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 2025-11-18.

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% 4 2 4 3 3 3 3 5% 7% 62%	135457
	Night	8% 5% 2 6% 4 3 3 3 5% 7% 55%	132377
Transparency	Day	23% 4 8% 21% 25% 20%	130376
	Night	28% 5% 10% 19% 22% 16%	128444
Seeing	Day	8% 10% 27% 34% 17% 4	16721
	Night	10% 5% 19% 38% 24% 4	26005

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% 8% 2 6% 4 5% 4 4 5% 8% 42%	3839
	11th to 20th	Day	13% 7% 4 7% 3 4 3 2 3 5% 50%	4100
	21st to end	Day	16% 7% 3 7% 4 4 4 3 4 6% 40%	4510
February	1st to 10th	Day	16% 6% 2 6% 3 4 5% 3 4 6% 44%	1800
	11th to 20th	Day	12% 8% 3 6% 4 4 4 4 5% 9% 41%	1831
	21st to end	Day	12% 5% 3 5% 4 4 4 4 5% 6% 47%	1650
March	1st to 10th	Day	10% 8% 3 9% 6% 6% 5% 4 6% 8% 35%	4863
	11th to 20th	Day	10% 5% 2 5% 4 4 4 4 5% 8% 49%	5000
	21st to end	Day	10% 5% 2 7% 4 6% 6% 5% 7% 10% 38%	5568
April	1st to 10th	Day	11% 7% 2 5% 5% 4 4 4 6% 8% 45%	2400
	11th to 20th	Day	7% 4 2 5% 3 3 4 4 5% 7% 56%	2369
	21st to end	Day	3 4 2 4 3 3 3 3 5% 10% 60%	2400
May	1st to 10th	Day	4 2 1 2 1 1 2 2 3 5% 78%	5660
	11th to 20th	Day	2 1 1 2 2 2 2 2 4 7% 76%	6014
	21st to end	Day	1 2 1 2 1 2 2 2 3 6% 79%	6902
June	1st to 10th	Day	3 4 1 4 3 4 3 3 5% 7% 64%	2800
	11th to 20th	Day	1 1 1 2 2 2 3 3 4 7% 75%	2800
	21st to end	Day	1 2 1 2 2 2 2 2 4 5% 76%	2800
July	1st to 10th	Day	1 1 2 2 2 1 2 3 4 83%	6319
	11th to 20th	Day	1 1 2 3 2 2 2 4 6% 76%	5975
	21st to end	Day	3 2 4 2 2 2 2 3 5% 74%	6402
August	1st to 10th	Day	1 1 1 2 2 2 3 3 6% 6% 73%	5777
	11th to 20th	Day	2 1 2 2 2 2 1 2 5% 82%	5543
	21st to end	Day	1 2 1 2 2 3 3 2 5% 6% 73%	5984
September	1st to 10th	Day	2 1 3 3 2 2 2 3 6% 75%	2400
	11th to 20th	Day	2 4 2 5% 3 3 3 3 3 8% 65%	2400
	21st to end	Day	3 2 1 2 3 3 2 2 4 6% 73%	2116
October	1st to 10th	Day	1 2 1 3 2 2 2 3 5% 6% 72%	3280
	11th to 20th	Day	4 4 1 5% 3 2 2 2 5% 6% 65%	3261
	21st to end	Day	4 4 1 5% 3 3 4 3 6% 8% 58%	3558
November	1st to 10th	Day	8% 4 2 4 3 3 3 3 5% 7% 57%	2006
	11th to 20th	Day	10% 8% 3 7% 4 4 4 3 5% 7% 45%	1863
	21st to end	Day	11% 6% 1 4 4 3 4 4 5% 7% 52%	1615
December	1st to 10th	Day	8% 5% 2 4 3 3 3 4 6% 11% 51%	2620
	11th to 20th	Day	19% 10% 3 9% 7% 7% 5% 4 5% 6% 24%	3028
	21st to end	Day	21% 11% 3 8% 5% 4 4 4 6% 7% 26%	4004
January	1st to 10th	Night	10% 6% 3 7% 5% 4 4 4 6% 8% 44%	6112
	11th to 20th	Night	12% 6% 3 7% 5% 5% 4 3 4 6% 45%	5880
	21st to end	Night	15% 8% 3 8% 4 3 3 3 4 6% 43%	6468
February	1st to 10th	Night	16% 6% 2 5% 4 4 3 3 4 6% 48%	2600
	11th to 20th	Night	11% 7% 2 6% 4 3 4 3 5% 8% 46%	2454
	21st to end	Night	11% 6% 1 4 3 4 4 3 5% 7% 52%	1980
March	1st to 10th	Night	11% 8% 3 8% 5% 4 4 3 6% 8% 40%	5026
	11th to 20th	Night	13% 5% 2 5% 3 3 3 3 4 7% 51%	5000
