Observatoire du Mont-Cosmos
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Observatoire du Mont-Cosmos clear sky chart from 2005-11-06 to 2025-09-04.

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	17% 12% 4 10% 6% 5% 5% 4 5% 6% 27%	125728
	Night	16% 13% 5% 11% 6% 5% 5% 3 5% 5% 27%	123021
Transparency	Day	55% 7% 12% 18% 8% 1	129183
	Night	56% 6% 12% 17% 8% 1	127654
Seeing	Day	24% 19% 30% 18% 7% 2	16293
	Night	23% 16% 29% 24% 8% 1	24756

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	12% 18% 9% 17% 9% 7% 6% 3 3 4 12%	3597
	11th to 20th	Day	7% 16% 7% 21% 11% 6% 4 3 3 3 18%	3570
	21st to end	Day	7% 11% 7% 17% 8% 7% 7% 4 5% 6% 22%	3960
February	1st to 10th	Day	12% 17% 7% 15% 9% 5% 4 3 3 5% 20%	1622
	11th to 20th	Day	11% 21% 8% 13% 6% 6% 5% 2 4 4 20%	1938
	21st to end	Day	11% 17% 5% 12% 7% 5% 4 3 5% 5% 25%	1650
March	1st to 10th	Day	16% 18% 5% 12% 6% 4 3 3 4 5% 23%	4476
	11th to 20th	Day	12% 18% 5% 11% 5% 5% 4 3 4 5% 27%	4782
	21st to end	Day	17% 15% 5% 11% 5% 4 4 3 3 4 29%	5781
April	1st to 10th	Day	22% 16% 3 9% 5% 3 4 3 4 5% 25%	2400
	11th to 20th	Day	19% 13% 3 8% 4 3 4 3 4 5% 34%	2472
	21st to end	Day	23% 14% 3 8% 4 4 4 3 4 5% 28%	2600
May	1st to 10th	Day	25% 12% 3 8% 5% 5% 4 4 4 6% 23%	5964
	11th to 20th	Day	21% 10% 3 8% 5% 5% 5% 4 5% 7% 27%	6318
	21st to end	Day	19% 9% 3 8% 6% 5% 6% 4 6% 6% 28%	6949
June	1st to 10th	Day	22% 9% 3 7% 5% 5% 5% 4 7% 7% 27%	3000
	11th to 20th	Day	20% 6% 2 6% 5% 4 5% 4 7% 8% 32%	3000
	21st to end	Day	23% 10% 3 8% 6% 5% 5% 3 5% 6% 25%	3000
July	1st to 10th	Day	16% 8% 3 9% 7% 6% 6% 5% 7% 8% 25%	6364
	11th to 20th	Day	14% 6% 2 7% 6% 5% 6% 5% 7% 9% 33%	6360
	21st to end	Day	12% 6% 2 6% 4 4 5% 4 7% 8% 41%	6926
August	1st to 10th	Day	17% 7% 2 7% 6% 5% 6% 5% 7% 9% 29%	4502
	11th to 20th	Day	17% 7% 3 8% 5% 6% 6% 4 7% 7% 28%	4175
	21st to end	Day	21% 7% 3 6% 4 5% 4 3 5% 7% 36%	4478
September	1st to 10th	Day	16% 7% 3 8% 4 5% 4 4 6% 7% 37%	2316
	11th to 20th	Day	18% 8% 3 5% 3 3 3 3 5% 6% 43%	2100
	21st to end	Day	19% 8% 3 5% 4 4 4 4 6% 8% 36%	2090
October	1st to 10th	Day	19% 11% 3 7% 4 4 5% 4 6% 7% 30%	2090
	11th to 20th	Day	19% 14% 3 9% 6% 6% 5% 4 3 4 26%	1749
	21st to end	Day	25% 15% 4 8% 6% 5% 4 2 4 5% 22%	1881
November	1st to 10th	Day	19% 15% 4 8% 5% 4 4 3 4 5% 29%	1761
	11th to 20th	Day	19% 17% 6% 10% 6% 4 3 4 4 5% 23%	1800
	21st to end	Day	18% 21% 6% 12% 6% 3 4 3 3 4 20%	1490
December	1st to 10th	Day	15% 17% 7% 15% 7% 5% 4 3 5% 5% 17%	2280
	11th to 20th	Day	15% 16% 6% 12% 6% 5% 4 3 4 5% 24%	2618
	21st to end	Day	17% 19% 6% 11% 6% 5% 4 3 4 4 20%	3669
January	1st to 10th	Night	11% 17% 8% 17% 9% 6% 4 3 3 4 16%	6349
	11th to 20th	Night	8% 18% 8% 21% 11% 6% 5% 3 3 3 15%	6315
	21st to end	Night	6% 12% 7% 14% 8% 7% 6% 4 6% 6% 24%	6996
February	1st to 10th	Night	10% 16% 5% 13% 9% 6% 5% 4 5% 4 22%	2776
