San Jose
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the San Jose clear sky chart from 2005-11-06 to 2025-11-03.

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	5% 4 2 4 3 3 3 3 4 6% 62%	135195
	Night	7% 7% 2 5% 5% 3 3 3 4 6% 54%	132312
Transparency	Day	22% 5% 7% 18% 26% 22%	130137
	Night	28% 6% 8% 17% 22% 21%	128355
Seeing	Day	7% 12% 24% 30% 20% 7%	16748
	Night	8% 6% 15% 34% 30% 7%	26087

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	13% 7% 2 5% 3 4 3 4 5% 8% 45%	3887
	11th to 20th	Day	10% 6% 2 7% 4 4 3 3 4 4 52%	4100
	21st to end	Day	14% 8% 3 6% 5% 4 4 3 5% 6% 41%	4510
February	1st to 10th	Day	14% 8% 2 6% 4 4 4 3 5% 6% 44%	1800
	11th to 20th	Day	12% 6% 3 7% 4 6% 4 3 6% 8% 41%	1868
	21st to end	Day	11% 8% 3 6% 5% 4 4 3 4 6% 46%	1650
March	1st to 10th	Day	11% 8% 3 9% 7% 6% 6% 5% 6% 7% 30%	4838
	11th to 20th	Day	10% 6% 2 6% 4 4 4 4 6% 8% 46%	5000
	21st to end	Day	9% 9% 3 8% 6% 6% 6% 5% 6% 8% 35%	5603
April	1st to 10th	Day	10% 8% 2 7% 5% 4 3 4 5% 7% 45%	2400
	11th to 20th	Day	9% 6% 2 5% 3 3 3 3 5% 6% 55%	2369
	21st to end	Day	6% 6% 2 4 3 3 3 3 5% 8% 57%	2400
May	1st to 10th	Day	5% 3 1 2 2 2 2 2 3 5% 73%	5525
	11th to 20th	Day	3 2 1 2 2 2 1 2 2 5% 78%	5988
	21st to end	Day	2 5% 1 3 3 2 2 2 3 5% 73%	6902
June	1st to 10th	Day	4 5% 1 4 3 2 2 3 5% 7% 64%	2800
	11th to 20th	Day	2 2 1 1 1 2 2 2 4 6% 78%	2800
	21st to end	Day	1 2 1 2 2 2 1 2 3 4 80%	2800
July	1st to 10th	Day	1 1 2 1 1 1 2 3 87%	6319
	11th to 20th	Day	1 1 2 2 2 2 2 3 3 81%	5925
	21st to end	Day	4 1 4 2 2 1 2 3 4 77%	6402
August	1st to 10th	Day	1 1 1 3 4 2 3 2 3 5% 76%	5777
	11th to 20th	Day	2 2 1 3 2 2 2 2 2 3 80%	5456
	21st to end	Day	1 1 3 3 2 2 2 3 5% 76%	5984
September	1st to 10th	Day	2 2 1 2 2 2 2 2 2 4 80%	2400
	11th to 20th	Day	2 3 1 4 3 3 3 3 4 7% 67%	2400
	21st to end	Day	3 2 1 2 2 2 2 2 3 5% 76%	2131
October	1st to 10th	Day	2 2 1 3 3 2 2 3 4 6% 74%	3280
	11th to 20th	Day	3 3 2 4 3 3 3 2 3 6% 69%	3261
	21st to end	Day	2 3 1 4 3 4 4 4 5% 8% 62%	3558
November	1st to 10th	Day	6% 5% 1 6% 5% 4 4 3 5% 7% 55%	1935
	11th to 20th	Day	7% 5% 2 6% 5% 4 4 3 6% 7% 51%	1800
	21st to end	Day	10% 6% 1 5% 4 3 4 3 5% 8% 52%	1675
December	1st to 10th	Day	5% 4 1 5% 5% 4 4 3 6% 9% 54%	2620
	11th to 20th	Day	14% 8% 3 9% 6% 6% 5% 4 6% 8% 31%	3028
	21st to end	Day	14% 12% 4 10% 6% 6% 5% 4 6% 7% 26%	4004
January	1st to 10th	Night	9% 8% 2 6% 7% 4 4 4 7% 7% 43%	6067
	11th to 20th	Night	10% 7% 3 7% 8% 4 4 2 4 4 47%	5880
	21st to end	Night	13% 9% 4 8% 6% 4 3 3 4 5% 41%	6468
February	1st to 10th	Night	12% 8% 2 6% 5% 4 4 3 4 5% 47%	2600
	11th to 20th	Night	11% 9% 2 6% 5% 4 4 4 6% 7% 42%	2417
	21st to end	Night	9% 7% 2 7% 4 4 4 3 5% 6% 48%	1980
March	1st to 10th	Night	17% 13% 4 8% 5% 4 4 4 4 5% 32%	5053
	11th to 20th	Night	14% 9% 2 6% 5% 3 3 3 4 6% 44%	5000
