Danville
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Danville clear sky chart from 2005-11-06 to 2025-09-19.

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	14% 9% 4 8% 5% 5% 5% 4 6% 7% 35%	126708
	Night	14% 9% 3 8% 5% 4 4 3 5% 7% 36%	122360
Transparency	Day	46% 6% 11% 22% 13% 1	130367
	Night	45% 6% 11% 22% 14% 2	126771
Seeing	Day	17% 10% 26% 24% 16% 7%	16313
	Night	18% 8% 26% 30% 15% 2	25937

By Month

Percentage Cloud Cover SE (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% 17% 8% 14% 5% 4 4 3 4 6% 21%	4056
	11th to 20th	Day	14% 15% 6% 15% 6% 7% 5% 3 4 4 24%	4060
	21st to end	Day	21% 12% 7% 9% 5% 6% 5% 3 5% 5% 21%	4466
February	1st to 10th	Day	19% 14% 4 9% 5% 5% 4 3 4 6% 27%	1860
	11th to 20th	Day	18% 12% 4 10% 6% 5% 4 3 4 6% 28%	1948
	21st to end	Day	19% 12% 5% 9% 5% 4 4 2 4 6% 30%	1650
March	1st to 10th	Day	17% 13% 7% 9% 5% 4 3 3 4 5% 30%	4608
	11th to 20th	Day	13% 7% 4 7% 5% 4 5% 3 5% 8% 39%	4980
	21st to end	Day	17% 10% 3 7% 5% 4 4 3 5% 7% 34%	5465
April	1st to 10th	Day	15% 10% 3 7% 4 4 5% 4 6% 8% 34%	2300
	11th to 20th	Day	16% 9% 3 6% 4 4 4 4 5% 6% 38%	2467
	21st to end	Day	17% 8% 3 7% 5% 5% 5% 4 5% 8% 34%	2600
May	1st to 10th	Day	18% 10% 3 8% 5% 5% 5% 4 6% 7% 29%	5862
	11th to 20th	Day	19% 10% 3 7% 4 4 5% 4 6% 7% 31%	5861
	21st to end	Day	14% 6% 3 7% 5% 5% 6% 5% 8% 9% 32%	6433
June	1st to 10th	Day	11% 6% 2 7% 5% 4 5% 5% 6% 8% 41%	2600
	11th to 20th	Day	9% 6% 2 7% 5% 4 6% 4 7% 10% 41%	2641
	21st to end	Day	9% 6% 2 7% 5% 5% 5% 4 7% 11% 40%	2800
July	1st to 10th	Day	10% 6% 2 6% 5% 5% 4 4 6% 8% 43%	6253
	11th to 20th	Day	10% 6% 2 7% 5% 6% 6% 4 7% 10% 36%	5900
	21st to end	Day	10% 7% 3 8% 6% 7% 6% 5% 7% 9% 32%	6424
August	1st to 10th	Day	8% 5% 3 6% 5% 5% 5% 4 6% 11% 43%	4234
	11th to 20th	Day	8% 6% 2 7% 5% 4 6% 5% 8% 11% 39%	4174
	21st to end	Day	5% 3 2 5% 4 4 5% 4 6% 9% 54%	4444
September	1st to 10th	Day	13% 6% 2 4 4 3 4 3 5% 7% 49%	2240
	11th to 20th	Day	8% 5% 2 6% 3 4 4 4 6% 8% 50%	2178
	21st to end	Day	16% 7% 2 6% 3 4 3 3 5% 6% 45%	2090
October	1st to 10th	Day	11% 5% 1 5% 4 4 4 4 5% 7% 49%	2085
	11th to 20th	Day	14% 7% 2 6% 4 4 3 3 4 7% 46%	1900
	21st to end	Day	19% 9% 3 6% 4 3 3 3 5% 7% 39%	1998
November	1st to 10th	Day	11% 7% 2 5% 3 3 4 4 4 7% 50%	1762
	11th to 20th	Day	14% 12% 3 8% 5% 4 4 3 5% 6% 36%	1800
