Castro Valley
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Castro Valley clear sky chart from 2005-11-06 to 2025-10-04.

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% 6% 2 5% 3 3 3 3 4 6% 59%	134941
	Night	9% 10% 4 7% 3 3 3 3 4 6% 48%	131896
Transparency	Day	27% 4 8% 19% 24% 18%	129872
	Night	36% 5% 9% 17% 20% 14%	127951
Seeing	Day	10% 10% 26% 32% 18% 5%	16598
	Night	13% 4 13% 35% 29% 6%	25902

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% 3 3 3 3 6% 9% 45%	3868
	11th to 20th	Day	12% 7% 3 7% 4 3 3 2 3 4 51%	4100
	21st to end	Day	17% 9% 3 7% 5% 4 3 3 4 6% 39%	4510
February	1st to 10th	Day	17% 8% 3 5% 4 3 3 3 5% 6% 44%	1800
	11th to 20th	Day	13% 8% 4 6% 4 4 3 3 6% 8% 41%	1849
	21st to end	Day	12% 7% 2 6% 4 4 3 3 5% 6% 46%	1650
March	1st to 10th	Day	11% 10% 4 10% 6% 6% 6% 4 6% 7% 32%	4860
	11th to 20th	Day	11% 6% 2 6% 3 4 4 3 5% 8% 47%	5000
	21st to end	Day	10% 8% 2 7% 5% 5% 6% 5% 7% 10% 35%	5571
April	1st to 10th	Day	12% 8% 2 6% 4 3 3 4 5% 7% 45%	2400
	11th to 20th	Day	9% 7% 1 5% 3 3 3 3 5% 6% 54%	2369
	21st to end	Day	5% 6% 2 5% 3 3 4 3 6% 9% 56%	2400
May	1st to 10th	Day	5% 3 1 2 2 1 2 2 3 5% 74%	5606
	11th to 20th	Day	3 3 1 3 2 2 2 2 4 6% 73%	5988
	21st to end	Day	2 7% 2 3 2 2 2 2 3 6% 68%	6902
June	1st to 10th	Day	5% 6% 2 4 3 3 3 3 5% 7% 60%	2800
	11th to 20th	Day	1 3 1 3 2 2 2 2 4 6% 73%	2800
	21st to end	Day	3 4 2 3 2 2 2 2 3 5% 72%	2800
July	1st to 10th	Day	2 1 3 2 2 2 2 2 4 79%	6319
	11th to 20th	Day	2 2 4 2 2 2 2 3 5% 74%	5931
	21st to end	Day	7% 3 5% 3 3 3 3 4 7% 63%	6402
August	1st to 10th	Day	1 3 2 4 2 3 3 3 4 6% 69%	5777
	11th to 20th	Day	1 5% 2 4 2 2 2 2 3 5% 72%	5500
	21st to end	Day	1 3 2 5% 3 3 3 2 4 6% 68%	5984
September	1st to 10th	Day	2 3 2 3 2 2 2 1 3 5% 74%	2400
	11th to 20th	Day	3 6% 2 4 3 3 3 2 5% 7% 61%	2400
	21st to end	Day	3 3 1 3 2 2 2 2 3 6% 73%	2124
October	1st to 10th	Day	2 3 1 3 2 2 3 2 4 6% 71%	3211
	11th to 20th	Day	4 5% 2 4 3 3 3 2 4 6% 64%	3161
	21st to end	Day	4 5% 2 5% 3 3 3 3 6% 7% 59%	3448
November	1st to 10th	Day	8% 5% 2 4 3 3 3 4 5% 8% 55%	1914
	11th to 20th	Day	9% 8% 3 6% 4 4 3 3 4 7% 49%	1800
	21st to end	Day	12% 6% 2 5% 2 3 3 4 5% 6% 52%	1645
December	1st to 10th	Day	7% 4 2 5% 3 3 3 4 6% 9% 53%	2620
	11th to 20th	Day	21% 8% 3 7% 4 4 5% 4 6% 7% 30%	3028
	21st to end	Day	20% 12% 3 7% 4 4 4 4 5% 7% 28%	4004
January	1st to 10th	Night	10% 7% 3 5% 3 4 3 3 6% 9% 47%	6071
	11th to 20th	Night	12% 8% 4 7% 5% 4 4 3 4 5% 45%	5880
	21st to end	Night	15% 12% 4 7% 4 3 3 3 3 4 40%	6468
February	1st to 10th	Night	18% 7% 2 5% 3 3 3 3 5% 5% 47%	2600
	11th to 20th	Night	13% 9% 3 6% 5% 4 4 3 5% 7% 42%	2437
	21st to end	Night	11% 8% 2 5% 4 3 3 3 4 7% 50%	1980
March	1st to 10th	Night	15% 14% 4 9% 4 4 4 3 4 5% 33%	5031
	11th to 20th	Night	15% 9% 4 6% 3 3 3 2 4 6% 45%	5000
