Jarnac Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Jarnac Observatory clear sky chart from 2005-11-06 to 2025-08-05.

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	2 2 1 3 3 3 3 3 5% 6% 67%	129798
	Night	3 3 1 3 3 3 3 3 5% 6% 68%	127267
Transparency	Day	17% 4 6% 12% 26% 35%	129523
	Night	17% 4 6% 13% 27% 34%	127057
Seeing	Day	5% 6% 22% 33% 24% 10%	15883
	Night	4 2 18% 42% 28% 5%	25980

By Month

Percentage Cloud Cover SW (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	4 5% 2 4 3 3 3 2 5% 6% 65%	4098
	11th to 20th	Day	2 3 1 3 2 2 2 2 3 4 76%	4100
	21st to end	Day	6% 5% 1 4 2 3 3 3 5% 8% 60%	4510
February	1st to 10th	Day	4 4 1 4 2 3 3 2 5% 6% 65%	1835
	11th to 20th	Day	5% 5% 2 4 4 4 4 3 4 7% 61%	1950
	21st to end	Day	6% 4 2 3 2 3 3 2 4 5% 66%	1770
March	1st to 10th	Day	4 3 1 3 3 2 3 3 4 6% 68%	4954
	11th to 20th	Day	2 3 1 3 2 3 4 3 5% 6% 69%	5000
	21st to end	Day	2 4 1 4 3 3 4 3 4 8% 65%	5486
April	1st to 10th	Day	1 1 1 3 2 3 3 3 4 6% 74%	2200
	11th to 20th	Day	2 2 1 2 2 1 2 2 3 4 79%	2336
	21st to end	Day	1 2 1 2 1 1 2 2 3 5% 82%	2400
May	1st to 10th	Day	1 2 1 1 2 1 2 3 86%	5850
	11th to 20th	Day	1 1 1 2 2 2 3 2 3 4 79%	5841
	21st to end	Day	1 1 2 1 2 2 1 3 3 85%	6423
June	1st to 10th	Day	2 1 1 2 2 2 2 2 4 6% 75%	2600
	11th to 20th	Day	1 1 1 1 2 2 2 3 4 83%	2600
	21st to end	Day	2 2 1 4 4 4 5% 3 5% 6% 63%	2600
July	1st to 10th	Day	1 2 1 4 4 4 5% 4 6% 6% 64%	5879
	11th to 20th	Day	2 2 1 5% 4 5% 6% 5% 9% 10% 51%	5880
	21st to end	Day	3 3 2 5% 5% 5% 6% 5% 9% 12% 43%	6401
August	1st to 10th	Day	3 3 1 5% 6% 6% 7% 6% 10% 11% 42%	4145
	11th to 20th	Day	2 3 1 4 5% 5% 6% 6% 8% 12% 48%	3980
	21st to end	Day	2 2 1 5% 4 6% 6% 5% 7% 8% 54%	4131
September	1st to 10th	Day	6% 4 2 6% 5% 5% 6% 4 6% 7% 51%	2090
	11th to 20th	Day	3 2 1 3 3 3 4 4 5% 6% 66%	2090
	21st to end	Day	2 1 1 3 2 3 4 3 6% 5% 70%	2090
October	1st to 10th	Day	1 1 1 2 1 2 2 2 4 4 81%	3333
	11th to 20th	Day	1 1 2 1 1 2 1 3 4 83%	3150
	21st to end	Day	2 1 2 1 1 2 2 3 5% 82%	3438
November	1st to 10th	Day	2 1 1 2 1 2 3 2 5% 6% 75%	1959
	11th to 20th	Day	2 2 1 2 3 2 4 2 4 5% 73%	2000
	21st to end	Day	4 4 1 4 4 3 4 3 4 6% 63%	1810
December	1st to 10th	Day	3 2 1 3 1 2 2 2 4 5% 76%	2950
	11th to 20th	Day	3 4 2 5% 3 2 3 3 5% 7% 63%	3409
	21st to end	Day	4 3 1 4 2 3 2 3 4 6% 68%	4510
January	1st to 10th	Night	3 3 1 3 2 2 3 3 5% 6% 67%	5867
	11th to 20th	Night	2 2 1 2 1 1 2 2 3 4 80%	5880
	21st to end	Night	8% 7% 1 4 3 3 3 3 4 7% 57%	6468
February	1st to 10th	Night	4 4 1 3 3 3 3 3 4 5% 66%	2562
	11th to 20th	Night	5% 5% 1 4 3 3 3 3 5% 6% 62%	2338
	21st to end	Night	4 4 1 4 3 3 3 3 5% 6% 64%	1857
March	1st to 10th	Night	3 3 1 3 3 3 3 3 4 5% 70%	4940
