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 2025-02-06.

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%	109051
	Night	10% 6% 2 5% 3 3 3 3 5% 7% 53%	109442
Transparency	Day	33% 5% 9% 21% 21% 11%	112908
	Night	31% 4 10% 21% 21% 14%	113712
Seeing	Day	13% 7% 25% 32% 19% 5%	15130
	Night	13% 8% 31% 34% 13% 1	25204

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	12% 7% 4 5% 2 2 3 2 3 5% 55%	4077
	11th to 20th	Day	12% 6% 2 4 3 2 3 3 4 6% 57%	4080
	21st to end	Day	19% 7% 2 5% 3 2 2 2 4 6% 47%	4948
February	1st to 10th	Day	14% 5% 2 7% 3 3 3 2 5% 5% 51%	1950
	11th to 20th	Day	13% 6% 2 5% 3 4 4 4 5% 6% 48%	1849
	21st to end	Day	14% 6% 2 5% 3 3 3 3 5% 5% 53%	1570
March	1st to 10th	Day	13% 8% 2 6% 5% 3 3 3 5% 6% 47%	3306
	11th to 20th	Day	15% 8% 4 8% 5% 5% 5% 4 5% 6% 36%	3480
	21st to end	Day	9% 6% 2 7% 4 4 4 3 5% 7% 49%	3814
April	1st to 10th	Day	11% 6% 2 6% 5% 4 4 4 5% 7% 47%	2090
	11th to 20th	Day	15% 8% 2 5% 4 3 4 3 4 6% 46%	2165
	21st to end	Day	10% 7% 2 6% 6% 4 4 3 4 6% 49%	2370
May	1st to 10th	Day	14% 8% 3 8% 4 4 5% 4 6% 7% 37%	4082
	11th to 20th	Day	12% 8% 3 6% 5% 3 4 4 5% 6% 45%	4082
	21st to end	Day	9% 5% 2 7% 5% 5% 6% 5% 7% 8% 40%	4477
June	1st to 10th	Day	4 4 2 5% 4 4 5% 4 7% 10% 51%	2470
	11th to 20th	Day	4 4 2 5% 4 4 4 4 7% 10% 51%	2470
	21st to end	Day	5% 5% 2 5% 5% 5% 6% 5% 7% 9% 45%	2470
July	1st to 10th	Day	5% 4 1 5% 5% 5% 6% 4 7% 9% 47%	4120
	11th to 20th	Day	3 3 1 4 4 4 4 4 6% 10% 56%	4120
	21st to end	Day	8% 5% 2 5% 5% 5% 5% 5% 7% 10% 44%	4465
August	1st to 10th	Day	1 1 1 3 3 3 3 3 6% 9% 68%	3951
	11th to 20th	Day	4 2 1 3 3 4 3 3 6% 10% 60%	3726
	21st to end	Day	3 3 1 5% 5% 5% 6% 5% 8% 11% 47%	4097
September	1st to 10th	Day	9% 5% 3 7% 5% 5% 4 3 5% 7% 46%	2090
	11th to 20th	Day	10% 6% 2 6% 5% 4 6% 4 6% 7% 44%	2090
	21st to end	Day	9% 3 1 5% 4 4 4 3 6% 7% 51%	2090
October	1st to 10th	Day	5% 4 2 4 4 4 3 3 6% 7% 58%	2090
	11th to 20th	Day	11% 5% 1 4 3 4 4 3 5% 5% 55%	2014
	21st to end	Day	10% 5% 2 4 3 3 3 3 4 5% 59%	2090
November	1st to 10th	Day	12% 7% 2 5% 4 3 3 2 5% 6% 50%	1939
	11th to 20th	Day	7% 4 2 5% 4 3 4 4 6% 6% 55%	1800
	21st to end	Day	13% 7% 3 6% 3 3 3 2 3 7% 51%	1800
December	1st to 10th	Day	20% 9% 3 6% 3 2 2 2 5% 4 43%	2940
	11th to 20th	Day	12% 9% 2 6% 4 4 4 3 5% 6% 45%	3391
	21st to end	Day	10% 6% 2 4 3 2 2 2 3 4 61%	4488
January	1st to 10th	Night	13% 5% 3 6% 2 2 2 2 3 5% 55%	5888
	11th to 20th	Night	10% 3 1 2 2 1 2 2 4 6% 65%	5900
	21st to end	Night	25% 6% 2 5% 3 3 3 2 4 5% 43%	6058
February	1st to 10th	Night	11% 6% 3 6% 4 3 3 2 4 5% 53%	2340
	11th to 20th	Night	13% 7% 2 5% 3 4 3 2 3 5% 52%	2218
	21st to end	Night	13% 7% 2 5% 3 3 4 3 5% 6% 50%	1884
March	1st to 10th	Night	8% 7% 3 7% 5% 5% 4 4 5% 7% 45%	3549
	11th to 20th	Night	15% 8% 2 7% 5% 5% 4 4 6% 6% 37%	3480
