Vallejo Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Vallejo Observatory clear sky chart from 2005-11-06 to 2024-05-14.

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	8% 5% 2 5% 3 3 3 3 5% 7% 55%	83779
	Night	9% 6% 2 6% 4 4 4 3 5% 7% 51%	85843
Transparency	Day	28% 4 9% 21% 23% 15%	83497
	Night	32% 5% 11% 20% 20% 13%	85646
Seeing	Day	8% 10% 26% 34% 18% 4	14667
	Night	10% 5% 18% 38% 25% 4	24102

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	17% 10% 3 8% 4 5% 4 4 5% 7% 35%	2669
	11th to 20th	Day	18% 10% 5% 9% 4 5% 3 2 4 6% 33%	2860
	21st to end	Day	20% 9% 3 7% 5% 5% 5% 4 5% 7% 30%	3146
February	1st to 10th	Day	15% 6% 2 7% 4 4 5% 3 4 6% 45%	1710
	11th to 20th	Day	13% 8% 3 6% 4 4 4 4 5% 8% 42%	1737
	21st to end	Day	13% 5% 3 5% 4 4 4 3 5% 6% 48%	1570
March	1st to 10th	Day	11% 8% 4 10% 6% 6% 6% 4 6% 7% 30%	3379
	11th to 20th	Day	9% 5% 2 4 4 3 3 3 5% 6% 55%	3490
	21st to end	Day	11% 6% 2 7% 4 5% 5% 4 7% 9% 38%	3881
April	1st to 10th	Day	11% 7% 2 5% 5% 4 3 4 7% 8% 45%	2280
	11th to 20th	Day	8% 4 2 5% 3 3 4 4 5% 7% 56%	2249
	21st to end	Day	3 4 2 3 3 3 3 3 5% 10% 61%	2280
May	1st to 10th	Day	4 4 1 3 3 2 3 3 5% 7% 65%	2375
	11th to 20th	Day	4 3 1 3 3 3 3 3 5% 8% 64%	2412
	21st to end	Day	3 4 2 5% 3 4 3 3 5% 7% 62%	2723
June	1st to 10th	Day	4 4 1 4 2 4 3 3 5% 7% 62%	2520
	11th to 20th	Day	1 2 1 2 2 2 3 3 4 7% 75%	2520
	21st to end	Day	2 2 1 2 2 2 2 2 4 6% 75%	2520
July	1st to 10th	Day	2 1 3 3 3 2 3 4 6% 74%	2520
	11th to 20th	Day	1 2 1 2 3 3 3 3 5% 8% 72%	2383
	21st to end	Day	1 2 1 2 2 3 2 3 4 6% 74%	2546
August	1st to 10th	Day	1 3 1 4 2 3 4 3 4 6% 70%	2328
	11th to 20th	Day	1 2 1 2 3 3 3 2 3 6% 73%	2201
	21st to end	Day	1 2 1 4 2 2 3 2 4 5% 72%	2376
September	1st to 10th	Day	2 1 3 2 2 2 2 3 5% 76%	2160
	11th to 20th	Day	2 4 2 5% 2 3 3 3 3 8% 65%	2160
	21st to end	Day	2 2 1 2 2 2 2 2 3 6% 74%	1905
October	1st to 10th	Day	2 3 1 3 2 3 3 3 4 6% 69%	1800
	11th to 20th	Day	5% 4 1 4 3 3 3 3 6% 6% 63%	1800
	21st to end	Day	7% 5% 1 4 3 3 3 2 5% 7% 60%	1980
November	1st to 10th	Day	8% 4 2 4 3 3 3 3 5% 8% 57%	1812
	11th to 20th	Day	9% 8% 3 7% 4 4 4 3 5% 7% 47%	1710
	21st to end	Day	10% 6% 1 4 4 3 4 4 5% 7% 53%	1535
December	1st to 10th	Day	13% 7% 3 6% 4 3 3 4 5% 7% 44%	1520
	11th to 20th	Day	23% 12% 3 8% 5% 4 3 3 4 5% 30%	1928
	21st to end	Day	19% 9% 3 7% 5% 4 4 4 5% 8% 32%	2794
January	1st to 10th	Night	14% 7% 3 8% 4 4 4 3 5% 6% 42%	4277
	11th to 20th	Night	18% 9% 4 9% 5% 5% 4 3 5% 7% 32%	4110
	21st to end	Night	21% 10% 3 8% 5% 4 4 3 4 6% 32%	4521
February	1st to 10th	Night	14% 6% 2 5% 4 4 3 3 4 6% 48%	2470
	11th to 20th	Night	11% 7% 2 6% 4 3 4 3 5% 8% 47%	2328
	21st to end	Night	11% 6% 1 4 3 4 4 3 5% 7% 52%	1884
March	1st to 10th	Night	12% 9% 3 9% 5% 4 4 3 5% 7% 38%	3490
