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 2024-11-04.

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 7% 5% 4 5% 4 6% 7% 33%	101569
	Night	17% 12% 4 8% 5% 5% 5% 4 5% 6% 29%	97472
Transparency	Day	49% 8% 15% 18% 8% 1	105034
	Night	53% 8% 14% 17% 7% 1	102098
Seeing	Day	23% 16% 31% 21% 8% 1	15611
	Night	23% 13% 31% 25% 8% 1	23787

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	13% 16% 5% 9% 6% 6% 6% 5% 6% 7% 22%	2362
	11th to 20th	Day	11% 13% 6% 10% 6% 5% 6% 4 5% 6% 29%	2520
	21st to end	Day	10% 9% 4 10% 7% 4 5% 5% 6% 6% 35%	2772
February	1st to 10th	Day	8% 14% 5% 10% 8% 5% 5% 5% 6% 7% 27%	1569
	11th to 20th	Day	13% 16% 5% 10% 6% 6% 4 3 5% 6% 25%	1757
	21st to end	Day	12% 11% 3 9% 6% 6% 6% 4 5% 6% 32%	1570
March	1st to 10th	Day	17% 13% 4 8% 6% 5% 5% 3 6% 6% 28%	3116
	11th to 20th	Day	16% 19% 4 9% 5% 5% 4 4 5% 5% 24%	3406
	21st to end	Day	15% 16% 4 8% 4 3 3 3 4 5% 34%	3935
April	1st to 10th	Day	21% 13% 3 8% 4 3 4 3 5% 6% 29%	2280
	11th to 20th	Day	19% 9% 2 6% 4 4 4 3 5% 6% 38%	2409
	21st to end	Day	22% 11% 3 5% 4 4 4 3 5% 7% 33%	2470
May	1st to 10th	Day	16% 9% 3 7% 5% 5% 5% 4 5% 7% 33%	4120
	11th to 20th	Day	17% 9% 2 7% 5% 5% 5% 4 7% 8% 32%	4398
	21st to end	Day	17% 7% 3 7% 5% 5% 5% 4 6% 8% 32%	5175
June	1st to 10th	Day	19% 8% 2 6% 5% 5% 5% 4 6% 8% 32%	2850
	11th to 20th	Day	18% 6% 2 6% 4 4 5% 4 7% 7% 37%	2850
	21st to end	Day	20% 8% 3 7% 5% 5% 5% 5% 5% 7% 31%	2850
July	1st to 10th	Day	14% 6% 3 7% 5% 5% 6% 5% 7% 9% 35%	4760
	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 8% 5% 4 4 4 5% 7% 32%	1698
	11th to 20th	Day	18% 11% 4 10% 6% 4 4 4 4 7% 27%	1601
	21st to end	Day	19% 14% 4 8% 6% 4 4 3 6% 5% 25%	1381
December	1st to 10th	Day	18% 15% 6% 11% 9% 5% 4 2 5% 4 20%	1330
	11th to 20th	Day	19% 15% 6% 9% 5% 5% 4 4 4 4 27%	1668
	21st to end	Day	21% 14% 4 8% 5% 3 3 3 4 5% 29%	2431
January	1st to 10th	Night	12% 17% 5% 11% 7% 6% 5% 4 6% 5% 21%	4587
	11th to 20th	Night	10% 15% 5% 10% 8% 7% 7% 4 6% 6% 21%	4440
	21st to end	Night	9% 13% 5% 11% 5% 6% 5% 4 6% 6% 29%	4884
February	1st to 10th	Night	10% 13% 5% 10% 8% 6% 5% 4 5% 6% 29%	2609
	11th to 20th	Night	12% 16% 5% 12% 8% 5% 4 3 5% 5% 26%	2305
	21st to end	Night	14% 14% 5% 9% 6% 4 4 3 5% 6% 31%	1884
March	1st to 10th	Night	18% 14% 4 8% 6% 5% 7% 4 5% 5% 26%	3724
	11th to 20th	Night	15% 21% 5% 11% 6% 5% 4 3 5% 4 20%	3553
	21st to end	Night	20% 17% 4 7% 4 3 4 3 4 5% 29%	3695
April	1st to 10th	Night	22% 16% 3 8% 4 3 4 3 5% 4 28%	1900
	11th to 20th	Night	21% 9% 3 7% 5% 4 4 3 5% 6% 32%	1719
	21st to end	Night	27% 9% 3 7% 4 4 3 2 3 6% 32%	1710
May	1st to 10th	Night	20% 9% 3 7% 5% 5% 4 4 6% 6% 30%	2834
	11th to 20th	Night	19% 9% 4 8% 6% 4 5% 4 6% 6% 29%	2509
	21st to end	Night	17% 7% 3 7% 4 4 5% 5% 7% 9% 32%	2410
June	1st to 10th	Night	20% 10% 3 7% 6% 4 5% 3 5% 7% 29%	1330
	11th to 20th	Night	16% 7% 2 6% 5% 4 4 5% 7% 7% 39%	1330
	21st to end	Night	22% 9% 2 8% 5% 5% 4 4 6% 6% 30%	1330
July	1st to 10th	Night	15% 7% 3 7% 5% 5% 5% 4 7% 8% 34%	2210
	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% 12% 3 7% 5% 5% 4 3 5% 6% 31%	2453
	11th to 20th	Night	19% 11% 4 9% 6% 4 4 4 5% 6% 27%	2576
	21st to end	Night	17% 16% 4 10% 6% 4 4 4 5% 5% 25%	2797
December	1st to 10th	Night	19% 18% 5% 10% 6% 5% 4 3 4 5% 20%	2850
	11th to 20th	Night	18% 15% 5% 10% 6% 6% 5% 4 4 6% 21%	3531
	21st to end	Night	19% 16% 5% 10% 5% 4 4 3 4 5% 24%	5236

