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 2025-11-18.

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% 3 1 3 2 2 2 2 3 5% 67%	135513
	Night	11% 4 1 3 2 3 3 2 4 5% 62%	132269
Transparency	Day	22% 3 7% 18% 25% 24%	130462
	Night	27% 3 8% 19% 23% 21%	128316
Seeing	Day	9% 5% 17% 30% 29% 9%	16548
	Night	11% 3 12% 38% 31% 5%	25848

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	21% 6% 2 3 3 3 3 3 5% 7% 44%	3639
	11th to 20th	Day	17% 6% 2 6% 3 3 2 2 2 3 53%	4007
	21st to end	Day	20% 6% 2 6% 3 4 4 3 4 6% 43%	4510
February	1st to 10th	Day	21% 6% 2 5% 2 3 3 3 4 5% 45%	1800
	11th to 20th	Day	17% 6% 1 5% 4 3 4 4 5% 6% 45%	1888
	21st to end	Day	17% 6% 2 5% 3 3 2 3 5% 6% 48%	1650
March	1st to 10th	Day	17% 9% 3 8% 5% 4 4 4 5% 6% 36%	4650
	11th to 20th	Day	17% 5% 2 5% 4 4 4 3 4 7% 47%	5000
	21st to end	Day	16% 7% 2 6% 4 4 5% 4 6% 8% 38%	5700
April	1st to 10th	Day	14% 6% 1 3 3 3 5% 4 5% 6% 49%	2400
	11th to 20th	Day	12% 4 2 4 3 3 3 3 4 6% 56%	2369
	21st to end	Day	8% 3 1 3 3 2 3 3 5% 8% 62%	2400
May	1st to 10th	Day	5% 2 1 2 1 1 1 2 2 5% 77%	5547
	11th to 20th	Day	3 1 1 2 1 1 2 2 3 4 80%	6277
	21st to end	Day	2 2 1 2 1 1 1 2 3 5% 81%	6902
June	1st to 10th	Day	5% 3 1 3 2 3 2 3 5% 6% 69%	2800
	11th to 20th	Day	1 1 1 1 1 2 2 2 3 4 82%	2800
	21st to end	Day	2 1 1 1 1 1 1 3 3 85%	2800
July	1st to 10th	Day	1 1 1 1 1 2 93%	6319
	11th to 20th	Day	1 1 1 1 1 1 1 2 2 89%	6320
	21st to end	Day	1 1 2 2 2 2 2 3 4 83%	6410
August	1st to 10th	Day	2 1 1 1 1 1 1 1 2 3 85%	5777
	11th to 20th	Day	1 1 1 1 1 1 2 3 89%	5418
	21st to end	Day	1 1 1 1 1 1 2 1 2 3 84%	5984
September	1st to 10th	Day	2 1 1 1 1 1 1 2 3 88%	2400
	11th to 20th	Day	3 2 1 2 2 2 3 2 3 5% 76%	2400
	21st to end	Day	4 2 1 2 2 2 1 2 2 4 80%	2161
October	1st to 10th	Day	3 2 1 3 2 2 2 2 4 5% 75%	3280
	11th to 20th	Day	4 2 1 2 2 2 3 2 3 5% 73%	3261
	21st to end	Day	5% 3 1 4 3 3 3 3 4 6% 65%	3558
November	1st to 10th	Day	11% 4 1 3 3 3 3 2 6% 8% 57%	1896
	11th to 20th	Day	13% 4 2 5% 4 4 4 2 4 7% 51%	1863
	21st to end	Day	14% 3 1 4 2 3 3 3 5% 7% 54%	1675
December	1st to 10th	Day	9% 4 1 3 3 4 3 3 4 8% 57%	2620
	11th to 20th	Day	28% 7% 2 5% 4 4 4 3 4 7% 32%	3028
	21st to end	Day	25% 7% 2 5% 4 5% 5% 4 7% 8% 27%	4004
January	1st to 10th	Night	13% 7% 2 5% 3 4 3 3 5% 9% 45%	6306
	11th to 20th	Night	16% 5% 2 4 3 3 3 2 4 4 54%	5954
	21st to end	Night	18% 6% 2 6% 4 3 3 3 4 5% 47%	6468
February	1st to 10th	Night	20% 6% 1 4 3 3 3 2 4 5% 49%	2600
	11th to 20th	Night	16% 5% 2 4 3 4 4 3 5% 8% 47%	2398
	21st to end	Night	16% 6% 2 4 3 3 4 3 4 5% 51%	1980
March	1st to 10th	Night	19% 9% 3 6% 4 4 4 3 4 6% 38%	5227
	11th to 20th	Night	22% 7% 2 4 3 3 3 2 3 4 48%	5000
	21st to end	Night	21% 6% 2 5% 3 4 4 3 5% 7% 40%	5275
