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 2024-11-30.

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 4 5% 66%	105183
	Night	9% 4 1 3 2 2 3 2 4 5% 65%	103149
Transparency	Day	21% 4 8% 18% 26% 23%	104932
	Night	22% 4 9% 20% 24% 21%	102991
Seeing	Day	11% 10% 29% 33% 13% 5%	15344
	Night	10% 3 16% 33% 28% 9%	24866

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	13% 9% 2 6% 3 3 4 4 6% 8% 42%	2539
	11th to 20th	Day	15% 7% 3 8% 5% 5% 5% 4 5% 6% 36%	2709
	21st to end	Day	17% 9% 2 6% 4 4 5% 4 7% 8% 35%	3146
February	1st to 10th	Day	16% 10% 1 4 2 2 3 3 5% 5% 48%	1817
	11th to 20th	Day	12% 11% 3 5% 3 3 4 3 4 6% 47%	1850
	21st to end	Day	17% 10% 4 6% 4 3 3 2 4 5% 41%	1570
March	1st to 10th	Day	14% 14% 3 8% 4 4 4 4 6% 7% 32%	3138
	11th to 20th	Day	10% 6% 2 4 3 3 3 3 4 6% 56%	3392
	21st to end	Day	18% 12% 3 7% 4 4 4 3 5% 5% 35%	4101
April	1st to 10th	Day	12% 7% 2 4 3 3 3 3 5% 6% 51%	2280
	11th to 20th	Day	11% 8% 2 4 4 3 3 3 4 6% 53%	2249
	21st to end	Day	9% 6% 1 5% 4 3 3 3 5% 6% 56%	2280
May	1st to 10th	Day	6% 4 1 3 2 2 2 2 4 4 71%	3919
	11th to 20th	Day	3 3 1 2 1 2 2 2 3 5% 76%	4071
	21st to end	Day	5% 2 1 2 2 2 2 2 3 4 76%	4801
June	1st to 10th	Day	3 3 1 4 3 3 3 3 5% 5% 68%	2660
	11th to 20th	Day	1 1 1 2 1 2 1 1 3 4 84%	2660
	21st to end	Day	1 2 1 2 2 1 1 1 2 3 84%	2660
July	1st to 10th	Day	1 1 1 1 1 1 1 2 91%	4420
	11th to 20th	Day	1 1 2 1 2 2 2 2 3 84%	4420
	21st to end	Day	1 1 1 1 1 1 1 2 2 3 85%	4563
August	1st to 10th	Day	2 2 2 1 1 1 1 2 2 86%	3858
	11th to 20th	Day	1 1 1 1 1 1 2 91%	3724
	21st to end	Day	1 1 1 1 1 1 1 2 2 89%	4180
September	1st to 10th	Day	2 1 1 1 2 2 1 2 2 86%	2280
	11th to 20th	Day	2 2 3 1 2 2 2 3 4 80%	2218
	21st to end	Day	3 2 2 1 2 2 2 3 4 80%	2090
October	1st to 10th	Day	3 3 1 2 2 2 3 3 3 4 75%	3335
	11th to 20th	Day	3 2 1 2 2 2 2 2 3 3 77%	3161
	21st to end	Day	4 3 1 3 3 3 4 3 4 7% 63%	3396
November	1st to 10th	Day	9% 5% 1 4 3 3 3 3 4 6% 58%	1764
	11th to 20th	Day	8% 4 1 3 2 3 3 3 6% 6% 61%	1800
	21st to end	Day	10% 6% 1 3 4 3 3 3 4 7% 57%	1795
December	1st to 10th	Day	13% 4 1 4 4 4 3 3 5% 7% 53%	1615
	11th to 20th	Day	24% 9% 3 7% 4 4 4 3 4 5% 35%	1928
	21st to end	Day	13% 8% 3 6% 5% 4 4 3 6% 9% 38%	2794
January	1st to 10th	Night	14% 8% 3 6% 3 3 3 3 7% 7% 43%	4406
	11th to 20th	Night	17% 8% 2 6% 4 4 4 3 6% 7% 38%	4253
	21st to end	Night	19% 7% 2 5% 4 3 4 4 6% 8% 38%	4521
February	1st to 10th	Night	16% 7% 2 4 2 3 3 3 5% 6% 50%	2361
	11th to 20th	Night	13% 6% 1 4 3 3 3 3 4 7% 54%	2218
	21st to end	Night	16% 8% 2 4 3 3 3 2 4 5% 50%	1884
