Observatoires du Mont Notre Dame
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Observatoires du Mont Notre Dame clear sky chart from 2005-11-06 to 2025-07-21.

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% 11% 4 9% 6% 6% 5% 4 5% 6% 27%	119979
	Night	16% 13% 5% 11% 7% 6% 5% 3 4 5% 24%	119287
Transparency	Day	58% 6% 11% 17% 7% 1	123452
	Night	61% 6% 11% 15% 6%	123976
Seeing	Day	25% 20% 30% 17% 6% 1	16870
	Night	23% 18% 28% 22% 7% 1	24764

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% 24% 8% 17% 11% 12% 4 2 2 2 4	3157
	11th to 20th	Day	9% 10% 7% 23% 19% 8% 5% 4 4 4 7%	3436
	21st to end	Day	4 8% 4 12% 10% 15% 12% 8% 7% 5% 15%	3960
February	1st to 10th	Day	7% 13% 6% 10% 12% 12% 8% 5% 4 5% 18%	1705
	11th to 20th	Day	12% 16% 6% 14% 9% 7% 7% 3 4 4 17%	1753
	21st to end	Day	10% 13% 5% 12% 9% 8% 8% 4 4 8% 20%	1650
March	1st to 10th	Day	14% 14% 5% 11% 8% 7% 8% 4 5% 6% 17%	4511
	11th to 20th	Day	17% 18% 5% 10% 6% 7% 5% 5% 5% 6% 16%	4980
	21st to end	Day	19% 18% 6% 13% 6% 5% 3 2 4 4 21%	5466
April	1st to 10th	Day	20% 15% 4 10% 6% 5% 4 3 4 5% 25%	2438
	11th to 20th	Day	18% 13% 4 8% 5% 4 3 4 5% 6% 30%	2572
	21st to end	Day	22% 14% 3 7% 4 4 3 3 4 4 32%	2600
May	1st to 10th	Day	19% 13% 3 8% 5% 4 4 3 5% 6% 30%	5886
	11th to 20th	Day	24% 10% 3 7% 4 5% 4 3 5% 6% 29%	6572
	21st to end	Day	20% 8% 2 6% 4 4 4 4 6% 7% 35%	7451
June	1st to 10th	Day	21% 9% 3 6% 5% 5% 4 4 6% 6% 32%	3000
	11th to 20th	Day	21% 6% 3 6% 5% 5% 4 4 6% 8% 34%	3000
	21st to end	Day	21% 8% 3 8% 5% 5% 5% 4 6% 7% 28%	3000
July	1st to 10th	Day	17% 7% 3 7% 4 4 5% 4 6% 8% 34%	4910
	11th to 20th	Day	14% 6% 2 6% 5% 5% 6% 5% 7% 8% 36%	4910
	21st to end	Day	12% 5% 2 6% 5% 5% 5% 4 7% 8% 41%	4855
August	1st to 10th	Day	14% 6% 2 6% 4 5% 6% 5% 7% 9% 36%	4243
	11th to 20th	Day	18% 6% 2 6% 5% 5% 5% 4 7% 9% 32%	4045
	21st to end	Day	17% 7% 2 6% 4 4 4 3 6% 8% 38%	4532
September	1st to 10th	Day	18% 7% 2 6% 4 4 4 3 5% 7% 40%	2289
	11th to 20th	Day	21% 7% 2 5% 3 4 3 2 4 5% 46%	2090
	21st to end	Day	19% 8% 2 7% 4 4 4 3 5% 6% 38%	2090
October	1st to 10th	Day	19% 10% 3 7% 5% 4 3 3 5% 6% 34%	1990
	11th to 20th	Day	25% 12% 3 7% 4 3 4 2 5% 6% 27%	1836
	21st to end	Day	24% 15% 3 7% 4 4 4 3 4 5% 26%	1881
November	1st to 10th	Day	19% 17% 5% 10% 5% 5% 3 4 5% 4 23%	1704
	11th to 20th	Day	20% 19% 6% 12% 5% 4 4 3 4 4 19%	1600
	21st to end	Day	19% 20% 5% 15% 7% 4 3 3 4 5% 16%	1493
December	1st to 10th	Day	14% 20% 6% 19% 14% 5% 3 1 2 3 13%	2280
	11th to 20th	Day	17% 15% 4 18% 11% 6% 4 2 3 4 16%	2618
	21st to end	Day	15% 13% 5% 14% 12% 11% 5% 3 3 5% 13%	3476
January	1st to 10th	Night	12% 24% 9% 18% 12% 9% 4 2 3 2 5%	6808
	11th to 20th	Night	9% 17% 8% 18% 12% 8% 5% 3 4 4 13%	6453
	21st to end	Night	4 10% 5% 12% 9% 10% 14% 7% 5% 5% 19%	6996
February	1st to 10th	Night	9% 13% 6% 13% 11% 9% 7% 4 4 4 20%	2695
