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 2025-07-06.

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 6% 7% 32%	119652
	Night	16% 12% 4 9% 6% 5% 5% 4 5% 6% 28%	119193
Transparency	Day	51% 8% 15% 17% 8% 1	123144
	Night	55% 8% 13% 17% 8% 1	123864
Seeing	Day	23% 16% 31% 20% 7% 1	16453
	Night	23% 13% 31% 25% 7% 1	24671

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	16% 21% 6% 9% 6% 5% 5% 4 5% 5% 18%	3377
	11th to 20th	Day	9% 15% 7% 14% 7% 5% 5% 3 5% 6% 23%	3570
	21st to end	Day	7% 8% 4 12% 7% 5% 6% 5% 6% 6% 35%	3960
February	1st to 10th	Day	8% 13% 5% 10% 8% 5% 6% 4 6% 6% 27%	1652
	11th to 20th	Day	12% 16% 5% 11% 6% 6% 4 3 6% 6% 25%	1853
	21st to end	Day	12% 11% 3 9% 6% 6% 6% 4 5% 6% 33%	1650
March	1st to 10th	Day	18% 16% 4 9% 5% 4 5% 3 5% 6% 25%	4476
	11th to 20th	Day	14% 15% 4 9% 5% 5% 5% 4 5% 6% 27%	4878
	21st to end	Day	16% 17% 5% 8% 4 3 3 3 4 5% 30%	5641
April	1st to 10th	Day	20% 13% 3 8% 4 3 4 3 5% 6% 30%	2400
	11th to 20th	Day	19% 10% 3 6% 4 4 4 3 5% 6% 37%	2537
	21st to end	Day	22% 11% 3 6% 4 4 4 3 4 7% 33%	2600
May	1st to 10th	Day	19% 11% 4 8% 5% 5% 5% 4 4 6% 30%	5900
	11th to 20th	Day	22% 8% 2 7% 5% 4 5% 4 6% 7% 30%	6318
	21st to end	Day	19% 8% 2 6% 5% 4 5% 4 6% 8% 32%	7441
June	1st to 10th	Day	19% 8% 2 6% 5% 5% 5% 4 6% 8% 32%	3000
	11th to 20th	Day	18% 6% 2 6% 4 4 5% 4 7% 7% 37%	3000
	21st to end	Day	20% 8% 3 7% 5% 5% 5% 5% 5% 7% 31%	3000
July	1st to 10th	Day	14% 6% 3 7% 5% 5% 6% 5% 7% 9% 35%	4835
	11th to 20th	Day	11% 6% 3 7% 5% 5% 5% 5% 7% 8% 38%	4489
	21st to end	Day	11% 7% 2 5% 4 4 4 5% 6% 8% 43%	4814
August	1st to 10th	Day	13% 6% 2 6% 5% 5% 6% 5% 7% 10% 35%	4313
	11th to 20th	Day	15% 7% 2 7% 5% 5% 5% 4 6% 8% 36%	4045
	21st to end	Day	18% 6% 2 6% 4 3 4 3 5% 7% 42%	4485
September	1st to 10th	Day	14% 6% 2 6% 3 3 5% 4 5% 8% 44%	2254
	11th to 20th	Day	16% 5% 2 5% 3 3 3 3 4 6% 51%	2090
	21st to end	Day	17% 6% 3 6% 4 3 4 3 6% 7% 41%	2090
October	1st to 10th	Day	17% 7% 3 7% 5% 4 4 4 6% 7% 37%	2058
	11th to 20th	Day	21% 9% 3 7% 4 5% 5% 4 6% 6% 30%	1772
	21st to end	Day	25% 11% 3 7% 5% 5% 4 3 5% 6% 27%	1881
November	1st to 10th	Day	18% 9% 3 9% 5% 4 4 4 5% 7% 31%	1761
	11th to 20th	Day	18% 12% 4 9% 6% 4 4 3 4 7% 27%	1685
	21st to end	Day	20% 15% 4 9% 6% 4 4 3 6% 5% 24%	1453
December	1st to 10th	Day	16% 14% 6% 11% 7% 5% 4 3 5% 5% 23%	2280
	11th to 20th	Day	17% 11% 4 8% 4 5% 5% 5% 5% 6% 27%	2618
	21st to end	Day	17% 13% 4 10% 5% 5% 5% 4 6% 5% 26%	3476
January	1st to 10th	Night	15% 22% 5% 11% 7% 5% 4 4 4 5% 18%	6563
	11th to 20th	Night	9% 17% 7% 17% 8% 6% 5% 4 5% 5% 17%	6315
	21st to end	Night	6% 10% 4 11% 5% 5% 6% 4 7% 8% 33%	6996
February	1st to 10th	Night	9% 13% 5% 10% 8% 6% 5% 4 5% 6% 29%	2746
	11th to 20th	Night	13% 16% 5% 12% 8% 5% 4 3 5% 5% 26%	2429
	21st to end	Night	14% 14% 5% 9% 6% 4 4 3 5% 6% 31%	1980
