Sierra Stars Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sierra Stars Observatory clear sky chart from 2005-11-06 to 2026-07-01.

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	1 3 1 3 3 3 3 3 5% 7% 68%	146346
	Night	1 3 1 4 3 3 3 3 5% 7% 68%	143461
Transparency	Day	18% 6% 9% 18% 28% 22%	141331
	Night	17% 6% 11% 22% 28% 17%	139458
Seeing	Day	10% 21% 31% 23% 12% 3	17207
	Night	5% 14% 27% 32% 18% 4	26592

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 1 4 2 3 5% 4 7% 11% 59%	4722
	11th to 20th	Day	2 1 3 3 3 4 3 5% 6% 69%	4899
	21st to end	Day	2 6% 3 6% 3 4 4 4 6% 10% 52%	5843
February	1st to 10th	Day	1 5% 1 4 3 3 4 4 7% 10% 58%	1916
	11th to 20th	Day	4 2 5% 4 4 4 5% 7% 11% 52%	2050
	21st to end	Day	1 3 2 5% 3 3 5% 4 6% 9% 59%	1730
March	1st to 10th	Day	1 6% 4 8% 4 5% 5% 5% 8% 10% 46%	4618
	11th to 20th	Day	4 2 4 3 3 4 4 6% 9% 62%	5015
	21st to end	Day	1 7% 2 5% 4 3 4 4 6% 8% 56%	5981
April	1st to 10th	Day	2 3 2 5% 4 4 5% 4 6% 8% 57%	2520
	11th to 20th	Day	2 4 1 3 4 3 4 5% 6% 8% 59%	2489
	21st to end	Day	3 5% 1 4 3 3 3 3 6% 8% 61%	2520
May	1st to 10th	Day	3 4 1 4 3 3 3 2 4 6% 67%	7391
	11th to 20th	Day	1 1 1 2 2 2 3 2 4 5% 77%	7934
	21st to end	Day	3 4 2 4 3 3 3 3 5% 6% 65%	8992
June	1st to 10th	Day	3 2 1 4 3 4 4 4 5% 7% 65%	2940
	11th to 20th	Day	1 1 1 2 1 2 2 2 3 5% 78%	2940
	21st to end	Day	2 2 1 3 2 2 3 2 3 4 75%	2940
July	1st to 10th	Day	1 2 2 2 2 1 2 3 85%	6320
	11th to 20th	Day	2 2 1 3 2 3 3 2 4 5% 73%	6320
	21st to end	Day	1 1 1 2 2 3 4 4 5% 6% 70%	6711
August	1st to 10th	Day	1 2 2 2 2 2 2 2 3 82%	5659
	11th to 20th	Day	1 1 1 1 2 2 2 3 86%	5364
	21st to end	Day	1 2 1 3 3 4 3 2 3 4 73%	5984
September	1st to 10th	Day	1 1 1 1 2 2 2 2 4 5% 78%	2400
	11th to 20th	Day	2 2 1 3 2 2 3 2 4 5% 73%	2360
	21st to end	Day	2 2 1 3 2 2 2 2 3 4 77%	2200
October	1st to 10th	Day	2 3 1 3 2 2 3 3 5% 7% 71%	3354
	11th to 20th	Day	1 2 1 3 2 1 2 1 3 3 80%	3261
	21st to end	Day	1 1 1 3 2 3 3 3 5% 7% 72%	3472
November	1st to 10th	Day	1 3 1 3 3 3 4 3 7% 8% 63%	1854
	11th to 20th	Day	2 3 1 3 2 3 3 3 6% 8% 65%	1890
	21st to end	Day	1 2 1 3 3 4 3 4 6% 10% 63%	1857
December	1st to 10th	Day	1 3 1 2 2 2 3 4 5% 8% 69%	2700
	11th to 20th	Day	1 4 2 5% 4 5% 5% 4 8% 9% 52%	3108
	21st to end	Day	1 4 3 6% 4 4 6% 4 7% 13% 48%	4092
January	1st to 10th	Night	1 4 2 4 3 3 4 3 6% 11% 60%	8172
	11th to 20th	Night	2 1 4 3 3 3 3 5% 7% 69%	8066
	21st to end	Night	1 5% 3 6% 4 4 5% 4 6% 8% 53%	8371
February	1st to 10th	Night	1 4 2 6% 3 3 5% 4 6% 8% 58%	2703
	11th to 20th	Night	1 4 2 4 3 3 5% 5% 8% 11% 54%	2458
	21st to end	Night	1 4 2 3 3 3 3 4 7% 10% 61%	2076
March	1st to 10th	Night	1 6% 3 6% 4 4 5% 4 7% 10% 51%	5496
	11th to 20th	Night	3 1 5% 3 4 4 4 6% 8% 60%	5202
	21st to end	Night	1 6% 2 6% 4 4 4 4 7% 9% 54%	5235
