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 2025-04-26.

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% 59%	117753
	Night	11% 5% 2 5% 3 3 3 3 4 6% 55%	119756
Transparency	Day	26% 5% 8% 19% 22% 18%	117512
	Night	29% 6% 9% 18% 20% 17%	119587
Seeing	Day	10% 10% 24% 30% 20% 6%	15910
	Night	11% 5% 16% 34% 29% 5%	25324

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	14% 6% 2 5% 4 3 4 3 5% 7% 46%	3851
	11th to 20th	Day	15% 6% 3 5% 4 3 3 2 3 4 51%	4100
	21st to end	Day	18% 6% 2 7% 4 4 4 3 4 6% 42%	4510
February	1st to 10th	Day	20% 4 2 7% 4 4 4 3 4 5% 44%	1800
	11th to 20th	Day	14% 8% 2 6% 5% 4 4 4 6% 7% 41%	1811
	21st to end	Day	15% 6% 2 5% 4 4 3 3 5% 6% 47%	1650
March	1st to 10th	Day	16% 10% 3 8% 5% 4 4 3 5% 7% 36%	4902
	11th to 20th	Day	15% 5% 2 5% 3 4 4 4 5% 7% 47%	5000
	21st to end	Day	17% 6% 2 5% 4 4 6% 4 6% 9% 36%	5543
April	1st to 10th	Day	13% 7% 2 6% 5% 4 5% 4 6% 7% 42%	2400
	11th to 20th	Day	9% 4 2 6% 4 4 4 4 5% 7% 51%	2369
	21st to end	Day	5% 5% 2 4 3 3 3 3 6% 9% 58%	2342
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	3 2 1 2 2 2 2 3 4 6% 73%	3180
	11th to 20th	Day	6% 4 1 4 3 2 3 2 4 5% 66%	3161
	21st to end	Day	7% 4 2 4 3 3 4 3 5% 8% 58%	3448
November	1st to 10th	Day	10% 3 1 4 3 3 4 3 5% 7% 56%	1922
	11th to 20th	Day	13% 4 3 6% 4 5% 4 3 5% 7% 47%	1789
	21st to end	Day	13% 4 1 4 4 4 3 4 5% 5% 52%	1600
December	1st to 10th	Day	10% 5% 1 4 2 2 3 3 7% 9% 54%	2620
	11th to 20th	Day	27% 7% 3 5% 4 3 4 4 6% 7% 30%	3028
	21st to end	Day	30% 8% 2 5% 3 3 4 4 5% 7% 29%	4004
January	1st to 10th	Night	11% 6% 2 4 4 4 4 3 6% 9% 48%	6086
	11th to 20th	Night	15% 6% 2 5% 3 4 3 2 4 5% 51%	5880
	21st to end	Night	17% 6% 2 6% 4 4 3 3 4 5% 45%	6468
February	1st to 10th	Night	19% 5% 2 4 3 4 4 2 4 5% 48%	2600
	11th to 20th	Night	13% 6% 3 6% 4 3 4 3 5% 8% 45%	2473
	21st to end	Night	12% 6% 2 5% 3 3 4 3 3 6% 53%	1980
March	1st to 10th	Night	16% 8% 2 6% 5% 4 5% 4 5% 7% 39%	4989
	11th to 20th	Night	16% 6% 2 5% 3 3 4 3 4 6% 49%	5000
	21st to end	Night	17% 8% 2 6% 3 3 3 3 5% 7% 43%	5430
April	1st to 10th	Night	10% 5% 2 6% 4 3 3 3 5% 7% 51%	2000
	11th to 20th	Night	9% 5% 2 4 3 4 4 3 4 6% 57%	1979
	21st to end	Night	6% 4 1 3 3 3 2 3 5% 9% 61%	1954
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	3 3 1 3 3 2 2 2 4 5% 73%	3800
	11th to 20th	Night	4 4 2 6% 3 4 4 3 4 6% 61%	3777
	21st to end	Night	7% 4 1 4 3 3 3 3 4 6% 62%	4121
November	1st to 10th	Night	9% 5% 1 4 3 3 4 3 5% 7% 56%	2375
	11th to 20th	Night	11% 5% 2 5% 3 3 3 3 5% 7% 52%	2608
	21st to end	Night	11% 5% 2 5% 3 3 3 2 4 6% 57%	2800
December	1st to 10th	Night	11% 4 1 4 3 3 3 3 5% 6% 56%	4560
	11th to 20th	Night	25% 8% 2 5% 4 4 4 3 7% 7% 30%	5176
	21st to end	Night	31% 7% 2 5% 3 3 3 3 4 5% 33%	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	37% 9% 16% 19% 12% 7%	3851
	11th to 20th	Day	39% 5% 7% 15% 18% 16%	4100
	21st to end	Day	44% 6% 7% 14% 12% 18%	4510
February	1st to 10th	Day	41% 6% 8% 19% 19% 8%	1800
	11th to 20th	Day	42% 7% 9% 19% 18% 6%	1820
	21st to end	Day	37% 6% 10% 23% 17% 7%	1650
March	1st to 10th	Day	46% 7% 8% 18% 19% 2	4902
	11th to 20th	Day	34% 7% 14% 27% 10% 8%	5000
	21st to end	Day	39% 11% 16% 23% 9% 2	5543
April	1st to 10th	Day	39% 7% 12% 21% 17% 4	2400
	11th to 20th	Day	31% 7% 14% 23% 19% 6%	2372
	21st to end	Day	22% 6% 14% 29% 24% 4	2342
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	14% 3 4 12% 28% 39%	3180
