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 2024-01-14.

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	4 5% 2 4 4 3 4 5% 6% 7% 55%	74641
	Night	4 5% 2 4 4 3 4 4 5% 7% 57%	75132
Transparency	Day	25% 7% 11% 20% 22% 14%	74366
	Night	24% 7% 11% 22% 24% 13%	74928
Seeing	Day	8% 13% 28% 30% 18% 4	13964
	Night	7% 7% 22% 35% 25% 5%	24329

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	5% 6% 3 4 5% 3 5% 5% 7% 9% 48%	1708
	11th to 20th	Day	4 6% 2 4 5% 2 3 4 5% 6% 57%	1647
	21st to end	Day	5% 7% 2 5% 4 4 5% 4 5% 6% 53%	1782
February	1st to 10th	Day	3 8% 3 6% 5% 3 3 4 6% 8% 52%	1620
	11th to 20th	Day	5% 7% 4 5% 5% 4 4 4 5% 8% 49%	1575
	21st to end	Day	6% 9% 4 4 5% 3 4 4 5% 6% 51%	1528
March	1st to 10th	Day	5% 9% 4 5% 4 3 4 4 5% 7% 50%	1980
	11th to 20th	Day	3 8% 3 5% 6% 3 4 5% 6% 8% 48%	1980
	21st to end	Day	5% 7% 4 6% 5% 3 4 5% 5% 7% 49%	2178
April	1st to 10th	Day	4 5% 3 6% 4 3 4 4 5% 7% 54%	2093
	11th to 20th	Day	3 6% 2 5% 4 3 5% 6% 6% 9% 50%	2131
	21st to end	Day	5% 4 2 5% 4 3 5% 6% 5% 7% 54%	2234
May	1st to 10th	Day	4 5% 3 6% 4 4 5% 6% 6% 8% 50%	2340
	11th to 20th	Day	5% 4 2 4 5% 3 5% 6% 6% 9% 52%	2301
	21st to end	Day	5% 3 2 5% 5% 2 5% 6% 6% 8% 54%	2579
June	1st to 10th	Day	3 3 1 3 3 2 3 3 4 9% 65%	2520
	11th to 20th	Day	3 4 1 3 3 2 3 3 4 6% 69%	2520
	21st to end	Day	2 2 1 3 4 2 4 4 5% 7% 67%	2498
July	1st to 10th	Day	4 3 2 4 4 4 6% 6% 6% 8% 52%	2340
	11th to 20th	Day	3 3 2 5% 6% 4 5% 7% 8% 11% 47%	2340
	21st to end	Day	4 4 2 5% 7% 5% 7% 8% 10% 10% 39%	2545
August	1st to 10th	Day	4 4 2 4 7% 4 7% 7% 9% 9% 43%	2328
	11th to 20th	Day	4 3 2 5% 5% 4 6% 6% 7% 9% 49%	2299
	21st to end	Day	4 3 2 4 5% 4 7% 6% 7% 9% 50%	2394
September	1st to 10th	Day	6% 3 1 4 4 3 5% 5% 6% 7% 58%	2160
	11th to 20th	Day	4 3 1 4 4 3 4 5% 5% 7% 59%	2025
	21st to end	Day	6% 3 2 3 2 2 3 3 4 6% 67%	1968
October	1st to 10th	Day	8% 3 2 4 4 3 3 4 6% 6% 58%	1800
	11th to 20th	Day	4 3 1 2 3 1 2 3 4 6% 72%	1800
	21st to end	Day	4 2 1 3 3 2 4 3 4 5% 67%	1980
November	1st to 10th	Day	4 4 2 3 3 1 2 3 4 6% 67%	1859
	11th to 20th	Day	5% 4 2 3 3 2 3 3 3 5% 67%	1687
	21st to end	Day	6% 5% 1 3 4 2 4 3 4 7% 62%	1520
December	1st to 10th	Day	6% 8% 3 5% 4 3 4 4 6% 7% 52%	1520
	11th to 20th	Day	5% 8% 3 7% 5% 3 6% 6% 6% 8% 44%	1926
	21st to end	Day	5% 7% 3 6% 5% 3 4 5% 5% 6% 50%	2936
January	1st to 10th	Night	6% 6% 2 4 3 2 3 4 4 8% 57%	2457
	11th to 20th	Night	5% 5% 2 5% 5% 3 4 4 4 6% 58%	2391
	21st to end	Night	6% 7% 2 5% 4 3 3 4 4 6% 56%	2574
February	1st to 10th	Night	3 7% 2 4 4 2 4 5% 6% 7% 57%	2340
	11th to 20th	Night	5% 6% 3 5% 4 2 5% 5% 6% 6% 54%	2272
	21st to end	Night	3 10% 3 5% 6% 4 4 4 5% 6% 51%	1724
March	1st to 10th	Night	5% 6% 2 5% 5% 3 5% 5% 5% 7% 53%	1980
	11th to 20th	Night	3 7% 2 5% 5% 2 3 5% 5% 6% 56%	1980
