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 2025-03-27.

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	10% 7% 2 7% 4 4 4 4 5% 7% 45%	109288
	Night	9% 6% 2 6% 4 4 4 3 5% 7% 49%	110208
Transparency	Day	37% 6% 13% 26% 17% 2	112968
	Night	34% 5% 12% 27% 19% 3	114645
Seeing	Day	14% 10% 32% 29% 14% 2	16231
	Night	12% 10% 34% 33% 10% 1	26304

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% 13% 4 14% 7% 6% 5% 4 5% 5% 28%	4077
	11th to 20th	Day	6% 9% 3 8% 5% 4 7% 5% 5% 5% 43%	4080
	21st to end	Day	6% 8% 4 11% 5% 4 5% 4 5% 6% 43%	4488
February	1st to 10th	Day	10% 12% 4 8% 6% 4 4 4 5% 8% 34%	1800
	11th to 20th	Day	8% 11% 3 8% 5% 4 5% 3 5% 7% 41%	1834
	21st to end	Day	9% 8% 3 6% 3 4 4 3 5% 7% 48%	1650
March	1st to 10th	Day	11% 9% 3 7% 4 2 3 2 3 5% 51%	3444
	11th to 20th	Day	11% 7% 2 6% 4 3 3 3 4 6% 51%	3590
	21st to end	Day	20% 11% 3 9% 4 4 4 4 6% 7% 28%	3880
April	1st to 10th	Day	14% 7% 3 5% 5% 4 3 3 4 6% 45%	2113
	11th to 20th	Day	15% 8% 3 5% 4 4 4 3 5% 6% 41%	2413
	21st to end	Day	19% 7% 2 7% 4 4 3 3 5% 7% 39%	2470
May	1st to 10th	Day	16% 8% 2 7% 5% 5% 4 3 5% 7% 37%	4082
	11th to 20th	Day	13% 7% 3 6% 4 4 3 3 4 6% 46%	4082
	21st to end	Day	14% 7% 3 6% 4 5% 4 4 6% 7% 40%	4477
June	1st to 10th	Day	6% 4 2 5% 4 5% 4 4 6% 9% 51%	2622
	11th to 20th	Day	10% 5% 2 5% 4 4 4 4 6% 8% 48%	2660
	21st to end	Day	7% 4 2 5% 5% 4 5% 4 6% 9% 51%	2716
July	1st to 10th	Day	13% 5% 2 5% 4 4 4 4 5% 8% 46%	4192
	11th to 20th	Day	7% 4 2 5% 4 4 4 4 6% 8% 53%	4120
	21st to end	Day	5% 4 1 4 4 4 5% 5% 8% 9% 53%	4465
August	1st to 10th	Day	7% 5% 2 6% 5% 4 5% 5% 7% 9% 46%	4104
	11th to 20th	Day	13% 6% 2 6% 4 4 4 4 6% 8% 43%	4041
	21st to end	Day	7% 4 2 6% 4 4 5% 4 6% 7% 53%	4510
September	1st to 10th	Day	12% 5% 1 5% 4 3 3 3 5% 6% 53%	2153
	11th to 20th	Day	9% 5% 2 5% 3 4 4 3 5% 6% 54%	2090
	21st to end	Day	9% 4 1 3 3 3 3 3 4 6% 58%	2090
October	1st to 10th	Day	11% 7% 2 5% 3 3 3 4 4 7% 52%	1976
	11th to 20th	Day	10% 6% 2 5% 4 3 4 2 3 5% 58%	1900
	21st to end	Day	13% 6% 3 6% 4 3 4 3 4 5% 50%	2066
November	1st to 10th	Day	9% 6% 3 5% 3 3 4 3 5% 7% 54%	1763
	11th to 20th	Day	9% 6% 2 6% 5% 3 4 2 5% 7% 50%	1800
	21st to end	Day	12% 8% 3 6% 4 4 4 2 5% 6% 46%	1618
December	1st to 10th	Day	5% 5% 1 5% 4 4 5% 4 5% 6% 55%	2610
	11th to 20th	Day	8% 8% 3 9% 5% 4 4 4 6% 6% 43%	3008
	21st to end	Day	15% 9% 3 13% 5% 4 4 3 5% 5% 34%	4304
January	1st to 10th	Night	9% 10% 4 15% 7% 5% 5% 4 5% 7% 28%	5888
	11th to 20th	Night	6% 7% 2 7% 4 4 6% 4 6% 8% 45%	5900
	21st to end	Night	7% 9% 5% 9% 4 3 3 3 4 5% 47%	6490
February	1st to 10th	Night	10% 10% 3 7% 6% 5% 5% 3 5% 6% 40%	2600
	11th to 20th	Night	8% 9% 3 7% 5% 5% 5% 4 6% 5% 43%	2470
	21st to end	Night	9% 7% 2 6% 3 3 4 3 5% 6% 52%	1980
March	1st to 10th	Night	11% 6% 2 5% 3 3 3 3 4 5% 55%	3631