	11th to 20th	Night	12% 16% 7% 12% 9% 8% 5% 4 4 5% 18%	2531
	21st to end	Night	10% 15% 5% 11% 9% 10% 5% 3 4 4 24%	1980
March	1st to 10th	Night	14% 14% 5% 13% 7% 6% 9% 3 3 4 22%	5328
	11th to 20th	Night	18% 19% 5% 10% 7% 5% 5% 3 4 5% 17%	4979
	21st to end	Night	21% 17% 4 10% 7% 5% 3 2 3 4 22%	5466
April	1st to 10th	Night	22% 15% 5% 10% 6% 4 4 3 4 3 25%	1960
	11th to 20th	Night	20% 14% 2 7% 4 4 4 4 6% 5% 31%	1778
	21st to end	Night	25% 10% 3 7% 4 4 3 3 4 6% 31%	1800
May	1st to 10th	Night	19% 9% 3 7% 5% 5% 5% 3 5% 6% 33%	4054
	11th to 20th	Night	23% 10% 3 7% 5% 3 4 4 5% 7% 31%	3378
	21st to end	Night	19% 7% 2 6% 4 5% 5% 5% 5% 6% 35%	3446
June	1st to 10th	Night	21% 8% 3 7% 5% 4 5% 3 7% 6% 32%	1400
	11th to 20th	Night	20% 8% 2 5% 4 5% 5% 4 6% 7% 33%	1400
	21st to end	Night	22% 8% 3 7% 6% 5% 5% 3 6% 7% 28%	1400
July	1st to 10th	Night	16% 8% 3 7% 5% 5% 5% 4 6% 7% 33%	2280
	11th to 20th	Night	14% 7% 3 8% 5% 5% 6% 5% 7% 9% 32%	2280
	21st to end	Night	13% 6% 2 6% 4 4 5% 5% 7% 8% 40%	2697
August	1st to 10th	Night	18% 6% 2 7% 5% 6% 6% 4 6% 8% 32%	2655
	11th to 20th	Night	14% 7% 3 9% 5% 6% 6% 5% 8% 8% 29%	2782
	21st to end	Night	20% 6% 2 5% 4 4 5% 3 5% 5% 41%	3124
September	1st to 10th	Night	18% 6% 3 6% 4 5% 4 3 5% 6% 41%	1888
	11th to 20th	Night	22% 7% 2 7% 4 3 3 4 4 6% 39%	2090
	21st to end	Night	19% 7% 2 6% 3 3 3 3 4 7% 42%	2090
October	1st to 10th	Night	20% 8% 2 6% 4 4 4 3 5% 5% 38%	2190
	11th to 20th	Night	26% 13% 3 6% 4 3 4 3 5% 5% 28%	2342
	21st to end	Night	23% 14% 3 8% 5% 3 4 3 4 6% 26%	2717
November	1st to 10th	Night	21% 15% 5% 9% 5% 5% 4 3 4 6% 25%	2592
	11th to 20th	Night	19% 20% 6% 11% 5% 4 3 2 4 4 22%	2800
	21st to end	Night	18% 20% 7% 16% 7% 5% 4 3 3 4 14%	2905
December	1st to 10th	Night	12% 17% 7% 20% 11% 6% 4 3 4 3 13%	4910
	11th to 20th	Night	17% 16% 5% 14% 13% 6% 4 3 3 4 14%	5591
	21st to end	Night	14% 15% 7% 14% 10% 8% 4 3 4 4 17%	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	87% 5% 5% 3 1	3158
	11th to 20th	Day	81% 4 8% 5% 2	3466
	21st to end	Day	65% 8% 9% 11% 7%	3960
February	1st to 10th	Day	68% 5% 9% 11% 6% 1	1705
	11th to 20th	Day	73% 4 10% 9% 5%	1761
	21st to end	Day	64% 6% 9% 15% 5%	1650
March	1st to 10th	Day	68% 7% 9% 11% 4 1	4514
	11th to 20th	Day	69% 7% 9% 11% 4	4980
	21st to end	Day	68% 3 6% 15% 6% 1	5466
April	1st to 10th	Day	64% 4 9% 13% 9% 1	2438
	11th to 20th	Day	57% 5% 10% 18% 10% 1	2572
	21st to end	Day	57% 5% 8% 18% 11% 2	2600
May	1st to 10th	Day	58% 7% 9% 17% 8% 1	5897
	11th to 20th	Day	56% 7% 12% 16% 8% 1	6576
	21st to end	Day	50% 6% 11% 25% 9%	7451
June	1st to 10th	Day	54% 7% 10% 17% 11% 2	3000
	11th to 20th	Day	49% 7% 12% 19% 12% 2	3000
	21st to end	Day	57% 8% 11% 14% 10% 1	3000
July	1st to 10th	Day	49% 6% 12% 23% 9%	4835
	11th to 20th	Day	45% 8% 20% 21% 5%	4829
	21st to end	Day	40% 9% 17% 25% 8%	4700
