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 2026-01-17.

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	13% 9% 3 8% 5% 4 4 4 6% 8% 36%	127190
	Night	14% 10% 4 8% 5% 4 4 3 5% 6% 38%	124700
Transparency	Day	46% 4 11% 23% 14% 2	130751
	Night	47% 4 10% 22% 15% 2	129213
Seeing	Day	17% 10% 26% 25% 16% 6%	17271
	Night	18% 9% 28% 30% 13% 1	27165

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	14% 13% 8% 13% 7% 4 4 3 5% 7% 22%	3697
	11th to 20th	Day	12% 12% 8% 15% 8% 7% 5% 5% 5% 4 20%	3668
	21st to end	Day	20% 19% 4 11% 5% 3 3 3 4 4 24%	4184
February	1st to 10th	Day	17% 12% 6% 10% 6% 4 4 3 4 6% 28%	1937
	11th to 20th	Day	15% 11% 3 9% 7% 6% 3 4 4 5% 31%	1948
	21st to end	Day	18% 13% 4 8% 5% 4 3 2 4 6% 33%	1650
March	1st to 10th	Day	19% 16% 4 7% 4 3 2 2 3 3 37%	4480
	11th to 20th	Day	9% 9% 3 6% 4 3 4 3 6% 9% 43%	4648
	21st to end	Day	16% 12% 4 8% 6% 4 5% 3 5% 6% 30%	5967
April	1st to 10th	Day	16% 10% 3 7% 5% 5% 3 4 6% 7% 34%	2400
	11th to 20th	Day	16% 9% 3 6% 5% 4 4 4 5% 6% 38%	2374
	21st to end	Day	20% 8% 3 7% 5% 4 4 3 5% 7% 35%	2490
May	1st to 10th	Day	18% 8% 3 8% 5% 5% 5% 4 7% 8% 29%	5862
	11th to 20th	Day	17% 8% 3 7% 4 5% 4 4 6% 7% 37%	5861
	21st to end	Day	10% 7% 3 8% 5% 5% 5% 5% 7% 9% 36%	6935
June	1st to 10th	Day	10% 6% 2 6% 5% 4 5% 5% 7% 8% 43%	2800
	11th to 20th	Day	9% 5% 2 6% 5% 4 5% 4 6% 9% 45%	2800
	21st to end	Day	9% 6% 3 7% 5% 5% 5% 4 7% 10% 41%	2800
July	1st to 10th	Day	10% 5% 2 6% 4 4 4 5% 7% 11% 40%	6360
	11th to 20th	Day	8% 7% 2 7% 5% 5% 5% 5% 8% 11% 36%	6360
	21st to end	Day	11% 6% 2 6% 5% 4 5% 5% 7% 11% 37%	6924
August	1st to 10th	Day	7% 4 2 5% 4 3 4 4 6% 10% 50%	4275
	11th to 20th	Day	8% 5% 2 6% 5% 4 6% 6% 8% 11% 40%	3849
	21st to end	Day	6% 4 2 5% 4 4 4 4 6% 11% 50%	4312
September	1st to 10th	Day	11% 5% 2 6% 4 5% 5% 4 5% 7% 47%	2400
	11th to 20th	Day	9% 5% 1 5% 4 4 4 4 6% 7% 51%	2240
	21st to end	Day	12% 5% 2 6% 4 5% 4 3 6% 7% 46%	2200
October	1st to 10th	Day	11% 6% 2 5% 4 4 4 4 6% 7% 48%	2200
	11th to 20th	Day	15% 7% 3 7% 4 4 5% 3 5% 6% 42%	2000
	21st to end	Day	18% 10% 3 8% 4 4 4 3 4 6% 36%	1980
November	1st to 10th	Day	11% 7% 2 6% 4 3 4 4 5% 6% 49%	1852
	11th to 20th	Day	18% 10% 3 7% 4 3 3 3 4 6% 39%	1890
	21st to end	Day	17% 12% 3 8% 5% 3 4 3 4 6% 35%	1890
December	1st to 10th	Day	12% 11% 4 8% 4 4 3 3 4 5% 41%	2799
	11th to 20th	Day	14% 15% 6% 11% 5% 5% 5% 4 6% 5% 25%	3088
	21st to end	Day	29% 16% 6% 11% 6% 4 5% 3 4 4 13%	4070
January	1st to 10th	Night	15% 15% 8% 12% 6% 4 3 3 5% 4 26%	6488
	11th to 20th	Night	13% 19% 6% 12% 7% 6% 6% 4 3 3 22%	6445
	21st to end	Night	20% 13% 6% 9% 5% 4 3 2 3 5% 28%	6789
February	1st to 10th	Night	19% 12% 4 8% 5% 5% 3 3 5% 6% 29%	2460
	11th to 20th	Night	15% 11% 4 10% 6% 5% 3 3 4 6% 34%	2338
	21st to end	Night	18% 10% 4 8% 4 4 4 3 4 5% 34%	1980
