San Ramon
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the San Ramon clear sky chart from 2005-11-06 to 2026-06-01.

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	5% 4 2 5% 3 3 3 3 5% 6% 61%	140831
	Night	7% 6% 3 7% 4 4 4 3 5% 6% 51%	139769
Transparency	Day	23% 4 8% 19% 25% 21%	135762
	Night	30% 5% 9% 18% 22% 16%	135824
Seeing	Day	9% 11% 27% 32% 17% 4	17387
	Night	10% 5% 17% 36% 26% 6%	26802

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% 7% 2 6% 4 3 4 4 6% 8% 42%	4962
	11th to 20th	Day	7% 5% 3 6% 4 3 3 2 4 6% 57%	5340
	21st to end	Day	13% 7% 3 8% 5% 4 4 4 6% 7% 40%	5843
February	1st to 10th	Day	15% 8% 2 6% 4 3 4 3 6% 6% 44%	1890
	11th to 20th	Day	12% 8% 3 6% 5% 4 5% 4 5% 8% 40%	1952
	21st to end	Day	11% 8% 2 6% 5% 4 4 3 6% 6% 45%	1730
March	1st to 10th	Day	10% 8% 3 10% 7% 5% 6% 5% 6% 6% 34%	4932
	11th to 20th	Day	10% 5% 2 5% 4 4 4 4 6% 7% 49%	5110
	21st to end	Day	10% 6% 3 7% 5% 5% 6% 5% 7% 10% 36%	5726
April	1st to 10th	Day	11% 7% 2 5% 5% 4 4 4 5% 7% 45%	2520
	11th to 20th	Day	8% 6% 2 5% 4 3 4 4 5% 6% 54%	2489
	21st to end	Day	5% 5% 2 4 4 3 3 4 5% 8% 58%	2520
May	1st to 10th	Day	4 2 1 2 2 2 2 2 3 5% 75%	5695
	11th to 20th	Day	2 2 1 3 2 2 2 2 3 6% 75%	6165
	21st to end	Day	2 2 1 4 2 2 2 2 3 6% 73%	7056
June	1st to 10th	Day	3 4 1 4 3 3 3 3 5% 7% 63%	2801
	11th to 20th	Day	1 2 1 2 2 2 2 3 5% 6% 75%	2800
	21st to end	Day	2 2 1 3 2 2 2 1 3 5% 77%	2800
July	1st to 10th	Day	2 1 2 2 1 3 4 84%	6319
	11th to 20th	Day	1 1 1 2 2 2 2 2 3 5% 80%	5977
	21st to end	Day	1 1 5% 3 2 2 2 4 6% 73%	6402
August	1st to 10th	Day	1 1 2 2 2 3 2 4 6% 75%	5777
	11th to 20th	Day	1 1 3 2 2 2 2 3 5% 79%	5493
	21st to end	Day	1 1 1 3 2 2 3 2 4 6% 74%	5984
September	1st to 10th	Day	1 2 1 3 2 2 2 2 3 5% 79%	2400
	11th to 20th	Day	2 3 2 5% 3 3 3 2 4 6% 67%	2400
	21st to end	Day	2 2 1 2 2 2 2 2 4 6% 75%	2129
October	1st to 10th	Day	1 2 1 3 2 2 2 2 4 6% 73%	3280
	11th to 20th	Day	3 3 2 5% 2 3 3 2 4 6% 68%	3261
	21st to end	Day	4 4 1 4 3 3 4 3 6% 8% 60%	3558
November	1st to 10th	Day	6% 5% 2 4 4 2 4 4 5% 9% 56%	1996
	11th to 20th	Day	8% 8% 2 6% 5% 4 4 3 5% 6% 48%	1890
	21st to end	Day	11% 6% 2 4 3 3 3 3 5% 7% 53%	1734
December	1st to 10th	Day	6% 5% 2 4 3 3 4 4 6% 9% 55%	2700
	11th to 20th	Day	18% 9% 3 8% 5% 5% 5% 4 7% 8% 28%	3108
	21st to end	Day	16% 11% 4 9% 4 4 5% 4 6% 7% 29%	4092
January	1st to 10th	Night	12% 8% 3 7% 4 3 3 3 5% 8% 44%	7927
	11th to 20th	Night	8% 6% 3 6% 4 4 3 3 4 6% 53%	7650
	21st to end	Night	11% 10% 4 9% 4 4 3 3 5% 7% 39%	8371
February	1st to 10th	Night	15% 8% 3 5% 3 3 3 3 5% 5% 48%	2730
	11th to 20th	Night	11% 8% 2 7% 5% 4 5% 3 5% 7% 43%	2553
	21st to end	Night	9% 8% 2 5% 4 3 4 3 6% 7% 50%	2076
March	1st to 10th	Night	13% 11% 4 9% 5% 4 5% 3 4 6% 35%	5166
	11th to 20th	Night	12% 7% 2 8% 5% 4 3 3 4 6% 47%	5110
