Penn Valley
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Penn Valley clear sky chart from 2005-11-06 to 2026-01-17.

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% 4 1 3 2 2 2 2 3 5% 66%	136086
	Night	13% 4 1 3 2 2 3 2 4 5% 60%	133003
Transparency	Day	24% 3 7% 18% 25% 24%	131010
	Night	29% 3 7% 17% 22% 22%	129064
Seeing	Day	11% 7% 20% 30% 25% 6%	16622
	Night	13% 3 15% 36% 29% 4	25998

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	23% 6% 1 4 3 3 3 2 4 6% 44%	3719
	11th to 20th	Day	18% 7% 2 5% 3 3 2 2 2 4 54%	4013
	21st to end	Day	22% 7% 2 4 2 2 3 3 3 6% 46%	4510
February	1st to 10th	Day	23% 7% 2 4 2 3 3 3 4 5% 45%	1800
	11th to 20th	Day	20% 7% 2 5% 4 3 4 3 4 7% 43%	1871
	21st to end	Day	20% 7% 2 3 3 3 3 3 4 6% 46%	1650
March	1st to 10th	Day	21% 13% 3 6% 4 4 4 2 4 6% 34%	4663
	11th to 20th	Day	21% 8% 1 4 3 3 3 3 3 5% 46%	5000
	21st to end	Day	22% 8% 2 5% 3 3 4 3 6% 7% 35%	5697
April	1st to 10th	Day	18% 6% 2 4 4 2 3 3 5% 6% 46%	2400
	11th to 20th	Day	14% 4 2 4 3 3 3 3 4 6% 54%	2369
	21st to end	Day	9% 5% 2 3 2 3 3 3 5% 6% 59%	2400
May	1st to 10th	Day	6% 2 1 3 1 2 2 2 2 5% 74%	5631
	11th to 20th	Day	4 2 1 2 2 2 2 2 3 4 76%	6277
	21st to end	Day	3 1 1 2 1 1 2 2 3 4 80%	6902
June	1st to 10th	Day	6% 4 1 3 2 2 2 3 4 7% 65%	2800
	11th to 20th	Day	2 2 1 2 1 2 2 1 3 4 80%	2800
	21st to end	Day	3 1 2 1 1 1 2 2 3 84%	2800
July	1st to 10th	Day	1 1 1 1 1 1 2 93%	6319
	11th to 20th	Day	1 1 1 1 1 1 1 2 2 90%	6320
	21st to end	Day	1 1 1 1 1 2 2 2 3 4 84%	6456
August	1st to 10th	Day	1 1 1 2 1 2 1 2 2 4 84%	5777
	11th to 20th	Day	1 1 1 1 1 1 2 2 91%	5474
	21st to end	Day	2 1 1 1 1 1 1 2 3 85%	5984
September	1st to 10th	Day	2 1 1 1 1 1 1 2 4 86%	2400
	11th to 20th	Day	5% 2 1 2 1 2 2 2 3 4 75%	2400
	21st to end	Day	5% 2 1 2 1 1 2 1 3 3 79%	2156
October	1st to 10th	Day	4 2 1 3 2 2 2 2 4 5% 74%	3280
	11th to 20th	Day	7% 2 1 3 1 2 2 2 4 5% 71%	3261
	21st to end	Day	6% 3 1 4 3 3 4 3 5% 6% 62%	3558
November	1st to 10th	Day	14% 4 1 4 3 3 4 2 5% 6% 54%	1887
	11th to 20th	Day	17% 4 1 5% 3 3 4 3 5% 6% 48%	1890
	21st to end	Day	14% 5% 1 4 3 3 3 3 4 5% 54%	1722
December	1st to 10th	Day	11% 3 1 4 3 3 3 3 4 7% 59%	2700
	11th to 20th	Day	31% 7% 2 4 3 3 3 3 5% 6% 32%	3108
	21st to end	Day	30% 6% 2 5% 3 4 4 3 6% 7% 30%	4092
January	1st to 10th	Night	16% 8% 2 5% 3 3 4 3 5% 7% 43%	6446
	11th to 20th	Night	18% 4 2 4 3 3 3 2 3 4 54%	6079
	21st to end	Night	20% 6% 2 5% 4 3 3 2 4 5% 46%	6468
February	1st to 10th	Night	23% 6% 1 3 3 3 3 3 4 5% 48%	2600
	11th to 20th	Night	18% 7% 2 5% 3 4 4 4 4 6% 45%	2415
	21st to end	Night	20% 6% 2 3 3 2 4 2 4 5% 48%	1980
March	1st to 10th	Night	25% 11% 3 6% 3 3 3 2 4 5% 35%	5227
