Lake Sonoma
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Lake Sonoma clear sky chart from 2005-11-06 to 2026-04-02.

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% 3 1 3 2 2 3 3 4 6% 63%	140269
	Night	11% 5% 2 4 3 3 3 3 4 6% 58%	139028
Transparency	Day	23% 4 8% 21% 25% 20%	135144
	Night	28% 5% 8% 20% 22% 17%	135146
Seeing	Day	13% 10% 24% 28% 19% 6%	17127
	Night	14% 6% 17% 32% 26% 6%	26476

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% 6% 2 5% 3 3 4 4 6% 7% 41%	5082
	11th to 20th	Day	14% 5% 2 4 3 2 3 2 4 5% 56%	5340
	21st to end	Day	15% 6% 2 6% 3 3 4 3 5% 7% 45%	5843
February	1st to 10th	Day	19% 6% 2 5% 4 3 4 4 3 5% 44%	1890
	11th to 20th	Day	15% 7% 3 5% 5% 4 4 4 5% 8% 41%	1856
	21st to end	Day	17% 6% 2 6% 3 4 4 3 4 7% 43%	1741
March	1st to 10th	Day	19% 11% 3 5% 4 4 4 3 4 6% 37%	5062
	11th to 20th	Day	15% 5% 2 5% 4 5% 4 4 5% 6% 47%	5110
	21st to end	Day	19% 7% 2 6% 4 4 5% 4 6% 9% 34%	5607
April	1st to 10th	Day	13% 8% 2 6% 5% 4 4 3 6% 7% 43%	2407
	11th to 20th	Day	10% 5% 2 5% 4 4 3 3 5% 8% 51%	2369
	21st to end	Day	6% 4 2 4 3 3 3 3 6% 9% 56%	2400
May	1st to 10th	Day	4 2 1 2 2 2 2 2 4 6% 74%	5877
	11th to 20th	Day	3 1 1 2 1 1 2 2 3 5% 78%	5917
	21st to end	Day	2 1 1 2 1 1 1 1 3 5% 81%	6902
June	1st to 10th	Day	5% 4 2 3 2 3 3 3 5% 7% 63%	2800
	11th to 20th	Day	2 1 2 2 2 2 2 4 6% 76%	2800
	21st to end	Day	2 1 1 2 2 2 2 2 3 4 82%	2800
July	1st to 10th	Day	1 1 1 1 1 1 1 2 3 88%	6302
	11th to 20th	Day	1 2 2 2 1 1 3 4 84%	5880
	21st to end	Day	1 1 3 1 1 1 2 3 4 83%	6402
August	1st to 10th	Day	1 1 1 1 2 2 2 4 5% 81%	5777
	11th to 20th	Day	1 1 1 1 1 1 2 4 87%	5693
	21st to end	Day	1 1 3 2 2 2 2 4 6% 78%	5984
September	1st to 10th	Day	1 1 1 2 1 2 1 2 3 5% 82%	2400
	11th to 20th	Day	3 2 1 3 2 2 3 2 4 7% 71%	2360
	21st to end	Day	4 2 1 2 1 2 2 2 3 6% 76%	2124
October	1st to 10th	Day	4 2 1 2 2 2 2 1 3 5% 75%	3280
	11th to 20th	Day	8% 2 1 3 2 2 2 3 3 6% 67%	3261
	21st to end	Day	7% 3 1 3 3 4 4 3 6% 8% 57%	3558
November	1st to 10th	Day	10% 4 1 3 4 4 4 4 5% 7% 54%	2054
	11th to 20th	Day	14% 5% 3 6% 4 3 4 4 6% 6% 46%	1811
	21st to end	Day	12% 5% 2 4 3 4 3 3 5% 7% 53%	1680
December	1st to 10th	Day	11% 4 1 4 2 4 3 3 7% 10% 52%	2700
	11th to 20th	Day	28% 8% 2 5% 4 4 5% 5% 6% 7% 27%	3108
	21st to end	Day	26% 8% 2 6% 4 4 4 4 6% 8% 28%	4092
January	1st to 10th	Night	21% 7% 2 5% 3 3 4 4 6% 8% 38%	7814
	11th to 20th	Night	12% 5% 2 5% 3 3 3 3 3 5% 55%	7650
	21st to end	Night	16% 7% 3 6% 3 4 4 3 5% 8% 41%	8371
February	1st to 10th	Night	17% 6% 2 5% 3 3 3 2 5% 6% 48%	2730
	11th to 20th	Night	15% 7% 2 5% 4 4 4 3 6% 8% 42%	2649
	21st to end	Night	15% 6% 2 4 3 3 3 2 5% 6% 51%	2060
March	1st to 10th	Night	17% 10% 3 6% 4 4 4 4 5% 7% 36%	5052
	11th to 20th	Night	17% 6% 2 5% 3 2 3 3 4 5% 51%	5110
