Observatoire Astronomique Jean-Marc-Richard
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Observatoire Astronomique Jean-Marc-Richard clear sky chart from 2005-11-06 to 2024-11-06.

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% 9% 3 8% 5% 4 5% 4 5% 7% 33%	101539
	Night	18% 10% 4 8% 5% 4 4 3 5% 6% 33%	97528
Transparency	Day	49% 5% 13% 22% 10% 1	105002
	Night	52% 5% 12% 20% 10% 1	102154
Seeing	Day	21% 10% 27% 25% 13% 4	14575
	Night	22% 7% 29% 31% 10% 1	23518

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	16% 16% 5% 12% 6% 6% 5% 3 4 4 24%	2517
	11th to 20th	Day	14% 13% 6% 9% 6% 4 4 3 6% 7% 28%	2520
	21st to end	Day	15% 14% 5% 10% 6% 6% 5% 4 5% 6% 24%	2772
February	1st to 10th	Day	15% 15% 5% 9% 6% 5% 3 4 4 6% 28%	1662
	11th to 20th	Day	13% 13% 5% 10% 5% 6% 5% 5% 5% 6% 28%	1762
	21st to end	Day	17% 16% 4 8% 5% 4 4 3 4 5% 27%	1570
March	1st to 10th	Day	20% 12% 4 9% 6% 6% 5% 3 4 5% 27%	3116
	11th to 20th	Day	14% 17% 5% 10% 5% 4 4 3 5% 5% 28%	3275
	21st to end	Day	15% 8% 4 8% 6% 4 4 4 4 5% 38%	4166
April	1st to 10th	Day	22% 12% 3 7% 6% 4 4 3 3 4 32%	2280
	11th to 20th	Day	21% 9% 3 7% 5% 4 3 3 4 5% 37%	2254
	21st to end	Day	22% 9% 3 8% 5% 4 5% 3 4 6% 32%	2437
May	1st to 10th	Day	20% 8% 3 8% 5% 4 5% 4 6% 7% 31%	4360
	11th to 20th	Day	18% 8% 3 8% 5% 4 5% 4 6% 7% 32%	4398
	21st to end	Day	15% 6% 2 7% 5% 4 5% 5% 6% 8% 36%	4837
June	1st to 10th	Day	20% 8% 3 7% 5% 5% 4 5% 5% 7% 31%	2660
	11th to 20th	Day	14% 6% 2 7% 5% 4 5% 5% 5% 8% 37%	2660
	21st to end	Day	20% 7% 2 7% 6% 5% 5% 4 6% 7% 30%	2660
July	1st to 10th	Day	12% 6% 2 7% 7% 5% 6% 5% 7% 9% 36%	4440
	11th to 20th	Day	13% 5% 2 6% 5% 5% 5% 5% 8% 9% 37%	4440
	21st to end	Day	11% 6% 2 6% 4 4 4 4 6% 8% 46%	4814
August	1st to 10th	Day	14% 5% 2 8% 5% 6% 6% 4 7% 9% 33%	4423
	11th to 20th	Day	14% 6% 3 7% 4 4 5% 4 6% 8% 38%	3998
	21st to end	Day	15% 5% 2 6% 5% 4 4 4 6% 8% 40%	4180
September	1st to 10th	Day	12% 5% 2 7% 4 4 5% 4 6% 9% 42%	2280
	11th to 20th	Day	15% 4 2 4 4 4 4 4 5% 7% 50%	2198
	21st to end	Day	17% 6% 2 6% 5% 4 5% 3 4 7% 41%	2090
October	1st to 10th	Day	20% 7% 2 7% 4 4 5% 4 5% 7% 36%	1966
	11th to 20th	Day	19% 7% 3 8% 6% 5% 5% 4 6% 6% 30%	1854
	21st to end	Day	24% 9% 3 7% 4 4 4 3 5% 6% 31%	1881
November	1st to 10th	Day	17% 10% 3 6% 5% 3 4 5% 5% 7% 35%	1717
	11th to 20th	Day	18% 11% 3 9% 4 4 4 4 4 7% 30%	1634
	21st to end	Day	22% 11% 3 6% 5% 6% 5% 3 5% 6% 27%	1520
December	1st to 10th	Day	25% 13% 6% 10% 5% 4 3 3 4 5% 22%	1520
	11th to 20th	Day	22% 14% 5% 7% 5% 3 4 4 5% 4 29%	1906
	21st to end	Day	27% 16% 4 10% 5% 3 3 3 3 4 21%	2772
January	1st to 10th	Night	17% 15% 5% 10% 7% 4 4 4 4 5% 24%	4429
	11th to 20th	Night	11% 14% 5% 12% 7% 6% 5% 4 6% 6% 25%	4440
	21st to end	Night	15% 16% 5% 11% 7% 5% 4 3 5% 5% 25%	4884
February	1st to 10th	Night	14% 15% 5% 12% 6% 5% 5% 4 5% 5% 24%	2515
