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 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	17% 11% 3 9% 6% 5% 5% 4 5% 6% 29%	101709
	Night	17% 13% 4 10% 7% 6% 5% 3 5% 5% 25%	97393
Transparency	Day	57% 7% 12% 17% 7% 1	105155
	Night	59% 7% 12% 15% 6% 1	102037
Seeing	Day	24% 20% 30% 18% 7% 1	15946
	Night	23% 18% 28% 22% 8% 1	23835

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% 7% 16% 13% 17% 5% 2 3 3 5%	2207
	11th to 20th	Day	10% 11% 5% 18% 20% 9% 6% 5% 4 4 8%	2425
	21st to end	Day	6% 10% 5% 12% 10% 14% 9% 5% 6% 6% 18%	2772
February	1st to 10th	Day	7% 13% 6% 10% 11% 12% 8% 4 4 5% 19%	1619
	11th to 20th	Day	12% 16% 5% 14% 9% 8% 7% 3 4 4 17%	1663
	21st to end	Day	10% 13% 5% 11% 9% 8% 8% 4 4 7% 20%	1570
March	1st to 10th	Day	14% 11% 4 9% 6% 7% 7% 4 6% 8% 23%	3137
	11th to 20th	Day	19% 21% 5% 11% 5% 5% 4 3 4 5% 18%	3480
	21st to end	Day	19% 18% 5% 12% 5% 4 2 2 3 4 25%	3816
April	1st to 10th	Day	20% 16% 5% 9% 6% 4 3 3 4 5% 25%	2316
	11th to 20th	Day	18% 13% 4 8% 5% 4 3 4 5% 6% 31%	2442
	21st to end	Day	22% 13% 3 7% 4 4 4 3 4 4 33%	2470
May	1st to 10th	Day	16% 12% 3 8% 6% 4 4 3 5% 7% 31%	4106
	11th to 20th	Day	20% 11% 3 7% 5% 5% 4 3 5% 7% 31%	4568
	21st to end	Day	20% 8% 3 7% 5% 4 4 4 6% 7% 33%	5185
June	1st to 10th	Day	21% 9% 3 6% 5% 5% 4 4 6% 6% 31%	2850
	11th to 20th	Day	21% 6% 2 6% 5% 5% 4 4 6% 8% 34%	2850
	21st to end	Day	22% 8% 3 8% 5% 5% 4 4 6% 7% 28%	2850
July	1st to 10th	Day	17% 8% 3 7% 4 4 5% 4 6% 8% 34%	4760
	11th to 20th	Day	14% 6% 2 6% 5% 5% 6% 5% 7% 8% 36%	4760
	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% 9% 5% 5% 3 4 5% 4 24%	1644
	11th to 20th	Day	19% 17% 7% 13% 5% 5% 4 3 4 4 19%	1520
	21st to end	Day	19% 18% 5% 15% 7% 4 3 3 4 5% 16%	1419
December	1st to 10th	Day	17% 20% 7% 17% 12% 7% 4 2 3 4 9%	1330
	11th to 20th	Day	17% 19% 5% 16% 10% 7% 6% 2 3 3 13%	1668
	21st to end	Day	18% 12% 5% 15% 10% 8% 4 3 3 4 17%	2431
January	1st to 10th	Night	10% 16% 8% 18% 15% 12% 5% 3 3 3 6%	4748
	11th to 20th	Night	11% 16% 7% 15% 11% 9% 6% 4 4 4 14%	4536
	21st to end	Night	6% 12% 7% 13% 10% 9% 7% 4 4 5% 23%	4884
February	1st to 10th	Night	9% 13% 6% 12% 11% 9% 6% 5% 4 4 21%	2561
	11th to 20th	Night	12% 16% 7% 12% 9% 7% 5% 4 4 5% 18%	2402
	21st to end	Night	10% 15% 5% 12% 9% 10% 5% 3 4 5% 23%	1884
March	1st to 10th	Night	12% 12% 4 11% 6% 6% 8% 3 4 5% 27%	3702
	11th to 20th	Night	20% 24% 5% 10% 6% 5% 4 2 3 3 17%	3479
	21st to end	Night	22% 15% 4 11% 5% 5% 3 3 4 5% 24%	3816
April	1st to 10th	Night	22% 15% 4 10% 6% 5% 4 3 4 3 25%	1862
	11th to 20th	Night	20% 13% 2 7% 4 4 4 4 6% 5% 31%	1688
	21st to end	Night	24% 10% 3 7% 4 4 3 3 4 6% 32%	1710
May	1st to 10th	Night	20% 9% 3 7% 4 5% 5% 4 5% 7% 32%	2834
	11th to 20th	Night	17% 10% 3 8% 5% 3 5% 4 5% 7% 33%	2360
