Vacaville
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Vacaville clear sky chart from 2005-11-06 to 2025-10-04.

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% 3 1 3 2 2 3 2 4 6% 68%	135076
	Night	7% 3 1 3 2 3 3 3 4 7% 64%	131750
Transparency	Day	19% 4 8% 20% 26% 24%	129993
	Night	22% 4 9% 19% 23% 23%	127819
Seeing	Day	7% 6% 21% 32% 24% 9%	16498
	Night	9% 4 19% 41% 25% 3	25765

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	14% 6% 2 6% 3 3 3 4 6% 8% 45%	3754
	11th to 20th	Day	13% 7% 3 6% 4 3 3 2 3 5% 53%	4100
	21st to end	Day	17% 6% 3 6% 4 3 4 3 5% 6% 44%	4510
February	1st to 10th	Day	16% 5% 3 6% 4 4 4 3 6% 5% 45%	1800
	11th to 20th	Day	12% 5% 2 5% 4 5% 4 4 5% 7% 45%	1840
	21st to end	Day	11% 5% 2 5% 4 4 4 3 6% 7% 49%	1650
March	1st to 10th	Day	9% 8% 3 9% 5% 5% 5% 4 6% 8% 39%	4820
	11th to 20th	Day	11% 5% 2 4 3 4 3 3 6% 7% 52%	5000
	21st to end	Day	10% 5% 2 5% 4 4 6% 4 8% 9% 42%	5604
April	1st to 10th	Day	10% 7% 2 4 4 4 4 4 7% 7% 47%	2400
	11th to 20th	Day	8% 3 2 3 2 3 3 4 5% 8% 58%	2369
	21st to end	Day	2 3 1 3 3 3 4 3 5% 8% 63%	2400
May	1st to 10th	Day	3 2 1 2 1 1 1 1 3 5% 80%	5650
	11th to 20th	Day	1 1 1 1 1 2 2 4 6% 80%	6095
	21st to end	Day	1 1 1 1 1 1 2 3 6% 83%	6902
June	1st to 10th	Day	3 3 1 2 2 2 2 3 4 8% 69%	2800
	11th to 20th	Day	1 1 1 2 2 2 4 6% 82%	2800
	21st to end	Day	1 1 1 1 1 1 1 3 5% 84%	2800
July	1st to 10th	Day	1 1 1 2 3 92%	6319
	11th to 20th	Day	1 1 1 1 1 1 1 2 3 89%	6066
	21st to end	Day	1 1 1 1 1 3 6% 85%	6402
August	1st to 10th	Day	1 1 1 1 1 2 2 4 5% 82%	5777
	11th to 20th	Day	1 1 1 1 2 3 91%	5535
	21st to end	Day	1 1 1 1 1 1 1 2 5% 86%	5984
September	1st to 10th	Day	1 1 1 1 2 1 3 4 86%	2400
	11th to 20th	Day	1 2 1 2 2 2 2 3 4 7% 74%	2400
	21st to end	Day	2 2 1 2 2 2 2 2 3 5% 79%	2127
October	1st to 10th	Day	1 2 1 3 2 2 2 2 4 6% 76%	3211
	11th to 20th	Day	3 2 1 2 2 2 2 2 4 6% 73%	3161
	21st to end	Day	4 2 1 4 3 4 3 3 4 7% 65%	3448
November	1st to 10th	Day	7% 4 1 4 2 3 4 3 5% 7% 60%	1880
	11th to 20th	Day	9% 6% 2 6% 4 5% 4 3 6% 6% 50%	1800
	21st to end	Day	10% 4 2 3 3 4 4 3 5% 6% 55%	1620
December	1st to 10th	Day	8% 4 2 4 2 4 3 3 6% 10% 55%	2620
	11th to 20th	Day	24% 8% 2 7% 5% 4 5% 4 5% 7% 29%	3028
	21st to end	Day	23% 10% 3 7% 4 3 4 3 6% 8% 30%	4004
January	1st to 10th	Night	9% 5% 2 5% 4 3 4 4 6% 10% 49%	6192
	11th to 20th	Night	14% 5% 2 5% 3 3 3 3 4 5% 54%	5880
	21st to end	Night	13% 6% 2 6% 4 3 3 3 5% 6% 48%	6468
February	1st to 10th	Night	16% 5% 2 5% 4 4 4 3 4 5% 50%	2600
	11th to 20th	Night	10% 4 2 6% 4 4 4 3 5% 8% 49%	2445
	21st to end	Night	11% 4 1 5% 3 3 3 2 4 8% 56%	1980
March	1st to 10th	Night	9% 6% 3 7% 4 4 4 4 6% 8% 44%	5070
	11th to 20th	Night	12% 4 2 4 2 3 3 3 4 7% 56%	5000
