The Cherrywood Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Cherrywood Observatory clear sky chart from 2005-11-06 to 2026-08-15.

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	5% 5% 2 5% 4 4 5% 4 6% 8% 53%	152738
	Night	5% 6% 2 5% 3 3 3 3 5% 7% 57%	146724
Transparency	Day	26% 6% 11% 23% 23% 11%	147677
	Night	27% 6% 11% 24% 24% 9%	142794
Seeing	Day	7% 6% 23% 35% 21% 7%	18094
	Night	7% 4 26% 44% 17% 2	28039

By Month

Percentage Cloud Cover SW (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	2 7% 3 7% 4 3 3 3 4 7% 56%	4720
	11th to 20th	Day	1 3 2 6% 5% 5% 4 4 5% 8% 57%	4837
	21st to end	Day	1 4 2 7% 4 3 3 4 6% 8% 59%	5844
February	1st to 10th	Day	3 6% 4 7% 3 4 5% 3 6% 7% 53%	1890
	11th to 20th	Day	1 5% 2 6% 3 4 4 4 5% 9% 55%	2010
	21st to end	Day	2 6% 2 5% 3 4 3 3 5% 6% 61%	1730
March	1st to 10th	Day	4 8% 2 4 3 2 3 3 5% 7% 60%	4600
	11th to 20th	Day	5% 6% 2 7% 4 4 4 4 6% 7% 50%	5050
	21st to end	Day	4 6% 2 6% 4 4 5% 4 7% 9% 49%	5955
April	1st to 10th	Day	5% 7% 2 4 4 3 4 3 6% 9% 55%	2520
	11th to 20th	Day	10% 8% 2 5% 3 3 4 4 6% 7% 47%	2491
	21st to end	Day	9% 8% 2 6% 4 4 5% 4 6% 8% 45%	2520
May	1st to 10th	Day	11% 7% 2 6% 4 4 5% 4 6% 6% 47%	7523
	11th to 20th	Day	9% 5% 2 6% 4 5% 5% 4 7% 8% 45%	8191
	21st to end	Day	7% 5% 2 5% 4 5% 5% 4 7% 9% 45%	9026
June	1st to 10th	Day	5% 4 1 4 4 5% 5% 5% 8% 11% 49%	2940
	11th to 20th	Day	5% 3 1 4 4 4 5% 5% 8% 8% 53%	2940
	21st to end	Day	3 3 1 4 4 5% 6% 5% 8% 9% 52%	2940
July	1st to 10th	Day	3 2 1 3 3 5% 6% 5% 7% 9% 56%	8249
	11th to 20th	Day	2 2 1 4 4 5% 5% 5% 7% 9% 55%	8250
	21st to end	Day	4 3 1 6% 5% 6% 7% 6% 7% 9% 45%	8864
August	1st to 10th	Day	7% 4 2 5% 5% 6% 6% 5% 8% 10% 43%	5909
	11th to 20th	Day	2 2 1 3 4 4 5% 5% 8% 10% 54%	5442
	21st to end	Day	6% 5% 2 6% 5% 5% 6% 5% 7% 8% 46%	5984
September	1st to 10th	Day	5% 3 1 4 3 3 5% 4 7% 6% 59%	2400
	11th to 20th	Day	5% 3 2 4 3 3 5% 3 6% 7% 60%	2379
	21st to end	Day	7% 4 1 4 2 2 3 3 3 6% 65%	2200
October	1st to 10th	Day	4 3 1 2 2 2 2 2 4 4 72%	3282
	11th to 20th	Day	5% 4 1 4 3 4 4 3 5% 7% 60%	3250
	21st to end	Day	4 6% 2 3 2 2 3 2 5% 7% 63%	3402
November	1st to 10th	Day	3 4 2 3 3 2 3 3 5% 7% 64%	1853
	11th to 20th	Day	3 5% 1 3 3 3 3 3 5% 8% 63%	1890
