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 2025-05-26.

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 6% 4 4 4 3 5% 7% 43%	114145
	Night	13% 9% 3 7% 4 4 4 3 5% 6% 43%	115311
Transparency	Day	39% 5% 12% 24% 17% 3	117783
	Night	41% 5% 11% 24% 18% 3	119815
Seeing	Day	17% 14% 29% 25% 13% 3	16342
	Night	17% 13% 31% 27% 11% 1	26270

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	13% 13% 5% 13% 5% 4 4 3 4 6% 30%	3617
	11th to 20th	Day	9% 11% 4 8% 5% 7% 4 3 4 6% 39%	4080
	21st to end	Day	17% 14% 6% 8% 4 3 3 3 4 4 36%	4488
February	1st to 10th	Day	16% 12% 4 8% 5% 4 4 3 4 6% 35%	1800
	11th to 20th	Day	15% 11% 3 7% 5% 4 3 3 5% 5% 39%	1856
	21st to end	Day	14% 12% 2 8% 4 4 4 4 4 5% 40%	1650
March	1st to 10th	Day	16% 11% 2 4 3 2 3 2 3 4 49%	4746
	11th to 20th	Day	9% 7% 2 5% 4 3 3 3 5% 7% 52%	4980
	21st to end	Day	14% 10% 3 8% 5% 4 5% 4 5% 8% 34%	5637
April	1st to 10th	Day	14% 8% 2 6% 5% 4 5% 3 5% 6% 43%	2400
	11th to 20th	Day	18% 8% 2 6% 4 4 3 3 4 6% 42%	2374
	21st to end	Day	21% 7% 3 6% 5% 4 3 4 6% 7% 35%	2400
May	1st to 10th	Day	12% 8% 3 7% 5% 5% 6% 4 7% 9% 33%	4008
	11th to 20th	Day	16% 9% 3 7% 4 4 4 3 5% 7% 38%	4506
	21st to end	Day	11% 6% 2 6% 5% 4 5% 3 6% 6% 45%	4895
June	1st to 10th	Day	8% 5% 2 5% 4 3 5% 4 7% 11% 47%	2660
	11th to 20th	Day	8% 4 2 5% 4 4 4 4 6% 11% 49%	2660
	21st to end	Day	6% 4 2 4 4 5% 5% 5% 7% 11% 47%	2660
July	1st to 10th	Day	14% 6% 2 6% 4 4 4 5% 6% 10% 39%	4440
	11th to 20th	Day	5% 4 2 5% 4 4 5% 4 7% 10% 50%	4394
	21st to end	Day	8% 5% 2 6% 5% 5% 6% 5% 8% 11% 38%	4465
August	1st to 10th	Day	6% 4 2 5% 4 4 5% 5% 7% 10% 49%	4104
	11th to 20th	Day	13% 5% 2 5% 4 4 5% 4 6% 9% 44%	3828
	21st to end	Day	6% 4 2 5% 4 4 5% 4 6% 10% 51%	4180
September	1st to 10th	Day	11% 5% 1 5% 3 4 4 3 5% 7% 51%	2280
	11th to 20th	Day	10% 5% 2 5% 4 3 3 3 6% 8% 51%	2280
	21st to end	Day	8% 5% 2 6% 3 4 4 4 5% 7% 53%	2022
October	1st to 10th	Day	9% 5% 2 5% 3 3 3 3 4 5% 56%	1900
	11th to 20th	Day	16% 6% 2 5% 3 3 3 2 4 4 53%	1900
	21st to end	Day	18% 8% 2 5% 4 3 3 2 4 5% 46%	2090
November	1st to 10th	Day	13% 6% 2 4 3 3 3 3 4 6% 54%	1810
	11th to 20th	Day	14% 9% 3 5% 4 3 3 3 6% 6% 46%	1800
	21st to end	Day	17% 10% 3 6% 4 3 3 3 4 5% 42%	1635
December	1st to 10th	Day	9% 10% 3 6% 3 3 3 2 4 6% 50%	2610
	11th to 20th	Day	13% 9% 5% 9% 5% 4 4 2 4 7% 39%	3008
	21st to end	Day	21% 18% 6% 11% 4 4 4 3 4 3 22%	3982
January	1st to 10th	Night	15% 11% 8% 12% 5% 4 3 3 4 4 31%	6330
	11th to 20th	Night	12% 11% 5% 12% 5% 5% 6% 4 4 3 34%	5900
	21st to end	Night	16% 14% 5% 8% 4 3 4 3 4 5% 35%	6490
February	1st to 10th	Night	15% 12% 5% 8% 5% 5% 4 3 5% 6% 34%	2600
	11th to 20th	Night	16% 9% 4 8% 5% 4 3 3 5% 5% 39%	2429
	21st to end	Night	15% 10% 3 7% 3 4 3 3 4 5% 43%	1980
March	1st to 10th	Night	19% 12% 3 5% 3 3 2 2 3 4 44%	5150
