Walnut Hill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Walnut Hill Observatory clear sky chart from 2005-11-06 to 2026-07-31.

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	11% 7% 2 7% 4 4 4 4 5% 7% 46%	138059
	Night	9% 6% 2 6% 4 4 4 3 5% 7% 49%	135216
Transparency	Day	37% 5% 12% 25% 17% 3	141700
	Night	34% 5% 12% 26% 20% 3	139632
Seeing	Day	14% 10% 32% 28% 13% 2	18214
	Night	12% 10% 35% 33% 10% 1	28034

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	9% 12% 4 12% 7% 5% 5% 4 5% 5% 32%	5289
	11th to 20th	Day	5% 8% 3 9% 6% 5% 7% 5% 5% 5% 42%	5310
	21st to end	Day	5% 10% 4 10% 5% 4 4 3 4 6% 43%	5812
February	1st to 10th	Day	9% 12% 4 8% 6% 4 4 4 5% 8% 35%	1890
	11th to 20th	Day	8% 11% 3 8% 5% 4 5% 3 5% 7% 42%	1928
	21st to end	Day	9% 8% 3 6% 3 4 4 3 5% 7% 49%	1730
March	1st to 10th	Day	13% 9% 3 6% 3 2 2 2 3 4 53%	4905
	11th to 20th	Day	10% 7% 2 5% 3 3 3 3 4 7% 52%	5090
	21st to end	Day	18% 10% 3 8% 4 4 5% 4 6% 7% 30%	5583
April	1st to 10th	Day	15% 8% 2 5% 5% 4 3 3 5% 6% 44%	2335
	11th to 20th	Day	15% 8% 3 5% 4 4 4 3 5% 6% 42%	2670
	21st to end	Day	19% 8% 3 7% 4 4 3 3 4 6% 39%	2730
May	1st to 10th	Day	12% 7% 2 6% 4 4 4 3 5% 7% 46%	7617
	11th to 20th	Day	12% 6% 2 6% 4 4 4 3 5% 6% 49%	7627
	21st to end	Day	18% 7% 3 6% 4 4 4 4 5% 6% 37%	8392
June	1st to 10th	Day	7% 5% 2 5% 4 5% 4 4 6% 9% 49%	2898
	11th to 20th	Day	10% 5% 2 5% 4 4 4 4 6% 8% 50%	2940
	21st to end	Day	8% 5% 2 5% 5% 4 5% 4 6% 8% 50%	3002
July	1st to 10th	Day	11% 5% 2 5% 3 4 4 4 6% 8% 49%	6131
	11th to 20th	Day	8% 5% 2 6% 4 4 4 4 6% 8% 49%	6030
	21st to end	Day	7% 5% 2 5% 4 3 5% 5% 7% 8% 49%	6553
August	1st to 10th	Day	7% 5% 2 6% 5% 4 5% 5% 7% 9% 45%	4234
	11th to 20th	Day	13% 6% 2 6% 4 4 4 4 6% 8% 43%	4171
	21st to end	Day	7% 4 2 6% 4 4 5% 4 6% 7% 52%	4652
September	1st to 10th	Day	12% 5% 1 5% 4 3 3 3 5% 6% 53%	2266
	11th to 20th	Day	9% 6% 2 5% 3 4 4 3 5% 7% 54%	2200
	21st to end	Day	9% 4 1 3 3 3 3 3 4 6% 59%	2200
October	1st to 10th	Day	11% 7% 2 5% 3 3 3 4 4 6% 52%	2080
	11th to 20th	Day	10% 6% 2 5% 4 3 4 2 3 5% 57%	2000
	21st to end	Day	14% 6% 3 6% 4 3 4 3 4 5% 49%	2175
November	1st to 10th	Day	8% 6% 3 5% 3 3 4 3 5% 7% 55%	1853
	11th to 20th	Day	9% 6% 2 6% 5% 3 4 2 5% 7% 51%	1890
	21st to end	Day	12% 9% 3 6% 4 4 4 2 5% 6% 46%	1699
December	1st to 10th	Day	5% 5% 1 5% 4 4 5% 4 5% 6% 55%	2690
	11th to 20th	Day	8% 8% 3 9% 5% 4 4 4 6% 6% 44%	3088
	21st to end	Day	15% 9% 3 13% 5% 4 4 3 5% 6% 34%	4399
January	1st to 10th	Night	9% 10% 3 13% 6% 5% 5% 4 6% 7% 33%	7634
	11th to 20th	Night	5% 6% 2 7% 4 5% 6% 5% 7% 9% 46%	7680
	21st to end	Night	7% 10% 5% 9% 4 3 3 3 4 5% 46%	8402
February	1st to 10th	Night	10% 9% 3 7% 6% 5% 5% 3 5% 6% 41%	2730
	11th to 20th	Night	8% 9% 3 7% 5% 5% 5% 4 6% 6% 43%	2597
	21st to end	Night	8% 6% 2 7% 3 3 4 3 5% 6% 53%	2076
