Cherry Mountain Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cherry Mountain Observatory clear sky chart from 2005-11-06 to 2026-06-01.

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% 6% 2 5% 4 4 4 3 5% 7% 50%	132513
	Night	10% 6% 2 5% 3 3 3 3 5% 7% 53%	130983
Transparency	Day	33% 4 9% 21% 21% 12%	136331
	Night	31% 4 10% 20% 21% 14%	135229
Seeing	Day	13% 8% 25% 31% 18% 5%	16891
	Night	13% 8% 31% 34% 13% 1	26883

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	10% 6% 3 4 2 3 3 3 4 5% 57%	5289
	11th to 20th	Day	12% 6% 2 4 3 2 3 3 4 5% 58%	5310
	21st to end	Day	19% 6% 3 6% 4 3 3 3 4 6% 45%	6408
February	1st to 10th	Day	14% 5% 2 6% 3 3 3 2 5% 5% 51%	2100
	11th to 20th	Day	13% 6% 2 5% 3 3 4 4 5% 6% 49%	2049
	21st to end	Day	14% 6% 1 5% 3 2 3 2 4 5% 54%	1730
March	1st to 10th	Day	10% 6% 2 5% 3 3 3 2 5% 6% 55%	4860
	11th to 20th	Day	10% 6% 3 6% 3 3 3 3 4 5% 54%	5090
	21st to end	Day	10% 6% 2 6% 4 4 4 3 5% 6% 51%	5583
April	1st to 10th	Day	11% 7% 2 6% 5% 4 4 4 5% 7% 46%	2310
	11th to 20th	Day	16% 7% 2 6% 4 3 4 3 5% 6% 45%	2396
	21st to end	Day	11% 7% 2 6% 5% 4 4 3 4 6% 48%	2620
May	1st to 10th	Day	14% 7% 2 7% 4 4 5% 4 6% 7% 39%	5992
	11th to 20th	Day	9% 6% 3 6% 5% 4 5% 4 6% 8% 45%	5992
	21st to end	Day	12% 6% 2 6% 5% 5% 5% 5% 7% 8% 37%	6577
June	1st to 10th	Day	4 4 2 5% 4 4 5% 4 7% 10% 51%	2601
	11th to 20th	Day	4 5% 2 5% 4 4 4 4 7% 10% 51%	2600
	21st to end	Day	5% 5% 2 5% 5% 5% 6% 5% 7% 9% 46%	2600
July	1st to 10th	Day	13% 7% 2 7% 5% 5% 6% 4 6% 9% 37%	5900
	11th to 20th	Day	4 5% 2 5% 4 4 5% 4 7% 10% 50%	5900
	21st to end	Day	6% 5% 2 5% 4 4 5% 5% 7% 9% 48%	6422
August	1st to 10th	Day	1 1 1 3 2 3 3 3 5% 9% 68%	4076
	11th to 20th	Day	4 2 1 3 3 4 4 3 6% 10% 60%	3846
	21st to end	Day	3 3 1 5% 5% 6% 6% 5% 8% 11% 47%	4226
September	1st to 10th	Day	10% 5% 3 6% 5% 5% 4 4 5% 7% 47%	2200
	11th to 20th	Day	9% 6% 2 6% 5% 4 6% 4 6% 8% 44%	2200
	21st to end	Day	9% 3 1 5% 5% 4 4 3 6% 7% 52%	2200
October	1st to 10th	Day	5% 4 2 4 4 4 3 3 5% 7% 59%	2200
	11th to 20th	Day	10% 5% 1 4 3 3 4 3 5% 5% 56%	2120
	21st to end	Day	10% 5% 2 4 3 3 3 3 4 5% 60%	2200
November	1st to 10th	Day	12% 7% 2 5% 4 3 3 2 5% 6% 52%	2038
	11th to 20th	Day	7% 4 2 5% 4 3 4 3 6% 6% 56%	1890
	21st to end	Day	13% 7% 3 6% 3 3 3 2 3 7% 50%	1890
December	1st to 10th	Day	20% 9% 3 6% 3 2 2 2 5% 4 43%	3030
	11th to 20th	Day	12% 9% 2 6% 4 4 4 3 5% 6% 45%	3481
	21st to end	Day	10% 6% 2 4 2 2 2 2 3 4 61%	4587
January	1st to 10th	Night	10% 5% 3 6% 3 2 3 2 3 5% 58%	7634
	11th to 20th	Night	9% 3 2 3 2 2 2 2 5% 6% 66%	7680
	21st to end	Night	24% 6% 2 5% 3 3 3 2 4 5% 43%	7848
February	1st to 10th	Night	11% 6% 3 6% 4 3 3 2 4 5% 53%	2520
	11th to 20th	Night	13% 7% 2 5% 3 4 3 3 3 6% 51%	2458
	21st to end	Night	13% 6% 2 5% 3 3 3 3 4 5% 51%	2076