	21st to end	Day	18% 11% 4 7% 5% 4 4 3 5% 6% 34%	1800
December	1st to 10th	Day	18% 11% 4 7% 5% 4 4 3 3 5% 35%	2930
	11th to 20th	Day	21% 13% 5% 10% 5% 5% 5% 3 4 5% 24%	3373
	21st to end	Day	21% 19% 5% 11% 6% 4 4 3 4 5% 18%	4466
January	1st to 10th	Night	15% 16% 6% 15% 8% 5% 5% 4 4 5% 18%	5889
	11th to 20th	Night	14% 14% 7% 14% 7% 5% 5% 3 4 5% 22%	5900
	21st to end	Night	20% 12% 5% 9% 5% 4 4 3 4 5% 30%	6490
February	1st to 10th	Night	21% 12% 4 8% 4 4 4 3 5% 5% 28%	2536
	11th to 20th	Night	19% 11% 3 9% 5% 4 4 4 4 7% 29%	2338
	21st to end	Night	19% 14% 4 8% 4 4 3 2 5% 6% 30%	1980
March	1st to 10th	Night	17% 13% 3 7% 4 4 4 3 4 5% 37%	5244
	11th to 20th	Night	12% 7% 3 6% 4 4 5% 4 6% 9% 42%	4980
	21st to end	Night	17% 9% 4 9% 5% 5% 4 4 4 6% 33%	5467
April	1st to 10th	Night	15% 8% 3 6% 5% 4 5% 4 6% 7% 36%	2116
	11th to 20th	Night	13% 8% 3 7% 4 4 5% 4 5% 7% 41%	1882
	21st to end	Night	16% 8% 3 7% 4 3 4 3 6% 8% 37%	1800
May	1st to 10th	Night	21% 11% 3 8% 5% 5% 4 4 5% 7% 28%	4033
	11th to 20th	Night	15% 7% 3 6% 5% 5% 5% 4 7% 9% 34%	4032
	21st to end	Night	13% 7% 3 7% 4 4 4 4 6% 9% 38%	4421
June	1st to 10th	Night	7% 6% 2 5% 4 4 4 4 6% 10% 49%	1800
	11th to 20th	Night	9% 4 2 5% 4 3 5% 3 7% 10% 47%	1749
	21st to end	Night	6% 4 1 5% 4 4 5% 5% 9% 11% 45%	1600
July	1st to 10th	Night	9% 6% 2 6% 5% 4 5% 3 5% 8% 47%	3701
	11th to 20th	Night	9% 6% 2 7% 5% 5% 5% 4 6% 11% 40%	4057
	21st to end	Night	9% 6% 2 7% 5% 5% 6% 4 7% 9% 38%	4427
August	1st to 10th	Night	8% 6% 3 5% 5% 5% 5% 4 6% 9% 45%	2915
	11th to 20th	Night	9% 8% 2 8% 5% 5% 5% 3 5% 8% 43%	2875
	21st to end	Night	4 2 1 4 3 3 4 4 6% 10% 58%	3449
September	1st to 10th	Night	10% 5% 2 5% 3 3 3 3 6% 6% 52%	2140
	11th to 20th	Night	8% 4 2 5% 4 4 4 4 5% 7% 56%	2179
	21st to end	Night	14% 6% 2 5% 4 4 3 4 5% 6% 48%	2090
October	1st to 10th	Night	8% 5% 2 5% 4 3 4 4 5% 6% 54%	2095
	11th to 20th	Night	15% 6% 2 6% 4 4 4 4 5% 7% 46%	2280
	21st to end	Night	18% 7% 3 5% 4 4 4 3 5% 6% 41%	2597
November	1st to 10th	Night	13% 7% 2 5% 4 3 3 3 5% 7% 46%	2536
	11th to 20th	Night	15% 9% 3 6% 5% 4 4 3 5% 7% 39%	2600
	21st to end	Night	18% 13% 3 7% 5% 5% 3 3 4 6% 33%	2600
December	1st to 10th	Night	17% 11% 4 8% 4 4 3 3 4 5% 37%	4250
	11th to 20th	Night	19% 13% 5% 10% 5% 4 4 3 4 6% 26%	4822
	21st to end	Night	21% 17% 7% 10% 6% 5% 5% 3 4 6% 17%	6490

