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 2025-03-12.

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	30% 14% 4 8% 5% 5% 4 3 5% 6% 16%	112237
	Night	34% 15% 4 8% 5% 4 4 3 4 4 16%	115424
Transparency	Day	67% 6% 11% 11% 4 1	111980
	Night	69% 6% 10% 10% 4 1	115327
Seeing	Day	35% 7% 19% 23% 12% 4	16526
	Night	43% 6% 13% 19% 15% 4	24391

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	42% 20% 4 9% 4 3 3 2 3 2 6%	3179
	11th to 20th	Day	33% 14% 3 8% 4 4 3 2 3 5% 20%	3159
	21st to end	Day	44% 16% 4 7% 4 3 2 2 3 4 11%	4000
February	1st to 10th	Day	40% 13% 4 7% 4 4 3 2 4 4 14%	1634
	11th to 20th	Day	33% 15% 3 6% 4 3 4 3 3 4 22%	1755
	21st to end	Day	33% 18% 4 8% 5% 4 3 3 2 3 16%	1583
March	1st to 10th	Day	30% 15% 3 9% 6% 5% 4 4 4 4 15%	3226
	11th to 20th	Day	34% 19% 4 9% 5% 4 4 3 3 3 12%	3503
	21st to end	Day	24% 14% 4 8% 6% 5% 4 3 5% 6% 20%	3825
April	1st to 10th	Day	35% 16% 5% 10% 6% 4 4 3 4 3 10%	2349
	11th to 20th	Day	31% 16% 3 8% 5% 5% 5% 2 4 4 18%	2438
	21st to end	Day	34% 15% 3 8% 5% 4 4 3 4 5% 15%	2470
May	1st to 10th	Day	28% 11% 3 6% 5% 4 4 4 5% 7% 23%	4267
	11th to 20th	Day	27% 15% 4 8% 5% 5% 4 4 7% 8% 14%	4696
	21st to end	Day	31% 13% 4 9% 5% 5% 5% 3 5% 5% 14%	5161
June	1st to 10th	Day	32% 14% 4 9% 5% 4 4 3 5% 5% 14%	2850
	11th to 20th	Day	32% 15% 4 9% 6% 5% 4 4 5% 4 11%	2850
	21st to end	Day	32% 12% 4 8% 5% 5% 5% 3 5% 6% 14%	2850
July	1st to 10th	Day	15% 10% 3 7% 5% 5% 4 4 6% 7% 34%	4740
	11th to 20th	Day	16% 14% 5% 12% 8% 6% 5% 5% 6% 8% 14%	4740
	21st to end	Day	28% 14% 4 9% 6% 5% 4 4 5% 7% 13%	5053
August	1st to 10th	Day	13% 8% 3 7% 4 5% 5% 5% 7% 10% 34%	4386
	11th to 20th	Day	20% 14% 5% 9% 5% 5% 5% 4 5% 7% 20%	4026
	21st to end	Day	24% 9% 4 8% 6% 6% 6% 4 7% 9% 18%	4521
September	1st to 10th	Day	25% 9% 4 9% 5% 5% 4 4 6% 9% 21%	2356
	11th to 20th	Day	23% 10% 3 8% 5% 4 5% 5% 5% 9% 23%	2090
	21st to end	Day	32% 11% 4 8% 4 4 4 4 5% 6% 17%	2090
October	1st to 10th	Day	36% 11% 3 7% 4 4 5% 4 5% 5% 15%	3315
	11th to 20th	Day	38% 11% 4 8% 5% 4 5% 4 5% 5% 10%	2980
	21st to end	Day	29% 11% 3 10% 6% 6% 5% 5% 6% 4 17%	3101
November	1st to 10th	Day	41% 13% 4 8% 4 4 4 2 4 5% 11%	1636
	11th to 20th	Day	41% 16% 3 7% 6% 4 3 2 3 4 12%	1575
	21st to end	Day	42% 16% 4 7% 5% 3 2 2 3 4 12%	1400
December	1st to 10th	Day	33% 17% 4 13% 6% 5% 3 3 2 2 12%	2290
	11th to 20th	Day	49% 14% 4 7% 4 3 3 2 3 3 8%	2645
	21st to end	Day	46% 17% 4 11% 5% 3 3 2 2 2 5%	3498
January	1st to 10th	Night	41% 20% 5% 8% 5% 3 3 2 3 2 8%	6766
	11th to 20th	Night	31% 14% 3 9% 5% 5% 4 4 5% 6% 16%	6739
	21st to end	Night	49% 17% 4 6% 3 2 2 1 2 2 12%	6974
February	1st to 10th	Night	43% 13% 4 7% 4 3 3 2 3 3 15%	2765
	11th to 20th	Night	33% 17% 3 6% 4 3 3 2 3 4 23%	2532
	21st to end	Night	31% 18% 3 7% 6% 3 4 2 3 3 19%	2045
