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 2024-01-15.

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	8% 4 1 3 3 2 3 3 4 5% 64%	74800
	Night	9% 4 1 3 3 2 3 3 4 6% 64%	74979
Transparency	Day	23% 4 8% 19% 26% 19%	74512
	Night	23% 4 9% 21% 26% 17%	74790
Seeing	Day	11% 9% 28% 33% 14% 5%	14116
	Night	10% 3 15% 33% 29% 10%	23735

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	18% 7% 2 3 3 2 4 4 6% 8% 43%	1519
	11th to 20th	Day	14% 5% 1 4 4 2 4 4 4 7% 52%	1571
	21st to end	Day	16% 10% 2 3 4 3 4 3 5% 6% 44%	1782
February	1st to 10th	Day	15% 8% 2 3 2 2 3 4 4 6% 51%	1722
	11th to 20th	Day	12% 10% 3 4 3 2 3 4 4 6% 48%	1750
	21st to end	Day	18% 10% 4 5% 4 3 3 3 4 5% 41%	1480
March	1st to 10th	Day	18% 10% 3 6% 4 3 3 4 5% 6% 37%	1800
	11th to 20th	Day	16% 9% 3 5% 5% 2 4 4 5% 8% 41%	1921
	21st to end	Day	15% 8% 3 4 4 2 4 4 5% 6% 45%	2338
April	1st to 10th	Day	13% 6% 3 4 4 2 3 4 5% 6% 51%	2160
	11th to 20th	Day	12% 6% 3 4 4 2 3 3 4 6% 53%	2129
	21st to end	Day	9% 6% 2 5% 4 2 3 3 5% 7% 56%	2160
May	1st to 10th	Day	7% 5% 2 3 3 2 2 3 4 5% 64%	2227
	11th to 20th	Day	6% 4 1 3 3 2 3 3 4 7% 64%	2301
	21st to end	Day	8% 3 1 3 4 2 3 3 4 6% 62%	2711
June	1st to 10th	Day	3 3 1 3 3 2 3 4 5% 6% 67%	2520
	11th to 20th	Day	1 1 1 2 1 1 2 3 4 83%	2520
	21st to end	Day	1 2 1 2 2 1 1 2 2 3 84%	2520
July	1st to 10th	Day	1 1 1 1 1 1 1 3 2 3 85%	2520
	11th to 20th	Day	2 1 1 1 1 1 2 2 3 3 82%	2520
	21st to end	Day	2 2 1 2 2 1 2 2 4 5% 77%	2615
August	1st to 10th	Day	2 2 2 1 1 2 2 2 3 84%	2199
	11th to 20th	Day	1 1 1 1 1 2 2 2 3 86%	2122
	21st to end	Day	1 1 1 1 1 1 1 1 2 3 86%	2376
September	1st to 10th	Day	2 1 1 1 1 2 2 2 3 86%	2160
	11th to 20th	Day	2 2 2 2 1 2 2 3 4 81%	2102
	21st to end	Day	3 2 1 2 1 1 2 3 3 4 79%	1980
October	1st to 10th	Day	5% 4 1 2 3 2 2 3 4 4 70%	1890
	11th to 20th	Day	4 2 2 3 2 3 3 4 4 72%	1800
	21st to end	Day	6% 2 1 2 2 1 2 3 4 5% 71%	1954
November	1st to 10th	Day	9% 5% 2 4 3 2 3 4 4 6% 57%	1674
	11th to 20th	Day	8% 3 1 3 3 2 3 4 6% 6% 62%	1710
	21st to end	Day	9% 5% 2 3 4 3 2 4 4 7% 58%	1710
December	1st to 10th	Day	13% 4 1 3 4 2 4 3 5% 7% 53%	1615
	11th to 20th	Day	24% 9% 3 6% 5% 3 4 3 4 5% 35%	1928
	21st to end	Day	13% 8% 3 5% 5% 3 4 4 6% 9% 38%	2794
January	1st to 10th	Night	17% 6% 2 4 3 2 4 4 5% 7% 47%	2646
	11th to 20th	Night	12% 4 2 3 4 3 4 4 6% 7% 52%	2474
	21st to end	Night	16% 6% 1 3 3 2 3 4 4 8% 50%	2574
February	1st to 10th	Night	15% 6% 2 3 2 2 3 3 5% 7% 52%	2236
	11th to 20th	Night	12% 6% 1 3 3 2 3 3 4 7% 56%	2098
	21st to end	Night	17% 8% 2 4 4 2 2 3 4 4 50%	1776
March	1st to 10th	Night	16% 8% 2 4 4 2 3 4 5% 6% 46%	2160
	11th to 20th	Night	14% 7% 2 4 3 2 4 4 4 8% 48%	2034
	21st to end	Night	14% 6% 2 3 2 2 3 2 3 6% 56%	2016
