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 2024-04-10.

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	10% 5% 2 4 4 2 3 4 4 6% 56%	83360
	Night	12% 5% 2 4 4 2 3 4 4 7% 53%	85507
Transparency	Day	29% 5% 8% 20% 22% 16%	83078
	Night	32% 5% 9% 19% 20% 15%	85310
Seeing	Day	11% 9% 23% 32% 21% 5%	14517
	Night	12% 5% 13% 36% 29% 5%	23949

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% 3 5% 5% 2 4 3 4 6% 39%	2667
	11th to 20th	Day	21% 9% 4 7% 5% 3 4 4 3 5% 34%	2860
	21st to end	Day	23% 8% 3 6% 5% 3 5% 4 4 7% 32%	3146
February	1st to 10th	Day	17% 6% 2 5% 5% 3 4 4 5% 6% 44%	1710
	11th to 20th	Day	15% 6% 3 6% 4 3 5% 4 4 6% 43%	1728
	21st to end	Day	15% 6% 3 4 5% 3 4 4 4 6% 47%	1570
March	1st to 10th	Day	18% 14% 4 8% 5% 3 4 4 4 6% 30%	3384
	11th to 20th	Day	12% 4 2 3 4 2 4 3 4 6% 55%	3490
	21st to end	Day	17% 8% 2 5% 5% 3 4 6% 6% 8% 36%	3876
April	1st to 10th	Day	14% 6% 3 4 4 3 4 5% 6% 7% 42%	2270
	11th to 20th	Day	9% 4 2 4 4 2 4 4 5% 7% 53%	2129
	21st to end	Day	5% 4 2 4 4 2 3 4 5% 9% 58%	2160
May	1st to 10th	Day	6% 4 2 4 3 2 3 3 4 7% 62%	2263
	11th to 20th	Day	6% 3 1 4 3 2 3 4 4 8% 62%	2370
	21st to end	Day	4 4 2 4 4 2 4 4 5% 8% 58%	2723
June	1st to 10th	Day	5% 4 2 3 3 2 3 4 4 9% 59%	2520
	11th to 20th	Day	1 2 1 2 3 2 2 3 4 7% 74%	2520
	21st to end	Day	2 1 1 2 3 1 3 3 4 5% 74%	2520
July	1st to 10th	Day	1 2 1 4 3 1 3 2 3 6% 75%	2520
	11th to 20th	Day	1 2 1 3 3 2 2 3 4 7% 73%	2388
	21st to end	Day	1 1 1 2 3 2 2 3 4 7% 74%	2546
August	1st to 10th	Day	2 3 1 3 4 2 3 3 4 6% 70%	2328
	11th to 20th	Day	1 2 1 3 2 2 3 3 3 5% 74%	2209
	21st to end	Day	1 2 2 3 3 1 2 2 3 7% 74%	2376
September	1st to 10th	Day	1 2 1 3 2 1 2 2 3 5% 78%	2160
	11th to 20th	Day	3 4 2 4 3 2 2 3 4 6% 67%	2160
	21st to end	Day	3 3 1 2 2 1 2 2 4 5% 74%	1905
October	1st to 10th	Day	3 3 1 3 3 2 2 3 4 5% 70%	1800
	11th to 20th	Day	7% 4 1 3 3 2 3 3 5% 5% 66%	1800
	21st to end	Day	9% 3 2 3 4 2 3 3 3 7% 61%	1980
November	1st to 10th	Day	11% 4 1 3 3 2 3 4 5% 7% 56%	1810
	11th to 20th	Day	12% 5% 3 5% 4 3 4 3 5% 7% 49%	1710
	21st to end	Day	12% 4 1 4 4 2 4 4 5% 7% 54%	1520
December	1st to 10th	Day	16% 7% 2 5% 3 3 3 5% 4 7% 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 5% 5% 2 3 5% 4 7% 36%	2794
January	1st to 10th	Night	15% 8% 3 5% 4 3 4 3 4 7% 45%	4280
	11th to 20th	Night	22% 7% 3 6% 5% 3 5% 4 5% 6% 34%	4110
	21st to end	Night	23% 10% 3 5% 5% 3 4 4 4 6% 33%	4521
February	1st to 10th	Night	17% 5% 3 4 4 2 4 3 4 5% 50%	2470
	11th to 20th	Night	13% 6% 3 6% 4 2 4 4 4 8% 46%	2336
	21st to end	Night	13% 6% 2 4 4 2 3 3 4 7% 54%	1884
March	1st to 10th	Night	20% 12% 3 5% 4 3 4 4 4 5% 36%	3477
	11th to 20th	Night	13% 4 2 4 4 2 3 3 3 5% 57%	3490
	21st to end	Night	16% 10% 4 5% 5% 3 3 3 4 6% 42%	3775
April	1st to 10th	Night	10% 5% 3 5% 5% 2 4 4 3 7% 51%	1900
