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 2024-04-10.

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	11% 5% 2 4 3 2 3 3 4 7% 54%	83364
	Night	13% 6% 2 5% 4 2 3 3 4 6% 50%	85513
Transparency	Day	33% 2 8% 21% 22% 13%	83084
	Night	37% 3 8% 19% 20% 13%	85315
Seeing	Day	12% 9% 24% 29% 19% 7%	14550
	Night	13% 5% 15% 32% 29% 7%	23963

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 6% 5% 3 3 3 5% 6% 40%	2708
	11th to 20th	Day	23% 9% 4 7% 5% 3 4 3 4 5% 32%	2860
	21st to end	Day	23% 9% 3 6% 5% 3 4 4 5% 6% 31%	3146
February	1st to 10th	Day	18% 6% 3 5% 5% 2 4 4 3 5% 46%	1710
	11th to 20th	Day	16% 7% 3 5% 5% 3 4 4 5% 8% 42%	1705
	21st to end	Day	17% 6% 2 4 5% 2 2 3 5% 6% 47%	1570
March	1st to 10th	Day	21% 12% 4 8% 5% 3 4 3 4 5% 30%	3430
	11th to 20th	Day	13% 5% 2 3 3 2 3 4 5% 7% 53%	3490
	21st to end	Day	20% 8% 2 5% 5% 3 5% 5% 6% 8% 35%	3846
April	1st to 10th	Day	16% 6% 3 4 5% 3 4 5% 5% 7% 42%	2270
	11th to 20th	Day	10% 5% 2 5% 4 2 4 5% 4 7% 52%	2129
	21st to end	Day	8% 4 2 3 3 2 3 4 6% 9% 56%	2160
May	1st to 10th	Day	7% 6% 2 4 3 2 3 3 4 7% 60%	2293
	11th to 20th	Day	7% 4 1 3 3 2 3 3 5% 8% 61%	2339
	21st to end	Day	6% 7% 2 4 3 2 3 3 5% 8% 57%	2723
June	1st to 10th	Day	8% 5% 2 3 3 2 3 3 5% 8% 59%	2520
	11th to 20th	Day	2 3 1 2 2 1 2 3 4 6% 73%	2520
	21st to end	Day	3 2 2 3 2 1 2 2 3 5% 75%	2520
July	1st to 10th	Day	2 3 2 3 2 1 2 3 4 6% 74%	2520
	11th to 20th	Day	2 3 1 3 2 1 2 3 4 8% 71%	2350
	21st to end	Day	1 3 1 3 2 1 2 2 3 7% 74%	2546
August	1st to 10th	Day	2 4 1 3 2 2 3 3 4 7% 68%	2328
	11th to 20th	Day	1 3 1 3 2 2 2 3 4 7% 72%	2231
	21st to end	Day	2 4 2 3 2 2 3 3 4 7% 68%	2376
September	1st to 10th	Day	1 2 2 2 2 1 2 2 3 7% 77%	2160
	11th to 20th	Day	5% 4 2 3 3 1 3 3 5% 8% 64%	2156
	21st to end	Day	5% 2 1 2 2 1 2 2 4 6% 73%	1895
October	1st to 10th	Day	7% 3 1 3 3 1 2 3 4 6% 68%	1800
	11th to 20th	Day	8% 4 1 2 3 2 3 3 4 6% 64%	1800
	21st to end	Day	11% 3 2 3 3 2 3 3 4 6% 60%	1980
November	1st to 10th	Day	12% 3 2 4 4 2 3 3 5% 8% 54%	1839
	11th to 20th	Day	13% 6% 3 5% 5% 2 3 4 5% 7% 47%	1682
	21st to end	Day	12% 5% 1 4 4 2 4 5% 5% 5% 53%	1520
December	1st to 10th	Day	18% 9% 1 4 4 3 3 4 4 6% 45%	1520
	11th to 20th	Day	33% 8% 3 5% 4 2 3 3 4 5% 31%	1928
	21st to end	Day	23% 9% 3 5% 4 2 3 4 4 8% 34%	2794
January	1st to 10th	Night	17% 8% 2 5% 4 2 4 4 5% 7% 42%	4237
	11th to 20th	Night	23% 9% 3 6% 6% 3 4 4 5% 5% 32%	4110
	21st to end	Night	24% 10% 3 6% 5% 3 4 4 4 5% 31%	4521
February	1st to 10th	Night	19% 5% 2 4 3 2 3 4 4 6% 48%	2470
	11th to 20th	Night	14% 8% 3 5% 4 3 4 3 5% 7% 44%	2360
	21st to end	Night	15% 6% 2 3 3 2 3 3 4 6% 53%	1884
March	1st to 10th	Night	22% 12% 3 5% 5% 2 4 4 3 5% 35%	3442
	11th to 20th	Night	14% 5% 1 5% 4 2 3 3 4 5% 54%	3490
	21st to end	Night	18% 12% 3 5% 4 2 3 3 4 6% 39%	3805
