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 2024-11-05.

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	7% 6% 2 5% 3 3 3 3 4 6% 58%	105038
	Night	9% 9% 3 7% 3 3 3 3 4 6% 49%	102764
Transparency	Day	27% 4 8% 18% 24% 19%	104796
	Night	35% 5% 9% 17% 19% 15%	102596
Seeing	Day	10% 9% 25% 32% 19% 5%	15410
	Night	13% 4 13% 35% 29% 6%	24729

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% 7% 3 7% 4 3 4 3 6% 7% 38%	2698
	11th to 20th	Day	17% 10% 4 9% 6% 5% 4 3 4 5% 33%	2860
	21st to end	Day	21% 10% 3 8% 5% 4 4 4 6% 7% 27%	3146
February	1st to 10th	Day	16% 8% 3 5% 4 3 3 3 5% 6% 45%	1710
	11th to 20th	Day	13% 8% 3 6% 4 4 3 3 5% 7% 42%	1754
	21st to end	Day	13% 7% 2 6% 5% 4 3 3 4 6% 47%	1570
March	1st to 10th	Day	12% 11% 5% 11% 6% 6% 6% 4 5% 7% 27%	3376
	11th to 20th	Day	10% 5% 2 5% 3 3 3 3 4 7% 53%	3490
	21st to end	Day	11% 9% 3 7% 5% 5% 6% 5% 7% 9% 34%	3884
April	1st to 10th	Day	12% 8% 2 6% 4 4 3 4 5% 7% 45%	2280
	11th to 20th	Day	9% 6% 1 5% 3 3 3 3 5% 6% 54%	2249
	21st to end	Day	5% 5% 2 4 3 3 3 3 6% 9% 57%	2280
May	1st to 10th	Day	6% 3 1 3 2 1 2 2 4 5% 70%	3914
	11th to 20th	Day	3 4 2 3 2 2 2 2 4 6% 71%	4179
	21st to end	Day	3 8% 2 4 2 2 2 2 3 5% 65%	4813
June	1st to 10th	Day	5% 6% 2 4 3 3 3 3 5% 7% 59%	2660
	11th to 20th	Day	2 3 1 3 2 2 2 2 4 6% 73%	2660
	21st to end	Day	3 4 2 3 2 2 2 2 3 5% 71%	2660
July	1st to 10th	Day	1 3 1 3 1 1 2 2 2 4 80%	4420
	11th to 20th	Day	1 3 2 4 2 3 2 3 4 5% 72%	4148
	21st to end	Day	4 2 4 2 2 2 2 4 6% 71%	4455
August	1st to 10th	Day	2 4 1 5% 3 4 3 3 5% 8% 62%	4091
	11th to 20th	Day	4 2 4 2 2 2 2 4 5% 73%	3821
	21st to end	Day	1 4 2 4 2 3 2 2 4 5% 70%	4180
September	1st to 10th	Day	2 3 1 3 2 2 2 1 3 4 76%	2280
	11th to 20th	Day	3 6% 2 4 3 3 3 2 5% 7% 61%	2280
	21st to end	Day	3 3 1 3 2 2 2 2 3 6% 74%	2018
October	1st to 10th	Day	2 3 1 3 2 2 3 2 4 6% 72%	3180
	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 5% 3 3 4 4 5% 9% 54%	1858
	11th to 20th	Day	9% 8% 3 6% 4 4 3 3 4 6% 49%	1710
	21st to end	Day	11% 6% 2 5% 2 3 3 4 6% 6% 53%	1563
December	1st to 10th	Day	13% 7% 3 7% 4 3 3 4 5% 6% 45%	1520
	11th to 20th	Day	23% 10% 3 8% 5% 3 4 3 5% 5% 31%	1928
	21st to end	Day	16% 11% 3 7% 5% 4 4 4 5% 8% 34%	2794
January	1st to 10th	Night	14% 8% 2 6% 4 4 4 3 5% 6% 44%	4249
	11th to 20th	Night	18% 10% 3 8% 5% 5% 5% 4 5% 5% 31%	4110
	21st to end	Night	21% 14% 4 7% 4 4 4 3 4 5% 30%	4521
February	1st to 10th	Night	17% 7% 2 5% 3 3 3 3 5% 5% 48%	2470
	11th to 20th	Night	12% 8% 3 6% 5% 4 4 3 5% 7% 43%	2312
	21st to end	Night	12% 8% 2 5% 4 3 3 3 4 6% 50%	1884
March	1st to 10th	Night	16% 14% 4 10% 5% 5% 4 4 5% 5% 31%	3495
	11th to 20th	Night	12% 8% 5% 6% 4 3 3 2 4 5% 49%	3490
