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 2025-02-06.

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	6% 4 2 5% 3 3 3 3 5% 6% 60%	112487
	Night	7% 6% 3 6% 4 4 4 3 5% 6% 52%	114567
Transparency	Day	23% 5% 8% 17% 24% 22%	112244
	Night	29% 6% 9% 17% 21% 17%	114399
Seeing	Day	9% 12% 27% 31% 17% 4	15621
	Night	9% 5% 17% 36% 27% 6%	25077

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	13% 7% 2 6% 4 3 4 4 6% 7% 45%	3823
	11th to 20th	Day	10% 6% 4 8% 5% 4 3 2 3 5% 51%	4100
	21st to end	Day	16% 8% 3 8% 4 4 4 3 5% 5% 40%	4510
February	1st to 10th	Day	16% 9% 2 6% 4 3 4 3 6% 6% 43%	1756
	11th to 20th	Day	12% 8% 3 6% 4 4 5% 4 5% 7% 43%	1761
	21st to end	Day	12% 8% 2 6% 4 4 4 3 5% 6% 47%	1570
March	1st to 10th	Day	11% 10% 4 11% 7% 6% 6% 5% 6% 6% 28%	3354
	11th to 20th	Day	9% 5% 2 4 4 3 3 3 5% 7% 54%	3490
	21st to end	Day	11% 8% 3 7% 5% 5% 6% 5% 7% 8% 34%	3903
April	1st to 10th	Day	12% 8% 2 6% 5% 4 4 3 5% 7% 45%	2280
	11th to 20th	Day	8% 6% 2 5% 3 3 4 4 5% 6% 55%	2249
	21st to end	Day	5% 4 1 4 3 3 3 4 5% 8% 60%	2280
May	1st to 10th	Day	5% 2 1 2 2 2 2 2 3 5% 72%	3893
	11th to 20th	Day	2 2 1 3 2 2 2 2 3 6% 75%	4209
	21st to end	Day	2 3 1 5% 3 2 2 2 4 6% 70%	4813
June	1st to 10th	Day	3 4 1 4 3 3 3 3 6% 7% 62%	2660
	11th to 20th	Day	1 2 1 2 2 2 2 3 5% 6% 75%	2660
	21st to end	Day	2 2 1 3 2 2 2 2 3 5% 76%	2660
July	1st to 10th	Day	1 1 2 1 1 2 1 3 4 85%	4420
	11th to 20th	Day	1 1 1 2 2 2 2 2 4 5% 78%	4181
	21st to end	Day	1 1 3 2 2 1 2 4 6% 78%	4455
August	1st to 10th	Day	1 2 1 3 2 2 4 3 6% 7% 69%	4091
	11th to 20th	Day	2 2 2 2 2 4 5% 80%	3814
	21st to end	Day	1 2 1 3 2 3 3 2 4 5% 76%	4180
September	1st to 10th	Day	1 2 1 3 2 2 2 1 3 5% 80%	2280
	11th to 20th	Day	2 3 2 4 3 3 3 2 4 6% 67%	2280
	21st to end	Day	2 2 1 2 2 2 2 2 4 6% 76%	2023
October	1st to 10th	Day	1 2 1 3 2 2 2 2 4 6% 73%	3180
	11th to 20th	Day	3 2 2 5% 2 3 3 2 4 6% 68%	3161
	21st to end	Day	4 4 1 4 3 3 4 3 6% 8% 60%	3448
November	1st to 10th	Day	6% 5% 2 4 4 3 4 4 4 9% 56%	1899
	11th to 20th	Day	8% 8% 2 6% 5% 3 4 3 5% 6% 50%	1800
	21st to end	Day	11% 7% 2 5% 3 3 3 3 5% 6% 52%	1652
December	1st to 10th	Day	7% 5% 2 4 3 3 3 4 6% 9% 55%	2620
	11th to 20th	Day	18% 9% 3 8% 5% 5% 5% 4 7% 8% 28%	3028
	21st to end	Day	16% 11% 4 9% 4 4 5% 4 6% 7% 29%	4004
January	1st to 10th	Night	9% 7% 2 6% 3 3 3 3 6% 9% 49%	6113
	11th to 20th	Night	10% 7% 3 8% 4 4 4 3 4 4 47%	5880
	21st to end	Night	13% 11% 4 9% 4 4 3 3 4 5% 41%	6468
February	1st to 10th	Night	16% 8% 2 5% 3 3 3 3 4 4 48%	2535
	11th to 20th	Night	10% 7% 2 7% 5% 4 5% 3 5% 7% 45%	2304
	21st to end	Night	9% 8% 2 5% 4 3 4 2 5% 7% 51%	1884
March	1st to 10th	Night	14% 12% 4 9% 6% 4 5% 4 4 5% 33%	3504
	11th to 20th	Night	10% 7% 2 7% 4 4 3 3 4 5% 51%	3490
	21st to end	Night	13% 12% 4 9% 5% 4 4 3 4 6% 35%	3748
