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 2026-08-30.

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%	152504
	Night	10% 5% 2 5% 3 3 3 3 4 6% 56%	147200
Transparency	Day	25% 4 7% 20% 24% 19%	147413
	Night	29% 5% 8% 18% 22% 17%	143282
Seeing	Day	10% 9% 24% 30% 21% 6%	17726
	Night	11% 5% 16% 34% 29% 5%	26964

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% 6% 2 5% 4 3 4 3 5% 7% 43%	5004
	11th to 20th	Day	12% 5% 2 4 3 3 3 2 4 5% 56%	5340
	21st to end	Day	15% 6% 3 8% 5% 4 4 3 5% 7% 40%	5843
February	1st to 10th	Day	19% 4 2 6% 4 4 4 3 4 5% 44%	1890
	11th to 20th	Day	15% 8% 2 7% 5% 4 4 4 6% 6% 40%	1904
	21st to end	Day	15% 7% 2 5% 5% 4 4 3 5% 6% 45%	1730
March	1st to 10th	Day	15% 10% 3 8% 5% 4 4 3 4 7% 36%	5011
	11th to 20th	Day	15% 5% 2 5% 3 4 4 4 5% 7% 48%	5110
	21st to end	Day	17% 6% 2 5% 4 4 6% 4 6% 9% 36%	5665
April	1st to 10th	Day	13% 7% 2 6% 4 4 5% 4 6% 7% 43%	2520
	11th to 20th	Day	9% 4 2 6% 4 4 4 4 5% 7% 51%	2489
	21st to end	Day	5% 5% 2 5% 4 3 3 3 6% 8% 57%	2520
May	1st to 10th	Day	4 4 2 4 2 2 2 2 3 5% 69%	7433
	11th to 20th	Day	2 2 1 2 2 2 2 2 3 7% 77%	7781
	21st to end	Day	2 2 1 3 2 2 2 2 4 7% 73%	8992
June	1st to 10th	Day	6% 4 1 3 4 2 3 3 5% 7% 61%	2940
	11th to 20th	Day	1 2 1 3 2 2 3 2 4 6% 74%	2940
	21st to end	Day	2 1 1 4 2 2 2 2 4 5% 75%	2940
July	1st to 10th	Day	1 1 3 2 2 2 2 3 4 80%	8219
	11th to 20th	Day	3 3 1 4 3 3 2 2 3 5% 70%	7731
	21st to end	Day	3 1 4 2 2 2 2 3 5% 77%	8304
August	1st to 10th	Day	1 1 1 3 3 3 3 3 4 6% 73%	5907
	11th to 20th	Day	1 2 1 2 2 2 2 1 2 4 81%	5713
	21st to end	Day	1 2 1 2 3 2 2 2 4 6% 74%	6093
September	1st to 10th	Day	1 2 1 2 2 2 2 2 3 6% 76%	2400
	11th to 20th	Day	3 4 1 5% 3 3 3 2 4 6% 66%	2400
	21st to end	Day	4 3 1 2 1 2 2 2 4 4 74%	2108
October	1st to 10th	Day	3 2 1 2 2 2 2 2 4 6% 72%	3280
	11th to 20th	Day	6% 4 1 4 3 2 3 2 3 5% 66%	3261
	21st to end	Day	7% 4 2 4 3 3 4 3 5% 8% 58%	3558
November	1st to 10th	Day	10% 3 1 4 3 3 4 3 5% 7% 56%	2020
	11th to 20th	Day	14% 5% 3 6% 5% 5% 4 3 5% 6% 45%	1878
	21st to end	Day	12% 4 1 4 4 4 4 4 5% 5% 53%	1680
December	1st to 10th	Day	10% 5% 1 4 2 3 3 3 7% 9% 54%	2700
	11th to 20th	Day	27% 7% 3 5% 4 3 4 4 6% 7% 31%	3108
	21st to end	Day	30% 8% 2 5% 3 3 4 4 5% 7% 29%	4092
January	1st to 10th	Night	20% 6% 2 4 3 3 4 3 5% 8% 44%	7887
	11th to 20th	Night	12% 5% 1 4 3 3 3 2 4 6% 57%	7650
	21st to end	Night	15% 7% 3 7% 4 4 4 3 5% 7% 43%	8371
February	1st to 10th	Night	18% 5% 2 4 3 4 4 2 5% 5% 48%	2730
	11th to 20th	Night	14% 7% 3 6% 4 3 4 3 5% 8% 44%	2600
	21st to end	Night	12% 6% 2 5% 3 3 4 4 3 7% 52%	2076
March	1st to 10th	Night	16% 8% 2 6% 5% 4 5% 4 5% 7% 39%	5100
	11th to 20th	Night	16% 5% 2 5% 3 3 4 3 4 6% 50%	5110
	21st to end	Night	17% 8% 2 6% 3 3 3 3 5% 8% 43%	5550