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	56% 13% 17% 11% 3	2363
	11th to 20th	Day	52% 11% 14% 18% 4 1	2520
	21st to end	Day	48% 12% 13% 17% 10%	2772
February	1st to 10th	Day	57% 8% 12% 14% 8% 1	1569
	11th to 20th	Day	60% 6% 12% 14% 7% 1	1765
	21st to end	Day	52% 7% 12% 20% 8% 2	1570
March	1st to 10th	Day	54% 12% 12% 12% 8% 2	3119
	11th to 20th	Day	59% 11% 13% 11% 5% 1	3406
	21st to end	Day	52% 5% 10% 18% 11% 4	3935
April	1st to 10th	Day	59% 4 11% 14% 9% 2	2280
	11th to 20th	Day	50% 5% 13% 19% 11% 2	2409
	21st to end	Day	52% 5% 11% 17% 13% 2	2470
May	1st to 10th	Day	46% 12% 13% 18% 10% 1	4134
	11th to 20th	Day	46% 10% 18% 18% 7%	4402
	21st to end	Day	45% 11% 14% 19% 10%	5175
June	1st to 10th	Day	50% 9% 12% 19% 9% 1	2850
	11th to 20th	Day	45% 8% 17% 20% 10% 1	2850
	21st to end	Day	54% 8% 13% 17% 8% 1	2850
July	1st to 10th	Day	45% 8% 18% 22% 6%	4685
	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	52% 8% 16% 19% 5%	1698
	11th to 20th	Day	58% 8% 14% 15% 5%	1601
	21st to end	Day	61% 8% 14% 14% 3	1381
December	1st to 10th	Day	67% 7% 11% 11% 4	1330
	11th to 20th	Day	61% 7% 10% 15% 7%	1668
	21st to end	Day	58% 8% 15% 15% 4	2431
January	1st to 10th	Night	60% 12% 12% 10% 5%	4587
	11th to 20th	Night	57% 13% 13% 12% 5%	4440
	21st to end	Night	52% 9% 13% 15% 11%	4884
February	1st to 10th	Night	56% 6% 12% 17% 8% 1	2609
	11th to 20th	Night	61% 6% 11% 14% 7%	2308
	21st to end	Night	55% 6% 10% 18% 10% 1	1884
March	1st to 10th	Night	57% 9% 11% 16% 7%	3734
	11th to 20th	Night	63% 9% 11% 11% 5% 1	3554
	21st to end	Night	57% 6% 9% 19% 8% 1	3695
April	1st to 10th	Night	59% 5% 8% 14% 11% 3	1900
	11th to 20th	Night	52% 8% 10% 18% 12% 1	1719
	21st to end	Night	55% 5% 8% 18% 12% 1	1710
May	1st to 10th	Night	48% 11% 13% 16% 11% 1	2840
	11th to 20th	Night	48% 12% 15% 16% 9% 1	2510
	21st to end	Night	42% 12% 15% 19% 11% 1	2411
June	1st to 10th	Night	53% 9% 12% 20% 5% 1	1330
	11th to 20th	Night	43% 8% 16% 19% 12% 2	1330
	21st to end	Night	55% 8% 13% 16% 8%	1330
July	1st to 10th	Night	46% 7% 19% 23% 4	2175
	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% 17% 5%	2453
	11th to 20th	Night	56% 10% 13% 15% 7%	2576
	21st to end	Night	63% 8% 12% 12% 5%	2797
December	1st to 10th	Night	68% 7% 11% 11% 3	2850
	11th to 20th	Night	61% 9% 12% 12% 6%	3532
	21st to end	Night	59% 10% 12% 13% 5%	5236

