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 2024-01-14.

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	14% 8% 4 6% 6% 3 4 5% 5% 7% 37%	74830
	Night	15% 8% 3 6% 5% 3 4 4 5% 7% 39%	74944
Transparency	Day	46% 6% 12% 21% 13% 2	74531
	Night	46% 5% 11% 22% 15% 2	74764
Seeing	Day	17% 9% 26% 25% 17% 6%	14507
	Night	18% 9% 28% 30% 14% 1	24459

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	17% 13% 6% 9% 6% 4 4 5% 4 6% 25%	1517
	11th to 20th	Day	19% 15% 7% 10% 8% 3 3 4 4 5% 22%	1464
	21st to end	Day	15% 16% 7% 9% 6% 3 5% 3 5% 6% 25%	1660
February	1st to 10th	Day	18% 12% 6% 10% 6% 3 5% 4 4 5% 27%	1743
	11th to 20th	Day	16% 11% 4 8% 8% 6% 3 5% 4 5% 29%	1748
	21st to end	Day	19% 13% 5% 7% 6% 4 4 3 4 6% 30%	1480
March	1st to 10th	Day	17% 10% 5% 8% 7% 3 4 5% 5% 5% 32%	1800
	11th to 20th	Day	16% 9% 5% 8% 6% 3 5% 4 5% 8% 32%	1844
	21st to end	Day	21% 12% 6% 7% 6% 3 4 4 4 6% 26%	2376
April	1st to 10th	Day	15% 8% 4 6% 6% 4 4 4 5% 8% 36%	2160
	11th to 20th	Day	17% 8% 3 5% 5% 3 4 4 4 6% 39%	2134
	21st to end	Day	21% 8% 3 5% 6% 3 4 3 5% 7% 35%	2240
May	1st to 10th	Day	17% 8% 3 6% 6% 4 5% 4 6% 8% 32%	2340
	11th to 20th	Day	20% 8% 3 6% 5% 3 4 4 5% 7% 35%	2301
	21st to end	Day	10% 6% 3 7% 6% 4 6% 6% 7% 8% 38%	2725
June	1st to 10th	Day	10% 5% 2 5% 5% 3 5% 6% 6% 9% 43%	2520
	11th to 20th	Day	8% 4 2 5% 5% 3 5% 5% 6% 10% 46%	2520
	21st to end	Day	9% 5% 3 6% 6% 4 5% 5% 6% 10% 40%	2520
July	1st to 10th	Day	10% 4 1 5% 6% 4 6% 6% 7% 10% 42%	2520
	11th to 20th	Day	7% 4 2 5% 5% 4 5% 6% 8% 10% 44%	2520
	21st to end	Day	9% 4 2 4 6% 3 5% 6% 7% 10% 45%	2743
August	1st to 10th	Day	7% 5% 2 4 6% 2 5% 6% 7% 10% 47%	2354
	11th to 20th	Day	8% 4 2 5% 6% 4 5% 8% 7% 10% 41%	2125
	21st to end	Day	7% 5% 2 4 5% 2 5% 4 6% 9% 51%	2376
September	1st to 10th	Day	12% 6% 2 5% 5% 4 5% 6% 5% 7% 45%	2160
	11th to 20th	Day	10% 5% 2 4 5% 3 4 5% 6% 7% 49%	2016
	21st to end	Day	11% 5% 2 5% 4 3 4 4 6% 7% 48%	1980
October	1st to 10th	Day	13% 6% 2 4 4 3 4 5% 5% 7% 47%	1980
	11th to 20th	Day	16% 7% 3 5% 5% 3 5% 4 5% 6% 40%	1802
	21st to end	Day	19% 10% 3 6% 5% 2 4 5% 4 6% 36%	1782
November	1st to 10th	Day	12% 5% 2 5% 4 3 3 5% 4 6% 51%	1672
	11th to 20th	Day	18% 8% 4 5% 5% 2 4 3 4 6% 40%	1710
	21st to end	Day	17% 10% 3 7% 5% 2 5% 4 4 6% 37%	1710
December	1st to 10th	Day	18% 12% 5% 9% 6% 4 4 5% 5% 6% 28%	1587
	11th to 20th	Day	15% 13% 6% 10% 6% 3 5% 4 5% 5% 30%	1918
	21st to end	Day	24% 18% 7% 11% 7% 3 5% 3 3 4 14%	2783
January	1st to 10th	Night	14% 13% 5% 8% 6% 3 4 4 5% 5% 32%	2648
	11th to 20th	Night	17% 14% 5% 9% 8% 3 4 4 3 4 28%	2575
	21st to end	Night	16% 13% 6% 9% 6% 3 4 4 4 6% 30%	2691
February	1st to 10th	Night	20% 12% 5% 8% 5% 3 4 4 5% 6% 28%	2214
	11th to 20th	Night	15% 10% 4 8% 7% 4 4 3 4 7% 34%	2098
	21st to end	Night	20% 10% 4 8% 5% 3 4 4 4 6% 32%	1776
