Sierra Skies Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sierra Skies Observatory clear sky chart from 2005-11-06 to 2024-11-08.

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 2 2 3 2 4 5% 65%	105155
	Night	11% 4 1 4 2 3 3 2 4 5% 61%	102683
Transparency	Day	23% 3 7% 18% 24% 25%	104907
	Night	27% 3 8% 18% 22% 22%	102520
Seeing	Day	9% 5% 17% 30% 30% 9%	15211
	Night	11% 3 12% 38% 31% 5%	24518

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	26% 9% 2 4 3 3 4 3 5% 6% 35%	2539
	11th to 20th	Day	25% 9% 3 8% 5% 4 3 3 3 4 32%	2795
	21st to end	Day	24% 9% 3 7% 4 4 4 4 5% 7% 29%	3146
February	1st to 10th	Day	20% 6% 2 5% 2 3 3 4 4 6% 45%	1710
	11th to 20th	Day	17% 7% 1 5% 4 3 4 3 4 6% 47%	1791
	21st to end	Day	18% 6% 2 5% 3 3 3 3 5% 5% 48%	1570
March	1st to 10th	Day	18% 10% 4 9% 5% 5% 5% 4 5% 5% 31%	3231
	11th to 20th	Day	14% 4 1 4 3 3 3 2 4 6% 56%	3490
	21st to end	Day	16% 8% 2 6% 5% 4 5% 4 6% 8% 36%	3974
April	1st to 10th	Day	14% 7% 1 4 3 3 5% 4 5% 6% 48%	2280
	11th to 20th	Day	12% 4 1 4 3 3 3 3 4 6% 56%	2249
	21st to end	Day	8% 3 1 2 3 2 3 3 5% 8% 62%	2280
May	1st to 10th	Day	7% 2 1 2 2 1 2 2 3 5% 73%	3868
	11th to 20th	Day	3 2 1 2 1 1 2 2 2 3 81%	4377
	21st to end	Day	3 3 1 2 2 2 2 2 3 6% 75%	4813
June	1st to 10th	Day	5% 3 1 3 2 3 2 3 5% 6% 68%	2660
	11th to 20th	Day	1 1 1 1 1 2 2 2 3 4 82%	2660
	21st to end	Day	2 1 1 1 1 1 1 3 4 84%	2660
July	1st to 10th	Day	1 1 1 1 1 2 93%	4420
	11th to 20th	Day	1 1 1 1 1 2 1 3 3 85%	4420
	21st to end	Day	1 1 1 1 1 1 2 3 88%	4463
August	1st to 10th	Day	2 2 1 2 1 1 2 2 3 5% 80%	4091
	11th to 20th	Day	1 1 1 1 1 1 2 3 88%	3765
	21st to end	Day	2 1 1 1 1 1 1 1 2 2 88%	4180
September	1st to 10th	Day	1 1 1 1 1 2 3 89%	2280
	11th to 20th	Day	3 2 1 2 1 2 3 2 3 5% 76%	2280
	21st to end	Day	3 2 1 2 1 1 1 2 2 4 80%	2053
October	1st to 10th	Day	2 2 1 3 2 2 2 2 4 5% 75%	3180
	11th to 20th	Day	4 2 1 2 2 2 3 2 3 5% 73%	3161
	21st to end	Day	5% 3 2 4 3 3 3 3 4 6% 65%	3448
November	1st to 10th	Day	11% 4 1 3 3 4 3 2 5% 8% 56%	1777
	11th to 20th	Day	12% 4 2 4 4 4 4 2 4 7% 53%	1710
	21st to end	Day	13% 4 1 4 2 3 3 4 5% 7% 55%	1592
December	1st to 10th	Day	16% 6% 2 4 4 5% 4 4 3 7% 45%	1520
	11th to 20th	Day	29% 8% 2 5% 4 3 4 2 4 6% 31%	1928
	21st to end	Day	15% 7% 3 6% 4 5% 5% 5% 7% 9% 34%	2794
January	1st to 10th	Night	17% 9% 3 6% 4 4 3 3 5% 7% 40%	4406
	11th to 20th	Night	23% 7% 3 6% 4 4 4 3 5% 6% 35%	4171
	21st to end	Night	25% 8% 3 7% 5% 4 4 3 4 6% 32%	4521
February	1st to 10th	Night	19% 6% 1 4 3 3 3 2 4 5% 50%	2470
	11th to 20th	Night	16% 4 2 4 3 3 4 3 5% 8% 48%	2275
	21st to end	Night	17% 6% 2 4 3 3 4 3 4 5% 52%	1884
March	1st to 10th	Night	22% 11% 3 7% 4 3 4 3 4 5% 34%	3626
	11th to 20th	Night	18% 5% 1 3 2 2 2 2 3 4 57%	3490