	11th to 20th	Night	11% 5% 2 4 3 3 3 2 4 6% 57%	3490
	21st to end	Night	11% 8% 3 8% 5% 4 4 3 5% 6% 43%	3772
April	1st to 10th	Night	8% 6% 2 6% 5% 4 3 3 6% 7% 51%	1900
	11th to 20th	Night	7% 5% 2 4 4 3 3 3 4 7% 59%	1879
	21st to end	Night	4 3 1 4 3 3 3 3 5% 8% 63%	1900
May	1st to 10th	Night	4 3 2 5% 3 2 3 3 5% 6% 63%	1800
	11th to 20th	Night	2 3 2 4 3 3 3 3 5% 9% 62%	1553
	21st to end	Night	3 3 2 4 4 4 4 3 5% 7% 61%	1552
June	1st to 10th	Night	3 3 1 5% 4 3 4 3 5% 10% 58%	1440
	11th to 20th	Night	1 1 1 3 3 3 3 2 5% 7% 71%	1440
	21st to end	Night	1 2 2 3 2 3 3 3 4 7% 70%	1440
July	1st to 10th	Night	1 2 1 3 4 4 3 3 5% 6% 69%	1440
	11th to 20th	Night	1 2 1 4 3 4 4 3 6% 8% 62%	1573
	21st to end	Night	2 2 1 5% 6% 6% 5% 4 7% 9% 52%	1773
August	1st to 10th	Night	2 3 2 4 6% 5% 5% 4 4 10% 54%	1611
	11th to 20th	Night	2 1 4 4 6% 5% 4 7% 9% 58%	1699
	21st to end	Night	1 3 2 7% 4 3 3 3 4 9% 61%	1980
September	1st to 10th	Night	1 2 3 2 2 3 3 5% 8% 71%	1800
	11th to 20th	Night	3 4 2 6% 3 4 3 3 5% 7% 61%	1800
	21st to end	Night	1 3 1 4 3 3 3 2 4 7% 69%	2050
October	1st to 10th	Night	4 4 2 3 3 2 3 2 5% 6% 66%	2160
	11th to 20th	Night	4 4 2 6% 4 3 2 3 5% 7% 60%	2160
	21st to end	Night	6% 4 1 5% 3 3 3 3 5% 8% 59%	2376
November	1st to 10th	Night	6% 6% 1 6% 4 3 4 2 5% 8% 56%	2266
	11th to 20th	Night	9% 6% 3 5% 3 3 4 3 5% 7% 51%	2470
	21st to end	Night	9% 5% 1 4 4 4 4 3 5% 6% 56%	2641
December	1st to 10th	Night	14% 6% 3 6% 4 4 3 3 4 6% 45%	2660
	11th to 20th	Night	24% 8% 3 7% 4 4 4 4 6% 7% 30%	3276
	21st to end	Night	17% 8% 3 8% 6% 5% 5% 4 5% 7% 33%	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	46% 7% 11% 27% 7% 2	2669
	11th to 20th	Day	53% 3 13% 14% 11% 6%	2860
	21st to end	Day	53% 5% 11% 16% 11% 4	3146
February	1st to 10th	Day	40% 4 7% 21% 20% 8%	1710
	11th to 20th	Day	40% 4 9% 19% 21% 6%	1746
	21st to end	Day	38% 4 9% 24% 19% 7%	1570
March	1st to 10th	Day	51% 6% 12% 17% 13% 2	3379
	11th to 20th	Day	30% 4 12% 30% 13% 11%	3490
	21st to end	Day	40% 7% 16% 23% 12% 2	3881
April	1st to 10th	Day	37% 4 13% 22% 19% 4	2280
	11th to 20th	Day	28% 4 15% 26% 21% 7%	2252
	21st to end	Day	21% 3 15% 30% 26% 4	2280
May	1st to 10th	Day	20% 3 9% 30% 29% 9%	2375
	11th to 20th	Day	19% 3 10% 34% 25% 10%	2415
	21st to end	Day	21% 4 15% 31% 25% 5%	2724
June	1st to 10th	Day	20% 5% 11% 28% 27% 8%	2520
	11th to 20th	Day	10% 3 10% 23% 37% 18%	2520
	21st to end	Day	12% 3 4 16% 35% 31%	2520
July	1st to 10th	Day	11% 3 3 12% 27% 44%	2451
	11th to 20th	Day	10% 4 5% 13% 27% 41%	2311
	21st to end	Day	10% 5% 2 11% 28% 45%	2405
August	1st to 10th	Day	15% 4 3 17% 31% 31%	2312
	11th to 20th	Day	12% 5% 2 13% 35% 34%	2201
	21st to end	Day	13% 4 2 15% 34% 33%	2376
September	1st to 10th	Day	11% 3 2 10% 32% 42%	2160
	11th to 20th	Day	19% 4 5% 24% 34% 15%	2160