	11th to 20th	Night	11% 6% 2 5% 2 2 3 2 4 6% 57%	3590
	21st to end	Night	17% 8% 3 8% 5% 4 4 3 6% 8% 35%	3889
April	1st to 10th	Night	12% 7% 2 5% 4 4 4 3 5% 7% 47%	2066
	11th to 20th	Night	11% 7% 2 6% 5% 4 3 3 5% 7% 47%	1715
	21st to end	Night	15% 8% 2 6% 4 3 3 3 5% 7% 45%	1710
May	1st to 10th	Night	18% 7% 2 5% 3 4 3 3 5% 6% 45%	2823
	11th to 20th	Night	11% 5% 2 5% 4 3 4 3 5% 7% 51%	2823
	21st to end	Night	15% 7% 2 6% 4 3 4 2 5% 7% 45%	3090
June	1st to 10th	Night	5% 3 2 4 4 4 4 4 6% 10% 55%	1553
	11th to 20th	Night	8% 5% 1 4 4 4 4 3 6% 9% 52%	1520
	21st to end	Night	3 3 1 3 3 4 3 5% 5% 9% 59%	1439
July	1st to 10th	Night	8% 5% 2 5% 3 4 3 3 5% 7% 55%	2752
	11th to 20th	Night	5% 3 1 3 3 3 4 3 6% 8% 61%	2850
	21st to end	Night	3 2 1 3 3 3 4 3 7% 9% 62%	3098
August	1st to 10th	Night	4 3 1 4 3 3 4 4 6% 11% 57%	2836
	11th to 20th	Night	12% 5% 2 5% 4 3 4 3 5% 7% 52%	2798
	21st to end	Night	6% 4 2 5% 4 4 4 3 5% 8% 56%	3156
September	1st to 10th	Night	8% 4 1 3 2 3 3 3 4 7% 62%	2023
	11th to 20th	Night	8% 3 1 4 3 2 3 3 5% 7% 60%	2090
	21st to end	Night	9% 4 1 5% 3 3 3 3 4 5% 60%	2090
October	1st to 10th	Night	12% 5% 2 4 3 4 3 4 5% 6% 52%	2183
	11th to 20th	Night	11% 5% 2 5% 3 2 3 2 3 5% 59%	2280
	21st to end	Night	13% 5% 2 5% 3 3 3 3 5% 5% 54%	2527
November	1st to 10th	Night	7% 4 2 5% 3 4 4 3 5% 6% 57%	2535
	11th to 20th	Night	9% 6% 2 5% 4 3 3 3 4 6% 55%	2600
	21st to end	Night	10% 6% 3 5% 3 3 4 4 5% 7% 52%	2779
December	1st to 10th	Night	5% 5% 1 5% 4 4 4 4 5% 7% 57%	4580
	11th to 20th	Night	9% 6% 2 7% 4 4 4 3 6% 8% 48%	5199
	21st to end	Night	14% 12% 4 9% 5% 4 4 3 4 5% 36%	6655

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	57% 6% 10% 19% 7% 1	4078
	11th to 20th	Day	42% 4 14% 21% 17% 2	4080
	21st to end	Day	42% 5% 12% 25% 15% 1	4488
February	1st to 10th	Day	48% 6% 15% 19% 11% 1	1800
	11th to 20th	Day	42% 5% 13% 22% 16% 3	1843
	21st to end	Day	36% 5% 13% 25% 17% 4	1650
March	1st to 10th	Day	37% 4 9% 27% 19% 3	3444
	11th to 20th	Day	35% 5% 10% 30% 18% 2	3590
	21st to end	Day	53% 8% 21% 13% 4 1	3880
April	1st to 10th	Day	40% 5% 13% 25% 15% 2	2113
	11th to 20th	Day	43% 5% 12% 22% 14% 3	2413
	21st to end	Day	45% 5% 11% 25% 13%	2470
May	1st to 10th	Day	44% 7% 11% 22% 14% 2	4120
	11th to 20th	Day	39% 6% 10% 25% 19% 1	4086
	21st to end	Day	41% 7% 13% 24% 14% 1	4489
June	1st to 10th	Day	29% 7% 17% 32% 14% 2	2622
	11th to 20th	Day	33% 7% 12% 30% 16% 2	2660
	21st to end	Day	29% 9% 15% 29% 14% 4	2716
July	1st to 10th	Day	36% 5% 13% 28% 16% 1	4128
	11th to 20th	Day	29% 6% 12% 31% 18% 4	4051
	21st to end	Day	24% 8% 17% 28% 21% 3	4325
August	1st to 10th	Day	31% 8% 15% 27% 16% 3	4090
	11th to 20th	Day	39% 7% 13% 25% 16% 1	4080
	21st to end	Day	31% 6% 14% 25% 18% 7%	4509
September	1st to 10th	Day	32% 5% 9% 30% 19% 5%	2153
