Sonoma
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sonoma clear sky chart from 2005-11-06 to 2024-10-25.

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% 4 2 4 3 3 3 3 4 6% 60%	101092
	Night	10% 5% 2 5% 3 3 3 3 4 6% 55%	97931
Transparency	Day	26% 5% 8% 19% 23% 19%	100851
	Night	29% 6% 10% 18% 20% 17%	97762
Seeing	Day	10% 10% 24% 31% 20% 6%	15328
	Night	11% 5% 16% 34% 29% 5%	24594

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	18% 8% 3 7% 4 4 4 3 4 6% 40%	2681
	11th to 20th	Day	22% 9% 4 7% 5% 4 4 3 4 5% 34%	2860
	21st to end	Day	22% 8% 3 8% 5% 4 4 4 4 6% 32%	3146
February	1st to 10th	Day	18% 4 2 7% 4 4 4 3 4 5% 44%	1710
	11th to 20th	Day	15% 8% 2 6% 5% 4 4 4 6% 6% 42%	1718
	21st to end	Day	15% 6% 2 5% 4 4 3 3 5% 6% 47%	1570
March	1st to 10th	Day	20% 12% 4 9% 5% 4 4 3 4 5% 30%	3405
	11th to 20th	Day	12% 4 2 4 3 3 4 3 5% 6% 54%	3490
	21st to end	Day	19% 7% 2 6% 4 4 5% 4 6% 8% 36%	3869
April	1st to 10th	Day	13% 8% 2 5% 5% 4 5% 4 6% 7% 42%	2280
	11th to 20th	Day	9% 4 2 6% 4 4 4 4 5% 7% 51%	2249
	21st to end	Day	5% 5% 2 4 3 3 3 3 6% 9% 58%	2280
May	1st to 10th	Day	6% 3 1 3 2 2 2 2 4 5% 72%	4007
	11th to 20th	Day	3 3 1 3 2 2 3 2 3 6% 71%	4179
	21st to end	Day	2 3 1 4 3 2 2 2 4 6% 71%	4813
June	1st to 10th	Day	6% 5% 2 3 4 2 3 3 5% 8% 59%	2660
	11th to 20th	Day	1 2 1 2 3 2 2 2 4 7% 73%	2660
	21st to end	Day	2 1 1 4 2 2 2 2 4 6% 74%	2660
July	1st to 10th	Day	2 1 2 1 1 1 1 2 4 83%	4420
	11th to 20th	Day	1 2 1 2 2 2 2 3 4 6% 75%	4152
	21st to end	Day	2 1 3 2 2 2 2 3 5% 78%	4455
August	1st to 10th	Day	1 2 1 4 4 4 4 4 5% 8% 64%	4091
	11th to 20th	Day	1 2 1 2 2 2 2 2 3 5% 80%	3882
	21st to end	Day	1 3 1 3 3 2 2 2 3 6% 74%	4180
September	1st to 10th	Day	1 2 1 2 2 2 2 2 3 6% 77%	2280
	11th to 20th	Day	3 4 1 5% 3 3 3 2 4 6% 66%	2280
	21st to end	Day	4 2 1 2 1 2 2 2 4 4 75%	2004
October	1st to 10th	Day	5% 2 1 3 3 2 3 2 4 5% 69%	1900
	11th to 20th	Day	7% 4 1 3 3 2 3 2 4 5% 65%	1900
	21st to end	Day	10% 4 2 4 3 3 2 2 3 7% 60%	2025
November	1st to 10th	Day	11% 3 1 4 3 3 4 3 5% 7% 55%	1824
	11th to 20th	Day	12% 4 3 6% 4 5% 4 3 5% 6% 48%	1700
	21st to end	Day	12% 4 1 4 4 4 3 5% 5% 5% 53%	1520
December	1st to 10th	Day	17% 8% 2 6% 3 3 4 3 5% 5% 45%	1520
	11th to 20th	Day	31% 9% 3 6% 4 3 3 2 4 4 31%	1928
	21st to end	Day	24% 8% 2 5% 4 3 4 4 5% 7% 34%	2794
January	1st to 10th	Night	16% 8% 2 5% 4 3 4 3 5% 7% 44%	4251
	11th to 20th	Night	21% 9% 2 7% 4 4 4 3 5% 6% 34%	4110
	21st to end	Night	23% 8% 3 7% 4 4 4 3 4 6% 33%	4521
February	1st to 10th	Night	17% 5% 2 4 3 4 4 2 5% 5% 48%	2470
	11th to 20th	Night	13% 7% 3 6% 3 3 4 4 5% 8% 45%	2346
	21st to end	Night	13% 6% 2 4 3 3 4 3 3 7% 53%	1884
March	1st to 10th	Night	20% 10% 3 6% 4 4 4 3 4 6% 35%	3466
	11th to 20th	Night	13% 4 2 5% 3 3 3 2 3 5% 56%	3490