	21st to end	Day	14% 2 4 23% 33% 24%	1905
October	1st to 10th	Day	17% 3 7% 18% 31% 24%	1800
	11th to 20th	Day	21% 4 9% 20% 24% 23%	1800
	21st to end	Day	23% 3 8% 22% 25% 18%	1980
November	1st to 10th	Day	27% 4 9% 28% 20% 11%	1812
	11th to 20th	Day	36% 5% 9% 22% 21% 6%	1710
	21st to end	Day	30% 4 10% 22% 23% 10%	1535
December	1st to 10th	Day	38% 3 12% 21% 18% 7%	1520
	11th to 20th	Day	55% 5% 8% 13% 14% 4	1928
	21st to end	Day	50% 5% 13% 18% 11% 3	2794
January	1st to 10th	Night	40% 7% 19% 23% 8% 3	4277
	11th to 20th	Night	52% 4 14% 13% 9% 7%	4110
	21st to end	Night	54% 4 12% 13% 11% 6%	4521
February	1st to 10th	Night	38% 4 8% 22% 20% 9%	2470
	11th to 20th	Night	36% 4 11% 21% 21% 8%	2330
	21st to end	Night	33% 5% 10% 29% 18% 5%	1884
March	1st to 10th	Night	46% 6% 17% 15% 14% 3	3490
	11th to 20th	Night	30% 4 13% 23% 19% 11%	3490
	21st to end	Night	41% 5% 17% 23% 11% 3	3772
April	1st to 10th	Night	33% 4 16% 21% 21% 5%	1900
	11th to 20th	Night	27% 4 13% 29% 18% 9%	1879
	21st to end	Night	20% 3 15% 32% 25% 6%	1900
May	1st to 10th	Night	22% 4 9% 27% 28% 11%	1800
	11th to 20th	Night	19% 4 15% 29% 25% 9%	1553
	21st to end	Night	21% 4 18% 30% 22% 5%	1552
June	1st to 10th	Night	21% 7% 11% 25% 27% 9%	1440
	11th to 20th	Night	13% 4 10% 21% 36% 18%	1440
	21st to end	Night	14% 3 4 18% 33% 27%	1440
July	1st to 10th	Night	15% 3 3 11% 29% 40%	1400
	11th to 20th	Night	16% 7% 5% 11% 25% 37%	1522
	21st to end	Night	20% 9% 1 11% 24% 34%	1674
August	1st to 10th	Night	23% 6% 2 16% 29% 23%	1602
	11th to 20th	Night	17% 7% 3 12% 31% 30%	1699
	21st to end	Night	20% 5% 2 14% 30% 29%	1980
September	1st to 10th	Night	12% 3 3 10% 30% 41%	1800
	11th to 20th	Night	20% 5% 7% 23% 29% 17%	1800
	21st to end	Night	15% 3 4 21% 30% 26%	2050
October	1st to 10th	Night	19% 4 7% 20% 26% 24%	2160
	11th to 20th	Night	24% 4 6% 21% 22% 23%	2160
	21st to end	Night	23% 4 7% 23% 23% 20%	2376
November	1st to 10th	Night	28% 4 11% 26% 18% 13%	2266
	11th to 20th	Night	31% 6% 10% 25% 20% 8%	2470
	21st to end	Night	28% 5% 10% 22% 24% 12%	2641
December	1st to 10th	Night	40% 4 9% 18% 18% 11%	2660
	11th to 20th	Night	52% 6% 10% 14% 15% 3	3276
	21st to end	Night	49% 7% 14% 16% 11% 3	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	27% 10% 28% 19% 12% 3	258
	11th to 20th	Day	19% 11% 20% 32% 15% 3	370
	21st to end	Day	13% 10% 25% 31% 19% 3	407
February	1st to 10th	Day	17% 11% 22% 29% 19% 2	370
	11th to 20th	Day	17% 16% 27% 23% 15% 3	399
	21st to end	Day	13% 15% 24% 31% 14% 3	382
March	1st to 10th	Day	18% 17% 30% 22% 11% 3	460
	11th to 20th	Day	17% 15% 26% 21% 15% 5%	460
	21st to end	Day	13% 18% 31% 26% 10% 2	506
April	1st to 10th	Day	14% 15% 32% 25% 10% 4	460
	11th to 20th	Day	12% 18% 29% 24% 11% 5%	452
	21st to end	Day	6% 18% 29% 29% 13% 5%	460