	11th to 20th	Night	15% 12% 4 9% 5% 5% 5% 4 5% 5% 31%	2302
	21st to end	Night	15% 15% 4 8% 6% 4 4 2 5% 4 31%	1884
March	1st to 10th	Night	21% 11% 4 7% 5% 4 3 3 4 5% 33%	3724
	11th to 20th	Night	18% 17% 5% 9% 5% 4 3 3 3 5% 28%	3681
	21st to end	Night	13% 10% 3 8% 6% 5% 4 3 4 5% 39%	3466
April	1st to 10th	Night	25% 8% 3 7% 4 4 4 4 5% 6% 31%	1900
	11th to 20th	Night	23% 8% 3 7% 4 4 3 3 4 5% 36%	1875
	21st to end	Night	23% 8% 2 6% 4 4 3 3 5% 5% 36%	1742
May	1st to 10th	Night	22% 9% 3 8% 5% 4 5% 3 5% 6% 31%	2596
	11th to 20th	Night	22% 8% 2 8% 4 4 4 3 5% 6% 35%	2495
	21st to end	Night	15% 8% 3 7% 5% 5% 4 4 5% 8% 36%	2738
June	1st to 10th	Night	21% 8% 3 6% 5% 4 5% 3 6% 7% 33%	1520
	11th to 20th	Night	16% 7% 3 6% 4 4 5% 3 7% 7% 40%	1520
	21st to end	Night	20% 7% 3 7% 4 5% 4 4 5% 8% 33%	1520
July	1st to 10th	Night	13% 7% 3 7% 6% 5% 5% 4 7% 8% 35%	2520
	11th to 20th	Night	14% 6% 2 6% 4 5% 4 4 6% 8% 41%	2520
	21st to end	Night	10% 6% 2 6% 5% 4 5% 4 7% 7% 44%	2739
August	1st to 10th	Night	14% 7% 5% 8% 5% 4 5% 4 5% 7% 37%	2506
	11th to 20th	Night	14% 7% 3 8% 4 5% 4 4 6% 7% 38%	2794
	21st to end	Night	16% 5% 3 7% 6% 5% 4 4 5% 7% 38%	3476
September	1st to 10th	Night	12% 5% 2 7% 4 4 5% 4 5% 7% 46%	1900
	11th to 20th	Night	14% 5% 3 6% 3 3 3 2 5% 7% 50%	1981
	21st to end	Night	16% 5% 2 6% 5% 4 4 3 5% 6% 44%	2090
October	1st to 10th	Night	18% 6% 2 7% 4 4 5% 4 5% 5% 40%	2213
	11th to 20th	Night	20% 8% 3 8% 4 4 3 3 4 6% 36%	2325
	21st to end	Night	22% 9% 3 8% 5% 4 5% 3 5% 5% 32%	2717
November	1st to 10th	Night	19% 9% 4 7% 5% 4 4 4 5% 6% 33%	2483
	11th to 20th	Night	19% 10% 3 8% 5% 4 4 4 5% 6% 33%	2541
	21st to end	Night	22% 13% 4 8% 5% 4 4 3 4 5% 29%	2660
December	1st to 10th	Night	21% 14% 5% 10% 6% 4 4 4 5% 5% 22%	2660
	11th to 20th	Night	22% 16% 4 9% 5% 3 3 3 4 5% 27%	3288
	21st to end	Night	28% 16% 5% 9% 3 3 3 2 3 4 23%	4884

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	63% 6% 12% 14% 4 1	2518
	11th to 20th	Day	55% 6% 14% 17% 8%	2520
	21st to end	Day	60% 5% 14% 14% 6% 1	2772
February	1st to 10th	Day	58% 5% 10% 19% 6% 2	1662
	11th to 20th	Day	56% 6% 12% 15% 10% 1	1771
	21st to end	Day	59% 3 10% 16% 10% 2	1570
March	1st to 10th	Day	60% 4 9% 18% 8% 1	3119
	11th to 20th	Day	57% 5% 11% 14% 11% 3	3275
	21st to end	Day	47% 4 8% 20% 18% 2	4166
April	1st to 10th	Day	56% 4 7% 19% 12% 2	2280
	11th to 20th	Day	50% 4 7% 22% 15% 2	2254
	21st to end	Day	54% 5% 8% 17% 13% 3	2437
May	1st to 10th	Day	52% 6% 9% 23% 10% 1	4374
	11th to 20th	Day	49% 6% 16% 18% 9% 2	4402
	21st to end	Day	43% 6% 12% 23% 14% 2	4837
June	1st to 10th	Day	51% 7% 10% 19% 12% 2	2660
	11th to 20th	Day	43% 5% 12% 23% 13% 3	2660
	21st to end	Day	52% 6% 13% 18% 10% 1	2660
July	1st to 10th	Day	44% 5% 14% 27% 10% 1	4370
	11th to 20th	Day	42% 4 17% 30% 8%	4365
	21st to end	Day	36% 5% 15% 33% 10%	4661
