Napa
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Napa clear sky chart from 2005-11-06 to 2025-10-19.

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% 4 2 4 3 3 3 3 4 6% 62%	135216
	Night	10% 5% 2 5% 3 3 3 3 4 6% 56%	131927
Transparency	Day	25% 4 7% 20% 24% 20%	130123
	Night	30% 4 8% 18% 22% 18%	128007
Seeing	Day	11% 9% 24% 31% 20% 5%	16597
	Night	11% 5% 14% 37% 28% 5%	25851

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	15% 8% 2 5% 3 3 4 3 5% 7% 47%	3823
	11th to 20th	Day	14% 7% 3 6% 3 3 3 2 2 4 52%	4100
	21st to end	Day	18% 6% 2 7% 4 3 4 2 4 6% 42%	4510
February	1st to 10th	Day	19% 6% 2 5% 4 4 3 3 5% 5% 44%	1800
	11th to 20th	Day	15% 7% 2 7% 4 4 4 4 5% 6% 41%	1822
	21st to end	Day	14% 6% 2 5% 4 3 4 3 4 6% 47%	1650
March	1st to 10th	Day	14% 12% 4 8% 4 4 4 3 5% 7% 35%	4868
	11th to 20th	Day	15% 5% 2 5% 4 4 4 3 5% 7% 47%	5000
	21st to end	Day	16% 8% 2 6% 4 5% 5% 4 7% 8% 36%	5563
April	1st to 10th	Day	13% 8% 2 5% 4 4 4 4 5% 7% 43%	2400
	11th to 20th	Day	9% 5% 2 5% 4 4 5% 3 5% 7% 52%	2369
	21st to end	Day	5% 5% 2 5% 4 3 3 3 5% 9% 58%	2400
May	1st to 10th	Day	4 2 1 2 1 2 1 2 3 5% 77%	5699
	11th to 20th	Day	3 2 1 3 2 2 2 2 3 6% 74%	6027
	21st to end	Day	2 2 1 3 2 2 2 2 4 6% 75%	6902
June	1st to 10th	Day	5% 5% 2 4 3 3 3 3 4 9% 60%	2800
	11th to 20th	Day	1 2 1 2 2 2 3 2 4 7% 74%	2800
	21st to end	Day	2 1 1 3 3 2 2 2 4 5% 75%	2800
July	1st to 10th	Day	1 1 3 2 2 2 1 2 4 82%	6319
	11th to 20th	Day	1 1 1 3 2 2 2 2 3 5% 77%	5980
	21st to end	Day	3 2 3 2 2 2 2 4 6% 75%	6402
August	1st to 10th	Day	1 2 1 3 2 2 3 3 4 6% 73%	5777
	11th to 20th	Day	2 1 2 2 2 2 1 2 4 82%	5577
	21st to end	Day	1 2 1 3 2 2 2 2 4 7% 75%	5984
September	1st to 10th	Day	1 2 1 3 2 2 2 2 3 5% 77%	2400
	11th to 20th	Day	3 4 2 5% 3 2 3 3 4 6% 66%	2400
	21st to end	Day	4 3 1 2 2 2 2 2 3 5% 74%	2115
October	1st to 10th	Day	2 3 1 3 2 2 2 2 4 5% 73%	3280
	11th to 20th	Day	5% 4 2 4 2 2 2 2 4 5% 67%	3242
	21st to end	Day	6% 4 2 4 3 3 3 3 4 8% 60%	3448
November	1st to 10th	Day	10% 4 1 3 3 3 3 3 5% 7% 57%	1907
	11th to 20th	Day	13% 6% 3 6% 3 4 3 3 5% 7% 48%	1800
	21st to end	Day	13% 4 1 4 3 3 3 3 6% 7% 53%	1600
December	1st to 10th	Day	9% 5% 1 4 2 3 3 4 5% 9% 55%	2620
	11th to 20th	Day	27% 8% 3 6% 4 3 4 4 5% 7% 30%	3028
	21st to end	Day	29% 8% 2 6% 3 3 3 4 4 7% 30%	4004
January	1st to 10th	Night	11% 6% 2 5% 3 4 4 3 5% 8% 49%	6115
	11th to 20th	Night	15% 5% 2 5% 3 3 3 2 4 4 52%	5880
	21st to end	Night	17% 8% 2 5% 4 3 3 3 4 6% 46%	6468
February	1st to 10th	Night	19% 6% 2 4 3 3 4 3 4 5% 49%	2600
	11th to 20th	Night	14% 6% 3 6% 4 3 4 3 4 8% 45%	2462
	21st to end	Night	12% 6% 1 4 3 2 3 2 5% 7% 53%	1980
March	1st to 10th	Night	16% 11% 3 6% 4 4 4 3 5% 6% 39%	5013
	11th to 20th	Night	16% 6% 2 5% 3 3 3 3 5% 6% 49%	5000