March	1st to 10th	Night	17% 13% 4 6% 4 3 3 3 4 5% 37%	5373
	11th to 20th	Night	12% 7% 2 6% 4 4 4 3 5% 8% 45%	5312
	21st to end	Night	17% 9% 4 9% 5% 5% 4 4 6% 7% 31%	4990
April	1st to 10th	Night	13% 9% 3 7% 5% 4 4 3 5% 6% 41%	2000
	11th to 20th	Night	17% 9% 2 5% 4 3 4 4 5% 6% 41%	1975
	21st to end	Night	17% 7% 3 7% 5% 4 5% 5% 6% 6% 35%	1907
May	1st to 10th	Night	19% 10% 3 7% 4 4 5% 3 6% 8% 32%	4033
	11th to 20th	Night	15% 7% 2 7% 3 4 4 3 6% 9% 39%	4016
	21st to end	Night	13% 8% 3 8% 5% 5% 5% 4 6% 7% 36%	3942
June	1st to 10th	Night	10% 5% 3 6% 4 4 4 3 5% 8% 48%	1600
	11th to 20th	Night	8% 6% 2 7% 4 4 4 4 5% 8% 49%	1600
	21st to end	Night	7% 5% 2 6% 6% 5% 5% 4 7% 10% 44%	1600
July	1st to 10th	Night	10% 5% 2 6% 4 4 5% 4 7% 9% 44%	3618
	11th to 20th	Night	6% 5% 2 6% 4 5% 5% 5% 8% 11% 44%	3617
	21st to end	Night	8% 7% 3 8% 4 5% 5% 4 6% 9% 40%	3947
August	1st to 10th	Night	7% 4 2 5% 4 4 3 4 5% 8% 54%	2870
	11th to 20th	Night	6% 4 2 6% 5% 5% 5% 4 6% 8% 48%	3200
	21st to end	Night	6% 4 2 5% 4 4 4 3 5% 8% 56%	3586
September	1st to 10th	Night	11% 5% 2 5% 3 3 3 3 6% 6% 52%	2000
	11th to 20th	Night	7% 4 2 4 4 3 4 3 5% 9% 56%	2138
	21st to end	Night	12% 6% 2 5% 4 4 4 4 5% 6% 49%	2200
October	1st to 10th	Night	10% 5% 1 5% 4 3 4 4 5% 5% 55%	2200
	11th to 20th	Night	15% 6% 1 4 4 4 3 3 4 6% 50%	2396
	21st to end	Night	17% 9% 3 6% 4 3 4 3 5% 6% 40%	2860
November	1st to 10th	Night	11% 7% 2 6% 3 3 4 3 4 6% 51%	2666
	11th to 20th	Night	13% 9% 3 7% 4 3 4 4 5% 6% 42%	2730
	21st to end	Night	20% 10% 4 8% 5% 4 3 3 5% 5% 35%	2730
December	1st to 10th	Night	13% 9% 3 8% 4 4 3 3 5% 5% 43%	4605
	11th to 20th	Night	17% 11% 4 10% 5% 5% 4 3 4 5% 31%	5339
	21st to end	Night	28% 17% 6% 9% 4 4 4 3 4 5% 16%	7150

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% 3 10% 17% 7% 1	3698
	11th to 20th	Day	68% 3 9% 8% 9% 3	3668
	21st to end	Day	66% 2 9% 16% 6% 1	4184
February	1st to 10th	Day	59% 4 11% 17% 8% 2	1937
	11th to 20th	Day	55% 3 11% 20% 9% 1	1958
	21st to end	Day	54% 4 10% 20% 9% 2	1650
March	1st to 10th	Day	55% 3 5% 20% 15% 3	4480
	11th to 20th	Day	39% 4 11% 34% 12% 1	4648
	21st to end	Day	55% 4 11% 19% 9% 1	5969
April	1st to 10th	Day	49% 6% 11% 20% 14% 1	2400
	11th to 20th	Day	46% 5% 11% 23% 11% 4	2374
	21st to end	Day	50% 4 11% 21% 11% 2	2490
May	1st to 10th	Day	51% 6% 12% 20% 10%	5897
	11th to 20th	Day	47% 6% 10% 24% 12% 1	5865
	21st to end	Day	43% 5% 13% 21% 16% 3	6949
June	1st to 10th	Day	37% 7% 12% 25% 16% 3	2800
	11th to 20th	Day	35% 6% 10% 24% 21% 4	2800
	21st to end	Day	38% 6% 15% 20% 17% 4	2800
July	1st to 10th	Day	38% 4 11% 31% 15% 2	6291
	11th to 20th	Day	42% 4 16% 25% 13% 1	6285
	21st to end	Day	42% 4 16% 24% 12% 2	6728
August	1st to 10th	Day	30% 5% 16% 33% 15% 1	4260
	11th to 20th	Day	36% 5% 13% 30% 16% 1	3885
	21st to end	Day	30% 4 15% 32% 16% 4	4311