April	1st to 10th	Night	12% 7% 2 5% 5% 2 3 4 4 6% 50%	1900
	11th to 20th	Night	10% 6% 2 3 4 2 4 3 4 6% 56%	1779
	21st to end	Night	7% 4 2 4 3 2 3 3 5% 7% 60%	1800
May	1st to 10th	Night	6% 4 2 4 5% 3 3 3 4 6% 59%	1664
	11th to 20th	Night	5% 4 2 4 4 3 4 3 5% 9% 57%	1550
	21st to end	Night	6% 5% 2 5% 3 3 3 3 4 7% 58%	1552
June	1st to 10th	Night	5% 5% 2 3 4 2 4 4 5% 9% 58%	1440
	11th to 20th	Night	2 1 1 4 3 2 2 3 4 6% 71%	1440
	21st to end	Night	4 4 2 4 4 2 3 3 3 4 68%	1440
July	1st to 10th	Night	2 2 1 4 3 2 3 2 3 6% 72%	1440
	11th to 20th	Night	2 2 2 4 3 2 4 3 4 7% 67%	1605
	21st to end	Night	2 4 2 6% 6% 3 4 4 4 9% 56%	1773
August	1st to 10th	Night	2 6% 2 6% 4 3 4 4 4 7% 58%	1611
	11th to 20th	Night	4 1 4 5% 3 5% 4 4 9% 60%	1669
	21st to end	Night	1 5% 2 6% 5% 3 3 3 4 7% 61%	1980
September	1st to 10th	Night	3 1 3 3 2 4 3 3 7% 72%	1800
	11th to 20th	Night	5% 5% 2 5% 4 3 4 3 4 7% 59%	1800
	21st to end	Night	4 4 1 3 2 2 3 2 3 5% 70%	2065
October	1st to 10th	Night	5% 4 2 4 4 2 2 3 3 4 67%	2160
	11th to 20th	Night	6% 4 2 4 4 3 4 4 4 6% 58%	2160
	21st to end	Night	10% 4 2 3 3 2 3 4 5% 7% 58%	2376
November	1st to 10th	Night	10% 5% 2 4 4 2 3 3 5% 8% 54%	2239
	11th to 20th	Night	13% 5% 2 4 5% 2 3 4 5% 7% 51%	2493
	21st to end	Night	11% 4 2 4 3 3 4 3 4 6% 57%	2660
December	1st to 10th	Night	20% 5% 3 5% 3 2 3 4 4 7% 44%	2660
	11th to 20th	Night	33% 6% 2 3 4 2 3 3 6% 6% 31%	3276
	21st to end	Night	22% 8% 3 6% 5% 3 4 4 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 12% 23% 16% 4	2270
	11th to 20th	Day	32% 3 13% 26% 21% 5%	2132
	21st to end	Day	26% 3 14% 32% 22% 4	2160
May	1st to 10th	Day	26% 3 8% 31% 27% 6%	2293
	11th to 20th	Day	24% 2 9% 35% 22% 7%	2342
	21st to end	Day	28% 3 16% 28% 22% 3	2724
June	1st to 10th	Day	26% 3 11% 29% 25% 6%	2520
	11th to 20th	Day	15% 2 10% 21% 36% 16%	2520
	21st to end	Day	16% 1 4 16% 38% 24%	2520
July	1st to 10th	Day	16% 2 3 12% 31% 37%	2451
	11th to 20th	Day	17% 3 4 13% 30% 33%	2280
	21st to end	Day	16% 1 2 11% 31% 38%	2405
August	1st to 10th	Day	21% 2 3 17% 32% 26%	2312
	11th to 20th	Day	17% 2 1 14% 38% 27%	2231
	21st to end	Day	22% 1 2 14% 36% 25%	2376
September	1st to 10th	Day	14% 1 1 12% 34% 37%	2160
	11th to 20th	Day	23% 2 4 26% 33% 11%	2156
	21st to end	Day	17% 1 4 24% 32% 22%	1895
October	1st to 10th	Day	21% 2 6% 18% 32% 22%	1800
	11th to 20th	Day	25% 3 7% 22% 24% 20%	1800
	21st to end	Day	29% 2 7% 22% 24% 16%	1980
November	1st to 10th	Day	30% 4 7% 29% 19% 10%	1839
	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	31% 3 11% 27% 18% 10%	1779
	21st to end	Night	25% 3 10% 31% 25% 6%	1800
May	1st to 10th	Night	28% 2 6% 22% 30% 10%	1664
	11th to 20th	Night	28% 3 7% 27% 27% 8%	1551
	21st to end	Night	28% 2 12% 29% 22% 5%	1552
June	1st to 10th	Night	26% 4 10% 22% 30% 8%	1440
	11th to 20th	Night	17% 2 6% 21% 35% 18%	1440