April	1st to 10th	Night	12% 5% 1 3 3 2 3 4 4 7% 57%	1800
	11th to 20th	Night	9% 4 1 3 4 2 3 3 4 5% 62%	1779
	21st to end	Night	7% 4 1 2 3 2 2 3 4 6% 65%	1800
May	1st to 10th	Night	6% 3 1 3 3 2 2 4 4 7% 66%	1731
	11th to 20th	Night	6% 2 1 2 2 1 2 3 4 6% 72%	1594
	21st to end	Night	5% 3 1 2 2 1 3 2 4 5% 73%	1560
June	1st to 10th	Night	5% 2 1 2 2 2 2 2 3 6% 74%	1440
	11th to 20th	Night	1 1 1 1 1 1 1 2 3 3 85%	1440
	21st to end	Night	1 1 1 1 1 2 2 2 3 88%	1440
July	1st to 10th	Night	1 1 1 1 1 3 4 88%	1440
	11th to 20th	Night	2 2 1 1 2 1 2 2 3 4 80%	1440
	21st to end	Night	2 1 1 2 3 1 3 3 4 4 75%	1699
August	1st to 10th	Night	2 3 1 1 1 1 1 1 2 3 85%	1736
	11th to 20th	Night	1 1 1 2 1 2 2 2 3 86%	1780
	21st to end	Night	2 1 1 2 1 1 1 2 3 87%	1980
September	1st to 10th	Night	1 1 1 1 1 1 2 2 90%	1800
	11th to 20th	Night	2 2 1 1 1 1 2 3 3 84%	1857
	21st to end	Night	2 1 1 2 1 2 2 3 3 83%	1980
October	1st to 10th	Night	4 3 1 2 2 2 2 3 3 4 74%	2065
	11th to 20th	Night	5% 1 1 1 2 1 3 3 4 5% 75%	2160
	21st to end	Night	4 1 1 2 2 1 2 2 2 4 79%	2401
November	1st to 10th	Night	10% 4 1 3 3 2 2 2 4 6% 64%	2405
	11th to 20th	Night	7% 3 1 3 3 1 3 3 5% 7% 64%	2470
	21st to end	Night	9% 5% 2 3 3 2 4 3 5% 6% 59%	2470
December	1st to 10th	Night	13% 5% 2 4 3 2 4 3 4 7% 52%	2560
	11th to 20th	Night	25% 6% 2 4 4 2 3 4 5% 6% 39%	3276
	21st to end	Night	13% 7% 3 6% 6% 3 5% 5% 7% 8% 37%	4862

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	37% 6% 14% 22% 17% 4	1519
	11th to 20th	Day	32% 7% 10% 20% 18% 13%	1571
	21st to end	Day	41% 5% 10% 17% 20% 7%	1782
February	1st to 10th	Day	34% 4 9% 26% 18% 9%	1722
	11th to 20th	Day	35% 5% 11% 22% 20% 7%	1760
	21st to end	Day	46% 5% 8% 23% 17% 2	1480
March	1st to 10th	Day	46% 7% 9% 17% 16% 4	1800
	11th to 20th	Day	40% 6% 12% 20% 17% 5%	1921
	21st to end	Day	39% 7% 14% 23% 13% 4	2338
April	1st to 10th	Day	33% 7% 13% 24% 20% 4	2160
	11th to 20th	Day	33% 6% 10% 24% 23% 4	2132
	21st to end	Day	28% 7% 10% 32% 21% 2	2160
May	1st to 10th	Day	23% 5% 10% 29% 29% 4	2227
	11th to 20th	Day	21% 5% 9% 29% 30% 6%	2304
	21st to end	Day	24% 6% 9% 33% 24% 4	2712
June	1st to 10th	Day	17% 6% 11% 21% 33% 13%	2520
	11th to 20th	Day	8% 3 6% 19% 41% 23%	2520
	21st to end	Day	8% 2 3 11% 38% 38%	2520
July	1st to 10th	Day	7% 1 4 8% 28% 52%	2451
	11th to 20th	Day	9% 2 4 11% 29% 45%	2445
	21st to end	Day	11% 3 7% 12% 28% 40%	2470
August	1st to 10th	Day	9% 2 4 10% 29% 47%	2183
	11th to 20th	Day	5% 1 4 11% 38% 40%	2122
	21st to end	Day	7% 1 3 9% 39% 41%	2376
September	1st to 10th	Day	8% 1 4 8% 33% 46%	2160
	11th to 20th	Day	10% 2 4 22% 37% 26%	2102
	21st to end	Day	11% 2 5% 19% 37% 26%	1980
October	1st to 10th	Day	18% 4 6% 17% 29% 26%	1890