	21st to end	Night	13% 11% 3 8% 5% 4 4 3 4 7% 38%	5489
April	1st to 10th	Night	8% 7% 3 6% 5% 4 4 3 5% 7% 47%	2100
	11th to 20th	Night	7% 7% 3 7% 3 3 4 3 4 7% 51%	2079
	21st to end	Night	5% 5% 2 4 4 3 3 4 6% 9% 56%	2100
May	1st to 10th	Night	4 5% 2 5% 2 2 3 2 3 4 67%	4501
	11th to 20th	Night	2 5% 1 7% 5% 6% 5% 3 4 6% 57%	3944
	21st to end	Night	1 4 2 9% 5% 6% 4 2 3 5% 58%	4060
June	1st to 10th	Night	4 5% 2 5% 3 3 4 4 5% 8% 57%	1600
	11th to 20th	Night	1 2 1 4 3 3 2 3 4 6% 70%	1600
	21st to end	Night	1 3 2 5% 3 3 3 2 3 5% 69%	1600
July	1st to 10th	Night	1 1 3 3 4 4 2 4 5% 72%	3640
	11th to 20th	Night	1 1 1 4 5% 7% 7% 3 5% 7% 58%	3994
	21st to end	Night	1 7% 8% 12% 7% 9% 6% 3 4 5% 39%	4472
August	1st to 10th	Night	2 2 2 6% 6% 9% 7% 4 5% 7% 50%	4035
	11th to 20th	Night	3 3 5% 5% 9% 7% 3 3 5% 58%	4368
	21st to end	Night	1 3 1 7% 6% 5% 4 4 5% 6% 57%	5015
September	1st to 10th	Night	1 4 2 5% 4 3 3 2 3 5% 69%	2000
	11th to 20th	Night	3 5% 3 8% 4 5% 4 2 5% 6% 56%	2000
	21st to end	Night	2 3 1 4 3 3 3 2 3 6% 70%	2266
October	1st to 10th	Night	2 2 1 3 3 2 2 2 4 5% 72%	3920
	11th to 20th	Night	3 6% 5% 9% 5% 4 3 3 4 5% 55%	3897
	21st to end	Night	5% 5% 2 5% 3 3 4 3 5% 6% 59%	4253
November	1st to 10th	Night	6% 5% 1 5% 3 4 3 3 6% 8% 55%	2519
	11th to 20th	Night	10% 6% 2 5% 4 3 4 3 5% 8% 49%	2730
	21st to end	Night	9% 4 2 5% 4 4 4 3 6% 7% 54%	2882
December	1st to 10th	Night	7% 4 2 5% 3 3 4 3 4 7% 59%	4700
	11th to 20th	Night	17% 10% 3 9% 5% 4 5% 4 9% 8% 26%	5316
	21st to end	Night	17% 10% 4 9% 6% 5% 5% 4 5% 6% 29%	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	37% 7% 17% 22% 13% 4	4963
	11th to 20th	Day	29% 5% 6% 14% 27% 19%	5340
	21st to end	Day	41% 6% 7% 16% 17% 12%	5843
February	1st to 10th	Day	38% 6% 8% 19% 19% 9%	1890
	11th to 20th	Day	41% 7% 10% 18% 19% 6%	1961
	21st to end	Day	37% 7% 9% 23% 17% 6%	1730
March	1st to 10th	Day	44% 10% 8% 16% 20% 2	4932
	11th to 20th	Day	32% 5% 15% 27% 13% 8%	5110
	21st to end	Day	35% 13% 16% 23% 11% 2	5726
April	1st to 10th	Day	35% 7% 13% 20% 20% 5%	2520
	11th to 20th	Day	29% 5% 14% 23% 22% 6%	2492
	21st to end	Day	23% 6% 14% 28% 25% 4	2520
May	1st to 10th	Day	14% 3 8% 27% 31% 19%	5695
	11th to 20th	Day	11% 2 7% 30% 39% 11%	6168
	21st to end	Day	11% 3 10% 28% 41% 6%	7058
June	1st to 10th	Day	20% 5% 11% 25% 29% 11%	2801
	11th to 20th	Day	10% 4 8% 21% 36% 23%	2800
	21st to end	Day	11% 2 4 13% 30% 40%	2800
July	1st to 10th	Day	6% 1 2 7% 22% 63%	6251
	11th to 20th	Day	8% 1 2 5% 11% 71%	5905
	21st to end	Day	13% 1 3 17% 26% 40%	6217
August	1st to 10th	Day	13% 2 2 9% 27% 46%	4206
	11th to 20th	Day	7% 1 3 11% 38% 40%	3975
	21st to end	Day	11% 2 5% 11% 34% 38%	4311
September	1st to 10th	Day	9% 1 5% 11% 30% 44%	2400
	11th to 20th	Day	17% 3 8% 21% 33% 18%	2400
	21st to end	Day	11% 2 7% 21% 33% 25%	2129
