Observatoire du Geai Bleu
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Observatoire du Geai Bleu clear sky chart from 2005-11-06 to 2025-10-04.

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	16% 9% 3 8% 5% 5% 5% 4 5% 7% 33%	126742
	Night	17% 11% 4 9% 5% 4 4 3 5% 6% 32%	122626
Transparency	Day	49% 6% 13% 20% 10% 1	130187
	Night	52% 6% 12% 19% 10% 1	127268
Seeing	Day	20% 11% 29% 25% 11% 3	15704
	Night	22% 8% 30% 30% 10% 1	24594

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% 5% 4 4 3 4 5% 25%	3597
	11th to 20th	Day	11% 15% 6% 10% 6% 5% 5% 3 5% 6% 27%	3570
	21st to end	Day	10% 14% 5% 12% 5% 5% 5% 4 5% 6% 29%	3960
February	1st to 10th	Day	13% 16% 4 9% 6% 4 5% 4 4 6% 28%	1742
	11th to 20th	Day	12% 12% 6% 12% 6% 6% 4 4 5% 6% 28%	1854
	21st to end	Day	16% 16% 4 10% 6% 3 4 3 5% 5% 28%	1650
March	1st to 10th	Day	19% 13% 4 10% 6% 5% 5% 3 5% 5% 26%	4476
	11th to 20th	Day	10% 12% 4 9% 5% 5% 4 3 5% 7% 34%	4690
	21st to end	Day	17% 10% 4 9% 5% 5% 4 3 4 5% 34%	5970
April	1st to 10th	Day	20% 11% 3 7% 5% 4 5% 3 4 5% 33%	2400
	11th to 20th	Day	21% 8% 2 6% 5% 5% 4 3 5% 5% 37%	2374
	21st to end	Day	21% 9% 3 7% 5% 4 4 4 5% 6% 33%	2570
May	1st to 10th	Day	26% 9% 3 8% 5% 5% 4 4 5% 6% 25%	6287
	11th to 20th	Day	20% 8% 3 8% 5% 4 5% 3 6% 7% 31%	6318
	21st to end	Day	21% 8% 3 6% 5% 5% 5% 4 6% 7% 31%	6949
June	1st to 10th	Day	20% 8% 2 7% 5% 5% 5% 4 6% 8% 30%	2800
	11th to 20th	Day	14% 6% 2 7% 5% 4 5% 5% 6% 9% 37%	2800
	21st to end	Day	19% 8% 3 7% 6% 5% 5% 5% 5% 8% 29%	2800
July	1st to 10th	Day	14% 6% 2 7% 6% 6% 6% 5% 6% 9% 33%	6360
	11th to 20th	Day	11% 5% 2 6% 4 5% 6% 5% 7% 9% 39%	6360
	21st to end	Day	9% 5% 2 5% 4 4 4 4 6% 10% 47%	6926
August	1st to 10th	Day	13% 5% 2 7% 6% 6% 6% 5% 7% 9% 33%	4563
	11th to 20th	Day	14% 6% 2 7% 5% 4 5% 4 7% 9% 37%	4155
	21st to end	Day	16% 5% 2 6% 4 4 5% 4 6% 8% 41%	4312
September	1st to 10th	Day	12% 5% 2 5% 4 4 4 5% 6% 9% 44%	2400
	11th to 20th	Day	12% 5% 2 4 4 4 4 3 6% 6% 51%	2314
	21st to end	Day	17% 6% 2 6% 5% 4 3 3 5% 7% 43%	2200
October	1st to 10th	Day	19% 7% 2 6% 4 4 5% 3 6% 8% 37%	1996
	11th to 20th	Day	19% 7% 4 7% 6% 5% 6% 4 6% 6% 31%	1851
	21st to end	Day	22% 10% 3 7% 4 4 4 3 5% 5% 32%	1881
November	1st to 10th	Day	15% 9% 3 7% 5% 4 5% 4 6% 7% 36%	1761
	11th to 20th	Day	17% 10% 4 7% 4 4 5% 4 6% 7% 33%	1710
	21st to end	Day	21% 11% 2 8% 5% 5% 6% 4 5% 7% 27%	1600
December	1st to 10th	Day	20% 15% 6% 12% 6% 4 5% 3 4 4 20%	2600
	11th to 20th	Day	19% 13% 4 8% 4 4 5% 4 5% 5% 28%	2986
	21st to end	Day	23% 13% 4 9% 5% 4 4 3 5% 5% 25%	3960
January	1st to 10th	Night	13% 19% 7% 15% 5% 4 4 3 4 4 23%	6349
	11th to 20th	Night	9% 14% 7% 15% 8% 6% 5% 3 5% 5% 24%	6315
	21st to end	Night	11% 13% 5% 12% 7% 5% 5% 4 5% 6% 27%	6996
February	1st to 10th	Night	13% 15% 5% 11% 6% 5% 4 4 5% 5% 27%	2655
	11th to 20th	Night	14% 11% 5% 10% 5% 5% 5% 4 5% 5% 31%	2431