August	1st to 10th	Day	47% 5% 18% 22% 8%	4409
	11th to 20th	Day	43% 5% 19% 27% 6%	4040
	21st to end	Day	43% 4 16% 22% 15% 1	4180
September	1st to 10th	Day	38% 6% 14% 30% 12% 1	2280
	11th to 20th	Day	35% 5% 13% 32% 14% 1	2198
	21st to end	Day	43% 5% 13% 27% 11% 1	2090
October	1st to 10th	Day	45% 4 14% 28% 9%	3265
	11th to 20th	Day	44% 5% 17% 27% 7%	3057
	21st to end	Day	43% 3 20% 23% 10% 1	3081
November	1st to 10th	Day	48% 5% 14% 24% 8% 1	1717
	11th to 20th	Day	53% 6% 11% 18% 11%	1634
	21st to end	Day	57% 6% 14% 16% 6% 1	1520
December	1st to 10th	Day	66% 5% 11% 14% 5%	1520
	11th to 20th	Day	59% 5% 11% 19% 7%	1906
	21st to end	Day	69% 3 12% 10% 6%	2772
January	1st to 10th	Night	62% 5% 16% 13% 4	4429
	11th to 20th	Night	58% 7% 13% 16% 5% 1	4440
	21st to end	Night	62% 5% 13% 13% 7% 1	4884
February	1st to 10th	Night	62% 4 9% 17% 7% 1	2515
	11th to 20th	Night	54% 4 11% 21% 9% 1	2304
	21st to end	Night	55% 5% 9% 17% 13% 1	1884
March	1st to 10th	Night	54% 4 11% 18% 9% 3	3734
	11th to 20th	Night	59% 5% 8% 13% 13% 2	3682
	21st to end	Night	49% 3 6% 23% 15% 3	3466
April	1st to 10th	Night	54% 4 8% 19% 14% 2	1900
	11th to 20th	Night	51% 4 7% 22% 14% 2	1875
	21st to end	Night	50% 4 8% 20% 14% 4	1742
May	1st to 10th	Night	53% 5% 11% 19% 9% 3	2602
	11th to 20th	Night	50% 6% 12% 19% 11% 2	2496
	21st to end	Night	47% 4 10% 20% 18% 1	2739
June	1st to 10th	Night	50% 6% 10% 21% 13% 1	1520
	11th to 20th	Night	43% 6% 13% 22% 14% 3	1520
	21st to end	Night	48% 8% 12% 18% 14% 1	1520
July	1st to 10th	Night	46% 5% 15% 21% 13% 1	2480
	11th to 20th	Night	40% 4 20% 27% 9%	2472
	21st to end	Night	38% 5% 19% 25% 13%	2651
August	1st to 10th	Night	48% 4 13% 26% 9%	2496
	11th to 20th	Night	45% 5% 19% 24% 7%	2818
	21st to end	Night	45% 5% 13% 22% 14% 1	3476
September	1st to 10th	Night	37% 6% 14% 26% 16% 1	1900
	11th to 20th	Night	35% 5% 14% 33% 12% 1	1981
	21st to end	Night	40% 6% 11% 28% 15%	2090
October	1st to 10th	Night	45% 4 14% 27% 10% 1	3694
	11th to 20th	Night	44% 3 12% 32% 9%	3861
	21st to end	Night	47% 3 16% 19% 12% 2	4467
November	1st to 10th	Night	51% 5% 12% 22% 9% 1	2483
	11th to 20th	Night	52% 5% 12% 20% 10% 1	2541
	21st to end	Night	59% 5% 11% 18% 7%	2660
December	1st to 10th	Night	64% 6% 11% 15% 5%	2660
	11th to 20th	Night	61% 4 9% 18% 8% 1	3288
	21st to end	Night	67% 3 9% 15% 5%	4884

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% 15% 24% 29% 10%	180
	11th to 20th	Day	27% 11% 33% 23% 6% 1	180
	21st to end	Day	18% 13% 34% 27% 7% 1	198
February	1st to 10th	Day	32% 16% 34% 17% 2	247
	11th to 20th	Day	21% 14% 37% 24% 4	369
	21st to end	Day	27% 13% 31% 25% 4	382
March	1st to 10th	Day	23% 12% 31% 24% 9%	460
	11th to 20th	Day	21% 13% 33% 25% 6% 1	460
	21st to end	Day	25% 11% 32% 20% 9% 2	506
April	1st to 10th	Day	27% 16% 26% 21% 8% 2	460
	11th to 20th	Day	27% 17% 23% 21% 10% 2	452
