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 2024-11-05.

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	9% 5% 2 4 3 3 3 3 4 6% 59%	105093
	Night	11% 6% 2 5% 4 3 3 3 4 6% 53%	102702
Transparency	Day	28% 2 7% 21% 23% 18%	104853
	Night	35% 2 8% 19% 20% 16%	102532
Seeing	Day	11% 9% 24% 29% 19% 7%	15386
	Night	12% 5% 15% 32% 29% 7%	24639

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	19% 8% 3 7% 4 3 3 2 5% 6% 40%	2708
	11th to 20th	Day	23% 9% 4 8% 4 4 4 2 4 5% 32%	2860
	21st to end	Day	23% 10% 3 7% 4 4 3 3 5% 6% 31%	3146
February	1st to 10th	Day	18% 6% 2 6% 4 3 4 3 3 5% 46%	1710
	11th to 20th	Day	16% 8% 3 6% 4 4 3 4 5% 8% 41%	1705
	21st to end	Day	16% 7% 2 4 5% 3 2 3 5% 6% 47%	1570
March	1st to 10th	Day	21% 13% 4 9% 5% 4 4 2 4 5% 30%	3430
	11th to 20th	Day	12% 5% 2 4 3 3 3 3 5% 7% 53%	3490
	21st to end	Day	19% 8% 2 6% 4 4 5% 4 6% 7% 35%	3846
April	1st to 10th	Day	15% 7% 3 5% 5% 4 4 5% 5% 7% 41%	2280
	11th to 20th	Day	10% 5% 2 5% 4 3 4 4 5% 7% 51%	2249
	21st to end	Day	8% 4 1 4 3 3 3 3 7% 9% 56%	2280
May	1st to 10th	Day	6% 4 1 3 2 2 2 2 3 5% 71%	4037
	11th to 20th	Day	4 3 1 3 2 2 2 2 4 7% 70%	4148
	21st to end	Day	4 4 1 3 2 2 1 2 4 6% 71%	4813
June	1st to 10th	Day	7% 5% 2 3 3 2 3 3 5% 7% 59%	2660
	11th to 20th	Day	2 3 1 2 2 2 2 2 4 6% 73%	2660
	21st to end	Day	3 3 1 3 2 2 1 2 3 5% 76%	2660
July	1st to 10th	Day	1 2 1 2 1 1 1 2 3 4 82%	4420
	11th to 20th	Day	1 2 1 3 2 2 2 2 4 6% 75%	4120
	21st to end	Day	1 2 1 3 2 2 2 2 3 6% 78%	4455
August	1st to 10th	Day	1 3 1 4 3 4 4 4 5% 8% 65%	4091
	11th to 20th	Day	1 2 1 2 2 2 2 2 3 6% 78%	3907
	21st to end	Day	2 4 1 3 2 3 3 2 3 5% 72%	4180
September	1st to 10th	Day	1 2 1 2 1 1 2 2 3 6% 77%	2280
	11th to 20th	Day	4 4 2 4 2 2 3 2 5% 8% 63%	2275
	21st to end	Day	5% 2 1 3 1 2 2 2 4 6% 74%	2000
October	1st to 10th	Day	4 3 1 3 2 2 2 2 4 6% 71%	3180
	11th to 20th	Day	8% 4 1 4 2 2 2 2 3 5% 66%	3161
	21st to end	Day	8% 4 1 4 3 3 4 3 6% 7% 57%	3448
November	1st to 10th	Day	12% 3 2 4 4 3 3 3 5% 8% 54%	1880
	11th to 20th	Day	13% 6% 2 5% 4 4 3 3 5% 7% 47%	1682
	21st to end	Day	11% 6% 1 4 4 3 4 4 5% 5% 53%	1520
December	1st to 10th	Day	18% 9% 2 4 4 3 3 4 4 6% 45%	1520
	11th to 20th	Day	33% 9% 2 5% 4 3 3 2 4 5% 31%	1928
	21st to end	Day	23% 10% 2 6% 3 3 3 3 4 8% 34%	2794
January	1st to 10th	Night	17% 8% 2 5% 4 3 4 3 5% 7% 42%	4237
	11th to 20th	Night	22% 9% 3 7% 4 4 4 4 5% 5% 32%	4110
	21st to end	Night	24% 11% 3 7% 4 4 4 3 4 5% 31%	4521
February	1st to 10th	Night	18% 6% 2 5% 3 3 3 3 4 6% 48%	2470
	11th to 20th	Night	14% 8% 3 5% 3 4 4 3 5% 7% 43%	2360
	21st to end	Night	14% 7% 1 3 3 2 4 2 4 6% 53%	1884
March	1st to 10th	Night	21% 12% 3 6% 4 3 4 3 3 5% 35%	3442
	11th to 20th	Night	14% 5% 1 6% 3 3 3 2 4 5% 54%	3490
