San Josef Bay
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the San Josef Bay clear sky chart from 2005-11-06 to 2026-07-16.

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	29% 14% 4 9% 5% 5% 4 4 5% 6% 16%	142202
	Night	33% 15% 4 9% 5% 4 4 3 4 4 16%	139213
Transparency	Day	67% 7% 12% 11% 4 1	141901
	Night	69% 6% 10% 10% 4 1	139096
Seeing	Day	35% 8% 20% 23% 12% 3	18426
	Night	42% 7% 13% 19% 15% 4	26016

By Month

Percentage Cloud Cover NW (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	44% 21% 4 9% 4 3 3 2 3 2 5%	4125
	11th to 20th	Day	32% 12% 3 7% 3 3 3 2 4 6% 25%	4119
	21st to end	Day	43% 17% 4 8% 3 3 2 2 3 4 10%	5181
February	1st to 10th	Day	40% 14% 4 7% 4 4 3 2 4 4 14%	1716
	11th to 20th	Day	32% 15% 3 7% 4 4 4 3 3 4 22%	1845
	21st to end	Day	33% 17% 4 8% 5% 4 3 3 2 3 16%	1660
March	1st to 10th	Day	33% 15% 4 9% 6% 5% 4 4 4 4 13%	4596
	11th to 20th	Day	29% 20% 5% 12% 6% 5% 4 3 3 3 9%	5110
	21st to end	Day	28% 15% 5% 9% 6% 6% 5% 3 4 5% 15%	5607
April	1st to 10th	Day	33% 16% 5% 10% 6% 4 4 3 4 4 11%	2597
	11th to 20th	Day	30% 15% 3 8% 5% 5% 5% 2 4 5% 18%	2698
	21st to end	Day	32% 14% 3 7% 5% 5% 4 3 5% 6% 16%	2730
May	1st to 10th	Day	23% 11% 3 9% 5% 4 5% 4 5% 6% 24%	7914
	11th to 20th	Day	25% 15% 5% 11% 6% 6% 5% 4 6% 7% 11%	8757
	21st to end	Day	29% 14% 3 8% 5% 5% 5% 3 6% 7% 16%	9649
June	1st to 10th	Day	31% 14% 4 9% 5% 4 4 3 5% 5% 15%	3150
	11th to 20th	Day	31% 15% 5% 9% 6% 5% 4 4 5% 5% 12%	3150
	21st to end	Day	31% 13% 4 9% 5% 5% 5% 3 5% 6% 14%	3150
July	1st to 10th	Day	16% 12% 4 9% 5% 6% 5% 4 6% 7% 26%	6930
	11th to 20th	Day	17% 14% 5% 12% 7% 7% 5% 5% 6% 8% 14%	6856
	21st to end	Day	20% 13% 5% 10% 6% 6% 6% 6% 6% 7% 14%	7251
August	1st to 10th	Day	13% 9% 3 7% 4 5% 5% 5% 7% 10% 34%	4525
	11th to 20th	Day	21% 15% 5% 9% 5% 5% 4 4 5% 6% 20%	4156
	21st to end	Day	23% 9% 4 8% 6% 6% 6% 4 7% 9% 18%	4664
September	1st to 10th	Day	25% 10% 4 9% 5% 5% 4 4 6% 9% 20%	2480
	11th to 20th	Day	23% 10% 3 8% 5% 4 5% 5% 5% 9% 24%	2200
	21st to end	Day	33% 11% 4 7% 4 4 4 4 5% 7% 17%	2200
October	1st to 10th	Day	36% 11% 3 7% 5% 4 5% 4 5% 6% 15%	3419
	11th to 20th	Day	38% 11% 4 7% 5% 4 5% 4 5% 6% 11%	3074
	21st to end	Day	29% 11% 3 10% 6% 5% 5% 5% 6% 5% 16%	3200
November	1st to 10th	Day	41% 12% 4 8% 4 4 4 2 4 5% 11%	1719
	11th to 20th	Day	41% 16% 3 8% 6% 4 4 2 3 4 11%	1654