	11th to 20th	Night	4 2 1 2 2 3 3 3 4 6% 69%	5000
	21st to end	Night	3 4 1 3 3 3 3 2 5% 6% 66%	5489
April	1st to 10th	Night	2 3 1 3 2 3 2 2 4 5% 73%	2200
	11th to 20th	Night	2 2 1 2 2 2 2 2 3 4 78%	2009
	21st to end	Night	2 2 2 1 1 1 1 2 5% 83%	2000
May	1st to 10th	Night	1 1 1 1 1 2 2 2 3 86%	4113
	11th to 20th	Night	1 1 1 2 2 2 2 2 4 4 78%	4074
	21st to end	Night	1 1 1 1 1 2 4 87%	4474
June	1st to 10th	Night	3 1 1 1 2 2 3 2 4 7% 75%	1800
	11th to 20th	Night	1 1 2 1 2 2 1 2 5% 82%	1800
	21st to end	Night	2 3 1 3 3 3 3 4 5% 6% 66%	1800
July	1st to 10th	Night	1 2 1 4 4 4 5% 5% 7% 8% 59%	4100
	11th to 20th	Night	1 2 1 5% 5% 6% 8% 7% 10% 14% 40%	4100
	21st to end	Night	2 2 1 5% 5% 5% 7% 7% 11% 15% 39%	4472
August	1st to 10th	Night	2 3 1 6% 5% 7% 6% 6% 10% 12% 42%	2883
	11th to 20th	Night	2 2 1 5% 5% 5% 6% 5% 9% 14% 46%	2856
	21st to end	Night	3 2 1 5% 4 4 7% 5% 8% 11% 50%	3533
September	1st to 10th	Night	6% 5% 2 5% 3 4 4 4 6% 7% 55%	2090
	11th to 20th	Night	2 2 1 4 2 2 3 2 5% 6% 71%	2090
	21st to end	Night	2 2 1 3 2 2 3 3 5% 5% 71%	2090
October	1st to 10th	Night	1 1 1 2 2 3 4 3 4 4 75%	3640
	11th to 20th	Night	2 1 2 2 2 2 2 3 4 82%	3778
	21st to end	Night	1 1 1 1 1 2 2 3 4 85%	4120
November	1st to 10th	Night	2 1 1 2 2 2 3 2 4 5% 76%	2340
	11th to 20th	Night	2 2 1 2 2 2 3 2 4 5% 74%	2400
	21st to end	Night	5% 4 2 4 3 2 3 3 5% 6% 63%	2585
December	1st to 10th	Night	3 2 1 2 2 2 2 2 3 4 77%	4240
	11th to 20th	Night	3 3 2 5% 3 3 3 2 4 5% 66%	4811
	21st to end	Night	4 5% 1 3 3 3 3 3 5% 6% 65%	6468

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	19% 3 4 10% 33% 31%	4099
	11th to 20th	Day	13% 3 3 9% 31% 41%	4100
	21st to end	Day	21% 3 6% 14% 27% 29%	4510
February	1st to 10th	Day	21% 3 4 10% 23% 39%	1835
	11th to 20th	Day	23% 4 4 13% 27% 28%	1960
	21st to end	Day	21% 2 3 10% 27% 37%	1770
March	1st to 10th	Day	17% 3 3 7% 35% 36%	4954
	11th to 20th	Day	13% 4 5% 15% 35% 27%	5000
	21st to end	Day	17% 4 5% 10% 37% 28%	5486
April	1st to 10th	Day	13% 2 3 8% 35% 40%	2200
	11th to 20th	Day	11% 2 1 6% 27% 53%	2339
	21st to end	Day	8% 2 2 8% 30% 50%	2400
May	1st to 10th	Day	7% 1 3 4 25% 60%	5850
	11th to 20th	Day	9% 2 4 7% 27% 52%	5844
	21st to end	Day	8% 1 2 3 14% 73%	6424
June	1st to 10th	Day	11% 2 2 8% 25% 52%	2600
	11th to 20th	Day	7% 2 2 5% 18% 67%	2600
	21st to end	Day	21% 4 6% 9% 18% 41%	2600
July	1st to 10th	Day	19% 5% 7% 16% 25% 28%	5816
	11th to 20th	Day	24% 8% 12% 29% 18% 8%	5811
	21st to end	Day	28% 9% 17% 20% 16% 10%	6217
August	1st to 10th	Day	30% 10% 19% 20% 13% 8%	4131
	11th to 20th	Day	25% 9% 14% 22% 23% 7%	4019
	21st to end	Day	25% 8% 13% 17% 23% 13%	4130
September	1st to 10th	Day	31% 6% 10% 16% 17% 21%	2090
	11th to 20th	Day	18% 5% 6% 14% 28% 29%	2090
