Observatoire de la Montagne Coupée
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Observatoire de la Montagne Coupée clear sky chart from 2005-11-06 to 2025-04-11.

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 5% 6% 33%	113429
	Night	16% 11% 4 9% 5% 4 4 3 5% 6% 33%	114858
Transparency	Day	49% 7% 13% 19% 10% 1	116909
	Night	51% 6% 11% 19% 11% 1	119542
Seeing	Day	22% 15% 31% 21% 8% 2	14977
	Night	22% 10% 30% 28% 9% 1	24074

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% 19% 6% 12% 6% 4 4 3 3 5% 26%	3597
	11th to 20th	Day	12% 14% 6% 13% 6% 5% 5% 4 4 5% 26%	3570
	21st to end	Day	8% 13% 7% 13% 6% 5% 4 3 5% 5% 31%	3960
February	1st to 10th	Day	12% 17% 6% 10% 6% 6% 4 3 4 5% 28%	1721
	11th to 20th	Day	12% 14% 5% 12% 5% 5% 4 3 5% 5% 28%	1853
	21st to end	Day	15% 17% 4 9% 7% 5% 4 3 3 4 29%	1650
March	1st to 10th	Day	20% 12% 5% 10% 7% 6% 4 3 4 4 24%	4476
	11th to 20th	Day	10% 15% 4 10% 5% 5% 4 3 5% 7% 33%	4687
	21st to end	Day	18% 12% 4 8% 5% 4 4 4 4 6% 34%	5970
April	1st to 10th	Day	20% 11% 3 8% 6% 4 4 3 4 5% 31%	2400
	11th to 20th	Day	22% 8% 2 6% 4 4 4 4 5% 6% 36%	2254
	21st to end	Day	21% 10% 4 7% 4 4 4 4 4 4 33%	2456
May	1st to 10th	Day	21% 8% 3 7% 5% 4 4 4 5% 5% 34%	4384
	11th to 20th	Day	17% 8% 3 7% 5% 5% 5% 4 6% 8% 33%	4398
	21st to end	Day	16% 6% 3 7% 5% 5% 5% 5% 6% 7% 35%	4837
June	1st to 10th	Day	21% 7% 3 7% 5% 5% 5% 4 6% 7% 29%	2660
	11th to 20th	Day	15% 6% 3 7% 5% 5% 6% 4 6% 8% 35%	2660
	21st to end	Day	20% 9% 3 8% 6% 5% 6% 4 5% 7% 27%	2660
July	1st to 10th	Day	12% 6% 2 7% 6% 5% 6% 5% 7% 8% 35%	4440
	11th to 20th	Day	14% 5% 2 6% 5% 5% 6% 5% 8% 9% 35%	4440
	21st to end	Day	11% 5% 2 6% 5% 4 5% 4 7% 8% 43%	4814
August	1st to 10th	Day	13% 6% 2 7% 6% 6% 7% 5% 7% 10% 31%	4423
	11th to 20th	Day	16% 6% 2 6% 5% 5% 5% 4 8% 9% 34%	4074
	21st to end	Day	16% 6% 2 6% 4 5% 5% 4 6% 8% 38%	4180
September	1st to 10th	Day	14% 5% 2 6% 4 4 5% 3 6% 8% 43%	2280
	11th to 20th	Day	14% 5% 2 5% 4 3 4 3 5% 7% 48%	2195
	21st to end	Day	20% 6% 3 5% 3 4 3 4 5% 6% 41%	2090
October	1st to 10th	Day	21% 7% 2 6% 4 4 5% 3 5% 6% 37%	1969
	11th to 20th	Day	21% 8% 4 8% 6% 5% 4 4 5% 7% 29%	1838
	21st to end	Day	24% 10% 3 7% 4 3 4 3 4 5% 32%	1881
November	1st to 10th	Day	15% 10% 3 7% 5% 5% 5% 4 5% 6% 36%	1761
	11th to 20th	Day	17% 10% 4 9% 5% 4 4 4 6% 6% 31%	1705
	21st to end	Day	21% 14% 3 8% 4 5% 5% 3 5% 6% 27%	1600
December	1st to 10th	Day	19% 17% 7% 12% 5% 5% 3 3 3 4 22%	2600
	11th to 20th	Day	18% 15% 4 9% 5% 4 4 4 4 4 29%	2986
	21st to end	Day	22% 13% 5% 10% 5% 4 4 3 4 4 27%	3960
January	1st to 10th	Night	13% 19% 7% 13% 5% 4 4 3 4 4 24%	6349
	11th to 20th	Night	9% 14% 7% 17% 8% 5% 5% 3 4 4 25%	6315
	21st to end	Night	9% 12% 5% 13% 7% 7% 6% 4 5% 5% 27%	6996
February	1st to 10th	Night	12% 15% 5% 10% 6% 5% 5% 4 5% 5% 29%	2675
	11th to 20th	Night	12% 13% 4 12% 7% 5% 5% 4 4 4 31%	2432