	21st to end	Night	13% 14% 3 8% 6% 4 4 4 5% 6% 33%	5370
April	1st to 10th	Night	10% 9% 3 5% 5% 3 4 4 6% 8% 44%	2000
	11th to 20th	Night	8% 7% 2 6% 4 3 3 3 5% 6% 52%	1979
	21st to end	Night	6% 6% 2 4 3 3 4 3 4 9% 55%	2000
May	1st to 10th	Night	4 7% 1 3 3 2 2 2 4 5% 65%	4454
	11th to 20th	Night	3 5% 1 3 3 3 3 3 4 6% 65%	3904
	21st to end	Night	2 6% 2 4 4 2 2 2 3 5% 67%	3972
June	1st to 10th	Night	4 5% 2 3 3 2 2 3 5% 8% 63%	1600
	11th to 20th	Night	1 3 1 3 2 1 2 2 4 7% 74%	1600
	21st to end	Night	1 3 2 3 2 2 2 2 3 4 77%	1600
July	1st to 10th	Night	2 2 2 1 1 1 2 3 85%	3640
	11th to 20th	Night	2 1 4 3 2 2 3 3 4 74%	4048
	21st to end	Night	12% 2 6% 5% 3 2 3 4 5% 57%	4472
August	1st to 10th	Night	1 3 1 4 9% 3 2 2 3 5% 67%	4035
	11th to 20th	Night	4 1 3 4 2 2 3 3 4 73%	4408
	21st to end	Night	1 4 2 5% 6% 3 2 3 3 5% 66%	5015
September	1st to 10th	Night	2 3 1 3 3 2 3 2 4 7% 72%	2000
	11th to 20th	Night	4 5% 2 6% 4 2 3 2 4 6% 63%	2000
	21st to end	Night	3 3 1 4 3 2 2 2 3 6% 72%	2264
October	1st to 10th	Night	3 2 1 2 2 2 2 2 3 6% 75%	3920
	11th to 20th	Night	3 7% 2 6% 5% 3 3 2 4 5% 60%	3897
	21st to end	Night	4 5% 2 5% 4 3 3 3 5% 6% 59%	4253
November	1st to 10th	Night	6% 5% 2 5% 4 3 3 3 6% 7% 55%	2408
	11th to 20th	Night	8% 6% 2 5% 5% 3 3 3 6% 8% 51%	2600
	21st to end	Night	8% 5% 2 5% 4 4 4 3 4 7% 54%	2720
December	1st to 10th	Night	6% 4 2 5% 3 4 4 4 5% 6% 58%	4560
	11th to 20th	Night	16% 9% 4 8% 8% 5% 5% 4 7% 8% 27%	5176
	21st to end	Night	16% 11% 4 9% 7% 7% 5% 5% 7% 6% 23%	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	34% 8% 16% 20% 17% 5%	3887
	11th to 20th	Day	34% 5% 7% 12% 23% 19%	4100
	21st to end	Day	42% 5% 7% 12% 19% 15%	4510
February	1st to 10th	Day	41% 5% 7% 18% 19% 11%	1800
	11th to 20th	Day	40% 7% 9% 17% 21% 8%	1877
	21st to end	Day	41% 5% 7% 21% 19% 7%	1650
March	1st to 10th	Day	47% 10% 7% 14% 18% 3	4838
	11th to 20th	Day	33% 8% 12% 26% 12% 8%	5000
	21st to end	Day	41% 12% 14% 21% 10% 2	5603
April	1st to 10th	Day	37% 7% 12% 20% 20% 5%	2400
	11th to 20th	Day	29% 7% 12% 23% 22% 7%	2372
	21st to end	Day	25% 6% 11% 30% 24% 5%	2400
May	1st to 10th	Day	14% 3 7% 27% 33% 15%	5525
	11th to 20th	Day	11% 3 5% 34% 36% 10%	5991
	21st to end	Day	13% 3 6% 29% 40% 8%	6904
June	1st to 10th	Day	20% 4 9% 25% 31% 11%	2800
	11th to 20th	Day	9% 3 7% 21% 38% 23%	2800
	21st to end	Day	9% 2 3 13% 30% 43%	2800
July	1st to 10th	Day	3 1 1 7% 21% 67%	6251
	11th to 20th	Day	5% 1 3 8% 15% 68%	5854
	21st to end	Day	9% 1 4 17% 29% 40%	6217
August	1st to 10th	Day	10% 2 3 12% 27% 45%	4206
	11th to 20th	Day	5% 1 3 8% 38% 45%	3949
	21st to end	Day	8% 2 3 12% 34% 41%	4311
September	1st to 10th	Day	8% 2 2 11% 31% 46%	2400
	11th to 20th	Day	15% 3 5% 23% 33% 20%	2400
	21st to end	Day	11% 3 4 20% 37% 24%	2131