	21st to end	Night	21% 7% 3 7% 4 5% 5% 5% 6% 7% 30%	2401
June	1st to 10th	Night	21% 8% 3 7% 5% 4 5% 3 6% 6% 32%	1330
	11th to 20th	Night	20% 8% 2 5% 5% 5% 5% 4 6% 7% 33%	1330
	21st to end	Night	21% 8% 4 7% 6% 5% 5% 3 6% 6% 29%	1330
July	1st to 10th	Night	17% 8% 2 7% 5% 5% 5% 4 6% 7% 33%	2210
	11th to 20th	Night	14% 7% 3 8% 5% 5% 6% 5% 7% 9% 32%	2210
	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% 14% 5% 10% 5% 4 4 3 4 6% 26%	2505
	11th to 20th	Night	19% 19% 6% 11% 5% 4 3 3 4 4 22%	2660
	21st to end	Night	18% 19% 7% 16% 7% 5% 4 3 3 4 15%	2759
December	1st to 10th	Night	18% 19% 6% 16% 10% 7% 4 3 4 3 11%	2850
	11th to 20th	Night	16% 18% 5% 15% 11% 8% 5% 3 3 4 12%	3531
	21st to end	Night	16% 16% 7% 14% 9% 6% 5% 3 4 4 17%	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	84% 6% 6% 3 1	2208
	11th to 20th	Day	80% 4 7% 6% 3	2425
	21st to end	Day	66% 7% 8% 11% 8%	2772
February	1st to 10th	Day	67% 5% 9% 12% 6% 1	1619
	11th to 20th	Day	72% 4 10% 9% 5%	1671
	21st to end	Day	64% 6% 9% 15% 5% 1	1570
March	1st to 10th	Day	60% 8% 11% 15% 4 1	3140
	11th to 20th	Day	71% 6% 7% 11% 4	3480
	21st to end	Day	64% 4 7% 16% 6% 2	3816
April	1st to 10th	Day	64% 4 9% 13% 9% 1	2316
	11th to 20th	Day	56% 5% 10% 19% 10% 1	2442
	21st to end	Day	56% 5% 8% 18% 11% 2	2470
May	1st to 10th	Day	56% 8% 11% 17% 8% 1	4120
	11th to 20th	Day	52% 8% 11% 18% 10% 1	4572
	21st to end	Day	52% 7% 10% 22% 10% 1	5185
June	1st to 10th	Day	54% 7% 10% 16% 11% 2	2850
	11th to 20th	Day	48% 7% 13% 19% 12% 2	2850
	21st to end	Day	57% 8% 11% 14% 10%	2850
July	1st to 10th	Day	49% 6% 12% 23% 9%	4685
	11th to 20th	Day	45% 8% 20% 22% 5%	4679
	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%	1644
	11th to 20th	Day	70% 5% 10% 11% 4	1520
	21st to end	Day	73% 6% 10% 9% 2	1419
December	1st to 10th	Day	83% 4 7% 5% 1	1330
	11th to 20th	Day	77% 6% 6% 8% 3	1668
	21st to end	Day	72% 7% 8% 7% 5% 1	2431
January	1st to 10th	Night	82% 6% 7% 4 1	4748
	11th to 20th	Night	71% 7% 9% 9% 4	4536
	21st to end	Night	61% 7% 11% 15% 6% 1	4884
February	1st to 10th	Night	65% 6% 10% 10% 7% 1	2561
	11th to 20th	Night	69% 5% 10% 11% 5%	2405
	21st to end	Night	63% 6% 10% 15% 6%	1884
March	1st to 10th	Night	57% 7% 12% 16% 8%	3712
	11th to 20th	Night	72% 5% 9% 10% 4	3480
	21st to end	Night	65% 4 8% 15% 7% 1	3816
April	1st to 10th	Night	64% 3 8% 12% 11% 2	1862
	11th to 20th	Night	53% 6% 9% 17% 14% 1	1688
	21st to end	Night	53% 6% 8% 19% 13% 1	1710
May	1st to 10th	Night	50% 7% 15% 14% 12% 2	2840
	11th to 20th	Night	47% 8% 12% 22% 10% 1	2361
	21st to end	Night	49% 7% 11% 22% 9% 1	2402
June	1st to 10th	Night	51% 7% 12% 19% 10% 2	1330
	11th to 20th	Night	46% 7% 13% 17% 13% 3	1330
	21st to end	Night	54% 8% 11% 17% 9%	1330
July	1st to 10th	Night	47% 7% 15% 25% 5%	2175
	11th to 20th	Night	46% 8% 23% 18% 4 1	2168
