Sierra La Sal Observatory
Clear Sky Chart
History

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

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	3 5% 2 5% 4 4 4 4 6% 7% 56%	135905
	Night	3 5% 2 5% 3 4 4 4 5% 7% 58%	133622
Transparency	Day	23% 7% 11% 20% 25% 15%	130747
	Night	23% 7% 10% 22% 25% 14%	129783
Seeing	Day	8% 13% 28% 30% 17% 4	16711
	Night	7% 7% 22% 36% 24% 4	27077

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 5% 3 7% 5% 5% 5% 3 6% 8% 51%	4188
	11th to 20th	Day	2 4 2 5% 4 4 4 4 5% 7% 60%	4190
	21st to end	Day	3 7% 3 6% 3 3 4 3 4 6% 58%	4609
February	1st to 10th	Day	3 8% 2 7% 5% 4 3 3 6% 8% 49%	1800
	11th to 20th	Day	5% 7% 4 7% 5% 5% 4 3 5% 7% 49%	1755
	21st to end	Day	5% 9% 3 4 4 4 4 3 5% 6% 53%	1705
March	1st to 10th	Day	2 9% 2 6% 4 4 4 3 5% 7% 54%	4954
	11th to 20th	Day	4 12% 4 8% 5% 5% 4 4 5% 7% 41%	5000
	21st to end	Day	4 11% 3 9% 6% 5% 6% 4 5% 7% 40%	5486
April	1st to 10th	Day	4 6% 3 7% 4 4 3 3 5% 8% 54%	2327
	11th to 20th	Day	3 6% 2 6% 4 4 5% 5% 6% 9% 50%	2371
	21st to end	Day	4 4 2 6% 4 4 5% 4 5% 7% 54%	2483
May	1st to 10th	Day	5% 4 2 5% 4 5% 5% 5% 7% 9% 50%	5880
	11th to 20th	Day	4 4 2 5% 4 4 4 4 6% 7% 57%	5841
	21st to end	Day	2 3 1 4 3 3 4 4 6% 8% 61%	6556
June	1st to 10th	Day	3 3 1 3 3 3 4 3 5% 9% 63%	2800
	11th to 20th	Day	2 4 1 3 2 2 2 2 4 6% 71%	2800
	21st to end	Day	2 2 1 3 3 4 4 3 5% 7% 67%	2774
July	1st to 10th	Day	2 2 1 3 3 3 4 3 4 6% 69%	5879
	11th to 20th	Day	2 3 1 4 3 4 5% 5% 9% 11% 53%	5880
	21st to end	Day	2 2 1 4 4 4 4 5% 7% 9% 58%	6401
August	1st to 10th	Day	2 2 1 4 5% 5% 6% 5% 8% 10% 52%	5779
	11th to 20th	Day	3 2 1 4 4 4 5% 5% 6% 8% 58%	5798
	21st to end	Day	4 3 2 4 4 5% 7% 5% 9% 10% 47%	6028
September	1st to 10th	Day	5% 2 1 4 3 4 5% 4 6% 7% 58%	2400
	11th to 20th	Day	4 3 1 4 3 4 5% 4 5% 7% 60%	2249
	21st to end	Day	5% 3 2 3 2 4 3 2 4 5% 66%	2186
October	1st to 10th	Day	4 2 1 3 2 2 2 2 4 4 75%	3270
	11th to 20th	Day	7% 4 2 5% 3 3 4 3 6% 8% 55%	3250
	21st to end	Day	4 2 1 4 2 2 4 3 4 5% 68%	3548
November	1st to 10th	Day	4 5% 2 4 3 2 3 3 4 6% 67%	2059
	11th to 20th	Day	4 5% 2 4 2 3 3 2 4 6% 66%	1864
	21st to end	Day	6% 6% 1 3 4 3 4 2 5% 6% 61%	1680