	21st to end	Day	4 5% 2 5% 4 2 3 3 6% 7% 59%	1759
December	1st to 10th	Day	1 5% 2 4 3 3 2 3 4 7% 68%	2700
	11th to 20th	Day	2 4 1 3 2 2 3 2 5% 7% 69%	3106
	21st to end	Day	4 7% 2 6% 4 4 4 4 6% 8% 53%	4092
January	1st to 10th	Night	2 9% 3 6% 3 3 3 2 4 8% 57%	8201
	11th to 20th	Night	1 6% 3 9% 3 3 3 2 4 6% 59%	8210
	21st to end	Night	2 5% 3 8% 4 4 3 2 4 5% 59%	8370
February	1st to 10th	Night	2 9% 3 6% 4 3 4 3 5% 8% 54%	2730
	11th to 20th	Night	2 8% 3 7% 5% 4 4 3 5% 7% 51%	2495
	21st to end	Night	3 9% 2 6% 4 3 2 2 4 6% 59%	2076
March	1st to 10th	Night	5% 9% 3 5% 3 3 4 3 4 7% 54%	5494
	11th to 20th	Night	8% 8% 2 5% 3 3 3 3 5% 6% 55%	5165
	21st to end	Night	8% 9% 3 6% 4 3 3 3 5% 8% 49%	5246
April	1st to 10th	Night	5% 6% 2 5% 4 4 4 3 5% 7% 55%	2100
	11th to 20th	Night	11% 7% 3 4 3 3 4 4 5% 7% 51%	2077
	21st to end	Night	14% 7% 2 5% 3 3 3 3 5% 8% 46%	2100
May	1st to 10th	Night	15% 7% 2 5% 3 3 3 4 5% 7% 45%	5420
	11th to 20th	Night	11% 6% 2 7% 4 4 4 3 5% 7% 47%	4701
	21st to end	Night	10% 6% 2 5% 4 3 3 3 5% 7% 50%	5181
June	1st to 10th	Night	6% 3 1 3 4 3 3 3 6% 10% 57%	1680
	11th to 20th	Night	5% 3 1 4 3 4 3 3 5% 7% 62%	1680
	21st to end	Night	4 3 2 4 3 3 4 4 6% 7% 61%	1680
July	1st to 10th	Night	3 2 1 4 3 3 4 3 6% 8% 63%	4740
	11th to 20th	Night	3 2 1 3 3 3 4 5% 7% 10% 60%	4740
	21st to end	Night	4 4 2 5% 5% 5% 6% 6% 8% 10% 46%	5228
August	1st to 10th	Night	7% 4 2 5% 4 5% 5% 5% 7% 8% 49%	4123
	11th to 20th	Night	3 2 1 3 3 4 5% 5% 8% 10% 56%	4495
	21st to end	Night	7% 5% 2 6% 4 4 5% 4 6% 9% 48%	4993
September	1st to 10th	Night	6% 3 1 4 3 3 4 3 5% 7% 61%	2000
	11th to 20th	Night	7% 2 1 3 2 3 4 3 5% 7% 64%	2020
	21st to end	Night	7% 3 1 3 3 3 3 3 4 6% 64%	2200
October	1st to 10th	Night	5% 3 1 3 2 2 2 2 3 4 73%	3904
	11th to 20th	Night	6% 4 2 5% 3 3 3 3 5% 7% 60%	3898
	21st to end	Night	6% 6% 2 3 2 2 3 2 4 6% 64%	4366
November	1st to 10th	Night	4 4 2 4 2 2 3 3 5% 6% 66%	2665
	11th to 20th	Night	3 5% 1 4 3 3 2 3 5% 6% 64%	2730
	21st to end	Night	4 6% 2 4 3 3 3 3 6% 6% 61%	2857
December	1st to 10th	Night	1 4 2 3 3 2 2 2 4 5% 72%	4710
	11th to 20th	Night	1 4 2 5% 2 2 3 3 5% 9% 64%	5321
	21st to end	Night	6% 9% 3 6% 3 3 3 3 5% 7% 52%	7128