	21st to end	Night	8% 7% 2 7% 4 4 4 3 4 6% 50%	3817
April	1st to 10th	Night	7% 6% 2 6% 5% 3 4 4 6% 6% 51%	2090
	11th to 20th	Night	10% 7% 3 7% 4 4 3 3 4 6% 49%	1960
	21st to end	Night	10% 6% 2 5% 5% 3 3 3 5% 7% 51%	1806
May	1st to 10th	Night	15% 10% 3 7% 4 4 3 3 4 6% 40%	2823
	11th to 20th	Night	8% 7% 3 6% 4 3 4 3 5% 7% 50%	2823
	21st to end	Night	9% 6% 3 6% 4 4 4 4 7% 8% 47%	3090
June	1st to 10th	Night	2 2 1 4 4 3 4 3 6% 10% 61%	1710
	11th to 20th	Night	4 3 1 4 4 3 3 3 6% 8% 62%	1710
	21st to end	Night	4 3 1 4 4 3 4 5% 5% 10% 56%	1710
July	1st to 10th	Night	4 3 2 4 4 4 4 4 6% 10% 56%	2850
	11th to 20th	Night	2 3 1 4 3 3 4 3 6% 9% 63%	2850
	21st to end	Night	6% 8% 2 5% 4 3 4 4 6% 10% 47%	3098
August	1st to 10th	Night	1 1 2 2 2 3 3 4 6% 76%	2984
	11th to 20th	Night	3 2 1 3 3 2 3 4 6% 10% 62%	3113
	21st to end	Night	3 4 1 4 4 3 4 4 7% 10% 55%	3566
September	1st to 10th	Night	7% 4 2 4 4 4 5% 4 6% 8% 52%	2090
	11th to 20th	Night	9% 6% 2 5% 4 4 4 4 7% 8% 47%	2090
	21st to end	Night	7% 4 2 4 3 4 4 3 6% 7% 55%	2090
October	1st to 10th	Night	4 3 1 4 3 3 3 3 4 7% 65%	2090
	11th to 20th	Night	9% 5% 2 4 3 4 3 3 4 6% 57%	2146
	21st to end	Night	12% 5% 2 4 3 3 2 2 3 3 61%	2508
November	1st to 10th	Night	11% 5% 2 5% 3 3 2 2 4 6% 56%	2355
	11th to 20th	Night	7% 4 2 5% 3 3 4 3 5% 6% 59%	2600
	21st to end	Night	13% 5% 2 5% 4 4 3 3 5% 6% 50%	2600
December	1st to 10th	Night	20% 6% 2 6% 4 4 3 3 4 5% 43%	4250
	11th to 20th	Night	16% 10% 3 5% 3 3 3 2 3 5% 48%	4816
	21st to end	Night	11% 7% 2 5% 2 2 2 2 3 4 60%	6490

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	35% 3 3 21% 24% 15%	4078
	11th to 20th	Day	31% 2 4 16% 27% 19%	4080
	21st to end	Day	41% 2 4 10% 27% 15%	4948
February	1st to 10th	Day	37% 3 7% 13% 22% 19%	1950
	11th to 20th	Day	37% 3 6% 18% 19% 16%	1859
	21st to end	Day	35% 3 6% 16% 22% 18%	1570
March	1st to 10th	Day	39% 4 7% 13% 20% 17%	3306
	11th to 20th	Day	48% 7% 7% 11% 16% 10%	3480
	21st to end	Day	34% 5% 9% 17% 21% 14%	3814
April	1st to 10th	Day	39% 3 8% 18% 18% 14%	2090
	11th to 20th	Day	40% 4 7% 17% 22% 11%	2165
	21st to end	Day	38% 4 6% 22% 20% 11%	2370
May	1st to 10th	Day	45% 6% 11% 14% 18% 7%	4120
	11th to 20th	Day	39% 6% 7% 17% 21% 10%	4086
	21st to end	Day	38% 8% 10% 27% 13% 4	4489
June	1st to 10th	Day	27% 7% 12% 28% 22% 4	2470
	11th to 20th	Day	29% 5% 12% 26% 22% 6%	2470
	21st to end	Day	33% 7% 13% 24% 18% 5%	2470
July	1st to 10th	Day	31% 6% 16% 38% 8%	4056
	11th to 20th	Day	22% 6% 15% 36% 18% 4	4051
	21st to end	Day	32% 7% 17% 30% 13% 1	4325
August	1st to 10th	Day	14% 4 10% 40% 31% 1	3937
	11th to 20th	Day	20% 4 21% 39% 13% 2	3762
	21st to end	Day	29% 10% 17% 24% 18% 2	4096
September	1st to 10th	Day	38% 5% 10% 20% 18% 9%	2090