December	1st to 10th	Day	3 5% 2 3 2 3 3 3 6% 7% 63%	2700
	11th to 20th	Day	3 6% 2 5% 3 3 5% 4 7% 9% 53%	3106
	21st to end	Day	4 8% 3 7% 4 4 4 4 5% 6% 50%	4309
January	1st to 10th	Night	3 7% 4 7% 4 4 4 3 5% 7% 53%	5997
	11th to 20th	Night	2 6% 2 6% 4 4 4 3 5% 7% 59%	6010
	21st to end	Night	4 9% 3 6% 4 3 3 3 4 6% 56%	6611
February	1st to 10th	Night	3 8% 3 5% 3 3 4 4 6% 6% 55%	2600
	11th to 20th	Night	4 6% 2 6% 3 4 4 5% 6% 6% 54%	2532
	21st to end	Night	3 10% 3 6% 5% 5% 4 3 5% 5% 52%	1921
March	1st to 10th	Night	4 7% 2 5% 4 4 4 4 4 6% 55%	4940
	11th to 20th	Night	3 10% 3 8% 4 4 5% 4 6% 8% 46%	5000
	21st to end	Night	4 9% 3 7% 5% 4 5% 4 6% 6% 48%	5489
April	1st to 10th	Night	3 8% 2 6% 5% 4 4 4 5% 6% 53%	2072
	11th to 20th	Night	4 5% 3 7% 5% 4 4 3 5% 7% 52%	1977
	21st to end	Night	4 4 2 5% 3 4 4 4 5% 8% 56%	1913
May	1st to 10th	Night	4 6% 2 6% 4 4 5% 4 6% 6% 53%	4100
	11th to 20th	Night	3 5% 2 5% 3 3 4 4 6% 8% 56%	4074
	21st to end	Night	4 2 1 3 3 3 3 3 4 7% 68%	4336
June	1st to 10th	Night	2 2 1 4 3 3 3 4 6% 7% 64%	1600
	11th to 20th	Night	3 3 1 4 3 3 4 3 4 6% 66%	1600
	21st to end	Night	3 2 1 3 2 2 4 4 6% 6% 67%	1617
July	1st to 10th	Night	1 2 1 3 3 3 4 3 5% 6% 69%	4100
	11th to 20th	Night	1 2 1 5% 5% 5% 6% 5% 9% 11% 50%	4100
	21st to end	Night	2 3 1 5% 4 4 5% 4 7% 10% 56%	4472
August	1st to 10th	Night	2 3 2 6% 5% 6% 6% 5% 8% 10% 48%	4033
	11th to 20th	Night	2 2 1 4 3 4 5% 5% 7% 8% 59%	4051
	21st to end	Night	4 4 2 6% 5% 5% 6% 4 7% 9% 47%	4949
September	1st to 10th	Night	4 4 1 4 3 3 4 4 5% 7% 61%	2000
	11th to 20th	Night	3 3 1 4 3 3 4 3 5% 7% 63%	2147
	21st to end	Night	5% 3 1 4 3 2 3 3 3 5% 70%	2213
October	1st to 10th	Night	4 2 1 2 2 2 2 2 3 6% 74%	3920
	11th to 20th	Night	6% 5% 2 6% 3 4 4 3 4 6% 56%	3898
	21st to end	Night	4 4 2 4 2 3 2 2 3 6% 68%	4252
November	1st to 10th	Night	3 4 2 4 2 3 3 2 3 6% 66%	2460
	11th to 20th	Night	4 3 1 4 2 3 3 3 5% 6% 67%	2727
	21st to end	Night	5% 6% 2 5% 3 3 2 3 5% 6% 60%	2940
December	1st to 10th	Night	3 4 1 3 2 2 3 3 5% 7% 67%	4710
	11th to 20th	Night	4 6% 2 6% 3 3 3 3 5% 8% 57%	5321
	21st to end	Night	5% 7% 2 5% 4 3 3 3 6% 7% 54%	6940