	21st to end	Day	27% 14% 28% 21% 8% 3	460
May	1st to 10th	Day	23% 10% 30% 24% 10% 2	463
	11th to 20th	Day	18% 8% 31% 26% 13% 4	487
	21st to end	Day	18% 11% 30% 26% 13% 2	529
June	1st to 10th	Day	22% 11% 25% 27% 13% 3	490
	11th to 20th	Day	16% 10% 25% 30% 15% 3	490
	21st to end	Day	21% 10% 26% 26% 15% 3	490
July	1st to 10th	Day	13% 6% 23% 31% 21% 6%	485
	11th to 20th	Day	14% 4 27% 31% 18% 5%	484
	21st to end	Day	13% 5% 22% 36% 19% 5%	532
August	1st to 10th	Day	14% 4 23% 30% 21% 7%	486
	11th to 20th	Day	12% 4 20% 30% 23% 11%	479
	21st to end	Day	14% 6% 24% 25% 20% 10%	539
September	1st to 10th	Day	12% 6% 27% 28% 19% 9%	490
	11th to 20th	Day	13% 7% 20% 26% 20% 14%	490
	21st to end	Day	18% 8% 24% 24% 20% 7%	490
October	1st to 10th	Day	20% 10% 27% 23% 14% 6%	490
	11th to 20th	Day	19% 12% 32% 23% 11% 3	465
	21st to end	Day	30% 10% 25% 24% 9%	429
November	1st to 10th	Day	27% 12% 25% 24% 11% 1	381
	11th to 20th	Day	24% 14% 32% 19% 9% 1	294
	21st to end	Day	28% 13% 27% 22% 8% 2	180
December	1st to 10th	Day	29% 11% 25% 23% 10% 2	180
	11th to 20th	Day	25% 12% 39% 19% 4 1	180
	21st to end	Day	32% 16% 28% 16% 7% 1	198
January	1st to 10th	Night	27% 10% 30% 27% 6%	760
	11th to 20th	Night	24% 10% 33% 26% 7%	760
	21st to end	Night	19% 7% 34% 30% 7% 1	836
February	1st to 10th	Night	21% 8% 36% 29% 6% 1	760
	11th to 20th	Night	20% 10% 36% 28% 5% 1	743
	21st to end	Night	24% 8% 31% 33% 4	628
March	1st to 10th	Night	24% 8% 33% 30% 6%	760
	11th to 20th	Night	26% 10% 31% 25% 8%	760
	21st to end	Night	21% 9% 31% 31% 8%	836
April	1st to 10th	Night	27% 10% 26% 30% 6%	760
	11th to 20th	Night	25% 11% 31% 27% 6% 1	751
	21st to end	Night	27% 7% 32% 24% 8%	602
May	1st to 10th	Night	24% 6% 32% 29% 8% 1	436
	11th to 20th	Night	22% 5% 31% 33% 8% 1	374
	21st to end	Night	18% 5% 36% 32% 9%	411
June	1st to 10th	Night	24% 5% 24% 36% 12%	380
	11th to 20th	Night	18% 5% 27% 37% 13% 1	380
	21st to end	Night	22% 4 25% 34% 15% 1	380
July	1st to 10th	Night	11% 5% 25% 42% 16% 1	370
	11th to 20th	Night	11% 3 24% 46% 16% 1	368
	21st to end	Night	13% 2 19% 47% 18% 1	416
August	1st to 10th	Night	15% 4 12% 47% 21% 1	378
	11th to 20th	Night	15% 2 20% 35% 25% 3	557
	21st to end	Night	16% 3 22% 35% 22% 2	836
September	1st to 10th	Night	15% 3 23% 41% 18% 1	760
	11th to 20th	Night	16% 5% 17% 36% 23% 3	760
	21st to end	Night	18% 4 25% 33% 17% 2	760
October	1st to 10th	Night	21% 6% 27% 33% 13% 1	760
	11th to 20th	Night	25% 7% 26% 30% 11% 1	760
	21st to end	Night	26% 8% 30% 29% 7%	836
November	1st to 10th	Night	25% 10% 28% 29% 9%	764
	11th to 20th	Night	23% 9% 32% 25% 10% 1	760
	21st to end	Night	29% 10% 29% 23% 9% 1	760
December	1st to 10th	Night	28% 9% 31% 24% 7%	760
	11th to 20th	Night	28% 10% 32% 23% 6% 1	760
	21st to end	Night	32% 11% 30% 21% 6%	836

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