	11th to 20th	Night	10% 5% 2 4 3 2 3 3 4 7% 58%	1779
	21st to end	Night	6% 3 2 3 3 2 3 4 5% 8% 62%	1800
May	1st to 10th	Night	5% 4 2 4 4 2 3 4 4 6% 62%	1693
	11th to 20th	Night	3 4 1 5% 4 2 4 4 4 8% 60%	1519
	21st to end	Night	4 4 3 4 4 2 3 4 5% 8% 60%	1552
June	1st to 10th	Night	4 3 2 4 4 2 3 4 4 9% 61%	1440
	11th to 20th	Night	1 2 1 2 2 1 3 3 4 7% 74%	1440
	21st to end	Night	2 3 1 3 5% 1 2 3 3 7% 70%	1440
July	1st to 10th	Night	1 1 2 5% 3 2 2 3 3 6% 73%	1440
	11th to 20th	Night	1 2 2 4 4 2 3 3 4 7% 68%	1568
	21st to end	Night	1 3 2 6% 6% 3 3 6% 5% 8% 58%	1773
August	1st to 10th	Night	2 4 3 6% 6% 3 3 4 4 8% 56%	1611
	11th to 20th	Night	2 2 4 5% 3 3 5% 6% 7% 63%	1692
	21st to end	Night	1 2 2 6% 4 2 3 4 3 7% 67%	1980
September	1st to 10th	Night	1 2 1 2 2 1 3 2 3 6% 77%	1800
	11th to 20th	Night	3 4 2 5% 4 2 3 3 4 6% 64%	1800
	21st to end	Night	2 2 2 3 3 2 2 2 4 6% 72%	2051
October	1st to 10th	Night	4 3 2 3 3 2 3 3 3 5% 69%	2160
	11th to 20th	Night	5% 4 2 4 3 2 3 3 5% 7% 62%	2160
	21st to end	Night	8% 4 2 3 3 2 2 3 4 7% 61%	2376
November	1st to 10th	Night	8% 6% 2 4 3 2 3 3 5% 8% 57%	2267
	11th to 20th	Night	11% 5% 2 4 3 2 3 4 4 7% 54%	2470
	21st to end	Night	10% 4 1 4 4 2 3 3 4 7% 58%	2655
December	1st to 10th	Night	18% 5% 2 5% 4 3 3 3 5% 6% 47%	2660
	11th to 20th	Night	31% 6% 3 3 4 2 4 4 5% 6% 32%	3276
	21st to end	Night	20% 6% 3 6% 5% 3 4 4 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	39% 7% 12% 20% 18% 6%	2270
	11th to 20th	Day	30% 5% 13% 24% 21% 7%	2132
	21st to end	Day	23% 5% 13% 30% 24% 6%	2160
May	1st to 10th	Day	22% 4 8% 28% 29% 8%	2263
	11th to 20th	Day	22% 5% 7% 34% 24% 9%	2373
	21st to end	Day	24% 4 14% 30% 22% 6%	2724
June	1st to 10th	Day	24% 5% 9% 29% 26% 8%	2520
	11th to 20th	Day	12% 3 9% 22% 36% 18%	2520
	21st to end	Day	14% 2 3 15% 36% 29%	2520
July	1st to 10th	Day	13% 2 3 11% 25% 45%	2451
	11th to 20th	Day	13% 2 5% 12% 27% 41%	2316
	21st to end	Day	12% 2 3 10% 27% 46%	2405
August	1st to 10th	Day	17% 3 3 15% 31% 31%	2312
	11th to 20th	Day	14% 2 3 11% 36% 34%	2209
	21st to end	Day	13% 2 3 14% 33% 35%	2376
September	1st to 10th	Day	12% 2 2 10% 31% 43%	2160
	11th to 20th	Day	19% 3 5% 23% 35% 14%	2160
	21st to end	Day	14% 3 4 20% 33% 26%	1905
October	1st to 10th	Day	17% 3 5% 17% 32% 25%	1800
	11th to 20th	Day	21% 4 7% 19% 24% 24%	1800
	21st to end	Day	25% 3 7% 21% 25% 19%	1980
November	1st to 10th	Day	27% 5% 8% 27% 20% 13%	1810
	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% 4 10% 28% 18% 11%	1779
	21st to end	Night	21% 4 12% 30% 25% 7%	1800
May	1st to 10th	Night	23% 5% 7% 24% 30% 11%	1693
	11th to 20th	Night	23% 4 9% 27% 27% 10%	1519
	21st to end	Night	23% 5% 13% 31% 22% 7%	1552
June	1st to 10th	Night	23% 5% 9% 24% 30% 9%	1440
	11th to 20th	Night	13% 4 7% 22% 35% 20%	1440
