Sonoma State University Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sonoma State University Observatory clear sky chart from 2005-11-06 to 2025-10-19.

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 2 4 3 3 3 3 4 6% 61%	135232
	Night	11% 6% 2 6% 4 3 3 3 4 6% 53%	131922
Transparency	Day	27% 2 7% 21% 24% 18%	130129
	Night	35% 3 8% 18% 21% 16%	128014
Seeing	Day	11% 9% 25% 28% 19% 7%	16630
	Night	13% 5% 15% 32% 28% 6%	25866

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	15% 6% 2 6% 3 3 4 3 6% 7% 45%	3892
	11th to 20th	Day	16% 7% 3 6% 3 3 3 2 3 4 50%	4100
	21st to end	Day	18% 7% 2 6% 4 3 3 2 5% 6% 43%	4510
February	1st to 10th	Day	20% 6% 2 6% 4 3 3 3 3 5% 46%	1800
	11th to 20th	Day	16% 8% 3 6% 4 4 4 4 5% 8% 40%	1798
	21st to end	Day	16% 7% 2 4 4 3 2 3 5% 6% 47%	1650
March	1st to 10th	Day	17% 11% 3 8% 4 4 4 3 4 6% 36%	4940
	11th to 20th	Day	16% 6% 2 5% 3 3 4 3 5% 7% 46%	5000
	21st to end	Day	18% 8% 2 6% 4 4 5% 4 6% 8% 34%	5520
April	1st to 10th	Day	15% 7% 3 5% 5% 4 4 5% 5% 7% 42%	2400
	11th to 20th	Day	10% 5% 2 6% 4 4 4 4 5% 6% 51%	2369
	21st to end	Day	7% 5% 1 4 3 3 3 3 7% 8% 56%	2400
May	1st to 10th	Day	4 3 1 2 1 1 2 2 3 5% 76%	5781
	11th to 20th	Day	3 3 1 2 2 2 2 2 4 7% 73%	5944
	21st to end	Day	3 3 1 2 2 2 1 2 3 5% 77%	6902
June	1st to 10th	Day	7% 5% 2 3 3 2 3 3 5% 7% 60%	2800
	11th to 20th	Day	1 3 1 2 2 2 2 2 4 6% 74%	2800
	21st to end	Day	2 2 1 3 2 2 1 2 3 5% 76%	2800
July	1st to 10th	Day	1 1 1 3 2 2 2 2 2 4 82%	6319
	11th to 20th	Day	1 2 1 4 2 2 2 2 3 5% 76%	5890
	21st to end	Day	4 2 4 2 2 2 2 3 6% 74%	6402
August	1st to 10th	Day	1 2 1 3 2 3 3 3 4 6% 74%	5777
	11th to 20th	Day	1 2 1 2 2 2 2 2 3 4 80%	5625
	21st to end	Day	1 3 1 3 3 3 3 2 3 7% 71%	5984
September	1st to 10th	Day	1 3 1 3 2 1 2 2 3 6% 75%	2400
	11th to 20th	Day	4 4 2 4 2 3 3 2 5% 8% 63%	2395
	21st to end	Day	5% 3 1 3 2 2 2 2 3 5% 73%	2105
October	1st to 10th	Day	4 3 1 3 2 2 2 2 4 6% 71%	3280
	11th to 20th	Day	8% 4 1 4 2 2 3 2 3 5% 65%	3242
	21st to end	Day	8% 4 1 4 3 3 4 3 6% 7% 57%	3448
November	1st to 10th	Day	11% 3 2 4 4 3 3 3 5% 8% 55%	1937
	11th to 20th	Day	13% 6% 2 5% 4 3 3 3 5% 7% 47%	1770
	21st to end	Day	12% 6% 1 4 3 3 4 4 5% 5% 52%	1600
December	1st to 10th	Day	11% 5% 1 3 3 3 3 4 6% 9% 52%	2620
	11th to 20th	Day	28% 7% 2 6% 4 3 4 4 6% 7% 29%	3028
	21st to end	Day	27% 10% 2 6% 3 3 4 4 5% 7% 28%	4004
January	1st to 10th	Night	12% 6% 2 5% 4 3 4 4 6% 9% 45%	6059
	11th to 20th	Night	16% 6% 2 7% 4 3 4 3 4 4 48%	5880
	21st to end	Night	18% 8% 2 6% 3 3 3 3 4 5% 43%	6468
February	1st to 10th	Night	20% 6% 2 5% 3 3 3 3 4 5% 47%	2600
	11th to 20th	Night	15% 8% 3 5% 3 4 4 3 5% 7% 43%	2487
	21st to end	Night	14% 7% 1 3 3 2 4 2 4 6% 53%	1980
March	1st to 10th	Night	18% 11% 3 7% 4 4 4 3 4 6% 37%	4952