October	1st to 10th	Day	13% 2 5% 11% 28% 41%	3280
	11th to 20th	Day	18% 3 8% 21% 29% 21%	3261
	21st to end	Day	21% 4 7% 20% 33% 15%	3558
November	1st to 10th	Day	26% 6% 11% 23% 22% 11%	1996
	11th to 20th	Day	33% 7% 11% 22% 21% 6%	1890
	21st to end	Day	30% 5% 11% 19% 26% 9%	1734
December	1st to 10th	Day	25% 5% 8% 28% 28% 6%	2700
	11th to 20th	Day	50% 7% 10% 15% 14% 4	3108
	21st to end	Day	51% 9% 12% 17% 10% 2	4092
January	1st to 10th	Night	37% 9% 16% 21% 14% 3	7927
	11th to 20th	Night	32% 6% 7% 12% 27% 15%	7650
	21st to end	Night	44% 6% 6% 14% 18% 11%	8371
February	1st to 10th	Night	38% 5% 9% 18% 19% 12%	2730
	11th to 20th	Night	39% 6% 11% 19% 19% 7%	2555
	21st to end	Night	33% 7% 12% 25% 18% 5%	2076
March	1st to 10th	Night	44% 11% 8% 19% 16% 3	5166
	11th to 20th	Night	34% 7% 13% 23% 15% 8%	5110
	21st to end	Night	39% 13% 15% 19% 12% 3	5489
April	1st to 10th	Night	32% 8% 15% 21% 18% 6%	2100
	11th to 20th	Night	30% 7% 12% 24% 18% 9%	2079
	21st to end	Night	23% 6% 12% 29% 24% 6%	2100
May	1st to 10th	Night	17% 6% 9% 24% 29% 15%	4498
	11th to 20th	Night	16% 3 12% 25% 30% 13%	3944
	21st to end	Night	17% 4 12% 26% 35% 6%	4060
June	1st to 10th	Night	22% 5% 10% 24% 27% 12%	1600
	11th to 20th	Night	13% 3 7% 16% 34% 26%	1600
	21st to end	Night	11% 3 5% 15% 31% 36%	1600
July	1st to 10th	Night	10% 1 1 9% 21% 57%	3600
	11th to 20th	Night	16% 1 2 6% 24% 50%	3943
	21st to end	Night	34% 1 6% 11% 23% 25%	4345
August	1st to 10th	Night	29% 3 2 12% 27% 28%	2929
	11th to 20th	Night	13% 1 4 11% 39% 32%	3155
	21st to end	Night	23% 2 4 12% 29% 31%	3608
September	1st to 10th	Night	16% 2 4 10% 23% 45%	2000
	11th to 20th	Night	23% 3 7% 21% 27% 19%	2000
	21st to end	Night	14% 2 7% 19% 30% 28%	2266
October	1st to 10th	Night	14% 2 6% 12% 22% 45%	3920
	11th to 20th	Night	28% 3 7% 19% 21% 22%	3897
	21st to end	Night	24% 3 6% 21% 27% 20%	4253
November	1st to 10th	Night	26% 6% 11% 25% 18% 14%	2519
	11th to 20th	Night	33% 5% 12% 22% 21% 8%	2730
	21st to end	Night	29% 5% 10% 21% 22% 12%	2882
December	1st to 10th	Night	26% 4 9% 26% 25% 11%	4700
	11th to 20th	Night	52% 7% 9% 14% 15% 3	5316
	21st to end	Night	52% 10% 13% 14% 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	25% 13% 28% 21% 13%	304
	11th to 20th	Day	15% 13% 21% 34% 15% 3	430
	21st to end	Day	17% 9% 23% 29% 20% 2	473
February	1st to 10th	Day	20% 13% 23% 27% 15% 2	430
	11th to 20th	Day	22% 16% 27% 23% 13% 1	483
	21st to end	Day	14% 15% 29% 30% 11% 1	446
March	1st to 10th	Day	19% 20% 29% 21% 9% 2	540
	11th to 20th	Day	19% 16% 24% 22% 17% 3	540
	21st to end	Day	15% 19% 31% 23% 12% 1	594
April	1st to 10th	Day	14% 17% 29% 24% 13% 3	540
	11th to 20th	Day	13% 19% 31% 23% 11% 3	532
	21st to end	Day	8% 19% 29% 28% 10% 6%	540
May	1st to 10th	Day	9% 17% 32% 28% 12% 3	543