	21st to end	Night	16% 9% 2 7% 4 4 3 3 5% 7% 41%	3782
April	1st to 10th	Night	11% 5% 2 6% 4 3 3 3 5% 6% 51%	1900
	11th to 20th	Night	10% 5% 2 4 3 3 4 3 4 6% 57%	1879
	21st to end	Night	6% 4 1 3 3 3 2 3 5% 9% 61%	1900
May	1st to 10th	Night	4 2 1 3 2 2 3 2 3 4 73%	2963
	11th to 20th	Night	2 3 2 5% 4 3 3 2 3 6% 66%	2720
	21st to end	Night	2 2 1 3 2 2 2 1 4 5% 75%	2762
June	1st to 10th	Night	4 4 1 4 3 3 4 4 5% 8% 61%	1520
	11th to 20th	Night	2 1 1 3 2 2 2 2 5% 7% 73%	1520
	21st to end	Night	2 3 2 3 3 2 2 2 4 6% 71%	1520
July	1st to 10th	Night	2 1 2 2 1 2 1 2 5% 82%	2540
	11th to 20th	Night	1 1 1 3 2 3 3 3 4 6% 73%	2813
	21st to end	Night	1 3 4 9% 5% 4 3 3 5% 9% 55%	3109
August	1st to 10th	Night	3 3 1 8% 6% 5% 5% 4 6% 10% 50%	2848
	11th to 20th	Night	2 1 4 2 2 2 2 4 7% 73%	2962
	21st to end	Night	2 2 1 4 3 2 2 2 3 5% 73%	3498
September	1st to 10th	Night	2 1 2 3 2 2 2 4 7% 74%	1900
	11th to 20th	Night	3 4 2 5% 3 4 3 2 4 8% 62%	1900
	21st to end	Night	2 2 1 4 2 3 3 2 3 5% 72%	2173
October	1st to 10th	Night	5% 4 1 3 3 2 3 2 3 5% 69%	2280
	11th to 20th	Night	6% 3 2 4 3 3 4 3 5% 6% 61%	2280
	21st to end	Night	8% 4 1 4 3 2 3 3 4 6% 61%	2436
November	1st to 10th	Night	9% 5% 2 4 3 3 4 3 5% 7% 56%	2253
	11th to 20th	Night	11% 5% 2 5% 3 3 3 3 5% 7% 52%	2477
	21st to end	Night	10% 5% 2 5% 3 3 3 2 4 6% 58%	2660
December	1st to 10th	Night	18% 6% 2 6% 4 3 3 3 5% 6% 45%	2660
	11th to 20th	Night	31% 7% 2 5% 4 3 3 3 6% 5% 32%	3276
	21st to end	Night	23% 6% 2 6% 4 4 4 3 4 5% 38%	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	44% 8% 14% 25% 7% 3	2681
	11th to 20th	Day	54% 7% 9% 13% 10% 7%	2860
	21st to end	Day	52% 7% 9% 16% 11% 4	3146
February	1st to 10th	Day	40% 6% 8% 19% 19% 8%	1710
	11th to 20th	Day	41% 7% 8% 19% 19% 7%	1727
	21st to end	Day	37% 6% 11% 23% 16% 7%	1570
March	1st to 10th	Day	52% 9% 10% 14% 12% 2	3405
	11th to 20th	Day	29% 7% 11% 29% 13% 11%	3490
	21st to end	Day	40% 12% 15% 20% 11% 2	3869
April	1st to 10th	Day	39% 7% 13% 20% 17% 4	2280
	11th to 20th	Day	30% 7% 15% 23% 19% 6%	2252
	21st to end	Day	22% 6% 14% 29% 24% 4	2280
May	1st to 10th	Day	16% 4 12% 30% 23% 16%	4007
	11th to 20th	Day	14% 5% 6% 31% 31% 13%	4182
	21st to end	Day	14% 5% 11% 36% 30% 4	4814
June	1st to 10th	Day	24% 5% 11% 28% 25% 7%	2660
	11th to 20th	Day	13% 4 10% 20% 36% 16%	2660
	21st to end	Day	14% 3 5% 15% 35% 29%	2660
July	1st to 10th	Day	9% 2 2 7% 15% 65%	4351
	11th to 20th	Day	11% 3 4 8% 19% 55%	4081
	21st to end	Day	13% 2 3 7% 29% 47%	4315
August	1st to 10th	Day	20% 3 4 10% 27% 37%	4076
	11th to 20th	Day	10% 2 3 11% 45% 30%	3921
	21st to end	Day	14% 3 4 15% 35% 29%	4179
September	1st to 10th	Day	12% 1 4 11% 30% 42%	2280
	11th to 20th	Day	19% 5% 8% 22% 34% 13%	2280
	21st to end	Day	13% 3 5% 20% 33% 25%	2004