	11th to 20th	Day	16% 3 7% 19% 30% 25%	1800
	21st to end	Day	16% 4 7% 20% 32% 21%	1954
November	1st to 10th	Day	27% 5% 10% 18% 23% 16%	1674
	11th to 20th	Day	22% 4 11% 23% 29% 11%	1710
	21st to end	Day	28% 4 9% 17% 27% 15%	1710
December	1st to 10th	Day	31% 5% 11% 21% 20% 11%	1615
	11th to 20th	Day	51% 5% 9% 14% 13% 8%	1928
	21st to end	Day	42% 6% 15% 23% 10% 4	2794
January	1st to 10th	Night	35% 5% 12% 24% 19% 4	2646
	11th to 20th	Night	28% 7% 13% 23% 19% 10%	2474
	21st to end	Night	34% 6% 11% 19% 23% 7%	2574
February	1st to 10th	Night	33% 4 9% 25% 19% 10%	2236
	11th to 20th	Night	29% 5% 10% 26% 23% 8%	2099
	21st to end	Night	38% 5% 11% 25% 17% 4	1776
March	1st to 10th	Night	37% 6% 13% 20% 20% 4	2160
	11th to 20th	Night	34% 6% 13% 22% 19% 5%	2034
	21st to end	Night	32% 5% 16% 28% 15% 4	2016
April	1st to 10th	Night	28% 5% 16% 27% 17% 6%	1800
	11th to 20th	Night	25% 6% 11% 28% 22% 7%	1779
	21st to end	Night	21% 5% 14% 36% 22% 2	1800
May	1st to 10th	Night	19% 5% 11% 30% 28% 6%	1731
	11th to 20th	Night	16% 4 8% 32% 34% 7%	1595
	21st to end	Night	16% 5% 11% 33% 30% 5%	1560
June	1st to 10th	Night	15% 4 11% 24% 36% 11%	1440
	11th to 20th	Night	7% 2 6% 23% 39% 24%	1440
	21st to end	Night	6% 2 2 12% 38% 39%	1440
July	1st to 10th	Night	4 1 4 7% 32% 52%	1400
	11th to 20th	Night	10% 2 4 9% 31% 43%	1392
	21st to end	Night	13% 4 5% 11% 29% 39%	1605
August	1st to 10th	Night	9% 1 4 14% 26% 47%	1727
	11th to 20th	Night	7% 2 4 10% 36% 41%	1780
	21st to end	Night	7% 1 2 9% 37% 44%	1980
September	1st to 10th	Night	5% 1 2 11% 29% 53%	1800
	11th to 20th	Night	8% 2 4 16% 45% 25%	1857
	21st to end	Night	8% 2 5% 20% 40% 24%	1980
October	1st to 10th	Night	15% 5% 6% 18% 30% 27%	2065
	11th to 20th	Night	12% 4 7% 20% 31% 26%	2160
	21st to end	Night	12% 2 5% 24% 32% 24%	2401
November	1st to 10th	Night	24% 4 10% 20% 25% 18%	2405
	11th to 20th	Night	21% 4 9% 23% 29% 14%	2470
	21st to end	Night	27% 4 8% 19% 30% 12%	2470
December	1st to 10th	Night	33% 5% 10% 20% 20% 13%	2560
	11th to 20th	Night	45% 5% 11% 16% 16% 7%	3276
	21st to end	Night	40% 7% 16% 22% 11% 5%	4862

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	31% 12% 24% 20% 12% 1	180
	11th to 20th	Day	18% 10% 24% 29% 15% 5%	267
	21st to end	Day	21% 7% 24% 30% 15% 4	374
February	1st to 10th	Day	23% 9% 25% 24% 15% 5%	386
	11th to 20th	Day	19% 13% 32% 25% 9% 2	415
	21st to end	Day	22% 15% 32% 21% 9% 1	346
March	1st to 10th	Day	25% 13% 33% 21% 7% 1	420
	11th to 20th	Day	23% 16% 27% 20% 11% 3	420
	21st to end	Day	23% 15% 32% 22% 7%	462
April	1st to 10th	Day	16% 15% 32% 25% 10% 2	420
	11th to 20th	Day	21% 17% 28% 26% 7% 1	412
	21st to end	Day	10% 15% 31% 28% 14% 3	420
May	1st to 10th	Day	13% 17% 31% 26% 8% 4	423