	11th to 20th	Night	10% 17% 6% 14% 8% 7% 5% 3 4 4 21%	2347
	21st to end	Night	13% 15% 5% 11% 7% 5% 4 3 5% 5% 27%	1980
March	1st to 10th	Night	17% 14% 5% 11% 6% 5% 6% 3 4 5% 24%	5364
	11th to 20th	Night	13% 15% 5% 11% 7% 6% 5% 4 5% 6% 24%	5176
	21st to end	Night	16% 15% 5% 11% 6% 4 4 3 4 4 28%	5150
April	1st to 10th	Night	20% 16% 4 8% 5% 4 3 4 5% 5% 28%	2000
	11th to 20th	Night	19% 11% 3 8% 5% 5% 4 3 5% 6% 31%	1877
	21st to end	Night	21% 12% 4 9% 5% 4 3 3 3 5% 32%	1800
May	1st to 10th	Night	23% 10% 3 9% 6% 5% 5% 4 6% 6% 25%	4018
	11th to 20th	Night	24% 9% 3 7% 4 5% 4 3 4 5% 31%	3575
	21st to end	Night	19% 7% 3 7% 4 3 5% 4 6% 7% 35%	3909
June	1st to 10th	Night	22% 10% 3 6% 5% 5% 4 4 5% 7% 29%	1400
	11th to 20th	Night	17% 7% 2 8% 5% 4 5% 5% 6% 7% 34%	1400
	21st to end	Night	19% 7% 3 8% 7% 6% 4 4 7% 6% 31%	1400
July	1st to 10th	Night	15% 7% 3 8% 6% 6% 6% 4 7% 9% 29%	3590
	11th to 20th	Night	18% 7% 2 7% 5% 5% 4 4 5% 7% 38%	3600
	21st to end	Night	11% 6% 3 7% 5% 5% 5% 4 5% 7% 41%	3926
August	1st to 10th	Night	18% 8% 3 6% 5% 5% 4 3 5% 8% 34%	2614
	11th to 20th	Night	16% 7% 3 8% 6% 6% 6% 4 6% 7% 30%	2872
	21st to end	Night	18% 6% 3 8% 4 4 4 3 5% 7% 38%	3416
September	1st to 10th	Night	16% 6% 3 7% 4 4 4 4 6% 6% 40%	1931
	11th to 20th	Night	17% 6% 2 5% 3 4 3 4 4 5% 46%	2079
	21st to end	Night	17% 8% 3 6% 4 4 3 3 4 7% 41%	2090
October	1st to 10th	Night	19% 8% 2 7% 4 4 5% 3 5% 5% 38%	2090
	11th to 20th	Night	21% 11% 3 7% 5% 5% 5% 3 5% 5% 31%	2427
	21st to end	Night	23% 14% 4 8% 5% 4 4 4 4 5% 27%	2717
November	1st to 10th	Night	19% 14% 4 8% 6% 5% 4 3 4 5% 28%	2537
	11th to 20th	Night	17% 13% 5% 11% 6% 4 4 3 4 4 29%	2600
	21st to end	Night	17% 19% 6% 11% 6% 4 4 3 4 4 23%	2904
December	1st to 10th	Night	14% 16% 7% 15% 11% 7% 5% 4 4 4 14%	4910
	11th to 20th	Night	15% 16% 6% 13% 8% 6% 4 3 4 5% 20%	5591
	21st to end	Night	17% 19% 6% 11% 5% 4 4 2 4 3 25%	7295

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	75% 4 6% 10% 4	3598
	11th to 20th	Day	70% 5% 6% 13% 5% 1	3600
	21st to end	Day	61% 7% 7% 12% 13% 1	3960
February	1st to 10th	Day	68% 4 6% 11% 9% 2	1622
	11th to 20th	Day	69% 4 9% 10% 7% 1	1947
	21st to end	Day	61% 5% 8% 17% 7% 3	1650
March	1st to 10th	Day	64% 5% 7% 11% 10% 3	4479
	11th to 20th	Day	57% 6% 14% 14% 9% 1	4782
	21st to end	Day	59% 4 6% 16% 13% 2	5781
April	1st to 10th	Day	61% 4 9% 14% 9% 3	2400
	11th to 20th	Day	53% 4 8% 21% 12% 2	2472
	21st to end	Day	58% 4 9% 16% 11% 2	2600
May	1st to 10th	Day	59% 6% 9% 16% 9% 1	5975
	11th to 20th	Day	54% 9% 13% 15% 6% 2	6322
	21st to end	Day	53% 8% 11% 16% 10% 2	6949
June	1st to 10th	Day	54% 8% 10% 20% 7% 1	3000
	11th to 20th	Day	47% 7% 14% 20% 11% 1	3000
	21st to end	Day	58% 8% 10% 16% 8%	3000
July	1st to 10th	Day	52% 8% 15% 20% 5%	6294
	11th to 20th	Day	43% 9% 18% 26% 5%	6285
	21st to end	Day	38% 7% 16% 31% 8%	6728