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	27% 5% 8% 30% 23% 6%	4723
	11th to 20th	Day	22% 6% 11% 28% 25% 9%	4837
	21st to end	Day	24% 4 10% 26% 26% 10%	5844
February	1st to 10th	Day	30% 4 12% 27% 20% 6%	1890
	11th to 20th	Day	25% 6% 11% 28% 25% 5%	2020
	21st to end	Day	24% 4 10% 24% 26% 12%	1730
March	1st to 10th	Day	24% 4 9% 19% 32% 13%	4600
	11th to 20th	Day	30% 7% 12% 21% 24% 6%	5050
	21st to end	Day	27% 8% 14% 34% 15% 2	5955
April	1st to 10th	Day	27% 5% 13% 25% 21% 8%	2520
	11th to 20th	Day	35% 6% 11% 20% 20% 8%	2494
	21st to end	Day	35% 7% 12% 24% 19% 3	2520
May	1st to 10th	Day	33% 8% 12% 25% 18% 4	7523
	11th to 20th	Day	33% 8% 12% 19% 20% 9%	8195
	21st to end	Day	30% 9% 14% 25% 16% 6%	9027
June	1st to 10th	Day	26% 10% 16% 23% 18% 8%	2940
	11th to 20th	Day	25% 7% 12% 18% 24% 14%	2940
	21st to end	Day	22% 7% 13% 22% 23% 13%	2940
July	1st to 10th	Day	21% 5% 9% 18% 30% 18%	8181
	11th to 20th	Day	21% 6% 11% 20% 20% 22%	8175
	21st to end	Day	30% 8% 14% 19% 16% 13%	8667
August	1st to 10th	Day	35% 8% 14% 18% 15% 9%	4338
	11th to 20th	Day	25% 8% 14% 19% 21% 14%	3946
	21st to end	Day	25% 6% 10% 18% 26% 14%	4311
September	1st to 10th	Day	21% 5% 9% 21% 26% 18%	2400
	11th to 20th	Day	21% 6% 9% 21% 26% 17%	2379
	21st to end	Day	22% 4 7% 20% 35% 12%	2200
October	1st to 10th	Day	17% 3 5% 16% 34% 24%	3282
	11th to 20th	Day	24% 3 10% 25% 28% 9%	3250
	21st to end	Day	23% 2 8% 24% 33% 10%	3402
November	1st to 10th	Day	21% 4 10% 26% 27% 13%	1853
	11th to 20th	Day	20% 5% 8% 25% 33% 8%	1890
	21st to end	Day	24% 4 9% 25% 29% 9%	1759
December	1st to 10th	Day	19% 3 9% 33% 27% 9%	2700
	11th to 20th	Day	17% 4 8% 32% 34% 6%	3104
	21st to end	Day	29% 6% 10% 25% 22% 8%	4092
January	1st to 10th	Night	28% 4 9% 31% 21% 7%	8202
	11th to 20th	Night	26% 6% 10% 23% 24% 11%	8210
	21st to end	Night	27% 5% 10% 23% 27% 8%	8370
February	1st to 10th	Night	28% 6% 11% 27% 23% 5%	2730
	11th to 20th	Night	32% 5% 11% 27% 21% 6%	2495
	21st to end	Night	28% 3 6% 24% 26% 13%	2076
March	1st to 10th	Night	30% 5% 6% 17% 33% 8%	5494
	11th to 20th	Night	29% 6% 11% 15% 27% 11%	5165
	21st to end	Night	33% 6% 12% 30% 16% 3	5246
April	1st to 10th	Night	28% 7% 11% 23% 22% 8%	2100
	11th to 20th	Night	33% 7% 12% 22% 19% 8%	2077
	21st to end	Night	38% 6% 10% 24% 18% 5%	2100
May	1st to 10th	Night	36% 9% 12% 25% 14% 4	5418
	11th to 20th	Night	34% 7% 15% 22% 15% 7%	4702
	21st to end	Night	33% 8% 18% 24% 14% 4	5182
June	1st to 10th	Night	23% 6% 19% 30% 14% 7%	1680
	11th to 20th	Night	22% 6% 13% 20% 22% 16%	1680
	21st to end	Night	21% 7% 11% 25% 27% 10%	1680
July	1st to 10th	Night	20% 4 10% 26% 32% 7%	4700
	11th to 20th	Night	19% 5% 16% 22% 23% 15%	4692
	21st to end	Night	30% 8% 18% 19% 17% 9%	5113
August	1st to 10th	Night	34% 8% 15% 20% 18% 5%	3018
	11th to 20th	Night	25% 9% 18% 21% 18% 10%	3267
	21st to end	Night	23% 6% 15% 23% 25% 8%	3597
September	1st to 10th	Night	22% 7% 10% 21% 22% 17%	2000
	11th to 20th	Night	20% 5% 11% 24% 23% 16%	2020
	21st to end	Night	22% 4 8% 22% 30% 13%	2200
October	1st to 10th	Night	18% 3 8% 18% 31% 23%	3904
	11th to 20th	Night	25% 4 10% 25% 27% 9%	3898
	21st to end	Night	24% 3 6% 24% 32% 11%	4366
November	1st to 10th	Night	20% 4 11% 25% 27% 13%	2665
	11th to 20th	Night	22% 4 7% 29% 31% 8%	2730
	21st to end	Night	24% 4 11% 24% 29% 8%	2857
December	1st to 10th	Night	17% 3 9% 35% 29% 6%	4710
	11th to 20th	Night	19% 4 7% 35% 31% 5%	5322
	21st to end	Night	31% 5% 10% 23% 26% 5%	7128