March	1st to 10th	Night	17% 9% 4 6% 6% 3 4 4 4 6% 36%	2160
	11th to 20th	Night	16% 8% 4 6% 5% 3 5% 4 5% 8% 38%	2116
	21st to end	Night	21% 9% 3 8% 4 3 4 4 5% 7% 33%	1994
April	1st to 10th	Night	12% 8% 3 7% 6% 3 4 4 5% 7% 42%	1800
	11th to 20th	Night	18% 8% 2 4 4 2 4 4 5% 7% 41%	1775
	21st to end	Night	18% 7% 3 6% 6% 3 4 6% 5% 6% 34%	1717
May	1st to 10th	Night	16% 8% 3 6% 5% 3 5% 5% 6% 8% 36%	1620
	11th to 20th	Night	17% 6% 2 6% 5% 3 4 4 5% 8% 40%	1589
	21st to end	Night	9% 6% 2 6% 7% 4 5% 5% 6% 10% 40%	1551
June	1st to 10th	Night	9% 4 2 5% 5% 3 5% 4 5% 8% 49%	1440
	11th to 20th	Night	7% 5% 2 6% 5% 3 4 5% 4 8% 51%	1440
	21st to end	Night	8% 4 2 5% 6% 3 5% 6% 7% 10% 44%	1440
July	1st to 10th	Night	8% 4 2 4 4 3 5% 5% 7% 10% 47%	1440
	11th to 20th	Night	6% 3 2 3 4 3 3 5% 6% 11% 55%	1440
	21st to end	Night	6% 4 2 4 3 3 4 4 5% 9% 55%	1575
August	1st to 10th	Night	7% 4 2 5% 5% 2 4 5% 6% 8% 53%	1578
	11th to 20th	Night	6% 4 2 5% 6% 4 5% 6% 7% 8% 48%	1776
	21st to end	Night	7% 3 2 3 4 3 4 4 4 8% 57%	1980
September	1st to 10th	Night	12% 5% 2 5% 3 2 3 4 6% 7% 51%	1800
	11th to 20th	Night	8% 4 2 4 4 2 4 4 5% 9% 54%	1924
	21st to end	Night	11% 5% 2 4 4 2 5% 5% 5% 7% 51%	1980
October	1st to 10th	Night	11% 5% 1 5% 4 3 4 4 5% 6% 53%	1980
	11th to 20th	Night	16% 6% 2 4 4 3 4 3 4 6% 48%	2154
	21st to end	Night	18% 9% 3 6% 5% 2 4 4 4 5% 39%	2574
November	1st to 10th	Night	11% 6% 2 6% 4 3 3 4 4 6% 51%	2406
	11th to 20th	Night	13% 9% 3 6% 4 3 3 4 5% 6% 42%	2470
	21st to end	Night	20% 9% 4 7% 5% 3 4 3 5% 5% 36%	2470
December	1st to 10th	Night	17% 11% 5% 9% 5% 3 4 4 5% 6% 32%	2590
	11th to 20th	Night	17% 10% 4 8% 6% 3 4 4 4 6% 33%	3279
	21st to end	Night	26% 18% 7% 9% 6% 3 4 4 3 4 16%	4884

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	61% 4 10% 15% 10% 1	1518
	11th to 20th	Day	65% 5% 12% 12% 5% 1	1464
	21st to end	Day	61% 4 12% 16% 6% 1	1660
February	1st to 10th	Day	61% 4 10% 16% 8% 1	1743
	11th to 20th	Day	57% 3 12% 19% 8% 1	1758
	21st to end	Day	58% 4 10% 19% 8% 2	1480
March	1st to 10th	Day	54% 5% 10% 21% 8% 1	1800
	11th to 20th	Day	51% 5% 13% 19% 10% 1	1844
	21st to end	Day	59% 4 11% 18% 6% 1	2376
April	1st to 10th	Day	47% 6% 12% 20% 15% 1	2160
	11th to 20th	Day	46% 5% 11% 23% 11% 4	2134
	21st to end	Day	51% 5% 11% 20% 12% 2	2240
May	1st to 10th	Day	50% 6% 14% 21% 9%	2340
	11th to 20th	Day	50% 7% 11% 18% 14% 1	2305
	21st to end	Day	43% 7% 12% 19% 15% 3	2725
June	1st to 10th	Day	37% 7% 12% 25% 16% 3	2520
	11th to 20th	Day	34% 7% 11% 24% 22% 4	2520
	21st to end	Day	40% 7% 15% 20% 15% 4	2520
July	1st to 10th	Day	36% 8% 13% 25% 15% 3	2451
	11th to 20th	Day	34% 8% 15% 27% 15% 1	2445
	21st to end	Day	33% 8% 15% 25% 16% 2	2591
August	1st to 10th	Day	31% 7% 15% 32% 13% 1	2339
	11th to 20th	Day	35% 8% 13% 27% 15% 2	2125