	21st to end	Night	10% 6% 2 6% 4 4 4 3 5% 8% 48%	5369
April	1st to 10th	Night	6% 5% 2 3 5% 4 5% 3 5% 7% 56%	2000
	11th to 20th	Night	7% 4 1 3 3 3 3 3 5% 6% 63%	1979
	21st to end	Night	4 3 1 3 2 2 2 3 5% 8% 67%	2000
May	1st to 10th	Night	2 1 1 2 1 2 2 1 2 6% 79%	4335
	11th to 20th	Night	1 1 2 2 2 2 2 3 5% 79%	3805
	21st to end	Night	1 1 1 1 1 1 3 5% 85%	3972
June	1st to 10th	Night	3 2 1 3 2 3 3 3 4 7% 71%	1600
	11th to 20th	Night	1 1 1 1 1 1 2 2 3 5% 84%	1600
	21st to end	Night	1 1 1 1 1 1 2 3 5% 83%	1600
July	1st to 10th	Night	1 1 1 1 1 3 92%	3640
	11th to 20th	Night	1 1 1 1 1 1 1 2 4 89%	3888
	21st to end	Night	1 1 1 1 2 2 5% 10% 77%	4472
August	1st to 10th	Night	2 1 1 2 1 1 1 2 3 6% 81%	4035
	11th to 20th	Night	1 1 1 2 5% 89%	4323
	21st to end	Night	1 1 1 1 1 2 2 3 6% 83%	5015
September	1st to 10th	Night	1 1 1 1 1 3 7% 84%	2000
	11th to 20th	Night	2 1 1 3 1 1 2 2 4 7% 75%	2000
	21st to end	Night	1 1 2 2 2 2 2 2 5% 80%	2269
October	1st to 10th	Night	2 1 1 2 2 2 2 2 4 6% 77%	3836
	11th to 20th	Night	2 1 1 3 2 2 3 2 4 7% 75%	3777
	21st to end	Night	4 2 1 3 3 3 4 3 5% 7% 66%	4121
November	1st to 10th	Night	6% 4 1 3 3 3 3 3 5% 8% 60%	2416
	11th to 20th	Night	7% 5% 2 4 3 3 3 3 5% 8% 57%	2600
	21st to end	Night	9% 4 2 4 3 3 4 4 4 6% 58%	2775
December	1st to 10th	Night	8% 3 1 5% 3 3 4 3 4 6% 60%	4560
	11th to 20th	Night	20% 7% 2 6% 5% 5% 5% 4 8% 9% 28%	5176
	21st to end	Night	21% 8% 3 7% 5% 4 4 3 5% 5% 34%	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	37% 9% 17% 21% 12% 6%	3754
	11th to 20th	Day	36% 4 8% 17% 20% 14%	4100
	21st to end	Day	41% 5% 8% 15% 15% 16%	4510
February	1st to 10th	Day	40% 4 8% 21% 19% 8%	1800
	11th to 20th	Day	36% 6% 11% 20% 20% 7%	1849
	21st to end	Day	33% 5% 10% 25% 19% 9%	1650
March	1st to 10th	Day	39% 7% 9% 21% 20% 4	4820
	11th to 20th	Day	30% 5% 15% 26% 16% 8%	5000
	21st to end	Day	33% 9% 18% 27% 12% 2	5604
April	1st to 10th	Day	33% 6% 12% 23% 20% 6%	2400
	11th to 20th	Day	24% 5% 14% 25% 23% 9%	2372
	21st to end	Day	19% 4 13% 30% 28% 7%	2400
May	1st to 10th	Day	10% 2 6% 26% 31% 25%	5650
	11th to 20th	Day	7% 2 5% 33% 39% 14%	6098
	21st to end	Day	6% 2 8% 33% 41% 11%	6904
June	1st to 10th	Day	15% 4 10% 29% 31% 11%	2800
	11th to 20th	Day	6% 2 8% 22% 36% 25%	2800
	21st to end	Day	6% 1 3 14% 37% 39%	2800
July	1st to 10th	Day	2 1 2 6% 21% 68%	6251
	11th to 20th	Day	4 1 2 6% 13% 75%	5992
	21st to end	Day	4 2 5% 16% 28% 45%	6217
August	1st to 10th	Day	8% 2 2 11% 28% 49%	4206
	11th to 20th	Day	3 1 2 10% 42% 42%	4006
	21st to end	Day	5% 1 3 11% 39% 41%	4311
September	1st to 10th	Day	6% 1 3 9% 33% 49%	2400
	11th to 20th	Day	12% 2 6% 22% 41% 17%	2400
	21st to end	Day	11% 3 4 18% 35% 29%	2127
October	1st to 10th	Day	12% 2 4 12% 28% 42%	3211