	21st to end	Day	40% 16% 5% 8% 4 3 2 2 3 4 12%	1470
December	1st to 10th	Day	34% 16% 4 12% 6% 5% 3 3 2 2 11%	2360
	11th to 20th	Day	49% 14% 4 7% 4 3 3 2 3 3 8%	2715
	21st to end	Day	45% 17% 4 11% 5% 3 3 2 2 2 5%	3575
January	1st to 10th	Night	45% 19% 4 8% 4 3 3 2 3 2 7%	8770
	11th to 20th	Night	31% 12% 2 8% 4 4 4 3 4 6% 21%	8779
	21st to end	Night	45% 17% 4 7% 3 3 2 2 2 3 12%	9028
February	1st to 10th	Night	42% 13% 4 7% 4 3 3 2 3 3 15%	2903
	11th to 20th	Night	32% 16% 3 6% 4 4 3 2 3 4 23%	2662
	21st to end	Night	31% 18% 3 8% 5% 4 4 2 3 3 18%	2144
March	1st to 10th	Night	33% 17% 4 9% 5% 4 4 2 3 4 13%	5498
	11th to 20th	Night	25% 19% 5% 12% 6% 5% 5% 3 5% 4 12%	5109
	21st to end	Night	31% 16% 5% 9% 5% 4 4 2 4 4 16%	5610
April	1st to 10th	Night	33% 17% 4 8% 5% 4 3 3 4 3 16%	2019
	11th to 20th	Night	29% 17% 3 7% 5% 4 4 3 4 5% 19%	1871
	21st to end	Night	30% 15% 4 8% 4 3 4 3 4 4 21%	1890
May	1st to 10th	Night	20% 10% 3 9% 5% 5% 5% 3 5% 7% 29%	4995
	11th to 20th	Night	24% 17% 5% 9% 6% 5% 4 3 5% 5% 16%	4105
	21st to end	Night	29% 15% 4 7% 4 4 3 3 4 6% 20%	4522
June	1st to 10th	Night	30% 14% 3 8% 4 4 4 3 5% 5% 21%	1470
	11th to 20th	Night	33% 12% 4 8% 4 4 4 3 4 6% 17%	1470
	21st to end	Night	30% 15% 3 8% 5% 4 4 2 4 5% 19%	1470
July	1st to 10th	Night	18% 14% 3 8% 4 5% 4 3 4 6% 32%	3250
	11th to 20th	Night	18% 19% 4 10% 6% 6% 4 3 5% 4 21%	3215
	21st to end	Night	21% 21% 6% 11% 6% 5% 4 3 4 5% 15%	3595
August	1st to 10th	Night	14% 15% 4 8% 3 3 4 3 5% 6% 34%	2634
	11th to 20th	Night	19% 12% 3 9% 5% 5% 4 3 6% 7% 25%	2902
	21st to end	Night	23% 11% 4 10% 7% 5% 5% 3 5% 5% 22%	3245
September	1st to 10th	Night	22% 11% 4 10% 4 4 4 4 7% 7% 24%	1917
	11th to 20th	Night	23% 11% 4 7% 5% 4 3 3 5% 7% 30%	2200
	21st to end	Night	35% 11% 3 7% 5% 5% 5% 3 4 5% 17%	2200
October	1st to 10th	Night	42% 9% 3 7% 4 4 3 3 4 5% 15%	3780
	11th to 20th	Night	40% 11% 3 8% 5% 4 4 3 4 5% 12%	4077
	21st to end	Night	33% 11% 3 8% 5% 4 4 2 4 5% 20%	4601
November	1st to 10th	Night	40% 14% 4 10% 6% 4 3 3 3 3 11%	2796
	11th to 20th	Night	41% 16% 3 8% 5% 3 3 2 3 4 12%	2963
	21st to end	Night	41% 14% 4 7% 4 3 3 2 3 4 12%	3150
December	1st to 10th	Night	40% 16% 4 9% 4 3 3 2 3 3 13%	5040
	11th to 20th	Night	44% 16% 4 8% 4 3 3 2 3 3 10%	5710
	21st to end	Night	42% 18% 5% 10% 5% 3 3 2 3 3 6%	7623