April	1st to 10th	Night	2 3 1 4 3 3 4 4 7% 10% 59%	2100
	11th to 20th	Night	1 4 1 3 3 3 4 3 6% 9% 63%	2079
	21st to end	Night	3 4 1 3 2 2 3 2 5% 8% 68%	2100
May	1st to 10th	Night	4 3 1 3 2 3 3 2 3 6% 70%	5566
	11th to 20th	Night	1 2 1 2 2 2 2 2 3 5% 79%	4921
	21st to end	Night	2 2 1 3 2 2 3 2 4 6% 73%	5181
June	1st to 10th	Night	3 3 1 2 2 3 3 3 5% 6% 70%	1680
	11th to 20th	Night	1 1 1 2 2 2 3 4 83%	1680
	21st to end	Night	2 2 1 1 1 2 2 1 2 3 83%	1680
July	1st to 10th	Night	1 1 1 1 1 2 92%	3640
	11th to 20th	Night	1 2 1 3 2 2 2 2 3 4 78%	3640
	21st to end	Night	1 1 1 2 2 2 2 2 4 6% 77%	4142
August	1st to 10th	Night	1 1 1 2 2 2 1 1 3 4 83%	4159
	11th to 20th	Night	1 1 1 1 1 1 1 2 3 87%	4506
	21st to end	Night	1 1 1 3 2 2 2 2 3 4 79%	5015
September	1st to 10th	Night	1 1 1 1 2 2 2 3 4 85%	2000
	11th to 20th	Night	2 2 1 2 1 2 2 2 3 5% 78%	2040
	21st to end	Night	1 1 1 3 2 2 2 3 3 5% 78%	2200
October	1st to 10th	Night	2 2 1 2 2 2 1 2 3 5% 80%	3843
	11th to 20th	Night	1 2 1 2 1 1 1 2 3 4 82%	3897
	21st to end	Night	1 2 1 3 2 3 3 2 4 7% 72%	4336
November	1st to 10th	Night	1 2 1 3 2 3 3 3 6% 9% 67%	2665
	11th to 20th	Night	1 3 1 3 2 3 4 3 6% 10% 65%	2730
	21st to end	Night	2 3 1 3 2 3 3 3 6% 8% 67%	2760
December	1st to 10th	Night	1 2 1 3 2 2 3 3 6% 8% 69%	4700
	11th to 20th	Night	1 3 1 4 3 3 4 4 8% 13% 54%	5316
	21st to end	Night	1 4 2 7% 5% 5% 6% 5% 9% 12% 45%	7106

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	19% 8% 19% 30% 22% 2	4723
	11th to 20th	Day	16% 6% 10% 18% 29% 22%	4899
	21st to end	Day	28% 8% 10% 18% 25% 11%	5843
February	1st to 10th	Day	23% 6% 15% 25% 22% 9%	1916
	11th to 20th	Day	26% 7% 13% 26% 22% 6%	2060
	21st to end	Day	22% 9% 17% 25% 19% 8%	1730
March	1st to 10th	Day	29% 9% 15% 19% 23% 5%	4618
	11th to 20th	Day	19% 7% 13% 30% 26% 4	5015
	21st to end	Day	27% 7% 15% 24% 23% 4	5981
April	1st to 10th	Day	25% 8% 16% 23% 22% 7%	2520
	11th to 20th	Day	21% 7% 15% 25% 24% 8%	2492
	21st to end	Day	22% 7% 15% 26% 24% 5%	2520
May	1st to 10th	Day	20% 5% 6% 21% 40% 8%	7391
	11th to 20th	Day	12% 5% 5% 20% 43% 15%	7937
	21st to end	Day	20% 7% 6% 19% 38% 10%	8994
June	1st to 10th	Day	19% 7% 11% 20% 32% 12%	2940
	11th to 20th	Day	11% 3 7% 18% 34% 27%	2940
	21st to end	Day	14% 3 5% 10% 32% 36%	2940
July	1st to 10th	Day	8% 2 2 3 16% 68%	6252
	11th to 20th	Day	16% 4 4 5% 11% 61%	6245
	21st to end	Day	14% 5% 4 12% 25% 40%	6515
August	1st to 10th	Day	14% 3 4 8% 24% 47%	4121
	11th to 20th	Day	8% 1 4 4 18% 65%	3880
	21st to end	Day	10% 2 3 7% 35% 43%	4311
September	1st to 10th	Day	12% 2 4 8% 33% 41%	2400
	11th to 20th	Day	15% 3 7% 17% 35% 23%	2360
	21st to end	Day	14% 2 5% 17% 37% 26%	2200
October	1st to 10th	Day	14% 4 5% 12% 33% 32%	3354
	11th to 20th	Day	12% 2 4 16% 50% 16%	3261