	21st to end	Night	5% 7% 3 5% 6% 3 5% 5% 5% 7% 49%	2178
April	1st to 10th	Night	3 8% 2 5% 5% 3 5% 5% 5% 6% 52%	1866
	11th to 20th	Night	5% 5% 3 6% 5% 3 5% 4 5% 7% 53%	1777
	21st to end	Night	5% 4 2 5% 4 3 4 4 5% 8% 56%	1723
May	1st to 10th	Night	5% 5% 2 4 5% 3 5% 5% 6% 7% 53%	1620
	11th to 20th	Night	4 5% 2 4 3 2 4 5% 5% 8% 55%	1594
	21st to end	Night	5% 4 2 4 4 2 4 3 5% 7% 59%	1692
June	1st to 10th	Night	3 1 1 3 3 2 3 5% 6% 8% 65%	1440
	11th to 20th	Night	3 3 1 3 3 2 3 4 5% 6% 66%	1440
	21st to end	Night	3 2 1 2 3 2 3 5% 6% 6% 68%	1455
July	1st to 10th	Night	2 3 2 3 5% 4 4 5% 6% 8% 58%	1620
	11th to 20th	Night	2 3 2 5% 6% 3 7% 6% 8% 10% 48%	1620
	21st to end	Night	3 3 2 6% 6% 5% 7% 7% 9% 12% 39%	1773
August	1st to 10th	Night	3 3 1 6% 4 4 5% 7% 8% 11% 48%	1611
	11th to 20th	Night	4 3 2 4 4 2 5% 6% 6% 9% 55%	1602
	21st to end	Night	3 4 2 4 4 3 5% 5% 7% 9% 54%	1962
September	1st to 10th	Night	5% 4 1 4 4 3 4 5% 6% 7% 59%	1800
	11th to 20th	Night	4 3 1 4 4 2 4 4 5% 7% 63%	1931
	21st to end	Night	5% 3 1 3 4 1 2 3 3 4 71%	1991
October	1st to 10th	Night	6% 3 1 3 3 2 4 4 4 7% 62%	2160
	11th to 20th	Night	3 3 2 3 2 1 3 3 4 5% 72%	2160
	21st to end	Night	4 4 1 3 3 2 2 3 3 6% 69%	2376
November	1st to 10th	Night	3 4 2 4 2 2 4 3 3 7% 66%	2220
	11th to 20th	Night	4 3 1 3 3 2 3 3 5% 6% 67%	2467
	21st to end	Night	5% 5% 2 4 3 2 2 4 5% 6% 61%	2660
December	1st to 10th	Night	6% 6% 2 4 4 2 4 4 4 7% 57%	2660
	11th to 20th	Night	6% 6% 2 6% 5% 3 5% 5% 6% 8% 47%	3271
	21st to end	Night	6% 8% 3 4 4 2 4 4 6% 8% 50%	4745

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	31% 8% 20% 29% 12% 1	1709
	11th to 20th	Day	25% 7% 18% 32% 16% 2	1647
	21st to end	Day	30% 9% 14% 32% 11% 5%	1782
February	1st to 10th	Day	29% 8% 19% 21% 20% 4	1620
	11th to 20th	Day	32% 7% 12% 24% 21% 4	1584
	21st to end	Day	32% 7% 11% 23% 23% 5%	1528
March	1st to 10th	Day	32% 6% 11% 25% 22% 4	1980
	11th to 20th	Day	28% 6% 12% 31% 17% 6%	1980
	21st to end	Day	29% 8% 13% 25% 20% 5%	2178
April	1st to 10th	Day	28% 5% 9% 21% 25% 12%	2093
	11th to 20th	Day	27% 7% 10% 22% 23% 10%	2134
	21st to end	Day	25% 6% 12% 21% 28% 8%	2234
May	1st to 10th	Day	27% 7% 11% 19% 26% 10%	2340
	11th to 20th	Day	26% 7% 11% 21% 24% 11%	2304
	21st to end	Day	23% 7% 10% 16% 29% 15%	2579
June	1st to 10th	Day	16% 4 7% 17% 30% 25%	2520
	11th to 20th	Day	17% 4 5% 9% 23% 42%	2520
	21st to end	Day	13% 5% 7% 12% 23% 41%	2498
July	1st to 10th	Day	26% 6% 10% 13% 15% 30%	2276
	11th to 20th	Day	26% 10% 13% 17% 15% 19%	2271
	21st to end	Day	31% 12% 17% 16% 11% 11%	2404
August	1st to 10th	Day	32% 10% 14% 14% 14% 17%	2313
	11th to 20th	Day	26% 9% 9% 15% 17% 22%	2299
	21st to end	Day	28% 8% 11% 17% 19% 17%	2394
September	1st to 10th	Day	25% 7% 9% 11% 22% 26%	2160
	11th to 20th	Day	23% 7% 10% 15% 20% 25%	2025
