Mountain Sky Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mountain Sky Observatory clear sky chart from 2005-11-06 to 2026-01-02.

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	3 4 2 5% 4 5% 5% 5% 7% 8% 53%	135869
	Night	3 4 2 5% 3 3 4 3 6% 7% 60%	132957
Transparency	Day	23% 9% 13% 25% 22% 8%	130819
	Night	20% 8% 14% 27% 23% 7%	129011
Seeing	Day	9% 21% 31% 25% 11% 2	16955
	Night	6% 14% 33% 32% 14% 2	27101

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	3 2 8% 4 4 4 4 5% 7% 60%	3646
	11th to 20th	Day	2 1 3 3 4 5% 4 7% 10% 59%	4097
	21st to end	Day	1 4 2 6% 4 4 4 3 6% 7% 60%	4510
February	1st to 10th	Day	5% 2 7% 3 3 4 3 6% 8% 59%	1800
	11th to 20th	Day	1 5% 3 5% 3 3 3 4 6% 9% 58%	1899
	21st to end	Day	1 5% 3 6% 4 3 4 4 5% 8% 58%	1650
March	1st to 10th	Day	1 4 2 5% 4 4 4 4 6% 8% 58%	4638
	11th to 20th	Day	1 9% 3 8% 5% 5% 4 4 6% 8% 46%	5000
	21st to end	Day	1 7% 2 5% 4 4 6% 4 6% 8% 53%	5700
April	1st to 10th	Day	2 6% 3 5% 4 4 4 4 6% 9% 55%	2400
	11th to 20th	Day	3 8% 2 6% 5% 4 5% 4 6% 8% 50%	2371
	21st to end	Day	3 6% 2 5% 5% 5% 5% 4 7% 9% 48%	2400
May	1st to 10th	Day	6% 8% 2 7% 5% 5% 5% 5% 7% 9% 41%	5454
	11th to 20th	Day	3 5% 2 6% 5% 5% 6% 5% 7% 8% 48%	6288
	21st to end	Day	2 3 2 5% 4 5% 6% 5% 8% 8% 52%	6925
June	1st to 10th	Day	3 4 1 5% 5% 4 6% 6% 9% 9% 47%	2800
	11th to 20th	Day	4 3 2 4 3 4 4 5% 7% 7% 58%	2800
	21st to end	Day	3 3 2 4 5% 5% 6% 5% 8% 9% 49%	2800
July	1st to 10th	Day	3 2 1 4 4 4 6% 5% 7% 8% 57%	6339
	11th to 20th	Day	2 3 1 5% 6% 7% 9% 8% 10% 10% 40%	6299
	21st to end	Day	3 4 2 7% 6% 7% 6% 6% 9% 9% 40%	6401
August	1st to 10th	Day	5% 4 2 6% 6% 7% 7% 6% 10% 10% 37%	5779
	11th to 20th	Day	2 3 1 5% 4 6% 7% 6% 9% 11% 46%	5395
	21st to end	Day	5% 5% 2 7% 6% 6% 7% 7% 9% 10% 36%	5984
September	1st to 10th	Day	5% 4 2 5% 5% 4 5% 5% 7% 8% 51%	2400
	11th to 20th	Day	5% 4 2 5% 4 4 5% 4 6% 7% 54%	2400
	21st to end	Day	6% 3 1 3 3 3 4 4 5% 6% 62%	2166
October	1st to 10th	Day	3 3 1 3 3 2 2 2 4 5% 72%	3270
	11th to 20th	Day	7% 4 1 3 3 4 4 4 5% 8% 57%	3250
	21st to end	Day	2 3 1 3 2 3 2 2 4 6% 71%	3548
November	1st to 10th	Day	1 3 1 3 3 2 3 2 5% 5% 71%	1887
	11th to 20th	Day	1 2 1 3 1 2 2 2 4 7% 75%	1890
	21st to end	Day	2 5% 2 5% 2 3 3 3 3 7% 65%	1785
December	1st to 10th	Day	1 3 2 4 2 2 3 3 3 6% 73%	2700
	11th to 20th	Day	1 3 1 3 2 2 2 3 5% 10% 68%	3106
	21st to end	Day	1 4 2 4 3 4 4 3 6% 9% 60%	4092
January	1st to 10th	Night	3 3 8% 4 4 4 4 5% 6% 57%	6342
	11th to 20th	Night	2 1 6% 4 3 3 3 4 6% 68%	5926
	21st to end	Night	1 5% 3 5% 3 4 3 3 5% 7% 63%	6468
February	1st to 10th	Night	1 5% 2 5% 3 2 3 3 4 7% 65%	2600
	11th to 20th	Night	2 6% 2 4 3 3 2 2 4 7% 65%	2385
	21st to end	Night	1 7% 2 5% 2 3 3 2 5% 8% 61%	1980
March	1st to 10th	Night	1 7% 2 5% 4 3 4 3 4 6% 62%	5225
