Sky Mountain Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sky Mountain Observatory clear sky chart from 2005-11-06 to 2026-07-31.

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	2 4 2 5% 4 4 5% 4 7% 8% 54%	146807
	Night	1 3 2 5% 3 3 4 4 6% 8% 61%	143732
Transparency	Day	25% 7% 11% 22% 24% 11%	141790
	Night	21% 6% 12% 25% 27% 9%	139758
Seeing	Day	9% 19% 33% 25% 12% 2	17824
	Night	5% 14% 37% 29% 13% 2	27817

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 7% 4 4 4 3 5% 7% 61%	4720
	11th to 20th	Day	1 1 4 3 3 3 3 4 6% 72%	5197
	21st to end	Day	4 3 7% 3 3 3 3 5% 7% 61%	5844
February	1st to 10th	Day	5% 3 6% 4 3 4 3 6% 7% 60%	1894
	11th to 20th	Day	1 3 2 6% 3 3 5% 3 6% 8% 62%	2049
	21st to end	Day	1 4 3 7% 4 3 4 3 6% 7% 59%	1730
March	1st to 10th	Day	3 3 7% 4 3 3 4 4 7% 61%	4618
	11th to 20th	Day	1 8% 5% 8% 6% 5% 5% 5% 6% 6% 44%	5110
	21st to end	Day	4 2 6% 3 4 5% 3 6% 10% 56%	5910
April	1st to 10th	Day	2 6% 2 5% 4 3 4 4 6% 9% 54%	2520
	11th to 20th	Day	2 7% 3 7% 5% 3 4 3 6% 8% 51%	2491
	21st to end	Day	4 7% 3 6% 5% 4 5% 5% 6% 8% 46%	2520
May	1st to 10th	Day	3 9% 3 8% 4 5% 5% 4 6% 7% 44%	7175
	11th to 20th	Day	2 4 1 5% 4 4 5% 5% 7% 9% 53%	8015
	21st to end	Day	2 3 1 4 4 4 4 5% 9% 10% 54%	9026
June	1st to 10th	Day	2 4 2 5% 4 5% 6% 6% 9% 11% 48%	2940
	11th to 20th	Day	2 2 1 4 3 4 6% 5% 7% 9% 57%	2940
	21st to end	Day	2 4 2 5% 4 5% 7% 5% 9% 8% 49%	2940
July	1st to 10th	Day	2 2 1 3 4 4 6% 5% 8% 10% 56%	6479
	11th to 20th	Day	2 2 2 6% 6% 8% 8% 7% 9% 9% 42%	6480
	21st to end	Day	3 4 2 7% 6% 7% 7% 7% 9% 9% 38%	6631
August	1st to 10th	Day	2 3 1 6% 5% 8% 9% 7% 10% 12% 37%	5700
	11th to 20th	Day	2 2 1 4 5% 6% 7% 6% 9% 11% 47%	5366
	21st to end	Day	4 5% 2 7% 6% 6% 7% 5% 9% 11% 39%	5984
September	1st to 10th	Day	4 5% 2 6% 4 4 6% 6% 7% 9% 50%	2400
	11th to 20th	Day	4 3 1 4 3 4 4 4 6% 8% 59%	2388
	21st to end	Day	3 3 1 5% 3 4 4 3 5% 5% 64%	2200
October	1st to 10th	Day	2 2 1 3 3 3 3 3 3 4 73%	3275
	11th to 20th	Day	2 2 2 4 3 3 3 4 6% 9% 62%	3250
	21st to end	Day	2 3 1 3 2 2 2 3 4 6% 73%	3526
November	1st to 10th	Day	1 3 2 4 2 2 3 2 6% 7% 69%	1853
	11th to 20th	Day	3 1 4 3 2 2 3 5% 7% 70%	1890
	21st to end	Day	1 3 2 5% 3 3 3 3 4 7% 66%	1848
December	1st to 10th	Day	2 1 4 2 2 2 2 4 5% 74%	2700
	11th to 20th	Day	2 1 3 2 3 3 3 6% 9% 68%	3106
	21st to end	Day	2 2 6% 4 4 3 4 6% 7% 61%	4092
January	1st to 10th	Night	2 2 8% 4 3 4 3 5% 8% 60%	8201
	11th to 20th	Night	1 1 2 2 2 3 3 4 6% 75%	7848
	21st to end	Night	2 2 5% 3 3 4 3 5% 7% 66%	8370
February	1st to 10th	Night	4 2 6% 4 3 2 3 3 6% 66%	2725
	11th to 20th	Night	1 4 1 6% 4 3 4 3 5% 7% 62%	2458
	21st to end	Night	3 2 5% 3 3 3 3 5% 7% 66%	2076
March	1st to 10th	Night	5% 2 6% 3 3 3 3 4 5% 64%	5460
	11th to 20th	Night	1 7% 3 5% 3 3 3 3 5% 7% 60%	5110