	11th to 20th	Day	13% 13% 30% 28% 11% 5%	448
	21st to end	Day	15% 10% 26% 32% 11% 7%	485
June	1st to 10th	Day	4 11% 28% 33% 16% 7%	450
	11th to 20th	Day	3 10% 26% 40% 14% 7%	450
	21st to end	Day	2 7% 31% 36% 16% 8%	450
July	1st to 10th	Day	2 1 24% 45% 18% 9%	446
	11th to 20th	Day	1 20% 47% 19% 13%	444
	21st to end	Day	3 1 22% 48% 15% 11%	487
August	1st to 10th	Day	2 4 33% 40% 12% 9%	447
	11th to 20th	Day	1 3 27% 44% 15% 10%	437
	21st to end	Day	1 3 26% 46% 17% 7%	495
September	1st to 10th	Day	3 4 30% 41% 16% 5%	450
	11th to 20th	Day	4 9% 32% 38% 15% 2	450
	21st to end	Day	6% 8% 28% 42% 15% 2	450
October	1st to 10th	Day	5% 10% 32% 38% 14% 1	450
	11th to 20th	Day	4 9% 30% 40% 15% 2	450
	21st to end	Day	6% 10% 29% 33% 20% 2	469
November	1st to 10th	Day	11% 10% 29% 28% 19% 4	270
	11th to 20th	Day	7% 8% 30% 31% 18% 5%	270
	21st to end	Day	12% 10% 27% 30% 16% 5%	270
December	1st to 10th	Day	16% 6% 29% 28% 18% 2	225
	11th to 20th	Day	15% 10% 29% 27% 16% 3	180
	21st to end	Day	23% 9% 30% 22% 11% 6%	198
January	1st to 10th	Night	19% 5% 19% 28% 18% 11%	760
	11th to 20th	Night	12% 3 22% 36% 18% 9%	736
	21st to end	Night	21% 4 18% 31% 19% 8%	792
February	1st to 10th	Night	18% 3 18% 34% 20% 8%	720
	11th to 20th	Night	15% 5% 19% 30% 23% 8%	704
	21st to end	Night	19% 4 18% 38% 17% 4	592
March	1st to 10th	Night	19% 6% 17% 34% 18% 6%	720
	11th to 20th	Night	16% 5% 16% 27% 26% 10%	720
	21st to end	Night	16% 6% 16% 36% 22% 4	792
April	1st to 10th	Night	14% 4 17% 38% 21% 6%	720
	11th to 20th	Night	10% 5% 21% 32% 26% 6%	711
	21st to end	Night	8% 3 20% 37% 23% 9%	720
May	1st to 10th	Night	6% 3 19% 37% 27% 7%	651
	11th to 20th	Night	7% 2 15% 38% 32% 5%	531
	21st to end	Night	7% 2 18% 30% 32% 11%	397
June	1st to 10th	Night	6% 12% 36% 39% 8%	360
	11th to 20th	Night	3 11% 39% 40% 8%	360
	21st to end	Night	1 2 11% 33% 45% 9%	360
July	1st to 10th	Night	9% 29% 53% 9%	350
	11th to 20th	Night	2 1 9% 27% 49% 12%	348
	21st to end	Night	1 1 7% 24% 49% 19%	517
August	1st to 10th	Night	3 1 8% 31% 45% 12%	662
	11th to 20th	Night	1 6% 29% 52% 12%	712
	21st to end	Night	3 6% 26% 53% 13%	792
September	1st to 10th	Night	2 1 6% 28% 49% 14%	720
	11th to 20th	Night	3 1 8% 35% 40% 14%	720
	21st to end	Night	3 12% 33% 36% 15%	720
October	1st to 10th	Night	5% 2 11% 44% 27% 11%	720
	11th to 20th	Night	6% 1 14% 35% 31% 13%	720
	21st to end	Night	5% 4 15% 30% 29% 18%	792
November	1st to 10th	Night	11% 3 13% 30% 31% 13%	740
	11th to 20th	Night	9% 2 18% 37% 25% 9%	760
	21st to end	Night	12% 2 19% 34% 23% 9%	760
December	1st to 10th	Night	15% 4 18% 36% 19% 7%	760
	11th to 20th	Night	21% 3 22% 31% 18% 6%	760
	21st to end	Night	19% 7% 21% 31% 17% 4	836

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