	21st to end	Night	22% 3 2 12% 33% 27%	1440
July	1st to 10th	Night	19% 3 1 9% 31% 38%	1400
	11th to 20th	Night	21% 3 4 9% 27% 35%	1552
	21st to end	Night	32% 3 7% 22% 35%	1674
August	1st to 10th	Night	32% 3 1 12% 26% 26%	1602
	11th to 20th	Night	27% 3 2 9% 29% 30%	1669
	21st to end	Night	29% 2 2 11% 27% 29%	1980
September	1st to 10th	Night	17% 2 1 10% 33% 37%	1800
	11th to 20th	Night	30% 2 3 20% 31% 15%	1800
	21st to end	Night	21% 2 2 22% 28% 26%	2065
October	1st to 10th	Night	23% 3 6% 17% 29% 23%	2160
	11th to 20th	Night	30% 2 5% 18% 23% 22%	2160
	21st to end	Night	28% 3 5% 20% 26% 18%	2376
November	1st to 10th	Night	31% 3 9% 27% 18% 11%	2239
	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% 16% 30% 23% 10% 4	457
	11th to 20th	Day	16% 15% 31% 23% 10% 5%	412
	21st to end	Day	9% 18% 29% 25% 12% 7%	420
May	1st to 10th	Day	11% 14% 30% 27% 15% 3	423
	11th to 20th	Day	12% 13% 30% 23% 16% 5%	448
	21st to end	Day	10% 12% 24% 28% 18% 7%	485
June	1st to 10th	Day	11% 8% 26% 31% 13% 9%	450
	11th to 20th	Day	4 11% 29% 35% 15% 6%	450
	21st to end	Day	2 4 27% 33% 26% 8%	450
July	1st to 10th	Day	4 4 21% 41% 22% 7%	446
	11th to 20th	Day	1 1 16% 38% 30% 14%	444
	21st to end	Day	1 2 12% 47% 25% 13%	487
August	1st to 10th	Day	3 4 19% 38% 26% 10%	447
	11th to 20th	Day	4 3 22% 33% 26% 11%	437
	21st to end	Day	3 3 19% 37% 26% 12%	495
September	1st to 10th	Day	3 4 18% 37% 27% 11%	450
	11th to 20th	Day	6% 6% 24% 33% 20% 10%	450
	21st to end	Day	6% 7% 24% 30% 23% 10%	450
October	1st to 10th	Day	7% 7% 26% 26% 22% 12%	450
	11th to 20th	Day	8% 8% 19% 27% 25% 13%	450
	21st to end	Day	13% 7% 20% 28% 22% 9%	495
November	1st to 10th	Day	12% 8% 19% 25% 28% 8%	435
	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% 9% 23% 32% 18% 4	760
	11th to 20th	Night	12% 12% 22% 28% 21% 5%	711
	21st to end	Night	9% 10% 28% 26% 23% 5%	720
May	1st to 10th	Night	8% 7% 21% 34% 25% 6%	584
	11th to 20th	Night	8% 3 21% 28% 34% 5%	488
	21st to end	Night	9% 10% 16% 29% 30% 6%	389
June	1st to 10th	Night	8% 4 21% 31% 33% 3	360
	11th to 20th	Night	2 5% 19% 42% 30% 2	360
	21st to end	Night	6% 1 14% 38% 38% 4	360
July	1st to 10th	Night	3 1 12% 33% 45% 6%	350
	11th to 20th	Night	4 1 3 27% 48% 17%	508
	21st to end	Night	3 2 5% 27% 52% 11%	591
August	1st to 10th	Night	6% 2 8% 28% 43% 13%	537
	11th to 20th	Night	3 1 8% 31% 46% 11%	601
	21st to end	Night	5% 3 7% 32% 43% 11%	792
September	1st to 10th	Night	2 2 9% 33% 44% 11%	720
	11th to 20th	Night	8% 2 11% 34% 35% 10%	720
	21st to end	Night	5% 4 12% 34% 34% 11%	720
October	1st to 10th	Night	8% 4 14% 33% 31% 10%	720
	11th to 20th	Night	9% 4 11% 32% 35% 9%	720
	21st to end	Night	11% 3 10% 31% 31% 13%	792
November	1st to 10th	Night	12% 4 11% 30% 33% 10%	740
	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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