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 10% 15% 6%	4057
	11th to 20th	Day	63% 4 9% 12% 10% 2	4060
	21st to end	Day	65% 4 9% 14% 6% 1	4466
February	1st to 10th	Day	59% 4 10% 16% 9% 1	1860
	11th to 20th	Day	58% 5% 11% 16% 10% 1	1958
	21st to end	Day	56% 5% 9% 20% 9% 2	1650
March	1st to 10th	Day	55% 4 8% 20% 11% 1	4608
	11th to 20th	Day	43% 5% 10% 28% 14% 1	4980
	21st to end	Day	49% 6% 11% 21% 12% 2	5465
April	1st to 10th	Day	46% 7% 13% 23% 11% 1	2300
	11th to 20th	Day	44% 7% 10% 22% 13% 3	2467
	21st to end	Day	48% 7% 11% 21% 12% 2	2600
May	1st to 10th	Day	52% 7% 12% 18% 10% 1	5897
	11th to 20th	Day	50% 7% 11% 22% 9%	5865
	21st to end	Day	42% 9% 14% 21% 12% 2	6447
June	1st to 10th	Day	38% 9% 12% 24% 16% 1	2600
	11th to 20th	Day	37% 7% 16% 23% 16% 1	2641
	21st to end	Day	38% 8% 14% 23% 14% 3	2800
July	1st to 10th	Day	38% 6% 11% 26% 18% 1	6186
	11th to 20th	Day	41% 8% 14% 25% 12%	5831
	21st to end	Day	47% 7% 12% 20% 12% 2	6240
August	1st to 10th	Day	33% 8% 15% 33% 11% 1	4221
	11th to 20th	Day	35% 7% 12% 30% 15%	4213
	21st to end	Day	28% 6% 15% 27% 21% 3	4443
September	1st to 10th	Day	35% 6% 12% 23% 20% 5%	2240
	11th to 20th	Day	30% 7% 12% 25% 24% 3	2178
	21st to end	Day	40% 6% 8% 23% 21% 2	2090
October	1st to 10th	Day	22% 5% 10% 30% 32% 2	3462
	11th to 20th	Day	30% 3 9% 30% 26% 2	3140
	21st to end	Day	35% 4 10% 25% 23% 3	3274
November	1st to 10th	Day	34% 5% 10% 28% 19% 3	1762
	11th to 20th	Day	49% 4 8% 21% 15% 3	1800
	21st to end	Day	51% 5% 10% 20% 13% 1	1800
December	1st to 10th	Day	53% 4 9% 25% 9%	2930
	11th to 20th	Day	62% 6% 8% 17% 7%	3370
	21st to end	Day	68% 5% 9% 12% 5% 1	4466
January	1st to 10th	Night	67% 6% 7% 13% 7%	5889
	11th to 20th	Night	65% 4 8% 12% 8% 3	5900
	21st to end	Night	57% 4 9% 20% 9% 1	6490
February	1st to 10th	Night	57% 5% 10% 18% 8% 1	2536
	11th to 20th	Night	55% 5% 10% 18% 12% 1	2339
	21st to end	Night	55% 5% 10% 17% 10% 3	1980
March	1st to 10th	Night	49% 4 7% 26% 11% 2	5244
	11th to 20th	Night	37% 6% 10% 30% 16% 1	4980
	21st to end	Night	52% 5% 8% 23% 12% 2	5467
April	1st to 10th	Night	44% 8% 11% 23% 12% 3	2116
	11th to 20th	Night	41% 5% 11% 20% 17% 5%	1882
	21st to end	Night	44% 6% 11% 23% 14% 3	1800
May	1st to 10th	Night	54% 8% 10% 18% 8% 3	4060
	11th to 20th	Night	44% 9% 14% 22% 9% 1	4033
	21st to end	Night	39% 8% 17% 23% 10% 3	4429
June	1st to 10th	Night	30% 7% 14% 26% 20% 3	1800
	11th to 20th	Night	30% 8% 16% 23% 19% 4	1749
	21st to end	Night	28% 11% 18% 22% 17% 4	1600
July	1st to 10th	Night	35% 5% 12% 24% 20% 4	3662
	11th to 20th	Night	37% 7% 15% 27% 13% 1	4006
	21st to end	Night	40% 7% 12% 24% 14% 3	4297
August	1st to 10th	Night	33% 7% 15% 32% 11% 1	2904
	11th to 20th	Night	38% 6% 11% 25% 18% 1	2902
	21st to end	Night	20% 7% 16% 32% 22% 3	3449
September	1st to 10th	Night	31% 6% 12% 24% 21% 6%	2140
	11th to 20th	Night	28% 6% 11% 25% 28% 3	2179
	21st to end	Night	36% 7% 9% 21% 26% 2	2090
October	1st to 10th	Night	23% 4 8% 25% 38% 3	3498
	11th to 20th	Night	28% 4 9% 26% 30% 4	3779
	21st to end	Night	33% 3 9% 22% 26% 7%	4271
November	1st to 10th	Night	37% 5% 11% 21% 22% 4	2536
	11th to 20th	Night	45% 4 9% 21% 18% 4	2600
	21st to end	Night	53% 5% 10% 20% 11% 2	2600
December	1st to 10th	Night	51% 4 11% 24% 10% 1	4250
	11th to 20th	Night	59% 4 11% 18% 7% 1	4824
	21st to end	Night	68% 5% 9% 14% 4	6490

