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 2024-11-06.

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	6% 5% 2 4 3 3 3 3 4 6% 61%	105025
	Night	7% 7% 2 5% 4 3 3 3 4 6% 54%	102793
Transparency	Day	23% 5% 7% 18% 24% 23%	104782
	Night	28% 6% 7% 17% 21% 22%	102626
Seeing	Day	7% 12% 24% 30% 20% 7%	15441
	Night	8% 6% 15% 34% 31% 7%	24795

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	16% 9% 3 6% 4 4 4 4 6% 7% 37%	2703
	11th to 20th	Day	14% 9% 4 10% 6% 5% 4 4 5% 6% 33%	2860
	21st to end	Day	19% 10% 4 7% 5% 4 5% 3 6% 7% 29%	3146
February	1st to 10th	Day	13% 8% 2 6% 4 4 4 3 5% 6% 44%	1710
	11th to 20th	Day	12% 5% 3 7% 4 6% 4 3 5% 8% 42%	1772
	21st to end	Day	12% 8% 3 7% 5% 4 3 3 4 6% 46%	1570
March	1st to 10th	Day	11% 10% 3 11% 7% 7% 6% 5% 6% 7% 26%	3367
	11th to 20th	Day	9% 6% 2 5% 4 4 3 3 5% 8% 52%	3490
	21st to end	Day	11% 10% 4 8% 6% 5% 6% 4 6% 7% 33%	3903
April	1st to 10th	Day	10% 8% 2 7% 5% 4 3 4 5% 7% 45%	2280
	11th to 20th	Day	10% 6% 2 5% 3 3 3 3 5% 6% 55%	2249
	21st to end	Day	6% 6% 2 3 3 3 3 3 5% 8% 58%	2280
May	1st to 10th	Day	6% 3 1 3 2 2 2 2 4 6% 69%	3859
	11th to 20th	Day	3 2 1 2 2 2 1 2 2 4 78%	4179
	21st to end	Day	3 5% 1 3 3 2 2 2 3 5% 70%	4813
June	1st to 10th	Day	4 5% 2 3 3 2 3 3 5% 7% 63%	2660
	11th to 20th	Day	2 2 1 1 1 2 2 2 4 6% 78%	2660
	21st to end	Day	1 3 1 2 2 2 1 2 3 4 80%	2660
July	1st to 10th	Day	1 1 1 1 1 1 2 3 87%	4420
	11th to 20th	Day	1 1 1 2 2 2 2 2 3 4 79%	4142
	21st to end	Day	2 1 3 2 1 1 2 3 4 81%	4455
August	1st to 10th	Day	1 2 1 3 5% 3 3 3 5% 6% 70%	4091
	11th to 20th	Day	1 2 2 2 2 2 2 4 84%	3790
	21st to end	Day	2 1 2 3 2 2 2 3 4 78%	4180
September	1st to 10th	Day	1 1 1 2 2 2 2 2 2 4 81%	2280
	11th to 20th	Day	2 3 1 4 3 3 3 3 4 7% 67%	2280
	21st to end	Day	3 2 1 2 2 2 2 2 3 5% 77%	2025
October	1st to 10th	Day	2 2 1 2 3 2 2 3 3 6% 74%	3180
	11th to 20th	Day	3 3 2 4 3 3 3 2 3 6% 69%	3161
	21st to end	Day	2 3 1 4 3 4 4 4 5% 8% 62%	3448
November	1st to 10th	Day	6% 5% 1 6% 4 4 4 3 5% 7% 54%	1868
	11th to 20th	Day	7% 6% 2 6% 5% 4 4 3 6% 7% 51%	1710
	21st to end	Day	9% 6% 1 5% 4 3 3 4 4 8% 53%	1592
December	1st to 10th	Day	8% 6% 2 7% 6% 5% 5% 3 5% 8% 45%	1520
	11th to 20th	Day	17% 9% 4 10% 5% 5% 4 3 5% 6% 32%	1928
	21st to end	Day	11% 11% 4 9% 5% 5% 5% 4 6% 7% 32%	2794
January	1st to 10th	Night	12% 10% 3 7% 5% 4 4 3 6% 7% 39%	4245
	11th to 20th	Night	15% 10% 4 10% 6% 5% 6% 3 5% 6% 31%	4110
	21st to end	Night	18% 12% 4 8% 5% 5% 4 4 4 5% 30%	4521
February	1st to 10th	Night	12% 8% 2 6% 4 4 4 3 4 5% 48%	2470
	11th to 20th	Night	10% 9% 2 6% 5% 4 4 4 6% 7% 43%	2293
	21st to end	Night	10% 7% 2 7% 4 4 4 3 5% 6% 48%	1884
March	1st to 10th	Night	14% 14% 4 9% 6% 5% 4 4 5% 5% 31%	3504
	11th to 20th	Night	10% 8% 2 5% 6% 3 3 2 4 6% 50%	3490