August	1st to 10th	Day	48% 9% 18% 20% 5%	4488
	11th to 20th	Day	50% 10% 18% 16% 5%	4214
	21st to end	Day	47% 8% 14% 19% 11% 1	4478
September	1st to 10th	Day	43% 9% 14% 22% 10% 1	2316
	11th to 20th	Day	42% 5% 13% 27% 12%	2100
	21st to end	Day	45% 8% 13% 25% 9%	2090
October	1st to 10th	Day	52% 7% 12% 24% 5%	3480
	11th to 20th	Day	52% 6% 16% 23% 3	2874
	21st to end	Day	55% 5% 15% 18% 7%	3081
November	1st to 10th	Day	58% 6% 12% 18% 7%	1761
	11th to 20th	Day	63% 6% 9% 15% 7% 1	1800
	21st to end	Day	68% 5% 11% 12% 3	1490
December	1st to 10th	Day	68% 5% 11% 10% 6%	2280
	11th to 20th	Day	62% 6% 8% 15% 9%	2618
	21st to end	Day	67% 5% 7% 9% 10% 1	3669
January	1st to 10th	Night	70% 6% 9% 9% 5%	6349
	11th to 20th	Night	72% 6% 8% 11% 3	6360
	21st to end	Night	58% 6% 9% 14% 13% 1	6996
February	1st to 10th	Night	64% 5% 8% 13% 9% 1	2776
	11th to 20th	Night	66% 6% 8% 12% 8% 1	2349
	21st to end	Night	58% 4 8% 16% 12% 2	1980
March	1st to 10th	Night	62% 4 9% 12% 11% 2	5374
	11th to 20th	Night	58% 5% 16% 13% 7% 1	5177
	21st to end	Night	59% 4 6% 19% 11% 1	5150
April	1st to 10th	Night	58% 4 9% 15% 12% 3	2000
	11th to 20th	Night	51% 5% 10% 20% 12% 2	1877
	21st to end	Night	55% 5% 9% 17% 12% 2	1800
May	1st to 10th	Night	57% 6% 12% 15% 9% 1	4024
	11th to 20th	Night	52% 8% 12% 20% 8%	3576
	21st to end	Night	46% 7% 15% 17% 14% 1	3910
June	1st to 10th	Night	53% 7% 11% 20% 9%	1400
	11th to 20th	Night	47% 6% 12% 22% 12% 2	1400
	21st to end	Night	51% 8% 14% 18% 9%	1400
July	1st to 10th	Night	46% 9% 20% 20% 5%	3550
	11th to 20th	Night	44% 7% 20% 23% 5%	3552
	21st to end	Night	39% 8% 18% 28% 7%	3809
August	1st to 10th	Night	48% 7% 17% 24% 5%	2604
	11th to 20th	Night	49% 10% 17% 18% 6%	2899
	21st to end	Night	45% 8% 14% 20% 13%	3416
September	1st to 10th	Night	42% 8% 14% 25% 10% 1	1931
	11th to 20th	Night	39% 7% 16% 28% 9% 1	2079
	21st to end	Night	43% 8% 12% 24% 13%	2090
October	1st to 10th	Night	46% 7% 12% 28% 8%	3480
	11th to 20th	Night	49% 8% 13% 25% 4	4041
	21st to end	Night	50% 5% 18% 16% 9%	4467
November	1st to 10th	Night	56% 6% 13% 18% 6%	2537
	11th to 20th	Night	57% 5% 12% 15% 10% 1	2600
	21st to end	Night	65% 6% 10% 14% 5%	2904
December	1st to 10th	Night	70% 8% 10% 11% 2	4910
	11th to 20th	Night	66% 5% 8% 16% 5%	5592
	21st to end	Night	63% 4 8% 15% 10%	7295

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% 21% 35% 15% 2 1	200
	11th to 20th	Day	27% 27% 31% 12% 4	200
	21st to end	Day	19% 28% 34% 14% 5%	220
February	1st to 10th	Day	23% 32% 34% 6% 3 1	222
	11th to 20th	Day	24% 29% 33% 12% 2	491
	21st to end	Day	23% 28% 30% 16% 2	414
March	1st to 10th	Day	29% 24% 30% 13% 4	500
	11th to 20th	Day	27% 26% 28% 15% 3 1	500
	21st to end	Day	28% 25% 27% 12% 7% 1	550
April	1st to 10th	Day	36% 19% 27% 12% 5%	500
	11th to 20th	Day	30% 26% 24% 13% 5% 1	492
	21st to end	Day	30% 21% 34% 9% 4 1	500