	11th to 20th	Night	24% 9% 2 3 2 2 2 2 3 5% 46%	5000
	21st to end	Night	25% 9% 2 5% 4 3 3 3 5% 6% 35%	5276
April	1st to 10th	Night	15% 7% 2 5% 3 3 3 3 5% 6% 48%	2000
	11th to 20th	Night	14% 4 2 4 3 2 3 3 4 7% 54%	1979
	21st to end	Night	9% 4 1 3 2 3 3 3 5% 6% 62%	2000
May	1st to 10th	Night	5% 3 1 3 2 2 3 2 3 5% 71%	4342
	11th to 20th	Night	4 2 1 2 2 2 2 3 4 5% 74%	3616
	21st to end	Night	3 1 2 2 2 2 2 2 4 81%	3972
June	1st to 10th	Night	6% 3 1 3 2 3 3 3 4 5% 67%	1600
	11th to 20th	Night	2 1 1 2 2 1 2 2 3 5% 79%	1600
	21st to end	Night	2 2 2 2 2 2 2 2 2 82%	1600
July	1st to 10th	Night	1 1 1 1 1 1 1 2 93%	3640
	11th to 20th	Night	2 1 1 2 1 1 1 1 1 2 88%	3640
	21st to end	Night	1 1 1 2 2 2 2 3 4 4 79%	4398
August	1st to 10th	Night	2 2 1 2 1 1 2 2 3 3 81%	4035
	11th to 20th	Night	1 1 1 1 1 2 2 91%	4390
	21st to end	Night	3 1 1 2 1 1 2 2 2 3 82%	5015
September	1st to 10th	Night	1 1 1 1 2 1 1 1 4 87%	2000
	11th to 20th	Night	4 2 1 2 1 2 2 2 3 5% 76%	2000
	21st to end	Night	3 1 1 2 1 2 2 1 3 5% 79%	2240
October	1st to 10th	Night	4 2 1 2 2 2 2 2 2 3 77%	3920
	11th to 20th	Night	4 2 1 2 2 1 2 2 3 4 77%	3897
	21st to end	Night	9% 2 1 3 2 3 3 3 5% 6% 62%	4253
November	1st to 10th	Night	15% 5% 1 3 3 2 2 3 5% 7% 54%	2630
	11th to 20th	Night	14% 5% 1 4 3 2 3 3 6% 7% 52%	2730
	21st to end	Night	14% 4 2 4 3 3 2 3 3 6% 56%	2893
December	1st to 10th	Night	13% 3 1 2 2 2 3 2 4 6% 62%	4700
	11th to 20th	Night	31% 7% 2 4 3 4 4 3 7% 7% 29%	5316
	21st to end	Night	31% 7% 2 6% 4 4 4 3 4 5% 30%	7106

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	43% 4 12% 20% 13% 8%	3719
	11th to 20th	Day	38% 2 5% 15% 23% 17%	4013
	21st to end	Day	41% 3 7% 14% 20% 16%	4510
February	1st to 10th	Day	42% 3 9% 17% 19% 10%	1800
	11th to 20th	Day	42% 4 9% 18% 21% 7%	1880
	21st to end	Day	40% 4 10% 22% 17% 8%	1650
March	1st to 10th	Day	52% 4 7% 18% 16% 4	4663
	11th to 20th	Day	41% 3 11% 26% 11% 7%	5000
	21st to end	Day	47% 6% 13% 22% 10% 2	5697
April	1st to 10th	Day	38% 5% 11% 25% 18% 4	2400
	11th to 20th	Day	31% 4 13% 24% 21% 7%	2372
	21st to end	Day	25% 4 13% 29% 25% 4	2400
May	1st to 10th	Day	15% 3 7% 26% 32% 17%	5631
	11th to 20th	Day	14% 2 7% 27% 40% 10%	6280
	21st to end	Day	10% 3 8% 30% 42% 8%	6904
June	1st to 10th	Day	20% 3 11% 25% 30% 12%	2800
	11th to 20th	Day	10% 2 8% 19% 36% 26%	2800
	21st to end	Day	8% 1 3 12% 37% 38%	2800
July	1st to 10th	Day	3 1 1 7% 19% 69%	6251
	11th to 20th	Day	5% 1 2 5% 11% 76%	6245
	21st to end	Day	8% 1 3 15% 26% 46%	6269
August	1st to 10th	Day	11% 1 2 7% 25% 54%	4206
	11th to 20th	Day	5% 1 1 7% 40% 46%	3955
	21st to end	Day	6% 1 2 9% 41% 42%	4311
September	1st to 10th	Day	6% 1 2 10% 27% 55%	2400
	11th to 20th	Day	14% 2 5% 20% 36% 23%	2400
