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 2024-11-05.

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% 60%	105103
	Night	10% 5% 2 5% 3 3 3 3 4 6% 56%	102682
Transparency	Day	25% 4 7% 19% 24% 21%	104861
	Night	30% 4 8% 18% 21% 19%	102513
Seeing	Day	11% 9% 23% 32% 21% 5%	15353
	Night	11% 5% 14% 37% 29% 5%	24624

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	19% 10% 2 6% 4 3 3 3 4 5% 39%	2667
	11th to 20th	Day	21% 9% 4 8% 4 4 4 3 3 5% 34%	2860
	21st to end	Day	23% 8% 3 7% 4 4 5% 3 5% 7% 31%	3146
February	1st to 10th	Day	17% 6% 2 5% 4 4 3 3 5% 6% 44%	1710
	11th to 20th	Day	15% 7% 2 7% 4 4 4 3 5% 6% 43%	1728
	21st to end	Day	15% 6% 2 5% 4 4 4 3 4 6% 47%	1570
March	1st to 10th	Day	18% 14% 4 9% 5% 4 4 3 5% 6% 29%	3384
	11th to 20th	Day	12% 4 2 4 4 3 3 3 5% 6% 55%	3490
	21st to end	Day	17% 8% 2 6% 4 4 4 5% 6% 7% 36%	3876
April	1st to 10th	Day	13% 8% 2 5% 4 4 4 4 6% 7% 42%	2280
	11th to 20th	Day	9% 5% 2 5% 4 4 4 3 5% 7% 52%	2249
	21st to end	Day	5% 4 2 4 3 3 3 3 5% 9% 59%	2280
May	1st to 10th	Day	6% 3 1 2 2 2 2 2 3 5% 72%	3981
	11th to 20th	Day	3 2 1 3 2 2 2 2 4 6% 72%	4205
	21st to end	Day	2 2 1 4 2 2 2 2 4 6% 71%	4813
June	1st to 10th	Day	5% 5% 2 4 3 3 3 3 4 9% 59%	2660
	11th to 20th	Day	1 2 1 2 2 2 3 2 4 7% 74%	2660
	21st to end	Day	2 2 1 3 3 2 2 2 4 5% 74%	2660
July	1st to 10th	Day	1 1 1 3 2 1 1 1 2 4 82%	4420
	11th to 20th	Day	1 1 1 3 2 2 2 2 4 6% 77%	4184
	21st to end	Day	2 1 3 2 2 2 2 4 5% 78%	4455
August	1st to 10th	Day	1 2 1 4 3 3 4 4 4 7% 64%	4091
	11th to 20th	Day	1 1 2 2 2 2 2 3 4 81%	3872
	21st to end	Day	1 2 1 3 2 2 2 2 3 6% 76%	4180
September	1st to 10th	Day	1 2 1 3 2 2 2 2 3 5% 79%	2280
	11th to 20th	Day	3 4 2 5% 3 2 3 3 4 6% 67%	2280
	21st to end	Day	3 3 1 2 2 2 2 2 3 5% 75%	2010
October	1st to 10th	Day	2 3 1 3 2 2 2 2 5% 5% 74%	3180
	11th to 20th	Day	5% 4 2 4 2 2 2 2 4 5% 67%	3161
	21st to end	Day	6% 4 2 4 3 3 3 3 4 8% 60%	3448
November	1st to 10th	Day	11% 4 1 4 3 3 3 3 5% 7% 56%	1851
	11th to 20th	Day	12% 6% 3 6% 3 4 3 3 5% 6% 49%	1710
	21st to end	Day	12% 4 1 4 4 3 3 3 5% 7% 53%	1520
December	1st to 10th	Day	16% 7% 2 5% 3 3 4 4 4 6% 45%	1520
	11th to 20th	Day	30% 9% 3 6% 4 2 2 3 4 5% 31%	1928
	21st to end	Day	23% 8% 2 6% 4 3 3 4 4 7% 36%	2794
January	1st to 10th	Night	15% 8% 2 5% 3 4 3 3 4 7% 45%	4280
	11th to 20th	Night	22% 8% 3 7% 4 4 4 3 6% 6% 34%	4110
	21st to end	Night	23% 10% 3 6% 4 4 4 3 4 6% 33%	4521
February	1st to 10th	Night	17% 6% 2 4 3 3 4 3 4 5% 49%	2470
	11th to 20th	Night	13% 6% 3 6% 4 3 4 4 4 9% 45%	2336
	21st to end	Night	13% 6% 1 4 4 2 3 2 5% 7% 53%	1884
March	1st to 10th	Night	19% 12% 3 6% 4 4 3 3 5% 5% 36%	3477
	11th to 20th	Night	13% 4 1 5% 3 3 3 2 4 5% 57%	3490
	21st to end	Night	16% 10% 3 6% 4 4 3 2 4 6% 41%	3775