May	1st to 10th	Day	29% 20% 31% 14% 5% 1	503
	11th to 20th	Day	24% 16% 30% 20% 8% 1	527
	21st to end	Day	22% 23% 33% 15% 6% 1	573
June	1st to 10th	Day	24% 16% 32% 21% 7% 1	730
	11th to 20th	Day	22% 16% 27% 24% 11% 1	730
	21st to end	Day	26% 15% 28% 21% 9% 1	730
July	1st to 10th	Day	15% 18% 34% 23% 9% 1	529
	11th to 20th	Day	17% 14% 38% 22% 8% 1	524
	21st to end	Day	18% 13% 36% 23% 8% 1	576
August	1st to 10th	Day	17% 13% 31% 23% 12% 2	526
	11th to 20th	Day	16% 11% 29% 27% 14% 3	519
	21st to end	Day	20% 16% 27% 19% 13% 5%	583
September	1st to 10th	Day	17% 15% 29% 24% 10% 4	502
	11th to 20th	Day	20% 12% 27% 22% 14% 5%	490
	21st to end	Day	16% 19% 28% 22% 9% 6%	490
October	1st to 10th	Day	21% 20% 28% 17% 13% 3	490
	11th to 20th	Day	29% 19% 29% 14% 8% 1	403
	21st to end	Day	33% 21% 25% 16% 4	429
November	1st to 10th	Day	32% 18% 27% 18% 5%	395
	11th to 20th	Day	27% 22% 28% 15% 8% 1	400
	21st to end	Day	33% 24% 28% 14% 1	235
December	1st to 10th	Day	30% 20% 26% 19% 4 2	200
	11th to 20th	Day	31% 27% 27% 13% 3 1	200
	21st to end	Day	30% 26% 27% 12% 4 1	220
January	1st to 10th	Night	24% 25% 30% 18% 3	800
	11th to 20th	Night	24% 22% 32% 18% 4	800
	21st to end	Night	15% 20% 35% 25% 5% 1	880
February	1st to 10th	Night	20% 23% 35% 18% 5%	800
	11th to 20th	Night	22% 25% 34% 16% 3	783
	21st to end	Night	24% 20% 32% 21% 4	660
March	1st to 10th	Night	24% 20% 33% 20% 3	800
	11th to 20th	Night	25% 20% 33% 18% 4 1	800
	21st to end	Night	24% 19% 29% 24% 5%	880
April	1st to 10th	Night	30% 17% 30% 18% 5%	800
	11th to 20th	Night	25% 16% 29% 23% 6%	693
	21st to end	Night	28% 19% 27% 21% 6%	600
May	1st to 10th	Night	25% 13% 30% 25% 7%	590
	11th to 20th	Night	25% 10% 28% 29% 8% 1	394
	21st to end	Night	21% 9% 32% 29% 8%	433
June	1st to 10th	Night	26% 8% 26% 31% 9%	400
	11th to 20th	Night	19% 10% 24% 35% 12%	400
	21st to end	Night	20% 10% 27% 34% 9% 1	400
July	1st to 10th	Night	17% 7% 32% 33% 10% 1	390
	11th to 20th	Night	16% 5% 25% 43% 11%	388
	21st to end	Night	14% 7% 25% 44% 10%	438
August	1st to 10th	Night	16% 5% 22% 38% 18%	413
	11th to 20th	Night	18% 6% 26% 33% 16% 1	592
	21st to end	Night	16% 9% 24% 33% 15% 2	775
September	1st to 10th	Night	18% 8% 31% 27% 15% 1	773
	11th to 20th	Night	20% 9% 21% 31% 17% 2	760
	21st to end	Night	20% 11% 27% 25% 15% 1	760
October	1st to 10th	Night	20% 13% 27% 29% 11% 1	760
	11th to 20th	Night	26% 16% 25% 23% 9% 1	760
	21st to end	Night	30% 18% 26% 20% 6%	836
November	1st to 10th	Night	28% 16% 28% 21% 6%	780
	11th to 20th	Night	24% 20% 27% 20% 8% 1	800
	21st to end	Night	29% 20% 29% 16% 5% 1	873
December	1st to 10th	Night	30% 19% 27% 18% 6%	900
	11th to 20th	Night	25% 24% 25% 19% 7% 1	900
	21st to end	Night	30% 21% 27% 17% 4 1	945

Page generated at 2025-09-01 01:51:07. Version ac07

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