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	26% 8% 16% 33% 14% 3	4189
	11th to 20th	Day	20% 9% 17% 22% 23% 9%	4190
	21st to end	Day	28% 7% 10% 24% 24% 7%	4609
February	1st to 10th	Day	30% 9% 17% 20% 19% 5%	1800
	11th to 20th	Day	32% 6% 12% 24% 21% 4	1764
	21st to end	Day	29% 7% 10% 22% 25% 7%	1705
March	1st to 10th	Day	24% 8% 11% 21% 29% 7%	4954
	11th to 20th	Day	34% 10% 12% 22% 19% 3	5000
	21st to end	Day	33% 11% 13% 23% 17% 2	5486
April	1st to 10th	Day	26% 5% 9% 21% 26% 12%	2327
	11th to 20th	Day	26% 7% 10% 22% 24% 11%	2374
	21st to end	Day	24% 7% 11% 20% 30% 7%	2483
May	1st to 10th	Day	22% 8% 13% 20% 26% 10%	5880
	11th to 20th	Day	20% 7% 8% 18% 32% 14%	5844
	21st to end	Day	14% 6% 8% 18% 40% 13%	6557
June	1st to 10th	Day	17% 5% 7% 18% 30% 24%	2800
	11th to 20th	Day	16% 3 5% 9% 24% 43%	2800
	21st to end	Day	14% 5% 7% 11% 23% 41%	2774
July	1st to 10th	Day	16% 4 7% 7% 13% 53%	5816
	11th to 20th	Day	19% 8% 9% 19% 25% 20%	5811
	21st to end	Day	18% 7% 11% 20% 21% 23%	6217
August	1st to 10th	Day	30% 10% 17% 18% 11% 14%	4208
	11th to 20th	Day	25% 9% 11% 14% 22% 18%	4197
	21st to end	Day	23% 9% 12% 19% 21% 16%	4343
September	1st to 10th	Day	24% 7% 9% 13% 23% 24%	2400
	11th to 20th	Day	21% 6% 10% 16% 21% 25%	2249
	21st to end	Day	19% 4 6% 15% 34% 21%	2186
October	1st to 10th	Day	14% 3 6% 12% 30% 35%	3270
	11th to 20th	Day	25% 5% 5% 24% 31% 9%	3250
	21st to end	Day	16% 6% 7% 28% 36% 6%	3548
November	1st to 10th	Day	18% 3 11% 28% 31% 9%	2059
	11th to 20th	Day	20% 4 8% 32% 32% 4	1864
	21st to end	Day	22% 5% 14% 26% 28% 6%	1680
December	1st to 10th	Day	20% 5% 12% 30% 28% 5%	2700
	11th to 20th	Day	23% 7% 16% 32% 21% 1	3104
	21st to end	Day	32% 7% 13% 21% 23% 5%	4309
January	1st to 10th	Night	27% 7% 15% 29% 18% 3	5997
	11th to 20th	Night	21% 9% 13% 23% 25% 9%	6010
	21st to end	Night	29% 6% 10% 24% 22% 8%	6611
February	1st to 10th	Night	26% 7% 16% 26% 19% 6%	2600
	11th to 20th	Night	27% 7% 14% 24% 22% 6%	2533
	21st to end	Night	29% 6% 10% 23% 25% 7%	1921
March	1st to 10th	Night	24% 7% 9% 17% 30% 12%	4940
	11th to 20th	Night	30% 9% 10% 24% 23% 4	5000
	21st to end	Night	28% 9% 11% 29% 20% 2	5489
April	1st to 10th	Night	27% 7% 9% 21% 25% 12%	2072
	11th to 20th	Night	28% 7% 8% 18% 26% 12%	1977
	21st to end	Night	23% 7% 9% 21% 31% 9%	1913
May	1st to 10th	Night	23% 8% 11% 18% 30% 10%	4097
	11th to 20th	Night	20% 6% 8% 19% 35% 12%	4075
	21st to end	Night	13% 5% 6% 17% 40% 19%	4336
June	1st to 10th	Night	16% 4 7% 16% 30% 27%	1600
	11th to 20th	Night	16% 5% 7% 11% 18% 43%	1600
	21st to end	Night	14% 4 6% 11% 21% 43%	1617
July	1st to 10th	Night	15% 5% 7% 8% 12% 53%	4055
	11th to 20th	Night	20% 6% 9% 19% 22% 23%	4046
	21st to end	Night	20% 6% 10% 18% 19% 27%	4344
August	1st to 10th	Night	33% 10% 16% 14% 13% 15%	2928
	11th to 20th	Night	22% 9% 13% 15% 17% 23%	2928
	21st to end	Night	24% 8% 11% 16% 22% 19%	3565
September	1st to 10th	Night	21% 6% 11% 13% 20% 30%	2000
	11th to 20th	Night	18% 6% 11% 18% 22% 27%	2147
	21st to end	Night	18% 4 6% 13% 35% 25%	2213
October	1st to 10th	Night	13% 3 6% 12% 27% 39%	3920
	11th to 20th	Night	27% 5% 7% 16% 33% 12%	3898
	21st to end	Night	18% 4 5% 29% 36% 7%	4252
November	1st to 10th	Night	19% 4 7% 24% 33% 13%	2460
	11th to 20th	Night	17% 5% 8% 28% 35% 6%	2727
	21st to end	Night	23% 6% 9% 23% 30% 9%	2940
December	1st to 10th	Night	16% 5% 9% 37% 27% 6%	4710
	11th to 20th	Night	23% 7% 11% 37% 20% 1	5322
	21st to end	Night	28% 7% 14% 24% 22% 5%	6940