	11th to 20th	Day	13% 4 7% 25% 29% 22%	3161
	21st to end	Day	19% 4 8% 24% 31% 15%	3448
November	1st to 10th	Day	23% 4 9% 29% 23% 12%	1880
	11th to 20th	Day	34% 5% 11% 25% 20% 6%	1800
	21st to end	Day	30% 4 11% 22% 24% 10%	1620
December	1st to 10th	Day	24% 4 10% 32% 22% 7%	2620
	11th to 20th	Day	51% 7% 11% 16% 11% 3	3028
	21st to end	Day	51% 7% 12% 17% 10% 2	4004
January	1st to 10th	Night	29% 9% 22% 21% 14% 5%	6192
	11th to 20th	Night	34% 5% 10% 17% 21% 13%	5880
	21st to end	Night	37% 5% 10% 13% 20% 15%	6468
February	1st to 10th	Night	37% 3 8% 21% 21% 10%	2600
	11th to 20th	Night	33% 4 10% 23% 20% 10%	2447
	21st to end	Night	29% 3 8% 27% 23% 9%	1980
March	1st to 10th	Night	35% 6% 13% 19% 22% 5%	5070
	11th to 20th	Night	29% 5% 14% 26% 18% 10%	5000
	21st to end	Night	33% 7% 14% 27% 16% 3	5369
April	1st to 10th	Night	27% 5% 14% 22% 23% 8%	2000
	11th to 20th	Night	22% 4 12% 28% 19% 14%	1979
	21st to end	Night	17% 3 12% 31% 31% 7%	2000
May	1st to 10th	Night	10% 2 7% 25% 32% 24%	4332
	11th to 20th	Night	9% 3 5% 27% 36% 21%	3805
	21st to end	Night	5% 1 7% 30% 39% 18%	3972
June	1st to 10th	Night	14% 4 10% 25% 34% 14%	1600
	11th to 20th	Night	6% 2 6% 19% 37% 30%	1600
	21st to end	Night	6% 2 3 11% 33% 44%	1600
July	1st to 10th	Night	3 1 5% 19% 72%	3600
	11th to 20th	Night	4 1 1 5% 10% 78%	3839
	21st to end	Night	5% 2 4 17% 25% 48%	4345
August	1st to 10th	Night	10% 2 2 8% 23% 55%	2929
	11th to 20th	Night	3 1 2 8% 33% 54%	3122
	21st to end	Night	6% 1 3 8% 34% 48%	3608
September	1st to 10th	Night	4 1 3 9% 29% 54%	2000
	11th to 20th	Night	11% 2 6% 20% 37% 25%	2000
	21st to end	Night	10% 2 4 18% 31% 36%	2269
October	1st to 10th	Night	11% 2 5% 11% 19% 52%	3836
	11th to 20th	Night	11% 3 7% 27% 27% 25%	3777
	21st to end	Night	18% 3 7% 21% 30% 22%	4121
November	1st to 10th	Night	23% 4 10% 26% 21% 16%	2416
	11th to 20th	Night	26% 3 13% 27% 22% 9%	2600
	21st to end	Night	27% 4 9% 22% 25% 14%	2775
December	1st to 10th	Night	26% 4 7% 27% 25% 12%	4560
	11th to 20th	Night	48% 7% 12% 18% 12% 2	5176
	21st to end	Night	51% 8% 12% 17% 10% 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	29% 6% 22% 24% 16% 3	250
	11th to 20th	Day	17% 6% 20% 31% 20% 7%	400
	21st to end	Day	15% 6% 19% 26% 26% 8%	440
February	1st to 10th	Day	19% 9% 20% 25% 21% 6%	400
	11th to 20th	Day	17% 10% 25% 26% 17% 6%	438
	21st to end	Day	12% 8% 28% 30% 17% 5%	414
March	1st to 10th	Day	17% 11% 25% 29% 12% 6%	500
	11th to 20th	Day	18% 7% 25% 24% 18% 8%	500
	21st to end	Day	14% 13% 25% 26% 18% 4	550
April	1st to 10th	Day	13% 9% 27% 25% 19% 8%	500
	11th to 20th	Day	8% 11% 31% 24% 18% 9%	492
	21st to end	Day	3 9% 27% 27% 24% 10%	500
May	1st to 10th	Day	6% 9% 29% 30% 21% 6%	503
	11th to 20th	Day	5% 6% 28% 28% 24% 9%	528
