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 2024-02-20.

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	33% 13% 4 7% 6% 3 4 4 4 5% 15%	77825
	Night	37% 14% 4 7% 5% 3 4 3 3 4 16%	81324
Transparency	Day	50% 38% 6% 5% 2	77529
	Night	51% 36% 6% 5% 2	81200
Seeing	Day	35% 7% 20% 22% 12% 4	15134
	Night	43% 6% 13% 19% 15% 4	23008

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% 20% 4 7% 5% 2 2 3 3 2 7%	2219
	11th to 20th	Day	37% 14% 3 6% 5% 2 3 3 3 5% 19%	2199
	21st to end	Day	53% 16% 3 6% 4 2 2 3 2 2 6%	2790
February	1st to 10th	Day	42% 13% 4 6% 5% 3 3 2 3 4 13%	1552
	11th to 20th	Day	34% 14% 4 6% 5% 3 5% 3 3 4 22%	1657
	21st to end	Day	33% 17% 4 7% 5% 3 4 3 2 3 18%	1420
March	1st to 10th	Day	35% 14% 3 7% 6% 3 4 4 3 5% 16%	1800
	11th to 20th	Day	36% 18% 5% 8% 7% 3 4 4 3 3 11%	1980
	21st to end	Day	35% 15% 5% 7% 6% 3 4 3 4 4 15%	2178
April	1st to 10th	Day	35% 16% 5% 9% 7% 3 4 4 3 4 10%	2225
	11th to 20th	Day	32% 14% 4 7% 6% 4 5% 3 4 4 16%	2308
	21st to end	Day	35% 13% 4 7% 6% 3 4 4 4 5% 15%	2340
May	1st to 10th	Day	30% 13% 4 7% 6% 3 4 6% 4 6% 17%	2430
	11th to 20th	Day	26% 12% 4 8% 6% 4 5% 4 5% 7% 18%	2656
	21st to end	Day	30% 13% 4 7% 6% 3 5% 4 5% 6% 16%	2917
June	1st to 10th	Day	33% 12% 4 7% 6% 3 5% 4 5% 6% 14%	2700
	11th to 20th	Day	34% 14% 5% 7% 7% 3 5% 5% 4 4 11%	2700
	21st to end	Day	31% 11% 4 8% 6% 4 5% 5% 5% 7% 15%	2700
July	1st to 10th	Day	25% 13% 5% 7% 6% 4 4 6% 6% 7% 18%	2700
	11th to 20th	Day	26% 13% 4 7% 6% 4 4 5% 6% 8% 17%	2700
	21st to end	Day	25% 12% 4 7% 5% 3 5% 5% 6% 8% 19%	2889
August	1st to 10th	Day	19% 10% 4 7% 6% 4 5% 5% 7% 9% 23%	2497
	11th to 20th	Day	21% 12% 5% 7% 7% 5% 5% 6% 6% 7% 20%	2298
	21st to end	Day	31% 11% 5% 8% 6% 4 5% 5% 5% 6% 13%	2574
September	1st to 10th	Day	27% 9% 4 7% 6% 3 4 4 6% 9% 20%	2233
	11th to 20th	Day	23% 10% 3 7% 6% 3 5% 5% 5% 8% 24%	1980
	21st to end	Day	31% 11% 4 7% 5% 3 5% 5% 5% 7% 18%	1980
October	1st to 10th	Day	31% 11% 3 6% 5% 3 5% 4 5% 6% 21%	1883
	11th to 20th	Day	36% 13% 4 7% 6% 4 4 4 5% 5% 12%	1701
	21st to end	Day	36% 10% 2 6% 7% 4 5% 5% 5% 4 16%	1782
November	1st to 10th	Day	42% 11% 5% 7% 5% 2 5% 3 4 5% 10%	1553
	11th to 20th	Day	42% 15% 3 7% 6% 3 3 3 2 4 12%	1497
	21st to end	Day	43% 14% 5% 7% 5% 2 3 3 3 4 12%	1330
December	1st to 10th	Day	33% 13% 4 8% 7% 3 4 4 3 3 18%	1330
	11th to 20th	Day	47% 12% 4 6% 5% 2 3 2 3 4 11%	1685
	21st to end	Day	48% 15% 3 8% 6% 2 3 3 3 2 6%	2442
January	1st to 10th	Night	42% 21% 5% 8% 5% 2 3 2 2 2 7%	4726
	11th to 20th	Night	34% 13% 3 8% 6% 3 4 4 4 5% 16%	4699
	21st to end	Night	56% 16% 3 5% 3 1 2 2 2 2 7%	4873
February	1st to 10th	Night	45% 12% 3 6% 4 3 3 3 3 3 14%	2627
	11th to 20th	Night	34% 16% 3 5% 5% 2 3 3 3 4 22%	2394
	21st to end	Night	32% 16% 4 6% 6% 2 4 3 2 4 20%	1834