	21st to end	Night	17% 10% 3 5% 3 3 3 2 4 7% 43%	5410
April	1st to 10th	Night	10% 6% 2 5% 4 4 3 4 4 7% 51%	2000
	11th to 20th	Night	9% 7% 2 5% 3 2 3 2 4 6% 57%	1979
	21st to end	Night	6% 4 1 4 3 3 3 3 5% 8% 61%	2000
May	1st to 10th	Night	4 2 1 3 2 2 2 2 3 5% 74%	4285
	11th to 20th	Night	2 3 2 4 3 3 4 2 4 5% 68%	3873
	21st to end	Night	2 2 1 3 2 2 2 2 4 5% 76%	3972
June	1st to 10th	Night	4 4 2 5% 3 3 3 3 5% 8% 61%	1600
	11th to 20th	Night	1 2 1 3 2 2 3 2 4 6% 75%	1600
	21st to end	Night	1 3 1 4 4 2 2 3 3 6% 71%	1600
July	1st to 10th	Night	1 1 1 6% 3 2 2 2 2 3 79%	3640
	11th to 20th	Night	1 4 4 8% 4 3 3 2 4 4 63%	3978
	21st to end	Night	1 11% 7% 10% 3 2 2 3 5% 6% 49%	4472
August	1st to 10th	Night	2 2 2 9% 6% 4 3 3 4 6% 59%	4035
	11th to 20th	Night	2 4 5% 3 2 2 3 3 5% 70%	4282
	21st to end	Night	2 2 1 4 3 3 4 3 3 5% 69%	5015
September	1st to 10th	Night	1 3 1 3 2 2 3 2 3 6% 76%	2000
	11th to 20th	Night	3 5% 2 6% 3 2 3 3 5% 6% 62%	2000
	21st to end	Night	3 3 1 4 3 2 1 2 4 5% 71%	2281
October	1st to 10th	Night	3 3 1 3 2 2 2 2 4 5% 73%	3920
	11th to 20th	Night	4 6% 3 5% 3 3 3 2 4 6% 62%	3873
	21st to end	Night	7% 4 1 3 2 3 3 3 4 6% 62%	4121
November	1st to 10th	Night	8% 6% 2 4 3 2 3 2 5% 8% 57%	2390
	11th to 20th	Night	11% 6% 2 5% 2 3 3 3 5% 7% 53%	2600
	21st to end	Night	11% 5% 2 4 3 3 3 3 4 6% 57%	2795
December	1st to 10th	Night	11% 3 1 4 3 4 3 3 5% 6% 58%	4560
	11th to 20th	Night	26% 9% 2 5% 4 5% 4 4 6% 7% 29%	5176
	21st to end	Night	29% 8% 2 6% 4 3 3 3 4 5% 33%	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% 7% 15% 21% 13% 7%	3823
	11th to 20th	Day	37% 5% 7% 15% 20% 16%	4100
	21st to end	Day	44% 5% 7% 14% 12% 19%	4510
February	1st to 10th	Day	42% 4 6% 19% 20% 9%	1800
	11th to 20th	Day	42% 6% 8% 18% 19% 7%	1831
	21st to end	Day	38% 4 8% 23% 18% 8%	1650
March	1st to 10th	Day	46% 7% 7% 19% 18% 3	4868
	11th to 20th	Day	36% 5% 12% 27% 11% 8%	5000
	21st to end	Day	42% 9% 14% 22% 11% 2	5563
April	1st to 10th	Day	38% 7% 12% 21% 18% 5%	2400
	11th to 20th	Day	31% 6% 13% 24% 20% 7%	2372
	21st to end	Day	23% 5% 13% 31% 23% 5%	2400
May	1st to 10th	Day	13% 2 7% 27% 31% 21%	5699
	11th to 20th	Day	12% 4 5% 34% 35% 10%	6030
	21st to end	Day	11% 3 9% 35% 37% 5%	6904
June	1st to 10th	Day	22% 5% 9% 30% 27% 8%	2800
	11th to 20th	Day	12% 3 8% 21% 37% 19%	2800
	21st to end	Day	14% 2 3 15% 36% 31%	2800
July	1st to 10th	Day	9% 1 1 7% 23% 58%	6251
	11th to 20th	Day	11% 2 2 6% 16% 63%	5908
	21st to end	Day	13% 2 4 16% 26% 39%	6217
August	1st to 10th	Day	18% 3 3 11% 27% 39%	4206
	11th to 20th	Day	9% 1 2 9% 44% 34%	4036
	21st to end	Day	12% 2 3 12% 37% 34%	4311
September	1st to 10th	Day	12% 2 2 11% 30% 43%	2400
	11th to 20th	Day	19% 3 5% 23% 36% 13%	2400
	21st to end	Day	15% 3 4 19% 33% 26%	2115