	21st to end	Night	12% 7% 2 7% 4 4 3 3 5% 8% 44%	5407
April	1st to 10th	Night	7% 6% 2 6% 5% 4 3 3 6% 7% 51%	2000
	11th to 20th	Night	7% 5% 2 5% 4 3 3 3 4 7% 58%	1979
	21st to end	Night	4 3 1 4 3 2 3 3 5% 8% 62%	2000
May	1st to 10th	Night	3 2 1 3 2 2 2 2 3 5% 75%	4312
	11th to 20th	Night	1 2 1 5% 3 3 3 3 4 7% 68%	3891
	21st to end	Night	1 1 1 2 2 2 2 2 3 4 80%	3972
June	1st to 10th	Night	3 3 1 5% 4 3 4 3 6% 9% 59%	1600
	11th to 20th	Night	1 1 1 3 3 2 3 2 5% 7% 74%	1600
	21st to end	Night	1 2 2 3 2 3 3 3 4 7% 72%	1600
July	1st to 10th	Night	1 1 3 4 2 2 2 4 4 78%	3640
	11th to 20th	Night	1 1 2 9% 6% 5% 4 3 5% 6% 59%	3997
	21st to end	Night	1 4 3 12% 5% 4 3 3 5% 6% 54%	4472
August	1st to 10th	Night	2 2 1 4 4 3 3 4 5% 7% 66%	4035
	11th to 20th	Night	2 1 4 3 3 3 2 4 5% 74%	4317
	21st to end	Night	1 2 1 4 3 3 3 3 5% 8% 66%	5015
September	1st to 10th	Night	1 2 1 3 2 2 4 3 5% 7% 71%	2000
	11th to 20th	Night	3 4 2 7% 3 4 3 3 5% 7% 61%	2000
	21st to end	Night	2 3 1 4 3 3 3 2 4 6% 69%	2278
October	1st to 10th	Night	2 2 2 3 3 2 3 2 4 6% 71%	3920
	11th to 20th	Night	3 5% 3 7% 3 3 2 2 4 6% 62%	3897
	21st to end	Night	5% 5% 2 4 3 3 4 3 5% 7% 59%	4253
November	1st to 10th	Night	6% 5% 1 6% 4 3 4 2 5% 8% 55%	2512
	11th to 20th	Night	10% 6% 3 5% 4 3 4 3 5% 7% 50%	2691
	21st to end	Night	10% 5% 1 5% 4 4 4 3 5% 7% 55%	2781
December	1st to 10th	Night	8% 4 2 5% 4 4 4 3 5% 7% 55%	4560
	11th to 20th	Night	20% 9% 3 8% 6% 6% 5% 5% 7% 7% 24%	5176
	21st to end	Night	22% 9% 3 8% 5% 4 5% 4 4 6% 29%	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	39% 7% 14% 21% 14% 5%	3839
	11th to 20th	Day	38% 3 9% 16% 20% 14%	4100
	21st to end	Day	45% 4 8% 13% 13% 17%	4510
February	1st to 10th	Day	41% 4 7% 20% 20% 8%	1800
	11th to 20th	Day	41% 4 10% 19% 21% 6%	1840
	21st to end	Day	37% 4 8% 24% 19% 8%	1650
March	1st to 10th	Day	43% 7% 9% 20% 18% 2	4863
	11th to 20th	Day	32% 4 16% 29% 11% 8%	5000
	21st to end	Day	37% 9% 17% 26% 10% 2	5568
April	1st to 10th	Day	37% 5% 13% 22% 19% 4	2400
	11th to 20th	Day	28% 4 14% 26% 21% 7%	2372
	21st to end	Day	21% 4 15% 31% 26% 4	2400
May	1st to 10th	Day	12% 2 6% 30% 31% 19%	5660
	11th to 20th	Day	10% 2 7% 37% 35% 9%	6017
	21st to end	Day	9% 2 10% 32% 43% 4	6904
June	1st to 10th	Day	19% 5% 10% 29% 28% 8%	2800
	11th to 20th	Day	9% 3 9% 22% 38% 18%	2800
	21st to end	Day	11% 2 4 16% 35% 33%	2800
July	1st to 10th	Day	5% 2 2 8% 23% 59%	6251
	11th to 20th	Day	7% 4 4 5% 15% 65%	5903
	21st to end	Day	10% 6% 4 16% 27% 37%	6217
August	1st to 10th	Day	12% 4 3 11% 28% 41%	4206
	11th to 20th	Day	7% 3 2 11% 43% 34%	4014
	21st to end	Day	10% 4 4 14% 36% 33%	4311
September	1st to 10th	Day	11% 3 2 11% 32% 41%	2400
	11th to 20th	Day	19% 4 5% 24% 34% 14%	2400
	21st to end	Day	14% 2 4 22% 34% 24%	2116