	21st to end	Night	18% 12% 3 6% 4 3 3 2 4 6% 39%	3805
April	1st to 10th	Night	12% 7% 2 6% 4 2 3 3 5% 6% 50%	1900
	11th to 20th	Night	10% 6% 2 4 4 3 3 2 4 6% 56%	1879
	21st to end	Night	7% 4 2 4 3 3 3 3 5% 7% 60%	1900
May	1st to 10th	Night	5% 3 1 4 2 2 2 2 3 4 71%	2932
	11th to 20th	Night	3 4 3 6% 5% 4 3 2 4 6% 60%	2748
	21st to end	Night	3 3 1 3 2 2 2 2 3 5% 74%	2762
June	1st to 10th	Night	5% 5% 2 3 3 3 3 3 5% 9% 58%	1520
	11th to 20th	Night	2 2 1 4 3 2 2 2 4 6% 71%	1520
	21st to end	Night	3 4 2 4 4 3 3 2 3 4 69%	1520
July	1st to 10th	Night	1 1 1 4 2 2 3 1 2 4 78%	2540
	11th to 20th	Night	1 2 2 8% 5% 4 2 2 3 4 67%	2845
	21st to end	Night	1 3 2 11% 6% 4 4 3 4 7% 53%	3109
August	1st to 10th	Night	3 5% 1 8% 9% 10% 6% 4 4 6% 45%	2848
	11th to 20th	Night	3 1 4 3 4 3 2 3 6% 70%	2935
	21st to end	Night	3 4 1 4 5% 5% 3 2 3 4 65%	3498
September	1st to 10th	Night	3 1 3 3 3 3 3 4 7% 71%	1900
	11th to 20th	Night	5% 5% 2 5% 4 4 3 3 4 7% 59%	1900
	21st to end	Night	3 4 1 5% 2 2 2 2 4 4 70%	2180
October	1st to 10th	Night	3 3 1 3 2 2 2 2 3 4 73%	3800
	11th to 20th	Night	5% 7% 3 6% 4 4 4 2 3 5% 57%	3777
	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%	2298
	11th to 20th	Night	13% 6% 2 5% 4 3 3 3 5% 7% 50%	2493
	21st to end	Night	11% 4 2 4 3 3 3 2 4 6% 57%	2660
December	1st to 10th	Night	20% 6% 2 6% 3 3 3 3 4 6% 43%	2660
	11th to 20th	Night	33% 7% 2 4 4 3 3 3 6% 6% 31%	3276
	21st to end	Night	22% 8% 3 6% 5% 3 4 3 4 5% 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	47% 3 11% 29% 8% 3	2708
	11th to 20th	Day	57% 2 11% 13% 10% 7%	2860
	21st to end	Day	55% 3 9% 18% 11% 4	3146
February	1st to 10th	Day	43% 3 7% 20% 20% 8%	1710
	11th to 20th	Day	43% 3 7% 22% 19% 7%	1714
	21st to end	Day	40% 3 9% 27% 15% 7%	1570
March	1st to 10th	Day	57% 3 9% 17% 12% 2	3430
	11th to 20th	Day	32% 2 11% 30% 13% 11%	3490
	21st to end	Day	47% 3 13% 24% 11% 2	3846
April	1st to 10th	Day	42% 3 13% 22% 16% 4	2280
	11th to 20th	Day	33% 3 13% 25% 20% 5%	2252
	21st to end	Day	25% 3 14% 32% 23% 4	2280
May	1st to 10th	Day	18% 2 11% 32% 22% 15%	4037
	11th to 20th	Day	18% 1 5% 32% 32% 11%	4151
	21st to end	Day	17% 2 11% 37% 29% 3	4814
June	1st to 10th	Day	25% 3 11% 30% 25% 6%	2660
	11th to 20th	Day	15% 2 10% 21% 37% 16%	2660
	21st to end	Day	15% 1 4 16% 38% 26%	2660
July	1st to 10th	Day	11% 1 2 8% 17% 61%	4351
	11th to 20th	Day	14% 1 3 9% 20% 53%	4050
	21st to end	Day	14% 1 2 8% 30% 45%	4315
August	1st to 10th	Day	21% 1 3 11% 27% 37%	4076
	11th to 20th	Day	13% 1 2 11% 43% 30%	3946
	21st to end	Day	19% 1 2 14% 36% 27%	4179
September	1st to 10th	Day	14% 1 2 11% 33% 39%	2280
	11th to 20th	Day	23% 2 4 26% 33% 11%	2275
	21st to end	Day	16% 1 4 23% 33% 23%	2000
October	1st to 10th	Day	17% 1 4 12% 26% 40%	3180