	21st to end	Day	19% 4 6% 15% 35% 21%	1968
October	1st to 10th	Day	25% 6% 10% 20% 24% 15%	1800
	11th to 20th	Day	14% 4 6% 22% 38% 16%	1800
	21st to end	Day	18% 7% 9% 27% 31% 8%	1980
November	1st to 10th	Day	18% 3 11% 28% 30% 10%	1859
	11th to 20th	Day	21% 4 8% 32% 32% 3	1687
	21st to end	Day	22% 5% 13% 26% 27% 6%	1520
December	1st to 10th	Day	30% 6% 17% 25% 17% 5%	1520
	11th to 20th	Day	33% 10% 17% 24% 14% 1	1924
	21st to end	Day	32% 9% 13% 15% 25% 7%	2936
January	1st to 10th	Night	24% 7% 20% 31% 17% 1	2457
	11th to 20th	Night	26% 6% 15% 32% 18% 2	2391
	21st to end	Night	29% 5% 14% 31% 16% 4	2574
February	1st to 10th	Night	25% 7% 16% 28% 20% 4	2340
	11th to 20th	Night	27% 7% 15% 23% 23% 5%	2273
	21st to end	Night	30% 6% 11% 24% 24% 5%	1724
March	1st to 10th	Night	28% 7% 11% 21% 28% 5%	1980
	11th to 20th	Night	26% 6% 8% 28% 25% 7%	1980
	21st to end	Night	31% 9% 11% 24% 20% 4	2178
April	1st to 10th	Night	29% 7% 9% 20% 23% 12%	1866
	11th to 20th	Night	27% 7% 9% 19% 27% 11%	1777
	21st to end	Night	24% 7% 9% 22% 29% 10%	1723
May	1st to 10th	Night	26% 7% 11% 17% 29% 10%	1620
	11th to 20th	Night	24% 6% 10% 23% 28% 11%	1595
	21st to end	Night	22% 6% 9% 18% 29% 16%	1692
June	1st to 10th	Night	15% 4 7% 16% 30% 28%	1440
	11th to 20th	Night	16% 5% 7% 11% 18% 42%	1440
	21st to end	Night	13% 4 6% 11% 23% 43%	1455
July	1st to 10th	Night	21% 7% 11% 12% 14% 34%	1575
	11th to 20th	Night	25% 8% 14% 17% 16% 20%	1566
	21st to end	Night	30% 10% 21% 17% 11% 11%	1674
August	1st to 10th	Night	26% 10% 14% 17% 16% 18%	1602
	11th to 20th	Night	21% 8% 11% 19% 15% 26%	1602
	21st to end	Night	23% 8% 11% 16% 21% 22%	1962
September	1st to 10th	Night	22% 6% 11% 12% 17% 32%	1800
	11th to 20th	Night	18% 6% 11% 18% 20% 27%	1931
	21st to end	Night	18% 3 6% 13% 36% 24%	1991
October	1st to 10th	Night	22% 5% 10% 19% 25% 19%	2160
	11th to 20th	Night	14% 4 7% 18% 35% 21%	2160
	21st to end	Night	19% 4 7% 28% 32% 11%	2376
November	1st to 10th	Night	19% 4 8% 24% 31% 13%	2220
	11th to 20th	Night	18% 5% 8% 29% 35% 6%	2467
	21st to end	Night	22% 6% 9% 23% 30% 10%	2660
December	1st to 10th	Night	26% 7% 14% 26% 21% 6%	2660
	11th to 20th	Night	30% 10% 15% 25% 19% 1	3272
	21st to end	Night	30% 9% 14% 17% 23% 7%	4745

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	16% 12% 31% 26% 12% 3	270
	11th to 20th	Day	12% 12% 32% 28% 12% 3	249
	21st to end	Day	7% 11% 31% 35% 14% 3	264
February	1st to 10th	Day	9% 18% 37% 25% 11%	240
	11th to 20th	Day	11% 18% 38% 23% 10%	240
	21st to end	Day	9% 25% 35% 24% 6% 1	296
March	1st to 10th	Day	12% 21% 33% 23% 10%	420
	11th to 20th	Day	11% 17% 34% 25% 11% 2	420
	21st to end	Day	13% 27% 34% 18% 6% 1	462
April	1st to 10th	Day	11% 21% 36% 21% 8% 3	420
	11th to 20th	Day	15% 24% 36% 17% 7% 1	412
	21st to end	Day	14% 23% 32% 20% 10% 1	420