August	1st to 10th	Day	43% 9% 20% 22% 6%	4229
	11th to 20th	Day	49% 9% 19% 19% 4	4084
	21st to end	Day	45% 7% 17% 19% 11%	4532
September	1st to 10th	Day	44% 7% 14% 23% 11%	2289
	11th to 20th	Day	43% 6% 15% 26% 10%	2090
	21st to end	Day	47% 7% 12% 24% 10%	2090
October	1st to 10th	Day	56% 6% 13% 19% 5% 1	3302
	11th to 20th	Day	58% 5% 12% 22% 3	3026
	21st to end	Day	58% 6% 13% 17% 6%	3081
November	1st to 10th	Day	65% 7% 10% 13% 5%	1704
	11th to 20th	Day	71% 5% 10% 11% 4	1600
	21st to end	Day	73% 6% 10% 9% 2	1493
December	1st to 10th	Day	80% 3 9% 7% 1	2280
	11th to 20th	Day	72% 5% 7% 11% 4	2618
	21st to end	Day	73% 6% 7% 8% 5%	3476
January	1st to 10th	Night	85% 5% 6% 4	6808
	11th to 20th	Night	74% 6% 9% 8% 3	6498
	21st to end	Night	61% 7% 11% 15% 5% 1	6996
February	1st to 10th	Night	65% 6% 10% 11% 7% 1	2695
	11th to 20th	Night	69% 5% 10% 11% 5%	2534
	21st to end	Night	62% 6% 10% 15% 6%	1980
March	1st to 10th	Night	65% 5% 9% 13% 8%	5338
	11th to 20th	Night	68% 5% 12% 10% 5%	4980
	21st to end	Night	67% 4 6% 13% 9% 1	5466
April	1st to 10th	Night	64% 3 8% 12% 11% 2	1960
	11th to 20th	Night	54% 6% 9% 18% 14% 1	1778
	21st to end	Night	55% 6% 8% 18% 12% 1	1800
May	1st to 10th	Night	50% 7% 12% 15% 14% 1	4060
	11th to 20th	Night	52% 6% 11% 21% 9% 1	3379
	21st to end	Night	47% 6% 13% 23% 11% 1	3447
June	1st to 10th	Night	50% 7% 12% 18% 11% 2	1400
	11th to 20th	Night	47% 7% 13% 17% 13% 2	1400
	21st to end	Night	54% 8% 11% 17% 10% 1	1400
July	1st to 10th	Night	47% 7% 15% 25% 5%	2245
	11th to 20th	Night	46% 8% 23% 18% 4	2238
	21st to end	Night	38% 9% 23% 20% 9%	2611
August	1st to 10th	Night	48% 8% 18% 22% 5%	2645
	11th to 20th	Night	48% 10% 22% 14% 5%	2809
	21st to end	Night	44% 7% 14% 22% 13%	3124
September	1st to 10th	Night	44% 6% 14% 25% 11% 1	1888
	11th to 20th	Night	47% 6% 17% 21% 9% 1	2090
	21st to end	Night	43% 7% 13% 24% 12% 1	2090
October	1st to 10th	Night	49% 6% 10% 26% 7%	3658
	11th to 20th	Night	56% 5% 13% 24% 3	3891
	21st to end	Night	55% 8% 15% 17% 4 1	4467
November	1st to 10th	Night	61% 6% 14% 14% 4 1	2592
	11th to 20th	Night	66% 7% 10% 12% 5%	2800
	21st to end	Night	75% 5% 10% 6% 3 1	2905
December	1st to 10th	Night	75% 5% 10% 8% 1	4910
	11th to 20th	Night	74% 6% 8% 8% 4	5592
	21st to end	Night	71% 6% 9% 10% 4	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	25% 29% 34% 10% 2 1	200
	11th to 20th	Day	24% 26% 28% 14% 7% 2	200
	21st to end	Day	19% 24% 35% 17% 6%	220
February	1st to 10th	Day	17% 29% 34% 14% 5% 1	305
	11th to 20th	Day	20% 27% 35% 14% 5%	396
	21st to end	Day	21% 26% 33% 13% 5% 2	414
March	1st to 10th	Day	24% 25% 31% 16% 4 1	500
	11th to 20th	Day	25% 24% 30% 12% 7% 1	500
	21st to end	Day	27% 23% 29% 15% 5% 1	550
April	1st to 10th	Day	31% 23% 29% 13% 4 1	500
	11th to 20th	Day	29% 18% 32% 15% 5% 1	492
	21st to end	Day	30% 21% 32% 13% 3 1	500
