Sierra Remote Observatories
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sierra Remote Observatories clear sky chart from 2005-11-06 to 2026-02-16.

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	7% 4 1 3 2 2 2 2 3 5% 68%	139326
	Night	8% 4 1 3 2 2 3 2 4 5% 65%	138988
Transparency	Day	21% 3 7% 18% 28% 22%	134362
	Night	22% 4 9% 20% 26% 19%	134930
Seeing	Day	11% 10% 29% 32% 13% 5%	16870
	Night	10% 3 17% 34% 27% 9%	26480

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	14% 7% 2 5% 3 3 3 3 5% 8% 47%	4722
	11th to 20th	Day	8% 4 2 5% 3 3 3 3 3 5% 62%	5077
	21st to end	Day	10% 6% 2 6% 3 4 4 3 6% 8% 47%	5843
February	1st to 10th	Day	16% 10% 2 4 2 2 3 3 5% 5% 48%	2009
	11th to 20th	Day	12% 10% 3 5% 3 3 4 3 5% 6% 46%	2001
	21st to end	Day	16% 10% 4 6% 4 4 3 3 4 5% 41%	1650
March	1st to 10th	Day	14% 13% 3 8% 4 4 4 4 6% 7% 34%	4518
	11th to 20th	Day	14% 7% 2 6% 3 3 3 3 4 7% 47%	4863
	21st to end	Day	17% 9% 3 6% 4 4 4 3 5% 6% 40%	5879
April	1st to 10th	Day	12% 7% 2 4 3 3 3 3 5% 6% 52%	2400
	11th to 20th	Day	11% 8% 2 5% 4 3 3 3 4 5% 53%	2369
	21st to end	Day	8% 6% 1 5% 4 3 3 3 5% 6% 56%	2400
May	1st to 10th	Day	6% 4 1 3 2 2 2 2 3 4 71%	5611
	11th to 20th	Day	3 3 1 3 2 2 2 2 3 5% 74%	5841
	21st to end	Day	3 2 1 2 2 1 2 2 3 4 79%	6890
June	1st to 10th	Day	3 3 1 3 3 3 3 3 5% 5% 69%	2800
	11th to 20th	Day	1 1 1 2 1 2 1 1 3 4 84%	2800
	21st to end	Day	1 2 1 2 2 1 1 1 2 3 85%	2800
July	1st to 10th	Day	1 1 1 1 2 93%	6319
	11th to 20th	Day	1 1 1 1 1 1 1 2 2 88%	6320
	21st to end	Day	1 1 1 1 1 1 2 3 3 86%	6562
August	1st to 10th	Day	1 1 1 1 1 1 2 2 90%	5460
	11th to 20th	Day	1 1 1 1 2 94%	5364
	21st to end	Day	1 1 1 2 1 1 1 1 2 3 84%	5984
September	1st to 10th	Day	2 1 1 1 2 2 1 2 2 87%	2400
	11th to 20th	Day	2 2 2 1 2 2 2 3 4 80%	2335
	21st to end	Day	3 2 2 1 2 2 2 3 4 80%	2200
October	1st to 10th	Day	3 3 1 2 2 2 3 3 3 4 75%	3440
	11th to 20th	Day	3 3 1 2 2 2 2 2 3 3 77%	3261
	21st to end	Day	4 3 1 3 3 3 3 3 4 7% 63%	3504
November	1st to 10th	Day	9% 5% 1 4 3 3 3 3 4 6% 59%	1854
	11th to 20th	Day	9% 4 1 4 2 3 3 3 6% 6% 59%	1890
	21st to end	Day	9% 6% 1 3 3 3 3 3 4 7% 58%	1890
December	1st to 10th	Day	7% 3 1 3 3 3 4 3 5% 6% 62%	2870
	11th to 20th	Day	19% 7% 2 5% 3 3 4 4 5% 6% 42%	3108
	21st to end	Day	16% 8% 3 5% 4 4 4 3 6% 10% 38%	4092
January	1st to 10th	Night	16% 7% 2 5% 3 3 3 3 6% 7% 46%	8172
	11th to 20th	Night	9% 4 1 3 2 3 3 3 5% 6% 61%	7897
	21st to end	Night	11% 7% 3 6% 4 3 3 3 5% 7% 47%	8371
February	1st to 10th	Night	16% 7% 2 4 2 3 3 3 5% 6% 50%	2609
	11th to 20th	Night	13% 7% 1 4 3 3 3 3 4 7% 52%	2398
	21st to end	Night	15% 8% 2 4 3 3 3 2 4 5% 51%	1980
March	1st to 10th	Night	16% 10% 2 5% 3 3 5% 4 6% 7% 39%	5376
	11th to 20th	Night	12% 7% 2 6% 3 3 4 3 4 6% 51%	5132