	21st to end	Night	15% 13% 4 10% 5% 4 4 3 4 6% 32%	1980
March	1st to 10th	Night	21% 14% 4 8% 5% 4 4 3 3 4 30%	5364
	11th to 20th	Night	12% 13% 4 9% 5% 4 5% 4 6% 7% 31%	5251
	21st to end	Night	15% 11% 4 9% 5% 5% 5% 3 4 5% 33%	4962
April	1st to 10th	Night	23% 10% 3 7% 4 4 4 3 4 7% 31%	2000
	11th to 20th	Night	20% 8% 2 7% 5% 3 4 4 5% 5% 36%	1975
	21st to end	Night	21% 9% 3 6% 4 3 4 3 5% 5% 37%	1828
May	1st to 10th	Night	26% 10% 4 8% 4 5% 5% 4 5% 5% 26%	3696
	11th to 20th	Night	21% 8% 3 7% 5% 4 5% 4 6% 7% 31%	3575
	21st to end	Night	19% 9% 3 8% 5% 5% 4 4 6% 7% 30%	3926
June	1st to 10th	Night	20% 6% 3 7% 4 5% 4 4 5% 8% 34%	1600
	11th to 20th	Night	16% 6% 2 6% 4 4 5% 4 7% 7% 38%	1600
	21st to end	Night	19% 8% 2 7% 5% 4 5% 4 6% 7% 35%	1600
July	1st to 10th	Night	12% 6% 2 7% 5% 5% 7% 6% 8% 9% 34%	3600
	11th to 20th	Night	14% 5% 2 6% 5% 4 5% 4 5% 8% 42%	3600
	21st to end	Night	9% 5% 2 6% 4 5% 4 4 7% 8% 46%	3926
August	1st to 10th	Night	13% 6% 3 8% 5% 4 5% 4 6% 8% 38%	2586
	11th to 20th	Night	15% 7% 3 7% 4 5% 4 3 6% 6% 41%	2843
	21st to end	Night	17% 7% 3 7% 4 4 4 4 5% 7% 40%	3586
September	1st to 10th	Night	11% 5% 2 7% 5% 4 4 4 5% 7% 48%	2000
	11th to 20th	Night	13% 5% 2 5% 4 3 4 3 5% 6% 52%	2085
	21st to end	Night	16% 6% 2 6% 4 3 3 3 5% 6% 46%	2200
October	1st to 10th	Night	17% 6% 3 6% 4 5% 4 3 4 5% 42%	2247
	11th to 20th	Night	19% 9% 3 7% 4 4 5% 3 5% 6% 37%	2327
	21st to end	Night	21% 9% 3 7% 5% 4 4 3 5% 6% 33%	2717
November	1st to 10th	Night	17% 9% 4 8% 5% 4 5% 3 5% 6% 34%	2537
	11th to 20th	Night	19% 10% 2 8% 4 5% 4 4 4 6% 35%	2685
	21st to end	Night	22% 13% 3 8% 5% 4 4 3 3 5% 30%	2800
December	1st to 10th	Night	18% 14% 5% 11% 7% 5% 5% 4 5% 5% 21%	4580
	11th to 20th	Night	23% 12% 4 8% 5% 4 3 3 4 5% 29%	5208
	21st to end	Night	24% 15% 4 9% 4 3 3 3 4 5% 25%	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	61% 7% 10% 17% 4 1	3598
	11th to 20th	Day	58% 6% 10% 17% 6% 2	3600
	21st to end	Day	55% 7% 11% 15% 12% 1	3960
February	1st to 10th	Day	58% 4 10% 18% 8% 2	1742
	11th to 20th	Day	57% 6% 12% 14% 10% 1	1863
	21st to end	Day	58% 4 10% 16% 10% 2	1650
March	1st to 10th	Day	60% 5% 8% 12% 11% 4	4479
	11th to 20th	Day	49% 6% 19% 12% 11% 3	4690
	21st to end	Day	52% 4 9% 15% 17% 2	5970
April	1st to 10th	Day	55% 4 8% 17% 13% 2	2400
	11th to 20th	Day	50% 5% 8% 22% 14% 2	2374
	21st to end	Day	52% 6% 9% 17% 12% 3	2570
May	1st to 10th	Day	59% 6% 8% 18% 7% 1	6298
	11th to 20th	Day	51% 7% 15% 17% 8% 2	6322
	21st to end	Day	50% 7% 12% 18% 12% 1	6949
June	1st to 10th	Day	50% 7% 11% 18% 12% 2	2800
	11th to 20th	Day	42% 7% 12% 23% 13% 3	2800
	21st to end	Day	52% 7% 12% 18% 10% 1	2800
July	1st to 10th	Day	46% 7% 14% 25% 8%	6290
	11th to 20th	Day	38% 7% 15% 29% 11%	6285
	21st to end	Day	33% 6% 16% 35% 10%	6728
August	1st to 10th	Day	44% 7% 19% 22% 8%	4549