	21st to end	Night	21% 7% 2 6% 3 4 4 3 5% 7% 37%	3679
April	1st to 10th	Night	13% 5% 2 5% 4 4 4 3 3 6% 50%	1900
	11th to 20th	Night	11% 4 1 4 4 2 3 3 4 6% 57%	1879
	21st to end	Night	7% 3 1 2 2 2 3 3 4 7% 65%	1900
May	1st to 10th	Night	5% 4 1 3 2 2 2 2 3 5% 71%	3103
	11th to 20th	Night	3 1 1 1 1 2 2 3 4 81%	2516
	21st to end	Night	3 1 2 1 1 2 1 3 5% 80%	2762
June	1st to 10th	Night	5% 3 2 3 3 2 2 2 4 6% 69%	1520
	11th to 20th	Night	1 1 1 2 2 2 1 1 3 4 82%	1520
	21st to end	Night	3 1 1 1 1 2 1 2 2 3 84%	1520
July	1st to 10th	Night	1 1 1 1 1 1 1 94%	2540
	11th to 20th	Night	2 2 1 2 1 2 1 1 3 4 82%	2540
	21st to end	Night	1 2 1 1 1 1 3 4 86%	3097
August	1st to 10th	Night	3 2 1 2 2 2 2 2 4 5% 77%	2848
	11th to 20th	Night	1 1 1 1 1 1 1 2 3 88%	3083
	21st to end	Night	3 1 1 1 1 1 1 1 1 2 88%	3498
September	1st to 10th	Night	1 1 1 1 1 1 1 2 3 89%	1900
	11th to 20th	Night	3 2 1 2 1 1 2 1 3 4 78%	1900
	21st to end	Night	2 1 1 1 2 2 2 3 4 80%	2123
October	1st to 10th	Night	3 2 1 2 2 2 2 1 3 4 77%	3800
	11th to 20th	Night	3 1 1 2 1 2 2 2 3 5% 78%	3777
	21st to end	Night	6% 3 1 3 3 3 3 2 4 5% 66%	4121
November	1st to 10th	Night	12% 5% 1 3 3 2 3 3 4 7% 57%	2459
	11th to 20th	Night	11% 4 1 4 3 3 3 3 5% 7% 57%	2470
	21st to end	Night	13% 5% 1 3 3 3 3 2 4 6% 58%	2587
December	1st to 10th	Night	18% 5% 1 5% 3 3 3 3 5% 6% 49%	2660
	11th to 20th	Night	28% 8% 2 4 4 3 3 3 4 6% 34%	3276
	21st to end	Night	20% 7% 3 7% 5% 5% 5% 4 5% 6% 35%	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	49% 4 13% 23% 10% 2	2539
	11th to 20th	Day	56% 3 10% 13% 10% 7%	2795
	21st to end	Day	54% 5% 11% 16% 11% 4	3146
February	1st to 10th	Day	41% 4 10% 20% 16% 9%	1710
	11th to 20th	Day	39% 4 10% 20% 20% 7%	1801
	21st to end	Day	39% 3 10% 24% 18% 6%	1570
March	1st to 10th	Day	53% 7% 11% 15% 12% 2	3231
	11th to 20th	Day	30% 3 10% 33% 14% 11%	3490
	21st to end	Day	43% 7% 16% 21% 10% 2	3974
April	1st to 10th	Day	36% 5% 11% 25% 16% 6%	2280
	11th to 20th	Day	30% 5% 12% 26% 20% 6%	2252
	21st to end	Day	22% 5% 14% 30% 26% 4	2280
May	1st to 10th	Day	16% 3 9% 29% 26% 17%	3868
	11th to 20th	Day	11% 2 5% 28% 41% 12%	4380
	21st to end	Day	13% 3 11% 36% 30% 7%	4814
June	1st to 10th	Day	18% 4 11% 26% 29% 13%	2660
	11th to 20th	Day	8% 2 8% 20% 35% 27%	2660
	21st to end	Day	8% 2 3 13% 35% 40%	2660
July	1st to 10th	Day	3 1 1 5% 14% 75%	4351
	11th to 20th	Day	8% 1 2 7% 16% 66%	4345
	21st to end	Day	5% 2 1 4 23% 65%	4322
August	1st to 10th	Day	11% 2 2 6% 21% 59%	4076
	11th to 20th	Day	5% 1 1 6% 35% 52%	3801
	21st to end	Day	7% 1 2 8% 36% 46%	4179
September	1st to 10th	Day	4 2 2 8% 29% 55%	2280
	11th to 20th	Day	13% 3 5% 21% 37% 22%	2280
	21st to end	Day	11% 2 4 18% 35% 30%	2053