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% 19% 36% 17% 5%	300
	11th to 20th	Day	24% 23% 36% 14% 2	300
	21st to end	Day	28% 18% 35% 14% 5%	330
February	1st to 10th	Day	28% 16% 37% 13% 6%	300
	11th to 20th	Day	29% 23% 32% 12% 3	300
	21st to end	Day	27% 17% 43% 10% 2	249
March	1st to 10th	Day	23% 22% 34% 17% 4	350
	11th to 20th	Day	23% 17% 36% 18% 6%	500
	21st to end	Day	29% 16% 32% 17% 5%	550
April	1st to 10th	Day	21% 16% 33% 19% 11% 1	500
	11th to 20th	Day	21% 19% 28% 20% 9% 4	492
	21st to end	Day	21% 13% 35% 18% 11% 2	500
May	1st to 10th	Day	24% 13% 30% 21% 10% 3	503
	11th to 20th	Day	19% 9% 26% 24% 15% 6%	527
	21st to end	Day	13% 8% 23% 27% 19% 10%	573
June	1st to 10th	Day	13% 4 21% 31% 22% 8%	530
	11th to 20th	Day	12% 4 20% 32% 22% 10%	571
	21st to end	Day	9% 6% 24% 31% 20% 10%	730
July	1st to 10th	Day	13% 4 14% 25% 25% 20%	681
	11th to 20th	Day	10% 4 18% 24% 28% 15%	524
	21st to end	Day	12% 1 13% 31% 26% 16%	576
August	1st to 10th	Day	11% 4 13% 26% 29% 17%	526
	11th to 20th	Day	11% 4 13% 33% 24% 14%	519
	21st to end	Day	8% 3 13% 31% 29% 17%	583
September	1st to 10th	Day	12% 4 15% 27% 29% 14%	530
	11th to 20th	Day	8% 2 14% 25% 34% 15%	522
	21st to end	Day	21% 7% 17% 26% 23% 6%	490
October	1st to 10th	Day	11% 4 21% 37% 19% 8%	490
	11th to 20th	Day	14% 12% 33% 22% 13% 6%	490
	21st to end	Day	23% 11% 33% 26% 7% 1	447
November	1st to 10th	Day	14% 8% 36% 27% 12% 3	300
	11th to 20th	Day	19% 18% 28% 22% 9% 3	300
	21st to end	Day	23% 14% 35% 24% 5%	300
December	1st to 10th	Day	24% 16% 37% 17% 6%	300
	11th to 20th	Day	21% 17% 39% 18% 4	300
	21st to end	Day	28% 19% 32% 18% 4	330
January	1st to 10th	Night	22% 17% 36% 21% 4	800
	11th to 20th	Night	27% 15% 35% 21% 3	800
	21st to end	Night	24% 14% 35% 24% 4	880
February	1st to 10th	Night	28% 11% 34% 25% 2	800
	11th to 20th	Night	26% 15% 36% 21% 2	783
	21st to end	Night	27% 12% 38% 22% 2	660
March	1st to 10th	Night	21% 13% 35% 25% 6%	800
	11th to 20th	Night	21% 14% 37% 22% 7%	800
	21st to end	Night	22% 11% 32% 28% 7%	880
April	1st to 10th	Night	18% 14% 36% 26% 7%	716
	11th to 20th	Night	16% 9% 35% 27% 11% 1	593
	21st to end	Night	16% 7% 38% 30% 8%	600
May	1st to 10th	Night	21% 8% 30% 30% 12% 1	600
	11th to 20th	Night	16% 6% 24% 33% 18% 2	591
	21st to end	Night	9% 4 20% 36% 25% 6%	650
June	1st to 10th	Night	8% 2 21% 42% 24% 3	600
	11th to 20th	Night	10% 3 16% 42% 27% 3	600
	21st to end	Night	7% 2 18% 43% 27% 3	600
July	1st to 10th	Night	12% 3 11% 35% 29% 11%	585
	11th to 20th	Night	6% 2 14% 37% 36% 5%	582
	21st to end	Night	11% 2 11% 40% 33% 4	657
August	1st to 10th	Night	8% 2 10% 38% 37% 5%	597
	11th to 20th	Night	9% 3 11% 40% 32% 5%	594
	21st to end	Night	6% 2 12% 35% 35% 9%	660
September	1st to 10th	Night	13% 2 12% 35% 31% 6%	740
	11th to 20th	Night	9% 4 11% 35% 34% 7%	793
	21st to end	Night	15% 4 18% 36% 24% 3	760
October	1st to 10th	Night	10% 5% 23% 35% 23% 5%	760
	11th to 20th	Night	18% 8% 26% 33% 14% 1	760
	21st to end	Night	21% 9% 29% 28% 12%	836
November	1st to 10th	Night	18% 7% 28% 29% 16% 1	780
	11th to 20th	Night	19% 12% 32% 26% 9% 1	800
	21st to end	Night	24% 12% 28% 30% 7% 1	800
December	1st to 10th	Night	27% 12% 33% 23% 5%	800
	11th to 20th	Night	24% 10% 35% 24% 6% 1	800
	21st to end	Night	28% 14% 29% 26% 3	880

Page generated at 2025-09-16 01:49:16. Version ac07

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