April	1st to 10th	Night	13% 5% 2 5% 4 4 4 4 4 6% 51%	2000
	11th to 20th	Night	11% 4 1 4 4 2 3 3 4 6% 57%	1979
	21st to end	Night	7% 3 1 2 2 2 3 3 4 7% 64%	2000
May	1st to 10th	Night	4 3 1 3 2 2 2 2 3 5% 75%	4441
	11th to 20th	Night	3 2 1 2 2 2 3 2 3 4 77%	3616
	21st to end	Night	2 1 2 1 1 2 1 3 4 83%	3972
June	1st to 10th	Night	5% 3 1 3 3 2 2 3 5% 6% 69%	1600
	11th to 20th	Night	1 1 1 2 1 2 1 1 3 4 83%	1600
	21st to end	Night	2 1 1 1 1 2 1 2 2 4 84%	1600
July	1st to 10th	Night	1 1 1 1 1 1 2 93%	3640
	11th to 20th	Night	1 1 1 1 1 1 1 2 3 87%	3640
	21st to end	Night	1 1 1 2 2 2 2 2 3 4 80%	4460
August	1st to 10th	Night	2 1 2 1 1 1 1 3 3 83%	4035
	11th to 20th	Night	1 1 1 1 2 2 91%	4435
	21st to end	Night	2 1 1 1 1 1 2 2 2 3 84%	5015
September	1st to 10th	Night	1 1 1 1 1 1 1 2 3 88%	2000
	11th to 20th	Night	3 3 1 2 1 2 2 1 3 4 77%	2000
	21st to end	Night	2 1 1 2 1 2 2 2 3 4 80%	2235
October	1st to 10th	Night	3 2 1 2 2 2 2 1 3 4 77%	3920
	11th to 20th	Night	4 2 1 2 1 2 2 2 3 5% 78%	3897
	21st to end	Night	6% 3 1 4 3 3 3 2 4 5% 66%	4253
November	1st to 10th	Night	11% 4 1 3 3 2 2 3 5% 7% 58%	2620
	11th to 20th	Night	11% 5% 1 4 3 3 3 3 5% 7% 55%	2691
	21st to end	Night	14% 4 1 3 2 3 3 2 4 6% 57%	2724
December	1st to 10th	Night	10% 3 1 3 3 3 3 3 5% 6% 61%	4560
	11th to 20th	Night	26% 7% 2 4 4 4 4 4 6% 8% 30%	5176
	21st to end	Night	26% 8% 3 7% 5% 5% 4 4 4 6% 30%	6952

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	40% 4 18% 21% 13% 5%	3639
	11th to 20th	Day	39% 2 7% 15% 22% 15%	4007
	21st to end	Day	43% 4 8% 12% 24% 9%	4510
February	1st to 10th	Day	42% 4 9% 19% 16% 9%	1800
	11th to 20th	Day	40% 4 10% 20% 19% 6%	1898
	21st to end	Day	39% 3 10% 24% 18% 7%	1650
March	1st to 10th	Day	49% 5% 8% 18% 17% 2	4650
	11th to 20th	Day	38% 4 12% 29% 10% 7%	5000
	21st to end	Day	42% 6% 15% 25% 10% 2	5700
April	1st to 10th	Day	35% 5% 11% 26% 17% 6%	2400
	11th to 20th	Day	30% 5% 12% 26% 21% 7%	2372
	21st to end	Day	22% 5% 14% 30% 25% 4	2400
May	1st to 10th	Day	14% 2 7% 27% 33% 16%	5547
	11th to 20th	Day	11% 3 5% 31% 40% 10%	6280
	21st to end	Day	10% 2 8% 29% 42% 10%	6904
June	1st to 10th	Day	18% 3 10% 27% 30% 12%	2800
	11th to 20th	Day	8% 2 8% 19% 35% 28%	2800
	21st to end	Day	8% 1 3 12% 36% 41%	2800
July	1st to 10th	Day	3 1 1 6% 18% 72%	6251
	11th to 20th	Day	5% 1 1 5% 12% 76%	6245
	21st to end	Day	8% 2 3 12% 26% 49%	6224
August	1st to 10th	Day	10% 1 2 6% 22% 58%	4206
	11th to 20th	Day	5% 1 1 6% 35% 52%	3922
	21st to end	Day	7% 1 2 8% 37% 46%	4311
September	1st to 10th	Day	5% 2 2 9% 29% 53%	2400
	11th to 20th	Day	13% 3 4 21% 37% 22%	2400
	21st to end	Day	12% 2 4 17% 35% 29%	2161
October	1st to 10th	Day	13% 2 4 11% 23% 46%	3280