October	1st to 10th	Day	18% 4 6% 16% 30% 25%	1900
	11th to 20th	Day	22% 5% 7% 19% 25% 22%	1900
	21st to end	Day	27% 4 7% 21% 24% 17%	2025
November	1st to 10th	Day	27% 8% 10% 26% 18% 11%	1824
	11th to 20th	Day	36% 6% 10% 22% 18% 7%	1700
	21st to end	Day	32% 6% 11% 19% 21% 12%	1520
December	1st to 10th	Day	39% 7% 9% 19% 18% 8%	1520
	11th to 20th	Day	56% 5% 9% 11% 15% 4	1928
	21st to end	Day	47% 8% 14% 19% 9% 3	2794
January	1st to 10th	Night	39% 9% 19% 22% 7% 3	4251
	11th to 20th	Night	50% 9% 12% 12% 9% 8%	4110
	21st to end	Night	51% 8% 11% 12% 12% 6%	4521
February	1st to 10th	Night	37% 6% 8% 20% 19% 11%	2470
	11th to 20th	Night	36% 7% 11% 19% 19% 8%	2348
	21st to end	Night	32% 5% 11% 27% 20% 5%	1884
March	1st to 10th	Night	45% 13% 12% 14% 14% 3	3466
	11th to 20th	Night	29% 6% 10% 22% 22% 11%	3490
	21st to end	Night	38% 12% 14% 22% 10% 4	3782
April	1st to 10th	Night	32% 7% 14% 20% 21% 6%	1900
	11th to 20th	Night	28% 6% 12% 27% 17% 10%	1879
	21st to end	Night	21% 6% 13% 30% 25% 6%	1900
May	1st to 10th	Night	16% 5% 8% 26% 28% 17%	2963
	11th to 20th	Night	17% 7% 10% 21% 25% 20%	2721
	21st to end	Night	14% 4 9% 36% 30% 6%	2762
June	1st to 10th	Night	22% 7% 11% 22% 30% 8%	1520
	11th to 20th	Night	12% 4 10% 20% 35% 19%	1520
	21st to end	Night	15% 5% 4 15% 31% 30%	1520
July	1st to 10th	Night	8% 2 4 6% 15% 64%	2500
	11th to 20th	Night	13% 3 5% 9% 17% 53%	2761
	21st to end	Night	25% 6% 4 6% 17% 42%	3011
August	1st to 10th	Night	30% 4 5% 11% 23% 29%	2839
	11th to 20th	Night	12% 5% 5% 8% 37% 34%	2989
	21st to end	Night	15% 3 5% 13% 33% 31%	3498
September	1st to 10th	Night	12% 2 5% 10% 30% 40%	1900
	11th to 20th	Night	23% 5% 7% 19% 31% 15%	1900
	21st to end	Night	16% 3 6% 19% 28% 27%	2173
October	1st to 10th	Night	18% 4 8% 17% 27% 27%	2280
	11th to 20th	Night	23% 5% 7% 19% 22% 24%	2280
	21st to end	Night	24% 5% 7% 19% 23% 20%	2436
November	1st to 10th	Night	27% 5% 11% 26% 17% 13%	2253
	11th to 20th	Night	31% 6% 12% 25% 18% 9%	2477
	21st to end	Night	29% 5% 9% 22% 22% 13%	2660
December	1st to 10th	Night	39% 6% 10% 17% 17% 11%	2660
	11th to 20th	Night	52% 6% 10% 13% 15% 4	3276
	21st to end	Night	46% 9% 14% 16% 11% 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	31% 11% 26% 17% 13% 2	270
	11th to 20th	Day	21% 11% 24% 27% 14% 3	370
	21st to end	Day	16% 8% 29% 29% 17% 2	407
February	1st to 10th	Day	19% 12% 20% 27% 19% 2	370
	11th to 20th	Day	21% 17% 24% 24% 14% 1	390
	21st to end	Day	15% 17% 25% 27% 15% 1	382
March	1st to 10th	Day	23% 18% 29% 19% 10% 1	460
	11th to 20th	Day	22% 13% 27% 20% 14% 4	460
	21st to end	Day	16% 20% 30% 24% 9% 1	506
April	1st to 10th	Day	16% 15% 30% 24% 10% 4	460
	11th to 20th	Day	13% 19% 27% 23% 13% 4	452
	21st to end	Day	6% 18% 31% 26% 12% 7%	460
