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 2025-11-03.

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	17% 9% 3 8% 5% 5% 5% 4 5% 7% 33%	126985
	Night	17% 11% 4 9% 5% 4 4 3 5% 6% 31%	123060
Transparency	Day	51% 4 12% 22% 10% 1	130430
	Night	54% 4 11% 20% 10% 1	127702
Seeing	Day	20% 10% 28% 25% 13% 4	15827
	Night	22% 7% 29% 31% 10% 1	24743

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	15% 19% 6% 13% 5% 5% 4 3 4 4 23%	3597
	11th to 20th	Day	12% 15% 6% 11% 6% 5% 4 3 5% 6% 25%	3570
	21st to end	Day	12% 14% 5% 10% 5% 6% 5% 5% 5% 6% 27%	3960
February	1st to 10th	Day	15% 15% 5% 9% 7% 5% 4 4 4 5% 28%	1750
	11th to 20th	Day	14% 13% 5% 10% 6% 6% 5% 5% 5% 6% 27%	1858
	21st to end	Day	17% 16% 4 8% 6% 4 4 4 4 5% 27%	1650
March	1st to 10th	Day	19% 13% 4 10% 6% 6% 4 3 4 5% 26%	4476
	11th to 20th	Day	11% 14% 4 9% 5% 5% 4 3 6% 7% 32%	4691
	21st to end	Day	18% 9% 4 9% 6% 4 4 3 4 4 34%	5970
April	1st to 10th	Day	22% 12% 3 6% 6% 4 4 3 4 5% 32%	2400
	11th to 20th	Day	20% 9% 3 7% 5% 4 3 3 4 5% 37%	2374
	21st to end	Day	22% 9% 3 8% 5% 4 5% 3 4 6% 32%	2565
May	1st to 10th	Day	25% 10% 4 9% 5% 5% 5% 3 5% 6% 24%	6252
	11th to 20th	Day	20% 9% 3 8% 5% 4 5% 3 6% 7% 30%	6318
	21st to end	Day	21% 8% 3 7% 4 4 4 4 6% 8% 31%	6949
June	1st to 10th	Day	20% 8% 3 7% 5% 5% 4 5% 5% 7% 31%	2800
	11th to 20th	Day	14% 6% 2 7% 5% 5% 5% 5% 5% 8% 38%	2800
	21st to end	Day	20% 7% 2 7% 6% 5% 5% 4 6% 8% 31%	2800
July	1st to 10th	Day	13% 6% 2 8% 6% 5% 6% 5% 7% 8% 33%	6360
	11th to 20th	Day	11% 5% 2 6% 5% 5% 5% 5% 7% 9% 40%	6360
	21st to end	Day	9% 6% 2 5% 3 4 4 4 6% 9% 47%	6926
August	1st to 10th	Day	13% 5% 2 8% 5% 6% 6% 4 7% 9% 34%	4563
	11th to 20th	Day	13% 6% 3 7% 4 4 5% 4 6% 8% 39%	4126
	21st to end	Day	15% 5% 2 6% 5% 4 4 4 6% 8% 40%	4312
September	1st to 10th	Day	11% 5% 2 6% 4 4 5% 4 6% 9% 43%	2400
	11th to 20th	Day	14% 4 2 4 4 4 3 4 4 7% 51%	2313
	21st to end	Day	17% 5% 2 6% 5% 4 5% 3 4 8% 41%	2200
October	1st to 10th	Day	19% 7% 2 7% 4 4 4 4 5% 7% 38%	2069
	11th to 20th	Day	19% 7% 3 8% 6% 5% 4 4 6% 7% 31%	1951
	21st to end	Day	24% 9% 3 7% 5% 4 4 3 5% 6% 31%	1980
November	1st to 10th	Day	16% 10% 3 7% 5% 3 4 5% 5% 6% 35%	1779
	11th to 20th	Day	17% 11% 3 8% 4 4 4 4 5% 7% 31%	1720
	21st to end	Day	22% 12% 3 6% 5% 6% 5% 3 5% 6% 26%	1600
December	1st to 10th	Day	20% 17% 7% 12% 5% 4 4 4 4 4 19%	2600
	11th to 20th	Day	18% 13% 5% 8% 5% 4 4 4 6% 4 28%	2986
	21st to end	Day	23% 13% 4 10% 5% 4 5% 4 5% 6% 22%	3960
January	1st to 10th	Night	14% 18% 7% 15% 6% 4 3 3 4 4 21%	6349
	11th to 20th	Night	9% 15% 7% 15% 7% 6% 5% 3 5% 5% 21%	6315
	21st to end	Night	13% 14% 5% 12% 7% 5% 5% 3 5% 5% 26%	6996
February	1st to 10th	Night	13% 15% 5% 12% 6% 5% 5% 4 5% 5% 24%	2647
	11th to 20th	Night	14% 12% 5% 9% 5% 5% 5% 4 5% 5% 31%	2426