October	1st to 10th	Day	13% 3 4 11% 23% 47%	3180
	11th to 20th	Day	14% 3 5% 21% 38% 19%	3161
	21st to end	Day	22% 2 6% 18% 35% 16%	3448
November	1st to 10th	Day	27% 4 11% 23% 23% 13%	1777
	11th to 20th	Day	32% 4 11% 24% 23% 7%	1710
	21st to end	Day	29% 4 11% 22% 24% 10%	1592
December	1st to 10th	Day	38% 4 11% 21% 17% 9%	1520
	11th to 20th	Day	54% 4 10% 15% 12% 5%	1928
	21st to end	Day	43% 6% 19% 19% 9% 3	2794
January	1st to 10th	Night	43% 4 17% 23% 10% 4	4406
	11th to 20th	Night	50% 4 14% 13% 10% 9%	4171
	21st to end	Night	54% 4 11% 14% 11% 6%	4521
February	1st to 10th	Night	38% 3 7% 22% 19% 10%	2470
	11th to 20th	Night	36% 4 8% 23% 20% 8%	2276
	21st to end	Night	35% 5% 9% 25% 19% 7%	1884
March	1st to 10th	Night	53% 5% 11% 15% 12% 4	3626
	11th to 20th	Night	32% 3 8% 27% 18% 12%	3490
	21st to end	Night	46% 7% 12% 21% 10% 4	3679
April	1st to 10th	Night	36% 4 12% 21% 20% 6%	1900
	11th to 20th	Night	29% 4 14% 26% 16% 10%	1879
	21st to end	Night	20% 5% 12% 32% 24% 6%	1900
May	1st to 10th	Night	15% 6% 14% 21% 25% 18%	3103
	11th to 20th	Night	9% 2 7% 27% 41% 12%	2516
	21st to end	Night	10% 3 12% 38% 30% 7%	2762
June	1st to 10th	Night	18% 4 11% 27% 28% 11%	1520
	11th to 20th	Night	9% 2 8% 21% 33% 28%	1520
	21st to end	Night	9% 1 3 13% 33% 41%	1520
July	1st to 10th	Night	3 1 1 6% 16% 74%	2500
	11th to 20th	Night	10% 2 2 6% 15% 66%	2492
	21st to end	Night	6% 2 1 5% 26% 60%	3000
August	1st to 10th	Night	12% 1 1 8% 25% 52%	2839
	11th to 20th	Night	5% 2 1 7% 38% 47%	3113
	21st to end	Night	8% 1 2 9% 35% 45%	3498
September	1st to 10th	Night	5% 2 2 7% 30% 54%	1900
	11th to 20th	Night	12% 2 4 18% 41% 22%	1900
	21st to end	Night	9% 2 6% 17% 32% 34%	2123
October	1st to 10th	Night	14% 2 3 10% 20% 51%	3800
	11th to 20th	Night	11% 3 6% 25% 33% 23%	3777
	21st to end	Night	22% 3 4 17% 33% 21%	4121
November	1st to 10th	Night	28% 4 10% 23% 20% 16%	2459
	11th to 20th	Night	28% 3 10% 26% 23% 10%	2470
	21st to end	Night	30% 3 8% 22% 24% 13%	2587
December	1st to 10th	Night	36% 4 11% 20% 18% 11%	2660
	11th to 20th	Night	51% 4 9% 16% 15% 6%	3276
	21st to end	Night	49% 6% 13% 18% 10% 4	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	37% 7% 14% 22% 16% 4	180
	11th to 20th	Day	20% 8% 15% 29% 21% 7%	331
	21st to end	Day	21% 4 19% 27% 23% 7%	407
February	1st to 10th	Day	22% 10% 13% 23% 25% 7%	370
	11th to 20th	Day	19% 11% 22% 22% 20% 5%	427
	21st to end	Day	18% 8% 24% 29% 18% 3	382
March	1st to 10th	Day	21% 13% 25% 25% 14% 3	460
	11th to 20th	Day	21% 9% 23% 23% 18% 6%	460
	21st to end	Day	17% 10% 28% 25% 17% 2	506
April	1st to 10th	Day	15% 9% 26% 27% 18% 6%	460
	11th to 20th	Day	13% 8% 28% 24% 21% 6%	452
	21st to end	Day	7% 7% 21% 31% 23% 11%	460
May	1st to 10th	Day	5% 10% 28% 28% 22% 7%	463