	21st to end	Day	30% 6% 13% 26% 18% 7%	2376
September	1st to 10th	Day	38% 7% 12% 23% 16% 4	2160
	11th to 20th	Day	34% 6% 11% 25% 21% 4	2016
	21st to end	Day	35% 5% 11% 24% 23% 2	1980
October	1st to 10th	Day	37% 5% 11% 25% 19% 3	1980
	11th to 20th	Day	45% 4 10% 22% 18% 1	1802
	21st to end	Day	51% 4 10% 21% 12% 1	1782
November	1st to 10th	Day	36% 5% 14% 24% 18% 4	1672
	11th to 20th	Day	47% 4 9% 22% 13% 4	1710
	21st to end	Day	51% 4 10% 21% 13% 1	1710
December	1st to 10th	Day	58% 5% 10% 19% 8% 1	1587
	11th to 20th	Day	58% 4 8% 24% 6% 1	1915
	21st to end	Day	74% 5% 8% 9% 4	2783
January	1st to 10th	Night	55% 4 9% 18% 12% 1	2648
	11th to 20th	Night	61% 4 10% 16% 7% 1	2575
	21st to end	Night	58% 4 11% 17% 9% 1	2691
February	1st to 10th	Night	59% 4 12% 15% 8% 1	2214
	11th to 20th	Night	53% 3 11% 21% 11% 2	2099
	21st to end	Night	55% 4 10% 19% 10% 2	1776
March	1st to 10th	Night	50% 5% 10% 21% 13% 1	2160
	11th to 20th	Night	45% 5% 12% 23% 13% 2	2116
	21st to end	Night	53% 5% 11% 20% 9% 2	1994
April	1st to 10th	Night	43% 6% 10% 24% 15% 2	1800
	11th to 20th	Night	44% 5% 8% 23% 16% 5%	1775
	21st to end	Night	48% 6% 10% 19% 12% 4	1717
May	1st to 10th	Night	46% 7% 11% 25% 12%	1620
	11th to 20th	Night	43% 6% 14% 21% 15% 1	1590
	21st to end	Night	39% 8% 14% 19% 15% 5%	1552
June	1st to 10th	Night	34% 5% 10% 27% 20% 3	1440
	11th to 20th	Night	32% 6% 11% 25% 22% 4	1440
	21st to end	Night	34% 7% 16% 22% 16% 5%	1440
July	1st to 10th	Night	32% 9% 13% 25% 15% 5%	1400
	11th to 20th	Night	23% 7% 16% 30% 22% 2	1392
	21st to end	Night	26% 6% 14% 30% 19% 4	1487
August	1st to 10th	Night	29% 7% 14% 30% 18% 2	1569
	11th to 20th	Night	32% 8% 12% 27% 20% 1	1776
	21st to end	Night	27% 6% 13% 25% 22% 7%	1980
September	1st to 10th	Night	34% 5% 11% 25% 18% 7%	1800
	11th to 20th	Night	29% 4 11% 26% 26% 5%	1924
	21st to end	Night	34% 4 9% 24% 24% 4	1980
October	1st to 10th	Night	33% 5% 10% 26% 23% 3	1980
	11th to 20th	Night	38% 3 9% 26% 22% 3	2154
	21st to end	Night	47% 4 9% 21% 17% 2	2574
November	1st to 10th	Night	35% 5% 10% 26% 20% 4	2406
	11th to 20th	Night	43% 4 11% 22% 17% 3	2470
	21st to end	Night	51% 3 10% 23% 13% 1	2470
December	1st to 10th	Night	55% 4 11% 19% 10% 1	2590
	11th to 20th	Night	53% 4 11% 19% 12% 1	3281
	21st to end	Night	73% 4 7% 10% 6%	4884

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	26% 15% 32% 21% 6%	270
	11th to 20th	Day	27% 14% 37% 17% 4 1	249
	21st to end	Day	24% 16% 36% 19% 4	264
February	1st to 10th	Day	30% 17% 35% 15% 3	240
	11th to 20th	Day	29% 20% 32% 14% 4 2	240
	21st to end	Day	31% 15% 39% 13% 2	198
March	1st to 10th	Day	21% 20% 38% 16% 5%	240
	11th to 20th	Day	29% 14% 37% 17% 4	284
	21st to end	Day	33% 14% 28% 18% 7% 1	462
April	1st to 10th	Day	19% 16% 32% 23% 8% 2	420
	11th to 20th	Day	20% 17% 28% 18% 14% 2	412