	11th to 20th	Day	42% 8% 19% 25% 6%	4197
	21st to end	Day	41% 7% 16% 21% 14% 1	4312
September	1st to 10th	Day	35% 8% 15% 30% 12% 1	2400
	11th to 20th	Day	33% 7% 13% 32% 14% 1	2314
	21st to end	Day	42% 5% 14% 26% 12% 1	2200
October	1st to 10th	Day	45% 6% 14% 26% 9%	3295
	11th to 20th	Day	43% 6% 18% 26% 7%	3054
	21st to end	Day	42% 5% 21% 22% 10% 1	3081
November	1st to 10th	Day	47% 6% 14% 22% 10% 1	1761
	11th to 20th	Day	51% 5% 13% 19% 11% 1	1710
	21st to end	Day	57% 7% 14% 15% 6% 1	1600
December	1st to 10th	Day	68% 5% 9% 15% 4	2600
	11th to 20th	Day	57% 5% 10% 17% 11%	2986
	21st to end	Day	62% 6% 11% 14% 8%	3960
January	1st to 10th	Night	66% 5% 12% 13% 4	6349
	11th to 20th	Night	61% 8% 11% 15% 4 1	6360
	21st to end	Night	58% 6% 11% 11% 13% 1	6996
February	1st to 10th	Night	59% 6% 10% 16% 9% 1	2655
	11th to 20th	Night	54% 6% 11% 19% 10% 1	2433
	21st to end	Night	54% 5% 9% 17% 13% 2	1980
March	1st to 10th	Night	58% 5% 8% 13% 12% 4	5374
	11th to 20th	Night	51% 6% 14% 15% 13% 2	5252
	21st to end	Night	53% 4 8% 20% 13% 2	4962
April	1st to 10th	Night	53% 4 9% 18% 14% 2	2000
	11th to 20th	Night	50% 5% 8% 21% 14% 2	1975
	21st to end	Night	49% 5% 9% 20% 14% 4	1828
May	1st to 10th	Night	59% 7% 9% 17% 7% 2	3702
	11th to 20th	Night	51% 8% 11% 18% 10% 2	3576
	21st to end	Night	52% 6% 11% 17% 14% 1	3927
June	1st to 10th	Night	49% 6% 11% 19% 14% 1	1600
	11th to 20th	Night	43% 7% 12% 21% 14% 3	1600
	21st to end	Night	48% 8% 11% 18% 14% 1	1600
July	1st to 10th	Night	43% 7% 19% 22% 9% 1	3560
	11th to 20th	Night	40% 6% 17% 28% 9%	3552
	21st to end	Night	35% 6% 17% 29% 12%	3809
August	1st to 10th	Night	44% 7% 13% 26% 9%	2576
	11th to 20th	Night	43% 7% 17% 25% 8%	2867
	21st to end	Night	44% 8% 13% 22% 13% 1	3586
September	1st to 10th	Night	36% 7% 14% 26% 16% 1	2000
	11th to 20th	Night	34% 6% 14% 33% 12% 1	2085
	21st to end	Night	39% 7% 11% 28% 15%	2200
October	1st to 10th	Night	43% 6% 14% 25% 11% 1	3728
	11th to 20th	Night	43% 5% 12% 31% 9%	3863
	21st to end	Night	45% 5% 17% 18% 13% 2	4467
November	1st to 10th	Night	50% 6% 13% 21% 9% 1	2537
	11th to 20th	Night	50% 6% 12% 19% 11% 1	2685
	21st to end	Night	58% 5% 11% 18% 8%	2800
December	1st to 10th	Night	65% 6% 12% 13% 4	4580
	11th to 20th	Night	59% 5% 9% 15% 11% 1	5208
	21st to end	Night	62% 4 8% 14% 11%	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	23% 15% 33% 22% 9%	200
	11th to 20th	Day	23% 15% 33% 25% 4 2	200
	21st to end	Day	19% 13% 35% 25% 7% 1	220
February	1st to 10th	Day	26% 22% 33% 16% 1	272
	11th to 20th	Day	19% 15% 40% 21% 4	401
	21st to end	Day	26% 11% 34% 24% 4	414
March	1st to 10th	Day	25% 12% 32% 23% 8%	500
	11th to 20th	Day	20% 14% 36% 25% 4 1	500
	21st to end	Day	27% 13% 32% 20% 8% 1	550
April	1st to 10th	Day	26% 17% 27% 21% 8% 2	500
	11th to 20th	Day	29% 17% 23% 21% 8% 2	492
