11th to 20th	Night	23% 11% 3 6% 4 5% 5% 3 4 6% 31%	2660
	21st to end	Night	22% 11% 3 7% 4 4 5% 3 5% 6% 30%	2965
November	1st to 10th	Night	20% 12% 3 7% 5% 4 4 3 5% 6% 30%	2667
	11th to 20th	Night	19% 12% 4 9% 6% 4 4 4 5% 6% 27%	2848
	21st to end	Night	18% 16% 4 10% 6% 5% 4 4 5% 4 23%	3092
December	1st to 10th	Night	13% 18% 8% 13% 7% 5% 5% 4 4 5% 19%	5060
	11th to 20th	Night	18% 13% 4 11% 7% 5% 5% 4 5% 6% 22%	5741
	21st to end	Night	16% 15% 5% 11% 7% 6% 5% 4 5% 5% 23%	7667

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	61% 8% 13% 14% 3	4379
	11th to 20th	Day	63% 8% 12% 13% 4 1	4680
	21st to end	Day	49% 8% 13% 17% 13%	5124
February	1st to 10th	Day	55% 7% 12% 16% 9% 2	1735
	11th to 20th	Day	60% 6% 12% 14% 7% 1	1956
	21st to end	Day	52% 7% 11% 21% 8% 2	1730
March	1st to 10th	Day	58% 9% 10% 9% 11% 4	4579
	11th to 20th	Day	56% 9% 15% 12% 7% 1	4985
	21st to end	Day	57% 4 8% 16% 12% 4	5766
April	1st to 10th	Day	59% 4 11% 14% 10% 2	2520
	11th to 20th	Day	51% 5% 13% 19% 10% 2	2665
	21st to end	Day	52% 5% 11% 18% 12% 2	2730
May	1st to 10th	Day	54% 8% 9% 18% 9% 1	7665
	11th to 20th	Day	50% 8% 14% 20% 7% 2	8225
	21st to end	Day	49% 8% 14% 17% 12%	9693
June	1st to 10th	Day	50% 9% 12% 20% 9% 1	3150
	11th to 20th	Day	46% 8% 16% 20% 10% 1	3150
	21st to end	Day	54% 8% 14% 16% 8% 1	3150
July	1st to 10th	Day	43% 7% 17% 24% 9%	8805
	11th to 20th	Day	44% 7% 18% 25% 6%	8308
	21st to end	Day	40% 8% 21% 26% 5%	8795
August	1st to 10th	Day	44% 9% 21% 19% 7% 1	6314
	11th to 20th	Day	46% 9% 22% 20% 3	5992
	21st to end	Day	40% 7% 20% 24% 8%	6532
September	1st to 10th	Day	39% 7% 18% 24% 11%	2491
	11th to 20th	Day	36% 7% 16% 29% 12% 1	2310
	21st to end	Day	43% 7% 15% 23% 13%	2310
October	1st to 10th	Day	53% 7% 12% 22% 5%	3638
	11th to 20th	Day	52% 5% 17% 22% 4	3109
	21st to end	Day	50% 6% 16% 20% 8%	3252
November	1st to 10th	Day	54% 8% 16% 18% 4	1851
	11th to 20th	Day	58% 7% 13% 17% 5%	1769
	21st to end	Day	63% 8% 14% 13% 3	1525
December	1st to 10th	Day	62% 7% 14% 9% 9%	2350
	11th to 20th	Day	58% 7% 12% 15% 7%	2688
	21st to end	Day	58% 7% 13% 14% 8%	3553
January	1st to 10th	Night	63% 8% 11% 13% 5%	8516
	11th to 20th	Night	67% 8% 10% 10% 4	8280
	21st to end	Night	50% 7% 11% 17% 14%	9057
February	1st to 10th	Night	55% 6% 12% 18% 8% 1	2883
	11th to 20th	Night	60% 6% 11% 15% 8%	2557
	21st to end	Night	54% 6% 10% 19% 11% 1	2076
March	1st to 10th	Night	60% 7% 9% 12% 12%	5494
	11th to 20th	Night	61% 8% 12% 12% 6% 1	5195
	21st to end	Night	57% 5% 7% 20% 11% 1	5406
April	1st to 10th	Night	59% 5% 8% 14% 11% 3	2100
	11th to 20th	Night	54% 7% 9% 18% 11% 1	1902
	21st to end	Night	55% 4 9% 19% 12% 1	1890
May	1st to 10th	Night	54% 8% 11% 17% 10% 1	5265
	11th to 20th	Night	53% 8% 11% 20% 8% 1	4681
	21st to end	Night	43% 9% 14% 19% 14% 2	4514
June	1st to 10th	Night	52% 8% 12% 20% 6% 1	1470
	11th to 20th	Night	45% 8% 15% 19% 11% 2	1470
	21st to end	Night	54% 8% 13% 17% 8%	1470
July	1st to 10th	Night	44% 6% 19% 23% 9%	4075
	11th to 20th	Night	46% 7% 20% 23% 4	4519
	21st to end	Night	40% 8% 22% 24% 6%	4967
August	1st to 10th	Night	49% 7% 18% 20% 5% 1	3764
	11th to 20th	Night	48% 11% 20% 15% 5%	4119
	21st to end	Night	45% 6% 16% 22% 10%	4578
September	1st to 10th	Night	43% 8% 15% 24% 9% 1	2128
	11th to 20th	Night	43% 7% 16% 24% 9% 1	2310
	21st to end	Night	42% 8% 12% 26% 12% 1	2310
October	1st to 10th	Night	49% 5% 12% 28% 5%	3761
	11th to 20th	Night	54% 5% 14% 22% 5%	4248
	21st to end	Night	52% 6% 17% 17% 7%	4715
November	1st to 10th	Night	56% 7% 14% 18% 5%	2667
	11th to 20th	Night	56% 9% 12% 16% 7%	2848
	21st to end	Night	65% 7% 11% 11% 5%	3092
December	1st to 10th	Night	68% 7% 10% 12% 4	5060
	11th to 20th	Night	62% 7% 10% 15% 6%	5742
	21st to end	Night	60% 9% 11% 15% 6%	7667