	11th to 20th	Night	12% 9% 2 4 3 3 4 3 4 6% 49%	4980
	21st to end	Night	16% 11% 4 9% 5% 4 4 3 4 5% 34%	5290
April	1st to 10th	Night	11% 7% 2 8% 3 4 4 3 4 6% 47%	2000
	11th to 20th	Night	14% 7% 3 6% 3 4 3 3 5% 6% 47%	1975
	21st to end	Night	19% 6% 2 5% 4 4 4 4 4 7% 41%	2000
May	1st to 10th	Night	16% 8% 3 6% 4 4 3 3 6% 9% 38%	3107
	11th to 20th	Night	14% 8% 2 7% 4 4 4 3 5% 7% 40%	2605
	21st to end	Night	10% 5% 2 5% 4 5% 4 4 6% 9% 46%	2788
June	1st to 10th	Night	8% 4 2 5% 4 4 4 4 7% 8% 51%	1520
	11th to 20th	Night	6% 5% 1 6% 4 4 5% 3 5% 10% 51%	1520
	21st to end	Night	4 4 2 3 4 5% 5% 5% 5% 11% 54%	1520
July	1st to 10th	Night	12% 6% 2 7% 4 4 4 4 6% 9% 44%	2530
	11th to 20th	Night	4 3 1 3 3 3 3 4 6% 10% 59%	2558
	21st to end	Night	7% 5% 2 6% 5% 5% 5% 4 7% 9% 45%	3098
August	1st to 10th	Night	5% 3 1 5% 3 3 4 3 5% 8% 61%	2836
	11th to 20th	Night	11% 6% 2 5% 3 4 3 3 5% 7% 50%	3003
	21st to end	Night	5% 3 1 3 3 3 3 3 6% 9% 59%	3476
September	1st to 10th	Night	12% 4 1 3 3 3 3 3 4 7% 56%	1900
	11th to 20th	Night	9% 5% 2 4 3 3 4 4 6% 7% 52%	1900
	21st to end	Night	7% 5% 2 6% 4 3 3 3 5% 6% 56%	2132
October	1st to 10th	Night	9% 3 2 4 3 3 3 3 4 6% 60%	2280
	11th to 20th	Night	14% 5% 2 4 3 3 3 3 4 6% 53%	2280
	21st to end	Night	18% 7% 2 5% 3 4 3 3 4 5% 47%	2508
November	1st to 10th	Night	11% 6% 2 4 3 3 4 3 4 5% 55%	2488
	11th to 20th	Night	12% 7% 3 5% 4 3 3 3 4 6% 49%	2600
	21st to end	Night	17% 9% 2 5% 4 3 3 3 4 5% 44%	2763
December	1st to 10th	Night	10% 9% 3 6% 4 3 4 3 4 6% 47%	4580
	11th to 20th	Night	15% 11% 4 7% 3 3 3 3 5% 6% 40%	5199
	21st to end	Night	24% 19% 5% 8% 5% 4 3 3 3 4 22%	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	56% 4 10% 18% 10% 2	3618
	11th to 20th	Day	46% 4 11% 21% 14% 4	4080
	21st to end	Day	52% 4 9% 14% 18% 3	4488
February	1st to 10th	Day	53% 4 11% 21% 10% 1	1800
	11th to 20th	Day	48% 5% 11% 19% 15% 4	1865
	21st to end	Day	46% 4 8% 22% 15% 4	1650
March	1st to 10th	Day	40% 3 6% 23% 24% 4	4746
	11th to 20th	Day	32% 4 9% 24% 25% 6%	4980
	21st to end	Day	47% 6% 14% 25% 7%	5639
April	1st to 10th	Day	43% 5% 10% 24% 17% 1	2400
	11th to 20th	Day	44% 5% 9% 23% 15% 3	2374
	21st to end	Day	48% 5% 12% 20% 13% 1	2400
May	1st to 10th	Day	44% 9% 14% 21% 11% 1	4043
	11th to 20th	Day	46% 7% 10% 24% 12% 1	4510
	21st to end	Day	36% 8% 10% 23% 21% 1	4908
June	1st to 10th	Day	31% 8% 15% 28% 15% 2	2660
	11th to 20th	Day	29% 8% 13% 27% 19% 4	2660
	21st to end	Day	29% 8% 17% 26% 15% 4	2660
July	1st to 10th	Day	40% 6% 13% 26% 13% 2	4371
	11th to 20th	Day	27% 7% 15% 32% 18% 2	4319
	21st to end	Day	36% 7% 20% 26% 10% 1	4325
August	1st to 10th	Day	27% 7% 16% 37% 13% 1	4090
	11th to 20th	Day	38% 6% 13% 25% 17% 2	3866
	21st to end	Day	29% 5% 16% 32% 14% 3	4179
September	1st to 10th	Day	33% 6% 10% 24% 22% 5%	2280