April	1st to 10th	Night	10% 6% 2 6% 4 4 3 4 4 7% 51%	1900
	11th to 20th	Night	9% 6% 2 5% 3 2 3 2 4 6% 58%	1879
	21st to end	Night	6% 3 1 4 3 3 2 3 5% 8% 62%	1900
May	1st to 10th	Night	5% 2 1 3 2 2 2 2 3 4 74%	2989
	11th to 20th	Night	2 3 2 4 3 3 4 2 4 6% 67%	2689
	21st to end	Night	2 2 2 3 2 2 2 2 3 5% 73%	2762
June	1st to 10th	Night	4 4 2 4 3 3 3 3 5% 8% 61%	1520
	11th to 20th	Night	1 2 1 3 2 2 3 2 4 6% 74%	1520
	21st to end	Night	2 3 1 4 4 2 2 3 3 6% 70%	1520
July	1st to 10th	Night	1 1 1 5% 2 1 1 1 2 4 81%	2540
	11th to 20th	Night	1 2 1 5% 3 3 3 3 4 5% 70%	2780
	21st to end	Night	1 4 4 10% 4 3 3 4 4 7% 56%	3109
August	1st to 10th	Night	3 3 2 11% 8% 5% 3 3 5% 8% 49%	2848
	11th to 20th	Night	1 2 4 3 2 2 3 4 5% 72%	2972
	21st to end	Night	3 2 1 5% 3 2 3 2 3 5% 71%	3498
September	1st to 10th	Night	1 2 1 3 2 2 2 2 3 6% 77%	1900
	11th to 20th	Night	3 4 2 6% 3 2 4 3 5% 6% 63%	1900
	21st to end	Night	2 2 1 4 3 2 1 2 4 6% 72%	2166
October	1st to 10th	Night	2 2 1 3 2 2 2 2 4 5% 73%	3800
	11th to 20th	Night	4 6% 3 5% 3 3 3 2 4 6% 62%	3777
	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%	2326
	11th to 20th	Night	11% 6% 2 5% 3 3 3 3 5% 7% 53%	2470
	21st to end	Night	10% 5% 2 4 4 3 3 3 4 6% 58%	2655
December	1st to 10th	Night	18% 5% 2 6% 3 4 2 3 5% 6% 47%	2660
	11th to 20th	Night	31% 7% 2 4 3 3 3 4 5% 6% 32%	3276
	21st to end	Night	20% 7% 3 7% 5% 4 4 3 5% 6% 37%	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% 6% 12% 26% 8% 3	2667
	11th to 20th	Day	52% 6% 10% 14% 11% 7%	2860
	21st to end	Day	53% 5% 9% 17% 11% 5%	3146
February	1st to 10th	Day	41% 4 7% 20% 19% 9%	1710
	11th to 20th	Day	41% 6% 7% 18% 20% 8%	1737
	21st to end	Day	39% 4 9% 23% 18% 8%	1570
March	1st to 10th	Day	52% 9% 9% 16% 13% 2	3384
	11th to 20th	Day	30% 5% 9% 30% 14% 12%	3490
	21st to end	Day	41% 10% 14% 20% 13% 3	3876
April	1st to 10th	Day	38% 7% 12% 20% 17% 5%	2280
	11th to 20th	Day	30% 6% 13% 24% 20% 7%	2252
	21st to end	Day	22% 5% 13% 30% 24% 5%	2280
May	1st to 10th	Day	16% 3 10% 28% 25% 18%	3981
	11th to 20th	Day	14% 4 5% 30% 34% 13%	4208
	21st to end	Day	14% 4 12% 39% 26% 5%	4814
June	1st to 10th	Day	23% 5% 9% 29% 26% 8%	2660
	11th to 20th	Day	12% 3 9% 21% 37% 18%	2660
	21st to end	Day	14% 2 3 15% 35% 31%	2660
July	1st to 10th	Day	9% 1 2 7% 15% 66%	4351
	11th to 20th	Day	10% 2 3 8% 18% 59%	4112
	21st to end	Day	11% 2 2 7% 26% 53%	4315
August	1st to 10th	Day	19% 3 3 10% 26% 39%	4076
	11th to 20th	Day	9% 1 2 9% 45% 33%	3911
	21st to end	Day	12% 2 3 13% 36% 34%	4179
September	1st to 10th	Day	11% 2 2 10% 31% 45%	2280
	11th to 20th	Day	19% 3 5% 23% 35% 14%	2280
	21st to end	Day	14% 3 4 19% 34% 27%	2010
October	1st to 10th	Day	14% 2 3 11% 29% 40%	3180