October	1st to 10th	Day	13% 3 5% 12% 29% 38%	3280
	11th to 20th	Day	18% 6% 6% 24% 27% 20%	3261
	21st to end	Day	22% 6% 7% 25% 27% 13%	3558
November	1st to 10th	Day	25% 5% 10% 28% 21% 11%	2006
	11th to 20th	Day	38% 5% 10% 22% 19% 6%	1863
	21st to end	Day	31% 4 10% 22% 23% 10%	1615
December	1st to 10th	Day	26% 3 9% 31% 25% 5%	2620
	11th to 20th	Day	58% 6% 9% 14% 11% 2	3028
	21st to end	Day	55% 5% 12% 17% 9% 2	4004
January	1st to 10th	Night	35% 9% 20% 19% 13% 5%	6112
	11th to 20th	Night	38% 7% 10% 16% 17% 12%	5880
	21st to end	Night	43% 4 10% 11% 18% 15%	6468
February	1st to 10th	Night	39% 4 7% 21% 20% 10%	2600
	11th to 20th	Night	37% 4 11% 21% 20% 8%	2456
	21st to end	Night	32% 5% 10% 29% 18% 6%	1980
March	1st to 10th	Night	40% 6% 13% 19% 19% 3	5026
	11th to 20th	Night	33% 5% 16% 23% 16% 8%	5000
	21st to end	Night	38% 6% 17% 25% 12% 2	5407
April	1st to 10th	Night	32% 4 15% 23% 21% 5%	2000
	11th to 20th	Night	28% 4 13% 28% 18% 9%	1979
	21st to end	Night	20% 3 15% 32% 24% 6%	2000
May	1st to 10th	Night	14% 2 8% 28% 31% 16%	4309
	11th to 20th	Night	16% 3 9% 35% 27% 10%	3891
	21st to end	Night	10% 2 14% 31% 39% 4	3972
June	1st to 10th	Night	20% 7% 11% 26% 28% 9%	1600
	11th to 20th	Night	12% 3 9% 20% 38% 18%	1600
	21st to end	Night	13% 3 4 17% 33% 29%	1600
July	1st to 10th	Night	6% 3 5% 6% 24% 56%	3600
	11th to 20th	Night	11% 9% 11% 5% 13% 51%	3946
	21st to end	Night	17% 12% 4 10% 23% 33%	4345
August	1st to 10th	Night	17% 6% 4 10% 30% 33%	2929
	11th to 20th	Night	10% 5% 2 9% 42% 32%	3116
	21st to end	Night	14% 4 4 12% 37% 29%	3608
September	1st to 10th	Night	12% 4 3 11% 30% 40%	2000
	11th to 20th	Night	21% 5% 7% 23% 28% 16%	2000
	21st to end	Night	15% 3 4 20% 31% 26%	2278
October	1st to 10th	Night	15% 3 4 13% 21% 44%	3920
	11th to 20th	Night	20% 7% 5% 25% 22% 21%	3897
	21st to end	Night	23% 5% 6% 23% 26% 18%	4253
November	1st to 10th	Night	27% 5% 11% 26% 18% 13%	2512
	11th to 20th	Night	31% 6% 11% 25% 19% 8%	2691
	21st to end	Night	29% 5% 10% 21% 23% 12%	2781
December	1st to 10th	Night	28% 5% 7% 25% 25% 10%	4560
	11th to 20th	Night	53% 7% 10% 16% 12% 2	5176
	21st to end	Night	54% 6% 12% 16% 9% 3	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% 11% 28% 21% 12% 3	283
	11th to 20th	Day	18% 11% 22% 33% 15% 3	400
	21st to end	Day	12% 10% 25% 32% 19% 3	440
February	1st to 10th	Day	19% 11% 21% 29% 19% 2	400
	11th to 20th	Day	16% 17% 27% 23% 15% 2	433
	21st to end	Day	12% 14% 24% 31% 15% 3	414
March	1st to 10th	Day	18% 17% 30% 21% 11% 3	500
	11th to 20th	Day	18% 15% 26% 21% 15% 5%	500
	21st to end	Day	14% 18% 30% 25% 11% 2	550
April	1st to 10th	Day	14% 15% 32% 25% 10% 4	500
	11th to 20th	Day	11% 18% 30% 25% 11% 5%	492
	21st to end	Day	5% 18% 30% 29% 13% 5%	500