April	1st to 10th	Night	8% 8% 3 5% 5% 4 4 3 5% 7% 47%	1900
	11th to 20th	Night	7% 7% 3 6% 3 3 4 3 4 7% 53%	1879
	21st to end	Night	4 4 2 4 4 3 3 4 6% 8% 58%	1900
May	1st to 10th	Night	4 6% 2 5% 3 2 2 2 3 4 67%	3066
	11th to 20th	Night	2 4 1 6% 5% 5% 5% 4 4 6% 58%	2688
	21st to end	Night	2 4 2 9% 3 4 3 2 3 5% 61%	2762
June	1st to 10th	Night	4 5% 2 5% 3 3 4 4 5% 8% 58%	1520
	11th to 20th	Night	1 2 1 5% 3 3 2 3 4 6% 70%	1520
	21st to end	Night	1 3 2 5% 3 3 3 2 3 5% 69%	1520
July	1st to 10th	Night	2 1 4 3 2 3 1 3 5% 77%	2540
	11th to 20th	Night	1 1 2 4 5% 6% 4 3 4 6% 63%	2782
	21st to end	Night	1 4 5% 8% 6% 7% 6% 3 5% 6% 49%	3109
August	1st to 10th	Night	3 2 3 7% 6% 6% 9% 6% 7% 9% 43%	2848
	11th to 20th	Night	2 2 4 4 8% 6% 2 3 6% 62%	3030
	21st to end	Night	2 4 1 7% 6% 4 3 3 4 5% 60%	3498
September	1st to 10th	Night	3 2 4 4 3 3 2 3 5% 70%	1900
	11th to 20th	Night	3 5% 2 7% 3 5% 4 2 5% 6% 57%	1900
	21st to end	Night	2 3 1 3 3 3 3 2 4 6% 71%	2152
October	1st to 10th	Night	2 2 1 3 3 2 2 2 4 6% 72%	3800
	11th to 20th	Night	3 6% 5% 9% 5% 4 3 3 4 5% 55%	3777
	21st to end	Night	5% 5% 2 4 3 3 4 3 5% 6% 60%	4121
November	1st to 10th	Night	6% 5% 1 5% 3 4 3 3 5% 8% 56%	2396
	11th to 20th	Night	9% 6% 2 5% 4 3 3 3 5% 8% 51%	2600
	21st to end	Night	9% 5% 2 5% 4 4 4 3 5% 7% 54%	2745
December	1st to 10th	Night	7% 4 2 5% 3 3 4 3 4 6% 59%	4560
	11th to 20th	Night	18% 11% 3 9% 5% 4 5% 4 8% 8% 26%	5176
	21st to end	Night	17% 10% 4 9% 6% 5% 5% 4 5% 6% 29%	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	34% 8% 18% 20% 15% 5%	3823
	11th to 20th	Day	35% 6% 8% 14% 23% 14%	4100
	21st to end	Day	43% 7% 7% 11% 17% 15%	4510
February	1st to 10th	Day	40% 6% 8% 19% 19% 8%	1756
	11th to 20th	Day	39% 7% 9% 18% 20% 7%	1770
	21st to end	Day	36% 7% 9% 23% 19% 6%	1570
March	1st to 10th	Day	49% 13% 9% 13% 14% 2	3354
	11th to 20th	Day	29% 6% 13% 26% 15% 11%	3490
	21st to end	Day	38% 16% 16% 17% 11% 2	3903
April	1st to 10th	Day	36% 8% 14% 20% 19% 4	2280
	11th to 20th	Day	29% 6% 14% 24% 22% 6%	2252
	21st to end	Day	21% 6% 14% 28% 26% 5%	2280
May	1st to 10th	Day	15% 4 10% 30% 25% 15%	3893
	11th to 20th	Day	11% 3 8% 23% 41% 15%	4212
	21st to end	Day	13% 5% 13% 30% 33% 6%	4814
June	1st to 10th	Day	21% 5% 11% 24% 27% 11%	2660
	11th to 20th	Day	10% 4 8% 21% 36% 21%	2660
	21st to end	Day	12% 2 4 13% 29% 40%	2660
July	1st to 10th	Day	6% 1 2 6% 13% 71%	4351
	11th to 20th	Day	10% 2 3 8% 14% 63%	4109
	21st to end	Day	8% 1 2 7% 27% 55%	4315
August	1st to 10th	Day	13% 2 2 9% 26% 47%	4076
	11th to 20th	Day	7% 1 3 11% 39% 40%	3852
	21st to end	Day	11% 2 5% 11% 33% 38%	4179
September	1st to 10th	Day	9% 1 4 10% 30% 46%	2280
	11th to 20th	Day	17% 3 8% 21% 32% 19%	2280
	21st to end	Day	10% 2 7% 22% 34% 25%	2023