	21st to end	Night	16% 12% 4 7% 4 4 4 3 5% 6% 36%	5402
April	1st to 10th	Night	9% 9% 3 6% 4 3 3 3 6% 8% 45%	2000
	11th to 20th	Night	10% 7% 3 5% 3 4 3 3 5% 6% 51%	1979
	21st to end	Night	6% 5% 2 4 3 4 4 3 6% 9% 55%	2000
May	1st to 10th	Night	6% 6% 2 4 2 2 2 2 3 4 67%	4381
	11th to 20th	Night	3 9% 4 9% 4 3 3 3 5% 5% 53%	3900
	21st to end	Night	3 16% 4 5% 3 2 2 2 3 4 56%	3972
June	1st to 10th	Night	6% 8% 3 6% 4 3 3 3 5% 7% 52%	1600
	11th to 20th	Night	1 4 2 5% 2 3 2 3 5% 6% 67%	1600
	21st to end	Night	2 9% 3 5% 2 3 3 2 3 5% 64%	1600
July	1st to 10th	Night	1 4 4 8% 3 2 3 2 3 4 67%	3640
	11th to 20th	Night	1 5% 8% 14% 4 3 4 2 4 5% 50%	4042
	21st to end	Night	2 30% 8% 7% 2 2 3 2 4 5% 34%	4472
August	1st to 10th	Night	2 11% 5% 16% 4 3 4 3 4 5% 42%	4035
	11th to 20th	Night	14% 5% 8% 3 2 3 2 4 4 54%	4362
	21st to end	Night	2 9% 6% 12% 3 3 3 3 4 5% 51%	5015
September	1st to 10th	Night	2 9% 4 4 2 3 3 2 4 6% 62%	2000
	11th to 20th	Night	4 11% 4 8% 3 3 4 2 4 6% 50%	2000
	21st to end	Night	3 6% 3 5% 2 2 2 2 4 6% 65%	2273
October	1st to 10th	Night	3 3 1 4 2 3 2 2 5% 6% 69%	3836
	11th to 20th	Night	4 15% 4 6% 3 3 2 2 4 4 52%	3777
	21st to end	Night	7% 5% 2 5% 3 3 3 3 5% 6% 58%	4121
November	1st to 10th	Night	8% 6% 2 4 3 4 4 4 5% 7% 53%	2382
	11th to 20th	Night	10% 7% 2 6% 3 3 3 2 6% 8% 50%	2600
	21st to end	Night	10% 5% 1 4 4 3 3 3 4 7% 54%	2752
December	1st to 10th	Night	8% 4 2 5% 3 3 3 3 5% 6% 58%	4560
	11th to 20th	Night	22% 11% 3 6% 4 3 5% 4 7% 8% 27%	5176
	21st to end	Night	21% 10% 3 8% 5% 5% 4 4 5% 6% 30%	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	35% 7% 18% 21% 14% 5%	3868
	11th to 20th	Day	37% 5% 8% 14% 21% 16%	4100
	21st to end	Day	45% 5% 6% 12% 15% 16%	4510
February	1st to 10th	Day	40% 4 7% 20% 20% 8%	1800
	11th to 20th	Day	41% 5% 11% 18% 20% 6%	1858
	21st to end	Day	37% 6% 8% 24% 19% 6%	1650
March	1st to 10th	Day	48% 7% 8% 17% 19% 2	4860
	11th to 20th	Day	34% 5% 16% 26% 11% 8%	5000
	21st to end	Day	39% 8% 17% 24% 10% 2	5571
April	1st to 10th	Day	37% 6% 13% 22% 19% 4	2400
	11th to 20th	Day	30% 5% 14% 25% 21% 5%	2372
	21st to end	Day	25% 5% 13% 29% 24% 4	2400
May	1st to 10th	Day	15% 3 8% 28% 31% 16%	5606
	11th to 20th	Day	15% 2 7% 33% 36% 7%	5991
	21st to end	Day	19% 4 6% 30% 38% 3	6904
June	1st to 10th	Day	25% 4 10% 27% 26% 8%	2800
	11th to 20th	Day	14% 3 8% 22% 37% 17%	2800
	21st to end	Day	17% 1 4 13% 33% 32%	2800
July	1st to 10th	Day	12% 1 2 7% 21% 56%	6251
	11th to 20th	Day	16% 1 4 4 14% 61%	5860
	21st to end	Day	23% 3 6% 13% 26% 29%	6217
August	1st to 10th	Day	22% 1 5% 9% 29% 33%	4206
	11th to 20th	Day	15% 2 4 8% 44% 27%	3982
	21st to end	Day	19% 2 3 10% 35% 30%	4311
September	1st to 10th	Day	16% 1 3 10% 31% 39%	2400
	11th to 20th	Day	25% 3 5% 22% 31% 15%	2400
	21st to end	Day	15% 3 5% 22% 33% 23%	2124