	21st to end	Night	19% 7% 2 4 2 2 3 2 5% 7% 46%	5094
April	1st to 10th	Night	11% 6% 1 3 3 2 2 3 5% 7% 57%	2000
	11th to 20th	Night	8% 5% 1 4 4 2 3 2 5% 5% 60%	1979
	21st to end	Night	6% 5% 1 3 3 3 2 3 4 6% 64%	2000
May	1st to 10th	Night	4 2 1 3 2 2 2 2 3 5% 74%	4379
	11th to 20th	Night	3 2 1 1 1 2 2 2 4 5% 78%	4074
	21st to end	Night	2 1 1 1 1 1 1 2 4 86%	3980
June	1st to 10th	Night	4 2 1 2 2 2 2 2 3 6% 74%	1600
	11th to 20th	Night	1 1 1 1 1 2 1 3 3 85%	1600
	21st to end	Night	1 1 1 1 1 1 1 2 3 88%	1600
July	1st to 10th	Night	1 1 2 95%	3640
	11th to 20th	Night	1 2 1 2 1 1 1 1 2 2 86%	3640
	21st to end	Night	1 1 1 1 1 1 1 2 3 87%	4286
August	1st to 10th	Night	1 1 1 1 1 1 1 2 3 87%	4363
	11th to 20th	Night	1 1 1 1 1 1 94%	4506
	21st to end	Night	2 1 2 1 1 2 1 2 4 83%	5015
September	1st to 10th	Night	1 1 1 1 1 1 2 2 90%	2000
	11th to 20th	Night	2 2 1 1 1 1 2 2 3 2 83%	2064
	21st to end	Night	2 1 1 1 1 2 2 3 3 83%	2200
October	1st to 10th	Night	2 2 1 1 1 2 2 2 3 82%	3755
	11th to 20th	Night	3 1 1 1 1 1 2 2 3 3 81%	3897
	21st to end	Night	4 2 1 3 1 1 1 2 3 5% 76%	4304
November	1st to 10th	Night	9% 4 1 3 2 2 2 2 4 6% 65%	2665
	11th to 20th	Night	8% 4 1 3 2 2 3 3 5% 7% 62%	2730
	21st to end	Night	9% 5% 2 3 3 3 3 3 5% 6% 58%	2730
December	1st to 10th	Night	8% 3 2 4 3 3 3 3 4 6% 62%	4530
	11th to 20th	Night	19% 6% 2 4 3 3 3 3 6% 9% 42%	5316
	21st to end	Night	12% 8% 3 8% 5% 5% 5% 4 8% 9% 34%	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	35% 4 17% 24% 16% 5%	4723
	11th to 20th	Day	26% 5% 7% 11% 26% 26%	5077
	21st to end	Day	34% 5% 9% 18% 26% 9%	5843
February	1st to 10th	Day	37% 4 9% 23% 17% 10%	2009
	11th to 20th	Day	36% 5% 11% 22% 19% 7%	2011
	21st to end	Day	44% 5% 8% 23% 16% 3	1650
March	1st to 10th	Day	44% 8% 10% 14% 20% 4	4518
	11th to 20th	Day	35% 7% 14% 27% 10% 6%	4863
	21st to end	Day	41% 8% 14% 21% 14% 2	5879
April	1st to 10th	Day	32% 6% 12% 26% 21% 3	2400
	11th to 20th	Day	33% 6% 10% 25% 23% 4	2372
	21st to end	Day	28% 7% 11% 33% 20% 2	2400
May	1st to 10th	Day	18% 4 7% 31% 34% 7%	5611
	11th to 20th	Day	14% 3 8% 33% 38% 4	5844
	21st to end	Day	12% 3 5% 28% 48% 4	6892
June	1st to 10th	Day	16% 6% 10% 23% 34% 12%	2800
	11th to 20th	Day	7% 3 6% 18% 42% 24%	2800
	21st to end	Day	8% 2 3 10% 37% 40%	2800
July	1st to 10th	Day	3 1 2 5% 19% 72%	6251
	11th to 20th	Day	6% 1 2 5% 23% 64%	6245
	21st to end	Day	6% 1 3 11% 38% 41%	6373
August	1st to 10th	Day	8% 1 3 6% 35% 48%	3966
	11th to 20th	Day	3 1 2 6% 31% 56%	3880
	21st to end	Day	6% 1 3 8% 34% 48%	4311
September	1st to 10th	Day	7% 1 4 9% 34% 45%	2400
	11th to 20th	Day	11% 2 4 22% 38% 24%	2335
	21st to end	Day	10% 2 5% 19% 37% 27%	2200
October	1st to 10th	Day	14% 2 4 11% 23% 46%	3440