March	1st to 10th	Night	33% 15% 4 8% 6% 3 4 3 4 4 17%	2160
	11th to 20th	Night	37% 17% 4 7% 5% 3 3 3 3 3 15%	1979
	21st to end	Night	37% 16% 4 7% 5% 2 3 2 3 3 16%	2178
April	1st to 10th	Night	35% 16% 4 8% 6% 3 3 3 4 3 14%	1731
	11th to 20th	Night	33% 16% 4 6% 5% 2 3 4 4 5% 17%	1601
	21st to end	Night	34% 14% 5% 8% 5% 2 4 4 3 4 19%	1620
May	1st to 10th	Night	29% 10% 4 6% 6% 3 5% 4 3 7% 22%	1527
	11th to 20th	Night	24% 15% 4 7% 7% 4 4 5% 4 6% 20%	1239
	21st to end	Night	28% 12% 5% 6% 6% 3 5% 6% 5% 5% 20%	1358
June	1st to 10th	Night	32% 13% 3 6% 5% 3 4 4 5% 5% 21%	1260
	11th to 20th	Night	36% 11% 4 7% 6% 3 5% 3 4 6% 16%	1260
	21st to end	Night	29% 14% 3 7% 5% 3 5% 3 5% 6% 20%	1260
July	1st to 10th	Night	24% 14% 3 7% 5% 3 4 3 4 5% 27%	1260
	11th to 20th	Night	25% 12% 4 7% 5% 3 5% 5% 5% 6% 23%	1260
	21st to end	Night	26% 16% 3 6% 5% 3 4 4 4 6% 24%	1425
August	1st to 10th	Night	21% 12% 4 7% 5% 3 4 5% 6% 7% 26%	1441
	11th to 20th	Night	18% 12% 3 7% 6% 4 5% 4 5% 8% 29%	1602
	21st to end	Night	30% 12% 4 7% 7% 3 5% 3 5% 6% 19%	1782
September	1st to 10th	Night	23% 9% 4 8% 6% 3 4 5% 7% 8% 25%	1724
	11th to 20th	Night	22% 11% 4 6% 6% 3 3 4 4 7% 30%	1980
	21st to end	Night	34% 10% 3 7% 6% 4 5% 4 4 5% 18%	1980
October	1st to 10th	Night	34% 10% 4 5% 5% 3 3 4 4 6% 22%	2076
	11th to 20th	Night	37% 10% 3 7% 7% 3 5% 4 4 5% 15%	2258
	21st to end	Night	37% 10% 4 7% 6% 3 4 3 3 5% 18%	2574
November	1st to 10th	Night	41% 13% 4 8% 7% 3 4 3 3 3 11%	2522
	11th to 20th	Night	42% 15% 4 6% 5% 2 4 3 3 4 13%	2680
	21st to end	Night	43% 13% 5% 6% 5% 2 3 3 3 5% 13%	2850
December	1st to 10th	Night	33% 13% 4 7% 6% 3 4 3 4 5% 18%	2850
	11th to 20th	Night	46% 14% 4 6% 4 2 3 3 3 3 13%	3520
	21st to end	Night	47% 17% 4 7% 5% 2 2 2 2 3 7%	5214

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	67% 28% 3 2 1	2219
	11th to 20th	Day	49% 27% 8% 8% 6% 1	2220
	21st to end	Day	71% 24% 2 2 1	2790
February	1st to 10th	Day	57% 34% 4 3 2	1552
	11th to 20th	Day	45% 37% 5% 9% 3 1	1666
	21st to end	Day	51% 37% 2 6% 2 1	1420
March	1st to 10th	Day	51% 38% 5% 4 1	1800
	11th to 20th	Day	53% 37% 3 3 2 1	1980
	21st to end	Day	51% 38% 5% 4 2	2178
April	1st to 10th	Day	53% 38% 5% 3 1	2225
	11th to 20th	Day	45% 37% 6% 6% 6%	2308
	21st to end	Day	49% 38% 4 6% 2 1	2340
May	1st to 10th	Day	48% 39% 7% 5% 1	2430
	11th to 20th	Day	47% 39% 8% 5% 1	2659
	21st to end	Day	48% 39% 7% 4 2	2918
June	1st to 10th	Day	48% 39% 6% 4 2 1	2700
	11th to 20th	Day	54% 39% 4 2 1	2700
	21st to end	Day	47% 41% 7% 4 2	2700
July	1st to 10th	Day	43% 41% 10% 5% 1	2626
	11th to 20th	Day	47% 41% 8% 4	2619
	21st to end	Day	41% 42% 10% 6% 1	2728
August	1st to 10th	Day	41% 42% 9% 7% 1	2480
	11th to 20th	Day	46% 38% 7% 5% 3 1	2298