	11th to 20th	Night	1 7% 2 5% 3 4 5% 4 6% 6% 56%	5000
	21st to end	Night	1 5% 2 5% 3 4 5% 4 5% 6% 59%	5265
April	1st to 10th	Night	2 5% 2 5% 3 3 4 3 5% 7% 61%	2000
	11th to 20th	Night	4 7% 2 5% 4 4 5% 3 5% 6% 56%	1977
	21st to end	Night	5% 7% 2 5% 3 3 3 4 5% 6% 57%	2000
May	1st to 10th	Night	6% 6% 2 4 4 4 4 3 6% 8% 52%	4510
	11th to 20th	Night	5% 5% 2 5% 3 3 4 4 5% 7% 56%	3624
	21st to end	Night	4 3 1 3 3 3 3 3 5% 6% 65%	3972
June	1st to 10th	Night	3 4 1 3 3 3 3 5% 7% 8% 60%	1600
	11th to 20th	Night	3 3 1 3 3 3 3 4 6% 6% 66%	1600
	21st to end	Night	3 3 2 4 4 5% 5% 5% 6% 7% 57%	1600
July	1st to 10th	Night	4 3 1 5% 4 4 5% 4 7% 8% 57%	3640
	11th to 20th	Night	2 3 1 4 4 5% 5% 5% 10% 12% 48%	3676
	21st to end	Night	3 3 2 5% 5% 5% 6% 6% 9% 10% 45%	4472
August	1st to 10th	Night	5% 4 2 5% 4 4 5% 5% 8% 11% 48%	4033
	11th to 20th	Night	2 2 1 4 4 5% 6% 5% 9% 12% 49%	4441
	21st to end	Night	7% 6% 2 7% 5% 5% 6% 6% 8% 10% 37%	4993
September	1st to 10th	Night	6% 3 2 5% 4 4 4 4 7% 8% 54%	2000
	11th to 20th	Night	6% 4 2 3 3 3 3 3 7% 8% 59%	2000
	21st to end	Night	6% 5% 1 4 3 2 3 2 4 7% 63%	2228
October	1st to 10th	Night	4 3 1 3 2 3 2 3 3 5% 71%	3920
	11th to 20th	Night	4 4 1 4 3 3 4 3 5% 7% 60%	3898
	21st to end	Night	4 3 1 3 1 2 2 2 3 5% 74%	4252
November	1st to 10th	Night	2 4 1 2 2 2 2 3 4 7% 72%	2610
	11th to 20th	Night	1 2 1 2 1 2 3 2 4 6% 76%	2730
	21st to end	Night	3 6% 2 4 2 2 2 2 4 7% 65%	2831
December	1st to 10th	Night	1 2 1 3 2 2 2 2 4 6% 76%	4710
	11th to 20th	Night	1 3 1 3 2 2 2 2 4 7% 73%	5321
	21st to end	Night	1 4 2 5% 3 3 4 4 5% 7% 62%	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	22% 6% 15% 33% 22% 2	3647
	11th to 20th	Day	14% 15% 14% 20% 27% 9%	4097
	21st to end	Day	20% 9% 9% 30% 23% 8%	4510
February	1st to 10th	Day	22% 9% 18% 26% 18% 6%	1800
	11th to 20th	Day	21% 9% 14% 29% 23% 4	1909
	21st to end	Day	24% 8% 9% 23% 28% 7%	1650
March	1st to 10th	Day	17% 9% 7% 21% 35% 12%	4638
	11th to 20th	Day	29% 9% 10% 29% 19% 4	5000
	21st to end	Day	22% 9% 18% 35% 15% 2	5700
April	1st to 10th	Day	24% 10% 12% 24% 23% 8%	2400
	11th to 20th	Day	26% 11% 11% 24% 22% 5%	2374
	21st to end	Day	26% 13% 12% 25% 23% 3	2400
May	1st to 10th	Day	30% 10% 14% 22% 17% 7%	5454
	11th to 20th	Day	25% 12% 11% 27% 18% 7%	6292
	21st to end	Day	19% 12% 13% 23% 27% 7%	6926
June	1st to 10th	Day	25% 12% 14% 22% 19% 8%	2800
	11th to 20th	Day	20% 9% 10% 17% 24% 20%	2800
	21st to end	Day	23% 11% 11% 20% 21% 14%	2800
July	1st to 10th	Day	20% 8% 8% 15% 23% 26%	6271
	11th to 20th	Day	28% 9% 22% 26% 11% 4	6224
	21st to end	Day	30% 10% 18% 20% 15% 6%	6217
August	1st to 10th	Day	38% 11% 20% 15% 11% 4	4208
	11th to 20th	Day	27% 11% 16% 18% 21% 7%	3899
	21st to end	Day	31% 11% 17% 17% 18% 5%	4311
September	1st to 10th	Day	24% 10% 13% 21% 22% 10%	2400
	11th to 20th	Day	23% 8% 12% 20% 27% 9%	2400