October	1st to 10th	Day	13% 2 5% 11% 28% 42%	3180
	11th to 20th	Day	18% 4 8% 21% 28% 21%	3161
	21st to end	Day	20% 4 7% 20% 33% 15%	3448
November	1st to 10th	Day	26% 6% 11% 24% 22% 12%	1899
	11th to 20th	Day	32% 7% 10% 23% 22% 6%	1800
	21st to end	Day	31% 5% 11% 18% 26% 9%	1652
December	1st to 10th	Day	26% 5% 8% 28% 28% 6%	2620
	11th to 20th	Day	50% 7% 10% 15% 14% 3	3028
	21st to end	Day	51% 9% 12% 17% 10% 2	4004
January	1st to 10th	Night	31% 10% 19% 21% 15% 4	6113
	11th to 20th	Night	39% 8% 9% 12% 20% 11%	5880
	21st to end	Night	45% 6% 7% 10% 18% 14%	6468
February	1st to 10th	Night	39% 5% 9% 18% 18% 11%	2535
	11th to 20th	Night	37% 6% 10% 18% 20% 8%	2306
	21st to end	Night	32% 7% 12% 26% 18% 4	1884
March	1st to 10th	Night	46% 15% 11% 12% 14% 3	3504
	11th to 20th	Night	30% 6% 12% 24% 17% 11%	3490
	21st to end	Night	40% 15% 16% 16% 10% 4	3748
April	1st to 10th	Night	33% 8% 15% 20% 18% 6%	1900
	11th to 20th	Night	29% 7% 12% 25% 19% 9%	1879
	21st to end	Night	21% 6% 13% 29% 25% 6%	1900
May	1st to 10th	Night	17% 8% 12% 22% 26% 14%	3066
	11th to 20th	Night	14% 3 14% 19% 32% 19%	2688
	21st to end	Night	14% 5% 15% 29% 30% 7%	2762
June	1st to 10th	Night	22% 5% 10% 23% 27% 13%	1520
	11th to 20th	Night	13% 3 8% 17% 33% 26%	1520
	21st to end	Night	11% 4 5% 15% 30% 36%	1520
July	1st to 10th	Night	7% 1 2 8% 14% 68%	2500
	11th to 20th	Night	15% 2 3 9% 19% 53%	2731
	21st to end	Night	25% 2 3 10% 25% 36%	3011
August	1st to 10th	Night	29% 3 2 12% 26% 28%	2839
	11th to 20th	Night	12% 1 4 11% 39% 32%	3058
	21st to end	Night	23% 2 4 11% 29% 31%	3498
September	1st to 10th	Night	14% 2 4 9% 24% 46%	1900
	11th to 20th	Night	22% 2 7% 22% 28% 20%	1900
	21st to end	Night	12% 2 7% 19% 31% 28%	2152
October	1st to 10th	Night	13% 3 6% 11% 21% 46%	3800
	11th to 20th	Night	28% 3 8% 19% 21% 22%	3777
	21st to end	Night	23% 3 6% 21% 27% 20%	4121
November	1st to 10th	Night	26% 6% 11% 25% 18% 15%	2396
	11th to 20th	Night	30% 6% 12% 23% 22% 8%	2600
	21st to end	Night	30% 5% 11% 19% 23% 12%	2745
December	1st to 10th	Night	26% 4 8% 25% 26% 11%	4560
	11th to 20th	Night	52% 7% 9% 14% 15% 3	5176
	21st to end	Night	51% 10% 13% 14% 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	25% 14% 28% 20% 14%	280
	11th to 20th	Day	16% 14% 22% 31% 15% 3	400
	21st to end	Day	18% 9% 23% 29% 20% 2	440
February	1st to 10th	Day	21% 15% 21% 25% 16% 2	386
	11th to 20th	Day	21% 17% 25% 23% 14% 1	412
	21st to end	Day	15% 16% 29% 27% 11% 2	382
March	1st to 10th	Day	20% 20% 29% 20% 9% 2	460
	11th to 20th	Day	19% 18% 25% 21% 15% 3	460
	21st to end	Day	15% 21% 32% 23% 9%	506
April	1st to 10th	Day	15% 18% 31% 21% 13% 3	460
	11th to 20th	Day	12% 21% 30% 23% 10% 4	452
	21st to end	Day	6% 22% 28% 27% 11% 6%	460
May	1st to 10th	Day	8% 19% 34% 27% 11% 2	463