March	1st to 10th	Night	19% 8% 2 5% 3 3 4 4 6% 7% 38%	3736
	11th to 20th	Night	10% 5% 1 4 2 2 3 2 3 6% 60%	3583
	21st to end	Night	19% 8% 2 4 3 3 4 3 4 7% 44%	3550
April	1st to 10th	Night	11% 5% 1 3 3 2 2 3 5% 7% 57%	1900
	11th to 20th	Night	8% 5% 1 4 4 2 3 2 5% 5% 61%	1879
	21st to end	Night	7% 4 1 3 3 3 2 3 4 6% 65%	1900
May	1st to 10th	Night	5% 3 2 2 1 2 2 3 4 75%	3055
	11th to 20th	Night	3 1 1 1 1 1 2 2 4 5% 79%	2834
	21st to end	Night	3 2 1 2 1 1 2 1 2 3 83%	2770
June	1st to 10th	Night	4 2 1 2 2 2 2 2 3 6% 74%	1520
	11th to 20th	Night	1 1 1 1 1 2 1 3 3 85%	1520
	21st to end	Night	1 1 1 1 1 2 1 2 3 88%	1520
July	1st to 10th	Night	1 1 2 2 93%	2540
	11th to 20th	Night	2 2 1 2 2 1 2 2 3 3 80%	2540
	21st to end	Night	1 1 1 1 2 1 2 2 3 3 85%	2979
August	1st to 10th	Night	1 2 1 2 1 2 2 1 3 4 82%	3085
	11th to 20th	Night	1 1 1 1 1 1 1 2 91%	3126
	21st to end	Night	2 1 1 1 1 1 1 1 2 89%	3498
September	1st to 10th	Night	1 1 1 1 1 1 2 2 90%	1900
	11th to 20th	Night	2 2 1 1 1 1 2 2 3 2 83%	1961
	21st to end	Night	2 1 1 1 1 2 2 3 3 84%	2090
October	1st to 10th	Night	2 2 2 1 1 2 2 2 3 82%	3640
	11th to 20th	Night	3 1 1 1 1 1 2 2 3 4 81%	3777
	21st to end	Night	5% 2 1 3 1 1 1 1 3 5% 76%	4170
November	1st to 10th	Night	9% 4 1 3 3 2 2 2 4 6% 64%	2535
	11th to 20th	Night	7% 3 1 3 2 2 3 2 5% 7% 64%	2600
	21st to end	Night	10% 5% 2 3 3 3 3 3 5% 6% 57%	2600
December	1st to 10th	Night	13% 5% 2 5% 3 3 3 2 4 7% 52%	2560
	11th to 20th	Night	24% 6% 2 4 3 3 3 3 5% 6% 39%	3276
	21st to end	Night	13% 7% 2 7% 4 5% 5% 4 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	36% 6% 21% 23% 12% 3	2539
	11th to 20th	Day	45% 9% 12% 16% 11% 8%	2709
	21st to end	Day	43% 8% 14% 20% 12% 4	3146
February	1st to 10th	Day	36% 5% 9% 25% 17% 9%	1817
	11th to 20th	Day	36% 5% 11% 22% 20% 7%	1860
	21st to end	Day	45% 6% 8% 23% 17% 2	1570
March	1st to 10th	Day	45% 11% 11% 16% 14% 3	3138
	11th to 20th	Day	26% 8% 13% 33% 12% 9%	3392
	21st to end	Day	45% 9% 13% 19% 11% 2	4101
April	1st to 10th	Day	33% 6% 12% 25% 20% 3	2280
	11th to 20th	Day	33% 6% 10% 25% 22% 4	2252
	21st to end	Day	27% 7% 10% 33% 21% 2	2280
May	1st to 10th	Day	16% 4 8% 31% 32% 9%	3919
	11th to 20th	Day	12% 3 7% 28% 43% 6%	4074
	21st to end	Day	14% 4 6% 32% 40% 4	4802
June	1st to 10th	Day	17% 6% 11% 21% 34% 12%	2660
	11th to 20th	Day	7% 3 6% 19% 43% 22%	2660
	21st to end	Day	8% 2 3 10% 37% 40%	2660
July	1st to 10th	Day	4 1 2 5% 16% 73%	4351
	11th to 20th	Day	8% 2 3 7% 19% 62%	4345
	21st to end	Day	7% 2 4 10% 27% 51%	4419
August	1st to 10th	Day	8% 1 3 6% 35% 47%	3843