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% 13% 32% 27% 11% 2	330
	11th to 20th	Day	11% 12% 32% 29% 13% 3	330
	21st to end	Day	6% 10% 32% 37% 12% 2	363
February	1st to 10th	Day	10% 20% 38% 22% 10%	300
	11th to 20th	Day	10% 19% 39% 23% 8%	300
	21st to end	Day	10% 23% 35% 24% 7% 1	359
March	1st to 10th	Day	13% 21% 31% 23% 11% 1	500
	11th to 20th	Day	12% 18% 33% 24% 11% 2	500
	21st to end	Day	14% 26% 33% 18% 7% 2	550
April	1st to 10th	Day	11% 20% 35% 21% 9% 3	500
	11th to 20th	Day	14% 23% 36% 19% 7% 1	492
	21st to end	Day	12% 23% 31% 23% 10% 1	500
May	1st to 10th	Day	10% 20% 38% 23% 9% 1	503
	11th to 20th	Day	9% 16% 31% 28% 13% 3	528
	21st to end	Day	7% 18% 27% 27% 17% 4	628
June	1st to 10th	Day	4 9% 25% 36% 23% 3	730
	11th to 20th	Day	6% 10% 27% 35% 18% 4	730
	21st to end	Day	3 8% 28% 38% 19% 4	704
July	1st to 10th	Day	4 4 24% 40% 24% 4	526
	11th to 20th	Day	3 2 17% 36% 35% 7%	524
	21st to end	Day	5% 2 13% 39% 34% 7%	575
August	1st to 10th	Day	5% 3 17% 40% 27% 6%	527
	11th to 20th	Day	4 4 22% 35% 28% 6%	517
	21st to end	Day	5% 3 19% 39% 29% 6%	583
September	1st to 10th	Day	5% 5% 18% 36% 28% 8%	530
	11th to 20th	Day	5% 10% 31% 33% 17% 4	530
	21st to end	Day	7% 12% 23% 33% 18% 6%	516
October	1st to 10th	Day	8% 15% 23% 22% 22% 9%	330
	11th to 20th	Day	8% 16% 26% 29% 17% 5%	330
	21st to end	Day	6% 15% 27% 30% 19% 3	363
November	1st to 10th	Day	8% 14% 28% 31% 16% 2	330
	11th to 20th	Day	6% 15% 33% 27% 16% 3	330
	21st to end	Day	11% 12% 29% 29% 17% 2	330
December	1st to 10th	Day	10% 15% 27% 28% 16% 3	330
	11th to 20th	Day	7% 13% 33% 29% 15% 4	330
	21st to end	Day	12% 14% 36% 29% 8% 1	363
January	1st to 10th	Night	9% 6% 25% 36% 20% 4	840
	11th to 20th	Night	8% 6% 23% 38% 23% 3	840
	21st to end	Night	8% 6% 26% 40% 17% 4	924
February	1st to 10th	Night	7% 9% 33% 34% 14% 3	800
	11th to 20th	Night	8% 8% 30% 37% 15% 2	784
	21st to end	Night	9% 10% 30% 38% 12% 1	660
March	1st to 10th	Night	7% 10% 29% 36% 16% 2	800
	11th to 20th	Night	6% 10% 24% 33% 25% 2	800
	21st to end	Night	9% 11% 28% 36% 15% 2	880
April	1st to 10th	Night	7% 13% 31% 35% 13% 1	672
	11th to 20th	Night	8% 14% 29% 34% 13% 2	593
	21st to end	Night	7% 12% 32% 28% 18% 4	600
May	1st to 10th	Night	8% 11% 27% 37% 15% 2	600
	11th to 20th	Night	6% 7% 22% 37% 23% 5%	591
	21st to end	Night	7% 10% 24% 32% 24% 3	650
June	1st to 10th	Night	3 5% 20% 38% 31% 3	600
	11th to 20th	Night	5% 7% 21% 40% 25% 4	600
	21st to end	Night	3 2 19% 40% 31% 5%	600
July	1st to 10th	Night	3 3 12% 34% 42% 6%	585
	11th to 20th	Night	3 1 9% 32% 44% 9%	582
	21st to end	Night	4 1 8% 31% 48% 8%	657
August	1st to 10th	Night	4 3 10% 33% 45% 6%	597
	11th to 20th	Night	4 2 10% 37% 41% 7%	594
	21st to end	Night	4 4 9% 35% 38% 10%	660
September	1st to 10th	Night	6% 1 11% 38% 37% 7%	600
	11th to 20th	Night	4 3 18% 34% 35% 7%	747
	21st to end	Night	6% 5% 17% 31% 34% 8%	800
October	1st to 10th	Night	8% 6% 20% 35% 27% 4	800
	11th to 20th	Night	5% 6% 22% 37% 25% 5%	800
	21st to end	Night	7% 6% 22% 35% 24% 6%	880
November	1st to 10th	Night	6% 5% 21% 37% 26% 4	820
	11th to 20th	Night	6% 5% 26% 39% 19% 5%	900
	21st to end	Night	9% 9% 23% 34% 21% 5%	1050
December	1st to 10th	Night	8% 6% 25% 39% 20% 2	1050
	11th to 20th	Night	11% 6% 28% 36% 18% 2	1050
	21st to end	Night	12% 9% 26% 38% 11% 2	1071

Page generated at 2026-01-28 23:21:45. Version ac07

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