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	87% 4 4 4 1	4125
	11th to 20th	Day	60% 5% 8% 13% 11% 4	4140
	21st to end	Day	78% 4 6% 8% 3	5182
February	1st to 10th	Day	77% 3 4 7% 7% 2	1716
	11th to 20th	Day	67% 4 9% 13% 5% 2	1854
	21st to end	Day	75% 5% 6% 8% 5% 2	1660
March	1st to 10th	Day	73% 5% 9% 8% 4 1	4596
	11th to 20th	Day	78% 5% 5% 9% 3 1	5110
	21st to end	Day	71% 7% 10% 10% 3	5607
April	1st to 10th	Day	78% 5% 8% 7% 2	2597
	11th to 20th	Day	69% 4 9% 11% 7% 1	2698
	21st to end	Day	69% 6% 10% 11% 3 1	2730
May	1st to 10th	Day	58% 6% 15% 18% 3	7914
	11th to 20th	Day	69% 8% 16% 6% 1	8760
	21st to end	Day	66% 7% 13% 12% 3	9650
June	1st to 10th	Day	71% 7% 11% 8% 3 1	3150
	11th to 20th	Day	74% 6% 12% 7% 1	3150
	21st to end	Day	68% 8% 12% 10% 2	3150
July	1st to 10th	Day	52% 9% 13% 13% 10% 3	6856
	11th to 20th	Day	52% 11% 21% 15% 1	6775
	21st to end	Day	58% 9% 19% 12% 1	7046
August	1st to 10th	Day	36% 8% 11% 31% 14%	4508
	11th to 20th	Day	62% 8% 17% 10% 2 1	4195
	21st to end	Day	54% 10% 19% 15% 2	4663
September	1st to 10th	Day	58% 8% 16% 13% 4 1	2480
	11th to 20th	Day	56% 7% 13% 16% 8%	2200
	21st to end	Day	67% 6% 12% 10% 5%	2200
October	1st to 10th	Day	69% 6% 12% 9% 3 1	3419
	11th to 20th	Day	73% 6% 9% 9% 3	3074
	21st to end	Day	67% 8% 8% 10% 7%	3200
November	1st to 10th	Day	77% 4 9% 5% 4	1719
	11th to 20th	Day	78% 4 6% 7% 4 1	1654
	21st to end	Day	79% 4 6% 6% 5% 1	1470
December	1st to 10th	Day	79% 4 7% 6% 2 1	2360
	11th to 20th	Day	84% 3 6% 4 2 1	2718
	21st to end	Day	87% 4 4 4 1 1	3575
January	1st to 10th	Night	85% 3 4 5% 2	8770
	11th to 20th	Night	61% 6% 10% 10% 12% 2	8820
	21st to end	Night	78% 3 6% 8% 4	9028
February	1st to 10th	Night	75% 4 5% 7% 7% 2	2903
	11th to 20th	Night	66% 3 9% 12% 8% 2	2664
	21st to end	Night	71% 5% 7% 10% 4 3	2144
March	1st to 10th	Night	73% 6% 7% 10% 3	5498
	11th to 20th	Night	72% 6% 7% 11% 4 1	5110
	21st to end	Night	73% 5% 7% 9% 5%	5610
April	1st to 10th	Night	73% 5% 9% 9% 4	2019
	11th to 20th	Night	67% 5% 9% 10% 8% 1	1871
	21st to end	Night	68% 4 10% 13% 3 1	1890
May	1st to 10th	Night	51% 8% 15% 20% 6%	4992
	11th to 20th	Night	67% 8% 15% 9% 1	4105
	21st to end	Night	65% 6% 16% 11% 2	4526
June	1st to 10th	Night	63% 7% 14% 12% 3 1	1470
	11th to 20th	Night	65% 9% 14% 10% 1	1470
	21st to end	Night	64% 6% 15% 13% 3	1470
July	1st to 10th	Night	49% 10% 13% 16% 11% 2	3215
	11th to 20th	Night	55% 12% 15% 16% 2	3173
	21st to end	Night	61% 12% 18% 7% 2	3495
August	1st to 10th	Night	35% 12% 16% 23% 14%	2626
	11th to 20th	Night	52% 11% 20% 14% 3 1	2929
	21st to end	Night	54% 9% 18% 16% 3	3245
September	1st to 10th	Night	52% 10% 19% 14% 4 2	1917
	11th to 20th	Night	52% 6% 15% 17% 10%	2200
	21st to end	Night	66% 8% 10% 11% 4 1	2200
October	1st to 10th	Night	70% 5% 12% 9% 3 1	3780
	11th to 20th	Night	74% 5% 10% 9% 2	4077
	21st to end	Night	67% 5% 9% 12% 6% 1	4601
November	1st to 10th	Night	78% 5% 7% 6% 4 1	2796
	11th to 20th	Night	78% 4 7% 6% 4 1	2963
	21st to end	Night	76% 4 7% 8% 4 2	3150
December	1st to 10th	Night	77% 4 8% 6% 4 1	5040
	11th to 20th	Night	81% 4 6% 6% 3 1	5706
	21st to end	Night	85% 5% 5% 4 1	7623