	11th to 20th	Day	15% 3 5% 21% 38% 19%	3261
	21st to end	Day	23% 2 6% 18% 36% 15%	3558
November	1st to 10th	Day	26% 4 11% 23% 24% 12%	1896
	11th to 20th	Day	34% 4 11% 24% 21% 6%	1863
	21st to end	Day	30% 4 11% 21% 23% 11%	1675
December	1st to 10th	Day	27% 3 8% 28% 25% 9%	2620
	11th to 20th	Day	52% 3 10% 17% 13% 5%	3028
	21st to end	Day	52% 5% 17% 16% 8% 2	4004
January	1st to 10th	Night	36% 4 20% 22% 11% 7%	6306
	11th to 20th	Night	35% 3 10% 15% 23% 14%	5954
	21st to end	Night	42% 3 8% 11% 23% 13%	6468
February	1st to 10th	Night	40% 3 7% 21% 19% 10%	2600
	11th to 20th	Night	37% 4 8% 23% 20% 8%	2399
	21st to end	Night	35% 5% 9% 25% 19% 7%	1980
March	1st to 10th	Night	48% 5% 8% 18% 15% 6%	5227
	11th to 20th	Night	41% 3 9% 24% 13% 8%	5000
	21st to end	Night	44% 6% 12% 23% 13% 3	5275
April	1st to 10th	Night	36% 5% 12% 22% 21% 6%	2000
	11th to 20th	Night	29% 4 14% 26% 17% 10%	1979
	21st to end	Night	21% 5% 12% 32% 24% 6%	2000
May	1st to 10th	Night	13% 5% 11% 25% 28% 18%	4438
	11th to 20th	Night	13% 3 6% 30% 38% 9%	3616
	21st to end	Night	8% 3 9% 32% 41% 8%	3972
June	1st to 10th	Night	18% 4 10% 29% 28% 11%	1600
	11th to 20th	Night	8% 2 8% 20% 34% 29%	1600
	21st to end	Night	9% 1 3 13% 34% 41%	1600
July	1st to 10th	Night	3 1 1 6% 23% 67%	3600
	11th to 20th	Night	7% 1 1 4 10% 76%	3592
	21st to end	Night	9% 2 2 14% 26% 46%	4334
August	1st to 10th	Night	12% 1 1 8% 26% 52%	2929
	11th to 20th	Night	5% 2 1 7% 38% 47%	3211
	21st to end	Night	8% 1 2 9% 36% 45%	3608
September	1st to 10th	Night	5% 2 2 9% 30% 53%	2000
	11th to 20th	Night	13% 2 5% 19% 39% 22%	2000
	21st to end	Night	10% 2 6% 17% 32% 33%	2235
October	1st to 10th	Night	14% 2 3 11% 20% 50%	3920
	11th to 20th	Night	12% 3 5% 26% 33% 22%	3897
	21st to end	Night	22% 3 4 17% 33% 20%	4253
November	1st to 10th	Night	27% 3 10% 23% 20% 16%	2620
	11th to 20th	Night	29% 3 11% 25% 22% 10%	2691
	21st to end	Night	31% 3 8% 21% 24% 13%	2724
December	1st to 10th	Night	25% 3 8% 26% 26% 12%	4560
	11th to 20th	Night	51% 4 8% 17% 16% 4	5176
	21st to end	Night	55% 5% 12% 16% 8% 3	6952

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	35% 8% 15% 23% 17% 4	200
	11th to 20th	Day	18% 8% 15% 29% 21% 8%	359
	21st to end	Day	20% 4 18% 28% 23% 8%	440
February	1st to 10th	Day	24% 10% 13% 22% 24% 7%	400
	11th to 20th	Day	19% 10% 23% 24% 19% 5%	464
	21st to end	Day	17% 7% 23% 30% 19% 3	414
March	1st to 10th	Day	21% 12% 24% 25% 15% 3	500
	11th to 20th	Day	22% 9% 23% 22% 17% 6%	500
	21st to end	Day	17% 10% 27% 25% 18% 3	550
April	1st to 10th	Day	15% 9% 25% 26% 19% 6%	500
	11th to 20th	Day	13% 7% 27% 25% 22% 6%	492
	21st to end	Day	7% 7% 20% 31% 24% 11%	500
May	1st to 10th	Day	5% 9% 28% 28% 23% 8%	503
	11th to 20th	Day	7% 6% 22% 28% 28% 9%	528