	21st to end	Day	49% 40% 7% 3 1	2574
September	1st to 10th	Day	40% 41% 11% 6% 1 1	2233
	11th to 20th	Day	39% 39% 9% 9% 4	1980
	21st to end	Day	47% 39% 6% 5% 2	1980
October	1st to 10th	Day	44% 40% 8% 7% 2	1883
	11th to 20th	Day	52% 39% 4 4 1	1701
	21st to end	Day	46% 40% 4 5% 4	1782
November	1st to 10th	Day	49% 42% 4 3 2	1553
	11th to 20th	Day	51% 40% 4 3 2	1497
	21st to end	Day	49% 41% 5% 4 1	1330
December	1st to 10th	Day	46% 41% 5% 4 2 1	1330
	11th to 20th	Day	56% 33% 4 4 2 1	1688
	21st to end	Day	68% 27% 2 3	2442
January	1st to 10th	Night	68% 27% 3 2	4726
	11th to 20th	Night	52% 29% 8% 6% 3 1	4740
	21st to end	Night	71% 22% 2 3 1	4873
February	1st to 10th	Night	56% 35% 3 4 2	2627
	11th to 20th	Night	45% 35% 4 10% 4 1	2396
	21st to end	Night	50% 37% 4 6% 2 2	1834
March	1st to 10th	Night	49% 37% 7% 5% 2	2160
	11th to 20th	Night	52% 37% 4 4 3 1	1980
	21st to end	Night	51% 37% 6% 4 3	2178
April	1st to 10th	Night	49% 38% 6% 4 2	1731
	11th to 20th	Night	46% 36% 4 7% 7%	1601
	21st to end	Night	48% 37% 4 8% 2 1	1620
May	1st to 10th	Night	45% 39% 8% 5% 3	1527
	11th to 20th	Night	45% 39% 8% 6% 1	1239
	21st to end	Night	45% 41% 7% 6% 1	1358
June	1st to 10th	Night	40% 40% 8% 9% 2 1	1260
	11th to 20th	Night	50% 39% 7% 3 1	1260
	21st to end	Night	42% 40% 10% 6% 2	1260
July	1st to 10th	Night	39% 40% 10% 9% 1	1225
	11th to 20th	Night	42% 41% 10% 6%	1218
	21st to end	Night	40% 39% 10% 8% 3	1346
August	1st to 10th	Night	38% 42% 12% 7% 1	1433
	11th to 20th	Night	36% 40% 12% 9% 2 1	1602
	21st to end	Night	44% 42% 8% 6% 1	1782
September	1st to 10th	Night	33% 42% 13% 10% 2 1	1724
	11th to 20th	Night	36% 39% 9% 10% 6%	1980
	21st to end	Night	46% 41% 5% 7% 1	1980
October	1st to 10th	Night	43% 39% 9% 7% 2	2076
	11th to 20th	Night	49% 40% 6% 5%	2258
	21st to end	Night	46% 39% 5% 6% 3 1	2574
November	1st to 10th	Night	50% 42% 2 3 3	2522
	11th to 20th	Night	51% 40% 4 3 2	2680
	21st to end	Night	50% 41% 5% 3 1	2850
December	1st to 10th	Night	46% 41% 5% 3 4	2850
	11th to 20th	Night	55% 33% 4 5% 2	3516
	21st to end	Night	66% 28% 4 2	5214

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	52% 11% 19% 9% 6% 3	180
	11th to 20th	Day	59% 8% 13% 12% 6% 1	180
	21st to end	Day	52% 10% 12% 14% 9% 2	403
February	1st to 10th	Day	49% 11% 19% 15% 5% 1	370
	11th to 20th	Day	42% 13% 20% 14% 8% 4	363
	21st to end	Day	46% 14% 18% 14% 7% 1	280
March	1st to 10th	Day	41% 9% 20% 15% 12% 3	340
	11th to 20th	Day	47% 9% 16% 15% 10% 3	420
	21st to end	Day	44% 8% 22% 17% 8% 2	462
April	1st to 10th	Day	43% 10% 20% 17% 8% 2	420
	11th to 20th	Day	37% 11% 19% 16% 14% 4	412
	21st to end	Day	40% 9% 23% 15% 10% 2	420
May	1st to 10th	Day	32% 6% 24% 25% 9% 5%	459
	11th to 20th	Day	28% 7% 23% 25% 13% 4	625
