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 2024-04-03.

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	18% 11% 4 8% 7% 4 5% 5% 5% 6% 27%	78323
	Night	17% 12% 5% 9% 8% 5% 5% 4 4 5% 24%	81829
Transparency	Day	59% 7% 11% 16% 7% 1	78014
	Night	62% 6% 11% 14% 6% 1	81673
Seeing	Day	25% 20% 30% 17% 7% 1	15046
	Night	23% 18% 28% 22% 8% 1	23207

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% 17% 8% 14% 14% 14% 7% 3 3 3 5%	2207
	11th to 20th	Day	10% 10% 6% 15% 22% 7% 7% 6% 4 4 8%	2425
	21st to end	Day	6% 10% 5% 10% 12% 11% 10% 7% 6% 6% 18%	2772
February	1st to 10th	Day	7% 12% 6% 9% 11% 9% 10% 6% 4 5% 19%	1619
	11th to 20th	Day	12% 15% 6% 13% 10% 5% 9% 5% 4 4 17%	1663
	21st to end	Day	11% 12% 6% 10% 9% 6% 9% 5% 4 8% 20%	1570
March	1st to 10th	Day	14% 11% 6% 10% 8% 6% 7% 5% 5% 6% 21%	1905
	11th to 20th	Day	16% 15% 6% 10% 9% 4 5% 4 4 5% 22%	2090
	21st to end	Day	15% 17% 6% 9% 6% 4 4 3 4 6% 27%	2299
April	1st to 10th	Day	20% 14% 5% 8% 8% 3 4 4 5% 5% 24%	2217
	11th to 20th	Day	19% 12% 4 7% 6% 3 4 4 5% 6% 31%	2312
	21st to end	Day	22% 12% 3 6% 5% 3 4 4 4 4 32%	2340
May	1st to 10th	Day	23% 12% 3 6% 6% 3 3 4 4 5% 31%	2340
	11th to 20th	Day	21% 10% 3 5% 4 3 4 4 5% 6% 35%	2564
	21st to end	Day	22% 8% 3 5% 5% 3 4 4 5% 7% 34%	2919
June	1st to 10th	Day	21% 8% 4 6% 5% 3 6% 5% 5% 6% 31%	2700
	11th to 20th	Day	22% 6% 2 5% 5% 3 4 4 6% 8% 35%	2700
	21st to end	Day	22% 7% 3 7% 6% 4 5% 5% 6% 7% 28%	2700
July	1st to 10th	Day	18% 7% 3 7% 6% 3 6% 5% 6% 8% 32%	2700
	11th to 20th	Day	16% 6% 2 6% 6% 3 6% 5% 6% 8% 37%	2700
	21st to end	Day	15% 6% 3 6% 6% 3 5% 6% 7% 7% 35%	2771
August	1st to 10th	Day	16% 6% 3 7% 5% 4 6% 7% 7% 9% 31%	2409
	11th to 20th	Day	17% 7% 3 4 5% 3 5% 5% 6% 8% 37%	2304
	21st to end	Day	18% 7% 2 6% 6% 3 4 5% 6% 8% 36%	2574
September	1st to 10th	Day	19% 7% 2 5% 5% 3 4 4 4 7% 40%	2170
	11th to 20th	Day	22% 6% 2 4 3 2 3 3 4 5% 44%	1980
	21st to end	Day	20% 7% 2 6% 5% 3 4 4 5% 6% 39%	1980
October	1st to 10th	Day	19% 10% 3 6% 5% 3 4 4 5% 6% 35%	1886
	11th to 20th	Day	25% 12% 3 6% 5% 2 4 3 5% 6% 27%	1740
	21st to end	Day	25% 15% 4 6% 5% 3 4 4 3 5% 25%	1782
November	1st to 10th	Day	20% 15% 6% 8% 6% 4 3 5% 4 5% 24%	1617
	11th to 20th	Day	20% 16% 7% 12% 6% 4 4 4 4 4 19%	1520
	21st to end	Day	19% 17% 6% 14% 9% 3 3 4 3 5% 16%	1419
December	1st to 10th	Day	17% 17% 8% 17% 12% 5% 5% 3 2 4 9%	1330
	11th to 20th	Day	17% 18% 5% 14% 12% 6% 6% 3 3 3 13%	1668
	21st to end	Day	18% 12% 6% 12% 12% 6% 6% 4 3 4 17%	2431
January	1st to 10th	Night	10% 16% 8% 15% 18% 10% 6% 4 3 3 6%	4748
	11th to 20th	Night	11% 15% 8% 13% 13% 7% 7% 5% 4 4 14%	4536
	21st to end	Night	6% 11% 7% 11% 11% 6% 9% 6% 4 6% 23%	4884
February	1st to 10th	Night	9% 12% 7% 11% 11% 8% 7% 6% 4 4 21%	2561
	11th to 20th	Night	12% 15% 7% 11% 10% 6% 6% 5% 4 5% 18%	2402
	21st to end	Night	10% 13% 5% 11% 10% 7% 7% 4 4 5% 23%	1884
March	1st to 10th	Night	12% 14% 5% 10% 9% 4 8% 5% 3 4 25%	2261
	11th to 20th	Night	17% 17% 6% 9% 8% 5% 5% 3 4 4 23%	2089
	21st to end	Night	17% 15% 6% 8% 5% 4 4 4 4 5% 27%	2299