	21st to end	Night	14% 14% 3 8% 5% 5% 4 3 5% 5% 34%	3767
April	1st to 10th	Night	10% 9% 3 6% 4 3 3 3 6% 8% 45%	1900
	11th to 20th	Night	10% 7% 3 4 3 4 3 3 5% 6% 52%	1879
	21st to end	Night	6% 4 2 4 3 4 4 3 6% 9% 56%	1900
May	1st to 10th	Night	6% 8% 2 4 2 3 2 2 4 4 65%	3057
	11th to 20th	Night	3 8% 4 10% 5% 3 3 3 5% 6% 51%	2716
	21st to end	Night	4 14% 3 4 3 2 3 2 3 5% 57%	2762
June	1st to 10th	Night	6% 8% 3 5% 3 3 3 3 5% 7% 53%	1520
	11th to 20th	Night	1 4 2 5% 2 3 2 3 5% 7% 66%	1520
	21st to end	Night	2 9% 2 5% 2 3 2 2 3 6% 63%	1520
July	1st to 10th	Night	1 5% 3 4 2 2 3 2 3 4 73%	2540
	11th to 20th	Night	1 7% 7% 8% 3 2 3 3 4 6% 56%	2816
	21st to end	Night	1 18% 7% 7% 2 3 3 3 6% 6% 43%	3109
August	1st to 10th	Night	3 8% 5% 21% 5% 4 5% 3 5% 6% 33%	2848
	11th to 20th	Night	11% 4 6% 3 2 3 3 4 6% 58%	3024
	21st to end	Night	2 11% 5% 9% 3 2 2 2 4 5% 55%	3498
September	1st to 10th	Night	1 8% 4 4 2 3 3 2 4 6% 63%	1900
	11th to 20th	Night	4 10% 4 8% 3 3 4 2 4 6% 51%	1900
	21st to end	Night	2 5% 2 5% 2 2 2 2 4 6% 67%	2159
October	1st to 10th	Night	3 3 1 4 2 2 2 2 5% 6% 69%	3800
	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	9% 6% 2 4 3 4 4 4 5% 7% 53%	2318
	11th to 20th	Night	10% 7% 2 6% 3 3 3 2 6% 8% 50%	2470
	21st to end	Night	9% 5% 1 4 3 3 3 3 4 7% 56%	2614
December	1st to 10th	Night	14% 7% 3 6% 3 4 3 3 4 6% 47%	2660
	11th to 20th	Night	26% 10% 2 5% 4 3 4 3 5% 5% 32%	3276
	21st to end	Night	17% 9% 3 8% 5% 5% 4 4 5% 6% 34%	4862

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	43% 7% 12% 28% 7% 3	2698
	11th to 20th	Day	51% 7% 11% 14% 10% 7%	2860
	21st to end	Day	54% 7% 8% 14% 12% 4	3146
February	1st to 10th	Day	39% 5% 8% 20% 20% 8%	1710
	11th to 20th	Day	40% 5% 10% 18% 21% 6%	1763
	21st to end	Day	37% 6% 9% 24% 19% 6%	1570
March	1st to 10th	Day	52% 9% 8% 15% 13% 3	3376
	11th to 20th	Day	31% 5% 12% 28% 14% 11%	3490
	21st to end	Day	41% 9% 16% 22% 10% 3	3884
April	1st to 10th	Day	38% 6% 13% 21% 19% 4	2280
	11th to 20th	Day	30% 5% 14% 25% 21% 5%	2252
	21st to end	Day	24% 5% 14% 29% 24% 5%	2280
May	1st to 10th	Day	17% 4 10% 30% 25% 14%	3914
	11th to 20th	Day	16% 3 5% 26% 39% 11%	4182
	21st to end	Day	22% 4 8% 32% 32% 3	4814
June	1st to 10th	Day	25% 4 11% 27% 25% 9%	2660
	11th to 20th	Day	14% 2 8% 22% 37% 16%	2660
	21st to end	Day	18% 1 4 13% 32% 32%	2660
July	1st to 10th	Day	11% 1 2 5% 14% 66%	4351
	11th to 20th	Day	17% 1 4 6% 14% 57%	4077
	21st to end	Day	16% 1 5% 6% 31% 42%	4315
August	1st to 10th	Day	22% 1 5% 9% 28% 34%	4076
	11th to 20th	Day	15% 2 4 8% 45% 27%	3859
	21st to end	Day	19% 2 3 11% 35% 30%	4179
September	1st to 10th	Day	15% 1 3 9% 31% 41%	2280
	11th to 20th	Day	24% 3 5% 22% 31% 16%	2280