	21st to end	Night	16% 2 3 14% 33% 32%	1440
July	1st to 10th	Night	14% 1 3 11% 27% 43%	1400
	11th to 20th	Night	17% 2 5% 12% 22% 42%	1517
	21st to end	Night	23% 3 3 11% 19% 41%	1674
August	1st to 10th	Night	27% 1 4 14% 24% 30%	1602
	11th to 20th	Night	19% 2 3 11% 28% 37%	1692
	21st to end	Night	19% 2 4 12% 28% 35%	1980
September	1st to 10th	Night	11% 2 3 9% 32% 44%	1800
	11th to 20th	Night	22% 3 4 20% 35% 17%	1800
	21st to end	Night	15% 3 4 19% 28% 31%	2051
October	1st to 10th	Night	19% 3 7% 17% 29% 26%	2160
	11th to 20th	Night	23% 3 6% 19% 24% 26%	2160
	21st to end	Night	24% 4 6% 20% 24% 22%	2376
November	1st to 10th	Night	27% 5% 9% 26% 19% 15%	2267
	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% 3	457
	11th to 20th	Day	15% 17% 28% 22% 13% 4	412
	21st to end	Day	8% 16% 26% 29% 15% 6%	420
May	1st to 10th	Day	10% 15% 30% 28% 14% 4	423
	11th to 20th	Day	10% 13% 30% 25% 18% 4	448
	21st to end	Day	6% 12% 26% 32% 20% 4	485
June	1st to 10th	Day	7% 11% 26% 33% 18% 5%	450
	11th to 20th	Day	2 10% 29% 41% 15% 2	450
	21st to end	Day	1 5% 24% 43% 24% 3	450
July	1st to 10th	Day	2 4 19% 47% 24% 4	446
	11th to 20th	Day	1 2 17% 43% 30% 6%	444
	21st to end	Day	1 2 15% 48% 28% 7%	487
August	1st to 10th	Day	2 4 18% 43% 26% 6%	447
	11th to 20th	Day	1 3 21% 38% 29% 9%	437
	21st to end	Day	1 2 17% 41% 30% 8%	495
September	1st to 10th	Day	2 3 17% 42% 28% 8%	450
	11th to 20th	Day	6% 6% 21% 37% 23% 7%	450
	21st to end	Day	3 7% 20% 36% 27% 6%	450
October	1st to 10th	Day	4 8% 24% 27% 26% 9%	450
	11th to 20th	Day	8% 7% 18% 29% 32% 7%	450
	21st to end	Day	12% 7% 20% 28% 27% 6%	495
November	1st to 10th	Day	14% 7% 17% 24% 33% 5%	406
	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% 38% 20% 3	760
	11th to 20th	Night	13% 10% 19% 32% 23% 4	711
	21st to end	Night	8% 5% 23% 36% 25% 4	720
May	1st to 10th	Night	7% 4 19% 38% 29% 4	613
	11th to 20th	Night	5% 5% 14% 37% 34% 5%	457
	21st to end	Night	6% 6% 15% 39% 31% 3	389
June	1st to 10th	Night	5% 5% 13% 44% 31% 2	360
	11th to 20th	Night	3 4 15% 49% 28% 1	360
	21st to end	Night	3 1 10% 51% 33% 1	360
July	1st to 10th	Night	1 1 7% 48% 41% 2	350
	11th to 20th	Night	2 1 3 45% 42% 7%	473
	21st to end	Night	2 2 5% 41% 44% 6%	591
August	1st to 10th	Night	3 5% 7% 34% 44% 7%	537
	11th to 20th	Night	2 1 8% 41% 41% 8%	624
	21st to end	Night	2 1 6% 40% 45% 7%	792
September	1st to 10th	Night	1 1 7% 40% 43% 7%	720
	11th to 20th	Night	5% 2 8% 40% 38% 7%	720
	21st to end	Night	4 2 11% 36% 41% 7%	720
October	1st to 10th	Night	6% 5% 11% 37% 34% 8%	720
	11th to 20th	Night	7% 5% 10% 36% 34% 7%	720
	21st to end	Night	9% 4 11% 31% 35% 9%	792
November	1st to 10th	Night	10% 4 13% 31% 33% 8%	740
	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 2024-04-07 06:28:15. Version ac07

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