	21st to end	Night	15% 15% 4 8% 6% 4 4 2 5% 4 31%	1980
March	1st to 10th	Night	22% 13% 4 8% 5% 4 4 3 4 5% 30%	5364
	11th to 20th	Night	14% 13% 4 8% 5% 4 5% 4 5% 7% 31%	5265
	21st to end	Night	15% 11% 4 9% 6% 5% 5% 3 5% 4 33%	4962
April	1st to 10th	Night	24% 9% 3 7% 4 4 3 4 5% 6% 31%	2000
	11th to 20th	Night	23% 8% 3 7% 5% 5% 3 3 4 5% 35%	1975
	21st to end	Night	23% 8% 2 6% 4 3 3 3 5% 5% 36%	1833
May	1st to 10th	Night	25% 10% 3 9% 5% 4 5% 3 5% 6% 25%	3718
	11th to 20th	Night	22% 9% 3 8% 4 4 4 3 5% 7% 30%	3575
	21st to end	Night	19% 10% 3 8% 5% 5% 4 4 5% 7% 31%	3926
June	1st to 10th	Night	21% 8% 3 6% 5% 4 5% 3 6% 7% 33%	1600
	11th to 20th	Night	16% 7% 3 6% 4 4 5% 3 7% 7% 40%	1600
	21st to end	Night	20% 6% 3 7% 4 5% 5% 4 6% 8% 34%	1600
July	1st to 10th	Night	13% 7% 2 7% 6% 5% 6% 5% 8% 9% 34%	3600
	11th to 20th	Night	13% 6% 2 6% 4 5% 5% 4 5% 7% 41%	3600
	21st to end	Night	9% 5% 2 6% 5% 4 5% 4 7% 8% 45%	3926
August	1st to 10th	Night	13% 7% 4 8% 5% 4 5% 4 5% 7% 38%	2586
	11th to 20th	Night	14% 7% 3 8% 4 5% 4 4 6% 7% 38%	2884
	21st to end	Night	16% 5% 3 7% 6% 5% 4 4 5% 7% 38%	3586
September	1st to 10th	Night	12% 4 2 7% 4 4 4 4 5% 7% 46%	2000
	11th to 20th	Night	13% 5% 2 6% 3 3 3 2 5% 7% 51%	2086
	21st to end	Night	15% 5% 2 6% 5% 4 4 3 5% 6% 44%	2200
October	1st to 10th	Night	17% 6% 2 7% 4 4 5% 4 5% 5% 42%	2330
	11th to 20th	Night	20% 8% 3 8% 4 4 4 3 5% 6% 37%	2448
	21st to end	Night	22% 8% 3 8% 5% 4 5% 3 5% 5% 32%	2860
November	1st to 10th	Night	19% 9% 4 7% 5% 4 4 4 5% 6% 33%	2564
	11th to 20th	Night	18% 10% 3 8% 5% 4 4 4 5% 5% 34%	2675
	21st to end	Night	22% 13% 3 9% 5% 4 4 3 4 5% 28%	2800
December	1st to 10th	Night	18% 16% 6% 10% 8% 6% 5% 4 5% 5% 19%	4580
	11th to 20th	Night	23% 13% 4 9% 5% 3 4 3 6% 4 26%	5208
	21st to end	Night	24% 15% 5% 9% 3 3 3 3 5% 5% 23%	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	65% 4 9% 17% 3 1	3598
	11th to 20th	Day	60% 4 10% 16% 7% 2	3600
	21st to end	Day	57% 4 11% 16% 11% 1	3960
February	1st to 10th	Day	58% 4 10% 19% 7% 2	1750
	11th to 20th	Day	57% 6% 11% 15% 10% 1	1867
	21st to end	Day	59% 3 10% 16% 11% 2	1650
March	1st to 10th	Day	61% 4 7% 14% 11% 4	4479
	11th to 20th	Day	51% 5% 17% 14% 11% 2	4691
	21st to end	Day	53% 3 7% 17% 18% 2	5970
April	1st to 10th	Day	55% 4 7% 19% 13% 2	2400
	11th to 20th	Day	50% 4 7% 22% 16% 2	2374
	21st to end	Day	54% 5% 8% 17% 13% 3	2565
May	1st to 10th	Day	61% 4 8% 19% 7% 1	6263
	11th to 20th	Day	53% 5% 14% 18% 8% 2	6322
	21st to end	Day	51% 5% 12% 19% 12% 1	6949
June	1st to 10th	Day	51% 6% 10% 19% 12% 2	2800
	11th to 20th	Day	43% 5% 11% 24% 13% 3	2800
	21st to end	Day	51% 5% 12% 19% 10% 1	2800
July	1st to 10th	Day	47% 4 14% 26% 8%	6290
	11th to 20th	Day	40% 4 15% 31% 11%	6285
	21st to end	Day	34% 4 15% 37% 9%	6728
August	1st to 10th	Day	46% 4 18% 23% 8%	4549