March	1st to 10th	Night	18% 14% 4 9% 6% 5% 6% 4 5% 5% 25%	5364
	11th to 20th	Night	14% 17% 4 10% 6% 5% 5% 4 6% 6% 22%	5081
	21st to end	Night	20% 18% 4 7% 4 3 4 3 4 5% 30%	5289
April	1st to 10th	Night	22% 16% 4 8% 4 3 4 3 5% 4 28%	2000
	11th to 20th	Night	22% 9% 3 7% 5% 5% 4 3 5% 6% 31%	1810
	21st to end	Night	28% 9% 3 7% 4 4 3 2 3 6% 32%	1800
May	1st to 10th	Night	21% 9% 3 7% 5% 5% 4 4 5% 6% 30%	4054
	11th to 20th	Night	26% 8% 3 6% 5% 4 4 4 5% 7% 28%	3603
	21st to end	Night	18% 7% 2 7% 4 4 5% 4 6% 8% 35%	3455
June	1st to 10th	Night	20% 10% 3 7% 6% 4 5% 3 5% 7% 30%	1400
	11th to 20th	Night	16% 7% 2 6% 5% 4 4 5% 7% 7% 38%	1400
	21st to end	Night	22% 9% 2 8% 5% 5% 4 4 6% 6% 30%	1400
July	1st to 10th	Night	15% 7% 3 7% 5% 5% 5% 4 7% 8% 34%	2245
	11th to 20th	Night	15% 6% 3 7% 5% 5% 5% 3 7% 7% 36%	2456
	21st to end	Night	11% 6% 2 5% 4 5% 5% 4 6% 9% 43%	2739
August	1st to 10th	Night	18% 8% 2 7% 5% 4 6% 4 6% 7% 34%	2584
	11th to 20th	Night	15% 8% 3 9% 5% 5% 6% 5% 8% 8% 28%	2782
	21st to end	Night	18% 8% 2 6% 5% 4 4 3 4 6% 40%	3156
September	1st to 10th	Night	17% 5% 2 6% 4 4 5% 4 6% 7% 39%	1925
	11th to 20th	Night	18% 8% 3 7% 4 4 3 3 4 6% 41%	2090
	21st to end	Night	17% 8% 2 5% 4 4 4 3 4 7% 42%	2090
October	1st to 10th	Night	19% 9% 3 7% 4 4 4 3 4 6% 39%	2121
	11th to 20th	Night	24% 11% 3 7% 4 5% 5% 3 4 6% 29%	2406
	21st to end	Night	23% 11% 3 7% 4 4 5% 3 5% 6% 29%	2717
November	1st to 10th	Night	19% 11% 3 7% 5% 5% 4 3 5% 6% 31%	2537
	11th to 20th	Night	19% 12% 4 9% 6% 4 4 4 5% 6% 27%	2712
	21st to end	Night	18% 16% 4 10% 6% 4 4 4 5% 4 23%	2945
December	1st to 10th	Night	14% 18% 8% 13% 7% 5% 5% 3 4 5% 19%	4910
	11th to 20th	Night	18% 13% 4 11% 7% 5% 5% 4 5% 6% 22%	5591
	21st to end	Night	16% 15% 5% 11% 7% 6% 5% 4 5% 5% 23%	7502

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% 9% 13% 10% 2	3378
	11th to 20th	Day	62% 10% 12% 13% 3 1	3600
	21st to end	Day	49% 9% 11% 15% 16%	3960
February	1st to 10th	Day	56% 7% 12% 14% 9% 2	1652
	11th to 20th	Day	61% 6% 12% 13% 7% 1	1861
	21st to end	Day	51% 7% 12% 20% 8% 2	1650
March	1st to 10th	Day	59% 9% 10% 9% 11% 4	4479
	11th to 20th	Day	56% 10% 15% 12% 7% 1	4878
	21st to end	Day	56% 4 8% 16% 11% 3	5641
April	1st to 10th	Day	58% 4 11% 14% 10% 2	2400
	11th to 20th	Day	50% 5% 13% 19% 11% 2	2537
	21st to end	Day	52% 5% 11% 17% 12% 2	2600
May	1st to 10th	Day	53% 10% 10% 17% 10% 1	5911
	11th to 20th	Day	51% 9% 15% 16% 7% 2	6322
	21st to end	Day	47% 9% 16% 17% 10%	7441
June	1st to 10th	Day	50% 9% 12% 20% 9% 1	3000
	11th to 20th	Day	45% 8% 16% 19% 11% 1	3000
	21st to end	Day	53% 8% 14% 16% 8% 1	3000
July	1st to 10th	Day	45% 8% 18% 23% 6%	4760
	11th to 20th	Day	42% 8% 23% 22% 4 1	4412
	21st to end	Day	39% 9% 21% 24% 8%	4661
August	1st to 10th	Day	44% 10% 21% 19% 5%	4299
	11th to 20th	Day	46% 9% 22% 19% 4	4084