March	1st to 10th	Night	31% 16% 4 9% 5% 4 5% 3 4 5% 14%	3858
	11th to 20th	Night	30% 19% 4 11% 5% 4 4 3 4 3 13%	3504
	21st to end	Night	28% 16% 4 9% 5% 4 3 2 3 5% 22%	3828
April	1st to 10th	Night	35% 17% 4 9% 5% 4 3 3 4 3 15%	1827
	11th to 20th	Night	31% 17% 3 7% 5% 3 4 3 4 5% 18%	1691
	21st to end	Night	33% 16% 4 8% 4 3 4 3 3 3 18%	1710
May	1st to 10th	Night	19% 11% 3 8% 6% 6% 6% 4 6% 8% 23%	2669
	11th to 20th	Night	23% 17% 4 10% 6% 5% 4 4 5% 5% 16%	2199
	21st to end	Night	34% 15% 4 7% 4 4 4 3 4 4 18%	2414
June	1st to 10th	Night	32% 14% 2 7% 4 3 4 3 5% 5% 21%	1330
	11th to 20th	Night	34% 12% 4 8% 5% 5% 4 3 4 6% 15%	1330
	21st to end	Night	30% 15% 3 8% 4 4 4 3 5% 6% 18%	1330
July	1st to 10th	Night	14% 12% 3 6% 4 4 3 2 4 7% 42%	2220
	11th to 20th	Night	16% 19% 5% 11% 7% 7% 5% 4 5% 5% 17%	2220
	21st to end	Night	27% 19% 4 9% 5% 5% 3 3 4 5% 15%	2499
August	1st to 10th	Night	13% 14% 4 8% 3 3 4 3 5% 7% 35%	2553
	11th to 20th	Night	19% 12% 4 9% 5% 6% 5% 3 6% 8% 25%	2812
	21st to end	Night	23% 11% 4 10% 7% 5% 5% 3 5% 5% 21%	3146
September	1st to 10th	Night	22% 9% 4 9% 4 4 4 4 7% 7% 25%	1821
	11th to 20th	Night	22% 11% 4 7% 5% 4 3 3 4 6% 30%	2090
	21st to end	Night	35% 11% 3 8% 4 5% 5% 3 4 5% 17%	2090
October	1st to 10th	Night	43% 9% 3 7% 4 4 3 3 4 5% 15%	3664
	11th to 20th	Night	41% 11% 3 8% 5% 4 4 3 4 5% 11%	3951
	21st to end	Night	33% 11% 3 8% 5% 4 4 2 4 5% 21%	4458
November	1st to 10th	Night	40% 14% 4 9% 6% 4 3 3 3 3 11%	2659
	11th to 20th	Night	41% 16% 3 7% 5% 3 3 2 3 4 13%	2822
	21st to end	Night	42% 14% 4 7% 4 3 3 2 3 4 13%	3000
December	1st to 10th	Night	39% 16% 4 9% 5% 4 3 2 3 3 13%	4890
	11th to 20th	Night	44% 16% 4 8% 4 3 3 2 3 3 10%	5560
	21st to end	Night	42% 18% 5% 10% 5% 3 3 2 3 3 6%	7458

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	85% 4 4 5% 2	3179
	11th to 20th	Day	65% 5% 10% 13% 5% 1	3180
	21st to end	Day	78% 4 6% 7% 4	4000
February	1st to 10th	Day	76% 3 4 7% 8% 2	1634
	11th to 20th	Day	68% 4 9% 13% 5% 2	1764
	21st to end	Day	74% 4 5% 8% 5% 2	1583
March	1st to 10th	Day	72% 4 8% 9% 6% 1	3226
	11th to 20th	Day	75% 5% 6% 10% 3 1	3503
	21st to end	Day	64% 8% 12% 13% 3	3825
April	1st to 10th	Day	79% 5% 8% 6% 3	2349
	11th to 20th	Day	70% 4 8% 10% 7% 1	2438
	21st to end	Day	73% 6% 8% 9% 3 1	2470
May	1st to 10th	Day	58% 7% 17% 15% 2	4267
	11th to 20th	Day	65% 8% 17% 8% 2	4699
	21st to end	Day	71% 6% 10% 10% 3	5162
June	1st to 10th	Day	72% 7% 11% 7% 2 1	2850
	11th to 20th	Day	76% 6% 11% 6% 1	2850
	21st to end	Day	68% 8% 12% 10% 2	2850
July	1st to 10th	Day	42% 8% 13% 17% 14% 5%	4666
	11th to 20th	Day	52% 11% 22% 13% 1	4659
	21st to end	Day	66% 8% 14% 10% 2	4893
August	1st to 10th	Day	35% 8% 11% 32% 15%	4369
	11th to 20th	Day	62% 8% 17% 10% 2 1	4065
	21st to end	Day	55% 10% 19% 14% 1	4520