	11th to 20th	Day	30% 6% 11% 31% 20% 2	2090
	21st to end	Day	27% 4 9% 32% 26% 1	2090
October	1st to 10th	Day	21% 2 6% 20% 46% 4	3288
	11th to 20th	Day	22% 3 12% 30% 25% 8%	3140
	21st to end	Day	33% 4 12% 27% 22% 3	3382
November	1st to 10th	Day	30% 4 15% 29% 20% 3	1763
	11th to 20th	Day	34% 5% 12% 27% 21% 1	1800
	21st to end	Day	40% 4 11% 30% 14% 2	1618
December	1st to 10th	Day	29% 5% 17% 30% 18% 1	2610
	11th to 20th	Day	40% 5% 19% 26% 9% 1	3005
	21st to end	Day	53% 5% 10% 23% 9% 1	4304
January	1st to 10th	Night	54% 6% 10% 16% 13%	5888
	11th to 20th	Night	36% 4 15% 25% 17% 2	5900
	21st to end	Night	41% 4 11% 21% 20% 2	6490
February	1st to 10th	Night	44% 6% 13% 21% 13% 3	2600
	11th to 20th	Night	41% 5% 14% 23% 15% 3	2471
	21st to end	Night	33% 5% 10% 26% 21% 5%	1980
March	1st to 10th	Night	32% 5% 8% 27% 24% 3	3631
	11th to 20th	Night	30% 4 9% 29% 23% 5%	3590
	21st to end	Night	45% 9% 21% 19% 6% 1	3889
April	1st to 10th	Night	36% 5% 11% 27% 18% 3	2066
	11th to 20th	Night	37% 5% 12% 25% 18% 3	1715
	21st to end	Night	39% 5% 12% 24% 19% 1	1710
May	1st to 10th	Night	40% 5% 12% 24% 17% 2	2850
	11th to 20th	Night	32% 6% 12% 26% 23% 1	2824
	21st to end	Night	39% 6% 13% 22% 19% 2	3099
June	1st to 10th	Night	24% 9% 16% 32% 17% 3	1553
	11th to 20th	Night	27% 7% 13% 27% 20% 5%	1520
	21st to end	Night	21% 8% 14% 34% 20% 4	1439
July	1st to 10th	Night	30% 5% 12% 33% 18% 3	2707
	11th to 20th	Night	21% 5% 15% 35% 20% 3	2796
	21st to end	Night	16% 7% 17% 32% 25% 2	2999
August	1st to 10th	Night	21% 6% 16% 35% 18% 3	2826
	11th to 20th	Night	33% 4 17% 26% 18% 1	2825
	21st to end	Night	27% 5% 15% 28% 17% 9%	3156
September	1st to 10th	Night	24% 4 11% 30% 23% 8%	2023
	11th to 20th	Night	24% 5% 12% 33% 22% 4	2090
	21st to end	Night	27% 4 8% 29% 28% 5%	2090
October	1st to 10th	Night	20% 3 7% 17% 47% 6%	3638
	11th to 20th	Night	21% 3 8% 31% 29% 8%	3779
	21st to end	Night	31% 3 9% 32% 24% 2	4151
November	1st to 10th	Night	26% 6% 14% 26% 24% 5%	2535
	11th to 20th	Night	32% 4 11% 30% 20% 4	2600
	21st to end	Night	33% 4 12% 28% 20% 3	2779
December	1st to 10th	Night	27% 5% 16% 33% 19% 1	4580
	11th to 20th	Night	35% 4 16% 31% 15%	5201
	21st to end	Night	51% 4 10% 25% 9% 1	6655

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% 17% 38% 20% 8% 1	300
	11th to 20th	Day	18% 14% 38% 25% 4 1	300
	21st to end	Day	13% 12% 46% 21% 8% 1	330
February	1st to 10th	Day	17% 18% 42% 21% 3	300
	11th to 20th	Day	14% 15% 40% 22% 7% 1	300
	21st to end	Day	15% 19% 44% 17% 4	249
March	1st to 10th	Day	18% 12% 44% 21% 4 1	300
	11th to 20th	Day	22% 12% 37% 21% 8%	300
	21st to end	Day	31% 16% 35% 15% 3	315
April	1st to 10th	Day	19% 17% 45% 16% 3	293
	11th to 20th	Day	20% 16% 37% 19% 8%	611
	21st to end	Day	22% 15% 34% 19% 8% 1	650
May	1st to 10th	Day	19% 14% 34% 24% 9% 1	653