	21st to end	Day	14% 5% 7% 24% 37% 13%	3472
November	1st to 10th	Day	17% 7% 13% 23% 28% 12%	1854
	11th to 20th	Day	17% 5% 16% 24% 31% 8%	1890
	21st to end	Day	17% 7% 13% 26% 27% 10%	1857
December	1st to 10th	Day	15% 5% 14% 25% 32% 10%	2700
	11th to 20th	Day	28% 10% 16% 18% 19% 9%	3108
	21st to end	Day	27% 9% 21% 23% 17% 3	4092
January	1st to 10th	Night	21% 6% 19% 30% 21% 2	8172
	11th to 20th	Night	16% 6% 10% 18% 31% 19%	8066
	21st to end	Night	28% 8% 10% 19% 26% 8%	8371
February	1st to 10th	Night	24% 7% 14% 25% 20% 10%	2703
	11th to 20th	Night	22% 7% 15% 28% 23% 5%	2459
	21st to end	Night	20% 7% 19% 27% 21% 6%	2076
March	1st to 10th	Night	27% 7% 15% 20% 26% 6%	5496
	11th to 20th	Night	23% 7% 12% 40% 15% 3	5202
	21st to end	Night	25% 9% 23% 28% 12% 4	5235
April	1st to 10th	Night	20% 10% 17% 28% 18% 7%	2100
	11th to 20th	Night	20% 7% 14% 30% 21% 8%	2079
	21st to end	Night	18% 7% 15% 34% 23% 2	2100
May	1st to 10th	Night	18% 5% 8% 26% 37% 6%	5564
	11th to 20th	Night	11% 4 7% 22% 44% 13%	4921
	21st to end	Night	14% 5% 7% 27% 43% 5%	5182
June	1st to 10th	Night	15% 7% 11% 24% 31% 11%	1680
	11th to 20th	Night	7% 4 7% 20% 36% 27%	1680
	21st to end	Night	10% 2 5% 13% 36% 35%	1680
July	1st to 10th	Night	3 1 2 4 21% 69%	3600
	11th to 20th	Night	12% 2 3 5% 15% 62%	3592
	21st to end	Night	10% 3 3 18% 24% 42%	4026
August	1st to 10th	Night	12% 3 3 10% 30% 43%	3017
	11th to 20th	Night	6% 1 3 7% 20% 63%	3256
	21st to end	Night	7% 1 3 11% 40% 39%	3608
September	1st to 10th	Night	6% 2 4 10% 38% 41%	2000
	11th to 20th	Night	10% 3 7% 19% 37% 24%	2040
	21st to end	Night	11% 2 7% 18% 37% 25%	2200
October	1st to 10th	Night	11% 3 5% 12% 38% 31%	3843
	11th to 20th	Night	10% 3 4 18% 49% 15%	3897
	21st to end	Night	14% 3 6% 23% 39% 14%	4336
November	1st to 10th	Night	14% 6% 13% 27% 27% 12%	2665
	11th to 20th	Night	16% 7% 13% 27% 30% 7%	2730
	21st to end	Night	17% 6% 12% 28% 28% 9%	2760
December	1st to 10th	Night	16% 6% 14% 25% 28% 12%	4700
	11th to 20th	Night	22% 11% 18% 23% 21% 5%	5316
	21st to end	Night	31% 10% 17% 20% 17% 5%	7106

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% 27% 25% 17% 9% 2	220
	11th to 20th	Day	13% 21% 25% 24% 12% 4	275
	21st to end	Day	8% 25% 29% 24% 11% 3	473
February	1st to 10th	Day	19% 26% 25% 19% 8% 3	444
	11th to 20th	Day	16% 27% 32% 20% 5%	535
	21st to end	Day	15% 25% 35% 21% 4 1	446
March	1st to 10th	Day	17% 31% 31% 15% 5% 1	540
	11th to 20th	Day	16% 29% 27% 16% 10% 1	540
	21st to end	Day	14% 34% 29% 17% 6%	594
April	1st to 10th	Day	14% 29% 31% 17% 8% 1	540
	11th to 20th	Day	18% 27% 30% 16% 7% 2	532
	21st to end	Day	13% 25% 31% 19% 9% 3	540
May	1st to 10th	Day	12% 28% 29% 21% 7% 3	543
	11th to 20th	Day	13% 24% 30% 20% 8% 4	568