September	1st to 10th	Day	58% 7% 16% 13% 4 1	2356
	11th to 20th	Day	57% 7% 13% 15% 8%	2090
	21st to end	Day	66% 6% 12% 10% 5%	2090
October	1st to 10th	Day	70% 6% 12% 9% 3 1	3315
	11th to 20th	Day	74% 6% 9% 9% 2	2980
	21st to end	Day	67% 8% 8% 10% 7%	3101
November	1st to 10th	Day	77% 4 8% 6% 4	1636
	11th to 20th	Day	78% 4 7% 6% 4 1	1575
	21st to end	Day	79% 4 6% 6% 5% 1	1400
December	1st to 10th	Day	79% 4 7% 6% 2 1	2290
	11th to 20th	Day	83% 3 6% 4 2 1	2648
	21st to end	Day	87% 4 4 4 1 1	3498
January	1st to 10th	Night	85% 3 4 6% 2	6766
	11th to 20th	Night	66% 7% 12% 11% 3 1	6780
	21st to end	Night	80% 3 6% 7% 4	6974
February	1st to 10th	Night	74% 4 5% 7% 7% 2	2765
	11th to 20th	Night	67% 3 8% 12% 8% 2	2534
	21st to end	Night	71% 4 7% 11% 4 3	2045
March	1st to 10th	Night	69% 7% 8% 11% 4	3858
	11th to 20th	Night	73% 5% 7% 11% 4 1	3505
	21st to end	Night	68% 6% 8% 11% 7%	3828
April	1st to 10th	Night	74% 5% 9% 8% 4	1827
	11th to 20th	Night	68% 5% 8% 10% 9% 1	1691
	21st to end	Night	71% 4 7% 13% 4 1	1710
May	1st to 10th	Night	50% 11% 19% 17% 3	2669
	11th to 20th	Night	64% 8% 18% 8% 2	2199
	21st to end	Night	69% 6% 11% 11% 3	2414
June	1st to 10th	Night	64% 7% 14% 12% 2 1	1330
	11th to 20th	Night	66% 10% 14% 9% 2	1330
	21st to end	Night	64% 6% 16% 12% 3	1330
July	1st to 10th	Night	41% 7% 12% 20% 16% 3	2185
	11th to 20th	Night	53% 15% 18% 11% 2	2178
	21st to end	Night	65% 9% 13% 9% 3	2420
August	1st to 10th	Night	34% 12% 16% 23% 15%	2545
	11th to 20th	Night	52% 11% 21% 13% 3 1	2839
	21st to end	Night	55% 9% 18% 16% 2	3146
September	1st to 10th	Night	51% 10% 19% 14% 4 2	1821
	11th to 20th	Night	51% 6% 14% 17% 10%	2090
	21st to end	Night	64% 8% 10% 12% 4 1	2090
October	1st to 10th	Night	71% 5% 12% 9% 2 1	3664
	11th to 20th	Night	75% 5% 10% 9% 2	3951
	21st to end	Night	66% 5% 9% 12% 6% 1	4458
November	1st to 10th	Night	77% 5% 6% 7% 4 1	2659
	11th to 20th	Night	77% 4 7% 6% 4 1	2822
	21st to end	Night	76% 4 7% 8% 3 2	3000
December	1st to 10th	Night	76% 4 8% 7% 4 1	4890
	11th to 20th	Night	80% 4 6% 7% 3 1	5556
	21st to end	Night	85% 5% 5% 3 1 1	7458

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% 18% 11% 5% 3	200
	11th to 20th	Day	58% 8% 15% 13% 6% 1	200
	21st to end	Day	51% 10% 12% 14% 10% 3	436
February	1st to 10th	Day	45% 10% 19% 18% 5% 2	400
	11th to 20th	Day	41% 13% 18% 14% 9% 4	395
	21st to end	Day	48% 14% 18% 14% 7% 1	331
March	1st to 10th	Day	41% 9% 19% 16% 12% 3	400
	11th to 20th	Day	46% 9% 17% 15% 10% 3	465
	21st to end	Day	41% 8% 21% 19% 8% 2	506
April	1st to 10th	Day	44% 11% 20% 16% 7% 2	460
	11th to 20th	Day	35% 12% 19% 17% 13% 4	452
	21st to end	Day	40% 9% 22% 17% 10% 2	460
May	1st to 10th	Day	31% 6% 24% 26% 9% 4	501
	11th to 20th	Day	28% 8% 24% 24% 13% 4	675