October	1st to 10th	Day	12% 3 5% 11% 28% 40%	3280
	11th to 20th	Day	16% 3 8% 22% 30% 21%	3261
	21st to end	Day	18% 5% 8% 20% 31% 19%	3558
November	1st to 10th	Day	28% 7% 8% 20% 25% 13%	1935
	11th to 20th	Day	30% 7% 10% 20% 23% 10%	1800
	21st to end	Day	31% 4 9% 15% 28% 11%	1675
December	1st to 10th	Day	26% 6% 6% 20% 34% 7%	2620
	11th to 20th	Day	47% 9% 8% 14% 15% 7%	3028
	21st to end	Day	53% 9% 10% 16% 10% 3	4004
January	1st to 10th	Night	31% 12% 15% 20% 16% 6%	6067
	11th to 20th	Night	36% 6% 12% 12% 21% 14%	5880
	21st to end	Night	41% 6% 7% 12% 15% 19%	6468
February	1st to 10th	Night	39% 4 7% 16% 19% 14%	2600
	11th to 20th	Night	39% 6% 8% 16% 21% 9%	2419
	21st to end	Night	35% 6% 9% 24% 18% 8%	1980
March	1st to 10th	Night	50% 9% 8% 14% 16% 4	5053
	11th to 20th	Night	38% 7% 8% 22% 17% 8%	5000
	21st to end	Night	47% 9% 10% 18% 11% 4	5370
April	1st to 10th	Night	35% 7% 12% 21% 19% 6%	2000
	11th to 20th	Night	32% 5% 10% 25% 19% 9%	1979
	21st to end	Night	24% 8% 9% 28% 24% 7%	2000
May	1st to 10th	Night	16% 6% 7% 21% 30% 19%	4451
	11th to 20th	Night	16% 5% 7% 31% 30% 11%	3905
	21st to end	Night	14% 4 6% 28% 35% 13%	3972
June	1st to 10th	Night	18% 5% 9% 23% 31% 13%	1600
	11th to 20th	Night	10% 3 8% 14% 32% 33%	1600
	21st to end	Night	10% 1 3 15% 28% 43%	1600
July	1st to 10th	Night	3 1 1 8% 19% 68%	3600
	11th to 20th	Night	7% 1 2 9% 17% 63%	3996
	21st to end	Night	20% 3 4 14% 26% 33%	4345
August	1st to 10th	Night	14% 1 4 16% 23% 41%	2929
	11th to 20th	Night	8% 1 3 9% 31% 47%	3183
	21st to end	Night	13% 2 6% 10% 31% 38%	3608
September	1st to 10th	Night	10% 2 3 10% 22% 52%	2000
	11th to 20th	Night	19% 3 5% 21% 30% 22%	2000
	21st to end	Night	13% 2 5% 18% 32% 30%	2264
October	1st to 10th	Night	12% 3 5% 11% 17% 52%	3920
	11th to 20th	Night	21% 4 9% 19% 24% 23%	3897
	21st to end	Night	22% 5% 7% 18% 25% 23%	4253
November	1st to 10th	Night	26% 6% 8% 20% 23% 17%	2408
	11th to 20th	Night	29% 6% 9% 20% 24% 11%	2600
	21st to end	Night	31% 5% 8% 17% 22% 17%	2720
December	1st to 10th	Night	26% 6% 6% 20% 28% 15%	4560
	11th to 20th	Night	47% 8% 10% 13% 17% 6%	5176
	21st to end	Night	54% 11% 11% 12% 8% 4	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	24% 18% 25% 19% 14% 1	305
	11th to 20th	Day	13% 15% 28% 28% 13% 4	400
	21st to end	Day	13% 14% 28% 24% 16% 4	440
February	1st to 10th	Day	16% 21% 25% 23% 12% 4	400
	11th to 20th	Day	17% 20% 29% 24% 9% 2	452
	21st to end	Day	12% 19% 31% 24% 12% 2	414
March	1st to 10th	Day	17% 19% 30% 24% 8% 1	500
	11th to 20th	Day	17% 19% 26% 22% 13% 3	500
	21st to end	Day	13% 23% 32% 22% 9% 2	550
April	1st to 10th	Day	11% 19% 31% 24% 13% 3	500
	11th to 20th	Day	11% 19% 28% 24% 13% 4	492
	21st to end	Day	6% 20% 27% 28% 11% 7%	500
May	1st to 10th	Day	8% 16% 31% 27% 15% 4	503
	11th to 20th	Day	8% 13% 30% 26% 16% 6%	528