	21st to end	Night	16% 11% 3 6% 4 3 4 3 5% 7% 38%	5610
April	1st to 10th	Night	10% 7% 3 6% 4 3 3 3 5% 7% 49%	2015
	11th to 20th	Night	8% 4 2 5% 4 3 3 4 5% 6% 55%	1979
	21st to end	Night	6% 4 1 3 3 2 4 3 5% 7% 63%	1999
May	1st to 10th	Night	3 3 1 4 2 2 2 1 3 6% 73%	4100
	11th to 20th	Night	3 2 1 4 4 2 2 2 4 5% 71%	3990
	21st to end	Night	2 1 1 2 1 2 2 1 3 5% 82%	3972
June	1st to 10th	Night	3 4 2 4 3 3 4 3 5% 9% 62%	1600
	11th to 20th	Night	1 2 1 1 1 2 3 2 4 6% 78%	1600
	21st to end	Night	2 3 1 2 2 1 2 2 2 4 79%	1600
July	1st to 10th	Night	1 2 1 1 1 1 2 3 88%	3640
	11th to 20th	Night	2 1 4 2 2 2 2 3 5% 78%	4099
	21st to end	Night	2 1 3 1 1 2 2 5% 6% 76%	4472
August	1st to 10th	Night	1 1 1 1 1 1 2 2 3 6% 81%	4035
	11th to 20th	Night	2 1 1 2 1 2 5% 86%	4158
	21st to end	Night	1 1 1 2 2 2 2 2 3 6% 78%	5015
September	1st to 10th	Night	1 1 2 1 1 1 1 4 6% 83%	2000
	11th to 20th	Night	4 3 2 3 2 2 2 3 4 8% 67%	2035
	21st to end	Night	3 2 1 3 2 2 2 2 3 5% 75%	2275
October	1st to 10th	Night	3 2 1 3 2 2 3 2 3 4 75%	3920
	11th to 20th	Night	4 3 2 5% 2 2 3 2 4 5% 67%	3897
	21st to end	Night	7% 3 1 3 2 3 4 3 5% 7% 61%	4253
November	1st to 10th	Night	11% 4 2 5% 3 3 3 3 6% 9% 52%	2462
	11th to 20th	Night	13% 6% 2 5% 3 3 4 4 6% 7% 46%	2804
	21st to end	Night	11% 4 2 5% 4 3 3 3 4 7% 54%	2940
December	1st to 10th	Night	12% 4 1 4 3 3 3 3 5% 7% 55%	4700
	11th to 20th	Night	27% 7% 2 5% 4 4 5% 4 6% 8% 28%	5316
	21st to end	Night	34% 8% 2 5% 3 3 3 2 4 5% 31%	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% 14% 22% 11% 6%	5083
	11th to 20th	Day	32% 4 5% 12% 24% 23%	5340
	21st to end	Day	38% 5% 8% 19% 12% 17%	5843
February	1st to 10th	Day	42% 4 8% 18% 17% 10%	1890
	11th to 20th	Day	41% 6% 11% 18% 18% 6%	1865
	21st to end	Day	41% 6% 11% 22% 15% 6%	1741
March	1st to 10th	Day	47% 6% 7% 19% 20% 2	5062
	11th to 20th	Day	36% 7% 13% 23% 12% 9%	5110
	21st to end	Day	45% 9% 14% 21% 10% 2	5607
April	1st to 10th	Day	40% 6% 13% 23% 15% 3	2407
	11th to 20th	Day	31% 6% 14% 24% 19% 6%	2372
	21st to end	Day	23% 6% 15% 30% 22% 4	2400
May	1st to 10th	Day	12% 4 10% 27% 30% 17%	5877
	11th to 20th	Day	10% 4 7% 31% 37% 11%	5920
	21st to end	Day	9% 2 8% 35% 42% 4	6904
June	1st to 10th	Day	21% 5% 11% 29% 29% 6%	2800
	11th to 20th	Day	10% 4 9% 23% 37% 18%	2800
	21st to end	Day	9% 3 4 16% 37% 31%	2800
July	1st to 10th	Day	4 2 1 9% 26% 57%	6234
	11th to 20th	Day	5% 4 2 7% 20% 62%	5811
	21st to end	Day	7% 2 5% 19% 31% 37%	6217
August	1st to 10th	Day	9% 3 4 13% 29% 42%	4206
	11th to 20th	Day	5% 2 2 16% 42% 34%	4116
	21st to end	Day	9% 3 4 17% 37% 30%	4311
September	1st to 10th	Day	7% 3 3 14% 32% 41%	2400
	11th to 20th	Day	16% 3 7% 27% 35% 12%	2360
	21st to end	Day	12% 3 5% 23% 34% 24%	2124
October	1st to 10th	Day	15% 2 4 14% 24% 42%	3280