	11th to 20th	Day	7% 5% 22% 27% 29% 10%	488
	21st to end	Day	5% 5% 18% 27% 30% 14%	529
June	1st to 10th	Day	4 5% 20% 31% 29% 11%	490
	11th to 20th	Day	2 4 20% 35% 32% 6%	490
	21st to end	Day	2 14% 32% 43% 9%	490
July	1st to 10th	Day	1 9% 37% 44% 9%	486
	11th to 20th	Day	1 1 7% 37% 46% 8%	484
	21st to end	Day	1 8% 37% 43% 12%	531
August	1st to 10th	Day	3 1 11% 39% 36% 9%	487
	11th to 20th	Day	1 1 13% 37% 38% 10%	477
	21st to end	Day	1 1 10% 33% 45% 11%	539
September	1st to 10th	Day	2 2 10% 38% 38% 11%	490
	11th to 20th	Day	5% 3 15% 35% 33% 9%	490
	21st to end	Day	2 2 17% 33% 34% 11%	490
October	1st to 10th	Day	2 5% 18% 30% 31% 14%	490
	11th to 20th	Day	6% 5% 12% 27% 37% 13%	490
	21st to end	Day	8% 6% 15% 30% 28% 12%	539
November	1st to 10th	Day	12% 8% 15% 26% 29% 11%	331
	11th to 20th	Day	11% 3 15% 30% 30% 11%	270
	21st to end	Day	13% 3 17% 27% 33% 7%	214
December	1st to 10th	Day	18% 9% 15% 26% 23% 9%	180
	11th to 20th	Day	18% 7% 18% 26% 21% 11%	180
	21st to end	Day	18% 11% 19% 25% 22% 5%	198
January	1st to 10th	Night	22% 4 14% 30% 22% 7%	760
	11th to 20th	Night	17% 4 11% 38% 23% 6%	760
	21st to end	Night	20% 3 13% 36% 22% 6%	836
February	1st to 10th	Night	20% 4 14% 36% 22% 4	760
	11th to 20th	Night	17% 6% 15% 37% 21% 5%	744
	21st to end	Night	19% 4 16% 40% 18% 3	628
March	1st to 10th	Night	22% 6% 16% 32% 20% 4	760
	11th to 20th	Night	20% 4 14% 33% 26% 4	760
	21st to end	Night	18% 5% 19% 36% 20% 3	836
April	1st to 10th	Night	14% 4 15% 40% 25% 2	760
	11th to 20th	Night	9% 5% 19% 36% 25% 6%	751
	21st to end	Night	7% 3 15% 41% 30% 4	760
May	1st to 10th	Night	7% 3 20% 38% 28% 4	722
	11th to 20th	Night	6% 2 10% 44% 36% 3	374
	21st to end	Night	6% 1 13% 41% 39% 1	411
June	1st to 10th	Night	4 2 14% 42% 36% 2	380
	11th to 20th	Night	1 1 13% 48% 35% 1	380
	21st to end	Night	3 1 7% 47% 41% 2	380
July	1st to 10th	Night	1 4 47% 48% 1	370
	11th to 20th	Night	2 1 4 49% 44% 1	368
	21st to end	Night	2 37% 55% 6%	612
August	1st to 10th	Night	3 5% 40% 49% 2	567
	11th to 20th	Night	1 5% 46% 43% 5%	721
	21st to end	Night	2 3 38% 52% 4	836
September	1st to 10th	Night	1 1 5% 38% 51% 4	760
	11th to 20th	Night	4 1 9% 38% 42% 6%	760
	21st to end	Night	2 1 11% 38% 39% 9%	760
October	1st to 10th	Night	5% 1 10% 41% 34% 8%	760
	11th to 20th	Night	7% 2 10% 36% 39% 6%	760
	21st to end	Night	6% 2 12% 37% 30% 13%	836
November	1st to 10th	Night	13% 3 11% 34% 31% 8%	770
	11th to 20th	Night	11% 2 12% 41% 28% 7%	760
	21st to end	Night	14% 2 13% 39% 25% 7%	760
December	1st to 10th	Night	18% 3 14% 35% 23% 7%	760
	11th to 20th	Night	26% 3 12% 33% 22% 4	760
	21st to end	Night	23% 4 18% 34% 17% 4	836

Page generated at 2024-11-05 05:15:47. Version ac07

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