	21st to end	Night	14% 15% 3 7% 6% 4 4 4 5% 6% 32%	3748
April	1st to 10th	Night	10% 9% 3 5% 5% 3 4 4 6% 8% 44%	1900
	11th to 20th	Night	9% 7% 2 5% 4 3 3 3 5% 6% 52%	1879
	21st to end	Night	6% 6% 2 4 3 3 4 3 4 9% 56%	1900
May	1st to 10th	Night	4 9% 2 4 3 2 2 2 6% 6% 59%	3102
	11th to 20th	Night	3 4 1 3 3 3 2 3 4 7% 68%	2720
	21st to end	Night	3 5% 2 4 3 2 2 2 4 6% 67%	2762
June	1st to 10th	Night	4 5% 2 3 3 2 2 3 5% 8% 63%	1520
	11th to 20th	Night	1 3 1 3 2 1 2 2 4 7% 73%	1520
	21st to end	Night	1 3 2 3 2 2 2 2 3 4 76%	1520
July	1st to 10th	Night	2 1 2 1 1 2 1 2 3 85%	2540
	11th to 20th	Night	1 2 1 5% 3 2 2 2 3 5% 72%	2822
	21st to end	Night	7% 2 4 4 3 2 3 4 6% 65%	3109
August	1st to 10th	Night	2 4 1 4 10% 4 3 2 4 7% 60%	2848
	11th to 20th	Night	3 1 3 3 2 2 2 4 5% 76%	3056
	21st to end	Night	1 5% 2 4 5% 2 2 2 3 5% 69%	3498
September	1st to 10th	Night	1 3 1 3 3 2 3 2 4 7% 73%	1900
	11th to 20th	Night	4 5% 1 6% 3 2 3 2 4 6% 64%	1900
	21st to end	Night	3 2 1 3 3 2 2 2 3 6% 74%	2150
October	1st to 10th	Night	3 2 1 2 2 2 2 2 3 6% 75%	3800
	11th to 20th	Night	3 7% 2 6% 5% 3 3 2 4 5% 60%	3777
	21st to end	Night	4 5% 2 5% 4 3 3 3 5% 6% 59%	4121
November	1st to 10th	Night	7% 6% 2 5% 4 3 3 3 6% 8% 54%	2332
	11th to 20th	Night	8% 6% 2 5% 5% 3 3 3 6% 8% 50%	2470
	21st to end	Night	7% 5% 1 4 4 4 4 3 4 7% 56%	2584
December	1st to 10th	Night	9% 7% 3 6% 4 4 4 4 5% 6% 47%	2660
	11th to 20th	Night	18% 10% 4 8% 6% 4 4 3 5% 6% 32%	3276
	21st to end	Night	12% 11% 4 8% 7% 6% 5% 5% 6% 6% 30%	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% 8% 10% 25% 11% 3	2703
	11th to 20th	Day	49% 7% 10% 15% 11% 8%	2860
	21st to end	Day	52% 7% 8% 15% 13% 5%	3146
February	1st to 10th	Day	40% 5% 7% 18% 19% 10%	1710
	11th to 20th	Day	39% 6% 8% 17% 21% 8%	1781
	21st to end	Day	41% 5% 7% 21% 20% 6%	1570
March	1st to 10th	Day	53% 13% 7% 13% 12% 3	3367
	11th to 20th	Day	30% 6% 9% 27% 17% 11%	3490
	21st to end	Day	46% 11% 12% 18% 11% 2	3903
April	1st to 10th	Day	38% 7% 11% 19% 19% 5%	2280
	11th to 20th	Day	29% 7% 12% 23% 22% 6%	2252
	21st to end	Day	24% 6% 11% 29% 24% 6%	2280
May	1st to 10th	Day	16% 4 9% 29% 28% 13%	3859
	11th to 20th	Day	12% 3 4 29% 38% 14%	4182
	21st to end	Day	16% 4 7% 32% 35% 6%	4814
June	1st to 10th	Day	21% 5% 9% 25% 30% 11%	2660
	11th to 20th	Day	9% 3 8% 21% 38% 22%	2660
	21st to end	Day	9% 2 3 13% 30% 42%	2660
July	1st to 10th	Day	4 1 2 6% 15% 73%	4351
	11th to 20th	Day	7% 2 4 9% 15% 63%	4071
	21st to end	Day	6% 1 2 8% 28% 54%	4315
August	1st to 10th	Day	10% 2 3 12% 26% 46%	4076
	11th to 20th	Day	5% 1 3 8% 38% 45%	3827
	21st to end	Day	8% 2 3 12% 33% 42%	4179
September	1st to 10th	Day	8% 2 2 10% 30% 49%	2280
	11th to 20th	Day	15% 3 5% 23% 33% 21%	2280
	21st to end	Day	11% 3 4 20% 38% 24%	2025
October	1st to 10th	Day	12% 3 5% 10% 28% 41%	3180