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	22% 25% 38% 12% 1 2	180
	11th to 20th	Day	24% 23% 36% 12% 4	180
	21st to end	Day	16% 23% 36% 17% 8% 1	198
February	1st to 10th	Day	21% 27% 33% 15% 3	229
	11th to 20th	Day	20% 21% 39% 18% 3	411
	21st to end	Day	20% 22% 35% 17% 6%	382
March	1st to 10th	Day	26% 19% 30% 20% 5%	460
	11th to 20th	Day	23% 16% 33% 18% 8% 2	460
	21st to end	Day	24% 19% 31% 18% 7% 2	506
April	1st to 10th	Day	29% 18% 30% 17% 4 1	460
	11th to 20th	Day	28% 19% 28% 20% 4 2	452
	21st to end	Day	28% 18% 32% 17% 4 2	460
May	1st to 10th	Day	28% 12% 34% 17% 7% 1	463
	11th to 20th	Day	22% 14% 32% 24% 6% 1	487
	21st to end	Day	23% 15% 37% 17% 7% 1	724
June	1st to 10th	Day	24% 13% 34% 20% 9% 1	680
	11th to 20th	Day	21% 11% 32% 23% 11% 1	680
	21st to end	Day	22% 12% 29% 28% 9%	680
July	1st to 10th	Day	19% 10% 35% 24% 10% 1	670
	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	29% 18% 31% 18% 2 1	374
	11th to 20th	Day	26% 19% 28% 19% 6% 1	370
	21st to end	Day	30% 23% 31% 14% 2	231
December	1st to 10th	Day	28% 19% 27% 19% 6% 1	180
	11th to 20th	Day	31% 21% 34% 9% 4 1	180
	21st to end	Day	29% 26% 25% 16% 4	198
January	1st to 10th	Night	24% 17% 35% 19% 4	804
	11th to 20th	Night	21% 16% 38% 22% 3	760
	21st to end	Night	14% 14% 36% 29% 7% 1	836
February	1st to 10th	Night	17% 16% 39% 22% 5% 1	760
	11th to 20th	Night	20% 15% 38% 21% 5% 1	743
	21st to end	Night	21% 17% 33% 25% 5%	628
March	1st to 10th	Night	21% 12% 37% 24% 6%	760
	11th to 20th	Night	23% 15% 35% 20% 7% 1	760
	21st to end	Night	23% 13% 35% 22% 7%	836
April	1st to 10th	Night	30% 11% 30% 23% 5%	760
	11th to 20th	Night	27% 11% 29% 26% 6%	595
	21st to end	Night	32% 12% 26% 22% 7% 1	570
May	1st to 10th	Night	26% 9% 30% 28% 7%	570
	11th to 20th	Night	23% 12% 25% 28% 12%	388
	21st to end	Night	27% 10% 30% 24% 8%	411
June	1st to 10th	Night	23% 9% 25% 35% 7% 1	380
	11th to 20th	Night	18% 6% 27% 35% 13% 1	380
	21st to end	Night	24% 7% 32% 27% 9% 1	380
July	1st to 10th	Night	16% 9% 34% 30% 10% 1	370
	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	27% 16% 29% 22% 5%	755
	11th to 20th	Night	24% 20% 29% 20% 6% 1	811
	21st to end	Night	25% 17% 32% 21% 4 1	850
December	1st to 10th	Night	28% 17% 29% 20% 5% 1	850
	11th to 20th	Night	24% 18% 34% 20% 4	850
	21st to end	Night	27% 18% 29% 18% 7% 1	935

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