	11th to 20th	Day	32% 5% 9% 28% 23% 4	2280
	21st to end	Day	31% 6% 9% 27% 24% 3	2022
October	1st to 10th	Day	19% 3 6% 19% 46% 7%	3160
	11th to 20th	Day	23% 3 6% 21% 39% 8%	3140
	21st to end	Day	34% 3 9% 26% 27% 2	3428
November	1st to 10th	Day	32% 5% 12% 25% 21% 5%	1810
	11th to 20th	Day	39% 3 9% 24% 21% 4	1800
	21st to end	Day	45% 4 9% 27% 15%	1635
December	1st to 10th	Day	38% 3 14% 30% 15% 1	2610
	11th to 20th	Day	46% 4 13% 23% 13% 1	3005
	21st to end	Day	67% 5% 10% 12% 6% 1	3982
January	1st to 10th	Night	58% 3 9% 21% 9% 1	6330
	11th to 20th	Night	55% 4 7% 20% 11% 3	5900
	21st to end	Night	52% 4 9% 18% 15% 2	6490
February	1st to 10th	Night	52% 5% 10% 19% 12% 2	2600
	11th to 20th	Night	47% 4 11% 20% 16% 3	2430
	21st to end	Night	45% 4 9% 23% 16% 3	1980
March	1st to 10th	Night	46% 3 7% 21% 21% 2	5150
	11th to 20th	Night	36% 4 8% 22% 23% 6%	4980
	21st to end	Night	51% 5% 10% 23% 9% 1	5290
April	1st to 10th	Night	37% 6% 10% 26% 18% 3	2000
	11th to 20th	Night	39% 5% 10% 24% 17% 5%	1975
	21st to end	Night	44% 4 11% 23% 17% 1	2000
May	1st to 10th	Night	41% 9% 17% 19% 14% 1	3137
	11th to 20th	Night	41% 8% 12% 26% 12% 1	2606
	21st to end	Night	33% 9% 15% 22% 20% 2	2796
June	1st to 10th	Night	29% 7% 16% 28% 18% 3	1520
	11th to 20th	Night	28% 6% 11% 27% 24% 3	1520
	21st to end	Night	24% 8% 16% 28% 19% 6%	1520
July	1st to 10th	Night	38% 5% 11% 30% 14% 2	2490
	11th to 20th	Night	19% 6% 13% 39% 20% 2	2510
	21st to end	Night	34% 6% 21% 26% 13% 1	2999
August	1st to 10th	Night	21% 6% 17% 35% 19% 2	2826
	11th to 20th	Night	34% 5% 13% 27% 19% 3	3031
	21st to end	Night	22% 5% 19% 35% 16% 3	3476
September	1st to 10th	Night	29% 5% 11% 25% 24% 6%	1900
	11th to 20th	Night	30% 7% 10% 24% 26% 4	1900
	21st to end	Night	30% 5% 9% 25% 27% 4	2132
October	1st to 10th	Night	16% 3 7% 20% 47% 7%	3800
	11th to 20th	Night	23% 3 7% 23% 34% 11%	3779
	21st to end	Night	34% 3 8% 25% 26% 3	4120
November	1st to 10th	Night	32% 5% 12% 26% 20% 6%	2488
	11th to 20th	Night	37% 4 8% 25% 22% 4	2600
	21st to end	Night	43% 4 11% 25% 16% 2	2763
December	1st to 10th	Night	39% 4 10% 33% 14% 1	4580
	11th to 20th	Night	45% 3 12% 22% 17%	5201
	21st to end	Night	67% 4 11% 12% 5% 1	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	27% 22% 34% 15% 2	300
	11th to 20th	Day	27% 27% 35% 10% 1	300
	21st to end	Day	21% 23% 42% 12% 2	330
February	1st to 10th	Day	30% 22% 34% 12% 2	300
	11th to 20th	Day	27% 30% 30% 11% 2	300
	21st to end	Day	31% 25% 33% 10% 2	249
March	1st to 10th	Day	23% 24% 40% 11% 1	300
	11th to 20th	Day	30% 23% 33% 12% 2	300
	21st to end	Day	33% 25% 31% 10% 2	402
April	1st to 10th	Day	23% 17% 38% 17% 5%	500
	11th to 20th	Day	28% 20% 29% 19% 5%	492
	21st to end	Day	27% 16% 34% 17% 5%	500
May	1st to 10th	Day	17% 18% 38% 20% 6% 1	574