	11th to 20th	Day	39% 6% 10% 20% 19% 6%	2090
	21st to end	Day	31% 4 10% 22% 19% 13%	2090
October	1st to 10th	Day	16% 2 6% 12% 27% 36%	3480
	11th to 20th	Day	25% 3 5% 19% 25% 23%	3332
	21st to end	Day	24% 3 4 18% 37% 13%	3428
November	1st to 10th	Day	38% 2 8% 19% 21% 12%	1939
	11th to 20th	Day	30% 4 8% 18% 22% 19%	1800
	21st to end	Day	37% 2 6% 18% 22% 15%	1800
December	1st to 10th	Day	46% 2 5% 17% 19% 10%	2940
	11th to 20th	Day	39% 4 8% 12% 23% 15%	3389
	21st to end	Day	30% 2 3 16% 27% 22%	4488
January	1st to 10th	Night	34% 2 4 17% 28% 15%	5888
	11th to 20th	Night	22% 2 3 19% 29% 26%	5900
	21st to end	Night	45% 2 3 10% 21% 18%	6058
February	1st to 10th	Night	36% 2 7% 12% 22% 20%	2340
	11th to 20th	Night	36% 3 5% 17% 21% 18%	2218
	21st to end	Night	37% 3 6% 12% 22% 19%	1884
March	1st to 10th	Night	37% 5% 8% 14% 23% 13%	3549
	11th to 20th	Night	46% 6% 8% 13% 15% 11%	3480
	21st to end	Night	35% 5% 9% 18% 18% 14%	3817
April	1st to 10th	Night	34% 4 9% 20% 17% 17%	2090
	11th to 20th	Night	38% 4 8% 15% 19% 16%	1960
	21st to end	Night	36% 4 9% 20% 22% 10%	1806
May	1st to 10th	Night	46% 4 7% 20% 13% 10%	2850
	11th to 20th	Night	32% 7% 10% 23% 18% 11%	2824
	21st to end	Night	33% 6% 13% 29% 13% 5%	3099
June	1st to 10th	Night	19% 7% 17% 34% 21% 3	1710
	11th to 20th	Night	21% 4 16% 30% 22% 6%	1710
	21st to end	Night	25% 5% 15% 31% 19% 4	1710
July	1st to 10th	Night	22% 8% 24% 39% 7% 1	2805
	11th to 20th	Night	19% 6% 18% 37% 18% 2	2796
	21st to end	Night	31% 7% 22% 30% 10%	2999
August	1st to 10th	Night	11% 5% 13% 42% 29% 1	2974
	11th to 20th	Night	17% 6% 27% 36% 12% 2	3143
	21st to end	Night	23% 10% 20% 26% 19% 2	3566
September	1st to 10th	Night	30% 5% 14% 23% 18% 9%	2090
	11th to 20th	Night	34% 6% 15% 22% 17% 6%	2090
	21st to end	Night	28% 5% 12% 22% 19% 14%	2090
October	1st to 10th	Night	13% 2 9% 15% 22% 38%	3480
	11th to 20th	Night	22% 3 8% 18% 30% 20%	3554
	21st to end	Night	26% 2 4 19% 34% 16%	4120
November	1st to 10th	Night	32% 2 7% 20% 23% 15%	2355
	11th to 20th	Night	28% 3 8% 21% 22% 19%	2600
	21st to end	Night	37% 3 7% 16% 21% 16%	2600
December	1st to 10th	Night	45% 2 6% 16% 19% 12%	4250
	11th to 20th	Night	41% 3 5% 11% 21% 19%	4817
	21st to end	Night	31% 2 3 16% 28% 19%	6490

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	21% 15% 40% 20% 3 1	300
	11th to 20th	Day	21% 14% 35% 24% 6% 1	300
	21st to end	Day	18% 17% 39% 22% 3	330
February	1st to 10th	Day	24% 16% 34% 21% 6%	285
	11th to 20th	Day	22% 19% 40% 16% 3	270
	21st to end	Day	24% 18% 40% 16% 2	225
March	1st to 10th	Day	17% 19% 41% 20% 3	270
	11th to 20th	Day	20% 18% 36% 20% 7%	270
	21st to end	Day	18% 19% 45% 14% 4	297
April	1st to 10th	Day	17% 18% 44% 18% 3	270
	11th to 20th	Day	21% 15% 41% 20% 4	363
	21st to end	Day	17% 14% 42% 20% 6%	550