	11th to 20th	Day	25% 2 5% 22% 26% 20%	3161
	21st to end	Day	28% 2 8% 24% 26% 12%	3448
November	1st to 10th	Day	30% 4 7% 29% 19% 10%	1880
	11th to 20th	Day	39% 2 10% 23% 17% 7%	1682
	21st to end	Day	34% 3 10% 21% 19% 12%	1520
December	1st to 10th	Day	42% 3 9% 19% 19% 8%	1520
	11th to 20th	Day	59% 2 7% 12% 15% 4	1928
	21st to end	Day	52% 2 12% 21% 9% 4	2794
January	1st to 10th	Night	44% 3 18% 24% 8% 3	4237
	11th to 20th	Night	55% 2 12% 13% 9% 7%	4110
	21st to end	Night	56% 2 10% 14% 12% 6%	4521
February	1st to 10th	Night	39% 3 8% 21% 19% 11%	2470
	11th to 20th	Night	41% 3 9% 20% 19% 8%	2362
	21st to end	Night	32% 4 10% 28% 20% 6%	1884
March	1st to 10th	Night	53% 3 12% 16% 14% 3	3442
	11th to 20th	Night	34% 3 8% 23% 23% 10%	3490
	21st to end	Night	46% 3 11% 25% 11% 3	3805
April	1st to 10th	Night	36% 4 13% 22% 21% 5%	1900
	11th to 20th	Night	32% 3 11% 28% 18% 9%	1879
	21st to end	Night	25% 3 10% 31% 25% 6%	1900
May	1st to 10th	Night	20% 1 6% 28% 29% 17%	2932
	11th to 20th	Night	27% 2 4 20% 27% 19%	2749
	21st to end	Night	16% 2 7% 39% 30% 6%	2762
June	1st to 10th	Night	25% 4 10% 22% 31% 8%	1520
	11th to 20th	Night	17% 2 6% 21% 37% 18%	1520
	21st to end	Night	21% 3 2 11% 33% 29%	1520
July	1st to 10th	Night	15% 2 1 6% 17% 58%	2500
	11th to 20th	Night	25% 2 3 7% 15% 48%	2792
	21st to end	Night	34% 2 3 6% 16% 40%	3011
August	1st to 10th	Night	45% 2 3 10% 18% 22%	2839
	11th to 20th	Night	19% 2 3 6% 38% 32%	2962
	21st to end	Night	27% 2 3 12% 31% 25%	3498
September	1st to 10th	Night	17% 2 2 10% 31% 38%	1900
	11th to 20th	Night	30% 2 3 20% 31% 15%	1900
	21st to end	Night	21% 2 3 21% 28% 26%	2180
October	1st to 10th	Night	18% 2 4 11% 21% 45%	3800
	11th to 20th	Night	33% 2 5% 18% 20% 21%	3777
	21st to end	Night	27% 3 5% 21% 27% 18%	4121
November	1st to 10th	Night	31% 3 9% 27% 19% 11%	2298
	11th to 20th	Night	34% 4 10% 27% 16% 9%	2493
	21st to end	Night	31% 3 9% 22% 22% 13%	2660
December	1st to 10th	Night	43% 3 8% 17% 19% 10%	2660
	11th to 20th	Night	55% 3 9% 14% 15% 4	3276
	21st to end	Night	51% 2 13% 19% 11% 3	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% 9% 27% 17% 12% 4	284
	11th to 20th	Day	23% 11% 21% 26% 15% 3	370
	21st to end	Day	17% 9% 26% 29% 15% 4	407
February	1st to 10th	Day	19% 11% 23% 21% 20% 6%	370
	11th to 20th	Day	21% 16% 28% 21% 14% 2	385
	21st to end	Day	16% 15% 26% 27% 13% 2	382
March	1st to 10th	Day	26% 13% 30% 19% 10% 2	460
	11th to 20th	Day	23% 11% 29% 20% 12% 5%	460
	21st to end	Day	19% 16% 32% 23% 9% 1	506
April	1st to 10th	Day	17% 17% 29% 24% 9% 5%	460
	11th to 20th	Day	15% 14% 30% 22% 12% 6%	452
	21st to end	Day	9% 18% 30% 24% 12% 8%	460
May	1st to 10th	Day	11% 14% 31% 27% 15% 3	463
	11th to 20th	Day	11% 12% 28% 24% 17% 7%	488