May	1st to 10th	Day	9% 21% 39% 23% 8% 1	423
	11th to 20th	Day	9% 16% 31% 26% 14% 3	448
	21st to end	Day	8% 18% 28% 25% 17% 4	534
June	1st to 10th	Day	5% 10% 25% 34% 24% 3	630
	11th to 20th	Day	6% 9% 26% 35% 19% 4	630
	21st to end	Day	3 7% 28% 39% 20% 4	608
July	1st to 10th	Day	4 3 22% 41% 25% 4	446
	11th to 20th	Day	3 2 18% 36% 35% 7%	444
	21st to end	Day	6% 1 10% 38% 37% 8%	487
August	1st to 10th	Day	6% 2 16% 43% 27% 6%	447
	11th to 20th	Day	5% 4 22% 35% 27% 7%	437
	21st to end	Day	5% 2 19% 38% 29% 6%	495
September	1st to 10th	Day	5% 5% 19% 36% 27% 8%	450
	11th to 20th	Day	5% 8% 29% 35% 18% 4	450
	21st to end	Day	8% 13% 22% 34% 17% 6%	438
October	1st to 10th	Day	10% 16% 24% 22% 21% 8%	270
	11th to 20th	Day	6% 14% 26% 30% 19% 6%	270
	21st to end	Day	7% 16% 27% 30% 19% 2	297
November	1st to 10th	Day	9% 16% 28% 31% 14% 2	270
	11th to 20th	Day	6% 14% 35% 26% 17% 3	270
	21st to end	Day	11% 14% 28% 29% 17% 1	270
December	1st to 10th	Day	11% 17% 28% 25% 16% 2	270
	11th to 20th	Day	8% 14% 34% 29% 12% 4	270
	21st to end	Day	13% 14% 34% 30% 8% 1	297
January	1st to 10th	Night	9% 6% 23% 36% 22% 5%	760
	11th to 20th	Night	9% 6% 23% 36% 23% 3	736
	21st to end	Night	9% 6% 24% 39% 17% 4	792
February	1st to 10th	Night	7% 7% 31% 36% 16% 3	720
	11th to 20th	Night	8% 8% 29% 36% 16% 3	704
	21st to end	Night	9% 10% 30% 37% 13% 1	592
March	1st to 10th	Night	7% 10% 29% 36% 16% 2	720
	11th to 20th	Night	6% 10% 23% 33% 26% 3	720
	21st to end	Night	9% 11% 27% 35% 15% 2	792
April	1st to 10th	Night	7% 13% 30% 35% 13% 1	606
	11th to 20th	Night	8% 15% 27% 34% 14% 3	533
	21st to end	Night	8% 12% 31% 27% 19% 4	540
May	1st to 10th	Night	8% 10% 27% 38% 15% 2	540
	11th to 20th	Night	7% 8% 21% 36% 24% 5%	531
	21st to end	Night	8% 10% 24% 30% 25% 3	584
June	1st to 10th	Night	3 5% 22% 37% 31% 3	540
	11th to 20th	Night	4 7% 21% 38% 26% 4	540
	21st to end	Night	3 2 17% 40% 32% 6%	540
July	1st to 10th	Night	3 3 11% 31% 45% 7%	525
	11th to 20th	Night	4 1 9% 31% 45% 10%	522
	21st to end	Night	4 6% 28% 52% 9%	591
August	1st to 10th	Night	4 2 9% 31% 47% 6%	537
	11th to 20th	Night	4 2 10% 35% 41% 7%	534
	21st to end	Night	4 3 8% 35% 39% 10%	594
September	1st to 10th	Night	7% 1 11% 36% 36% 8%	540
	11th to 20th	Night	4 3 15% 33% 38% 8%	671
	21st to end	Night	6% 5% 16% 29% 34% 9%	720
October	1st to 10th	Night	8% 6% 21% 34% 27% 5%	720
	11th to 20th	Night	4 5% 20% 38% 27% 5%	720
	21st to end	Night	8% 5% 22% 34% 25% 7%	792
November	1st to 10th	Night	6% 6% 20% 36% 27% 5%	740
	11th to 20th	Night	6% 5% 24% 39% 20% 6%	814
	21st to end	Night	9% 9% 22% 33% 22% 5%	950
December	1st to 10th	Night	9% 7% 24% 38% 19% 2	950
	11th to 20th	Night	12% 6% 27% 34% 18% 2	950
	21st to end	Night	13% 10% 24% 39% 12% 2	969

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