	11th to 20th	Day	14% 2 4 18% 40% 22%	3261
	21st to end	Day	21% 2 6% 16% 37% 19%	3504
November	1st to 10th	Day	26% 4 10% 20% 25% 16%	1854
	11th to 20th	Day	25% 4 11% 23% 27% 10%	1890
	21st to end	Day	28% 4 9% 18% 27% 14%	1890
December	1st to 10th	Day	24% 3 8% 24% 28% 13%	2870
	11th to 20th	Day	42% 4 10% 16% 22% 6%	3108
	21st to end	Day	42% 5% 16% 24% 9% 4	4092
January	1st to 10th	Night	34% 4 17% 22% 18% 5%	8172
	11th to 20th	Night	24% 4 9% 15% 31% 17%	7897
	21st to end	Night	34% 5% 8% 20% 23% 10%	8371
February	1st to 10th	Night	35% 4 8% 23% 18% 11%	2609
	11th to 20th	Night	31% 5% 10% 25% 21% 7%	2399
	21st to end	Night	36% 5% 12% 25% 17% 6%	1980
March	1st to 10th	Night	41% 6% 10% 17% 22% 4	5376
	11th to 20th	Night	34% 5% 13% 30% 11% 7%	5132
	21st to end	Night	37% 6% 20% 20% 15% 2	5094
April	1st to 10th	Night	28% 5% 16% 28% 18% 6%	2000
	11th to 20th	Night	25% 7% 11% 29% 21% 7%	1979
	21st to end	Night	21% 5% 13% 36% 22% 2	2000
May	1st to 10th	Night	14% 4 10% 38% 29% 6%	4376
	11th to 20th	Night	10% 3 8% 34% 40% 5%	4075
	21st to end	Night	7% 2 5% 31% 51% 3	3980
June	1st to 10th	Night	14% 4 10% 25% 36% 11%	1600
	11th to 20th	Night	6% 2 6% 21% 39% 26%	1600
	21st to end	Night	5% 2 2 11% 38% 41%	1600
July	1st to 10th	Night	2 2 3 20% 73%	3600
	11th to 20th	Night	8% 1 3 4 25% 60%	3592
	21st to end	Night	5% 1 2 13% 34% 43%	4164
August	1st to 10th	Night	9% 1 3 10% 29% 48%	3173
	11th to 20th	Night	4 1 2 6% 23% 64%	3256
	21st to end	Night	7% 1 2 6% 32% 52%	3608
September	1st to 10th	Night	5% 1 2 12% 29% 51%	2000
	11th to 20th	Night	10% 2 4 16% 45% 23%	2064
	21st to end	Night	8% 2 5% 20% 40% 25%	2200
October	1st to 10th	Night	10% 3 4 11% 27% 47%	3755
	11th to 20th	Night	10% 3 4 20% 41% 23%	3897
	21st to end	Night	14% 2 4 20% 35% 24%	4304
November	1st to 10th	Night	23% 3 10% 21% 26% 17%	2665
	11th to 20th	Night	23% 4 9% 22% 29% 13%	2730
	21st to end	Night	27% 4 8% 19% 30% 11%	2730
December	1st to 10th	Night	24% 3 7% 24% 26% 16%	4530
	11th to 20th	Night	39% 4 12% 17% 21% 6%	5316
	21st to end	Night	43% 6% 15% 22% 10% 4	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	29% 14% 25% 20% 11% 2	220
	11th to 20th	Day	15% 10% 25% 29% 15% 5%	336
	21st to end	Day	18% 10% 25% 28% 14% 4	473
February	1st to 10th	Day	25% 11% 25% 23% 13% 4	493
	11th to 20th	Day	20% 14% 33% 23% 8% 2	516
	21st to end	Day	20% 15% 37% 20% 7% 1	414
March	1st to 10th	Day	23% 15% 34% 20% 6% 1	500
	11th to 20th	Day	22% 18% 28% 20% 10% 2	500
	21st to end	Day	23% 17% 31% 21% 7%	550
April	1st to 10th	Day	16% 16% 31% 25% 10% 2	500
	11th to 20th	Day	20% 17% 27% 27% 8% 1	492
	21st to end	Day	10% 14% 30% 29% 13% 4	500
May	1st to 10th	Day	13% 17% 31% 25% 8% 5%	503
	11th to 20th	Day	11% 14% 30% 29% 12% 5%	528