April	1st to 10th	Night	10% 5% 2 6% 4 3 3 3 5% 6% 51%	2100
	11th to 20th	Night	9% 5% 2 5% 3 4 4 3 4 6% 56%	2079
	21st to end	Night	6% 4 1 3 3 3 2 3 5% 8% 60%	2100
May	1st to 10th	Night	6% 5% 3 6% 2 2 2 2 3 5% 64%	5508
	11th to 20th	Night	2 2 1 3 2 3 3 2 3 5% 73%	5084
	21st to end	Night	2 2 1 3 2 2 2 2 3 5% 74%	5181
June	1st to 10th	Night	3 3 1 4 3 3 4 4 5% 8% 62%	1680
	11th to 20th	Night	1 1 1 3 2 3 2 2 5% 7% 73%	1680
	21st to end	Night	2 3 2 3 3 2 2 2 4 6% 71%	1680
July	1st to 10th	Night	1 1 3 2 2 2 2 2 5% 79%	4740
	11th to 20th	Night	4 3 3 7% 3 3 3 3 4 6% 62%	5237
	21st to end	Night	6% 6% 8% 4 3 3 2 5% 7% 57%	5806
August	1st to 10th	Night	2 2 1 6% 5% 4 3 3 5% 8% 62%	4125
	11th to 20th	Night	3 2 4 2 2 2 2 3 7% 72%	4366
	21st to end	Night	1 2 1 4 2 2 3 2 4 7% 72%	5105
September	1st to 10th	Night	1 2 1 2 3 2 2 2 4 7% 73%	2000
	11th to 20th	Night	3 4 2 5% 3 4 3 2 4 8% 61%	2000
	21st to end	Night	3 3 1 4 2 2 2 2 3 5% 71%	2287
October	1st to 10th	Night	3 3 1 3 3 2 2 2 4 5% 73%	3920
	11th to 20th	Night	4 4 2 6% 3 4 4 3 4 6% 61%	3897
	21st to end	Night	7% 4 1 4 3 3 3 3 4 6% 62%	4253
November	1st to 10th	Night	9% 5% 1 4 3 3 4 3 5% 8% 56%	2497
	11th to 20th	Night	12% 6% 2 6% 3 3 3 3 5% 7% 51%	2739
	21st to end	Night	10% 5% 2 5% 3 3 3 2 5% 6% 56%	2940
December	1st to 10th	Night	10% 4 1 4 3 3 3 3 5% 7% 56%	4700
	11th to 20th	Night	25% 8% 2 5% 4 4 4 3 7% 7% 30%	5316
	21st to end	Night	32% 7% 2 5% 3 3 3 3 4 5% 33%	7106

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	40% 8% 15% 21% 11% 5%	5005
	11th to 20th	Day	32% 5% 5% 15% 23% 20%	5340
	21st to end	Day	44% 5% 7% 19% 10% 15%	5843
February	1st to 10th	Day	40% 6% 8% 19% 20% 8%	1890
	11th to 20th	Day	43% 7% 9% 18% 17% 6%	1913
	21st to end	Day	38% 6% 11% 22% 17% 7%	1730
March	1st to 10th	Day	45% 7% 8% 19% 19% 2	5011
	11th to 20th	Day	34% 7% 13% 27% 11% 9%	5110
	21st to end	Day	39% 11% 16% 23% 10% 2	5665
April	1st to 10th	Day	38% 6% 12% 21% 18% 4	2520
	11th to 20th	Day	31% 7% 14% 23% 19% 6%	2492
	21st to end	Day	24% 6% 14% 30% 23% 4	2520
May	1st to 10th	Day	19% 3 8% 28% 28% 14%	7433
	11th to 20th	Day	9% 3 5% 32% 37% 14%	7784
	21st to end	Day	12% 4 8% 34% 38% 4	8994
June	1st to 10th	Day	22% 4 10% 28% 28% 7%	2940
	11th to 20th	Day	12% 4 9% 20% 38% 17%	2940
	21st to end	Day	14% 3 5% 15% 35% 29%	2940
July	1st to 10th	Day	11% 2 2 10% 26% 50%	8151
	11th to 20th	Day	19% 2 2 6% 16% 55%	7660
	21st to end	Day	14% 1 3 14% 26% 42%	8119
August	1st to 10th	Day	19% 3 4 10% 29% 36%	4336
	11th to 20th	Day	11% 2 3 11% 44% 30%	4172
	21st to end	Day	15% 3 4 15% 35% 28%	4420
September	1st to 10th	Day	12% 2 4 12% 30% 40%	2400
	11th to 20th	Day	19% 4 8% 22% 34% 12%	2400
	21st to end	Day	14% 3 5% 21% 32% 24%	2108
October	1st to 10th	Day	15% 3 4 12% 28% 38%	3280