	21st to end	Day	14% 2 5% 22% 33% 23%	2018
October	1st to 10th	Day	16% 2 5% 12% 27% 39%	3180
	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% 24% 21% 12%	1858
	11th to 20th	Day	35% 5% 9% 22% 22% 6%	1710
	21st to end	Day	31% 4 10% 19% 27% 9%	1563
December	1st to 10th	Day	38% 7% 9% 19% 18% 9%	1520
	11th to 20th	Day	53% 6% 9% 14% 14% 4	1928
	21st to end	Day	48% 7% 12% 19% 12% 3	2794
January	1st to 10th	Night	39% 9% 19% 23% 7% 3	4249
	11th to 20th	Night	53% 8% 11% 12% 9% 7%	4110
	21st to end	Night	57% 6% 9% 13% 11% 6%	4521
February	1st to 10th	Night	39% 4 8% 20% 18% 10%	2470
	11th to 20th	Night	39% 5% 10% 18% 20% 8%	2314
	21st to end	Night	35% 5% 12% 28% 16% 4	1884
March	1st to 10th	Night	50% 10% 11% 14% 12% 3	3495
	11th to 20th	Night	35% 6% 10% 24% 14% 11%	3490
	21st to end	Night	47% 10% 12% 18% 10% 3	3767
April	1st to 10th	Night	36% 7% 14% 22% 17% 4	1900
	11th to 20th	Night	32% 5% 12% 26% 18% 7%	1879
	21st to end	Night	24% 7% 13% 31% 20% 5%	1900
May	1st to 10th	Night	22% 6% 9% 27% 25% 11%	3057
	11th to 20th	Night	27% 3 9% 25% 27% 9%	2717
	21st to end	Night	27% 4 12% 28% 26% 2	2762
June	1st to 10th	Night	28% 6% 11% 26% 22% 9%	1520
	11th to 20th	Night	17% 4 9% 20% 33% 16%	1520
	21st to end	Night	23% 1 5% 14% 29% 27%	1520
July	1st to 10th	Night	15% 1 5% 5% 14% 61%	2500
	11th to 20th	Night	28% 1 6% 7% 13% 46%	2764
	21st to end	Night	37% 1 7% 7% 20% 28%	3011
August	1st to 10th	Night	47% 1 9% 7% 20% 16%	2839
	11th to 20th	Night	27% 1 6% 7% 37% 22%	3052
	21st to end	Night	31% 2 6% 9% 28% 24%	3498
September	1st to 10th	Night	22% 2 4 7% 26% 38%	1900
	11th to 20th	Night	32% 3 7% 19% 25% 14%	1900
	21st to end	Night	19% 3 5% 20% 30% 23%	2159
October	1st to 10th	Night	17% 2 5% 11% 22% 42%	3800
	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	31% 5% 11% 23% 18% 12%	2318
	11th to 20th	Night	33% 6% 10% 24% 19% 8%	2470
	21st to end	Night	29% 4 11% 21% 24% 11%	2614
December	1st to 10th	Night	38% 7% 9% 18% 16% 12%	2660
	11th to 20th	Night	53% 5% 9% 14% 15% 3	3276
	21st to end	Night	47% 9% 14% 15% 11% 4	4862

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	24% 12% 25% 22% 13% 4	274
	11th to 20th	Day	19% 8% 22% 29% 19% 3	370
	21st to end	Day	16% 6% 24% 30% 19% 4	407
February	1st to 10th	Day	19% 11% 22% 25% 20% 4	370
	11th to 20th	Day	19% 14% 29% 24% 13% 2	407
	21st to end	Day	13% 14% 27% 34% 11% 2	382
March	1st to 10th	Day	20% 16% 29% 22% 10% 2	460
	11th to 20th	Day	18% 17% 23% 24% 14% 4	460
	21st to end	Day	16% 15% 35% 22% 11% 1	506
April	1st to 10th	Day	17% 14% 31% 24% 10% 4	460
	11th to 20th	Day	15% 16% 31% 25% 10% 4	452
	21st to end	Day	8% 18% 30% 27% 10% 6%	460
May	1st to 10th	Day	10% 14% 32% 30% 12% 2	463