	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% 5% 1	2505
	11th to 20th	Night	66% 7% 10% 12% 4	2660
	21st to end	Night	74% 6% 11% 6% 3 1	2759
December	1st to 10th	Night	79% 6% 7% 6% 2	2850
	11th to 20th	Night	78% 5% 7% 7% 3	3532
	21st to end	Night	71% 7% 10% 8% 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% 29% 35% 11% 2 1	180
	11th to 20th	Day	26% 26% 31% 13% 4 1	180
	21st to end	Day	19% 22% 35% 16% 7% 1	198
February	1st to 10th	Day	18% 29% 33% 15% 4 1	279
	11th to 20th	Day	20% 26% 36% 14% 5%	366
	21st to end	Day	21% 26% 33% 13% 5% 2	382
March	1st to 10th	Day	23% 24% 32% 16% 4 1	460
	11th to 20th	Day	24% 24% 30% 13% 8% 2	460
	21st to end	Day	27% 24% 28% 15% 5% 1	506
April	1st to 10th	Day	29% 24% 29% 13% 4 1	460
	11th to 20th	Day	28% 18% 33% 14% 6% 2	452
	21st to end	Day	28% 22% 32% 14% 3 2	460
May	1st to 10th	Day	32% 21% 28% 15% 4 1	463
	11th to 20th	Day	26% 19% 27% 20% 7% 1	626
	21st to end	Day	28% 23% 26% 17% 5% 1	735
June	1st to 10th	Day	26% 17% 29% 21% 6% 2	680
	11th to 20th	Day	24% 18% 28% 21% 8% 1	680
	21st to end	Day	24% 16% 29% 22% 8% 1	680
July	1st to 10th	Day	21% 14% 35% 22% 7% 1	670
	11th to 20th	Day	17% 14% 34% 25% 9% 1	668
	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	31% 26% 26% 12% 4	374
	11th to 20th	Day	27% 25% 28% 13% 7% 1	370
	21st to end	Day	30% 29% 26% 11% 3 1	269
December	1st to 10th	Day	29% 17% 34% 10% 8% 2	180
	11th to 20th	Day	31% 26% 32% 7% 4 1	180
	21st to end	Day	23% 36% 26% 13% 2	198
January	1st to 10th	Night	22% 22% 36% 16% 3	850
	11th to 20th	Night	24% 26% 28% 17% 5%	787
	21st to end	Night	13% 25% 34% 21% 7%	836
February	1st to 10th	Night	14% 28% 33% 18% 7% 1	760
	11th to 20th	Night	22% 24% 29% 18% 6% 1	743
	21st to end	Night	17% 21% 35% 18% 7% 1	628
March	1st to 10th	Night	20% 22% 31% 22% 6% 1	760
	11th to 20th	Night	25% 22% 27% 17% 8% 1	760
	21st to end	Night	24% 20% 26% 22% 7% 1	836
April	1st to 10th	Night	29% 20% 27% 18% 6% 1	717
	11th to 20th	Night	27% 15% 27% 21% 9% 1	564
	21st to end	Night	29% 16% 25% 22% 7% 1	570
May	1st to 10th	Night	30% 13% 26% 23% 7% 1	570
	11th to 20th	Night	22% 10% 30% 28% 9% 1	436
	21st to end	Night	25% 14% 30% 24% 7% 1	411
June	1st to 10th	Night	24% 7% 27% 34% 7% 1	380
	11th to 20th	Night	19% 10% 27% 32% 11% 1	380
	21st to end	Night	22% 9% 28% 29% 10% 1	380
July	1st to 10th	Night	19% 9% 34% 28% 9% 2	370
	11th to 20th	Night	14% 10% 28% 37% 10% 1	368
	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% 18% 4	779
	11th to 20th	Night	27% 24% 27% 16% 5% 1	850
	21st to end	Night	25% 21% 32% 16% 5%	850
December	1st to 10th	Night	26% 22% 29% 16% 5% 1	850
	11th to 20th	Night	26% 24% 27% 17% 5% 1	850
	21st to end	Night	24% 25% 26% 19% 4 1	935

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