	21st to end	Night	1 4 1 4 3 3 4 3 5% 7% 64%	5292
April	1st to 10th	Night	1 4 1 4 3 3 4 2 6% 8% 63%	2100
	11th to 20th	Night	2 7% 2 5% 3 3 4 3 5% 8% 58%	2077
	21st to end	Night	3 6% 2 6% 3 3 4 4 6% 7% 55%	2100
May	1st to 10th	Night	3 7% 3 7% 5% 5% 4 3 6% 7% 50%	5756
	11th to 20th	Night	3 5% 1 4 3 3 4 3 5% 9% 60%	4873
	21st to end	Night	3 3 1 5% 4 4 5% 4 5% 9% 57%	5181
June	1st to 10th	Night	3 4 1 4 3 5% 5% 4 6% 9% 57%	1680
	11th to 20th	Night	1 3 1 3 2 3 3 4 5% 8% 66%	1680
	21st to end	Night	2 3 2 4 4 5% 5% 5% 7% 10% 52%	1680
July	1st to 10th	Night	2 2 1 4 4 4 5% 6% 7% 9% 56%	3720
	11th to 20th	Night	2 2 1 4 4 5% 8% 7% 10% 13% 44%	3720
	21st to end	Night	2 4 2 6% 5% 6% 6% 6% 9% 12% 42%	4459
August	1st to 10th	Night	3 3 2 6% 4 6% 6% 6% 9% 10% 45%	4109
	11th to 20th	Night	2 2 1 4 3 4 6% 6% 10% 14% 49%	4483
	21st to end	Night	3 5% 2 7% 5% 6% 7% 6% 9% 11% 37%	4993
September	1st to 10th	Night	4 4 2 5% 4 5% 4 4 8% 8% 53%	2000
	11th to 20th	Night	4 4 1 3 3 3 5% 3 5% 8% 61%	2010
	21st to end	Night	3 3 1 5% 3 3 4 3 5% 7% 64%	2200
October	1st to 10th	Night	2 2 1 3 2 2 2 2 4 5% 75%	3910
	11th to 20th	Night	2 2 1 4 3 3 4 3 6% 7% 64%	3898
	21st to end	Night	2 3 1 3 2 1 2 2 3 5% 75%	4252
November	1st to 10th	Night	1 2 1 3 2 2 2 2 5% 7% 72%	2655
	11th to 20th	Night	2 1 4 2 2 3 3 4 6% 73%	2730
	21st to end	Night	1 4 1 4 2 2 3 3 5% 7% 67%	2767
December	1st to 10th	Night	2 1 4 2 2 2 2 3 6% 75%	4710
	11th to 20th	Night	2 2 4 2 2 2 3 5% 7% 72%	5321
	21st to end	Night	3 2 6% 3 3 4 4 6% 8% 60%	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	23% 3 9% 30% 27% 7%	4723
	11th to 20th	Day	14% 4 10% 25% 29% 17%	5197
	21st to end	Day	24% 5% 8% 24% 24% 14%	5844
February	1st to 10th	Day	23% 4 13% 28% 22% 11%	1894
	11th to 20th	Day	21% 5% 11% 26% 28% 9%	2059
	21st to end	Day	23% 5% 9% 20% 27% 15%	1730
March	1st to 10th	Day	19% 7% 7% 13% 35% 19%	4618
	11th to 20th	Day	34% 8% 8% 22% 22% 5%	5110
	21st to end	Day	21% 6% 12% 41% 17% 3	5910
April	1st to 10th	Day	27% 6% 11% 22% 22% 12%	2520
	11th to 20th	Day	31% 6% 8% 25% 20% 10%	2494
	21st to end	Day	33% 7% 12% 21% 21% 6%	2520
May	1st to 10th	Day	34% 8% 10% 23% 18% 7%	7175
	11th to 20th	Day	25% 9% 10% 19% 30% 8%	8019
	21st to end	Day	23% 7% 11% 22% 24% 13%	9027
June	1st to 10th	Day	26% 11% 14% 20% 20% 10%	2940
	11th to 20th	Day	21% 8% 10% 16% 25% 20%	2940
	21st to end	Day	28% 9% 12% 19% 20% 13%	2940
July	1st to 10th	Day	23% 8% 9% 15% 22% 23%	6411
	11th to 20th	Day	34% 9% 16% 24% 12% 5%	6405
	21st to end	Day	37% 10% 15% 16% 15% 7%	6444
August	1st to 10th	Day	40% 11% 20% 17% 9% 4	4151
	11th to 20th	Day	30% 8% 18% 17% 18% 10%	3882
	21st to end	Day	37% 9% 16% 17% 18% 4	4311
September	1st to 10th	Day	28% 8% 12% 22% 20% 10%	2400
	11th to 20th	Day	22% 6% 11% 17% 29% 14%	2388