	21st to end	Day	20% 7% 8% 24% 34% 7%	2166
October	1st to 10th	Day	15% 5% 5% 19% 40% 15%	3270
	11th to 20th	Day	23% 5% 8% 33% 25% 6%	3250
	21st to end	Day	14% 5% 7% 37% 30% 8%	3548
November	1st to 10th	Day	14% 6% 11% 28% 36% 5%	1887
	11th to 20th	Day	10% 7% 11% 38% 30% 5%	1890
	21st to end	Day	21% 5% 11% 32% 24% 8%	1785
December	1st to 10th	Day	14% 6% 14% 40% 22% 5%	2700
	11th to 20th	Day	14% 6% 18% 38% 22% 2	3104
	21st to end	Day	18% 8% 16% 29% 22% 8%	4092
January	1st to 10th	Night	23% 9% 13% 31% 21% 4	6342
	11th to 20th	Night	13% 11% 14% 25% 24% 12%	5926
	21st to end	Night	20% 9% 11% 27% 23% 10%	6468
February	1st to 10th	Night	21% 6% 18% 27% 20% 8%	2600
	11th to 20th	Night	21% 7% 14% 30% 24% 4	2385
	21st to end	Night	20% 7% 9% 25% 32% 7%	1980
March	1st to 10th	Night	18% 10% 7% 20% 38% 7%	5225
	11th to 20th	Night	24% 9% 10% 28% 26% 4	5000
	21st to end	Night	21% 8% 17% 37% 15% 1	5265
April	1st to 10th	Night	22% 8% 11% 29% 25% 6%	2000
	11th to 20th	Night	25% 9% 12% 26% 22% 6%	1977
	21st to end	Night	24% 9% 13% 27% 23% 3	2000
May	1st to 10th	Night	25% 8% 19% 21% 22% 4	4507
	11th to 20th	Night	23% 8% 13% 30% 17% 8%	3625
	21st to end	Night	17% 7% 19% 20% 30% 8%	3972
June	1st to 10th	Night	19% 9% 16% 25% 19% 12%	1600
	11th to 20th	Night	16% 6% 11% 21% 23% 23%	1600
	21st to end	Night	22% 10% 11% 19% 23% 15%	1600
July	1st to 10th	Night	22% 8% 11% 16% 22% 21%	3600
	11th to 20th	Night	23% 9% 31% 25% 6% 5%	3628
	21st to end	Night	28% 11% 23% 21% 13% 5%	4344
August	1st to 10th	Night	37% 12% 25% 12% 9% 6%	2928
	11th to 20th	Night	26% 14% 22% 17% 16% 5%	3213
	21st to end	Night	32% 13% 17% 20% 15% 3	3597
September	1st to 10th	Night	25% 10% 14% 22% 20% 9%	2000
	11th to 20th	Night	21% 9% 13% 20% 26% 12%	2000
	21st to end	Night	22% 6% 9% 24% 31% 8%	2228
October	1st to 10th	Night	15% 6% 6% 24% 32% 16%	3920
	11th to 20th	Night	22% 5% 10% 28% 28% 7%	3898
	21st to end	Night	15% 4 5% 35% 32% 8%	4252
November	1st to 10th	Night	12% 6% 11% 30% 33% 7%	2610
	11th to 20th	Night	11% 5% 10% 38% 30% 6%	2730
	21st to end	Night	20% 5% 9% 30% 29% 7%	2831
December	1st to 10th	Night	12% 4 12% 48% 21% 3	4710
	11th to 20th	Night	13% 6% 12% 43% 24% 3	5322
	21st to end	Night	19% 7% 15% 27% 26% 6%	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	12% 23% 37% 23% 4 1	303
	11th to 20th	Day	12% 31% 37% 17% 3	300
	21st to end	Day	5% 24% 43% 22% 5% 1	330
February	1st to 10th	Day	5% 41% 42% 11% 1 1	300
	11th to 20th	Day	15% 37% 32% 13% 3	300
	21st to end	Day	16% 37% 34% 11% 2	249
March	1st to 10th	Day	10% 37% 29% 22% 2	360
	11th to 20th	Day	13% 34% 30% 18% 5%	500
	21st to end	Day	14% 41% 31% 11% 2	550
April	1st to 10th	Day	15% 37% 32% 12% 3	500
	11th to 20th	Day	19% 38% 29% 9% 4	492
	21st to end	Day	13% 37% 33% 13% 3 1	500