	11th to 20th	Day	16% 3 8% 22% 30% 21%	3161
	21st to end	Day	18% 5% 7% 20% 31% 19%	3448
November	1st to 10th	Day	29% 6% 8% 20% 24% 13%	1868
	11th to 20th	Day	30% 7% 10% 19% 24% 11%	1710
	21st to end	Day	31% 4 10% 15% 29% 12%	1592
December	1st to 10th	Day	36% 8% 9% 18% 17% 12%	1520
	11th to 20th	Day	53% 6% 8% 13% 14% 6%	1928
	21st to end	Day	48% 7% 10% 20% 12% 4	2794
January	1st to 10th	Night	39% 12% 12% 21% 12% 5%	4245
	11th to 20th	Night	50% 8% 11% 13% 10% 8%	4110
	21st to end	Night	52% 8% 7% 13% 11% 8%	4521
February	1st to 10th	Night	38% 4 7% 17% 20% 14%	2470
	11th to 20th	Night	38% 6% 8% 16% 22% 10%	2295
	21st to end	Night	35% 6% 9% 25% 18% 6%	1884
March	1st to 10th	Night	50% 11% 9% 12% 13% 4	3504
	11th to 20th	Night	31% 6% 7% 23% 22% 11%	3490
	21st to end	Night	48% 9% 11% 16% 12% 4	3748
April	1st to 10th	Night	35% 7% 13% 20% 19% 6%	1900
	11th to 20th	Night	32% 5% 9% 25% 19% 9%	1879
	21st to end	Night	23% 8% 9% 28% 24% 8%	1900
May	1st to 10th	Night	19% 8% 9% 19% 29% 17%	3102
	11th to 20th	Night	13% 5% 5% 32% 29% 16%	2721
	21st to end	Night	15% 5% 6% 31% 32% 12%	2762
June	1st to 10th	Night	18% 6% 10% 23% 30% 14%	1520
	11th to 20th	Night	11% 3 8% 14% 32% 31%	1520
	21st to end	Night	10% 1 3 15% 27% 43%	1520
July	1st to 10th	Night	4 1 1 5% 15% 74%	2500
	11th to 20th	Night	9% 2 3 9% 18% 59%	2770
	21st to end	Night	14% 2 2 12% 25% 44%	3011
August	1st to 10th	Night	14% 1 5% 16% 22% 42%	2839
	11th to 20th	Night	8% 1 3 8% 32% 48%	3085
	21st to end	Night	13% 2 6% 10% 31% 38%	3498
September	1st to 10th	Night	9% 2 2 9% 22% 54%	1900
	11th to 20th	Night	18% 3 4 20% 31% 23%	1900
	21st to end	Night	12% 2 5% 18% 32% 31%	2150
October	1st to 10th	Night	12% 3 5% 10% 17% 53%	3800
	11th to 20th	Night	21% 4 9% 19% 24% 24%	3777
	21st to end	Night	22% 5% 6% 19% 25% 23%	4121
November	1st to 10th	Night	27% 6% 8% 20% 22% 18%	2332
	11th to 20th	Night	29% 6% 9% 20% 25% 11%	2470
	21st to end	Night	29% 5% 8% 18% 23% 17%	2584
December	1st to 10th	Night	36% 6% 8% 17% 19% 14%	2660
	11th to 20th	Night	53% 5% 9% 12% 16% 6%	3276
	21st to end	Night	48% 11% 11% 14% 12% 5%	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% 19% 24% 19% 13% 1	279
	11th to 20th	Day	14% 15% 27% 29% 12% 3	370
	21st to end	Day	14% 14% 28% 24% 16% 4	407
February	1st to 10th	Day	15% 19% 25% 23% 13% 4	370
	11th to 20th	Day	18% 19% 28% 24% 9% 2	416
	21st to end	Day	12% 20% 31% 23% 12% 2	382
March	1st to 10th	Day	18% 19% 32% 23% 8% 1	460
	11th to 20th	Day	17% 20% 25% 22% 13% 4	460
	21st to end	Day	13% 23% 33% 23% 8% 1	506
April	1st to 10th	Day	12% 19% 31% 22% 13% 3	460
	11th to 20th	Day	12% 19% 28% 24% 13% 3	452
	21st to end	Day	5% 21% 28% 28% 11% 6%	460
May	1st to 10th	Day	8% 16% 33% 27% 13% 3	463
	11th to 20th	Day	8% 14% 29% 27% 16% 7%	488
