Danville Conservation Area
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Danville Conservation Area clear sky chart from 2005-11-06 to 2026-10-14.

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	12% 8% 3 7% 4 4 4 4 5% 8% 42%	149410
	Night	13% 8% 3 6% 4 4 4 3 5% 6% 44%	142659
Transparency	Day	39% 5% 12% 24% 17% 2	153007
	Night	40% 4 12% 24% 17% 3	147141
Seeing	Day	17% 13% 29% 25% 12% 3	18343
	Night	17% 13% 31% 27% 11% 1	28057

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	12% 13% 5% 12% 5% 4 4 3 4 6% 33%	4691
	11th to 20th	Day	7% 10% 3 8% 6% 6% 5% 3 4 6% 41%	5310
	21st to end	Day	16% 13% 5% 8% 4 3 3 3 4 4 37%	5812
February	1st to 10th	Day	16% 12% 4 8% 5% 4 4 3 4 6% 36%	1890
	11th to 20th	Day	15% 10% 3 7% 5% 4 3 4 5% 5% 39%	1951
	21st to end	Day	14% 11% 2 8% 4 4 4 4 4 5% 40%	1730
March	1st to 10th	Day	16% 11% 2 4 3 2 3 2 3 4 49%	4851
	11th to 20th	Day	9% 7% 2 5% 4 3 3 3 5% 7% 52%	5090
	21st to end	Day	14% 9% 3 8% 5% 4 5% 4 5% 8% 34%	5762
April	1st to 10th	Day	14% 8% 2 6% 5% 4 5% 3 5% 6% 42%	2520
	11th to 20th	Day	17% 8% 2 7% 4 4 4 3 4 6% 42%	2494
	21st to end	Day	20% 7% 3 6% 5% 4 3 4 6% 7% 35%	2520
May	1st to 10th	Day	15% 8% 3 7% 5% 5% 5% 4 6% 9% 34%	7251
	11th to 20th	Day	13% 8% 3 7% 4 4 4 3 6% 7% 40%	8163
	21st to end	Day	17% 8% 3 7% 5% 5% 5% 4 6% 6% 35%	9047
June	1st to 10th	Day	9% 5% 2 5% 4 3 5% 4 7% 10% 46%	2940
	11th to 20th	Day	9% 4 2 5% 4 4 4 4 7% 11% 47%	2940
	21st to end	Day	7% 4 2 5% 4 5% 5% 5% 6% 10% 47%	2940
July	1st to 10th	Day	10% 5% 2 6% 4 4 4 4 6% 10% 45%	8280
	11th to 20th	Day	7% 5% 2 6% 4 4 5% 4 6% 9% 46%	8206
	21st to end	Day	9% 5% 2 6% 4 5% 6% 5% 8% 12% 40%	8337
August	1st to 10th	Day	9% 4 2 5% 4 4 5% 5% 6% 10% 46%	6014
	11th to 20th	Day	11% 4 2 5% 4 4 5% 4 7% 10% 43%	5630
	21st to end	Day	4 3 2 5% 4 3 5% 3 6% 9% 56%	6080
September	1st to 10th	Day	11% 5% 1 5% 3 4 3 3 5% 7% 52%	2520
	11th to 20th	Day	10% 5% 2 5% 4 3 4 3 6% 8% 50%	2520
	21st to end	Day	9% 6% 2 6% 4 4 4 3 5% 7% 51%	2234
October	1st to 10th	Day	9% 5% 2 5% 3 3 3 3 4 5% 57%	2100
	11th to 20th	Day	16% 6% 2 5% 3 3 3 2 4 5% 54%	2030
	21st to end	Day	19% 8% 2 5% 4 3 3 2 4 5% 46%	2200
November	1st to 10th	Day	12% 5% 2 4 3 3 3 3 4 6% 54%	1902
	11th to 20th	Day	14% 9% 3 5% 4 3 3 3 6% 6% 46%	1890
	21st to end	Day	17% 10% 3 7% 4 3 3 2 4 5% 42%	1717
December	1st to 10th	Day	9% 10% 3 6% 3 3 3 2 4 6% 50%	2690
	11th to 20th	Day	13% 9% 5% 9% 5% 4 3 2 4 6% 39%	3088
	21st to end	Day	21% 18% 6% 11% 4 4 4 3 4 3 22%	4070
January	1st to 10th	Night	13% 11% 7% 11% 5% 4 3 3 3 4 36%	8214
	11th to 20th	Night	10% 9% 4 11% 5% 5% 5% 4 4 4 40%	7680
	21st to end	Night	15% 13% 5% 7% 5% 4 4 3 4 5% 35%	8402
February	1st to 10th	Night	14% 12% 4 8% 5% 5% 4 3 5% 6% 35%	2730
	11th to 20th	Night	16% 8% 3 8% 5% 4 3 3 5% 6% 39%	2554