	11th to 20th	Day	3 1 3 6% 30% 57%	3760
	21st to end	Day	6% 1 3 8% 34% 49%	4179
September	1st to 10th	Day	7% 1 4 7% 34% 46%	2280
	11th to 20th	Day	10% 2 4 21% 38% 25%	2218
	21st to end	Day	10% 2 5% 18% 37% 28%	2090
October	1st to 10th	Day	14% 2 4 10% 23% 46%	3335
	11th to 20th	Day	14% 2 4 18% 40% 22%	3161
	21st to end	Day	21% 2 6% 16% 37% 18%	3396
November	1st to 10th	Day	26% 4 10% 20% 24% 16%	1764
	11th to 20th	Day	23% 4 11% 24% 28% 11%	1800
	21st to end	Day	29% 4 9% 17% 27% 14%	1795
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	33% 6% 19% 24% 14% 2	4406
	11th to 20th	Night	42% 7% 16% 18% 11% 6%	4253
	21st to end	Night	40% 9% 12% 21% 13% 4	4521
February	1st to 10th	Night	35% 5% 8% 24% 18% 10%	2361
	11th to 20th	Night	30% 6% 10% 26% 22% 7%	2219
	21st to end	Night	37% 6% 12% 25% 17% 4	1884
March	1st to 10th	Night	41% 8% 12% 19% 18% 3	3736
	11th to 20th	Night	26% 6% 11% 35% 14% 9%	3583
	21st to end	Night	39% 7% 17% 24% 12% 2	3550
April	1st to 10th	Night	28% 5% 16% 27% 18% 6%	1900
	11th to 20th	Night	24% 7% 11% 30% 21% 7%	1879
	21st to end	Night	20% 5% 13% 36% 23% 2	1900
May	1st to 10th	Night	14% 4 8% 37% 28% 8%	3055
	11th to 20th	Night	9% 3 6% 30% 44% 7%	2835
	21st to end	Night	9% 3 7% 32% 45% 3	2770
June	1st to 10th	Night	14% 4 11% 24% 36% 11%	1520
	11th to 20th	Night	6% 2 6% 22% 41% 23%	1520
	21st to end	Night	6% 2 2 12% 38% 41%	1520
July	1st to 10th	Night	2 1 2 4 18% 73%	2500
	11th to 20th	Night	11% 1 4 6% 21% 57%	2492
	21st to end	Night	7% 2 3 11% 24% 54%	2886
August	1st to 10th	Night	9% 1 3 10% 30% 47%	3076
	11th to 20th	Night	4 1 2 6% 22% 65%	3156
	21st to end	Night	7% 1 2 6% 32% 53%	3498
September	1st to 10th	Night	5% 1 1 11% 29% 53%	1900
	11th to 20th	Night	9% 2 4 15% 45% 24%	1961
	21st to end	Night	8% 2 5% 20% 40% 26%	2090
October	1st to 10th	Night	10% 3 3 10% 26% 48%	3640
	11th to 20th	Night	10% 3 4 20% 41% 23%	3777
	21st to end	Night	14% 2 4 20% 36% 24%	4170
November	1st to 10th	Night	23% 3 10% 21% 25% 17%	2535
	11th to 20th	Night	21% 4 9% 23% 30% 14%	2600
	21st to end	Night	28% 4 8% 19% 30% 12%	2600
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% 14% 5%	284
	21st to end	Day	21% 8% 24% 29% 14% 4	407
February	1st to 10th	Day	24% 10% 25% 23% 13% 5%	421
	11th to 20th	Day	19% 13% 32% 25% 9% 2	455
	21st to end	Day	21% 15% 35% 21% 8% 1	382
March	1st to 10th	Day	24% 14% 34% 21% 6% 1	460
	11th to 20th	Day	21% 17% 28% 21% 10% 3	460
	21st to end	Day	23% 17% 32% 21% 7%	506
April	1st to 10th	Day	16% 16% 31% 25% 10% 2	460
	11th to 20th	Day	20% 17% 27% 27% 8% 1	452