	21st to end	Day	6% 12% 25% 33% 17% 8%	529
June	1st to 10th	Day	5% 12% 26% 29% 20% 8%	490
	11th to 20th	Day	3 11% 28% 34% 19% 5%	490
	21st to end	Day	1 4 23% 37% 25% 10%	490
July	1st to 10th	Day	1 2 17% 39% 31% 10%	486
	11th to 20th	Day	1 1 13% 40% 32% 12%	484
	21st to end	Day	1 14% 39% 30% 15%	531
August	1st to 10th	Day	1 3 19% 38% 29% 10%	487
	11th to 20th	Day	1 3 18% 39% 24% 15%	477
	21st to end	Day	1 2 14% 39% 31% 13%	539
September	1st to 10th	Day	1 5% 16% 39% 29% 10%	490
	11th to 20th	Day	2 7% 23% 38% 20% 10%	490
	21st to end	Day	4 6% 19% 36% 28% 8%	490
October	1st to 10th	Day	3 10% 22% 31% 24% 11%	490
	11th to 20th	Day	5% 12% 20% 28% 28% 9%	490
	21st to end	Day	6% 10% 25% 32% 22% 6%	539
November	1st to 10th	Day	7% 13% 24% 24% 26% 6%	434
	11th to 20th	Day	3 11% 21% 29% 27% 9%	270
	21st to end	Day	8% 10% 20% 34% 22% 6%	214
December	1st to 10th	Day	7% 16% 22% 27% 20% 8%	180
	11th to 20th	Day	9% 21% 21% 24% 17% 8%	180
	21st to end	Day	12% 25% 22% 24% 13% 4	198
January	1st to 10th	Night	16% 8% 17% 30% 22% 6%	760
	11th to 20th	Night	12% 8% 15% 35% 22% 8%	760
	21st to end	Night	17% 7% 15% 33% 23% 6%	836
February	1st to 10th	Night	14% 9% 15% 34% 23% 5%	760
	11th to 20th	Night	14% 10% 19% 34% 18% 6%	744
	21st to end	Night	12% 10% 21% 35% 19% 3	628
March	1st to 10th	Night	14% 11% 20% 32% 19% 3	760
	11th to 20th	Night	14% 9% 19% 31% 23% 4	760
	21st to end	Night	11% 11% 22% 34% 19% 3	836
April	1st to 10th	Night	12% 7% 24% 33% 22% 3	760
	11th to 20th	Night	10% 12% 20% 32% 21% 5%	751
	21st to end	Night	6% 8% 25% 31% 23% 7%	760
May	1st to 10th	Night	5% 5% 27% 33% 25% 5%	721
	11th to 20th	Night	6% 2 14% 36% 36% 6%	500
	21st to end	Night	4 4 18% 37% 33% 4	411
June	1st to 10th	Night	4 4 16% 42% 28% 5%	380
	11th to 20th	Night	1 4 12% 49% 33% 1	380
	21st to end	Night	1 1 8% 41% 46% 3	380
July	1st to 10th	Night	1 5% 36% 53% 4	370
	11th to 20th	Night	1 1 1 30% 54% 12%	529
	21st to end	Night	1 2 2 25% 59% 11%	624
August	1st to 10th	Night	4 1 3 34% 51% 8%	567
	11th to 20th	Night	1 9% 37% 46% 7%	706
	21st to end	Night	2 1 6% 31% 48% 11%	836
September	1st to 10th	Night	2 1 8% 33% 45% 11%	760
	11th to 20th	Night	4 3 10% 37% 38% 8%	760
	21st to end	Night	3 3 10% 33% 41% 10%	760
October	1st to 10th	Night	4 2 11% 36% 35% 10%	760
	11th to 20th	Night	6% 5% 11% 31% 37% 11%	760
	21st to end	Night	6% 5% 11% 33% 30% 15%	836
November	1st to 10th	Night	7% 5% 15% 32% 32% 9%	764
	11th to 20th	Night	10% 3 16% 32% 31% 8%	760
	21st to end	Night	9% 4 16% 39% 25% 7%	760
December	1st to 10th	Night	11% 8% 17% 34% 24% 6%	760
	11th to 20th	Night	17% 6% 19% 33% 20% 5%	760
	21st to end	Night	15% 12% 21% 30% 19% 3	836

Page generated at 2024-11-03 10:19:04. Version ac07

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