	21st to end	Night	14% 13% 3 9% 5% 4 5% 3 5% 6% 32%	1980
March	1st to 10th	Night	22% 13% 4 10% 5% 4 4 3 3 5% 27%	5364
	11th to 20th	Night	12% 14% 4 8% 5% 5% 5% 4 5% 6% 32%	5267
	21st to end	Night	16% 11% 4 10% 5% 5% 5% 3 5% 5% 32%	4962
April	1st to 10th	Night	21% 11% 4 7% 5% 4 4 2 4 7% 31%	2000
	11th to 20th	Night	20% 7% 2 6% 4 4 5% 3 4 5% 38%	1882
	21st to end	Night	20% 10% 3 5% 4 3 3 4 5% 6% 36%	1722
May	1st to 10th	Night	20% 9% 4 8% 5% 4 4 4 6% 6% 31%	2584
	11th to 20th	Night	20% 7% 2 6% 4 4 5% 4 4 7% 38%	2495
	21st to end	Night	18% 6% 3 7% 5% 5% 5% 4 6% 8% 33%	2738
June	1st to 10th	Night	19% 7% 2 5% 5% 5% 4 4 7% 7% 35%	1520
	11th to 20th	Night	18% 7% 3 6% 4 5% 4 4 6% 7% 37%	1520
	21st to end	Night	18% 7% 4 7% 5% 4 5% 4 5% 7% 35%	1520
July	1st to 10th	Night	13% 6% 3 7% 5% 5% 6% 5% 7% 8% 35%	2520
	11th to 20th	Night	14% 5% 2 6% 4 4 5% 4 6% 7% 43%	2520
	21st to end	Night	11% 4 2 4 4 4 5% 5% 6% 8% 48%	2739
August	1st to 10th	Night	12% 6% 2 8% 5% 5% 5% 4 5% 8% 39%	2506
	11th to 20th	Night	17% 5% 2 6% 5% 4 4 3 5% 7% 41%	2720
	21st to end	Night	19% 6% 2 6% 4 4 4 3 5% 6% 41%	3476
September	1st to 10th	Night	11% 5% 2 5% 4 4 4 4 5% 7% 49%	1900
	11th to 20th	Night	14% 5% 2 5% 3 3 4 3 5% 6% 52%	1985
	21st to end	Night	19% 5% 2 5% 3 3 3 3 5% 7% 45%	2090
October	1st to 10th	Night	17% 7% 2 7% 5% 4 4 3 5% 4 42%	2209
	11th to 20th	Night	21% 8% 2 6% 4 4 4 4 5% 6% 37%	2342
	21st to end	Night	22% 9% 3 8% 5% 4 4 3 5% 6% 32%	2717
November	1st to 10th	Night	17% 9% 4 8% 4 4 4 4 5% 6% 35%	2537
	11th to 20th	Night	18% 10% 3 7% 5% 4 4 3 5% 6% 35%	2692
	21st to end	Night	21% 13% 4 8% 5% 4 4 3 4 5% 30%	2800
December	1st to 10th	Night	17% 14% 5% 10% 7% 5% 5% 4 5% 7% 22%	4580
	11th to 20th	Night	24% 12% 4 9% 5% 4 3 2 4 4 29%	5208
	21st to end	Night	22% 17% 4 10% 4 3 3 2 3 4 29%	6996

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	59% 8% 10% 18% 5% 1	3598
	11th to 20th	Day	58% 6% 11% 15% 7% 2	3600
	21st to end	Day	54% 7% 9% 16% 13% 1	3960
February	1st to 10th	Day	59% 6% 8% 17% 8% 3	1721
	11th to 20th	Day	57% 7% 12% 13% 11% 1	1862
	21st to end	Day	59% 5% 8% 15% 10% 3	1650
March	1st to 10th	Day	60% 6% 8% 12% 11% 3	4479
	11th to 20th	Day	50% 7% 18% 11% 11% 3	4687
	21st to end	Day	52% 4 8% 15% 18% 3	5970
April	1st to 10th	Day	54% 5% 10% 16% 12% 2	2400
	11th to 20th	Day	49% 5% 9% 21% 13% 2	2254
	21st to end	Day	54% 6% 9% 16% 13% 3	2456
May	1st to 10th	Day	50% 6% 11% 20% 11% 2	4398
	11th to 20th	Day	44% 12% 17% 17% 9% 2	4402
	21st to end	Day	44% 8% 12% 20% 14% 2	4837
June	1st to 10th	Day	52% 7% 11% 17% 10% 2	2660
	11th to 20th	Day	44% 7% 13% 21% 13% 2	2660
	21st to end	Day	54% 9% 10% 16% 10% 1	2660
July	1st to 10th	Day	43% 10% 14% 23% 10%	4370
	11th to 20th	Day	41% 10% 18% 24% 7%	4365
	21st to end	Day	37% 9% 15% 29% 10%	4661
August	1st to 10th	Day	45% 10% 16% 21% 8%	4409