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	53% 11% 17% 11% 5% 3	220
	11th to 20th	Day	56% 7% 14% 13% 7% 3	220
	21st to end	Day	50% 10% 13% 14% 10% 3	469
February	1st to 10th	Day	47% 10% 19% 18% 5% 2	430
	11th to 20th	Day	39% 13% 19% 15% 10% 4	425
	21st to end	Day	48% 14% 17% 13% 6% 1	355
March	1st to 10th	Day	41% 10% 18% 16% 12% 2	430
	11th to 20th	Day	48% 9% 17% 14% 9% 3	540
	21st to end	Day	41% 9% 21% 19% 8% 2	594
April	1st to 10th	Day	41% 11% 20% 18% 9% 1	540
	11th to 20th	Day	34% 13% 19% 17% 13% 4	532
	21st to end	Day	35% 8% 25% 19% 11% 2	540
May	1st to 10th	Day	30% 6% 24% 26% 9% 4	585
	11th to 20th	Day	29% 9% 23% 24% 12% 3	775
	21st to end	Day	28% 9% 27% 26% 9% 1	845
June	1st to 10th	Day	38% 8% 22% 20% 10% 2	780
	11th to 20th	Day	29% 8% 25% 26% 10% 2	780
	21st to end	Day	30% 6% 21% 29% 12% 2	780
July	1st to 10th	Day	23% 5% 20% 33% 16% 4	771
	11th to 20th	Day	22% 5% 21% 35% 14% 4	744
	21st to end	Day	22% 3 20% 32% 19% 4	794
August	1st to 10th	Day	20% 3 14% 35% 21% 7%	714
	11th to 20th	Day	25% 5% 18% 36% 13% 4	517
	21st to end	Day	28% 4 21% 26% 17% 3	583
September	1st to 10th	Day	30% 5% 19% 23% 19% 5%	530
	11th to 20th	Day	31% 6% 17% 24% 16% 6%	530
	21st to end	Day	39% 8% 17% 21% 10% 5%	530
October	1st to 10th	Day	37% 7% 18% 19% 13% 5%	530
	11th to 20th	Day	41% 11% 18% 18% 8% 4	468
	21st to end	Day	39% 13% 22% 12% 9% 5%	462
November	1st to 10th	Day	47% 10% 18% 16% 5% 4	291
	11th to 20th	Day	44% 8% 18% 15% 10% 5%	220
	21st to end	Day	48% 12% 8% 18% 10% 4	220
December	1st to 10th	Day	47% 10% 15% 16% 6% 6%	220
	11th to 20th	Day	52% 10% 15% 11% 7% 5%	220
	21st to end	Day	51% 10% 13% 19% 5% 1	242
January	1st to 10th	Night	57% 10% 14% 10% 7% 2	950
	11th to 20th	Night	54% 8% 11% 14% 12% 1	950
	21st to end	Night	53% 7% 10% 14% 13% 3	1045
February	1st to 10th	Night	52% 7% 14% 17% 8% 1	931
	11th to 20th	Night	43% 8% 15% 17% 13% 3	824
	21st to end	Night	45% 11% 15% 17% 9% 3	692
March	1st to 10th	Night	43% 9% 14% 16% 15% 3	840
	11th to 20th	Night	48% 8% 14% 18% 10% 2	840
	21st to end	Night	46% 6% 14% 19% 12% 3	924
April	1st to 10th	Night	47% 7% 13% 17% 14% 3	630
	11th to 20th	Night	38% 7% 19% 18% 14% 4	623
	21st to end	Night	39% 6% 15% 24% 13% 2	630
May	1st to 10th	Night	33% 4 13% 25% 22% 5%	566
	11th to 20th	Night	31% 6% 13% 25% 21% 4	414
	21st to end	Night	33% 5% 13% 27% 19% 4	455
June	1st to 10th	Night	36% 5% 12% 21% 20% 5%	420
	11th to 20th	Night	35% 4 11% 26% 18% 6%	420
	21st to end	Night	37% 5% 14% 22% 18% 5%	420
July	1st to 10th	Night	30% 3 12% 28% 20% 8%	410
	11th to 20th	Night	27% 4 9% 29% 25% 7%	398
	21st to end	Night	27% 4 8% 24% 25% 12%	490
August	1st to 10th	Night	23% 2 6% 24% 31% 14%	597
	11th to 20th	Night	24% 4 10% 27% 26% 9%	594
	21st to end	Night	32% 3 8% 27% 25% 5%	660
September	1st to 10th	Night	27% 2 12% 26% 26% 7%	645
	11th to 20th	Night	28% 3 12% 21% 25% 12%	800
	21st to end	Night	41% 6% 12% 22% 16% 4	800
October	1st to 10th	Night	40% 5% 15% 21% 14% 6%	800
	11th to 20th	Night	44% 6% 13% 20% 13% 4	800
	21st to end	Night	44% 8% 16% 18% 13% 2	880
November	1st to 10th	Night	48% 10% 15% 16% 9% 1	820
	11th to 20th	Night	51% 9% 13% 16% 9% 2	853
	21st to end	Night	52% 7% 13% 16% 10% 2	950
December	1st to 10th	Night	45% 10% 13% 19% 11% 2	950
	11th to 20th	Night	47% 9% 17% 17% 7% 3	950
	21st to end	Night	52% 9% 16% 16% 6% 1	1045

Page generated at 2026-07-13 00:14:52. Version ac07

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