May	1st to 10th	Day	17% 32% 31% 16% 4	503
	11th to 20th	Day	12% 20% 30% 26% 10% 1	715
	21st to end	Day	10% 21% 29% 28% 11% 2	789
June	1st to 10th	Day	4 15% 28% 32% 18% 3	730
	11th to 20th	Day	7% 14% 35% 28% 13% 3	730
	21st to end	Day	5% 15% 30% 29% 18% 4	730
July	1st to 10th	Day	6% 7% 23% 38% 22% 4	721
	11th to 20th	Day	7% 3 18% 39% 27% 6%	703
	21st to end	Day	8% 4 22% 37% 25% 3	575
August	1st to 10th	Day	9% 6% 30% 36% 17% 3	527
	11th to 20th	Day	5% 8% 34% 33% 16% 3	517
	21st to end	Day	8% 8% 30% 33% 17% 4	583
September	1st to 10th	Day	8% 9% 27% 33% 18% 5%	530
	11th to 20th	Day	8% 15% 35% 26% 13% 3	530
	21st to end	Day	7% 17% 24% 32% 16% 5%	352
October	1st to 10th	Day	9% 21% 27% 23% 17% 4	330
	11th to 20th	Day	11% 25% 31% 20% 10% 4	330
	21st to end	Day	12% 25% 30% 24% 8% 1	363
November	1st to 10th	Day	8% 27% 36% 22% 7% 1	330
	11th to 20th	Day	7% 21% 41% 24% 6%	330
	21st to end	Day	7% 24% 43% 22% 4	330
December	1st to 10th	Day	9% 24% 40% 20% 6% 1	330
	11th to 20th	Day	5% 32% 38% 18% 7% 1	330
	21st to end	Day	13% 27% 40% 16% 4	363
January	1st to 10th	Night	4 21% 38% 27% 8% 2	1006
	11th to 20th	Night	5% 19% 40% 27% 7% 2	820
	21st to end	Night	3 17% 40% 32% 8% 1	880
February	1st to 10th	Night	3 22% 47% 24% 4 1	800
	11th to 20th	Night	5% 24% 43% 23% 6%	784
	21st to end	Night	6% 21% 48% 23% 2	660
March	1st to 10th	Night	4 19% 45% 27% 5% 1	800
	11th to 20th	Night	5% 16% 39% 31% 9% 1	800
	21st to end	Night	4 22% 43% 27% 5%	785
April	1st to 10th	Night	8% 23% 41% 24% 5%	600
	11th to 20th	Night	9% 22% 36% 24% 8% 1	593
	21st to end	Night	9% 18% 41% 25% 6% 1	600
May	1st to 10th	Night	10% 15% 32% 33% 10% 1	600
	11th to 20th	Night	10% 10% 28% 37% 13% 2	591
	21st to end	Night	8% 11% 29% 37% 13% 1	650
June	1st to 10th	Night	3 8% 23% 43% 21% 3	600
	11th to 20th	Night	4 7% 30% 43% 16% 1	600
	21st to end	Night	3 5% 23% 39% 27% 4	600
July	1st to 10th	Night	7% 4 14% 40% 32% 3	585
	11th to 20th	Night	6% 2 10% 38% 37% 7%	582
	21st to end	Night	7% 2 10% 31% 44% 6%	657
August	1st to 10th	Night	7% 3 14% 36% 35% 5%	597
	11th to 20th	Night	5% 3 11% 45% 31% 5%	594
	21st to end	Night	7% 3 13% 40% 33% 4	660
September	1st to 10th	Night	8% 3 15% 43% 26% 6%	600
	11th to 20th	Night	7% 3 21% 46% 19% 4	600
	21st to end	Night	7% 6% 25% 40% 18% 4	652
October	1st to 10th	Night	7% 8% 33% 37% 13% 2	800
	11th to 20th	Night	4 12% 37% 33% 12% 3	800
	21st to end	Night	7% 11% 39% 29% 12% 2	880
November	1st to 10th	Night	4 12% 40% 33% 10% 1	970
	11th to 20th	Night	3 18% 40% 29% 8% 2	1050
	21st to end	Night	7% 16% 35% 32% 10%	1050
December	1st to 10th	Night	4 20% 42% 28% 6%	1050
	11th to 20th	Night	5% 20% 41% 25% 9%	1050
	21st to end	Night	6% 21% 44% 24% 5% 1	1155

Page generated at 2025-12-29 23:14:10. Version ac07

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