	21st to end	Day	28% 12% 32% 19% 8% 1	500
May	1st to 10th	Day	24% 13% 32% 22% 8% 2	503
	11th to 20th	Day	19% 10% 31% 24% 13% 3	527
	21st to end	Day	20% 13% 31% 25% 10% 2	573
June	1st to 10th	Day	23% 10% 28% 27% 10% 2	530
	11th to 20th	Day	16% 12% 28% 30% 13% 2	530
	21st to end	Day	21% 11% 25% 27% 12% 3	530
July	1st to 10th	Day	14% 7% 28% 30% 17% 5%	525
	11th to 20th	Day	14% 7% 28% 32% 16% 3	524
	21st to end	Day	11% 6% 28% 34% 17% 4	576
August	1st to 10th	Day	15% 5% 23% 34% 19% 5%	526
	11th to 20th	Day	11% 5% 21% 33% 20% 10%	519
	21st to end	Day	15% 8% 25% 25% 19% 9%	583
September	1st to 10th	Day	13% 8% 27% 27% 18% 8%	530
	11th to 20th	Day	11% 7% 21% 30% 20% 11%	530
	21st to end	Day	18% 10% 26% 23% 17% 6%	530
October	1st to 10th	Day	20% 11% 27% 23% 15% 4	502
	11th to 20th	Day	20% 13% 32% 22% 12% 2	463
	21st to end	Day	29% 10% 28% 23% 8%	429
November	1st to 10th	Day	25% 14% 27% 24% 9% 1	395
	11th to 20th	Day	24% 12% 34% 20% 10% 1	310
	21st to end	Day	29% 16% 29% 17% 10% 1	200
December	1st to 10th	Day	28% 13% 25% 23% 10% 3	200
	11th to 20th	Day	24% 17% 32% 24% 3 1	200
	21st to end	Day	31% 14% 29% 16% 8% 2	220
January	1st to 10th	Night	25% 11% 33% 26% 6%	800
	11th to 20th	Night	23% 11% 32% 26% 8%	800
	21st to end	Night	17% 8% 36% 32% 7% 1	880
February	1st to 10th	Night	20% 10% 37% 27% 5% 1	800
	11th to 20th	Night	19% 11% 37% 26% 5% 1	783
	21st to end	Night	22% 9% 34% 30% 4	660
March	1st to 10th	Night	26% 9% 32% 27% 6%	800
	11th to 20th	Night	24% 11% 33% 26% 7%	800
	21st to end	Night	22% 9% 33% 30% 7%	880
April	1st to 10th	Night	28% 11% 28% 27% 6%	800
	11th to 20th	Night	25% 11% 32% 26% 5%	791
	21st to end	Night	29% 8% 33% 24% 7%	628
May	1st to 10th	Night	25% 8% 30% 30% 7% 1	450
	11th to 20th	Night	23% 7% 29% 32% 9% 1	394
	21st to end	Night	19% 8% 35% 30% 8%	433
June	1st to 10th	Night	24% 5% 25% 37% 8%	400
	11th to 20th	Night	19% 8% 28% 35% 12% 1	400
	21st to end	Night	22% 4 27% 34% 13%	400
July	1st to 10th	Night	11% 6% 27% 39% 16% 1	390
	11th to 20th	Night	12% 5% 22% 45% 16%	388
	21st to end	Night	12% 3 22% 46% 15% 1	438
August	1st to 10th	Night	13% 5% 14% 48% 20% 1	398
	11th to 20th	Night	14% 4 22% 35% 24% 2	572
	21st to end	Night	15% 4 23% 35% 20% 2	880
September	1st to 10th	Night	15% 4 24% 39% 17% 1	800
	11th to 20th	Night	15% 4 20% 36% 23% 2	800
	21st to end	Night	18% 5% 27% 33% 16% 2	800
October	1st to 10th	Night	21% 5% 28% 34% 11% 1	773
	11th to 20th	Night	23% 8% 28% 30% 10% 1	760
	21st to end	Night	27% 9% 31% 27% 7%	836
November	1st to 10th	Night	23% 11% 30% 27% 8% 1	780
	11th to 20th	Night	23% 9% 34% 23% 10% 1	800
	21st to end	Night	28% 11% 31% 22% 8% 1	800
December	1st to 10th	Night	28% 8% 32% 24% 7%	800
	11th to 20th	Night	29% 11% 31% 24% 5% 1	800
	21st to end	Night	32% 10% 31% 20% 7%	880

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