	11th to 20th	Day	9% 15% 33% 27% 14% 2	568
	21st to end	Day	5% 14% 31% 34% 14% 3	617
June	1st to 10th	Day	5% 12% 35% 30% 14% 4	531
	11th to 20th	Day	1 14% 33% 37% 13% 1	530
	21st to end	Day	3 7% 30% 39% 18% 3	530
July	1st to 10th	Day	1 4 32% 44% 15% 4	526
	11th to 20th	Day	2 25% 48% 22% 4	524
	21st to end	Day	1 2 27% 44% 18% 7%	575
August	1st to 10th	Day	3 6% 28% 40% 17% 6%	527
	11th to 20th	Day	1 6% 28% 39% 18% 9%	517
	21st to end	Day	1 3 22% 42% 24% 8%	583
September	1st to 10th	Day	2 4 21% 42% 24% 7%	530
	11th to 20th	Day	6% 10% 25% 36% 16% 7%	530
	21st to end	Day	3 8% 24% 35% 23% 7%	530
October	1st to 10th	Day	3 10% 27% 30% 25% 6%	530
	11th to 20th	Day	6% 9% 21% 29% 27% 7%	530
	21st to end	Day	7% 10% 23% 30% 25% 5%	583
November	1st to 10th	Day	8% 11% 23% 24% 28% 7%	472
	11th to 20th	Day	11% 10% 18% 32% 21% 7%	330
	21st to end	Day	13% 9% 25% 31% 22% 1	247
December	1st to 10th	Day	13% 13% 20% 26% 21% 6%	220
	11th to 20th	Day	15% 13% 20% 24% 25% 3	220
	21st to end	Day	26% 12% 26% 22% 12% 2	242
January	1st to 10th	Night	19% 5% 16% 32% 20% 7%	840
	11th to 20th	Night	13% 5% 15% 35% 26% 6%	840
	21st to end	Night	17% 5% 12% 38% 23% 5%	924
February	1st to 10th	Night	19% 7% 13% 35% 21% 4	840
	11th to 20th	Night	16% 8% 18% 36% 18% 5%	824
	21st to end	Night	12% 10% 19% 38% 19% 2	692
March	1st to 10th	Night	18% 11% 19% 30% 20% 3	840
	11th to 20th	Night	16% 10% 18% 31% 22% 4	840
	21st to end	Night	14% 9% 24% 34% 18% 2	924
April	1st to 10th	Night	10% 9% 22% 36% 20% 3	840
	11th to 20th	Night	10% 12% 26% 29% 19% 4	831
	21st to end	Night	7% 8% 26% 35% 20% 5%	840
May	1st to 10th	Night	6% 6% 24% 39% 21% 3	772
	11th to 20th	Night	6% 4 18% 43% 25% 3	532
	21st to end	Night	4 8% 25% 39% 22% 2	455
June	1st to 10th	Night	5% 5% 20% 46% 22% 2	400
	11th to 20th	Night	1 3 21% 52% 22% 2	400
	21st to end	Night	1 4 15% 46% 32% 2	400
July	1st to 10th	Night	1 1 17% 44% 34% 2	390
	11th to 20th	Night	2 1 9% 46% 35% 8%	531
	21st to end	Night	1 13% 39% 38% 8%	657
August	1st to 10th	Night	3 3 18% 34% 34% 8%	597
	11th to 20th	Night	1 1 15% 43% 33% 6%	729
	21st to end	Night	2 11% 42% 37% 8%	880
September	1st to 10th	Night	2 1 15% 35% 39% 9%	800
	11th to 20th	Night	5% 4 15% 38% 33% 6%	800
	21st to end	Night	4 3 14% 34% 36% 10%	800
October	1st to 10th	Night	5% 3 13% 38% 32% 9%	800
	11th to 20th	Night	8% 3 12% 35% 33% 9%	800
	21st to end	Night	7% 4 14% 33% 31% 12%	880
November	1st to 10th	Night	8% 5% 15% 30% 32% 10%	820
	11th to 20th	Night	12% 2 15% 36% 28% 7%	840
	21st to end	Night	11% 4 14% 35% 29% 7%	840
December	1st to 10th	Night	14% 5% 15% 35% 25% 6%	840
	11th to 20th	Night	22% 5% 13% 33% 23% 5%	840
	21st to end	Night	21% 8% 19% 32% 18% 2	924

Page generated at 2026-05-28 22:53:27. Version ac07

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