September	1st to 10th	Day	36% 6% 11% 25% 19% 4	2400
	11th to 20th	Day	32% 5% 10% 25% 24% 4	2240
	21st to end	Day	37% 4 10% 22% 24% 2	2200
October	1st to 10th	Day	25% 3 7% 28% 34% 2	3590
	11th to 20th	Day	34% 2 9% 24% 30% 1	3228
	21st to end	Day	44% 2 8% 23% 21% 1	3180
November	1st to 10th	Day	37% 4 13% 25% 17% 4	1852
	11th to 20th	Day	48% 4 9% 22% 13% 4	1890
	21st to end	Day	53% 3 9% 21% 12% 1	1890
December	1st to 10th	Day	47% 3 11% 29% 11%	2799
	11th to 20th	Day	63% 3 10% 19% 5%	3085
	21st to end	Day	76% 3 8% 9% 4	4070
January	1st to 10th	Night	62% 3 9% 17% 8%	6488
	11th to 20th	Night	69% 3 6% 10% 10% 2	6445
	21st to end	Night	61% 2 9% 18% 10% 1	6789
February	1st to 10th	Night	57% 4 12% 17% 9% 2	2460
	11th to 20th	Night	53% 3 10% 21% 12% 2	2339
	21st to end	Night	52% 4 10% 20% 12% 2	1980
March	1st to 10th	Night	51% 3 7% 23% 12% 4	5373
	11th to 20th	Night	39% 4 11% 27% 17% 2	5312
	21st to end	Night	53% 4 9% 24% 8% 1	4990
April	1st to 10th	Night	44% 6% 10% 23% 16% 2	2000
	11th to 20th	Night	44% 5% 8% 23% 16% 4	1975
	21st to end	Night	47% 6% 10% 21% 12% 4	1907
May	1st to 10th	Night	51% 6% 10% 19% 12% 2	4060
	11th to 20th	Night	42% 6% 12% 25% 14% 1	4017
	21st to end	Night	45% 5% 13% 20% 14% 3	3950
June	1st to 10th	Night	34% 5% 10% 27% 21% 3	1600
	11th to 20th	Night	34% 6% 10% 24% 21% 4	1600
	21st to end	Night	34% 6% 16% 23% 16% 5%	1600
July	1st to 10th	Night	38% 4 10% 28% 16% 4	3580
	11th to 20th	Night	34% 4 16% 28% 18% 1	3572
	21st to end	Night	41% 4 15% 25% 13% 3	3832
August	1st to 10th	Night	29% 4 16% 34% 16% 1	2861
	11th to 20th	Night	33% 5% 15% 28% 19% 1	3230
	21st to end	Night	28% 4 12% 32% 20% 4	3586
September	1st to 10th	Night	33% 5% 10% 26% 20% 7%	2000
	11th to 20th	Night	28% 3 10% 25% 29% 5%	2138
	21st to end	Night	37% 4 9% 23% 24% 4	2200
October	1st to 10th	Night	21% 3 6% 29% 39% 2	3590
	11th to 20th	Night	29% 2 8% 26% 33% 2	3907
	21st to end	Night	38% 2 7% 23% 25% 4	4610
November	1st to 10th	Night	36% 4 9% 27% 21% 4	2666
	11th to 20th	Night	44% 4 11% 22% 17% 3	2730
	21st to end	Night	53% 3 10% 22% 12% 1	2730
December	1st to 10th	Night	45% 2 12% 28% 13% 1	4605
	11th to 20th	Night	56% 2 12% 18% 11% 1	5341
	21st to end	Night	73% 3 9% 10% 5%	7150

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	25% 15% 33% 22% 5%	330
	11th to 20th	Day	25% 20% 35% 16% 3 1	318
	21st to end	Day	26% 17% 35% 18% 3	330
February	1st to 10th	Day	27% 17% 35% 17% 3	300
	11th to 20th	Day	27% 21% 33% 13% 4 2	300
	21st to end	Day	30% 16% 39% 13% 2	249
March	1st to 10th	Day	21% 20% 39% 15% 4	300
	11th to 20th	Day	28% 17% 35% 17% 3	350
	21st to end	Day	32% 14% 30% 17% 6% 1	550
April	1st to 10th	Day	21% 17% 33% 21% 7% 1	500
	11th to 20th	Day	21% 18% 27% 19% 12% 2	492
	21st to end	Day	25% 13% 32% 19% 9% 2	500
May	1st to 10th	Day	21% 10% 31% 24% 11% 3	503