	21st to end	Day	27% 6% 28% 28% 9% 2	680
June	1st to 10th	Day	41% 6% 21% 20% 10% 2	630
	11th to 20th	Day	32% 7% 24% 25% 10% 2	630
	21st to end	Day	28% 5% 22% 29% 13% 3	630
July	1st to 10th	Day	24% 4 20% 31% 16% 5%	621
	11th to 20th	Day	24% 4 20% 35% 13% 4	618
	21st to end	Day	22% 3 19% 33% 18% 5%	684
August	1st to 10th	Day	20% 3 14% 36% 19% 8%	616
	11th to 20th	Day	24% 4 18% 36% 13% 4	437
	21st to end	Day	31% 3 23% 26% 14% 3	495
September	1st to 10th	Day	33% 4 19% 22% 16% 5%	450
	11th to 20th	Day	32% 6% 17% 22% 16% 7%	450
	21st to end	Day	36% 7% 16% 23% 12% 6%	450
October	1st to 10th	Day	37% 6% 18% 19% 14% 6%	450
	11th to 20th	Day	42% 10% 18% 17% 8% 5%	400
	21st to end	Day	39% 12% 23% 12% 9% 5%	396
November	1st to 10th	Day	46% 10% 17% 16% 6% 4	245
	11th to 20th	Day	43% 7% 18% 13% 12% 7%	180
	21st to end	Day	51% 11% 9% 14% 11% 5%	180
December	1st to 10th	Day	42% 9% 17% 17% 7% 8%	180
	11th to 20th	Day	48% 9% 17% 12% 8% 6%	180
	21st to end	Day	50% 9% 14% 20% 6% 2	198
January	1st to 10th	Night	57% 9% 13% 10% 8% 2	850
	11th to 20th	Night	56% 8% 11% 13% 11% 1	850
	21st to end	Night	55% 6% 10% 13% 12% 3	935
February	1st to 10th	Night	54% 7% 14% 16% 8% 1	835
	11th to 20th	Night	45% 8% 15% 16% 12% 3	742
	21st to end	Night	46% 9% 14% 16% 10% 4	592
March	1st to 10th	Night	43% 9% 14% 16% 16% 3	720
	11th to 20th	Night	48% 8% 12% 19% 11% 2	720
	21st to end	Night	48% 5% 14% 18% 12% 3	792
April	1st to 10th	Night	49% 6% 11% 17% 14% 3	540
	11th to 20th	Night	42% 6% 17% 17% 14% 4	533
	21st to end	Night	42% 6% 15% 22% 13% 2	540
May	1st to 10th	Night	35% 4 10% 24% 22% 5%	486
	11th to 20th	Night	31% 4 10% 27% 24% 4	354
	21st to end	Night	32% 4 13% 26% 20% 4	389
June	1st to 10th	Night	38% 5% 11% 19% 22% 6%	360
	11th to 20th	Night	37% 3 11% 25% 19% 6%	360
	21st to end	Night	37% 4 13% 21% 19% 6%	360
July	1st to 10th	Night	30% 2 11% 27% 20% 9%	350
	11th to 20th	Night	28% 3 8% 28% 26% 7%	348
	21st to end	Night	27% 3 7% 25% 25% 12%	440
August	1st to 10th	Night	24% 1 4 25% 31% 15%	537
	11th to 20th	Night	24% 3 9% 27% 28% 9%	534
	21st to end	Night	35% 2 8% 27% 24% 4	594
September	1st to 10th	Night	28% 2 11% 26% 26% 7%	580
	11th to 20th	Night	28% 2 12% 20% 25% 13%	720
	21st to end	Night	39% 5% 11% 23% 18% 4	720
October	1st to 10th	Night	40% 4 15% 20% 14% 6%	720
	11th to 20th	Night	45% 4 12% 20% 14% 5%	720
	21st to end	Night	44% 7% 16% 18% 14% 2	792
November	1st to 10th	Night	49% 10% 14% 17% 9% 1	740
	11th to 20th	Night	51% 8% 13% 17% 9% 3	770
	21st to end	Night	53% 7% 12% 14% 11% 3	850
December	1st to 10th	Night	42% 9% 13% 21% 12% 2	850
	11th to 20th	Night	46% 7% 18% 17% 8% 3	850
	21st to end	Night	52% 8% 16% 16% 6% 1	935

Page generated at 2024-02-17 03:28:39. Version ac07

Copyright 2024 Attilla Danko