	21st to end	Day	22% 5% 8% 23% 32% 10%	2200
October	1st to 10th	Day	16% 5% 6% 16% 40% 18%	3275
	11th to 20th	Day	22% 5% 8% 28% 28% 9%	3250
	21st to end	Day	16% 4 4 21% 43% 12%	3526
November	1st to 10th	Day	15% 4 10% 22% 36% 13%	1853
	11th to 20th	Day	15% 5% 8% 32% 31% 10%	1890
	21st to end	Day	21% 4 9% 23% 31% 14%	1848
December	1st to 10th	Day	14% 4 9% 29% 35% 10%	2700
	11th to 20th	Day	15% 5% 10% 30% 32% 9%	3104
	21st to end	Day	20% 6% 11% 26% 25% 12%	4092
January	1st to 10th	Night	22% 4 8% 31% 28% 6%	8202
	11th to 20th	Night	12% 3 10% 31% 27% 17%	7848
	21st to end	Night	18% 6% 10% 22% 29% 15%	8370
February	1st to 10th	Night	21% 4 11% 29% 24% 10%	2725
	11th to 20th	Night	23% 5% 11% 28% 26% 7%	2458
	21st to end	Night	17% 5% 9% 24% 32% 13%	2076
March	1st to 10th	Night	20% 5% 7% 16% 37% 16%	5460
	11th to 20th	Night	24% 6% 9% 22% 32% 7%	5110
	21st to end	Night	19% 6% 15% 41% 18% 2	5292
April	1st to 10th	Night	19% 6% 14% 27% 26% 8%	2100
	11th to 20th	Night	25% 6% 10% 26% 26% 6%	2077
	21st to end	Night	27% 7% 12% 27% 22% 5%	2100
May	1st to 10th	Night	29% 7% 12% 30% 16% 6%	5754
	11th to 20th	Night	22% 6% 11% 29% 27% 5%	4874
	21st to end	Night	23% 7% 15% 21% 26% 9%	5182
June	1st to 10th	Night	23% 8% 18% 21% 17% 13%	1680
	11th to 20th	Night	16% 7% 11% 22% 25% 20%	1680
	21st to end	Night	26% 9% 13% 18% 21% 14%	1680
July	1st to 10th	Night	22% 7% 11% 19% 26% 15%	3680
	11th to 20th	Night	28% 9% 24% 25% 10% 4	3672
	21st to end	Night	34% 10% 17% 19% 16% 4	4334
August	1st to 10th	Night	39% 12% 19% 16% 9% 4	2982
	11th to 20th	Night	29% 9% 24% 16% 17% 5%	3243
	21st to end	Night	36% 9% 17% 22% 13% 2	3597
September	1st to 10th	Night	26% 7% 15% 24% 20% 7%	2000
	11th to 20th	Night	21% 6% 12% 22% 26% 14%	2010
	21st to end	Night	21% 6% 10% 23% 33% 8%	2200
October	1st to 10th	Night	14% 4 8% 24% 38% 11%	3910
	11th to 20th	Night	21% 5% 11% 25% 28% 9%	3898
	21st to end	Night	15% 3 5% 25% 39% 13%	4252
November	1st to 10th	Night	14% 4 9% 27% 33% 13%	2655
	11th to 20th	Night	14% 5% 7% 32% 31% 11%	2730
	21st to end	Night	18% 5% 10% 23% 32% 12%	2767
December	1st to 10th	Night	13% 4 9% 29% 36% 9%	4710
	11th to 20th	Night	13% 5% 9% 29% 37% 8%	5322
	21st to end	Night	21% 6% 13% 20% 30% 10%	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	15% 22% 36% 18% 8% 1	330
	11th to 20th	Day	16% 23% 40% 18% 3	330
	21st to end	Day	6% 25% 43% 20% 6% 1	363
February	1st to 10th	Day	10% 35% 38% 13% 2 2	330
	11th to 20th	Day	11% 39% 37% 10% 3	330
	21st to end	Day	19% 29% 37% 14% 1	273
March	1st to 10th	Day	13% 29% 33% 19% 5%	335
	11th to 20th	Day	13% 32% 34% 16% 5% 1	540
	21st to end	Day	15% 36% 36% 10% 3	594
April	1st to 10th	Day	15% 32% 35% 15% 3 1	540
	11th to 20th	Day	17% 36% 33% 12% 2	532
	21st to end	Day	15% 34% 34% 12% 4	540