	11th to 20th	Day	19% 4 6% 25% 29% 16%	3261
	21st to end	Day	24% 4 10% 22% 28% 11%	3558
November	1st to 10th	Day	28% 6% 10% 27% 18% 11%	2054
	11th to 20th	Day	37% 7% 11% 22% 16% 8%	1811
	21st to end	Day	31% 5% 11% 20% 20% 14%	1680
December	1st to 10th	Day	27% 4 7% 26% 26% 9%	2700
	11th to 20th	Day	54% 5% 8% 16% 12% 4	3108
	21st to end	Day	52% 7% 11% 19% 8% 3	4092
January	1st to 10th	Night	42% 7% 18% 17% 13% 3	7814
	11th to 20th	Night	31% 5% 5% 17% 22% 20%	7650
	21st to end	Night	40% 6% 8% 13% 19% 14%	8371
February	1st to 10th	Night	38% 4 8% 18% 20% 12%	2730
	11th to 20th	Night	40% 6% 10% 21% 17% 6%	2651
	21st to end	Night	35% 4 14% 23% 20% 5%	2060
March	1st to 10th	Night	44% 8% 9% 20% 17% 2	5052
	11th to 20th	Night	35% 5% 11% 21% 20% 8%	5110
	21st to end	Night	44% 8% 14% 21% 11% 2	5610
April	1st to 10th	Night	35% 7% 13% 24% 17% 5%	2015
	11th to 20th	Night	29% 6% 13% 25% 19% 8%	1979
	21st to end	Night	23% 5% 10% 34% 24% 5%	1999
May	1st to 10th	Night	15% 3 4 25% 34% 18%	4097
	11th to 20th	Night	14% 6% 5% 32% 30% 14%	3991
	21st to end	Night	9% 2 7% 35% 42% 5%	3972
June	1st to 10th	Night	22% 4 9% 25% 31% 9%	1600
	11th to 20th	Night	10% 3 8% 22% 36% 22%	1600
	21st to end	Night	12% 2 4 15% 32% 34%	1600
July	1st to 10th	Night	4 3 1 9% 27% 56%	3600
	11th to 20th	Night	6% 6% 3 10% 19% 58%	4046
	21st to end	Night	10% 3 5% 17% 27% 38%	4345
August	1st to 10th	Night	11% 2 2 14% 26% 45%	2929
	11th to 20th	Night	6% 1 2 14% 40% 38%	3009
	21st to end	Night	10% 3 3 15% 35% 34%	3608
September	1st to 10th	Night	6% 2 3 14% 31% 44%	2000
	11th to 20th	Night	17% 3 7% 20% 37% 16%	2035
	21st to end	Night	13% 4 4 22% 28% 29%	2275
October	1st to 10th	Night	14% 3 5% 13% 23% 44%	3920
	11th to 20th	Night	18% 6% 7% 25% 27% 17%	3897
	21st to end	Night	23% 4 7% 25% 26% 15%	4253
November	1st to 10th	Night	29% 6% 10% 24% 19% 11%	2462
	11th to 20th	Night	34% 8% 13% 21% 17% 7%	2804
	21st to end	Night	31% 5% 9% 19% 22% 13%	2940
December	1st to 10th	Night	28% 5% 6% 29% 21% 11%	4700
	11th to 20th	Night	51% 7% 9% 15% 14% 4	5316
	21st to end	Night	56% 5% 10% 17% 10% 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	33% 14% 21% 15% 15% 2	346
	11th to 20th	Day	23% 10% 22% 20% 20% 5%	430
	21st to end	Day	19% 10% 23% 27% 16% 4	473
February	1st to 10th	Day	23% 12% 24% 20% 18% 4	430
	11th to 20th	Day	29% 16% 21% 22% 11% 1	431
	21st to end	Day	24% 15% 22% 26% 9% 2	446
March	1st to 10th	Day	31% 16% 24% 19% 9% 1	540
	11th to 20th	Day	24% 16% 28% 18% 10% 4	540
	21st to end	Day	22% 20% 28% 20% 8% 2	594
April	1st to 10th	Day	23% 15% 30% 21% 7% 4	505
	11th to 20th	Day	18% 18% 29% 21% 11% 3	492
	21st to end	Day	13% 19% 34% 17% 12% 5%	500
May	1st to 10th	Day	13% 18% 29% 25% 11% 3	503
	11th to 20th	Day	13% 16% 31% 21% 13% 6%	528