	11th to 20th	Night	17% 8% 1 6% 3 3 3 2 4 6% 47%	5000
	21st to end	Night	20% 10% 3 5% 3 3 3 2 4 7% 40%	5453
April	1st to 10th	Night	11% 8% 2 6% 4 3 3 4 5% 6% 50%	2000
	11th to 20th	Night	10% 7% 2 5% 4 3 3 2 4 6% 55%	1979
	21st to end	Night	7% 5% 2 5% 3 3 3 3 5% 7% 59%	2000
May	1st to 10th	Night	5% 3 1 4 2 2 2 2 3 5% 72%	4200
	11th to 20th	Night	4 4 2 5% 4 3 3 2 3 5% 63%	3946
	21st to end	Night	2 2 1 3 2 2 2 2 2 4 77%	3972
June	1st to 10th	Night	5% 5% 2 4 4 3 3 3 5% 8% 58%	1600
	11th to 20th	Night	2 2 1 4 3 2 3 2 4 6% 72%	1600
	21st to end	Night	3 4 2 4 4 3 3 2 3 4 70%	1600
July	1st to 10th	Night	1 1 1 7% 3 5% 3 1 2 3 73%	3640
	11th to 20th	Night	1 3 4 13% 7% 5% 3 2 2 3 56%	4071
	21st to end	Night	1 9% 5% 14% 6% 3 4 3 3 6% 46%	4472
August	1st to 10th	Night	2 3 1 6% 7% 7% 5% 3 3 5% 59%	4035
	11th to 20th	Night	3 2 6% 4 4 3 2 3 5% 66%	4231
	21st to end	Night	2 3 1 5% 8% 7% 4 3 4 5% 60%	5015
September	1st to 10th	Night	1 3 1 3 3 3 3 3 4 7% 70%	2000
	11th to 20th	Night	5% 5% 2 5% 3 4 3 3 4 7% 59%	2000
	21st to end	Night	4 4 1 5% 2 3 3 2 3 4 69%	2295
October	1st to 10th	Night	3 3 1 3 2 2 2 2 3 4 72%	3920
	11th to 20th	Night	5% 7% 3 6% 4 4 4 2 3 5% 57%	3873
	21st to end	Night	9% 4 1 3 3 3 3 3 5% 6% 60%	4121
November	1st to 10th	Night	10% 6% 2 4 3 2 3 3 5% 8% 54%	2360
	11th to 20th	Night	13% 6% 2 5% 4 3 3 3 5% 7% 50%	2625
	21st to end	Night	12% 4 2 5% 3 4 3 2 4 6% 55%	2800
December	1st to 10th	Night	12% 4 2 4 3 3 4 3 5% 7% 55%	4560
	11th to 20th	Night	28% 8% 2 5% 4 4 4 3 7% 7% 29%	5176
	21st to end	Night	32% 8% 2 6% 4 3 3 3 3 4 32%	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	39% 6% 14% 22% 12% 7%	3892
	11th to 20th	Day	42% 1 9% 15% 19% 15%	4100
	21st to end	Day	45% 2 7% 15% 13% 18%	4510
February	1st to 10th	Day	43% 3 7% 20% 20% 8%	1800
	11th to 20th	Day	44% 3 7% 21% 18% 6%	1807
	21st to end	Day	39% 3 9% 26% 16% 7%	1650
March	1st to 10th	Day	49% 2 8% 21% 18% 2	4940
	11th to 20th	Day	37% 3 14% 27% 11% 8%	5000
	21st to end	Day	45% 6% 14% 24% 10% 1	5520
April	1st to 10th	Day	41% 3 13% 23% 16% 4	2400
	11th to 20th	Day	34% 3 13% 25% 20% 5%	2372
	21st to end	Day	26% 3 14% 32% 22% 4	2400
May	1st to 10th	Day	14% 1 8% 29% 30% 17%	5781
	11th to 20th	Day	15% 1 6% 36% 33% 8%	5947
	21st to end	Day	13% 2 8% 35% 39% 3	6904
June	1st to 10th	Day	25% 3 10% 29% 27% 6%	2800
	11th to 20th	Day	14% 2 9% 21% 37% 17%	2800
	21st to end	Day	15% 1 4 16% 38% 26%	2800
July	1st to 10th	Day	11% 1 2 9% 25% 53%	6251
	11th to 20th	Day	14% 1 3 8% 17% 57%	5820
	21st to end	Day	17% 1 4 16% 29% 33%	6217
August	1st to 10th	Day	20% 1 3 12% 28% 36%	4206
	11th to 20th	Day	13% 1 2 11% 43% 30%	4072
	21st to end	Day	19% 1 2 14% 37% 27%	4311
September	1st to 10th	Day	15% 2 2 12% 33% 37%	2400
	11th to 20th	Day	23% 2 4 26% 33% 11%	2395