	21st to end	Day	12% 2 4 18% 32% 31%	2156
October	1st to 10th	Day	15% 2 4 13% 24% 42%	3280
	11th to 20th	Day	17% 2 4 19% 38% 20%	3261
	21st to end	Day	23% 3 6% 19% 31% 16%	3558
November	1st to 10th	Day	30% 4 8% 21% 23% 14%	1887
	11th to 20th	Day	36% 4 11% 20% 21% 8%	1890
	21st to end	Day	32% 4 9% 19% 23% 14%	1722
December	1st to 10th	Day	27% 3 7% 24% 27% 12%	2700
	11th to 20th	Day	54% 3 8% 16% 13% 6%	3108
	21st to end	Day	54% 4 12% 18% 10% 3	4092
January	1st to 10th	Night	40% 4 15% 19% 13% 9%	6446
	11th to 20th	Night	36% 3 8% 13% 23% 17%	6079
	21st to end	Night	43% 2 7% 10% 19% 19%	6468
February	1st to 10th	Night	40% 3 7% 18% 18% 13%	2600
	11th to 20th	Night	39% 5% 7% 20% 20% 8%	2417
	21st to end	Night	37% 4 9% 22% 19% 8%	1980
March	1st to 10th	Night	53% 4 6% 19% 12% 7%	5227
	11th to 20th	Night	43% 3 8% 23% 16% 7%	5000
	21st to end	Night	50% 5% 11% 20% 12% 3	5276
April	1st to 10th	Night	37% 5% 12% 19% 21% 6%	2000
	11th to 20th	Night	31% 5% 12% 24% 18% 10%	1979
	21st to end	Night	24% 5% 12% 30% 23% 6%	2000
May	1st to 10th	Night	17% 4 10% 23% 27% 19%	4339
	11th to 20th	Night	15% 3 7% 26% 37% 12%	3616
	21st to end	Night	10% 2 8% 30% 40% 10%	3972
June	1st to 10th	Night	20% 5% 10% 27% 27% 11%	1600
	11th to 20th	Night	10% 2 7% 19% 33% 28%	1600
	21st to end	Night	10% 3 3 12% 33% 40%	1600
July	1st to 10th	Night	3 1 1 6% 22% 67%	3600
	11th to 20th	Night	7% 1 5% 9% 76%	3592
	21st to end	Night	10% 2 2 14% 24% 48%	4273
August	1st to 10th	Night	14% 1 1 9% 23% 52%	2929
	11th to 20th	Night	4 1 1 9% 35% 50%	3178
	21st to end	Night	8% 1 2 11% 31% 47%	3608
September	1st to 10th	Night	6% 2 2 8% 30% 53%	2000
	11th to 20th	Night	13% 2 6% 17% 38% 23%	2000
	21st to end	Night	11% 2 5% 17% 31% 35%	2240
October	1st to 10th	Night	15% 2 4 11% 19% 49%	3920
	11th to 20th	Night	14% 2 5% 24% 33% 23%	3897
	21st to end	Night	23% 2 5% 18% 32% 20%	4253
November	1st to 10th	Night	30% 4 9% 21% 19% 17%	2630
	11th to 20th	Night	31% 4 11% 22% 22% 10%	2730
	21st to end	Night	32% 3 7% 20% 21% 16%	2893
December	1st to 10th	Night	25% 3 8% 24% 24% 17%	4700
	11th to 20th	Night	53% 4 6% 13% 16% 7%	5316
	21st to end	Night	57% 5% 10% 14% 10% 4	7106

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	38% 7% 21% 19% 13% 2	220
	11th to 20th	Day	23% 7% 18% 27% 20% 6%	355
	21st to end	Day	21% 8% 21% 25% 20% 6%	440
February	1st to 10th	Day	27% 8% 18% 26% 18% 4	400
	11th to 20th	Day	21% 14% 24% 26% 13% 2	455
	21st to end	Day	18% 12% 23% 30% 15% 1	414
March	1st to 10th	Day	26% 16% 26% 19% 11% 2	500
	11th to 20th	Day	27% 12% 23% 20% 16% 3	500
	21st to end	Day	21% 14% 30% 21% 12% 2	550
April	1st to 10th	Day	19% 11% 28% 24% 16% 2	500
	11th to 20th	Day	16% 13% 27% 26% 15% 4	492