October	1st to 10th	Day	14% 3 4 11% 29% 39%	3280
	11th to 20th	Day	20% 3 5% 23% 27% 22%	3242
	21st to end	Day	24% 3 7% 23% 29% 14%	3448
November	1st to 10th	Day	26% 5% 8% 27% 21% 13%	1907
	11th to 20th	Day	36% 5% 9% 24% 19% 7%	1800
	21st to end	Day	31% 4 9% 20% 23% 13%	1600
December	1st to 10th	Day	26% 4 7% 31% 25% 7%	2620
	11th to 20th	Day	52% 5% 8% 17% 15% 3	3028
	21st to end	Day	54% 6% 10% 18% 10% 2	4004
January	1st to 10th	Night	32% 9% 21% 20% 14% 6%	6115
	11th to 20th	Night	35% 6% 9% 16% 21% 14%	5880
	21st to end	Night	40% 5% 8% 11% 19% 18%	6468
February	1st to 10th	Night	39% 4 8% 18% 20% 12%	2600
	11th to 20th	Night	38% 5% 10% 19% 19% 9%	2464
	21st to end	Night	31% 5% 9% 26% 20% 8%	1980
March	1st to 10th	Night	42% 9% 9% 17% 20% 3	5013
	11th to 20th	Night	35% 6% 10% 23% 18% 8%	5000
	21st to end	Night	40% 7% 12% 25% 13% 3	5410
April	1st to 10th	Night	34% 5% 12% 21% 21% 6%	2000
	11th to 20th	Night	29% 5% 10% 28% 17% 11%	1979
	21st to end	Night	22% 5% 12% 31% 25% 6%	2000
May	1st to 10th	Night	16% 2 6% 27% 29% 19%	4282
	11th to 20th	Night	17% 3 6% 28% 30% 16%	3873
	21st to end	Night	12% 3 6% 33% 37% 10%	3972
June	1st to 10th	Night	22% 5% 9% 25% 30% 9%	1600
	11th to 20th	Night	12% 3 6% 21% 37% 21%	1600
	21st to end	Night	16% 2 2 14% 32% 33%	1600
July	1st to 10th	Night	12% 2 1 7% 23% 55%	3600
	11th to 20th	Night	25% 2 2 7% 12% 53%	3927
	21st to end	Night	36% 2 2 10% 18% 32%	4345
August	1st to 10th	Night	33% 2 3 9% 22% 31%	2929
	11th to 20th	Night	15% 2 2 8% 38% 35%	3094
	21st to end	Night	16% 4 3 10% 33% 34%	3608
September	1st to 10th	Night	12% 2 3 9% 31% 44%	2000
	11th to 20th	Night	23% 3 4 21% 33% 16%	2000
	21st to end	Night	17% 3 4 18% 29% 30%	2281
October	1st to 10th	Night	15% 2 4 11% 21% 47%	3920
	11th to 20th	Night	25% 3 5% 22% 22% 23%	3873
	21st to end	Night	23% 3 6% 20% 27% 20%	4121
November	1st to 10th	Night	27% 5% 9% 26% 19% 15%	2390
	11th to 20th	Night	31% 4 11% 26% 19% 9%	2600
	21st to end	Night	30% 4 8% 22% 23% 14%	2795
December	1st to 10th	Night	28% 3 7% 23% 26% 14%	4560
	11th to 20th	Night	52% 6% 9% 16% 15% 3	5176
	21st to end	Night	54% 6% 10% 16% 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	33% 9% 22% 24% 11% 1	280
	11th to 20th	Day	20% 11% 23% 29% 15% 4	400
	21st to end	Day	18% 7% 24% 28% 21% 2	440
February	1st to 10th	Day	23% 11% 20% 26% 18% 2	400
	11th to 20th	Day	22% 15% 27% 22% 12% 2	430
	21st to end	Day	18% 12% 23% 30% 14% 2	414
March	1st to 10th	Day	26% 14% 26% 22% 10% 1	500
	11th to 20th	Day	24% 14% 25% 19% 14% 4	500
	21st to end	Day	19% 16% 29% 25% 10% 2	550
April	1st to 10th	Day	18% 14% 28% 24% 12% 4	500
	11th to 20th	Day	14% 17% 28% 23% 14% 4	492
	21st to end	Day	8% 15% 27% 28% 16% 6%	500
May	1st to 10th	Day	10% 14% 31% 28% 13% 4	503
	11th to 20th	Day	10% 12% 29% 27% 19% 3	528