	11th to 20th	Day	11% 12% 29% 31% 14% 2	488
	21st to end	Day	12% 11% 28% 31% 14% 5%	529
June	1st to 10th	Day	7% 11% 30% 32% 16% 4	490
	11th to 20th	Day	3 10% 30% 39% 15% 3	490
	21st to end	Day	3 7% 28% 38% 19% 5%	490
July	1st to 10th	Day	4 4 21% 46% 20% 5%	486
	11th to 20th	Day	1 2 22% 43% 28% 4	484
	21st to end	Day	3 2 21% 42% 24% 8%	531
August	1st to 10th	Day	5% 5% 22% 40% 21% 7%	487
	11th to 20th	Day	4 3 23% 42% 20% 8%	477
	21st to end	Day	4 3 21% 38% 26% 8%	539
September	1st to 10th	Day	4 4 18% 41% 26% 9%	490
	11th to 20th	Day	6% 10% 25% 33% 19% 7%	490
	21st to end	Day	3 7% 22% 33% 28% 7%	490
October	1st to 10th	Day	4 7% 26% 32% 22% 9%	490
	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% 27% 7%	430
	11th to 20th	Day	7% 9% 15% 34% 26% 10%	270
	21st to end	Day	10% 5% 23% 30% 28% 3	201
December	1st to 10th	Day	16% 12% 20% 22% 23% 7%	180
	11th to 20th	Day	16% 11% 20% 26% 23% 5%	180
	21st to end	Day	22% 10% 25% 21% 19% 3	198
January	1st to 10th	Night	19% 5% 13% 34% 22% 7%	760
	11th to 20th	Night	16% 5% 12% 36% 26% 5%	760
	21st to end	Night	20% 4 12% 37% 22% 5%	836
February	1st to 10th	Night	19% 4 14% 36% 23% 4	760
	11th to 20th	Night	14% 6% 19% 35% 22% 4	744
	21st to end	Night	14% 7% 16% 41% 19% 3	628
March	1st to 10th	Night	20% 7% 18% 31% 22% 3	760
	11th to 20th	Night	20% 6% 14% 34% 23% 4	760
	21st to end	Night	17% 7% 18% 37% 18% 2	836
April	1st to 10th	Night	14% 5% 18% 40% 22% 2	760
	11th to 20th	Night	14% 7% 20% 33% 22% 5%	751
	21st to end	Night	8% 5% 19% 39% 23% 5%	760
May	1st to 10th	Night	11% 4 16% 40% 24% 4	689
	11th to 20th	Night	12% 2 13% 37% 31% 5%	496
	21st to end	Night	15% 5% 14% 36% 26% 5%	411
June	1st to 10th	Night	11% 2 13% 42% 29% 3	380
	11th to 20th	Night	6% 1 12% 50% 30% 2	380
	21st to end	Night	6% 4 10% 36% 41% 3	380
July	1st to 10th	Night	6% 2 5% 32% 51% 4	370
	11th to 20th	Night	6% 4 37% 41% 11%	523
	21st to end	Night	8% 1 8% 25% 47% 11%	624
August	1st to 10th	Night	14% 1 8% 29% 39% 8%	567
	11th to 20th	Night	8% 1 10% 34% 40% 8%	686
	21st to end	Night	7% 9% 34% 41% 9%	836
September	1st to 10th	Night	7% 1 8% 32% 43% 10%	760
	11th to 20th	Night	11% 3 13% 32% 34% 7%	760
	21st to end	Night	5% 2 10% 31% 41% 12%	760
October	1st to 10th	Night	8% 2 9% 37% 37% 8%	760
	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% 33% 32% 9%	760
	11th to 20th	Night	13% 2 11% 36% 31% 7%	760
	21st to end	Night	12% 2 14% 38% 27% 6%	760
December	1st to 10th	Night	17% 4 14% 36% 24% 5%	760
	11th to 20th	Night	25% 4 12% 35% 20% 5%	760
	21st to end	Night	21% 5% 21% 32% 19% 2	836

Page generated at 2024-11-02 23:11:23. Version ac07

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