	11th to 20th	Day	43% 10% 18% 22% 6%	4116
	21st to end	Day	41% 9% 15% 20% 14% 1	4180
September	1st to 10th	Day	38% 8% 16% 25% 13% 1	2280
	11th to 20th	Day	36% 7% 14% 30% 12% 1	2195
	21st to end	Day	43% 7% 13% 25% 11%	2090
October	1st to 10th	Day	44% 7% 15% 25% 8%	3268
	11th to 20th	Day	45% 6% 17% 25% 6% 1	3028
	21st to end	Day	42% 5% 22% 20% 9% 1	3081
November	1st to 10th	Day	47% 7% 14% 21% 10% 1	1761
	11th to 20th	Day	52% 6% 14% 16% 11% 1	1705
	21st to end	Day	58% 6% 14% 15% 6% 1	1600
December	1st to 10th	Day	66% 5% 10% 15% 5%	2600
	11th to 20th	Day	57% 5% 9% 17% 12%	2986
	21st to end	Day	61% 5% 11% 13% 10%	3960
January	1st to 10th	Night	63% 7% 10% 14% 5%	6349
	11th to 20th	Night	62% 7% 11% 16% 5% 1	6360
	21st to end	Night	57% 7% 10% 12% 14% 1	6996
February	1st to 10th	Night	57% 6% 9% 17% 10% 1	2675
	11th to 20th	Night	56% 5% 9% 17% 11% 1	2434
	21st to end	Night	52% 6% 8% 19% 13% 2	1980
March	1st to 10th	Night	60% 5% 8% 13% 11% 4	5374
	11th to 20th	Night	51% 6% 14% 14% 13% 3	5268
	21st to end	Night	52% 4 8% 21% 12% 2	4962
April	1st to 10th	Night	53% 5% 9% 16% 15% 2	2000
	11th to 20th	Night	47% 6% 9% 20% 15% 3	1882
	21st to end	Night	50% 6% 7% 19% 16% 2	1722
May	1st to 10th	Night	52% 8% 12% 17% 9% 3	2590
	11th to 20th	Night	44% 10% 13% 18% 11% 2	2496
	21st to end	Night	46% 6% 14% 17% 16% 1	2739
June	1st to 10th	Night	46% 8% 12% 20% 13% 1	1520
	11th to 20th	Night	45% 8% 11% 20% 13% 3	1520
	21st to end	Night	49% 8% 11% 19% 13% 1	1520
July	1st to 10th	Night	44% 9% 17% 19% 11% 1	2480
	11th to 20th	Night	39% 9% 18% 28% 7%	2472
	21st to end	Night	33% 8% 20% 27% 12%	2651
August	1st to 10th	Night	44% 8% 13% 26% 8%	2496
	11th to 20th	Night	43% 8% 17% 24% 8%	2744
	21st to end	Night	44% 8% 12% 22% 13% 1	3476
September	1st to 10th	Night	35% 9% 15% 27% 14% 1	1900
	11th to 20th	Night	33% 7% 15% 32% 11% 1	1985
	21st to end	Night	40% 7% 10% 30% 13%	2090
October	1st to 10th	Night	45% 7% 13% 24% 11%	3690
	11th to 20th	Night	41% 6% 13% 31% 9%	3891
	21st to end	Night	44% 5% 17% 18% 13% 2	4467
November	1st to 10th	Night	50% 6% 14% 21% 8% 1	2537
	11th to 20th	Night	51% 6% 12% 18% 12% 1	2692
	21st to end	Night	58% 6% 11% 17% 8% 1	2800
December	1st to 10th	Night	62% 7% 13% 14% 5%	4580
	11th to 20th	Night	59% 5% 8% 15% 11% 1	5208
	21st to end	Night	61% 4 7% 15% 12% 1	6996

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	21% 20% 32% 24% 4	200
	11th to 20th	Day	24% 13% 38% 21% 4 1	200
	21st to end	Day	15% 21% 38% 18% 7%	220
February	1st to 10th	Day	27% 24% 38% 9% 2	261
	11th to 20th	Day	21% 20% 43% 14% 3	400
	21st to end	Day	25% 19% 35% 18% 4	414
March	1st to 10th	Day	25% 18% 34% 19% 4 1	500
	11th to 20th	Day	22% 20% 35% 19% 3 1	500
	21st to end	Day	28% 20% 29% 16% 5% 2	550
April	1st to 10th	Day	29% 19% 28% 17% 6% 1	500
	11th to 20th	Day	29% 20% 27% 15% 8% 1	452