	21st to end	Day	17% 1 4 23% 32% 23%	2105
October	1st to 10th	Day	17% 1 4 13% 26% 39%	3280
	11th to 20th	Day	25% 2 5% 22% 26% 20%	3242
	21st to end	Day	28% 2 8% 24% 26% 12%	3448
November	1st to 10th	Day	29% 4 7% 29% 20% 10%	1937
	11th to 20th	Day	40% 3 10% 24% 17% 7%	1770
	21st to end	Day	35% 3 10% 20% 19% 13%	1600
December	1st to 10th	Day	29% 2 8% 29% 23% 9%	2620
	11th to 20th	Day	55% 4 8% 16% 14% 3	3028
	21st to end	Day	57% 3 11% 19% 8% 3	4004
January	1st to 10th	Night	38% 6% 20% 19% 12% 6%	6059
	11th to 20th	Night	42% 2 12% 14% 17% 13%	5880
	21st to end	Night	45% 2 9% 11% 16% 17%	6468
February	1st to 10th	Night	40% 3 7% 20% 19% 11%	2600
	11th to 20th	Night	42% 3 9% 20% 18% 7%	2489
	21st to end	Night	32% 4 9% 27% 20% 7%	1980
March	1st to 10th	Night	49% 2 11% 19% 17% 3	4952
	11th to 20th	Night	39% 4 10% 21% 19% 8%	5000
	21st to end	Night	44% 4 13% 25% 12% 2	5453
April	1st to 10th	Night	36% 4 13% 23% 21% 4	2000
	11th to 20th	Night	33% 2 11% 27% 17% 10%	1979
	21st to end	Night	25% 3 10% 32% 25% 5%	2000
May	1st to 10th	Night	18% 1 5% 24% 32% 20%	4197
	11th to 20th	Night	24% 2 4 28% 28% 13%	3947
	21st to end	Night	14% 2 7% 31% 40% 7%	3972
June	1st to 10th	Night	26% 4 10% 22% 31% 8%	1600
	11th to 20th	Night	16% 2 6% 20% 37% 19%	1600
	21st to end	Night	21% 3 2 12% 33% 29%	1600
July	1st to 10th	Night	20% 1 2 8% 23% 46%	3600
	11th to 20th	Night	38% 1 2 7% 11% 41%	4018
	21st to end	Night	42% 1 4 7% 17% 28%	4345
August	1st to 10th	Night	44% 2 3 10% 20% 21%	2929
	11th to 20th	Night	20% 2 3 7% 37% 31%	3056
	21st to end	Night	27% 2 3 12% 30% 26%	3608
September	1st to 10th	Night	18% 2 2 10% 31% 37%	2000
	11th to 20th	Night	30% 2 3 20% 30% 14%	2000
	21st to end	Night	22% 2 3 20% 28% 25%	2295
October	1st to 10th	Night	18% 2 4 12% 21% 44%	3920
	11th to 20th	Night	33% 2 5% 19% 20% 21%	3873
	21st to end	Night	27% 3 5% 21% 27% 18%	4121
November	1st to 10th	Night	31% 3 9% 27% 19% 11%	2360
	11th to 20th	Night	34% 4 11% 27% 16% 8%	2625
	21st to end	Night	33% 3 9% 21% 22% 13%	2800
December	1st to 10th	Night	31% 3 9% 23% 23% 12%	4560
	11th to 20th	Night	54% 3 9% 15% 14% 4	5176
	21st to end	Night	57% 2 10% 18% 9% 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	29% 9% 28% 18% 12% 4	310
	11th to 20th	Day	21% 11% 23% 27% 16% 3	400
	21st to end	Day	16% 8% 26% 29% 16% 5%	440
February	1st to 10th	Day	22% 10% 24% 20% 19% 5%	400
	11th to 20th	Day	21% 16% 28% 21% 12% 1	418
	21st to end	Day	16% 14% 26% 28% 13% 2	414
March	1st to 10th	Day	25% 14% 30% 19% 10% 2	500
	11th to 20th	Day	24% 11% 29% 21% 11% 4	500
	21st to end	Day	20% 16% 31% 22% 9% 1	550
April	1st to 10th	Day	17% 16% 29% 23% 9% 5%	500
	11th to 20th	Day	15% 14% 30% 23% 12% 6%	492
	21st to end	Day	9% 17% 30% 24% 13% 7%	500
May	1st to 10th	Day	10% 14% 31% 26% 15% 4	503