	21st to end	Day	12% 9% 28% 32% 12% 6%	573
June	1st to 10th	Day	4 10% 28% 35% 17% 7%	530
	11th to 20th	Day	3 11% 28% 37% 13% 7%	530
	21st to end	Day	3 6% 32% 36% 14% 8%	530
July	1st to 10th	Day	1 3 28% 41% 17% 10%	526
	11th to 20th	Day	1 1 21% 46% 17% 13%	524
	21st to end	Day	2 2 24% 46% 14% 12%	575
August	1st to 10th	Day	2 5% 34% 38% 12% 9%	527
	11th to 20th	Day	2 5% 27% 42% 14% 10%	517
	21st to end	Day	2 4 26% 45% 16% 8%	583
September	1st to 10th	Day	3 6% 31% 40% 16% 5%	530
	11th to 20th	Day	5% 10% 32% 36% 15% 3	530
	21st to end	Day	5% 9% 30% 40% 14% 2	530
October	1st to 10th	Day	5% 11% 31% 38% 14% 1	530
	11th to 20th	Day	5% 11% 30% 37% 15% 2	530
	21st to end	Day	6% 11% 29% 35% 18% 2	553
November	1st to 10th	Day	10% 11% 30% 26% 20% 3	330
	11th to 20th	Day	11% 12% 30% 27% 15% 4	330
	21st to end	Day	13% 11% 30% 28% 13% 5%	330
December	1st to 10th	Day	13% 7% 30% 28% 19% 4	275
	11th to 20th	Day	14% 10% 30% 26% 16% 5%	220
	21st to end	Day	25% 11% 29% 21% 10% 5%	242
January	1st to 10th	Night	19% 6% 21% 27% 16% 10%	840
	11th to 20th	Night	11% 4 25% 36% 16% 8%	840
	21st to end	Night	19% 5% 21% 31% 17% 7%	924
February	1st to 10th	Night	20% 5% 20% 32% 18% 7%	840
	11th to 20th	Night	16% 6% 22% 29% 20% 7%	804
	21st to end	Night	18% 5% 22% 37% 15% 4	660
March	1st to 10th	Night	19% 7% 20% 33% 17% 5%	800
	11th to 20th	Night	16% 6% 19% 27% 24% 9%	800
	21st to end	Night	17% 7% 16% 37% 20% 4	880
April	1st to 10th	Night	13% 6% 18% 37% 20% 5%	800
	11th to 20th	Night	10% 6% 22% 32% 25% 6%	791
	21st to end	Night	8% 4 21% 38% 22% 8%	800
May	1st to 10th	Night	6% 4 21% 38% 25% 6%	723
	11th to 20th	Night	7% 3 18% 38% 30% 5%	591
	21st to end	Night	6% 2 21% 32% 30% 10%	441
June	1st to 10th	Night	5% 14% 38% 36% 7%	400
	11th to 20th	Night	3 13% 40% 37% 7%	400
	21st to end	Night	1 2 12% 37% 41% 8%	400
July	1st to 10th	Night	12% 31% 49% 8%	390
	11th to 20th	Night	2 1 9% 31% 46% 11%	388
	21st to end	Night	1 1 7% 28% 45% 17%	575
August	1st to 10th	Night	3 1 10% 34% 42% 11%	737
	11th to 20th	Night	1 7% 31% 50% 11%	792
	21st to end	Night	2 7% 28% 51% 11%	880
September	1st to 10th	Night	2 1 8% 31% 45% 13%	800
	11th to 20th	Night	3 1 10% 37% 37% 12%	800
	21st to end	Night	3 14% 36% 33% 14%	800
October	1st to 10th	Night	5% 2 13% 46% 25% 10%	800
	11th to 20th	Night	6% 2 15% 36% 30% 12%	800
	21st to end	Night	5% 4 17% 32% 27% 16%	880
November	1st to 10th	Night	10% 3 15% 31% 30% 11%	820
	11th to 20th	Night	10% 3 20% 36% 22% 9%	840
	21st to end	Night	12% 3 21% 34% 21% 8%	840
December	1st to 10th	Night	13% 4 21% 36% 19% 7%	840
	11th to 20th	Night	20% 3 23% 32% 17% 5%	840
	21st to end	Night	19% 8% 23% 30% 16% 4	924

Page generated at 2026-02-12 22:42:13. Version ac07

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