May	1st to 10th	Day	10% 16% 29% 28% 14% 3	463
	11th to 20th	Day	8% 14% 28% 27% 19% 3	488
	21st to end	Day	7% 11% 27% 31% 18% 5%	529
June	1st to 10th	Day	9% 10% 26% 31% 18% 6%	490
	11th to 20th	Day	4 10% 30% 37% 15% 4	490
	21st to end	Day	2 4 26% 37% 26% 6%	490
July	1st to 10th	Day	3 4 21% 42% 24% 6%	486
	11th to 20th	Day	2 2 16% 41% 31% 9%	484
	21st to end	Day	2 1 16% 46% 27% 8%	531
August	1st to 10th	Day	2 3 20% 42% 26% 7%	487
	11th to 20th	Day	3 4 23% 36% 24% 9%	477
	21st to end	Day	4 2 18% 40% 27% 9%	539
September	1st to 10th	Day	3 3 19% 38% 28% 9%	490
	11th to 20th	Day	5% 7% 23% 36% 21% 8%	490
	21st to end	Day	4 5% 24% 32% 26% 8%	490
October	1st to 10th	Day	5% 6% 25% 28% 25% 11%	490
	11th to 20th	Day	7% 9% 20% 27% 28% 9%	490
	21st to end	Day	12% 6% 20% 29% 26% 6%	514
November	1st to 10th	Day	13% 7% 19% 23% 31% 7%	420
	11th to 20th	Day	10% 7% 15% 27% 32% 9%	265
	21st to end	Day	13% 7% 22% 30% 23% 5%	180
December	1st to 10th	Day	20% 12% 21% 22% 17% 7%	180
	11th to 20th	Day	22% 13% 19% 24% 16% 6%	180
	21st to end	Day	22% 13% 23% 25% 16% 2	198
January	1st to 10th	Night	21% 6% 15% 31% 23% 6%	760
	11th to 20th	Night	19% 4 17% 35% 21% 4	760
	21st to end	Night	18% 6% 15% 33% 25% 3	836
February	1st to 10th	Night	18% 6% 15% 35% 22% 3	760
	11th to 20th	Night	15% 8% 18% 33% 21% 5%	744
	21st to end	Night	15% 8% 21% 33% 20% 2	628
March	1st to 10th	Night	19% 9% 19% 30% 19% 3	760
	11th to 20th	Night	19% 6% 18% 31% 24% 3	760
	21st to end	Night	15% 8% 26% 32% 16% 2	836
April	1st to 10th	Night	14% 6% 26% 33% 19% 3	760
	11th to 20th	Night	12% 11% 22% 30% 22% 4	751
	21st to end	Night	8% 7% 29% 31% 22% 4	760
May	1st to 10th	Night	6% 6% 23% 35% 25% 4	634
	11th to 20th	Night	4 3 20% 34% 35% 4	500
	21st to end	Night	5% 6% 21% 35% 28% 5%	411
June	1st to 10th	Night	6% 4 21% 37% 30% 2	380
	11th to 20th	Night	2 5% 17% 48% 27% 2	380
	21st to end	Night	3 2 14% 43% 37% 2	380
July	1st to 10th	Night	2 9% 38% 46% 4	370
	11th to 20th	Night	3 1 4 35% 48% 10%	520
	21st to end	Night	3 1 6% 34% 50% 6%	624
August	1st to 10th	Night	6% 2 8% 30% 47% 7%	567
	11th to 20th	Night	3 1 9% 39% 40% 8%	649
	21st to end	Night	4 1 9% 37% 43% 7%	836
September	1st to 10th	Night	1 1 9% 38% 43% 8%	760
	11th to 20th	Night	6% 3 12% 36% 36% 8%	760
	21st to end	Night	3 3 13% 34% 38% 9%	760
October	1st to 10th	Night	6% 3 16% 33% 33% 9%	760
	11th to 20th	Night	8% 4 13% 33% 34% 8%	760
	21st to end	Night	10% 3 12% 31% 32% 12%	812
November	1st to 10th	Night	10% 5% 15% 29% 33% 8%	740
	11th to 20th	Night	12% 3 14% 34% 30% 6%	760
	21st to end	Night	11% 5% 16% 36% 26% 6%	760
December	1st to 10th	Night	20% 5% 14% 33% 22% 6%	760
	11th to 20th	Night	25% 5% 15% 32% 20% 4	760
	21st to end	Night	21% 6% 21% 32% 18% 2	836

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