	21st to end	Night	15% 10% 3 7% 3 3 3 3 4 5% 44%	2076
March	1st to 10th	Night	19% 12% 3 5% 3 3 2 2 3 4 44%	5265
	11th to 20th	Night	12% 9% 2 4 3 3 4 3 4 6% 49%	5090
	21st to end	Night	16% 11% 4 9% 5% 4 4 3 4 6% 34%	5407
April	1st to 10th	Night	11% 7% 3 8% 3 4 4 3 4 6% 46%	2100
	11th to 20th	Night	14% 7% 3 6% 3 4 3 3 5% 6% 46%	2075
	21st to end	Night	19% 7% 3 5% 4 3 4 3 4 7% 42%	2100
May	1st to 10th	Night	17% 8% 3 7% 4 4 4 4 5% 7% 37%	5610
	11th to 20th	Night	12% 6% 2 5% 4 4 4 4 6% 9% 44%	4715
	21st to end	Night	17% 7% 3 7% 5% 5% 4 4 6% 7% 35%	5139
June	1st to 10th	Night	9% 4 2 5% 4 4 4 4 6% 8% 51%	1680
	11th to 20th	Night	6% 5% 1 6% 4 4 5% 3 6% 11% 50%	1680
	21st to end	Night	5% 4 1 3 4 5% 5% 4 5% 10% 53%	1680
July	1st to 10th	Night	9% 5% 2 7% 4 4 4 4 6% 9% 47%	4708
	11th to 20th	Night	6% 4 2 5% 4 4 4 4 6% 10% 51%	4763
	21st to end	Night	6% 4 2 5% 4 5% 5% 5% 7% 10% 46%	5770
August	1st to 10th	Night	10% 3 1 5% 3 3 4 3 5% 7% 57%	4156
	11th to 20th	Night	11% 5% 2 5% 3 4 4 3 5% 9% 49%	4408
	21st to end	Night	4 3 1 3 3 3 3 3 6% 9% 62%	5046
September	1st to 10th	Night	11% 4 2 4 3 3 3 3 4 7% 56%	2100
	11th to 20th	Night	8% 5% 2 4 4 3 5% 4 6% 8% 51%	2100
	21st to end	Night	8% 5% 2 6% 4 3 3 3 4 6% 54%	2358
October	1st to 10th	Night	8% 3 2 4 3 3 3 3 4 5% 61%	2520
	11th to 20th	Night	14% 5% 2 4 3 3 4 3 4 7% 53%	2437
	21st to end	Night	18% 8% 2 5% 3 4 3 3 4 5% 47%	2640
November	1st to 10th	Night	11% 6% 2 5% 3 3 4 3 4 5% 54%	2616
	11th to 20th	Night	12% 7% 3 5% 4 3 3 3 4 7% 50%	2730
	21st to end	Night	17% 10% 2 5% 4 3 3 3 4 5% 43%	2901
December	1st to 10th	Night	11% 10% 3 6% 4 3 4 3 4 6% 47%	4720
	11th to 20th	Night	15% 11% 4 7% 3 3 3 3 5% 6% 40%	5339
	21st to end	Night	23% 19% 5% 8% 5% 4 3 3 3 4 22%	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	53% 4 10% 21% 9% 2	4692
	11th to 20th	Day	43% 4 11% 23% 16% 4	5310
	21st to end	Day	51% 3 10% 16% 16% 3	5812
February	1st to 10th	Day	51% 4 11% 22% 10% 1	1890
	11th to 20th	Day	48% 4 10% 19% 14% 4	1960
	21st to end	Day	46% 4 8% 23% 15% 4	1730
March	1st to 10th	Day	41% 3 6% 23% 24% 4	4851
	11th to 20th	Day	32% 4 10% 24% 24% 6%	5090
	21st to end	Day	47% 6% 14% 26% 7%	5764
April	1st to 10th	Day	43% 5% 10% 24% 17% 1	2520
	11th to 20th	Day	44% 5% 9% 23% 15% 3	2494
	21st to end	Day	48% 5% 12% 21% 13% 1	2520
May	1st to 10th	Day	45% 7% 13% 21% 13%	7286
	11th to 20th	Day	44% 6% 10% 23% 16% 2	8167
	21st to end	Day	48% 6% 10% 20% 16% 1	9061
June	1st to 10th	Day	32% 8% 15% 28% 16% 2	2940
	11th to 20th	Day	30% 7% 13% 27% 19% 4	2940
	21st to end	Day	31% 8% 17% 25% 15% 4	2940
July	1st to 10th	Day	35% 5% 15% 26% 18% 1	8211
	11th to 20th	Day	32% 7% 17% 25% 17% 2	8131
	21st to end	Day	35% 5% 23% 29% 7% 1	8155
August	1st to 10th	Day	31% 6% 16% 32% 15%	6000
	11th to 20th	Day	35% 5% 18% 29% 12% 1	5668