	11th to 20th	Day	22% 4 6% 22% 25% 21%	3261
	21st to end	Day	26% 4 8% 24% 27% 12%	3558
November	1st to 10th	Day	27% 7% 10% 26% 19% 11%	2020
	11th to 20th	Day	38% 6% 10% 22% 17% 7%	1878
	21st to end	Day	32% 5% 11% 19% 21% 12%	1680
December	1st to 10th	Day	26% 5% 7% 31% 24% 7%	2700
	11th to 20th	Day	53% 4 11% 15% 14% 3	3108
	21st to end	Day	53% 7% 13% 17% 8% 2	4092
January	1st to 10th	Night	39% 8% 18% 18% 12% 4	7887
	11th to 20th	Night	30% 5% 7% 16% 24% 18%	7650
	21st to end	Night	41% 5% 8% 15% 17% 14%	8371
February	1st to 10th	Night	37% 6% 8% 19% 19% 11%	2730
	11th to 20th	Night	38% 7% 11% 19% 18% 7%	2602
	21st to end	Night	33% 5% 11% 25% 19% 6%	2076
March	1st to 10th	Night	41% 10% 10% 18% 19% 3	5100
	11th to 20th	Night	32% 7% 13% 21% 19% 8%	5110
	21st to end	Night	36% 11% 14% 24% 13% 3	5550
April	1st to 10th	Night	32% 7% 13% 21% 21% 5%	2100
	11th to 20th	Night	30% 6% 12% 26% 16% 10%	2079
	21st to end	Night	22% 6% 13% 31% 24% 6%	2100
May	1st to 10th	Night	24% 4 5% 20% 31% 15%	5506
	11th to 20th	Night	13% 4 6% 26% 32% 19%	5085
	21st to end	Night	14% 4 9% 28% 39% 6%	5182
June	1st to 10th	Night	21% 6% 11% 23% 30% 10%	1680
	11th to 20th	Night	12% 4 9% 19% 36% 20%	1680
	21st to end	Night	15% 4 4 15% 31% 30%	1680
July	1st to 10th	Night	10% 2 2 10% 26% 49%	4700
	11th to 20th	Night	22% 4 3 7% 17% 47%	5185
	21st to end	Night	26% 5% 4 8% 17% 40%	5679
August	1st to 10th	Night	28% 3 5% 11% 24% 29%	3019
	11th to 20th	Night	13% 4 5% 9% 36% 33%	3178
	21st to end	Night	15% 3 5% 13% 33% 30%	3698
September	1st to 10th	Night	12% 2 6% 11% 30% 39%	2000
	11th to 20th	Night	23% 5% 7% 20% 31% 15%	2000
	21st to end	Night	17% 3 6% 19% 29% 26%	2287
October	1st to 10th	Night	15% 3 5% 11% 22% 44%	3920
	11th to 20th	Night	25% 4 6% 21% 22% 21%	3897
	21st to end	Night	24% 4 6% 21% 26% 18%	4253
November	1st to 10th	Night	27% 5% 11% 26% 18% 12%	2497
	11th to 20th	Night	33% 6% 12% 24% 17% 8%	2739
	21st to end	Night	30% 5% 9% 22% 21% 13%	2940
December	1st to 10th	Night	28% 4 7% 27% 22% 11%	4700
	11th to 20th	Night	51% 5% 11% 16% 13% 4	5316
	21st to end	Night	54% 7% 11% 16% 9% 3	7106

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% 19% 13% 3	320
	11th to 20th	Day	18% 10% 23% 26% 19% 4	430
	21st to end	Day	14% 7% 27% 29% 20% 3	473
February	1st to 10th	Day	20% 11% 19% 28% 19% 2	430
	11th to 20th	Day	21% 18% 24% 24% 13% 1	456
	21st to end	Day	15% 16% 24% 29% 15% 1	446
March	1st to 10th	Day	21% 17% 30% 19% 10% 2	540
	11th to 20th	Day	21% 12% 26% 20% 17% 4	540
	21st to end	Day	16% 18% 28% 24% 11% 2	594
April	1st to 10th	Day	15% 15% 29% 25% 12% 5%	540
	11th to 20th	Day	12% 17% 27% 24% 14% 5%	532
	21st to end	Day	6% 17% 29% 27% 14% 8%	540
May	1st to 10th	Day	9% 16% 30% 26% 15% 5%	543