	21st to end	Day	6% 11% 28% 32% 20% 3	573
June	1st to 10th	Day	6% 10% 29% 33% 18% 5%	530
	11th to 20th	Day	3 11% 32% 39% 14% 2	530
	21st to end	Day	1 6% 26% 41% 23% 2	530
July	1st to 10th	Day	1 5% 23% 45% 23% 3	526
	11th to 20th	Day	1 2 20% 44% 27% 6%	524
	21st to end	Day	1 2 19% 47% 25% 6%	575
August	1st to 10th	Day	2 5% 20% 41% 25% 6%	527
	11th to 20th	Day	2 4 23% 37% 26% 8%	517
	21st to end	Day	2 3 19% 40% 29% 8%	583
September	1st to 10th	Day	2 4 21% 40% 26% 7%	530
	11th to 20th	Day	6% 7% 22% 37% 22% 7%	530
	21st to end	Day	5% 7% 22% 35% 25% 7%	530
October	1st to 10th	Day	5% 9% 24% 26% 27% 10%	530
	11th to 20th	Day	9% 8% 19% 28% 30% 7%	523
	21st to end	Day	11% 8% 22% 27% 26% 6%	539
November	1st to 10th	Day	13% 9% 17% 24% 32% 5%	443
	11th to 20th	Day	13% 7% 18% 27% 29% 6%	300
	21st to end	Day	15% 7% 20% 32% 24% 3	200
December	1st to 10th	Day	21% 8% 23% 25% 16% 8%	200
	11th to 20th	Day	25% 13% 15% 27% 16% 6%	200
	21st to end	Day	28% 10% 22% 23% 15% 1	220
January	1st to 10th	Night	22% 4 15% 33% 22% 6%	800
	11th to 20th	Night	19% 5% 14% 37% 21% 5%	800
	21st to end	Night	19% 4 14% 35% 24% 4	880
February	1st to 10th	Night	21% 7% 12% 35% 21% 4	800
	11th to 20th	Night	16% 9% 16% 33% 22% 4	784
	21st to end	Night	16% 6% 18% 37% 19% 2	660
March	1st to 10th	Night	22% 8% 16% 30% 21% 3	800
	11th to 20th	Night	20% 6% 15% 32% 24% 3	800
	21st to end	Night	17% 7% 21% 36% 18% 2	880
April	1st to 10th	Night	12% 8% 20% 38% 20% 3	800
	11th to 20th	Night	13% 9% 19% 33% 22% 4	791
	21st to end	Night	8% 6% 23% 36% 24% 4	800
May	1st to 10th	Night	7% 5% 21% 37% 27% 4	681
	11th to 20th	Night	5% 5% 15% 37% 33% 5%	508
	21st to end	Night	5% 6% 16% 41% 29% 3	433
June	1st to 10th	Night	5% 4 15% 45% 29% 2	400
	11th to 20th	Night	2 5% 18% 49% 25% 1	400
	21st to end	Night	3 2 13% 51% 32% 1	400
July	1st to 10th	Night	1 1 9% 49% 38% 2	390
	11th to 20th	Night	2 1 7% 45% 39% 6%	528
	21st to end	Night	2 2 8% 42% 41% 5%	657
August	1st to 10th	Night	3 4 10% 36% 42% 6%	597
	11th to 20th	Night	1 2 11% 41% 37% 7%	694
	21st to end	Night	2 1 8% 40% 42% 6%	880
September	1st to 10th	Night	2 2 10% 41% 40% 6%	800
	11th to 20th	Night	5% 2 10% 39% 36% 7%	800
	21st to end	Night	4 2 13% 35% 39% 7%	800
October	1st to 10th	Night	6% 4 11% 37% 34% 8%	800
	11th to 20th	Night	8% 5% 12% 35% 33% 7%	792
	21st to end	Night	9% 4 12% 32% 34% 9%	836
November	1st to 10th	Night	10% 5% 13% 31% 33% 8%	780
	11th to 20th	Night	14% 4 13% 35% 28% 6%	800
	21st to end	Night	14% 5% 13% 37% 25% 7%	800
December	1st to 10th	Night	19% 6% 13% 34% 23% 6%	800
	11th to 20th	Night	26% 5% 12% 34% 20% 3	800
	21st to end	Night	23% 6% 20% 31% 19% 1	880

Page generated at 2025-10-15 22:50:29. Version ac07

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