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	5% 8% 18% 44% 18% 5%	330
	11th to 20th	Day	6% 6% 28% 43% 15% 1	330
	21st to end	Day	3 4 29% 45% 17% 2	363
February	1st to 10th	Day	5% 9% 37% 39% 8% 2	330
	11th to 20th	Day	3 7% 34% 46% 8% 2	330
	21st to end	Day	4 10% 34% 42% 8% 1	273
March	1st to 10th	Day	5% 8% 32% 35% 16% 3	330
	11th to 20th	Day	10% 12% 30% 34% 10% 3	506
	21st to end	Day	10% 13% 37% 28% 10% 2	594
April	1st to 10th	Day	10% 13% 32% 30% 13% 2	540
	11th to 20th	Day	13% 16% 31% 30% 9% 2	532
	21st to end	Day	14% 13% 28% 29% 14% 2	540
May	1st to 10th	Day	13% 9% 25% 35% 14% 3	690
	11th to 20th	Day	16% 5% 20% 34% 18% 6%	775
	21st to end	Day	11% 6% 24% 30% 24% 5%	844
June	1st to 10th	Day	5% 4 18% 34% 28% 11%	780
	11th to 20th	Day	6% 3 24% 39% 23% 5%	780
	21st to end	Day	5% 4 21% 37% 26% 8%	780
July	1st to 10th	Day	5% 2 14% 30% 35% 13%	771
	11th to 20th	Day	4 1 12% 33% 36% 14%	768
	21st to end	Day	6% 2 13% 26% 40% 14%	819
August	1st to 10th	Day	7% 2 21% 34% 27% 9%	567
	11th to 20th	Day	3 2 18% 34% 30% 12%	534
	21st to end	Day	7% 3 17% 34% 30% 9%	583
September	1st to 10th	Day	7% 3 17% 33% 29% 12%	530
	11th to 20th	Day	5% 2 20% 35% 28% 10%	509
	21st to end	Day	8% 3 19% 30% 24% 16%	330
October	1st to 10th	Day	10% 5% 17% 26% 27% 15%	330
	11th to 20th	Day	3 5% 24% 43% 17% 8%	330
	21st to end	Day	8% 5% 27% 36% 20% 4	363
November	1st to 10th	Day	6% 7% 22% 42% 18% 7%	330
	11th to 20th	Day	5% 5% 26% 43% 16% 5%	330
	21st to end	Day	8% 5% 23% 45% 16% 3	330
December	1st to 10th	Day	3 9% 24% 43% 15% 5%	330
	11th to 20th	Day	3 7% 28% 49% 12% 2	330
	21st to end	Day	6% 8% 30% 44% 10% 3	363
January	1st to 10th	Night	4 8% 32% 43% 11% 2	1050
	11th to 20th	Night	3 8% 37% 40% 11% 1	1036
	21st to end	Night	5% 4 33% 50% 8%	924
February	1st to 10th	Night	6% 4 43% 40% 7% 1	840
	11th to 20th	Night	4 6% 38% 44% 9%	824
	21st to end	Night	5% 5% 40% 45% 4	692
March	1st to 10th	Night	4 5% 35% 46% 10%	840
	11th to 20th	Night	9% 6% 29% 44% 12% 1	840
	21st to end	Night	8% 7% 30% 47% 7% 1	767
April	1st to 10th	Night	8% 5% 35% 46% 6%	630
	11th to 20th	Night	13% 6% 33% 38% 9% 1	623
	21st to end	Night	16% 5% 29% 37% 12% 1	630
May	1st to 10th	Night	15% 5% 26% 39% 14% 1	630
	11th to 20th	Night	18% 3 16% 41% 19% 3	621
	21st to end	Night	10% 4 22% 39% 23% 2	683
June	1st to 10th	Night	7% 2 14% 49% 26% 3	630
	11th to 20th	Night	6% 2 21% 53% 18% 1	630
	21st to end	Night	4 3 14% 48% 28% 2	630
July	1st to 10th	Night	7% 1 10% 41% 36% 4	615
	11th to 20th	Night	3 1 7% 45% 40% 4	612
	21st to end	Night	6% 1 7% 36% 45% 5%	690
August	1st to 10th	Night	5% 1 11% 42% 37% 3	627
	11th to 20th	Night	4 1 7% 47% 38% 3	606
	21st to end	Night	4 1 9% 44% 37% 6%	660
September	1st to 10th	Night	6% 2 9% 46% 31% 6%	600
	11th to 20th	Night	7% 1 13% 47% 28% 4	600
	21st to end	Night	8% 2 15% 49% 24% 3	600
October	1st to 10th	Night	10% 2 18% 45% 20% 4	784
	11th to 20th	Night	7% 3 21% 50% 17% 2	800
	21st to end	Night	10% 3 24% 47% 14% 3	945
November	1st to 10th	Night	5% 3 29% 47% 13% 2	1025
	11th to 20th	Night	5% 6% 33% 45% 9% 1	1050
	21st to end	Night	6% 4 29% 47% 13% 1	1050
December	1st to 10th	Night	5% 6% 32% 46% 11%	1050
	11th to 20th	Night	5% 4 38% 43% 9% 1	1050
	21st to end	Night	6% 4 37% 42% 8% 1	1155

Page generated at 2026-08-12 01:37:17. Version ac07

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