March	1st to 10th	Night	12% 6% 2 5% 3 3 4 3 4 5% 53%	5173
	11th to 20th	Night	10% 6% 2 5% 2 3 3 2 4 6% 57%	5090
	21st to end	Night	13% 8% 3 8% 4 4 4 3 6% 9% 38%	5588
April	1st to 10th	Night	12% 7% 2 5% 4 4 4 3 5% 7% 47%	2284
	11th to 20th	Night	10% 7% 2 6% 5% 4 4 3 5% 7% 47%	1897
	21st to end	Night	15% 8% 2 6% 3 3 4 3 5% 6% 45%	1890
May	1st to 10th	Night	11% 6% 2 5% 4 4 3 3 4 6% 52%	5266
	11th to 20th	Night	11% 4 2 4 3 3 4 3 4 7% 54%	5265
	21st to end	Night	18% 7% 2 5% 3 3 4 3 5% 7% 43%	5793
June	1st to 10th	Night	6% 3 2 5% 4 4 4 4 6% 9% 54%	1717
	11th to 20th	Night	7% 4 1 4 4 4 4 3 6% 9% 53%	1680
	21st to end	Night	5% 4 1 4 3 4 3 5% 5% 8% 58%	1589
July	1st to 10th	Night	7% 5% 2 5% 3 4 3 3 5% 7% 56%	4028
	11th to 20th	Night	6% 4 2 5% 4 4 5% 4 6% 8% 53%	4167
	21st to end	Night	6% 4 1 4 3 4 5% 3 7% 8% 55%	4539
August	1st to 10th	Night	4 3 1 4 3 3 4 4 6% 11% 57%	2926
	11th to 20th	Night	12% 5% 2 4 4 3 4 3 5% 7% 52%	2888
	21st to end	Night	7% 4 2 5% 4 4 4 3 5% 8% 55%	3256
September	1st to 10th	Night	9% 4 1 4 2 3 3 3 4 7% 61%	2130
	11th to 20th	Night	8% 3 1 4 3 3 3 4 5% 7% 59%	2200
	21st to end	Night	8% 4 1 5% 3 3 3 3 4 5% 60%	2200
October	1st to 10th	Night	13% 5% 2 4 3 4 3 4 5% 6% 51%	2298
	11th to 20th	Night	11% 5% 2 5% 3 3 3 2 3 5% 58%	2400
	21st to end	Night	14% 5% 2 5% 3 2 3 3 5% 5% 54%	2660
November	1st to 10th	Night	7% 4 2 5% 3 4 4 3 5% 6% 58%	2665
	11th to 20th	Night	8% 6% 2 5% 4 3 3 3 4 6% 54%	2730
	21st to end	Night	10% 6% 3 5% 3 3 4 4 5% 7% 51%	2918
December	1st to 10th	Night	5% 5% 2 5% 4 4 4 4 5% 7% 57%	4720
	11th to 20th	Night	9% 6% 2 7% 4 4 4 3 5% 8% 48%	5339
	21st to end	Night	14% 12% 4 9% 5% 4 4 3 4 5% 37%	6801

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	54% 5% 11% 19% 10% 1	5290
	11th to 20th	Day	43% 4 14% 20% 18% 2	5310
	21st to end	Day	43% 4 13% 25% 14% 1	5812
February	1st to 10th	Day	47% 6% 15% 20% 11% 1	1890
	11th to 20th	Day	42% 5% 12% 23% 15% 4	1937
	21st to end	Day	36% 5% 13% 26% 17% 3	1730
March	1st to 10th	Day	37% 3 8% 26% 19% 7%	4905
	11th to 20th	Day	33% 4 8% 26% 25% 4	5090
	21st to end	Day	50% 7% 20% 16% 5% 2	5585
April	1st to 10th	Day	41% 5% 12% 25% 14% 2	2335
	11th to 20th	Day	41% 5% 12% 23% 15% 3	2670
	21st to end	Day	46% 5% 10% 25% 13% 1	2730
May	1st to 10th	Day	37% 5% 9% 26% 21% 2	7655
	11th to 20th	Day	36% 5% 8% 23% 24% 4	7631
	21st to end	Day	46% 6% 11% 22% 14% 1	8405
June	1st to 10th	Day	31% 7% 16% 31% 13% 2	2898
	11th to 20th	Day	32% 6% 13% 30% 17% 2	2940
	21st to end	Day	31% 8% 14% 28% 15% 4	3002
July	1st to 10th	Day	33% 5% 13% 31% 17% 1	6067
	11th to 20th	Day	32% 6% 13% 30% 16% 3	5961
	21st to end	Day	29% 7% 18% 28% 17% 2	6371
August	1st to 10th	Day	32% 8% 15% 27% 15% 3	4220
	11th to 20th	Day	38% 7% 14% 25% 16% 1	4210
	21st to end	Day	31% 6% 14% 24% 18% 7%	4651