March	1st to 10th	Night	8% 7% 2 5% 4 4 4 3 6% 8% 48%	5219
	11th to 20th	Night	10% 6% 2 5% 4 4 3 3 5% 5% 53%	5090
	21st to end	Night	10% 10% 2 5% 3 3 3 3 4 6% 51%	5588
April	1st to 10th	Night	8% 7% 2 6% 4 3 4 4 5% 6% 50%	2310
	11th to 20th	Night	12% 8% 3 7% 4 4 3 3 4 5% 48%	2168
	21st to end	Night	12% 7% 2 5% 4 3 3 3 5% 7% 48%	1996
May	1st to 10th	Night	16% 9% 3 6% 4 4 3 3 4 6% 41%	4143
	11th to 20th	Night	6% 6% 2 6% 4 4 4 4 6% 7% 50%	4143
	21st to end	Night	11% 7% 3 6% 4 4 4 4 6% 10% 41%	4542
June	1st to 10th	Night	3 2 1 4 4 3 4 3 6% 11% 60%	1800
	11th to 20th	Night	4 3 1 4 4 3 3 3 6% 8% 62%	1800
	21st to end	Night	4 3 1 4 4 4 4 5% 5% 10% 56%	1800
July	1st to 10th	Night	10% 5% 2 5% 4 4 5% 4 6% 9% 44%	4078
	11th to 20th	Night	3 3 1 4 3 4 4 3 7% 10% 58%	4077
	21st to end	Night	5% 6% 2 4 4 3 4 4 7% 10% 51%	4449
August	1st to 10th	Night	1 1 2 2 2 3 3 4 6% 77%	3079
	11th to 20th	Night	3 2 1 3 3 2 3 4 6% 10% 63%	3213
	21st to end	Night	3 4 1 4 4 3 4 4 7% 10% 55%	3678
September	1st to 10th	Night	7% 5% 2 4 4 4 5% 4 6% 8% 51%	2200
	11th to 20th	Night	9% 6% 2 5% 4 4 4 4 7% 8% 48%	2200
	21st to end	Night	7% 4 2 4 3 4 4 3 7% 7% 55%	2200
October	1st to 10th	Night	4 3 1 4 3 3 3 3 4 7% 66%	2200
	11th to 20th	Night	9% 5% 1 4 3 3 3 3 4 6% 59%	2259
	21st to end	Night	11% 5% 1 4 3 3 2 2 3 3 62%	2640
November	1st to 10th	Night	10% 5% 2 5% 3 3 2 2 4 6% 58%	2476
	11th to 20th	Night	7% 4 2 5% 3 3 3 3 5% 6% 59%	2730
	21st to end	Night	14% 5% 2 5% 4 4 3 3 5% 6% 49%	2730
December	1st to 10th	Night	20% 6% 2 6% 3 4 3 3 4 5% 44%	4380
	11th to 20th	Night	16% 10% 3 5% 3 3 3 2 3 5% 48%	4946
	21st to end	Night	11% 7% 2 5% 2 2 2 2 3 4 60%	6633

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	32% 2 4 22% 27% 12%	5290
	11th to 20th	Day	31% 2 3 15% 28% 21%	5310
	21st to end	Day	43% 2 4 13% 25% 12%	6408
February	1st to 10th	Day	37% 3 7% 13% 22% 19%	2100
	11th to 20th	Day	36% 4 7% 20% 19% 15%	2059
	21st to end	Day	34% 3 5% 16% 25% 18%	1730
March	1st to 10th	Day	31% 3 5% 16% 18% 27%	4860
	11th to 20th	Day	33% 5% 5% 9% 15% 33%	5090
	21st to end	Day	34% 4 7% 19% 22% 14%	5585
April	1st to 10th	Day	40% 3 8% 17% 17% 14%	2310
	11th to 20th	Day	41% 4 7% 16% 21% 11%	2396
	21st to end	Day	39% 4 7% 21% 18% 10%	2620
May	1st to 10th	Day	42% 5% 10% 17% 19% 6%	6030
	11th to 20th	Day	36% 5% 10% 16% 22% 12%	5996
	21st to end	Day	42% 7% 9% 25% 14% 3	6590
June	1st to 10th	Day	27% 7% 12% 29% 21% 4	2601
	11th to 20th	Day	29% 5% 12% 26% 23% 6%	2600
	21st to end	Day	33% 7% 13% 25% 18% 5%	2600
July	1st to 10th	Day	42% 6% 16% 29% 6%	5836
	11th to 20th	Day	28% 7% 15% 33% 15% 2	5831
	21st to end	Day	29% 7% 17% 27% 19% 1	6240
August	1st to 10th	Day	14% 4 10% 40% 31% 1	4062
	11th to 20th	Day	21% 4 21% 39% 13% 2	3882
	21st to end	Day	29% 10% 17% 24% 18% 3	4225