October	1st to 10th	Day	16% 2 5% 12% 27% 39%	3211
	11th to 20th	Day	23% 3 7% 21% 26% 19%	3161
	21st to end	Day	25% 4 6% 22% 28% 15%	3448
November	1st to 10th	Day	27% 5% 10% 25% 21% 12%	1914
	11th to 20th	Day	35% 5% 10% 23% 22% 6%	1800
	21st to end	Day	32% 4 10% 19% 27% 9%	1645
December	1st to 10th	Day	27% 4 6% 27% 29% 6%	2620
	11th to 20th	Day	51% 6% 10% 15% 15% 4	3028
	21st to end	Day	54% 7% 10% 17% 10% 2	4004
January	1st to 10th	Night	33% 9% 20% 21% 13% 4	6071
	11th to 20th	Night	42% 6% 9% 13% 18% 12%	5880
	21st to end	Night	47% 4 7% 10% 17% 15%	6468
February	1st to 10th	Night	40% 4 8% 19% 19% 10%	2600
	11th to 20th	Night	40% 5% 10% 18% 19% 7%	2439
	21st to end	Night	35% 5% 12% 28% 17% 5%	1980
March	1st to 10th	Night	49% 7% 9% 19% 13% 2	5031
	11th to 20th	Night	39% 5% 12% 23% 12% 8%	5000
	21st to end	Night	45% 10% 13% 20% 11% 2	5402
April	1st to 10th	Night	35% 7% 13% 23% 17% 4	2000
	11th to 20th	Night	33% 6% 12% 25% 18% 7%	1979
	21st to end	Night	25% 7% 13% 31% 20% 5%	2000
May	1st to 10th	Night	20% 5% 8% 28% 28% 12%	4378
	11th to 20th	Night	29% 3 9% 29% 25% 6%	3901
	21st to end	Night	30% 4 9% 24% 31% 1	3972
June	1st to 10th	Night	28% 6% 11% 25% 22% 8%	1600
	11th to 20th	Night	17% 4 9% 19% 34% 17%	1600
	21st to end	Night	22% 1 5% 14% 30% 27%	1600
July	1st to 10th	Night	21% 6% 6% 17% 49%	3600
	11th to 20th	Night	35% 1 8% 5% 12% 40%	3990
	21st to end	Night	50% 2 5% 7% 16% 19%	4345
August	1st to 10th	Night	47% 1 9% 7% 20% 15%	2929
	11th to 20th	Night	27% 1 6% 7% 36% 22%	3149
	21st to end	Night	31% 2 6% 9% 28% 24%	3608
September	1st to 10th	Night	24% 2 4 7% 26% 37%	2000
	11th to 20th	Night	33% 3 7% 19% 24% 14%	2000
	21st to end	Night	20% 3 5% 20% 29% 23%	2273
October	1st to 10th	Night	18% 2 5% 11% 22% 42%	3836
	11th to 20th	Night	36% 3 5% 16% 18% 21%	3777
	21st to end	Night	26% 3 7% 21% 23% 19%	4121
November	1st to 10th	Night	30% 5% 11% 23% 19% 12%	2382
	11th to 20th	Night	33% 6% 11% 24% 19% 8%	2600
	21st to end	Night	30% 4 11% 20% 23% 11%	2752
December	1st to 10th	Night	28% 5% 6% 23% 27% 11%	4560
	11th to 20th	Night	52% 4 9% 14% 17% 3	5176
	21st to end	Night	53% 8% 13% 15% 8% 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	23% 11% 26% 22% 14% 3	299
	11th to 20th	Day	18% 8% 23% 28% 20% 4	400
	21st to end	Day	16% 6% 25% 30% 19% 4	440
February	1st to 10th	Day	21% 11% 22% 24% 20% 3	400
	11th to 20th	Day	19% 14% 28% 25% 13% 2	442
	21st to end	Day	12% 14% 27% 33% 12% 2	414
March	1st to 10th	Day	20% 16% 29% 22% 10% 3	500
	11th to 20th	Day	20% 17% 23% 23% 13% 4	500
	21st to end	Day	17% 15% 34% 21% 11% 2	550
April	1st to 10th	Day	17% 14% 31% 24% 10% 4	500
	11th to 20th	Day	14% 16% 32% 24% 10% 4	492
	21st to end	Day	9% 18% 30% 27% 10% 6%	500
May	1st to 10th	Day	10% 14% 32% 30% 12% 2	503