	11th to 20th	Day	8% 15% 33% 28% 14% 2	488
	21st to end	Day	6% 15% 29% 34% 14% 2	529
June	1st to 10th	Day	5% 13% 36% 29% 14% 4	490
	11th to 20th	Day	1 14% 32% 38% 14% 1	490
	21st to end	Day	3 7% 31% 40% 18% 2	490
July	1st to 10th	Day	1 4 30% 46% 16% 4	486
	11th to 20th	Day	2 26% 47% 21% 4	484
	21st to end	Day	1 2 27% 43% 19% 8%	531
August	1st to 10th	Day	3 5% 28% 41% 18% 5%	487
	11th to 20th	Day	1 6% 28% 39% 17% 9%	477
	21st to end	Day	1 4 22% 44% 23% 7%	539
September	1st to 10th	Day	2 4 21% 42% 25% 7%	490
	11th to 20th	Day	6% 11% 25% 37% 16% 6%	490
	21st to end	Day	2 9% 23% 36% 23% 7%	490
October	1st to 10th	Day	2 9% 28% 30% 24% 7%	490
	11th to 20th	Day	6% 9% 21% 29% 28% 7%	490
	21st to end	Day	6% 10% 23% 30% 25% 5%	539
November	1st to 10th	Day	9% 11% 23% 25% 27% 6%	435
	11th to 20th	Day	8% 11% 18% 33% 23% 7%	300
	21st to end	Day	14% 9% 25% 30% 21%	225
December	1st to 10th	Day	15% 14% 21% 24% 21% 7%	200
	11th to 20th	Day	15% 14% 22% 24% 23% 3	200
	21st to end	Day	24% 12% 26% 24% 12% 2	220
January	1st to 10th	Night	18% 5% 16% 32% 21% 7%	800
	11th to 20th	Night	13% 6% 16% 34% 25% 6%	800
	21st to end	Night	17% 5% 12% 36% 24% 5%	880
February	1st to 10th	Night	20% 7% 14% 33% 22% 5%	780
	11th to 20th	Night	13% 8% 17% 35% 20% 6%	744
	21st to end	Night	12% 11% 19% 36% 20% 2	628
March	1st to 10th	Night	18% 11% 18% 30% 21% 3	760
	11th to 20th	Night	16% 11% 18% 30% 22% 4	760
	21st to end	Night	14% 10% 24% 33% 17% 2	836
April	1st to 10th	Night	11% 9% 22% 36% 19% 3	760
	11th to 20th	Night	9% 13% 26% 29% 19% 4	751
	21st to end	Night	5% 8% 27% 35% 21% 5%	760
May	1st to 10th	Night	5% 7% 24% 38% 22% 3	698
	11th to 20th	Night	6% 3 19% 43% 26% 3	481
	21st to end	Night	4 8% 25% 39% 22% 2	411
June	1st to 10th	Night	5% 6% 19% 46% 22% 2	380
	11th to 20th	Night	1 3 21% 52% 22% 2	380
	21st to end	Night	1 4 15% 45% 32% 2	380
July	1st to 10th	Night	1 1 17% 44% 35% 2	370
	11th to 20th	Night	2 1 9% 45% 36% 8%	503
	21st to end	Night	1 13% 38% 40% 8%	624
August	1st to 10th	Night	3 2 18% 35% 34% 8%	567
	11th to 20th	Night	1 1 15% 42% 34% 7%	692
	21st to end	Night	2 11% 42% 37% 8%	836
September	1st to 10th	Night	1 1 14% 35% 40% 9%	760
	11th to 20th	Night	4 4 14% 39% 33% 6%	760
	21st to end	Night	3 3 13% 34% 37% 11%	760
October	1st to 10th	Night	4 3 13% 38% 33% 9%	760
	11th to 20th	Night	8% 3 12% 35% 33% 9%	760
	21st to end	Night	7% 4 13% 33% 31% 12%	836
November	1st to 10th	Night	8% 5% 16% 31% 31% 9%	780
	11th to 20th	Night	11% 3 15% 36% 29% 7%	800
	21st to end	Night	12% 4 15% 35% 28% 7%	800
December	1st to 10th	Night	14% 5% 15% 35% 24% 6%	800
	11th to 20th	Night	22% 5% 13% 33% 22% 5%	800
	21st to end	Night	19% 8% 19% 33% 19% 2	880

Page generated at 2025-02-03 00:05:56. Version ac07

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