	21st to end	Day	25% 5% 15% 35% 17% 2	6079
September	1st to 10th	Day	32% 6% 10% 26% 22% 4	2520
	11th to 20th	Day	31% 5% 10% 29% 21% 3	2520
	21st to end	Day	33% 6% 9% 26% 24% 3	2234
October	1st to 10th	Day	19% 3 6% 20% 46% 7%	3360
	11th to 20th	Day	24% 3 6% 21% 38% 8%	3270
	21st to end	Day	35% 2 9% 26% 27% 2	3538
November	1st to 10th	Day	32% 5% 12% 26% 21% 5%	1902
	11th to 20th	Day	39% 3 9% 25% 20% 4	1890
	21st to end	Day	46% 4 9% 26% 15%	1717
December	1st to 10th	Day	38% 3 14% 30% 15% 1	2690
	11th to 20th	Day	46% 4 12% 24% 13% 1	3085
	21st to end	Day	67% 4 10% 12% 6% 1	4070
January	1st to 10th	Night	53% 3 12% 23% 9%	8215
	11th to 20th	Night	48% 4 6% 24% 15% 3	7680
	21st to end	Night	52% 3 11% 18% 14% 2	8402
February	1st to 10th	Night	51% 5% 11% 20% 12% 2	2730
	11th to 20th	Night	46% 4 10% 21% 16% 3	2555
	21st to end	Night	44% 4 9% 24% 16% 3	2076
March	1st to 10th	Night	46% 3 7% 21% 21% 2	5265
	11th to 20th	Night	36% 4 8% 22% 23% 6%	5090
	21st to end	Night	51% 5% 10% 24% 9% 1	5407
April	1st to 10th	Night	38% 6% 11% 25% 17% 3	2100
	11th to 20th	Night	39% 5% 11% 23% 17% 5%	2075
	21st to end	Night	43% 4 11% 23% 17% 1	2100
May	1st to 10th	Night	45% 7% 14% 22% 12% 1	5640
	11th to 20th	Night	37% 6% 12% 24% 17% 4	4716
	21st to end	Night	46% 7% 12% 18% 15% 1	5149
June	1st to 10th	Night	30% 6% 15% 28% 17% 3	1680
	11th to 20th	Night	29% 6% 11% 26% 24% 3	1680
	21st to end	Night	25% 7% 16% 28% 18% 6%	1680
July	1st to 10th	Night	34% 5% 14% 28% 18% 1	4670
	11th to 20th	Night	27% 7% 17% 30% 18% 2	4718
	21st to end	Night	31% 5% 23% 31% 9%	5641
August	1st to 10th	Night	26% 5% 16% 33% 19% 1	4146
	11th to 20th	Night	33% 4 15% 31% 15% 2	4436
	21st to end	Night	20% 5% 17% 38% 18% 2	5046
September	1st to 10th	Night	29% 5% 10% 25% 25% 5%	2100
	11th to 20th	Night	30% 7% 11% 26% 24% 4	2100
	21st to end	Night	32% 5% 9% 24% 26% 4	2358
October	1st to 10th	Night	16% 3 6% 20% 47% 8%	4040
	11th to 20th	Night	23% 3 7% 23% 34% 11%	3936
	21st to end	Night	35% 3 8% 25% 26% 3	4252
November	1st to 10th	Night	32% 5% 12% 26% 20% 5%	2616
	11th to 20th	Night	36% 3 9% 26% 22% 4	2730
	21st to end	Night	44% 4 10% 25% 15% 2	2901
December	1st to 10th	Night	40% 4 10% 32% 14% 1	4720
	11th to 20th	Night	45% 3 12% 22% 17%	5341
	21st to end	Night	67% 4 11% 12% 5% 1	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	27% 21% 36% 14% 2	330
	11th to 20th	Day	28% 27% 35% 9% 1	330
	21st to end	Day	21% 23% 42% 12% 2	363
February	1st to 10th	Day	29% 22% 35% 12% 2	330
	11th to 20th	Day	27% 29% 31% 10% 2	330
	21st to end	Day	29% 26% 33% 10% 1	273
March	1st to 10th	Day	25% 25% 39% 10% 1	330
	11th to 20th	Day	30% 23% 34% 11% 2	330
	21st to end	Day	31% 24% 33% 10% 2	439
April	1st to 10th	Day	24% 17% 38% 16% 5%	540
	11th to 20th	Day	27% 21% 29% 18% 4	532
	21st to end	Day	26% 16% 34% 18% 5%	540