	21st to end	Day	2 6% 25% 32% 25% 9%	573
June	1st to 10th	Day	4 6% 26% 35% 22% 8%	530
	11th to 20th	Day	1 8% 23% 39% 24% 5%	530
	21st to end	Day	1 3 22% 39% 29% 6%	530
July	1st to 10th	Day	1 2 17% 45% 29% 7%	526
	11th to 20th	Day	1 13% 44% 33% 8%	524
	21st to end	Day	1 1 13% 44% 32% 10%	575
August	1st to 10th	Day	2 2 14% 43% 28% 10%	527
	11th to 20th	Day	1 2 18% 37% 29% 13%	517
	21st to end	Day	1 1 13% 37% 33% 15%	583
September	1st to 10th	Day	1 2 13% 35% 34% 15%	530
	11th to 20th	Day	2 5% 17% 33% 28% 15%	530
	21st to end	Day	3 4 19% 32% 28% 14%	530
October	1st to 10th	Day	2 7% 19% 27% 27% 17%	503
	11th to 20th	Day	6% 6% 18% 27% 27% 17%	490
	21st to end	Day	7% 5% 20% 27% 27% 13%	539
November	1st to 10th	Day	8% 6% 21% 23% 29% 13%	416
	11th to 20th	Day	12% 6% 19% 30% 25% 9%	300
	21st to end	Day	13% 5% 20% 35% 25% 2	210
December	1st to 10th	Day	19% 5% 19% 29% 21% 9%	200
	11th to 20th	Day	21% 5% 19% 30% 22% 5%	200
	21st to end	Day	21% 7% 26% 23% 19% 4	220
January	1st to 10th	Night	18% 4 16% 36% 21% 5%	800
	11th to 20th	Night	15% 6% 16% 39% 21% 4	800
	21st to end	Night	16% 4 16% 35% 25% 4	880
February	1st to 10th	Night	18% 6% 16% 37% 20% 3	800
	11th to 20th	Night	12% 5% 24% 33% 22% 4	784
	21st to end	Night	12% 7% 20% 40% 19% 2	660
March	1st to 10th	Night	15% 6% 21% 33% 23% 2	800
	11th to 20th	Night	16% 5% 20% 35% 22% 2	800
	21st to end	Night	12% 5% 25% 39% 17% 1	880
April	1st to 10th	Night	8% 5% 25% 42% 18% 2	800
	11th to 20th	Night	9% 6% 28% 35% 19% 3	791
	21st to end	Night	5% 4 27% 41% 22% 2	800
May	1st to 10th	Night	5% 3 29% 39% 22% 2	705
	11th to 20th	Night	3 2 23% 47% 23% 2	479
	21st to end	Night	3 3 29% 45% 20%	433
June	1st to 10th	Night	4 2 25% 50% 19%	400
	11th to 20th	Night	1 2 28% 53% 16%	400
	21st to end	Night	1 1 27% 51% 20%	400
July	1st to 10th	Night	1 1 18% 59% 21%	390
	11th to 20th	Night	1 15% 56% 26% 2	492
	21st to end	Night	1 1 14% 54% 28% 2	657
August	1st to 10th	Night	2 19% 48% 28% 3	597
	11th to 20th	Night	1 17% 51% 29% 2	709
	21st to end	Night	1 1 14% 49% 35% 1	880
September	1st to 10th	Night	1 2 13% 48% 34% 3	800
	11th to 20th	Night	2 1 18% 44% 30% 4	800
	21st to end	Night	2 2 17% 40% 35% 5%	800
October	1st to 10th	Night	3 3 14% 42% 32% 6%	772
	11th to 20th	Night	4 3 17% 38% 32% 5%	760
	21st to end	Night	6% 3 18% 35% 31% 8%	836
November	1st to 10th	Night	7% 4 18% 32% 34% 6%	780
	11th to 20th	Night	9% 3 17% 39% 28% 5%	800
	21st to end	Night	11% 6% 16% 40% 24% 5%	800
December	1st to 10th	Night	16% 5% 16% 35% 23% 5%	800
	11th to 20th	Night	22% 4 16% 34% 22% 3	800
	21st to end	Night	19% 7% 21% 32% 19% 1	880

Page generated at 2025-09-30 22:39:08. Version ac07

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