May	1st to 10th	Day	9% 14% 30% 34% 12% 1	503
	11th to 20th	Day	7% 14% 33% 31% 15% 1	528
	21st to end	Day	4 10% 31% 38% 15% 2	573
June	1st to 10th	Day	5% 10% 34% 36% 12% 2	530
	11th to 20th	Day	2 12% 35% 38% 12% 1	530
	21st to end	Day	1 6% 31% 44% 17% 1	530
July	1st to 10th	Day	1 5% 31% 48% 15% 1	526
	11th to 20th	Day	1 2 23% 52% 22% 1	524
	21st to end	Day	1 2 27% 50% 19% 2	575
August	1st to 10th	Day	2 5% 29% 46% 16% 3	527
	11th to 20th	Day	1 5% 28% 44% 20% 3	517
	21st to end	Day	1 3 22% 47% 24% 3	583
September	1st to 10th	Day	1 5% 25% 43% 21% 6%	530
	11th to 20th	Day	2 10% 24% 41% 16% 7%	530
	21st to end	Day	3 8% 24% 33% 25% 7%	530
October	1st to 10th	Day	3 7% 26% 32% 22% 10%	530
	11th to 20th	Day	6% 8% 22% 29% 27% 8%	530
	21st to end	Day	8% 8% 23% 28% 26% 6%	583
November	1st to 10th	Day	9% 9% 21% 24% 30% 7%	482
	11th to 20th	Day	11% 10% 18% 32% 23% 6%	321
	21st to end	Day	15% 5% 26% 34% 17% 2	207
December	1st to 10th	Day	16% 10% 23% 28% 17% 7%	200
	11th to 20th	Day	19% 14% 23% 29% 16% 2	200
	21st to end	Day	22% 12% 26% 26% 13% 1	220
January	1st to 10th	Night	19% 5% 14% 35% 23% 5%	800
	11th to 20th	Night	15% 5% 15% 39% 22% 4	800
	21st to end	Night	16% 5% 13% 37% 24% 4	880
February	1st to 10th	Night	19% 6% 14% 37% 20% 4	800
	11th to 20th	Night	14% 8% 18% 34% 21% 5%	784
	21st to end	Night	15% 7% 20% 35% 21% 2	660
March	1st to 10th	Night	18% 9% 19% 32% 21% 3	800
	11th to 20th	Night	16% 9% 20% 31% 22% 3	800
	21st to end	Night	14% 9% 26% 33% 17% 1	880
April	1st to 10th	Night	10% 8% 26% 36% 17% 3	800
	11th to 20th	Night	10% 11% 27% 32% 18% 3	791
	21st to end	Night	6% 8% 30% 35% 19% 2	800
May	1st to 10th	Night	6% 6% 27% 40% 20% 2	695
	11th to 20th	Night	3 4 24% 44% 23% 2	513
	21st to end	Night	3 7% 27% 42% 18% 2	433
June	1st to 10th	Night	4 5% 30% 45% 16% 1	400
	11th to 20th	Night	1 5% 28% 50% 15%	400
	21st to end	Night	2 4 27% 45% 23%	400
July	1st to 10th	Night	1 1 19% 56% 22% 1	390
	11th to 20th	Night	2 1 17% 52% 24% 4	534
	21st to end	Night	2 2 19% 45% 29% 2	657
August	1st to 10th	Night	4 3 18% 45% 28% 3	597
	11th to 20th	Night	1 2 22% 46% 27% 3	703
	21st to end	Night	2 1 17% 47% 30% 4	880
September	1st to 10th	Night	2 2 16% 45% 31% 5%	800
	11th to 20th	Night	4 3 20% 42% 27% 5%	800
	21st to end	Night	3 4 16% 39% 33% 5%	800
October	1st to 10th	Night	4 5% 16% 37% 33% 6%	800
	11th to 20th	Night	7% 6% 16% 34% 31% 7%	800
	21st to end	Night	8% 4 13% 34% 33% 9%	880
November	1st to 10th	Night	7% 6% 15% 30% 35% 7%	820
	11th to 20th	Night	13% 3 15% 37% 28% 5%	828
	21st to end	Night	13% 4 13% 39% 26% 5%	800
December	1st to 10th	Night	15% 5% 14% 36% 25% 5%	800
	11th to 20th	Night	23% 6% 14% 33% 21% 3	800
	21st to end	Night	21% 6% 21% 32% 18% 1	880

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