	21st to end	Day	5% 5% 19% 28% 30% 13%	573
June	1st to 10th	Day	4 5% 18% 32% 30% 11%	530
	11th to 20th	Day	2 4 21% 35% 32% 5%	530
	21st to end	Day	2 14% 32% 42% 10%	530
July	1st to 10th	Day	1 11% 37% 42% 9%	526
	11th to 20th	Day	1 1 6% 37% 47% 8%	524
	21st to end	Day	1 1 8% 36% 43% 12%	575
August	1st to 10th	Day	2 1 12% 39% 35% 11%	527
	11th to 20th	Day	1 1 12% 37% 39% 11%	517
	21st to end	Day	1 1 9% 32% 45% 12%	583
September	1st to 10th	Day	2 2 11% 38% 37% 11%	530
	11th to 20th	Day	5% 3 14% 35% 32% 10%	530
	21st to end	Day	3 2 17% 34% 33% 11%	530
October	1st to 10th	Day	3 5% 18% 29% 31% 14%	530
	11th to 20th	Day	6% 5% 13% 26% 36% 13%	530
	21st to end	Day	9% 6% 14% 28% 30% 12%	583
November	1st to 10th	Day	11% 7% 13% 26% 30% 13%	372
	11th to 20th	Day	15% 3 15% 31% 27% 10%	321
	21st to end	Day	16% 3 16% 25% 32% 8%	237
December	1st to 10th	Day	17% 8% 14% 27% 25% 11%	200
	11th to 20th	Day	20% 6% 18% 26% 22% 10%	200
	21st to end	Day	22% 10% 17% 26% 20% 4	220
January	1st to 10th	Night	21% 4 16% 32% 21% 7%	800
	11th to 20th	Night	16% 4 12% 39% 23% 6%	800
	21st to end	Night	19% 3 13% 37% 22% 6%	880
February	1st to 10th	Night	22% 4 15% 35% 21% 4	800
	11th to 20th	Night	17% 5% 16% 37% 20% 5%	784
	21st to end	Night	18% 4 16% 41% 18% 3	660
March	1st to 10th	Night	21% 6% 17% 33% 21% 4	800
	11th to 20th	Night	21% 5% 14% 32% 25% 4	800
	21st to end	Night	18% 4 19% 37% 20% 2	880
April	1st to 10th	Night	14% 4 16% 40% 25% 2	800
	11th to 20th	Night	9% 5% 18% 38% 24% 5%	791
	21st to end	Night	8% 3 14% 41% 30% 4	800
May	1st to 10th	Night	7% 3 21% 38% 28% 4	759
	11th to 20th	Night	6% 2 11% 45% 34% 3	394
	21st to end	Night	6% 1 13% 41% 39% 1	433
June	1st to 10th	Night	4 2 14% 42% 36% 2	400
	11th to 20th	Night	1 1 13% 49% 35% 1	400
	21st to end	Night	3 1 7% 47% 42% 2	400
July	1st to 10th	Night	1 4 48% 47% 1	390
	11th to 20th	Night	2 1 3 48% 45% 1	388
	21st to end	Night	2 37% 55% 6%	645
August	1st to 10th	Night	3 6% 40% 49% 3	597
	11th to 20th	Night	1 5% 46% 44% 4	759
	21st to end	Night	2 1 3 39% 52% 4	880
September	1st to 10th	Night	1 1 6% 39% 49% 4	800
	11th to 20th	Night	4 2 9% 37% 42% 6%	800
	21st to end	Night	2 1 11% 38% 39% 9%	800
October	1st to 10th	Night	6% 1 10% 42% 34% 8%	800
	11th to 20th	Night	7% 2 10% 37% 38% 6%	800
	21st to end	Night	6% 2 11% 37% 31% 12%	880
November	1st to 10th	Night	12% 3 10% 34% 32% 8%	820
	11th to 20th	Night	13% 2 12% 40% 26% 6%	828
	21st to end	Night	15% 2 14% 39% 24% 7%	800
December	1st to 10th	Night	17% 3 14% 36% 23% 7%	800
	11th to 20th	Night	27% 3 12% 33% 22% 4	800
	21st to end	Night	24% 4 18% 34% 16% 4	880

Page generated at 2025-11-14 21:55:57. Version ac07

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