September	1st to 10th	Day	32% 5% 10% 30% 18% 5%	2266
	11th to 20th	Day	31% 6% 12% 31% 19% 2	2200
	21st to end	Day	27% 4 9% 32% 26% 2	2200
October	1st to 10th	Day	21% 3 6% 21% 46% 4	3392
	11th to 20th	Day	22% 3 13% 30% 25% 8%	3240
	21st to end	Day	34% 4 12% 26% 22% 3	3491
November	1st to 10th	Day	29% 4 14% 29% 21% 3	1853
	11th to 20th	Day	34% 5% 12% 28% 21% 1	1890
	21st to end	Day	40% 3 11% 29% 14% 2	1699
December	1st to 10th	Day	29% 5% 17% 29% 18% 1	2690
	11th to 20th	Day	40% 5% 19% 26% 10% 1	3085
	21st to end	Day	53% 5% 10% 23% 9% 1	4399
January	1st to 10th	Night	50% 6% 12% 17% 14% 1	7635
	11th to 20th	Night	34% 4 14% 27% 19% 2	7680
	21st to end	Night	42% 4 13% 20% 20% 2	8402
February	1st to 10th	Night	44% 5% 14% 22% 13% 2	2730
	11th to 20th	Night	40% 5% 13% 23% 15% 3	2598
	21st to end	Night	33% 5% 10% 26% 22% 5%	2076
March	1st to 10th	Night	34% 4 7% 28% 24% 3	5173
	11th to 20th	Night	30% 3 7% 24% 29% 7%	5090
	21st to end	Night	42% 7% 18% 25% 7% 1	5588
April	1st to 10th	Night	36% 5% 11% 27% 17% 3	2284
	11th to 20th	Night	37% 5% 12% 25% 19% 3	1897
	21st to end	Night	40% 6% 12% 24% 18% 1	1890
May	1st to 10th	Night	34% 4 9% 26% 25% 3	5293
	11th to 20th	Night	31% 5% 12% 22% 25% 6%	5266
	21st to end	Night	42% 5% 12% 25% 16% 1	5805
June	1st to 10th	Night	26% 9% 15% 31% 17% 3	1717
	11th to 20th	Night	27% 7% 13% 27% 21% 5%	1680
	21st to end	Night	23% 8% 14% 32% 20% 3	1589
July	1st to 10th	Night	28% 5% 12% 34% 17% 3	3985
	11th to 20th	Night	29% 6% 13% 31% 19% 3	4116
	21st to end	Night	25% 7% 17% 27% 22% 2	4410
August	1st to 10th	Night	22% 6% 17% 35% 18% 3	2916
	11th to 20th	Night	32% 4 17% 27% 18% 1	2915
	21st to end	Night	27% 5% 14% 28% 16% 9%	3256
September	1st to 10th	Night	24% 4 12% 29% 23% 7%	2130
	11th to 20th	Night	24% 5% 13% 33% 21% 4	2200
	21st to end	Night	27% 4 8% 28% 28% 5%	2200
October	1st to 10th	Night	21% 3 7% 17% 46% 6%	3753
	11th to 20th	Night	22% 3 8% 30% 29% 8%	3899
	21st to end	Night	31% 3 8% 32% 24% 2	4284
November	1st to 10th	Night	26% 5% 14% 27% 24% 4	2665
	11th to 20th	Night	32% 4 11% 31% 19% 3	2730
	21st to end	Night	34% 4 12% 28% 19% 3	2918
December	1st to 10th	Night	27% 5% 15% 33% 18% 1	4720
	11th to 20th	Night	35% 4 15% 31% 15%	5341
	21st to end	Night	50% 4 10% 25% 9% 1	6801

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	16% 16% 39% 20% 8% 1	330
	11th to 20th	Day	17% 16% 38% 23% 4 1	330
	21st to end	Day	13% 12% 47% 20% 7% 1	363
February	1st to 10th	Day	15% 18% 43% 21% 3	330
	11th to 20th	Day	15% 15% 39% 22% 7% 1	330
	21st to end	Day	14% 19% 45% 16% 4	273
March	1st to 10th	Day	18% 12% 45% 21% 4 1	330
	11th to 20th	Day	23% 13% 36% 21% 8%	330
	21st to end	Day	31% 15% 35% 17% 2	363
April	1st to 10th	Day	21% 19% 43% 14% 3	355
	11th to 20th	Day	20% 17% 37% 19% 7%	708
	21st to end	Day	22% 15% 35% 19% 7% 1	750