September	1st to 10th	Day	38% 5% 9% 20% 18% 9%	2200
	11th to 20th	Day	38% 6% 11% 20% 19% 5%	2200
	21st to end	Day	31% 4 10% 23% 19% 14%	2200
October	1st to 10th	Day	16% 2 6% 13% 27% 36%	3590
	11th to 20th	Day	25% 3 5% 19% 25% 23%	3438
	21st to end	Day	24% 3 4 18% 37% 14%	3538
November	1st to 10th	Day	36% 2 7% 18% 23% 13%	2038
	11th to 20th	Day	30% 3 8% 19% 22% 19%	1890
	21st to end	Day	38% 2 6% 18% 22% 15%	1890
December	1st to 10th	Day	46% 2 5% 17% 20% 10%	3030
	11th to 20th	Day	39% 4 8% 11% 23% 15%	3479
	21st to end	Day	30% 2 3 16% 27% 22%	4587
January	1st to 10th	Night	31% 2 5% 18% 29% 15%	7635
	11th to 20th	Night	22% 2 3 17% 30% 26%	7680
	21st to end	Night	46% 2 3 10% 23% 16%	7848
February	1st to 10th	Night	36% 2 7% 12% 23% 20%	2520
	11th to 20th	Night	36% 4 6% 18% 20% 16%	2458
	21st to end	Night	36% 3 6% 13% 23% 20%	2076
March	1st to 10th	Night	34% 4 7% 18% 21% 18%	5219
	11th to 20th	Night	33% 4 6% 12% 16% 29%	5090
	21st to end	Night	35% 4 7% 19% 20% 15%	5588
April	1st to 10th	Night	35% 4 9% 19% 16% 17%	2310
	11th to 20th	Night	40% 4 8% 15% 18% 15%	2168
	21st to end	Night	39% 4 9% 19% 20% 10%	1996
May	1st to 10th	Night	45% 4 8% 19% 14% 10%	4170
	11th to 20th	Night	32% 6% 11% 19% 18% 15%	4144
	21st to end	Night	39% 6% 14% 26% 12% 4	4551
June	1st to 10th	Night	19% 7% 16% 34% 20% 3	1800
	11th to 20th	Night	22% 4 16% 30% 22% 6%	1800
	21st to end	Night	25% 5% 16% 31% 19% 4	1800
July	1st to 10th	Night	35% 8% 23% 29% 5% 1	4035
	11th to 20th	Night	22% 7% 21% 34% 15% 1	4026
	21st to end	Night	28% 7% 21% 27% 15% 3	4320
August	1st to 10th	Night	10% 4 13% 42% 29% 1	3069
	11th to 20th	Night	17% 6% 28% 36% 12% 2	3243
	21st to end	Night	23% 10% 20% 25% 19% 2	3678
September	1st to 10th	Night	31% 5% 13% 23% 18% 9%	2200
	11th to 20th	Night	34% 6% 15% 23% 17% 6%	2200
	21st to end	Night	28% 5% 12% 23% 19% 14%	2200
October	1st to 10th	Night	13% 2 9% 15% 23% 37%	3590
	11th to 20th	Night	21% 3 8% 19% 30% 20%	3667
	21st to end	Night	26% 2 4 19% 34% 16%	4252
November	1st to 10th	Night	31% 2 7% 20% 25% 15%	2476
	11th to 20th	Night	27% 3 8% 21% 22% 19%	2730
	21st to end	Night	37% 3 7% 16% 21% 16%	2730
December	1st to 10th	Night	44% 2 6% 15% 19% 12%	4380
	11th to 20th	Night	41% 2 5% 11% 21% 19%	4947
	21st to end	Night	31% 2 4 16% 28% 19%	6633

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	20% 15% 40% 21% 3 1	330
	11th to 20th	Day	22% 14% 35% 24% 6% 1	330
	21st to end	Day	19% 17% 40% 21% 3	363
February	1st to 10th	Day	23% 15% 36% 20% 6% 1	330
	11th to 20th	Day	20% 20% 40% 18% 2	330
	21st to end	Day	23% 16% 42% 16% 3	273
March	1st to 10th	Day	20% 19% 39% 19% 2	330
	11th to 20th	Day	19% 18% 37% 19% 6%	330
	21st to end	Day	16% 17% 44% 18% 5% 1	363
April	1st to 10th	Day	17% 17% 43% 19% 3	330
	11th to 20th	Day	22% 16% 40% 19% 3	434