	11th to 20th	Day	13% 11% 30% 23% 17% 6%	528
	21st to end	Day	9% 11% 26% 29% 18% 8%	573
June	1st to 10th	Day	10% 8% 28% 31% 15% 9%	530
	11th to 20th	Day	4 11% 31% 34% 14% 5%	530
	21st to end	Day	2 5% 27% 33% 26% 8%	530
July	1st to 10th	Day	4 4 22% 41% 22% 7%	526
	11th to 20th	Day	1 1 17% 38% 30% 13%	524
	21st to end	Day	2 2 14% 46% 25% 12%	575
August	1st to 10th	Day	2 4 21% 37% 26% 10%	527
	11th to 20th	Day	4 4 25% 32% 25% 10%	517
	21st to end	Day	3 4 19% 36% 26% 12%	583
September	1st to 10th	Day	3 5% 22% 35% 25% 10%	530
	11th to 20th	Day	6% 7% 25% 32% 21% 9%	530
	21st to end	Day	6% 8% 24% 31% 23% 9%	530
October	1st to 10th	Day	8% 7% 25% 26% 22% 12%	530
	11th to 20th	Day	10% 9% 20% 26% 24% 11%	523
	21st to end	Day	12% 8% 21% 28% 22% 9%	539
November	1st to 10th	Day	11% 10% 19% 25% 27% 7%	473
	11th to 20th	Day	12% 9% 17% 22% 27% 12%	285
	21st to end	Day	14% 10% 20% 28% 23% 7%	200
December	1st to 10th	Day	20% 10% 20% 21% 20% 11%	200
	11th to 20th	Day	25% 11% 22% 19% 19% 6%	200
	21st to end	Day	24% 12% 24% 19% 19% 2	220
January	1st to 10th	Night	20% 6% 13% 32% 23% 6%	800
	11th to 20th	Night	20% 4 15% 35% 21% 5%	800
	21st to end	Night	19% 6% 13% 33% 25% 4	880
February	1st to 10th	Night	21% 6% 14% 35% 21% 3	800
	11th to 20th	Night	17% 8% 18% 31% 20% 5%	784
	21st to end	Night	17% 8% 19% 35% 18% 3	660
March	1st to 10th	Night	21% 10% 18% 30% 18% 4	800
	11th to 20th	Night	20% 7% 15% 32% 22% 3	800
	21st to end	Night	18% 9% 25% 30% 16% 2	880
April	1st to 10th	Night	15% 9% 24% 31% 18% 3	800
	11th to 20th	Night	12% 12% 22% 29% 20% 5%	791
	21st to end	Night	8% 10% 27% 27% 23% 5%	800
May	1st to 10th	Night	8% 7% 21% 33% 25% 5%	648
	11th to 20th	Night	8% 4 21% 28% 34% 6%	542
	21st to end	Night	8% 9% 18% 29% 30% 6%	433
June	1st to 10th	Night	8% 4 21% 33% 32% 3	400
	11th to 20th	Night	2 6% 20% 42% 29% 2	400
	21st to end	Night	5% 1 14% 37% 39% 4	400
July	1st to 10th	Night	3 1 12% 35% 43% 6%	390
	11th to 20th	Night	3 1 4 29% 47% 15%	567
	21st to end	Night	3 2 6% 26% 51% 11%	657
August	1st to 10th	Night	6% 2 8% 28% 43% 13%	597
	11th to 20th	Night	3 2 11% 32% 42% 10%	669
	21st to end	Night	5% 3 7% 33% 41% 10%	880
September	1st to 10th	Night	2 2 10% 33% 43% 10%	800
	11th to 20th	Night	8% 3 12% 33% 35% 10%	800
	21st to end	Night	6% 4 12% 35% 34% 10%	800
October	1st to 10th	Night	8% 4 13% 34% 32% 10%	800
	11th to 20th	Night	9% 4 13% 31% 34% 9%	792
	21st to end	Night	11% 4 11% 32% 31% 13%	836
November	1st to 10th	Night	12% 5% 12% 30% 32% 9%	780
	11th to 20th	Night	13% 4 13% 35% 29% 6%	800
	21st to end	Night	13% 5% 16% 34% 26% 6%	800
December	1st to 10th	Night	21% 3 13% 34% 23% 6%	800
	11th to 20th	Night	26% 4 13% 33% 20% 4	800
	21st to end	Night	24% 6% 18% 33% 17% 2	880

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