	11th to 20th	Day	18% 17% 27% 27% 10% 1	725
	21st to end	Day	15% 13% 30% 27% 10% 4	760
June	1st to 10th	Day	9% 7% 28% 34% 18% 5%	680
	11th to 20th	Day	9% 4 29% 41% 14% 3	680
	21st to end	Day	7% 10% 33% 35% 12% 3	680
July	1st to 10th	Day	13% 3 20% 31% 27% 7%	671
	11th to 20th	Day	7% 6% 24% 37% 23% 4	668
	21st to end	Day	10% 4 21% 32% 29% 5%	739
August	1st to 10th	Day	8% 4 21% 40% 22% 5%	676
	11th to 20th	Day	10% 5% 23% 35% 21% 6%	542
	21st to end	Day	8% 3 20% 32% 29% 8%	539
September	1st to 10th	Day	12% 3 18% 35% 25% 6%	490
	11th to 20th	Day	10% 3 20% 32% 25% 10%	490
	21st to end	Day	10% 12% 25% 27% 21% 5%	395
October	1st to 10th	Day	10% 12% 32% 28% 13% 4	300
	11th to 20th	Day	20% 26% 27% 22% 6%	300
	21st to end	Day	29% 24% 29% 15% 2	330
November	1st to 10th	Day	15% 16% 40% 18% 9% 2	300
	11th to 20th	Day	22% 22% 29% 21% 4 2	300
	21st to end	Day	30% 16% 30% 20% 3	300
December	1st to 10th	Day	17% 22% 39% 14% 7% 1	300
	11th to 20th	Day	23% 20% 37% 17% 4	300
	21st to end	Day	27% 26% 34% 12% 1	330
January	1st to 10th	Night	20% 20% 38% 17% 5%	1000
	11th to 20th	Night	24% 20% 36% 17% 3	1000
	21st to end	Night	19% 21% 38% 20% 1	1100
February	1st to 10th	Night	23% 20% 35% 19% 3	1000
	11th to 20th	Night	24% 20% 34% 19% 2	875
	21st to end	Night	25% 20% 35% 18% 2	660
March	1st to 10th	Night	24% 17% 39% 18% 2	707
	11th to 20th	Night	24% 16% 35% 22% 4	600
	21st to end	Night	26% 17% 36% 17% 3	660
April	1st to 10th	Night	18% 19% 37% 23% 4	600
	11th to 20th	Night	20% 15% 35% 25% 6%	593
	21st to end	Night	22% 12% 39% 23% 5%	600
May	1st to 10th	Night	15% 12% 34% 32% 7%	600
	11th to 20th	Night	19% 8% 29% 33% 11% 1	591
	21st to end	Night	12% 8% 31% 31% 16% 2	634
June	1st to 10th	Night	8% 4 24% 39% 23% 1	570
	11th to 20th	Night	8% 3 27% 41% 19% 1	570
	21st to end	Night	5% 4 30% 43% 17% 1	570
July	1st to 10th	Night	10% 3 14% 37% 31% 4	555
	11th to 20th	Night	5% 2 18% 42% 30% 2	552
	21st to end	Night	8% 2 18% 40% 31% 2	624
August	1st to 10th	Night	7% 3 13% 45% 31% 1	567
	11th to 20th	Night	9% 2 15% 40% 31% 3	564
	21st to end	Night	6% 2 18% 38% 33% 3	627
September	1st to 10th	Night	13% 4 18% 35% 24% 6%	570
	11th to 20th	Night	11% 3 19% 35% 28% 5%	570
	21st to end	Night	9% 8% 24% 34% 21% 4	712
October	1st to 10th	Night	10% 8% 29% 29% 22% 3	760
	11th to 20th	Night	18% 12% 33% 28% 9% 1	760
	21st to end	Night	22% 14% 34% 23% 6%	836
November	1st to 10th	Night	13% 13% 36% 28% 9%	780
	11th to 20th	Night	17% 18% 35% 24% 7%	800
	21st to end	Night	23% 17% 34% 22% 5%	963
December	1st to 10th	Night	21% 19% 35% 21% 5% 1	1000
	11th to 20th	Night	25% 15% 32% 24% 4	1000
	21st to end	Night	25% 21% 34% 18% 2	1100

Page generated at 2025-05-23 08:20:56. Version ac07

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