	11th to 20th	Day	43% 4 19% 27% 7%	4168
	21st to end	Day	43% 4 16% 22% 15% 1	4312
September	1st to 10th	Day	37% 6% 14% 31% 12% 1	2400
	11th to 20th	Day	34% 4 13% 33% 15% 1	2313
	21st to end	Day	43% 5% 14% 26% 12% 1	2200
October	1st to 10th	Day	44% 4 14% 28% 10%	3368
	11th to 20th	Day	44% 4 17% 27% 7%	3154
	21st to end	Day	44% 3 20% 24% 9% 1	3180
November	1st to 10th	Day	48% 5% 13% 24% 8% 1	1779
	11th to 20th	Day	52% 6% 11% 19% 12% 1	1720
	21st to end	Day	58% 6% 14% 15% 6% 1	1600
December	1st to 10th	Day	70% 3 9% 16% 3	2600
	11th to 20th	Day	59% 4 9% 18% 10%	2986
	21st to end	Day	65% 3 12% 14% 7%	3960
January	1st to 10th	Night	68% 3 12% 13% 4	6349
	11th to 20th	Night	64% 5% 11% 15% 5%	6360
	21st to end	Night	60% 4 10% 12% 13% 1	6996
February	1st to 10th	Night	61% 4 9% 17% 8% 1	2647
	11th to 20th	Night	55% 4 11% 21% 9% 1	2428
	21st to end	Night	55% 5% 8% 18% 13% 1	1980
March	1st to 10th	Night	58% 3 8% 14% 12% 5%	5374
	11th to 20th	Night	53% 4 13% 15% 13% 2	5266
	21st to end	Night	54% 3 5% 22% 13% 2	4962
April	1st to 10th	Night	54% 4 7% 20% 14% 2	2000
	11th to 20th	Night	52% 4 7% 21% 14% 2	1975
	21st to end	Night	50% 4 8% 20% 14% 4	1833
May	1st to 10th	Night	61% 4 9% 17% 7% 2	3724
	11th to 20th	Night	55% 4 10% 19% 10% 2	3576
	21st to end	Night	54% 3 10% 18% 14% 1	3927
June	1st to 10th	Night	50% 6% 10% 20% 14% 1	1600
	11th to 20th	Night	43% 5% 13% 22% 14% 3	1600
	21st to end	Night	48% 7% 12% 19% 14% 1	1600
July	1st to 10th	Night	46% 3 18% 23% 10%	3560
	11th to 20th	Night	41% 3 18% 28% 9%	3552
	21st to end	Night	37% 4 17% 30% 13%	3809
August	1st to 10th	Night	47% 4 13% 27% 9%	2576
	11th to 20th	Night	44% 4 19% 25% 8%	2908
	21st to end	Night	45% 5% 13% 22% 13% 1	3586
September	1st to 10th	Night	37% 6% 13% 28% 15% 1	2000
	11th to 20th	Night	35% 5% 14% 34% 13% 1	2086
	21st to end	Night	41% 5% 11% 27% 16%	2200
October	1st to 10th	Night	44% 4 14% 27% 11% 1	3811
	11th to 20th	Night	44% 3 12% 32% 9%	3984
	21st to end	Night	47% 3 16% 20% 12% 2	4610
November	1st to 10th	Night	51% 5% 12% 23% 9% 1	2564
	11th to 20th	Night	52% 5% 12% 20% 11% 1	2675
	21st to end	Night	59% 4 11% 19% 7%	2800
December	1st to 10th	Night	68% 3 12% 13% 4	4580
	11th to 20th	Night	61% 4 8% 15% 12%	5208
	21st to end	Night	64% 3 8% 15% 10%	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	24% 15% 26% 27% 9%	200
	11th to 20th	Day	26% 11% 34% 23% 6% 2	200
	21st to end	Day	18% 14% 35% 26% 6% 1	220
February	1st to 10th	Day	30% 16% 35% 17% 2	275
	11th to 20th	Day	21% 15% 36% 23% 4	405
	21st to end	Day	26% 13% 32% 25% 4	414
March	1st to 10th	Day	24% 13% 31% 23% 9%	500
	11th to 20th	Day	21% 13% 35% 25% 6% 1	500
	21st to end	Day	26% 12% 32% 20% 9% 2	550
April	1st to 10th	Day	26% 17% 26% 20% 8% 2	500
	11th to 20th	Day	27% 17% 23% 21% 9% 2	492
	21st to end	Day	27% 14% 29% 20% 7% 3	500