	21st to end	Day	10% 14% 30% 29% 14% 3	460
May	1st to 10th	Day	13% 17% 32% 25% 8% 4	463
	11th to 20th	Day	11% 12% 29% 30% 13% 5%	488
	21st to end	Day	13% 9% 26% 33% 12% 7%	529
June	1st to 10th	Day	4 11% 27% 34% 16% 7%	490
	11th to 20th	Day	3 10% 28% 38% 13% 7%	490
	21st to end	Day	2 6% 32% 36% 15% 8%	490
July	1st to 10th	Day	1 2 26% 44% 17% 10%	486
	11th to 20th	Day	1 1 21% 46% 18% 13%	484
	21st to end	Day	2 2 23% 47% 14% 11%	531
August	1st to 10th	Day	2 4 33% 39% 12% 9%	487
	11th to 20th	Day	2 5% 27% 42% 14% 10%	477
	21st to end	Day	2 4 25% 45% 16% 8%	539
September	1st to 10th	Day	3 5% 31% 40% 17% 4	490
	11th to 20th	Day	4 11% 32% 36% 15% 2	490
	21st to end	Day	5% 8% 29% 42% 14% 2	490
October	1st to 10th	Day	4 10% 32% 39% 14% 1	490
	11th to 20th	Day	4 10% 30% 39% 15% 2	490
	21st to end	Day	6% 10% 30% 34% 19% 2	511
November	1st to 10th	Day	10% 11% 30% 27% 18% 3	300
	11th to 20th	Day	7% 11% 31% 29% 16% 5%	300
	21st to end	Day	15% 11% 26% 28% 14% 5%	299
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 23% 36% 18% 8%	760
	21st to end	Night	20% 5% 19% 31% 18% 7%	836
February	1st to 10th	Night	19% 4 18% 33% 19% 7%	760
	11th to 20th	Night	16% 5% 20% 30% 22% 8%	744
	21st to end	Night	19% 5% 21% 36% 16% 4	628
March	1st to 10th	Night	19% 7% 18% 33% 17% 6%	760
	11th to 20th	Night	15% 6% 18% 27% 25% 9%	760
	21st to end	Night	17% 7% 16% 36% 21% 4	836
April	1st to 10th	Night	14% 5% 18% 38% 20% 6%	760
	11th to 20th	Night	10% 6% 22% 32% 25% 6%	751
	21st to end	Night	8% 4 21% 37% 22% 9%	760
May	1st to 10th	Night	6% 4 21% 37% 26% 6%	687
	11th to 20th	Night	7% 2 16% 39% 31% 5%	561
	21st to end	Night	6% 2 20% 30% 31% 11%	419
June	1st to 10th	Night	5% 14% 37% 37% 7%	380
	11th to 20th	Night	3 12% 39% 38% 7%	380
	21st to end	Night	1 2 11% 36% 43% 8%	380
July	1st to 10th	Night	10% 31% 51% 8%	370
	11th to 20th	Night	2 1 10% 29% 47% 12%	368
	21st to end	Night	1 1 7% 26% 47% 18%	546
August	1st to 10th	Night	3 1 9% 33% 43% 11%	700
	11th to 20th	Night	1 7% 31% 49% 11%	752
	21st to end	Night	3 7% 27% 51% 12%	836
September	1st to 10th	Night	2 1 7% 30% 47% 13%	760
	11th to 20th	Night	3 1 10% 36% 38% 13%	760
	21st to end	Night	3 13% 35% 34% 14%	760
October	1st to 10th	Night	5% 1 12% 46% 26% 10%	760
	11th to 20th	Night	5% 1 15% 35% 31% 12%	760
	21st to end	Night	5% 4 17% 30% 28% 17%	836
November	1st to 10th	Night	11% 3 14% 30% 30% 12%	780
	11th to 20th	Night	9% 2 20% 37% 23% 9%	800
	21st to end	Night	13% 3 21% 33% 22% 9%	800
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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