	21st to end	Day	18% 15% 42% 19% 6%	640
May	1st to 10th	Day	17% 10% 43% 25% 5%	753
	11th to 20th	Day	17% 10% 36% 25% 11% 1	775
	21st to end	Day	18% 5% 30% 34% 12% 1	844
June	1st to 10th	Day	6% 1 25% 43% 21% 3	731
	11th to 20th	Day	6% 3 23% 49% 16% 2	730
	21st to end	Day	7% 1 15% 42% 29% 6%	730
July	1st to 10th	Day	10% 2 9% 41% 28% 10%	721
	11th to 20th	Day	5% 1 7% 46% 34% 7%	718
	21st to end	Day	4 1 7% 43% 35% 11%	794
August	1st to 10th	Day	2 3 43% 39% 12%	633
	11th to 20th	Day	9% 1 4 38% 36% 13%	517
	21st to end	Day	5% 1 6% 30% 42% 16%	536
September	1st to 10th	Day	11% 2 5% 31% 35% 16%	330
	11th to 20th	Day	6% 6% 32% 41% 15%	330
	21st to end	Day	10% 1 6% 37% 36% 10%	330
October	1st to 10th	Day	7% 2 12% 39% 32% 8%	330
	11th to 20th	Day	10% 3 27% 38% 18% 3	330
	21st to end	Day	19% 12% 27% 29% 12% 2	363
November	1st to 10th	Day	20% 6% 29% 32% 11% 2	330
	11th to 20th	Day	18% 9% 30% 28% 11% 4	330
	21st to end	Day	22% 16% 35% 19% 7% 2	330
December	1st to 10th	Day	18% 10% 35% 27% 7% 2	330
	11th to 20th	Day	18% 15% 35% 23% 7% 2	330
	21st to end	Day	16% 12% 39% 24% 9%	363
January	1st to 10th	Night	16% 12% 37% 28% 6%	1050
	11th to 20th	Night	16% 13% 36% 29% 5%	1050
	21st to end	Night	20% 12% 37% 24% 7%	961
February	1st to 10th	Night	18% 13% 40% 23% 5%	840
	11th to 20th	Night	19% 16% 40% 22% 3	824
	21st to end	Night	19% 16% 44% 20% 1	692
March	1st to 10th	Night	18% 17% 40% 24% 1	721
	11th to 20th	Night	17% 12% 41% 26% 5%	630
	21st to end	Night	13% 12% 49% 23% 3	693
April	1st to 10th	Night	12% 14% 49% 23% 3	630
	11th to 20th	Night	18% 14% 46% 20% 3	623
	21st to end	Night	17% 11% 48% 18% 6%	630
May	1st to 10th	Night	16% 9% 47% 25% 3	630
	11th to 20th	Night	13% 7% 41% 30% 8% 1	621
	21st to end	Night	13% 4 39% 33% 10%	683
June	1st to 10th	Night	4 1 30% 47% 18% 1	600
	11th to 20th	Night	6% 3 38% 42% 11%	600
	21st to end	Night	6% 1 21% 50% 20% 2	600
July	1st to 10th	Night	8% 13% 51% 25% 2	585
	11th to 20th	Night	4 1 12% 57% 25% 2	582
	21st to end	Night	4 12% 57% 24% 3	657
August	1st to 10th	Night	1 1 7% 60% 29% 3	597
	11th to 20th	Night	5% 1 7% 55% 29% 4	594
	21st to end	Night	5% 1 6% 50% 33% 5%	660
September	1st to 10th	Night	9% 10% 43% 32% 5%	600
	11th to 20th	Night	12% 7% 44% 34% 3	600
	21st to end	Night	9% 14% 49% 27% 2	600
October	1st to 10th	Night	5% 1 16% 49% 26% 2	600
	11th to 20th	Night	12% 2 27% 41% 16% 2	659
	21st to end	Night	15% 8% 30% 29% 15% 1	880
November	1st to 10th	Night	13% 6% 30% 36% 14% 1	836
	11th to 20th	Night	11% 7% 35% 33% 13% 1	1050
	21st to end	Night	17% 12% 33% 29% 9% 1	1050
December	1st to 10th	Night	16% 8% 35% 32% 8%	1050
	11th to 20th	Night	20% 9% 35% 29% 7%	1050
	21st to end	Night	15% 12% 37% 30% 7%	1155

Page generated at 2026-05-29 00:53:24. Version ac07

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