	11th to 20th	Day	18% 11% 30% 26% 13% 2	675
	21st to end	Day	17% 9% 31% 31% 12% 2	734
June	1st to 10th	Day	6% 6% 25% 38% 20% 4	680
	11th to 20th	Day	9% 6% 33% 33% 15% 2	680
	21st to end	Day	6% 8% 32% 35% 16% 3	680
July	1st to 10th	Day	8% 4 17% 38% 26% 7%	671
	11th to 20th	Day	6% 3 23% 39% 22% 6%	668
	21st to end	Day	6% 3 21% 40% 25% 5%	739
August	1st to 10th	Day	7% 3 23% 41% 22% 4	676
	11th to 20th	Day	9% 3 21% 43% 22% 3	665
	21st to end	Day	10% 3 23% 37% 24% 3	739
September	1st to 10th	Day	13% 4 28% 34% 17% 3	363
	11th to 20th	Day	7% 3 31% 38% 18% 3	300
	21st to end	Day	13% 9% 32% 30% 13% 3	300
October	1st to 10th	Day	17% 6% 31% 27% 14% 5%	300
	11th to 20th	Day	13% 14% 40% 23% 9% 1	300
	21st to end	Day	18% 12% 42% 17% 9% 1	330
November	1st to 10th	Day	19% 13% 37% 22% 7% 3	300
	11th to 20th	Day	14% 16% 43% 20% 7% 1	300
	21st to end	Day	18% 16% 34% 21% 9% 1	300
December	1st to 10th	Day	10% 13% 43% 26% 6% 1	300
	11th to 20th	Day	15% 14% 37% 26% 8%	300
	21st to end	Day	20% 15% 40% 20% 4	330
January	1st to 10th	Night	11% 13% 37% 32% 6% 1	1000
	11th to 20th	Night	13% 14% 39% 27% 7% 1	1000
	21st to end	Night	14% 11% 41% 29% 5%	1100
February	1st to 10th	Night	15% 12% 43% 26% 4	1000
	11th to 20th	Night	13% 13% 42% 28% 4	916
	21st to end	Night	13% 14% 43% 26% 4	660
March	1st to 10th	Night	15% 13% 37% 29% 6%	668
	11th to 20th	Night	20% 9% 38% 27% 6%	600
	21st to end	Night	20% 11% 39% 24% 5%	647
April	1st to 10th	Night	15% 16% 39% 23% 6%	570
	11th to 20th	Night	16% 14% 38% 28% 4	563
	21st to end	Night	17% 11% 37% 27% 7% 1	570
May	1st to 10th	Night	17% 8% 36% 30% 9%	570
	11th to 20th	Night	13% 9% 33% 34% 11%	561
	21st to end	Night	16% 7% 32% 33% 11% 1	617
June	1st to 10th	Night	7% 4 26% 46% 16% 1	570
	11th to 20th	Night	9% 6% 31% 42% 12% 1	570
	21st to end	Night	5% 5% 34% 41% 15%	570
July	1st to 10th	Night	7% 2 19% 44% 26% 1	555
	11th to 20th	Night	3 1 22% 49% 23% 2	552
	21st to end	Night	3 1 17% 53% 25% 1	624
August	1st to 10th	Night	6% 2 20% 48% 22% 2	567
	11th to 20th	Night	7% 2 20% 49% 22% 1	564
	21st to end	Night	9% 2 24% 42% 22% 2	627
September	1st to 10th	Night	8% 4 26% 39% 21% 1	570
	11th to 20th	Night	10% 3 31% 39% 16% 1	570
	21st to end	Night	10% 6% 34% 34% 16% 1	570
October	1st to 10th	Night	16% 8% 30% 33% 12% 2	663
	11th to 20th	Night	14% 12% 37% 30% 7%	760
	21st to end	Night	16% 12% 30% 32% 9%	855
November	1st to 10th	Night	10% 10% 37% 35% 7% 1	975
	11th to 20th	Night	13% 12% 41% 27% 6% 1	1000
	21st to end	Night	13% 13% 37% 29% 8% 1	1000
December	1st to 10th	Night	12% 13% 40% 30% 5%	1000
	11th to 20th	Night	15% 12% 37% 31% 6%	1000
	21st to end	Night	17% 14% 35% 29% 5%	1100

Page generated at 2025-03-24 08:04:31. Version ac07

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