April	1st to 10th	Night	22% 15% 5% 9% 7% 4 4 4 4 4 23%	1786
	11th to 20th	Night	21% 12% 2 6% 4 3 4 5% 6% 5% 32%	1598
	21st to end	Night	25% 9% 4 6% 4 3 4 4 4 6% 32%	1620
May	1st to 10th	Night	26% 10% 4 7% 5% 3 5% 5% 4 5% 27%	1620
	11th to 20th	Night	20% 9% 4 6% 5% 3 5% 4 4 6% 35%	1342
	21st to end	Night	23% 7% 3 6% 5% 3 6% 5% 6% 5% 30%	1356
June	1st to 10th	Night	22% 8% 3 7% 5% 3 5% 4 6% 6% 32%	1260
	11th to 20th	Night	21% 7% 3 4 5% 3 5% 5% 5% 7% 34%	1260
	21st to end	Night	22% 7% 4 6% 7% 4 6% 4 6% 6% 29%	1260
July	1st to 10th	Night	18% 8% 3 6% 6% 5% 5% 5% 5% 8% 30%	1260
	11th to 20th	Night	15% 5% 3 6% 6% 3 6% 7% 8% 9% 32%	1260
	21st to end	Night	17% 7% 3 6% 6% 4 6% 7% 7% 8% 31%	1545
August	1st to 10th	Night	16% 6% 2 5% 6% 4 6% 6% 7% 9% 33%	1511
	11th to 20th	Night	15% 7% 3 7% 5% 3 6% 6% 7% 7% 34%	1596
	21st to end	Night	19% 6% 2 5% 5% 4 6% 6% 6% 7% 36%	1782
September	1st to 10th	Night	18% 6% 3 5% 5% 3 5% 4 5% 6% 41%	1788
	11th to 20th	Night	23% 6% 3 6% 4 3 3 4 4 6% 38%	1980
	21st to end	Night	20% 6% 2 5% 4 2 4 3 4 6% 43%	1980
October	1st to 10th	Night	20% 8% 3 5% 5% 3 4 4 5% 6% 38%	2074
	11th to 20th	Night	26% 13% 3 5% 5% 2 4 4 5% 5% 27%	2218
	21st to end	Night	24% 14% 3 7% 5% 2 4 4 4 7% 26%	2574
November	1st to 10th	Night	21% 13% 6% 9% 6% 4 4 3 4 6% 25%	2459
	11th to 20th	Night	19% 17% 7% 10% 6% 3 4 3 4 4 22%	2660
	21st to end	Night	18% 17% 7% 15% 9% 4 4 3 3 4 15%	2759
December	1st to 10th	Night	18% 17% 7% 15% 11% 5% 5% 4 4 3 11%	2850
	11th to 20th	Night	16% 17% 6% 13% 13% 6% 5% 4 3 4 12%	3531
	21st to end	Night	16% 15% 7% 12% 11% 5% 5% 4 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	66% 6% 9% 13% 5% 1	1908
	11th to 20th	Day	67% 4 8% 13% 7% 1	2090
	21st to end	Day	61% 5% 8% 17% 9% 1	2299
April	1st to 10th	Day	65% 4 10% 12% 8% 1	2217
	11th to 20th	Day	55% 5% 10% 19% 10% 1	2312
	21st to end	Day	56% 5% 8% 17% 11% 2	2340
May	1st to 10th	Day	59% 4 9% 16% 10% 2	2340
	11th to 20th	Day	52% 5% 10% 20% 12% 1	2568
	21st to end	Day	52% 6% 9% 21% 11% 1	2919
June	1st to 10th	Day	55% 7% 10% 15% 11% 2	2700
	11th to 20th	Day	49% 7% 12% 19% 12% 2	2700
	21st to end	Day	57% 8% 11% 14% 10% 1	2700
July	1st to 10th	Day	51% 9% 15% 19% 7%	2625
	11th to 20th	Day	47% 9% 14% 21% 8% 1	2619
	21st to end	Day	48% 11% 14% 19% 8%	2616
August	1st to 10th	Day	49% 11% 16% 18% 5%	2395
	11th to 20th	Day	47% 10% 15% 22% 6%	2304
	21st to end	Day	47% 9% 15% 22% 7%	2574
September	1st to 10th	Day	45% 7% 14% 23% 10%	2170
	11th to 20th	Day	45% 5% 14% 25% 10%	1980
	21st to end	Day	47% 7% 12% 23% 10%	1980
October	1st to 10th	Day	52% 6% 14% 17% 9% 1	1886
	11th to 20th	Day	60% 7% 13% 15% 4	1740
	21st to end	Day	65% 6% 9% 14% 6%	1782
November	1st to 10th	Day	65% 7% 10% 13% 5%	1617
	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	61% 5% 9% 17% 8%	2271
	11th to 20th	Night	67% 4 8% 14% 7% 1	2090
	21st to end	Night	61% 4 8% 17% 9% 1	2299
April	1st to 10th	Night	66% 3 9% 10% 11% 1	1786
	11th to 20th	Night	52% 6% 9% 17% 14% 1	1598
	21st to end	Night	54% 5% 9% 18% 13% 1	1620