	11th to 20th	Day	20% 3 5% 23% 27% 23%	3161
	21st to end	Day	24% 3 7% 23% 29% 14%	3448
November	1st to 10th	Day	27% 5% 8% 27% 20% 12%	1851
	11th to 20th	Day	35% 5% 9% 23% 19% 8%	1710
	21st to end	Day	30% 4 9% 21% 23% 13%	1520
December	1st to 10th	Day	39% 6% 9% 20% 19% 8%	1520
	11th to 20th	Day	56% 4 8% 12% 16% 4	1928
	21st to end	Day	47% 7% 11% 19% 11% 3	2794
January	1st to 10th	Night	38% 7% 19% 23% 9% 4	4280
	11th to 20th	Night	50% 7% 12% 12% 11% 8%	4110
	21st to end	Night	52% 5% 11% 14% 13% 6%	4521
February	1st to 10th	Night	38% 4 8% 19% 20% 11%	2470
	11th to 20th	Night	37% 5% 10% 19% 20% 10%	2338
	21st to end	Night	31% 5% 10% 26% 21% 6%	1884
March	1st to 10th	Night	46% 10% 11% 15% 15% 3	3477
	11th to 20th	Night	30% 6% 8% 25% 22% 11%	3490
	21st to end	Night	41% 8% 12% 24% 12% 4	3775
April	1st to 10th	Night	34% 5% 13% 21% 21% 6%	1900
	11th to 20th	Night	28% 5% 10% 29% 17% 11%	1879
	21st to end	Night	21% 4 12% 31% 26% 6%	1900
May	1st to 10th	Night	16% 3 8% 26% 29% 18%	2989
	11th to 20th	Night	17% 4 7% 20% 30% 22%	2689
	21st to end	Night	13% 4 8% 39% 26% 10%	2762
June	1st to 10th	Night	22% 5% 9% 25% 30% 9%	1520
	11th to 20th	Night	12% 4 6% 22% 36% 20%	1520
	21st to end	Night	16% 3 3 14% 32% 33%	1520
July	1st to 10th	Night	10% 1 2 6% 16% 65%	2500
	11th to 20th	Night	16% 2 3 9% 14% 57%	2729
	21st to end	Night	28% 2 2 7% 18% 44%	3011
August	1st to 10th	Night	33% 2 3 9% 21% 32%	2839
	11th to 20th	Night	15% 2 2 8% 39% 35%	2999
	21st to end	Night	16% 3 3 10% 33% 34%	3498
September	1st to 10th	Night	11% 2 3 8% 31% 46%	1900
	11th to 20th	Night	22% 3 4 20% 34% 17%	1900
	21st to end	Night	16% 3 4 18% 29% 31%	2166
October	1st to 10th	Night	15% 2 4 10% 21% 48%	3800
	11th to 20th	Night	25% 3 5% 22% 22% 23%	3777
	21st to end	Night	23% 3 6% 20% 27% 20%	4121
November	1st to 10th	Night	27% 5% 9% 26% 19% 14%	2326
	11th to 20th	Night	31% 4 11% 26% 19% 9%	2470
	21st to end	Night	28% 4 8% 22% 23% 14%	2655
December	1st to 10th	Night	39% 4 10% 18% 17% 12%	2660
	11th to 20th	Night	52% 6% 9% 13% 16% 4	3276
	21st to end	Night	47% 8% 13% 16% 13% 4	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	34% 10% 21% 22% 12% 2	256
	11th to 20th	Day	21% 10% 22% 28% 15% 4	370
	21st to end	Day	19% 7% 24% 27% 21% 2	407
February	1st to 10th	Day	20% 12% 20% 27% 19% 2	370
	11th to 20th	Day	22% 14% 26% 23% 13% 3	396
	21st to end	Day	19% 13% 23% 29% 14% 2	382
March	1st to 10th	Day	27% 14% 25% 22% 10% 1	460
	11th to 20th	Day	23% 13% 25% 20% 15% 5%	460
	21st to end	Day	17% 16% 30% 25% 9% 2	506
April	1st to 10th	Day	19% 13% 28% 24% 13% 4	460
	11th to 20th	Day	15% 17% 27% 23% 14% 5%	452
	21st to end	Day	8% 15% 27% 28% 15% 7%	460
May	1st to 10th	Day	11% 14% 30% 28% 13% 4	463
	11th to 20th	Day	9% 12% 29% 27% 19% 3	488