	21st to end	Day	43% 8% 19% 19% 12%	4485
September	1st to 10th	Day	39% 8% 18% 24% 11% 1	2254
	11th to 20th	Day	37% 7% 16% 27% 12% 1	2090
	21st to end	Day	44% 7% 16% 23% 11%	2090
October	1st to 10th	Day	54% 7% 12% 22% 5%	3422
	11th to 20th	Day	53% 5% 16% 22% 4	2923
	21st to end	Day	51% 6% 16% 19% 8%	3081
November	1st to 10th	Day	53% 8% 15% 18% 5%	1761
	11th to 20th	Day	58% 7% 13% 16% 5%	1685
	21st to end	Day	63% 8% 14% 13% 3	1453
December	1st to 10th	Day	62% 7% 14% 9% 9%	2280
	11th to 20th	Day	58% 7% 12% 16% 7%	2618
	21st to end	Day	57% 7% 13% 14% 8%	3476
January	1st to 10th	Night	67% 9% 11% 10% 3	6563
	11th to 20th	Night	65% 10% 12% 9% 4 1	6360
	21st to end	Night	46% 8% 12% 19% 14%	6996
February	1st to 10th	Night	56% 6% 12% 16% 9% 1	2746
	11th to 20th	Night	61% 6% 11% 14% 8%	2432
	21st to end	Night	55% 6% 10% 18% 10% 1	1980
March	1st to 10th	Night	60% 7% 9% 12% 12% 1	5374
	11th to 20th	Night	61% 8% 12% 12% 6% 1	5082
	21st to end	Night	57% 5% 7% 20% 11% 1	5289
April	1st to 10th	Night	59% 5% 8% 14% 12% 3	2000
	11th to 20th	Night	53% 7% 10% 18% 11% 1	1810
	21st to end	Night	56% 5% 8% 19% 12% 1	1800
May	1st to 10th	Night	51% 9% 11% 16% 12% 1	4060
	11th to 20th	Night	52% 9% 12% 19% 7%	3604
	21st to end	Night	43% 11% 15% 18% 11% 2	3456
June	1st to 10th	Night	53% 8% 12% 20% 6% 1	1400
	11th to 20th	Night	43% 8% 15% 19% 12% 2	1400
	21st to end	Night	54% 8% 12% 17% 9%	1400
July	1st to 10th	Night	46% 7% 20% 23% 3	2210
	11th to 20th	Night	46% 9% 21% 20% 3	2411
	21st to end	Night	36% 10% 26% 21% 8%	2651
August	1st to 10th	Night	49% 8% 17% 21% 4	2574
	11th to 20th	Night	50% 12% 20% 12% 7%	2809
	21st to end	Night	45% 6% 16% 21% 12%	3156
September	1st to 10th	Night	43% 8% 16% 22% 10% 1	1925
	11th to 20th	Night	44% 7% 17% 23% 9%	2090
	21st to end	Night	43% 8% 12% 26% 11% 1	2090
October	1st to 10th	Night	49% 5% 12% 28% 5%	3537
	11th to 20th	Night	55% 5% 14% 21% 4	3994
	21st to end	Night	52% 6% 18% 17% 7%	4467
November	1st to 10th	Night	55% 7% 15% 18% 5%	2537
	11th to 20th	Night	56% 9% 12% 15% 7%	2712
	21st to end	Night	65% 7% 11% 11% 5%	2945
December	1st to 10th	Night	68% 7% 10% 12% 3	4910
	11th to 20th	Night	62% 7% 10% 15% 6%	5592
	21st to end	Night	60% 9% 11% 15% 6%	7502

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	24% 23% 39% 12% 2 2	200
	11th to 20th	Day	24% 24% 33% 14% 6%	200
	21st to end	Day	15% 24% 37% 16% 7% 1	220
February	1st to 10th	Day	21% 26% 34% 15% 4 1	252
	11th to 20th	Day	21% 21% 38% 17% 2	447
	21st to end	Day	20% 22% 36% 16% 6%	414
March	1st to 10th	Day	27% 19% 30% 19% 5%	500
	11th to 20th	Day	23% 17% 33% 18% 7% 1	500
	21st to end	Day	25% 20% 30% 17% 7% 1	550
April	1st to 10th	Day	31% 19% 30% 16% 4 1	500
	11th to 20th	Day	29% 20% 28% 18% 4 1	492
	21st to end	Day	30% 18% 31% 16% 4 1	500
May	1st to 10th	Day	29% 13% 35% 16% 6% 1	503