	21st to end	Day	6% 12% 25% 32% 17% 8%	573
June	1st to 10th	Day	4 11% 25% 30% 21% 8%	530
	11th to 20th	Day	2 11% 29% 34% 18% 6%	530
	21st to end	Day	2 5% 22% 36% 25% 11%	530
July	1st to 10th	Day	1 2 17% 39% 31% 10%	526
	11th to 20th	Day	1 1 12% 40% 33% 13%	524
	21st to end	Day	1 2 14% 38% 30% 15%	575
August	1st to 10th	Day	1 3 20% 37% 28% 11%	527
	11th to 20th	Day	1 4 18% 38% 24% 15%	517
	21st to end	Day	1 2 14% 38% 32% 14%	583
September	1st to 10th	Day	1 5% 17% 38% 28% 10%	530
	11th to 20th	Day	3 8% 22% 37% 20% 10%	530
	21st to end	Day	4 7% 19% 36% 27% 7%	530
October	1st to 10th	Day	3 10% 22% 30% 23% 12%	530
	11th to 20th	Day	4 12% 20% 28% 27% 9%	530
	21st to end	Day	6% 10% 25% 31% 23% 6%	583
November	1st to 10th	Day	7% 12% 23% 24% 26% 7%	459
	11th to 20th	Day	5% 11% 23% 29% 24% 8%	300
	21st to end	Day	10% 9% 22% 33% 21% 5%	237
December	1st to 10th	Day	7% 15% 22% 28% 21% 9%	200
	11th to 20th	Day	9% 21% 20% 23% 19% 9%	200
	21st to end	Day	12% 25% 23% 24% 12% 4	220
January	1st to 10th	Night	15% 8% 18% 31% 22% 6%	800
	11th to 20th	Night	12% 8% 16% 36% 21% 8%	800
	21st to end	Night	16% 6% 15% 34% 23% 6%	880
February	1st to 10th	Night	14% 10% 16% 34% 22% 5%	800
	11th to 20th	Night	15% 10% 18% 34% 17% 5%	784
	21st to end	Night	12% 10% 21% 36% 18% 3	660
March	1st to 10th	Night	15% 12% 20% 32% 19% 3	800
	11th to 20th	Night	15% 9% 20% 30% 22% 4	800
	21st to end	Night	11% 11% 22% 34% 19% 2	880
April	1st to 10th	Night	12% 7% 24% 33% 22% 3	800
	11th to 20th	Night	10% 12% 20% 33% 21% 5%	791
	21st to end	Night	6% 9% 25% 31% 23% 7%	800
May	1st to 10th	Night	5% 5% 26% 34% 25% 5%	759
	11th to 20th	Night	6% 3 14% 37% 35% 6%	526
	21st to end	Night	4 5% 17% 38% 33% 4	433
June	1st to 10th	Night	4 4 15% 42% 30% 5%	400
	11th to 20th	Night	1 4 12% 50% 33% 1	400
	21st to end	Night	1 1 9% 40% 46% 3	400
July	1st to 10th	Night	1 5% 37% 53% 4	390
	11th to 20th	Night	1 1 1 29% 54% 13%	558
	21st to end	Night	2 3 2 25% 58% 11%	657
August	1st to 10th	Night	4 1 3 34% 50% 8%	597
	11th to 20th	Night	1 1 9% 36% 46% 7%	744
	21st to end	Night	2 2 6% 31% 49% 10%	880
September	1st to 10th	Night	2 1 9% 32% 44% 11%	800
	11th to 20th	Night	4 4 10% 36% 38% 8%	800
	21st to end	Night	4 4 10% 33% 40% 10%	800
October	1st to 10th	Night	5% 3 12% 36% 36% 10%	800
	11th to 20th	Night	6% 5% 12% 31% 36% 11%	800
	21st to end	Night	6% 5% 11% 33% 30% 14%	880
November	1st to 10th	Night	6% 5% 15% 31% 33% 9%	788
	11th to 20th	Night	9% 3 18% 32% 30% 8%	800
	21st to end	Night	10% 5% 16% 38% 24% 7%	800
December	1st to 10th	Night	11% 7% 17% 35% 25% 6%	800
	11th to 20th	Night	17% 6% 19% 33% 20% 5%	800
	21st to end	Night	15% 12% 22% 30% 18% 3	880

Page generated at 2025-10-30 23:02:37. Version ac07

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