May	1st to 10th	Day	13% 28% 37% 16% 6% 1	585
	11th to 20th	Day	9% 19% 34% 25% 11% 2	775
	21st to end	Day	7% 19% 32% 28% 12% 2	844
June	1st to 10th	Day	4 13% 32% 30% 17% 3	780
	11th to 20th	Day	5% 15% 36% 27% 14% 3	780
	21st to end	Day	4 13% 35% 27% 18% 3	780
July	1st to 10th	Day	5% 6% 26% 37% 21% 5%	771
	11th to 20th	Day	3 3 17% 40% 29% 8%	768
	21st to end	Day	5% 3 18% 40% 27% 7%	663
August	1st to 10th	Day	7% 5% 25% 39% 20% 5%	527
	11th to 20th	Day	5% 4 30% 39% 18% 4	517
	21st to end	Day	7% 6% 26% 39% 18% 3	583
September	1st to 10th	Day	6% 8% 28% 37% 17% 5%	530
	11th to 20th	Day	4 12% 40% 28% 14% 2	518
	21st to end	Day	5% 11% 29% 30% 21% 4	330
October	1st to 10th	Day	8% 16% 28% 29% 16% 3	330
	11th to 20th	Day	8% 27% 29% 24% 9% 3	330
	21st to end	Day	10% 20% 37% 22% 10% 1	363
November	1st to 10th	Day	8% 27% 34% 22% 8% 1	330
	11th to 20th	Day	6% 25% 39% 20% 9% 2	330
	21st to end	Day	7% 20% 41% 26% 5% 1	330
December	1st to 10th	Day	12% 24% 37% 18% 7% 1	330
	11th to 20th	Day	7% 32% 38% 16% 6% 1	330
	21st to end	Day	11% 34% 39% 12% 4	363
January	1st to 10th	Night	4 23% 39% 25% 8% 1	1050
	11th to 20th	Night	5% 23% 42% 23% 6% 1	908
	21st to end	Night	2 16% 50% 25% 6% 1	924
February	1st to 10th	Night	3 20% 56% 17% 4	840
	11th to 20th	Night	5% 23% 52% 16% 5%	824
	21st to end	Night	4 22% 54% 19% 1	692
March	1st to 10th	Night	5% 24% 46% 21% 4 1	840
	11th to 20th	Night	6% 14% 45% 26% 9% 1	840
	21st to end	Night	5% 21% 46% 24% 4	787
April	1st to 10th	Night	8% 18% 46% 24% 4	630
	11th to 20th	Night	9% 18% 43% 24% 5%	623
	21st to end	Night	7% 18% 39% 28% 7%	630
May	1st to 10th	Night	7% 13% 39% 32% 8% 1	630
	11th to 20th	Night	9% 10% 35% 33% 12% 1	621
	21st to end	Night	7% 13% 36% 30% 12% 1	683
June	1st to 10th	Night	4 9% 26% 39% 20% 1	630
	11th to 20th	Night	1 10% 34% 41% 13% 1	630
	21st to end	Night	3 6% 30% 38% 21% 2	630
July	1st to 10th	Night	6% 2 17% 42% 29% 3	615
	11th to 20th	Night	4 1 11% 39% 39% 6%	612
	21st to end	Night	4 1 10% 38% 41% 6%	687
August	1st to 10th	Night	5% 3 14% 40% 34% 5%	597
	11th to 20th	Night	4 4 13% 45% 31% 3	594
	21st to end	Night	6% 3 16% 41% 30% 5%	660
September	1st to 10th	Night	4 4 19% 43% 26% 6%	600
	11th to 20th	Night	5% 7% 27% 36% 20% 4	600
	21st to end	Night	4 8% 28% 42% 16% 3	600
October	1st to 10th	Night	5% 10% 34% 35% 14% 2	790
	11th to 20th	Night	2 13% 38% 30% 14% 2	800
	21st to end	Night	6% 13% 37% 29% 13% 2	880
November	1st to 10th	Night	3 16% 41% 28% 10% 1	1015
	11th to 20th	Night	4 18% 43% 25% 8% 2	1050
	21st to end	Night	6% 15% 43% 28% 8% 1	1050
December	1st to 10th	Night	5% 19% 43% 26% 7%	1050
	11th to 20th	Night	4 21% 47% 21% 6% 1	1050
	21st to end	Night	5% 25% 45% 21% 4 1	1155

Page generated at 2026-07-28 01:25:21. Version ac07

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