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	23% 23% 38% 14% 2 1	220
	11th to 20th	Day	27% 25% 31% 13% 5%	220
	21st to end	Day	15% 25% 36% 17% 7% 1	242
February	1st to 10th	Day	20% 25% 32% 17% 5% 1	275
	11th to 20th	Day	20% 21% 38% 18% 3	482
	21st to end	Day	20% 23% 36% 16% 6%	446
March	1st to 10th	Day	28% 21% 28% 18% 5%	540
	11th to 20th	Day	25% 18% 32% 17% 7% 1	540
	21st to end	Day	27% 21% 29% 16% 6% 2	594
April	1st to 10th	Day	32% 19% 30% 15% 4 1	540
	11th to 20th	Day	30% 20% 28% 18% 4 1	532
	21st to end	Day	29% 18% 31% 17% 4 1	540
May	1st to 10th	Day	30% 13% 33% 16% 6% 1	543
	11th to 20th	Day	23% 16% 31% 23% 6% 1	567
	21st to end	Day	24% 15% 36% 17% 7% 1	833
June	1st to 10th	Day	24% 13% 33% 20% 9% 1	780
	11th to 20th	Day	22% 12% 33% 22% 10% 1	780
	21st to end	Day	22% 13% 29% 27% 8% 1	780
July	1st to 10th	Day	18% 12% 35% 24% 9% 1	770
	11th to 20th	Day	18% 12% 34% 26% 9% 2	602
	21st to end	Day	16% 11% 32% 28% 11% 1	620
August	1st to 10th	Day	15% 12% 32% 27% 11% 3	566
	11th to 20th	Day	16% 8% 31% 29% 14% 2	559
	21st to end	Day	19% 14% 28% 22% 14% 4	627
September	1st to 10th	Day	19% 15% 30% 23% 10% 3	570
	11th to 20th	Day	19% 14% 29% 24% 11% 4	570
	21st to end	Day	20% 12% 29% 24% 11% 5%	570
October	1st to 10th	Day	21% 17% 32% 19% 9% 2	549
	11th to 20th	Day	29% 18% 26% 19% 7% 1	450
	21st to end	Day	32% 18% 29% 16% 5% 1	486
November	1st to 10th	Day	32% 19% 29% 17% 2 1	425
	11th to 20th	Day	29% 17% 28% 18% 6% 1	430
	21st to end	Day	33% 23% 31% 12% 1	275
December	1st to 10th	Day	28% 20% 28% 18% 5% 1	220
	11th to 20th	Day	32% 23% 32% 10% 4	220
	21st to end	Day	27% 25% 25% 18% 5%	242
January	1st to 10th	Night	24% 16% 35% 19% 4	893
	11th to 20th	Night	21% 16% 37% 21% 4	840
	21st to end	Night	13% 16% 36% 28% 7% 1	924
February	1st to 10th	Night	16% 16% 38% 23% 7% 1	840
	11th to 20th	Night	20% 15% 38% 21% 5% 1	823
	21st to end	Night	21% 16% 33% 25% 5%	692
March	1st to 10th	Night	22% 14% 35% 23% 5%	840
	11th to 20th	Night	23% 16% 34% 19% 7% 1	840
	21st to end	Night	24% 14% 35% 20% 7%	924
April	1st to 10th	Night	31% 12% 30% 21% 5%	840
	11th to 20th	Night	28% 12% 29% 25% 6%	658
	21st to end	Night	32% 12% 26% 23% 7%	630
May	1st to 10th	Night	27% 9% 31% 27% 6%	630
	11th to 20th	Night	23% 12% 25% 29% 11%	430
	21st to end	Night	27% 10% 29% 26% 7%	455
June	1st to 10th	Night	22% 10% 25% 36% 7%	420
	11th to 20th	Night	19% 6% 28% 34% 13%	420
	21st to end	Night	23% 7% 33% 27% 9% 1	420
July	1st to 10th	Night	16% 10% 34% 30% 9%	410
	11th to 20th	Night	15% 8% 29% 36% 12%	408
	21st to end	Night	17% 4 25% 42% 12% 1	460
August	1st to 10th	Night	15% 5% 23% 42% 14% 1	461
	11th to 20th	Night	16% 5% 26% 36% 16% 1	622
	21st to end	Night	18% 8% 27% 34% 12% 1	714
September	1st to 10th	Night	18% 8% 32% 30% 11% 1	840
	11th to 20th	Night	21% 8% 29% 29% 12% 2	840
	21st to end	Night	18% 8% 31% 29% 13% 1	840
October	1st to 10th	Night	23% 11% 31% 25% 7% 2	840
	11th to 20th	Night	30% 10% 31% 20% 9% 1	840
	21st to end	Night	28% 12% 31% 21% 7%	913
November	1st to 10th	Night	28% 17% 29% 20% 5%	820
	11th to 20th	Night	26% 19% 28% 20% 6% 1	903
	21st to end	Night	27% 17% 31% 20% 4 1	950
December	1st to 10th	Night	26% 18% 30% 20% 5% 1	950
	11th to 20th	Night	25% 19% 33% 20% 4	950
	21st to end	Night	27% 18% 29% 20% 7%	1045

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