May	1st to 10th	Day	17% 13% 34% 24% 10% 1	753
	11th to 20th	Day	18% 13% 30% 25% 13% 2	775
	21st to end	Day	17% 9% 31% 30% 11% 2	844
June	1st to 10th	Day	7% 6% 26% 37% 19% 4	780
	11th to 20th	Day	9% 7% 34% 33% 14% 3	780
	21st to end	Day	8% 8% 32% 34% 14% 2	780
July	1st to 10th	Day	8% 4 19% 37% 26% 6%	771
	11th to 20th	Day	6% 3 23% 37% 23% 8%	768
	21st to end	Day	8% 3 22% 39% 23% 5%	845
August	1st to 10th	Day	7% 3 25% 40% 21% 4	726
	11th to 20th	Day	8% 3 20% 43% 23% 3	715
	21st to end	Day	10% 4 23% 37% 24% 3	793
September	1st to 10th	Day	13% 5% 28% 33% 17% 3	396
	11th to 20th	Day	8% 4 32% 36% 17% 4	330
	21st to end	Day	13% 8% 31% 30% 14% 3	330
October	1st to 10th	Day	16% 6% 31% 28% 13% 5%	330
	11th to 20th	Day	13% 14% 42% 22% 8% 1	330
	21st to end	Day	20% 13% 41% 16% 8% 1	363
November	1st to 10th	Day	18% 14% 38% 22% 7% 2	330
	11th to 20th	Day	13% 15% 44% 21% 6% 1	330
	21st to end	Day	20% 16% 34% 20% 9% 1	330
December	1st to 10th	Day	10% 14% 43% 25% 6% 1	330
	11th to 20th	Day	15% 15% 37% 25% 8%	330
	21st to end	Day	19% 15% 41% 21% 4	363
January	1st to 10th	Night	11% 13% 38% 32% 6% 1	1050
	11th to 20th	Night	12% 14% 40% 27% 6%	1050
	21st to end	Night	14% 12% 41% 28% 5%	1155
February	1st to 10th	Night	14% 12% 43% 27% 4	1050
	11th to 20th	Night	12% 13% 42% 28% 4	963
	21st to end	Night	12% 14% 44% 26% 4	692
March	1st to 10th	Night	15% 13% 37% 28% 6%	701
	11th to 20th	Night	20% 9% 39% 27% 6%	630
	21st to end	Night	19% 11% 40% 25% 5%	693
April	1st to 10th	Night	16% 16% 40% 22% 6%	630
	11th to 20th	Night	15% 16% 38% 27% 4	623
	21st to end	Night	18% 12% 37% 26% 7% 1	630
May	1st to 10th	Night	16% 8% 35% 31% 9%	630
	11th to 20th	Night	13% 10% 33% 33% 11%	621
	21st to end	Night	17% 7% 31% 33% 11% 1	683
June	1st to 10th	Night	7% 6% 25% 45% 16% 1	630
	11th to 20th	Night	8% 7% 32% 41% 12%	630
	21st to end	Night	5% 7% 34% 40% 14%	630
July	1st to 10th	Night	7% 2 19% 45% 26% 1	615
	11th to 20th	Night	3 1 22% 48% 25% 2	612
	21st to end	Night	4 1 18% 52% 24% 1	687
August	1st to 10th	Night	6% 2 20% 49% 21% 2	597
	11th to 20th	Night	7% 3 19% 49% 22% 1	594
	21st to end	Night	9% 2 23% 43% 22% 2	660
September	1st to 10th	Night	9% 4 26% 39% 21% 1	600
	11th to 20th	Night	10% 3 32% 39% 16% 1	600
	21st to end	Night	10% 6% 32% 35% 17% 1	600
October	1st to 10th	Night	16% 8% 30% 32% 12% 2	698
	11th to 20th	Night	14% 12% 38% 29% 7%	800
	21st to end	Night	17% 12% 31% 32% 8%	900
November	1st to 10th	Night	10% 10% 38% 34% 6% 1	1025
	11th to 20th	Night	13% 12% 42% 28% 6%	1050
	21st to end	Night	14% 12% 36% 29% 8%	1050
December	1st to 10th	Night	12% 13% 40% 30% 5%	1050
	11th to 20th	Night	15% 12% 37% 30% 6%	1050
	21st to end	Night	17% 14% 35% 29% 5%	1155

Page generated at 2026-07-28 02:25:20. Version ac07

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