	11th to 20th	Day	23% 7% 25% 26% 13% 6%	527
	21st to end	Day	12% 7% 31% 24% 17% 8%	789
June	1st to 10th	Day	13% 7% 26% 30% 20% 5%	730
	11th to 20th	Day	11% 5% 26% 31% 21% 6%	730
	21st to end	Day	12% 6% 27% 29% 20% 6%	730
July	1st to 10th	Day	12% 2 15% 29% 27% 14%	721
	11th to 20th	Day	10% 4 18% 31% 26% 11%	718
	21st to end	Day	11% 2 16% 32% 30% 9%	794
August	1st to 10th	Day	8% 2 16% 31% 29% 14%	574
	11th to 20th	Day	10% 3 16% 30% 31% 10%	519
	21st to end	Day	8% 3 15% 27% 33% 13%	583
September	1st to 10th	Day	12% 4 16% 30% 29% 10%	530
	11th to 20th	Day	8% 3 12% 30% 29% 17%	530
	21st to end	Day	14% 9% 22% 21% 22% 12%	530
October	1st to 10th	Day	11% 5% 24% 33% 19% 7%	530
	11th to 20th	Day	12% 17% 30% 28% 9% 2	338
	21st to end	Day	28% 14% 34% 19% 5%	363
November	1st to 10th	Day	13% 15% 35% 23% 11% 4	330
	11th to 20th	Day	26% 18% 31% 16% 9% 1	330
	21st to end	Day	27% 11% 36% 22% 5%	330
December	1st to 10th	Day	18% 17% 40% 17% 6% 1	330
	11th to 20th	Day	21% 14% 41% 17% 5% 1	330
	21st to end	Day	31% 15% 32% 21% 1	363
January	1st to 10th	Night	22% 14% 37% 24% 4	1050
	11th to 20th	Night	23% 16% 35% 21% 4	1031
	21st to end	Night	23% 13% 35% 25% 3	1100
February	1st to 10th	Night	27% 12% 36% 23% 3	860
	11th to 20th	Night	21% 16% 35% 25% 3	783
	21st to end	Night	25% 13% 37% 22% 3	660
March	1st to 10th	Night	23% 14% 34% 25% 4	800
	11th to 20th	Night	19% 13% 37% 25% 6% 1	800
	21st to end	Night	25% 8% 36% 25% 5% 1	675
April	1st to 10th	Night	18% 14% 35% 27% 6%	600
	11th to 20th	Night	22% 12% 31% 26% 9%	593
	21st to end	Night	20% 10% 35% 28% 7% 1	600
May	1st to 10th	Night	20% 6% 33% 33% 9%	600
	11th to 20th	Night	19% 6% 27% 30% 16% 2	591
	21st to end	Night	13% 5% 26% 35% 19% 2	650
June	1st to 10th	Night	12% 3 24% 43% 18%	600
	11th to 20th	Night	12% 2 22% 45% 18% 1	600
	21st to end	Night	8% 3 22% 40% 25% 1	600
July	1st to 10th	Night	12% 2 14% 36% 31% 6%	585
	11th to 20th	Night	5% 3 17% 41% 30% 4	582
	21st to end	Night	8% 2 11% 43% 34% 2	657
August	1st to 10th	Night	8% 2 9% 43% 33% 5%	597
	11th to 20th	Night	7% 3 12% 42% 32% 4	594
	21st to end	Night	7% 1 14% 33% 41% 3	660
September	1st to 10th	Night	14% 3 15% 35% 31% 3	600
	11th to 20th	Night	8% 3 13% 39% 32% 4	738
	21st to end	Night	13% 6% 20% 30% 29% 2	800
October	1st to 10th	Night	13% 5% 22% 38% 20% 3	800
	11th to 20th	Night	19% 9% 29% 28% 14%	800
	21st to end	Night	21% 14% 35% 24% 6%	880
November	1st to 10th	Night	16% 10% 38% 25% 10% 1	820
	11th to 20th	Night	20% 15% 32% 26% 7% 1	840
	21st to end	Night	27% 12% 30% 25% 7%	840
December	1st to 10th	Night	24% 11% 37% 24% 5%	974
	11th to 20th	Night	25% 10% 34% 26% 6%	1050
	21st to end	Night	32% 13% 30% 21% 3	1155

Page generated at 2026-01-14 00:14:28. Version ac07

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