	11th to 20th	Day	8% 15% 29% 26% 20% 3	568
	21st to end	Day	6% 11% 28% 31% 18% 6%	617
June	1st to 10th	Day	8% 9% 28% 32% 18% 6%	570
	11th to 20th	Day	4 10% 30% 38% 15% 4	570
	21st to end	Day	2 6% 25% 37% 25% 6%	570
July	1st to 10th	Day	2 4 22% 42% 23% 6%	566
	11th to 20th	Day	2 2 15% 40% 31% 9%	564
	21st to end	Day	2 2 18% 44% 26% 9%	619
August	1st to 10th	Day	2 3 19% 41% 27% 9%	567
	11th to 20th	Day	4 4 22% 37% 25% 9%	557
	21st to end	Day	4 2 18% 38% 27% 10%	620
September	1st to 10th	Day	3 3 20% 38% 28% 8%	530
	11th to 20th	Day	5% 7% 23% 35% 21% 9%	530
	21st to end	Day	6% 5% 24% 32% 26% 8%	530
October	1st to 10th	Day	6% 7% 25% 27% 25% 11%	530
	11th to 20th	Day	8% 9% 19% 27% 27% 10%	530
	21st to end	Day	11% 7% 22% 28% 26% 6%	583
November	1st to 10th	Day	12% 6% 21% 23% 30% 8%	496
	11th to 20th	Day	13% 7% 17% 27% 28% 7%	323
	21st to end	Day	14% 6% 21% 27% 26% 6%	220
December	1st to 10th	Day	17% 10% 20% 23% 21% 10%	220
	11th to 20th	Day	23% 10% 17% 24% 19% 7%	220
	21st to end	Day	25% 14% 21% 24% 15% 1	242
January	1st to 10th	Night	21% 6% 15% 32% 22% 5%	840
	11th to 20th	Night	17% 4 17% 36% 22% 4	840
	21st to end	Night	17% 6% 15% 35% 25% 3	924
February	1st to 10th	Night	19% 5% 16% 36% 21% 3	840
	11th to 20th	Night	17% 8% 18% 33% 19% 4	824
	21st to end	Night	15% 8% 21% 35% 18% 2	692
March	1st to 10th	Night	18% 9% 20% 31% 18% 3	840
	11th to 20th	Night	18% 5% 18% 33% 23% 3	840
	21st to end	Night	15% 8% 25% 33% 17% 2	924
April	1st to 10th	Night	13% 6% 25% 33% 19% 3	840
	11th to 20th	Night	11% 10% 22% 31% 22% 4	831
	21st to end	Night	8% 7% 27% 32% 22% 4	840
May	1st to 10th	Night	6% 6% 23% 34% 26% 5%	700
	11th to 20th	Night	4 3 20% 35% 34% 4	553
	21st to end	Night	5% 6% 20% 34% 30% 5%	455
June	1st to 10th	Night	5% 4 20% 38% 31% 2	420
	11th to 20th	Night	2 5% 18% 47% 27% 2	420
	21st to end	Night	2 2 14% 42% 36% 3	420
July	1st to 10th	Night	2 9% 39% 46% 4	410
	11th to 20th	Night	3 1 4 36% 47% 9%	576
	21st to end	Night	3 1 6% 34% 49% 6%	690
August	1st to 10th	Night	5% 2 7% 29% 48% 10%	627
	11th to 20th	Night	3 1 9% 39% 40% 8%	718
	21st to end	Night	3 1 9% 38% 43% 7%	916
September	1st to 10th	Night	2 1 10% 37% 43% 7%	800
	11th to 20th	Night	5% 3 12% 36% 36% 8%	800
	21st to end	Night	4 2 13% 34% 37% 9%	800
October	1st to 10th	Night	6% 3 15% 34% 33% 9%	800
	11th to 20th	Night	8% 4 14% 33% 34% 8%	800
	21st to end	Night	10% 3 12% 31% 33% 11%	880
November	1st to 10th	Night	10% 5% 14% 29% 34% 8%	820
	11th to 20th	Night	13% 3 16% 34% 28% 6%	840
	21st to end	Night	12% 5% 15% 35% 27% 5%	840
December	1st to 10th	Night	18% 4 14% 34% 23% 6%	840
	11th to 20th	Night	25% 5% 13% 32% 21% 3	840
	21st to end	Night	24% 6% 20% 32% 17% 2	924

Page generated at 2026-08-27 01:07:11. Version ac07

Copyright © 2026 Attilla Danko.