	11th to 20th	Day	12% 12% 30% 30% 14% 2	528
	21st to end	Day	12% 11% 29% 31% 13% 5%	573
June	1st to 10th	Day	6% 10% 31% 33% 16% 4	530
	11th to 20th	Day	3 10% 32% 38% 14% 3	530
	21st to end	Day	4 7% 27% 38% 18% 5%	530
July	1st to 10th	Day	4 5% 23% 45% 19% 5%	526
	11th to 20th	Day	1 2 22% 44% 28% 4	524
	21st to end	Day	5% 2 22% 42% 22% 7%	575
August	1st to 10th	Day	5% 5% 24% 39% 20% 7%	527
	11th to 20th	Day	5% 3 23% 42% 20% 7%	517
	21st to end	Day	4 3 20% 39% 26% 8%	583
September	1st to 10th	Day	4 5% 19% 40% 24% 8%	530
	11th to 20th	Day	7% 10% 25% 32% 19% 7%	530
	21st to end	Day	5% 8% 23% 32% 26% 7%	530
October	1st to 10th	Day	4 8% 26% 31% 22% 9%	503
	11th to 20th	Day	8% 8% 19% 31% 27% 8%	490
	21st to end	Day	8% 8% 25% 30% 20% 8%	539
November	1st to 10th	Day	8% 11% 22% 25% 26% 8%	450
	11th to 20th	Day	9% 9% 17% 33% 24% 9%	300
	21st to end	Day	13% 5% 23% 30% 26% 3	223
December	1st to 10th	Day	14% 11% 21% 24% 22% 9%	200
	11th to 20th	Day	17% 11% 18% 27% 23% 5%	200
	21st to end	Day	24% 10% 25% 22% 17% 2	220
January	1st to 10th	Night	19% 6% 13% 35% 21% 6%	800
	11th to 20th	Night	16% 5% 13% 37% 26% 5%	800
	21st to end	Night	19% 4 12% 38% 22% 5%	880
February	1st to 10th	Night	20% 4 14% 36% 22% 4	800
	11th to 20th	Night	15% 5% 18% 36% 21% 4	784
	21st to end	Night	14% 8% 16% 42% 18% 3	660
March	1st to 10th	Night	20% 7% 18% 30% 21% 3	800
	11th to 20th	Night	21% 6% 14% 34% 22% 4	800
	21st to end	Night	18% 7% 18% 37% 19% 2	880
April	1st to 10th	Night	13% 6% 18% 39% 22% 2	800
	11th to 20th	Night	15% 6% 20% 34% 21% 4	791
	21st to end	Night	9% 5% 19% 39% 23% 5%	800
May	1st to 10th	Night	11% 4 16% 40% 25% 4	725
	11th to 20th	Night	12% 2 13% 39% 30% 5%	522
	21st to end	Night	15% 5% 14% 36% 26% 4	433
June	1st to 10th	Night	10% 2 13% 43% 30% 3	400
	11th to 20th	Night	5% 1 12% 51% 29% 2	400
	21st to end	Night	6% 4 10% 36% 41% 3	400
July	1st to 10th	Night	6% 2 6% 33% 49% 3	390
	11th to 20th	Night	6% 4 37% 42% 11%	552
	21st to end	Night	11% 2 8% 24% 45% 10%	657
August	1st to 10th	Night	14% 2 8% 29% 38% 9%	597
	11th to 20th	Night	9% 1 10% 34% 39% 7%	723
	21st to end	Night	7% 9% 35% 41% 9%	880
September	1st to 10th	Night	8% 1 9% 31% 41% 10%	800
	11th to 20th	Night	12% 2 13% 32% 34% 8%	800
	21st to end	Night	6% 2 10% 31% 40% 11%	800
October	1st to 10th	Night	8% 2 9% 37% 36% 8%	772
	11th to 20th	Night	11% 3 10% 31% 37% 9%	760
	21st to end	Night	9% 3 10% 35% 31% 11%	836
November	1st to 10th	Night	10% 3 13% 32% 33% 8%	780
	11th to 20th	Night	13% 3 12% 36% 29% 7%	800
	21st to end	Night	13% 2 14% 38% 26% 6%	800
December	1st to 10th	Night	16% 4 14% 36% 25% 6%	800
	11th to 20th	Night	25% 4 12% 35% 20% 4	800
	21st to end	Night	22% 5% 22% 31% 18% 2	880

Page generated at 2025-09-30 22:39:08. Version ac07

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