May	1st to 10th	Day	15% 11% 43% 25% 6%	653
	11th to 20th	Day	18% 8% 34% 26% 12% 1	675
	21st to end	Day	18% 5% 30% 33% 13% 1	734
June	1st to 10th	Day	6% 1 23% 43% 23% 4	680
	11th to 20th	Day	6% 2 23% 50% 17% 3	680
	21st to end	Day	7% 1 14% 43% 28% 6%	680
July	1st to 10th	Day	8% 1 9% 42% 29% 10%	671
	11th to 20th	Day	4 1 6% 46% 35% 8%	668
	21st to end	Day	4 1 6% 43% 35% 11%	739
August	1st to 10th	Day	2 4 44% 39% 12%	588
	11th to 20th	Day	9% 1 4 38% 36% 12%	477
	21st to end	Day	5% 1 5% 30% 43% 16%	495
September	1st to 10th	Day	10% 2 4 29% 37% 18%	300
	11th to 20th	Day	7% 6% 30% 41% 16%	300
	21st to end	Day	11% 1 5% 36% 37% 10%	300
October	1st to 10th	Day	7% 2 12% 40% 30% 8%	300
	11th to 20th	Day	11% 3 27% 39% 17% 3	300
	21st to end	Day	20% 12% 27% 28% 12% 2	330
November	1st to 10th	Day	21% 7% 30% 32% 9% 2	300
	11th to 20th	Day	18% 10% 31% 25% 11% 5%	300
	21st to end	Day	21% 16% 34% 20% 7% 2	300
December	1st to 10th	Day	19% 9% 35% 28% 8% 2	300
	11th to 20th	Day	18% 17% 36% 23% 6% 1	300
	21st to end	Day	17% 13% 40% 22% 8%	330
January	1st to 10th	Night	17% 12% 37% 28% 6% 1	1000
	11th to 20th	Night	17% 14% 36% 28% 5%	1000
	21st to end	Night	19% 11% 38% 24% 7%	915
February	1st to 10th	Night	19% 13% 39% 23% 5%	780
	11th to 20th	Night	18% 15% 42% 22% 2	744
	21st to end	Night	20% 16% 45% 18% 1	628
March	1st to 10th	Night	17% 16% 41% 25% 1	653
	11th to 20th	Night	17% 11% 41% 27% 5%	570
	21st to end	Night	13% 13% 51% 21% 3	627
April	1st to 10th	Night	10% 15% 50% 22% 3	570
	11th to 20th	Night	16% 13% 48% 20% 3	563
	21st to end	Night	15% 9% 51% 19% 6%	570
May	1st to 10th	Night	15% 9% 48% 25% 3	570
	11th to 20th	Night	12% 6% 41% 31% 9% 1	561
	21st to end	Night	14% 4 40% 33% 10%	617
June	1st to 10th	Night	3 1 28% 48% 19% 1	570
	11th to 20th	Night	5% 3 38% 43% 11%	570
	21st to end	Night	6% 1 21% 49% 21% 2	570
July	1st to 10th	Night	8% 13% 52% 26% 2	555
	11th to 20th	Night	3 12% 58% 25% 2	552
	21st to end	Night	4 10% 59% 23% 3	624
August	1st to 10th	Night	1 1 7% 60% 29% 3	567
	11th to 20th	Night	5% 6% 55% 29% 4	564
	21st to end	Night	5% 1 6% 50% 32% 5%	627
September	1st to 10th	Night	10% 10% 44% 31% 5%	570
	11th to 20th	Night	13% 7% 42% 34% 3	570
	21st to end	Night	9% 14% 47% 28% 2	570
October	1st to 10th	Night	6% 1 17% 50% 24% 2	570
	11th to 20th	Night	12% 2 27% 42% 15% 2	626
	21st to end	Night	16% 8% 31% 29% 16% 1	836
November	1st to 10th	Night	14% 5% 30% 36% 14% 1	795
	11th to 20th	Night	11% 8% 34% 33% 13% 1	1000
	21st to end	Night	16% 12% 33% 29% 9% 1	1000
December	1st to 10th	Night	16% 8% 35% 33% 8% 1	1000
	11th to 20th	Night	20% 9% 35% 29% 6%	1000
	21st to end	Night	15% 12% 37% 30% 6%	1100

Page generated at 2025-02-03 02:05:54. Version ac07

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