	21st to end	Day	9% 12% 28% 25% 19% 6%	573
June	1st to 10th	Day	9% 11% 30% 29% 16% 4	530
	11th to 20th	Day	8% 12% 33% 30% 14% 2	530
	21st to end	Day	2 6% 27% 35% 22% 8%	530
July	1st to 10th	Day	4 3 21% 42% 22% 7%	526
	11th to 20th	Day	1 3 17% 43% 29% 8%	524
	21st to end	Day	1 1 15% 41% 31% 11%	575
August	1st to 10th	Day	1 4 20% 40% 26% 8%	527
	11th to 20th	Day	1 4 23% 37% 26% 9%	517
	21st to end	Day	1 3 17% 41% 28% 9%	583
September	1st to 10th	Day	2 5% 19% 38% 26% 9%	530
	11th to 20th	Day	5% 8% 22% 32% 25% 8%	530
	21st to end	Day	6% 7% 23% 32% 23% 9%	530
October	1st to 10th	Day	8% 8% 24% 26% 24% 10%	530
	11th to 20th	Day	12% 8% 18% 23% 27% 11%	530
	21st to end	Day	14% 8% 22% 26% 23% 9%	583
November	1st to 10th	Day	13% 11% 21% 24% 25% 7%	530
	11th to 20th	Day	19% 9% 16% 24% 25% 8%	289
	21st to end	Day	17% 7% 20% 23% 22% 11%	220
December	1st to 10th	Day	21% 6% 17% 25% 21% 10%	220
	11th to 20th	Day	32% 7% 16% 20% 15% 9%	220
	21st to end	Day	31% 11% 20% 19% 14% 5%	242
January	1st to 10th	Night	25% 6% 15% 29% 19% 4	840
	11th to 20th	Night	21% 5% 14% 34% 21% 4	840
	21st to end	Night	19% 6% 14% 35% 22% 4	924
February	1st to 10th	Night	21% 7% 16% 33% 20% 3	840
	11th to 20th	Night	21% 10% 19% 30% 15% 4	824
	21st to end	Night	21% 9% 20% 34% 14% 2	692
March	1st to 10th	Night	25% 9% 20% 29% 14% 3	840
	11th to 20th	Night	21% 9% 18% 32% 18% 3	840
	21st to end	Night	22% 9% 24% 30% 14% 2	924
April	1st to 10th	Night	18% 10% 24% 28% 17% 3	804
	11th to 20th	Night	14% 12% 23% 29% 18% 5%	791
	21st to end	Night	9% 16% 27% 25% 20% 3	799
May	1st to 10th	Night	9% 10% 27% 29% 21% 4	600
	11th to 20th	Night	5% 6% 25% 33% 26% 4	560
	21st to end	Night	9% 10% 23% 28% 26% 4	433
June	1st to 10th	Night	7% 10% 24% 33% 25% 2	400
	11th to 20th	Night	3 12% 26% 38% 20% 2	400
	21st to end	Night	4 3 19% 38% 35% 2	400
July	1st to 10th	Night	2 4 16% 36% 37% 5%	390
	11th to 20th	Night	1 1 12% 32% 43% 10%	582
	21st to end	Night	1 1 10% 25% 53% 11%	657
August	1st to 10th	Night	2 1 14% 33% 39% 11%	597
	11th to 20th	Night	1 1 17% 31% 39% 10%	635
	21st to end	Night	2 2 9% 31% 48% 8%	880
September	1st to 10th	Night	2 3 11% 35% 41% 9%	800
	11th to 20th	Night	6% 5% 14% 35% 34% 8%	800
	21st to end	Night	6% 4 15% 31% 35% 10%	800
October	1st to 10th	Night	7% 5% 15% 34% 32% 8%	800
	11th to 20th	Night	7% 6% 14% 30% 35% 8%	800
	21st to end	Night	12% 3 13% 31% 31% 10%	880
November	1st to 10th	Night	14% 5% 14% 29% 31% 6%	820
	11th to 20th	Night	17% 5% 15% 36% 22% 5%	840
	21st to end	Night	13% 7% 16% 35% 23% 6%	840
December	1st to 10th	Night	23% 4 13% 33% 23% 6%	840
	11th to 20th	Night	30% 5% 11% 30% 20% 4	840
	21st to end	Night	28% 6% 17% 30% 15% 3	924

Page generated at 2026-03-29 23:18:43. Version ac07

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