	21st to end	Day	12% 18% 30% 21% 13% 5%	617
June	1st to 10th	Day	8% 21% 31% 25% 10% 5%	570
	11th to 20th	Day	8% 22% 36% 21% 8% 5%	570
	21st to end	Day	5% 14% 38% 26% 11% 5%	570
July	1st to 10th	Day	2 15% 39% 26% 14% 3	527
	11th to 20th	Day	2 11% 36% 30% 15% 5%	524
	21st to end	Day	2 11% 37% 28% 17% 5%	575
August	1st to 10th	Day	4 14% 32% 30% 15% 5%	527
	11th to 20th	Day	3 14% 36% 24% 16% 7%	517
	21st to end	Day	3 13% 36% 29% 15% 4	583
September	1st to 10th	Day	3 11% 37% 29% 17% 3	530
	11th to 20th	Day	6% 17% 33% 28% 13% 3	530
	21st to end	Day	5% 16% 34% 34% 10% 2	530
October	1st to 10th	Day	6% 21% 26% 28% 17% 2	530
	11th to 20th	Day	8% 18% 26% 30% 15% 4	530
	21st to end	Day	9% 18% 28% 25% 17% 3	533
November	1st to 10th	Day	11% 18% 25% 19% 22% 5%	330
	11th to 20th	Day	11% 19% 25% 26% 13% 6%	330
	21st to end	Day	8% 20% 27% 27% 15% 3	312
December	1st to 10th	Day	10% 20% 24% 22% 16% 7%	220
	11th to 20th	Day	13% 25% 26% 17% 12% 6%	220
	21st to end	Day	24% 16% 27% 21% 10% 2	242
January	1st to 10th	Night	7% 19% 30% 23% 14% 7%	840
	11th to 20th	Night	4 16% 29% 32% 13% 6%	840
	21st to end	Night	5% 13% 33% 29% 15% 5%	924
February	1st to 10th	Night	9% 15% 29% 30% 15% 3	840
	11th to 20th	Night	7% 19% 30% 25% 14% 5%	824
	21st to end	Night	7% 23% 28% 30% 11% 2	692
March	1st to 10th	Night	7% 24% 32% 24% 11% 3	840
	11th to 20th	Night	7% 21% 28% 24% 17% 3	840
	21st to end	Night	6% 24% 32% 26% 10% 1	924
April	1st to 10th	Night	7% 18% 31% 29% 14% 1	840
	11th to 20th	Night	8% 19% 29% 28% 13% 3	831
	21st to end	Night	7% 15% 29% 30% 15% 4	840
May	1st to 10th	Night	6% 17% 28% 30% 16% 3	719
	11th to 20th	Night	4 12% 27% 37% 17% 2	494
	21st to end	Night	6% 7% 26% 35% 25% 2	455
June	1st to 10th	Night	5% 10% 23% 39% 21% 2	420
	11th to 20th	Night	2 10% 26% 36% 25% 1	420
	21st to end	Night	2 6% 25% 44% 20% 2	420
July	1st to 10th	Night	1 3 23% 46% 26% 1	390
	11th to 20th	Night	2 4 25% 46% 24%	388
	21st to end	Night	3 3 18% 40% 32% 4	509
August	1st to 10th	Night	3 3 20% 43% 29% 3	646
	11th to 20th	Night	2 4 25% 39% 26% 4	792
	21st to end	Night	3 4 22% 43% 25% 3	880
September	1st to 10th	Night	2 5% 24% 38% 28% 4	800
	11th to 20th	Night	3 8% 21% 34% 28% 5%	800
	21st to end	Night	2 9% 24% 33% 26% 7%	800
October	1st to 10th	Night	5% 10% 22% 36% 22% 6%	800
	11th to 20th	Night	6% 9% 20% 38% 23% 5%	800
	21st to end	Night	5% 12% 22% 28% 23% 10%	880
November	1st to 10th	Night	5% 15% 21% 29% 23% 7%	820
	11th to 20th	Night	5% 15% 29% 30% 14% 6%	840
	21st to end	Night	8% 15% 26% 29% 17% 5%	840
December	1st to 10th	Night	6% 15% 29% 31% 15% 5%	840
	11th to 20th	Night	8% 19% 26% 30% 11% 6%	840
	21st to end	Night	11% 18% 32% 26% 10% 2	924

Page generated at 2026-06-27 23:10:43. Version ac07

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