May	1st to 10th	Day	17% 19% 38% 19% 6% 1	618
	11th to 20th	Day	18% 16% 29% 26% 10% 1	775
	21st to end	Day	16% 12% 31% 27% 9% 4	844
June	1st to 10th	Day	10% 7% 29% 32% 17% 4	780
	11th to 20th	Day	10% 5% 29% 40% 13% 3	780
	21st to end	Day	8% 9% 32% 35% 12% 3	780
July	1st to 10th	Day	12% 3 19% 32% 27% 8%	771
	11th to 20th	Day	7% 6% 24% 37% 23% 4	768
	21st to end	Day	10% 4 21% 34% 27% 4	849
August	1st to 10th	Day	8% 4 22% 40% 22% 5%	776
	11th to 20th	Day	9% 5% 23% 36% 22% 6%	628
	21st to end	Day	7% 4 20% 34% 27% 7%	627
September	1st to 10th	Day	12% 4 17% 36% 25% 6%	570
	11th to 20th	Day	9% 2 20% 33% 25% 10%	570
	21st to end	Day	11% 12% 24% 26% 20% 6%	465
October	1st to 10th	Day	10% 11% 30% 32% 14% 3	360
	11th to 20th	Day	19% 24% 28% 22% 7%	339
	21st to end	Day	30% 24% 29% 15% 2	363
November	1st to 10th	Day	15% 18% 41% 17% 8% 2	330
	11th to 20th	Day	22% 21% 30% 21% 4 2	330
	21st to end	Day	31% 16% 29% 20% 3	330
December	1st to 10th	Day	18% 21% 40% 13% 6% 1	330
	11th to 20th	Day	23% 20% 37% 16% 4	330
	21st to end	Day	26% 25% 36% 11% 1	363
January	1st to 10th	Night	20% 20% 38% 18% 5%	1050
	11th to 20th	Night	23% 21% 36% 17% 3	1050
	21st to end	Night	19% 22% 38% 20% 1	1155
February	1st to 10th	Night	22% 20% 36% 20% 3	1050
	11th to 20th	Night	24% 20% 35% 19% 2	920
	21st to end	Night	25% 20% 36% 18% 1	692
March	1st to 10th	Night	26% 16% 38% 18% 2	742
	11th to 20th	Night	24% 17% 34% 21% 4	630
	21st to end	Night	26% 18% 37% 17% 3	693
April	1st to 10th	Night	18% 19% 37% 22% 3	630
	11th to 20th	Night	21% 16% 35% 24% 5%	623
	21st to end	Night	21% 12% 39% 23% 5%	630
May	1st to 10th	Night	15% 12% 34% 31% 7%	630
	11th to 20th	Night	19% 9% 29% 32% 11% 1	621
	21st to end	Night	12% 8% 30% 32% 16% 2	683
June	1st to 10th	Night	9% 4 25% 38% 22% 1	630
	11th to 20th	Night	8% 4 27% 40% 19% 1	630
	21st to end	Night	5% 4 30% 43% 16% 1	630
July	1st to 10th	Night	9% 2 14% 38% 32% 4	615
	11th to 20th	Night	5% 2 17% 42% 31% 2	612
	21st to end	Night	8% 2 18% 40% 30% 1	690
August	1st to 10th	Night	7% 3 15% 44% 29% 1	627
	11th to 20th	Night	8% 2 14% 41% 32% 3	624
	21st to end	Night	6% 2 16% 41% 33% 3	693
September	1st to 10th	Night	13% 4 18% 36% 24% 5%	630
	11th to 20th	Night	11% 3 19% 34% 29% 5%	630
	21st to end	Night	11% 7% 23% 33% 21% 4	788
October	1st to 10th	Night	10% 7% 28% 32% 21% 3	840
	11th to 20th	Night	17% 12% 33% 28% 10%	813
	21st to end	Night	23% 14% 34% 23% 6%	880
November	1st to 10th	Night	14% 13% 37% 27% 8%	820
	11th to 20th	Night	17% 18% 36% 23% 7%	840
	21st to end	Night	24% 16% 33% 22% 5%	1011
December	1st to 10th	Night	21% 18% 35% 20% 4	1050
	11th to 20th	Night	24% 16% 32% 24% 4	1050
	21st to end	Night	25% 21% 34% 18% 2	1155

Page generated at 2026-10-11 04:06:29. Version ac07

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