	21st to end	Day	27% 13% 31% 19% 8% 3	420
May	1st to 10th	Day	22% 10% 32% 24% 10% 3	423
	11th to 20th	Day	24% 6% 25% 26% 13% 6%	447
	21st to end	Day	12% 6% 28% 25% 18% 10%	679
June	1st to 10th	Day	12% 6% 25% 31% 22% 5%	630
	11th to 20th	Day	11% 5% 26% 30% 22% 7%	630
	21st to end	Day	12% 7% 27% 28% 19% 7%	630
July	1st to 10th	Day	11% 3 14% 29% 28% 15%	621
	11th to 20th	Day	8% 4 17% 32% 27% 12%	618
	21st to end	Day	10% 2 17% 31% 30% 10%	684
August	1st to 10th	Day	8% 2 14% 31% 30% 16%	490
	11th to 20th	Day	10% 3 15% 29% 31% 12%	439
	21st to end	Day	8% 3 16% 25% 34% 14%	495
September	1st to 10th	Day	14% 3 15% 30% 29% 10%	450
	11th to 20th	Day	9% 3 13% 30% 28% 16%	450
	21st to end	Day	12% 9% 22% 20% 23% 12%	450
October	1st to 10th	Day	13% 5% 26% 31% 16% 8%	450
	11th to 20th	Day	14% 18% 33% 26% 8%	278
	21st to end	Day	31% 14% 33% 18% 4	297
November	1st to 10th	Day	12% 13% 34% 23% 13% 4	270
	11th to 20th	Day	24% 16% 31% 17% 10% 1	270
	21st to end	Day	24% 10% 38% 23% 6%	270
December	1st to 10th	Day	19% 17% 37% 17% 8% 1	270
	11th to 20th	Day	20% 13% 42% 17% 7% 1	270
	21st to end	Day	28% 17% 32% 22% 2	297
January	1st to 10th	Night	22% 14% 36% 23% 4	950
	11th to 20th	Night	25% 14% 34% 22% 5%	920
	21st to end	Night	23% 13% 35% 26% 3	990
February	1st to 10th	Night	28% 11% 35% 22% 3	774
	11th to 20th	Night	22% 15% 34% 26% 4	703
	21st to end	Night	28% 12% 35% 21% 3	592
March	1st to 10th	Night	23% 14% 34% 24% 5%	720
	11th to 20th	Night	20% 11% 38% 25% 6% 1	720
	21st to end	Night	25% 8% 37% 25% 6% 1	608
April	1st to 10th	Night	16% 14% 35% 29% 7%	540
	11th to 20th	Night	21% 12% 30% 27% 10%	533
	21st to end	Night	21% 9% 34% 28% 8% 1	540
May	1st to 10th	Night	20% 6% 33% 32% 9%	540
	11th to 20th	Night	19% 6% 26% 31% 17% 2	531
	21st to end	Night	12% 5% 25% 35% 21% 2	584
June	1st to 10th	Night	11% 3 23% 44% 19%	540
	11th to 20th	Night	11% 3 22% 44% 19% 1	540
	21st to end	Night	9% 3 22% 40% 25% 1	540
July	1st to 10th	Night	10% 2 13% 36% 31% 6%	525
	11th to 20th	Night	5% 3 16% 41% 31% 4	522
	21st to end	Night	7% 2 12% 43% 34% 2	591
August	1st to 10th	Night	8% 2 9% 44% 33% 5%	537
	11th to 20th	Night	7% 2 12% 41% 33% 5%	534
	21st to end	Night	8% 2 15% 32% 41% 3	594
September	1st to 10th	Night	15% 3 14% 34% 30% 3	540
	11th to 20th	Night	9% 4 15% 38% 31% 4	664
	21st to end	Night	13% 6% 20% 30% 29% 2	720
October	1st to 10th	Night	13% 5% 22% 37% 19% 3	720
	11th to 20th	Night	20% 9% 30% 27% 13%	720
	21st to end	Night	22% 14% 33% 25% 6%	792
November	1st to 10th	Night	15% 9% 38% 26% 11% 1	740
	11th to 20th	Night	20% 14% 30% 27% 8% 1	760
	21st to end	Night	25% 11% 31% 26% 8%	760
December	1st to 10th	Night	23% 11% 36% 24% 5%	880
	11th to 20th	Night	25% 9% 33% 27% 6%	950
	21st to end	Night	31% 13% 30% 22% 4	1045

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