	21st to end	Day	16% 4 7% 12% 30% 32%	2090
October	1st to 10th	Day	8% 2 4 10% 17% 58%	3333
	11th to 20th	Day	8% 2 3 7% 31% 49%	3150
	21st to end	Day	8% 1 3 6% 24% 58%	3438
November	1st to 10th	Day	10% 2 3 13% 36% 35%	1959
	11th to 20th	Day	15% 2 4 11% 31% 36%	2000
	21st to end	Day	22% 3 5% 13% 25% 31%	1810
December	1st to 10th	Day	13% 2 3 10% 38% 34%	2950
	11th to 20th	Day	21% 2 4 10% 27% 37%	3408
	21st to end	Day	17% 3 3 17% 37% 23%	4510
January	1st to 10th	Night	16% 4 5% 9% 43% 24%	5867
	11th to 20th	Night	10% 2 3 14% 36% 36%	5880
	21st to end	Night	26% 4 6% 14% 25% 25%	6468
February	1st to 10th	Night	19% 4 4 12% 22% 39%	2562
	11th to 20th	Night	23% 3 6% 14% 28% 27%	2339
	21st to end	Night	21% 3 4 10% 28% 34%	1857
March	1st to 10th	Night	16% 3 4 9% 36% 33%	4940
	11th to 20th	Night	15% 3 5% 17% 34% 26%	5000
	21st to end	Night	17% 4 5% 13% 36% 25%	5489
April	1st to 10th	Night	14% 2 3 10% 35% 37%	2200
	11th to 20th	Night	12% 2 2 8% 27% 49%	2009
	21st to end	Night	9% 1 2 8% 30% 51%	2000
May	1st to 10th	Night	7% 1 2 6% 32% 52%	4110
	11th to 20th	Night	8% 2 5% 6% 21% 57%	4075
	21st to end	Night	5% 1 3 16% 74%	4474
June	1st to 10th	Night	11% 3 2 6% 25% 54%	1800
	11th to 20th	Night	8% 2 2 3 17% 67%	1800
	21st to end	Night	18% 5% 6% 8% 18% 45%	1800
July	1st to 10th	Night	20% 6% 9% 16% 23% 26%	4055
	11th to 20th	Night	28% 10% 18% 22% 15% 7%	4046
	21st to end	Night	26% 10% 22% 19% 12% 11%	4344
August	1st to 10th	Night	30% 11% 19% 18% 13% 8%	2874
	11th to 20th	Night	25% 7% 17% 24% 20% 7%	2883
	21st to end	Night	24% 7% 17% 19% 21% 12%	3533
September	1st to 10th	Night	26% 7% 11% 19% 18% 19%	2090
	11th to 20th	Night	15% 3 7% 16% 29% 30%	2090
	21st to end	Night	13% 5% 7% 14% 30% 31%	2090
October	1st to 10th	Night	12% 3 6% 10% 17% 53%	3640
	11th to 20th	Night	9% 1 3 12% 24% 51%	3778
	21st to end	Night	7% 1 3 8% 25% 57%	4120
November	1st to 10th	Night	11% 2 3 13% 36% 34%	2340
	11th to 20th	Night	13% 2 4 13% 31% 37%	2400
	21st to end	Night	21% 3 5% 13% 28% 30%	2585
December	1st to 10th	Night	13% 2 4 12% 36% 33%	4240
	11th to 20th	Night	22% 2 4 11% 26% 36%	4811
	21st to end	Night	20% 3 4 16% 32% 25%	6468

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	12% 6% 33% 32% 14% 2	300
	11th to 20th	Day	10% 4 30% 41% 13% 2	300
	21st to end	Day	6% 7% 33% 38% 14% 3	330
February	1st to 10th	Day	6% 5% 36% 33% 16% 4	335
	11th to 20th	Day	10% 12% 35% 33% 10% 1	495
	21st to end	Day	10% 15% 39% 26% 8% 1	414
March	1st to 10th	Day	7% 15% 35% 29% 11% 2	500
	11th to 20th	Day	6% 12% 34% 29% 14% 5%	500
	21st to end	Day	6% 15% 33% 31% 12% 2	550
April	1st to 10th	Day	3 12% 29% 32% 18% 6%	500
	11th to 20th	Day	5% 13% 32% 28% 18% 3	492
	21st to end	Day	3 12% 32% 30% 17% 7%	500