May	1st to 10th	Day	32% 20% 28% 14% 4 1	503
	11th to 20th	Day	26% 19% 27% 20% 7% 1	674
	21st to end	Day	27% 22% 26% 18% 5% 1	790
June	1st to 10th	Day	26% 17% 30% 20% 5% 2	730
	11th to 20th	Day	25% 18% 29% 20% 8% 1	730
	21st to end	Day	25% 16% 30% 21% 7% 1	730
July	1st to 10th	Day	22% 14% 34% 22% 7% 1	720
	11th to 20th	Day	17% 14% 34% 25% 9% 1	718
	21st to end	Day	18% 11% 37% 25% 8% 1	560
August	1st to 10th	Day	19% 15% 33% 24% 8% 1	486
	11th to 20th	Day	18% 15% 30% 27% 9% 1	479
	21st to end	Day	20% 16% 29% 21% 11% 2	539
September	1st to 10th	Day	23% 17% 30% 20% 8% 2	490
	11th to 20th	Day	22% 17% 31% 18% 9% 2	490
	21st to end	Day	21% 20% 33% 14% 10% 3	490
October	1st to 10th	Day	24% 19% 29% 17% 9% 2	437
	11th to 20th	Day	33% 20% 24% 12% 8% 2	390
	21st to end	Day	32% 23% 24% 14% 7%	429
November	1st to 10th	Day	32% 26% 26% 11% 4	395
	11th to 20th	Day	28% 25% 28% 13% 6% 1	400
	21st to end	Day	33% 28% 25% 11% 3 1	293
December	1st to 10th	Day	29% 19% 33% 11% 8% 2	200
	11th to 20th	Day	31% 25% 32% 9% 4 1	200
	21st to end	Day	24% 35% 25% 14% 3	220
January	1st to 10th	Night	23% 22% 36% 16% 3	900
	11th to 20th	Night	23% 25% 27% 18% 5% 1	830
	21st to end	Night	13% 24% 34% 21% 7%	880
February	1st to 10th	Night	14% 28% 33% 18% 7% 1	800
	11th to 20th	Night	23% 24% 29% 17% 6% 1	783
	21st to end	Night	17% 21% 35% 19% 7% 1	660
March	1st to 10th	Night	20% 23% 31% 21% 5% 1	800
	11th to 20th	Night	25% 22% 27% 17% 7% 1	800
	21st to end	Night	25% 19% 26% 23% 7% 1	880
April	1st to 10th	Night	30% 20% 27% 17% 6% 1	755
	11th to 20th	Night	27% 15% 29% 21% 9% 1	594
	21st to end	Night	30% 15% 25% 22% 7% 1	600
May	1st to 10th	Night	30% 14% 27% 23% 7% 1	600
	11th to 20th	Night	23% 10% 30% 27% 9% 1	459
	21st to end	Night	24% 13% 30% 24% 7% 1	433
June	1st to 10th	Night	23% 8% 28% 34% 7% 1	400
	11th to 20th	Night	20% 11% 27% 31% 11% 1	400
	21st to end	Night	22% 10% 29% 29% 9% 1	400
July	1st to 10th	Night	18% 9% 33% 29% 9% 2	390
	11th to 20th	Night	14% 10% 29% 37% 10% 1	388
	21st to end	Night	18% 6% 28% 35% 13% 1	416
August	1st to 10th	Night	17% 9% 27% 31% 14% 1	462
	11th to 20th	Night	18% 6% 27% 35% 14% 1	562
	21st to end	Night	19% 9% 27% 29% 15% 1	627
September	1st to 10th	Night	21% 12% 29% 26% 12%	672
	11th to 20th	Night	23% 10% 27% 26% 12% 2	760
	21st to end	Night	21% 13% 28% 27% 10% 1	760
October	1st to 10th	Night	23% 17% 27% 25% 8% 1	760
	11th to 20th	Night	32% 17% 22% 20% 8% 1	760
	21st to end	Night	29% 22% 21% 22% 6%	836
November	1st to 10th	Night	29% 20% 28% 17% 4	807
	11th to 20th	Night	27% 23% 27% 16% 5% 1	900
	21st to end	Night	26% 22% 32% 15% 5%	900
December	1st to 10th	Night	25% 22% 29% 17% 5% 1	900
	11th to 20th	Night	26% 24% 27% 17% 5% 1	900
	21st to end	Night	24% 24% 26% 20% 5% 1	990

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