	11th to 20th	Day	23% 14% 31% 24% 6% 1	527
	21st to end	Day	24% 15% 35% 18% 8% 1	779
June	1st to 10th	Day	24% 14% 34% 19% 8% 1	730
	11th to 20th	Day	21% 12% 33% 22% 11% 1	730
	21st to end	Day	22% 13% 30% 27% 8%	730
July	1st to 10th	Day	19% 11% 36% 24% 10% 1	695
	11th to 20th	Day	17% 11% 33% 28% 9% 2	518
	21st to end	Day	15% 10% 32% 30% 11% 1	532
August	1st to 10th	Day	16% 13% 30% 27% 11% 2	486
	11th to 20th	Day	17% 7% 31% 29% 14% 2	479
	21st to end	Day	19% 13% 28% 22% 13% 4	539
September	1st to 10th	Day	18% 14% 30% 24% 11% 3	490
	11th to 20th	Day	19% 12% 29% 25% 11% 3	490
	21st to end	Day	20% 12% 29% 23% 10% 5%	490
October	1st to 10th	Day	21% 15% 31% 21% 10% 3	473
	11th to 20th	Day	28% 18% 25% 19% 7% 2	390
	21st to end	Day	33% 18% 28% 16% 4 1	429
November	1st to 10th	Day	31% 19% 30% 17% 2 1	395
	11th to 20th	Day	28% 18% 29% 18% 6% 1	400
	21st to end	Day	33% 22% 30% 13% 2	253
December	1st to 10th	Day	28% 19% 28% 19% 6% 1	200
	11th to 20th	Day	32% 21% 34% 10% 4 1	200
	21st to end	Day	28% 25% 25% 17% 5%	220
January	1st to 10th	Night	25% 17% 35% 19% 4	848
	11th to 20th	Night	21% 16% 36% 22% 5% 1	800
	21st to end	Night	14% 14% 37% 28% 6% 1	880
February	1st to 10th	Night	17% 16% 40% 22% 5% 1	800
	11th to 20th	Night	21% 15% 38% 21% 5% 1	783
	21st to end	Night	22% 16% 32% 25% 5%	660
March	1st to 10th	Night	21% 13% 36% 24% 5%	800
	11th to 20th	Night	23% 15% 35% 19% 7% 1	800
	21st to end	Night	24% 13% 34% 21% 7%	880
April	1st to 10th	Night	30% 12% 31% 22% 5%	800
	11th to 20th	Night	27% 12% 30% 26% 6%	626
	21st to end	Night	33% 12% 26% 22% 7% 1	600
May	1st to 10th	Night	27% 9% 31% 27% 6%	600
	11th to 20th	Night	23% 12% 24% 28% 11%	409
	21st to end	Night	27% 10% 30% 25% 8%	433
June	1st to 10th	Night	23% 10% 25% 35% 7% 1	400
	11th to 20th	Night	19% 6% 28% 34% 13% 1	400
	21st to end	Night	24% 7% 34% 26% 9% 1	400
July	1st to 10th	Night	16% 9% 35% 30% 10% 1	380
	11th to 20th	Night	15% 7% 29% 36% 13%	368
	21st to end	Night	16% 4 25% 43% 12% 1	416
August	1st to 10th	Night	15% 5% 24% 41% 14% 1	417
	11th to 20th	Night	17% 5% 26% 35% 16% 1	562
	21st to end	Night	18% 8% 26% 33% 13% 1	646
September	1st to 10th	Night	18% 7% 33% 29% 12% 1	760
	11th to 20th	Night	22% 8% 29% 28% 12% 2	760
	21st to end	Night	18% 8% 31% 29% 12% 1	760
October	1st to 10th	Night	23% 11% 30% 26% 8% 2	760
	11th to 20th	Night	31% 10% 30% 20% 9% 1	760
	21st to end	Night	29% 13% 32% 21% 6%	836
November	1st to 10th	Night	28% 16% 29% 21% 5%	780
	11th to 20th	Night	25% 19% 29% 20% 6% 1	857
	21st to end	Night	27% 17% 31% 20% 4 1	900
December	1st to 10th	Night	27% 17% 30% 20% 5% 1	900
	11th to 20th	Night	25% 18% 33% 20% 4	900
	21st to end	Night	27% 17% 29% 19% 7% 1	990

Page generated at 2025-07-03 01:30:13. Version ac07

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