May	1st to 10th	Day	3 10% 29% 33% 19% 6%	503
	11th to 20th	Day	2 8% 26% 34% 24% 7%	528
	21st to end	Day	2 6% 27% 36% 22% 7%	573
June	1st to 10th	Day	3 3 22% 37% 25% 10%	530
	11th to 20th	Day	2 3 21% 38% 29% 8%	530
	21st to end	Day	4 1 16% 36% 32% 12%	530
July	1st to 10th	Day	2 6% 32% 36% 23%	526
	11th to 20th	Day	5% 1 4 26% 40% 24%	524
	21st to end	Day	8% 1 3 26% 37% 26%	575
August	1st to 10th	Day	6% 2 23% 42% 27%	504
	11th to 20th	Day	6% 1 3 25% 42% 24%	477
	21st to end	Day	3 1 3 27% 40% 26%	539
September	1st to 10th	Day	9% 2 9% 33% 32% 15%	490
	11th to 20th	Day	4 11% 35% 35% 15%	490
	21st to end	Day	4 3 17% 33% 31% 12%	490
October	1st to 10th	Day	2 4 17% 37% 29% 11%	398
	11th to 20th	Day	4 3 17% 37% 29% 10%	300
	21st to end	Day	2 2 19% 37% 31% 9%	330
November	1st to 10th	Day	3 6% 24% 40% 23% 4	300
	11th to 20th	Day	3 3 30% 40% 20% 4	300
	21st to end	Day	8% 5% 37% 31% 17% 2	300
December	1st to 10th	Day	10% 3 25% 44% 16% 2	300
	11th to 20th	Day	7% 6% 31% 41% 14% 2	300
	21st to end	Day	10% 6% 32% 32% 17% 2	330
January	1st to 10th	Night	8% 2 25% 46% 16% 3	800
	11th to 20th	Night	6% 3 25% 44% 20% 2	800
	21st to end	Night	9% 4 25% 42% 18% 3	880
February	1st to 10th	Night	5% 3 23% 44% 21% 5%	800
	11th to 20th	Night	7% 4 25% 46% 15% 2	784
	21st to end	Night	7% 3 32% 42% 16% 1	660
March	1st to 10th	Night	4 5% 31% 43% 15% 1	800
	11th to 20th	Night	6% 4 26% 36% 22% 6%	800
	21st to end	Night	4 4 22% 47% 21% 3	880
April	1st to 10th	Night	3 3 22% 49% 21% 2	800
	11th to 20th	Night	3 4 27% 48% 17% 1	625
	21st to end	Night	3 3 25% 48% 19% 2	600
May	1st to 10th	Night	3 3 20% 51% 22% 1	600
	11th to 20th	Night	1 2 20% 49% 25% 3	591
	21st to end	Night	1 3 18% 49% 28% 1	650
June	1st to 10th	Night	3 12% 50% 32% 3	600
	11th to 20th	Night	1 1 13% 52% 32% 2	600
	21st to end	Night	2 2 9% 40% 43% 4	600
July	1st to 10th	Night	2 1 4 38% 48% 6%	585
	11th to 20th	Night	3 1 3 28% 55% 10%	582
	21st to end	Night	5% 2 25% 52% 15%	657
August	1st to 10th	Night	3 1 1 20% 63% 13%	579
	11th to 20th	Night	5% 1 3 21% 59% 11%	564
	21st to end	Night	5% 1 2 25% 55% 13%	647
September	1st to 10th	Night	6% 1 7% 29% 46% 11%	760
	11th to 20th	Night	4 1 6% 40% 42% 8%	760
	21st to end	Night	3 1 11% 44% 36% 7%	760
October	1st to 10th	Night	2 15% 47% 31% 5%	760
	11th to 20th	Night	2 1 18% 45% 28% 6%	760
	21st to end	Night	2 1 18% 44% 27% 8%	836
November	1st to 10th	Night	3 3 20% 44% 25% 5%	780
	11th to 20th	Night	4 2 24% 43% 26% 3	800
	21st to end	Night	7% 4 27% 42% 17% 4	800
December	1st to 10th	Night	7% 2 22% 47% 18% 3	800
	11th to 20th	Night	8% 4 26% 44% 16% 2	800
	21st to end	Night	9% 3 24% 41% 18% 4	880

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