	21st to end	Day	6% 11% 26% 33% 22% 4	529
June	1st to 10th	Day	6% 11% 26% 33% 19% 5%	490
	11th to 20th	Day	2 11% 30% 40% 14% 2	490
	21st to end	Day	1 5% 26% 42% 23% 3	490
July	1st to 10th	Day	1 4 20% 47% 24% 3	486
	11th to 20th	Day	1 2 20% 43% 28% 6%	484
	21st to end	Day	1 2 17% 48% 26% 6%	531
August	1st to 10th	Day	2 5% 19% 42% 25% 6%	487
	11th to 20th	Day	2 4 23% 36% 27% 8%	477
	21st to end	Day	2 3 18% 40% 29% 8%	539
September	1st to 10th	Day	2 3 19% 40% 28% 8%	490
	11th to 20th	Day	6% 7% 22% 36% 22% 7%	490
	21st to end	Day	3 7% 21% 36% 26% 7%	490
October	1st to 10th	Day	4 8% 24% 27% 27% 10%	490
	11th to 20th	Day	8% 8% 19% 28% 30% 7%	490
	21st to end	Day	11% 8% 22% 27% 26% 6%	539
November	1st to 10th	Day	13% 8% 18% 24% 32% 5%	423
	11th to 20th	Day	11% 6% 15% 28% 32% 7%	270
	21st to end	Day	13% 7% 19% 32% 26% 3	180
December	1st to 10th	Day	23% 8% 22% 26% 14% 8%	180
	11th to 20th	Day	24% 13% 14% 27% 16% 6%	180
	21st to end	Day	25% 10% 22% 25% 17% 2	198
January	1st to 10th	Night	22% 4 14% 32% 22% 6%	760
	11th to 20th	Night	20% 5% 13% 36% 21% 5%	760
	21st to end	Night	20% 4 14% 35% 23% 4	836
February	1st to 10th	Night	20% 7% 12% 35% 23% 4	760
	11th to 20th	Night	16% 9% 15% 32% 23% 5%	744
	21st to end	Night	16% 7% 18% 37% 20% 3	628
March	1st to 10th	Night	23% 8% 16% 30% 21% 3	760
	11th to 20th	Night	19% 6% 15% 32% 25% 3	760
	21st to end	Night	16% 6% 21% 37% 17% 2	836
April	1st to 10th	Night	12% 7% 20% 37% 20% 3	760
	11th to 20th	Night	12% 10% 18% 33% 23% 4	751
	21st to end	Night	8% 6% 23% 36% 24% 4	760
May	1st to 10th	Night	7% 4 20% 37% 28% 4	647
	11th to 20th	Night	4 5% 14% 38% 34% 5%	482
	21st to end	Night	5% 6% 15% 41% 30% 3	411
June	1st to 10th	Night	5% 4 13% 46% 30% 2	380
	11th to 20th	Night	2 4 17% 49% 26% 1	380
	21st to end	Night	3 1 11% 51% 32% 1	380
July	1st to 10th	Night	1 1 7% 49% 39% 2	370
	11th to 20th	Night	2 1 6% 44% 41% 6%	501
	21st to end	Night	2 2 6% 42% 42% 5%	624
August	1st to 10th	Night	3 4 9% 35% 42% 7%	567
	11th to 20th	Night	2 2 10% 41% 39% 7%	659
	21st to end	Night	2 1 7% 39% 43% 7%	836
September	1st to 10th	Night	1 1 8% 41% 42% 6%	760
	11th to 20th	Night	5% 2 10% 40% 36% 7%	760
	21st to end	Night	3 2 12% 35% 40% 7%	760
October	1st to 10th	Night	5% 4 11% 37% 35% 8%	760
	11th to 20th	Night	7% 5% 12% 35% 33% 7%	760
	21st to end	Night	9% 4 12% 32% 34% 9%	836
November	1st to 10th	Night	11% 5% 13% 31% 33% 8%	760
	11th to 20th	Night	13% 4 12% 36% 29% 6%	760
	21st to end	Night	12% 5% 12% 37% 26% 7%	760
December	1st to 10th	Night	20% 6% 13% 33% 23% 6%	760
	11th to 20th	Night	25% 5% 12% 34% 21% 3	760
	21st to end	Night	22% 6% 19% 32% 19% 1	836

Page generated at 2024-11-02 23:11:09. Version ac07

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