May	1st to 10th	Day	24% 10% 31% 24% 10% 2	503
	11th to 20th	Day	20% 8% 30% 26% 13% 4	527
	21st to end	Day	20% 11% 29% 26% 13% 2	573
June	1st to 10th	Day	22% 11% 26% 27% 12% 2	530
	11th to 20th	Day	16% 10% 26% 29% 15% 3	530
	21st to end	Day	20% 9% 28% 25% 14% 3	530
July	1st to 10th	Day	13% 6% 25% 30% 20% 5%	525
	11th to 20th	Day	14% 4 28% 32% 18% 4	524
	21st to end	Day	12% 5% 24% 34% 19% 5%	576
August	1st to 10th	Day	13% 4 22% 32% 21% 7%	526
	11th to 20th	Day	12% 4 19% 30% 23% 11%	519
	21st to end	Day	15% 6% 25% 24% 21% 10%	583
September	1st to 10th	Day	12% 6% 27% 27% 19% 9%	530
	11th to 20th	Day	12% 7% 18% 27% 21% 14%	530
	21st to end	Day	18% 8% 25% 24% 19% 7%	530
October	1st to 10th	Day	19% 9% 27% 24% 15% 6%	530
	11th to 20th	Day	18% 12% 32% 24% 11% 3	502
	21st to end	Day	30% 10% 25% 25% 10%	462
November	1st to 10th	Day	26% 12% 27% 23% 11% 1	401
	11th to 20th	Day	23% 14% 32% 21% 8% 1	320
	21st to end	Day	30% 13% 28% 20% 8% 2	200
December	1st to 10th	Day	29% 11% 26% 23% 10% 3	200
	11th to 20th	Day	26% 13% 38% 19% 4 1	200
	21st to end	Day	30% 14% 29% 17% 8% 1	220
January	1st to 10th	Night	26% 9% 32% 27% 6%	800
	11th to 20th	Night	24% 10% 33% 27% 7%	800
	21st to end	Night	19% 7% 35% 31% 7% 1	880
February	1st to 10th	Night	20% 8% 37% 28% 6% 1	800
	11th to 20th	Night	21% 11% 37% 27% 5% 1	783
	21st to end	Night	24% 8% 30% 33% 4	660
March	1st to 10th	Night	25% 8% 33% 29% 6%	800
	11th to 20th	Night	25% 10% 32% 25% 8%	800
	21st to end	Night	22% 9% 30% 31% 7%	880
April	1st to 10th	Night	27% 10% 27% 29% 6%	800
	11th to 20th	Night	25% 11% 31% 27% 6% 1	791
	21st to end	Night	28% 7% 33% 24% 8%	633
May	1st to 10th	Night	24% 7% 32% 29% 7% 1	459
	11th to 20th	Night	23% 5% 30% 34% 8% 1	394
	21st to end	Night	19% 5% 35% 32% 9%	433
June	1st to 10th	Night	24% 5% 24% 36% 11%	400
	11th to 20th	Night	18% 5% 27% 38% 12% 1	400
	21st to end	Night	22% 4 26% 34% 15% 1	400
July	1st to 10th	Night	11% 5% 26% 41% 17% 1	390
	11th to 20th	Night	11% 3 24% 45% 16% 1	388
	21st to end	Night	13% 3 20% 47% 17% 1	438
August	1st to 10th	Night	14% 4 12% 47% 22% 1	398
	11th to 20th	Night	15% 2 20% 35% 26% 2	587
	21st to end	Night	17% 2 22% 36% 22% 2	880
September	1st to 10th	Night	14% 3 23% 40% 18% 1	800
	11th to 20th	Night	15% 5% 17% 37% 23% 3	800
	21st to end	Night	18% 4 25% 34% 17% 2	800
October	1st to 10th	Night	20% 6% 27% 33% 14% 1	800
	11th to 20th	Night	24% 7% 27% 31% 11% 1	800
	21st to end	Night	26% 8% 30% 30% 7%	880
November	1st to 10th	Night	24% 10% 28% 29% 8%	789
	11th to 20th	Night	23% 9% 32% 26% 10% 1	800
	21st to end	Night	29% 10% 29% 22% 9% 1	800
December	1st to 10th	Night	28% 9% 31% 25% 7%	800
	11th to 20th	Night	28% 11% 32% 23% 6% 1	800
	21st to end	Night	31% 11% 29% 21% 7%	880

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