	21st to end	Day	29% 17% 33% 14% 6% 1	460
May	1st to 10th	Day	27% 18% 35% 15% 4 2	463
	11th to 20th	Day	19% 18% 33% 20% 8% 2	487
	21st to end	Day	21% 19% 33% 19% 6% 2	529
June	1st to 10th	Day	23% 13% 34% 21% 8% 1	490
	11th to 20th	Day	17% 18% 32% 21% 10% 2	490
	21st to end	Day	25% 13% 29% 23% 9% 1	490
July	1st to 10th	Day	14% 9% 33% 30% 10% 3	485
	11th to 20th	Day	14% 10% 34% 30% 10% 2	484
	21st to end	Day	12% 8% 29% 35% 12% 4	532
August	1st to 10th	Day	14% 10% 27% 30% 17% 1	486
	11th to 20th	Day	15% 8% 24% 32% 15% 7%	479
	21st to end	Day	16% 13% 27% 24% 14% 5%	539
September	1st to 10th	Day	16% 11% 32% 24% 12% 6%	490
	11th to 20th	Day	13% 10% 27% 27% 15% 8%	490
	21st to end	Day	21% 12% 27% 21% 13% 5%	490
October	1st to 10th	Day	24% 13% 26% 23% 11% 3	490
	11th to 20th	Day	25% 16% 30% 19% 9% 2	457
	21st to end	Day	31% 16% 26% 22% 5%	429
November	1st to 10th	Day	24% 17% 29% 22% 7% 1	395
	11th to 20th	Day	23% 18% 33% 16% 9%	305
	21st to end	Day	29% 21% 31% 17% 4	200
December	1st to 10th	Day	30% 12% 31% 18% 8% 2	200
	11th to 20th	Day	26% 22% 32% 17% 4 1	200
	21st to end	Day	30% 16% 28% 18% 5% 2	220
January	1st to 10th	Night	25% 12% 33% 25% 4	800
	11th to 20th	Night	24% 13% 33% 24% 7%	800
	21st to end	Night	15% 10% 37% 31% 7%	880
February	1st to 10th	Night	19% 12% 41% 24% 5% 1	800
	11th to 20th	Night	19% 15% 38% 23% 4 1	783
	21st to end	Night	20% 14% 35% 26% 4	660
March	1st to 10th	Night	25% 14% 34% 23% 4	800
	11th to 20th	Night	23% 14% 34% 23% 5% 1	800
	21st to end	Night	21% 14% 33% 25% 6% 1	880
April	1st to 10th	Night	28% 13% 30% 24% 5%	800
	11th to 20th	Night	23% 14% 33% 24% 6% 1	754
	21st to end	Night	26% 14% 32% 22% 5% 1	582
May	1st to 10th	Night	27% 12% 30% 24% 6% 1	424
	11th to 20th	Night	22% 10% 31% 29% 7%	374
	21st to end	Night	23% 6% 38% 25% 9%	411
June	1st to 10th	Night	23% 7% 29% 32% 9%	380
	11th to 20th	Night	21% 6% 26% 34% 12% 1	380
	21st to end	Night	21% 5% 25% 36% 13%	380
July	1st to 10th	Night	12% 7% 27% 42% 12%	370
	11th to 20th	Night	13% 2 26% 46% 13% 1	368
	21st to end	Night	14% 5% 22% 44% 14% 2	416
August	1st to 10th	Night	13% 4 18% 48% 16% 2	378
	11th to 20th	Night	15% 2 24% 36% 21% 2	522
	21st to end	Night	17% 5% 26% 31% 18% 2	836
September	1st to 10th	Night	13% 6% 29% 37% 14% 1	760
	11th to 20th	Night	17% 6% 23% 32% 20% 2	760
	21st to end	Night	20% 8% 26% 29% 16% 2	760
October	1st to 10th	Night	22% 9% 29% 28% 12% 1	760
	11th to 20th	Night	27% 10% 27% 26% 10% 1	760
	21st to end	Night	27% 11% 31% 25% 6%	836
November	1st to 10th	Night	25% 10% 29% 27% 8% 1	780
	11th to 20th	Night	24% 13% 31% 22% 10% 1	800
	21st to end	Night	28% 16% 26% 24% 6% 1	800
December	1st to 10th	Night	27% 10% 31% 26% 6%	800
	11th to 20th	Night	27% 14% 30% 23% 6% 1	800
	21st to end	Night	29% 13% 31% 23% 5%	880

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