May	1st to 10th	Day	9% 14% 30% 34% 11% 1	463
	11th to 20th	Day	6% 14% 33% 30% 15% 2	461
	21st to end	Day	5% 11% 30% 37% 15% 2	485
June	1st to 10th	Day	6% 11% 32% 34% 14% 3	450
	11th to 20th	Day	2 12% 33% 39% 12% 2	450
	21st to end	Day	6% 32% 43% 18% 1	450
July	1st to 10th	Day	1 4 31% 49% 15% 1	446
	11th to 20th	Day	1 2 22% 50% 23% 1	444
	21st to end	Day	1 1 26% 50% 20% 2	487
August	1st to 10th	Day	1 5% 29% 47% 15% 2	447
	11th to 20th	Day	1 3 26% 46% 20% 4	437
	21st to end	Day	1 3 22% 46% 25% 3	495
September	1st to 10th	Day	1 4 20% 45% 23% 6%	450
	11th to 20th	Day	3 9% 23% 42% 16% 7%	450
	21st to end	Day	3 7% 24% 34% 26% 6%	450
October	1st to 10th	Day	3 7% 27% 33% 22% 9%	450
	11th to 20th	Day	5% 8% 20% 30% 28% 8%	450
	21st to end	Day	9% 8% 23% 28% 27% 6%	495
November	1st to 10th	Day	10% 9% 19% 23% 33% 6%	408
	11th to 20th	Day	8% 9% 16% 34% 27% 7%	270
	21st to end	Day	13% 5% 25% 35% 18% 3	187
December	1st to 10th	Day	17% 11% 23% 27% 16% 6%	180
	11th to 20th	Day	18% 13% 22% 29% 16% 2	180
	21st to end	Day	21% 10% 27% 27% 14% 1	198
January	1st to 10th	Night	19% 5% 14% 33% 24% 5%	760
	11th to 20th	Night	16% 5% 14% 38% 21% 5%	760
	21st to end	Night	17% 5% 14% 36% 24% 4	836
February	1st to 10th	Night	17% 6% 14% 37% 21% 4	760
	11th to 20th	Night	13% 8% 18% 34% 22% 5%	744
	21st to end	Night	15% 7% 20% 33% 22% 3	628
March	1st to 10th	Night	18% 9% 19% 31% 21% 3	760
	11th to 20th	Night	16% 8% 19% 32% 23% 3	760
	21st to end	Night	13% 9% 26% 34% 17% 1	836
April	1st to 10th	Night	11% 8% 26% 36% 17% 3	760
	11th to 20th	Night	10% 11% 26% 32% 18% 3	751
	21st to end	Night	6% 8% 30% 34% 20% 2	760
May	1st to 10th	Night	6% 6% 26% 39% 21% 2	660
	11th to 20th	Night	3 4 22% 45% 24% 3	471
	21st to end	Night	3 7% 27% 41% 19% 2	389
June	1st to 10th	Night	4 4 27% 46% 18% 1	360
	11th to 20th	Night	1 5% 26% 51% 17%	360
	21st to end	Night	2 3 26% 45% 24%	360
July	1st to 10th	Night	1 1 17% 56% 24% 1	350
	11th to 20th	Night	2 15% 53% 26% 4	478
	21st to end	Night	2 2 17% 45% 31% 3	591
August	1st to 10th	Night	4 3 17% 44% 30% 3	537
	11th to 20th	Night	1 2 19% 46% 29% 3	631
	21st to end	Night	2 1 16% 46% 31% 4	792
September	1st to 10th	Night	1 1 13% 45% 34% 5%	720
	11th to 20th	Night	3 3 19% 43% 28% 5%	720
	21st to end	Night	2 3 15% 40% 34% 6%	720
October	1st to 10th	Night	4 4 16% 37% 32% 7%	720
	11th to 20th	Night	6% 5% 14% 35% 33% 7%	720
	21st to end	Night	8% 4 12% 34% 33% 9%	792
November	1st to 10th	Night	7% 6% 15% 31% 34% 7%	740
	11th to 20th	Night	12% 3 14% 37% 29% 6%	760
	21st to end	Night	12% 4 13% 38% 27% 5%	760
December	1st to 10th	Night	16% 5% 14% 36% 24% 5%	760
	11th to 20th	Night	23% 6% 14% 33% 21% 3	760
	21st to end	Night	20% 6% 21% 32% 19% 1	836

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