	21st to end	Day	9% 11% 23% 29% 19% 8%	529
June	1st to 10th	Day	11% 9% 26% 31% 14% 9%	490
	11th to 20th	Day	3 11% 30% 35% 15% 6%	490
	21st to end	Day	1 4 28% 33% 26% 8%	490
July	1st to 10th	Day	4 4 20% 42% 23% 7%	486
	11th to 20th	Day	1 1 17% 38% 29% 14%	484
	21st to end	Day	1 2 13% 47% 25% 12%	531
August	1st to 10th	Day	2 4 20% 38% 26% 10%	487
	11th to 20th	Day	5% 4 24% 31% 25% 11%	477
	21st to end	Day	3 4 19% 36% 25% 12%	539
September	1st to 10th	Day	3 4 20% 36% 26% 11%	490
	11th to 20th	Day	7% 7% 25% 32% 20% 9%	490
	21st to end	Day	5% 7% 23% 31% 24% 9%	490
October	1st to 10th	Day	7% 6% 26% 27% 23% 11%	490
	11th to 20th	Day	9% 9% 20% 27% 24% 12%	490
	21st to end	Day	12% 8% 21% 28% 22% 9%	539
November	1st to 10th	Day	12% 10% 19% 25% 27% 8%	452
	11th to 20th	Day	10% 7% 16% 24% 30% 13%	257
	21st to end	Day	12% 8% 22% 28% 23% 8%	180
December	1st to 10th	Day	21% 11% 18% 22% 18% 10%	180
	11th to 20th	Day	24% 11% 21% 19% 19% 6%	180
	21st to end	Day	22% 11% 23% 20% 21% 3	198
January	1st to 10th	Night	21% 6% 12% 31% 23% 6%	760
	11th to 20th	Night	21% 4 14% 34% 22% 5%	760
	21st to end	Night	20% 6% 13% 33% 24% 4	836
February	1st to 10th	Night	20% 5% 14% 36% 22% 3	760
	11th to 20th	Night	17% 9% 17% 31% 21% 5%	744
	21st to end	Night	18% 8% 19% 33% 19% 3	628
March	1st to 10th	Night	21% 9% 18% 30% 18% 4	760
	11th to 20th	Night	19% 7% 15% 33% 23% 3	760
	21st to end	Night	18% 9% 25% 30% 16% 2	836
April	1st to 10th	Night	15% 8% 24% 31% 18% 4	760
	11th to 20th	Night	12% 12% 21% 28% 21% 5%	751
	21st to end	Night	8% 10% 28% 26% 22% 5%	760
May	1st to 10th	Night	8% 7% 22% 33% 24% 6%	616
	11th to 20th	Night	7% 3 20% 29% 35% 6%	515
	21st to end	Night	9% 9% 17% 29% 30% 6%	411
June	1st to 10th	Night	8% 4 21% 31% 32% 3	380
	11th to 20th	Night	2 6% 18% 42% 30% 2	380
	21st to end	Night	5% 1 14% 37% 39% 4	380
July	1st to 10th	Night	3 1 12% 34% 45% 6%	370
	11th to 20th	Night	3 1 4 28% 47% 16%	538
	21st to end	Night	3 2 5% 26% 53% 11%	624
August	1st to 10th	Night	6% 2 8% 28% 43% 12%	567
	11th to 20th	Night	3 2 10% 31% 44% 10%	635
	21st to end	Night	5% 3 7% 32% 42% 11%	836
September	1st to 10th	Night	2 2 9% 33% 44% 10%	760
	11th to 20th	Night	8% 3 12% 34% 35% 10%	760
	21st to end	Night	5% 4 12% 34% 34% 11%	760
October	1st to 10th	Night	7% 4 14% 33% 32% 10%	760
	11th to 20th	Night	9% 4 12% 31% 35% 9%	760
	21st to end	Night	11% 4 11% 32% 31% 13%	836
November	1st to 10th	Night	12% 4 12% 30% 32% 9%	760
	11th to 20th	Night	13% 3 12% 35% 30% 7%	760
	21st to end	Night	12% 4 16% 34% 27% 6%	760
December	1st to 10th	Night	22% 3 13% 33% 23% 6%	760
	11th to 20th	Night	26% 4 13% 33% 20% 4	760
	21st to end	Night	23% 6% 18% 34% 18% 2	836

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