	11th to 20th	Day	22% 4 6% 22% 25% 22%	3161
	21st to end	Day	25% 4 8% 24% 27% 13%	3448
November	1st to 10th	Day	26% 7% 10% 26% 19% 11%	1922
	11th to 20th	Day	37% 6% 10% 22% 18% 7%	1789
	21st to end	Day	33% 6% 10% 19% 21% 12%	1600
December	1st to 10th	Day	26% 5% 7% 31% 24% 7%	2620
	11th to 20th	Day	53% 4 11% 15% 14% 3	3028
	21st to end	Day	53% 7% 13% 17% 8% 2	4004
January	1st to 10th	Night	33% 9% 22% 18% 13% 5%	6086
	11th to 20th	Night	37% 6% 9% 17% 18% 13%	5880
	21st to end	Night	40% 6% 8% 11% 19% 16%	6468
February	1st to 10th	Night	38% 6% 8% 19% 18% 11%	2600
	11th to 20th	Night	37% 7% 11% 19% 18% 8%	2475
	21st to end	Night	31% 5% 11% 26% 20% 7%	1980
March	1st to 10th	Night	41% 10% 10% 18% 18% 3	4989
	11th to 20th	Night	33% 7% 13% 21% 18% 8%	5000
	21st to end	Night	37% 11% 14% 25% 12% 2	5430
April	1st to 10th	Night	32% 7% 13% 21% 21% 5%	2000
	11th to 20th	Night	28% 6% 12% 26% 17% 11%	1979
	21st to end	Night	21% 5% 13% 30% 25% 6%	1954
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	15% 3 5% 11% 22% 46%	3800
	11th to 20th	Night	25% 5% 6% 21% 21% 22%	3777
	21st to end	Night	24% 4 6% 21% 26% 19%	4121
November	1st to 10th	Night	27% 5% 11% 26% 18% 13%	2375
	11th to 20th	Night	31% 6% 13% 25% 18% 8%	2608
	21st to end	Night	30% 5% 9% 21% 22% 13%	2800
December	1st to 10th	Night	28% 4 7% 27% 23% 12%	4560
	11th to 20th	Night	51% 5% 11% 16% 13% 3	5176
	21st to end	Night	54% 7% 11% 16% 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	28% 11% 25% 20% 13% 2	295
	11th to 20th	Day	20% 11% 24% 27% 16% 3	400
	21st to end	Day	15% 7% 28% 29% 18% 3	440
February	1st to 10th	Day	22% 12% 19% 27% 19% 2	400
	11th to 20th	Day	20% 17% 24% 24% 13% 1	423
	21st to end	Day	14% 16% 24% 29% 16% 1	414
March	1st to 10th	Day	22% 17% 30% 19% 10% 2	500
	11th to 20th	Day	23% 13% 27% 20% 13% 4	500
	21st to end	Day	16% 19% 29% 24% 10% 2	550
April	1st to 10th	Day	15% 16% 30% 25% 10% 4	500
	11th to 20th	Day	12% 18% 28% 24% 14% 5%	492
	21st to end	Day	6% 17% 31% 26% 12% 8%	481
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	11% 6% 22% 29% 26% 6%	539
November	1st to 10th	Day	12% 7% 21% 23% 30% 7%	458
	11th to 20th	Day	11% 7% 17% 27% 30% 8%	294
	21st to end	Day	15% 7% 22% 28% 24% 5%	200
December	1st to 10th	Day	19% 11% 21% 23% 19% 9%	200
	11th to 20th	Day	23% 12% 19% 24% 19% 5%	200
	21st to end	Day	23% 14% 22% 25% 15% 1	220
January	1st to 10th	Night	20% 6% 15% 32% 22% 5%	800
	11th to 20th	Night	18% 4 18% 35% 21% 4	800
	21st to end	Night	17% 6% 15% 33% 25% 4	880
February	1st to 10th	Night	20% 6% 15% 35% 21% 3	800
	11th to 20th	Night	16% 8% 18% 34% 20% 4	784
	21st to end	Night	15% 7% 21% 35% 19% 2	660
March	1st to 10th	Night	19% 9% 20% 31% 19% 3	800
	11th to 20th	Night	19% 6% 18% 31% 23% 3	800
	21st to end	Night	16% 8% 26% 32% 16% 2	880
April	1st to 10th	Night	14% 6% 26% 33% 18% 3	800
	11th to 20th	Night	11% 10% 22% 30% 22% 4	791
	21st to end	Night	8% 7% 28% 31% 22% 4	782
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% 11%	836
November	1st to 10th	Night	10% 5% 15% 29% 33% 8%	780
	11th to 20th	Night	12% 4 16% 34% 29% 6%	800
	21st to end	Night	12% 6% 16% 36% 25% 6%	800
December	1st to 10th	Night	19% 5% 14% 34% 22% 7%	800
	11th to 20th	Night	25% 5% 14% 33% 20% 3	800
	21st to end	Night	23% 7% 20% 32% 17% 2	880

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