May	1st to 10th	Night	59% 5% 8% 15% 10% 2	1620
	11th to 20th	Night	51% 5% 11% 21% 11% 1	1343
	21st to end	Night	53% 7% 8% 18% 12% 2	1357
June	1st to 10th	Night	52% 7% 12% 19% 9% 2	1260
	11th to 20th	Night	47% 7% 12% 18% 13% 3	1260
	21st to end	Night	54% 9% 11% 17% 10%	1260
July	1st to 10th	Night	50% 8% 16% 20% 6%	1225
	11th to 20th	Night	44% 10% 17% 21% 7% 1	1218
	21st to end	Night	48% 11% 15% 20% 7%	1459
August	1st to 10th	Night	45% 11% 20% 20% 4	1501
	11th to 20th	Night	45% 11% 18% 18% 7%	1596
	21st to end	Night	46% 9% 15% 23% 7%	1782
September	1st to 10th	Night	43% 7% 14% 24% 11% 1	1788
	11th to 20th	Night	49% 6% 16% 20% 9% 1	1980
	21st to end	Night	43% 7% 13% 24% 13% 1	1980
October	1st to 10th	Night	48% 7% 12% 22% 10% 1	2074
	11th to 20th	Night	59% 7% 13% 17% 4	2218
	21st to end	Night	59% 7% 13% 14% 5% 1	2574
November	1st to 10th	Night	61% 6% 14% 13% 5% 1	2459
	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	428
	11th to 20th	Day	27% 18% 33% 14% 6% 2	412
	21st to end	Day	27% 20% 32% 15% 4 2	420
May	1st to 10th	Day	33% 21% 27% 14% 4 1	423
	11th to 20th	Day	27% 20% 26% 20% 6% 1	578
	21st to end	Day	28% 23% 25% 17% 5% 1	680
June	1st to 10th	Day	26% 18% 29% 20% 5% 1	630
	11th to 20th	Day	25% 17% 28% 22% 8% 1	630
	21st to end	Day	24% 15% 29% 23% 8% 1	630
July	1st to 10th	Day	21% 14% 34% 22% 7% 1	620
	11th to 20th	Day	17% 14% 32% 25% 10% 1	618
	21st to end	Day	18% 10% 37% 25% 9% 1	515
August	1st to 10th	Day	19% 15% 32% 25% 8% 2	446
	11th to 20th	Day	18% 15% 29% 27% 10% 1	439
	21st to end	Day	20% 16% 30% 20% 11% 2	495
September	1st to 10th	Day	23% 17% 29% 20% 9% 2	450
	11th to 20th	Day	24% 18% 30% 18% 8% 2	450
	21st to end	Day	22% 21% 34% 13% 9% 2	450
October	1st to 10th	Day	24% 20% 29% 17% 8% 2	403
	11th to 20th	Day	33% 19% 24% 12% 9% 3	360
	21st to end	Day	33% 23% 23% 13% 7% 1	396
November	1st to 10th	Day	31% 26% 26% 12% 4	365
	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% 17% 6% 1	689
	11th to 20th	Night	27% 15% 28% 21% 9% 1	534
	21st to end	Night	29% 16% 25% 22% 8% 1	540
May	1st to 10th	Night	31% 13% 25% 23% 7% 1	540
	11th to 20th	Night	23% 9% 31% 28% 8% 1	413
	21st to end	Night	25% 14% 29% 24% 7% 1	389
June	1st to 10th	Night	24% 7% 26% 34% 8% 1	360
	11th to 20th	Night	20% 10% 26% 32% 11% 1	360
	21st to end	Night	22% 9% 28% 30% 10% 1	360
July	1st to 10th	Night	19% 9% 32% 28% 9% 2	350
	11th to 20th	Night	15% 9% 27% 37% 11% 1	348
	21st to end	Night	18% 6% 26% 36% 13% 1	394
August	1st to 10th	Night	17% 8% 27% 32% 15% 1	437
	11th to 20th	Night	18% 5% 27% 34% 14% 1	532
	21st to end	Night	19% 9% 27% 29% 14% 1	594
September	1st to 10th	Night	21% 11% 28% 27% 12%	636
	11th to 20th	Night	24% 11% 26% 26% 11% 2	720
	21st to end	Night	22% 14% 29% 26% 9% 1	720
October	1st to 10th	Night	22% 17% 27% 25% 7% 1	720
	11th to 20th	Night	31% 18% 22% 20% 9% 1	720
	21st to end	Night	29% 22% 20% 22% 7%	792
November	1st to 10th	Night	29% 21% 29% 17% 5%	764
	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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