	21st to end	Day	27% 8% 27% 27% 9% 1	735
June	1st to 10th	Day	41% 7% 21% 20% 10% 2	680
	11th to 20th	Day	31% 6% 24% 26% 11% 2	680
	21st to end	Day	30% 6% 21% 28% 12% 3	680
July	1st to 10th	Day	22% 4 21% 32% 16% 5%	671
	11th to 20th	Day	22% 4 19% 36% 15% 4	668
	21st to end	Day	23% 3 20% 32% 17% 5%	739
August	1st to 10th	Day	19% 3 14% 36% 21% 7%	665
	11th to 20th	Day	23% 4 18% 36% 14% 5%	477
	21st to end	Day	30% 4 22% 27% 14% 3	539
September	1st to 10th	Day	31% 5% 19% 23% 17% 5%	490
	11th to 20th	Day	31% 7% 17% 23% 16% 6%	490
	21st to end	Day	38% 8% 17% 22% 11% 6%	490
October	1st to 10th	Day	39% 7% 18% 18% 13% 5%	490
	11th to 20th	Day	41% 10% 18% 18% 8% 5%	434
	21st to end	Day	39% 13% 22% 12% 9% 5%	429
November	1st to 10th	Day	46% 11% 17% 16% 6% 4	268
	11th to 20th	Day	43% 8% 19% 14% 11% 6%	200
	21st to end	Day	50% 11% 9% 14% 12% 5%	200
December	1st to 10th	Day	45% 9% 16% 17% 7% 7%	200
	11th to 20th	Day	51% 10% 16% 11% 8% 6%	200
	21st to end	Day	51% 10% 13% 20% 5% 1	220
January	1st to 10th	Night	57% 10% 14% 10% 8% 2	900
	11th to 20th	Night	55% 8% 12% 14% 11% 1	900
	21st to end	Night	54% 7% 10% 14% 12% 3	990
February	1st to 10th	Night	52% 7% 14% 18% 9% 1	883
	11th to 20th	Night	44% 8% 15% 16% 13% 3	784
	21st to end	Night	46% 10% 15% 17% 10% 3	660
March	1st to 10th	Night	43% 9% 14% 16% 16% 3	800
	11th to 20th	Night	48% 8% 14% 18% 11% 2	766
	21st to end	Night	47% 5% 14% 19% 12% 3	836
April	1st to 10th	Night	49% 6% 12% 16% 14% 3	570
	11th to 20th	Night	40% 7% 18% 18% 14% 4	563
	21st to end	Night	42% 6% 14% 22% 13% 2	570
May	1st to 10th	Night	34% 4 11% 24% 23% 5%	512
	11th to 20th	Night	31% 4 11% 27% 23% 4	374
	21st to end	Night	33% 5% 12% 27% 19% 4	411
June	1st to 10th	Night	38% 5% 11% 20% 21% 5%	380
	11th to 20th	Night	36% 3 11% 25% 19% 6%	380
	21st to end	Night	37% 4 13% 22% 18% 6%	380
July	1st to 10th	Night	29% 2 11% 29% 20% 9%	370
	11th to 20th	Night	27% 3 8% 29% 26% 7%	368
	21st to end	Night	28% 4 8% 25% 24% 11%	465
August	1st to 10th	Night	23% 2 5% 25% 32% 14%	567
	11th to 20th	Night	24% 3 9% 27% 28% 9%	564
	21st to end	Night	34% 2 8% 28% 23% 4	627
September	1st to 10th	Night	27% 2 11% 26% 26% 7%	613
	11th to 20th	Night	28% 2 12% 21% 24% 12%	760
	21st to end	Night	40% 5% 11% 23% 17% 4	760
October	1st to 10th	Night	41% 5% 14% 20% 14% 6%	760
	11th to 20th	Night	45% 4 13% 19% 14% 4	760
	21st to end	Night	43% 8% 16% 18% 14% 2	836
November	1st to 10th	Night	49% 9% 15% 17% 9% 1	780
	11th to 20th	Night	51% 8% 13% 17% 9% 2	812
	21st to end	Night	52% 8% 12% 15% 11% 2	900
December	1st to 10th	Night	44% 9% 13% 20% 12% 2	900
	11th to 20th	Night	46% 8% 18% 17% 8% 3	900
	21st to end	Night	52% 9% 16% 16% 6% 1	990

Page generated at 2025-03-09 05:08:44. Version ac07

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