	21st to end	Day	7% 12% 25% 30% 22% 5%	500
May	1st to 10th	Day	8% 12% 31% 26% 18% 5%	503
	11th to 20th	Day	9% 9% 25% 27% 25% 6%	528
	21st to end	Day	6% 7% 21% 31% 27% 9%	573
June	1st to 10th	Day	4 8% 19% 34% 29% 7%	530
	11th to 20th	Day	3 8% 22% 36% 25% 5%	530
	21st to end	Day	2 3 18% 38% 31% 8%	530
July	1st to 10th	Day	1 1 16% 41% 34% 7%	526
	11th to 20th	Day	1 1 10% 41% 40% 8%	524
	21st to end	Day	1 1 11% 41% 38% 9%	575
August	1st to 10th	Day	2 2 11% 41% 35% 10%	527
	11th to 20th	Day	1 1 16% 35% 36% 10%	517
	21st to end	Day	1 1 11% 35% 43% 8%	583
September	1st to 10th	Day	2 2 13% 42% 32% 9%	530
	11th to 20th	Day	5% 5% 21% 32% 30% 7%	530
	21st to end	Day	4 5% 20% 33% 30% 8%	530
October	1st to 10th	Day	4 9% 21% 28% 29% 10%	530
	11th to 20th	Day	7% 7% 17% 29% 31% 9%	530
	21st to end	Day	11% 7% 19% 28% 27% 8%	583
November	1st to 10th	Day	15% 5% 17% 25% 30% 8%	363
	11th to 20th	Day	17% 5% 18% 29% 24% 8%	330
	21st to end	Day	14% 4 19% 28% 26% 10%	242
December	1st to 10th	Day	18% 7% 16% 26% 23% 10%	220
	11th to 20th	Day	25% 5% 19% 23% 18% 10%	220
	21st to end	Day	28% 8% 18% 26% 17% 3	242
January	1st to 10th	Night	25% 6% 16% 28% 20% 5%	840
	11th to 20th	Night	18% 4 17% 38% 21% 3	824
	21st to end	Night	22% 2 16% 32% 24% 4	880
February	1st to 10th	Night	23% 4 17% 34% 21% 3	800
	11th to 20th	Night	19% 6% 20% 33% 19% 4	784
	21st to end	Night	21% 5% 21% 37% 15% 2	660
March	1st to 10th	Night	24% 7% 19% 29% 18% 3	800
	11th to 20th	Night	23% 7% 16% 28% 24% 3	800
	21st to end	Night	21% 5% 21% 32% 20% 1	880
April	1st to 10th	Night	16% 5% 20% 38% 19% 2	800
	11th to 20th	Night	12% 6% 21% 34% 23% 4	791
	21st to end	Night	9% 4 18% 37% 27% 4	800
May	1st to 10th	Night	8% 5% 24% 35% 25% 3	712
	11th to 20th	Night	7% 4 18% 36% 34% 2	394
	21st to end	Night	7% 3 16% 36% 36% 1	433
June	1st to 10th	Night	6% 1 18% 38% 35% 2	400
	11th to 20th	Night	2 2 17% 47% 32% 2	400
	21st to end	Night	3 1 10% 47% 37% 3	400
July	1st to 10th	Night	7% 46% 44% 3	390
	11th to 20th	Night	2 6% 50% 40% 2	388
	21st to end	Night	1 6% 37% 50% 6%	622
August	1st to 10th	Night	3 6% 42% 44% 5%	597
	11th to 20th	Night	1 1 6% 45% 42% 5%	739
	21st to end	Night	1 1 5% 41% 49% 4	880
September	1st to 10th	Night	2 8% 40% 45% 5%	800
	11th to 20th	Night	4 2 11% 37% 40% 7%	800
	21st to end	Night	3 2 14% 38% 35% 8%	800
October	1st to 10th	Night	6% 3 12% 42% 31% 6%	800
	11th to 20th	Night	7% 4 11% 37% 36% 5%	800
	21st to end	Night	9% 3 15% 34% 28% 10%	880
November	1st to 10th	Night	14% 4 14% 30% 31% 7%	820
	11th to 20th	Night	14% 3 16% 37% 25% 5%	840
	21st to end	Night	14% 3 16% 37% 25% 5%	840
December	1st to 10th	Night	20% 4 14% 34% 23% 6%	840
	11th to 20th	Night	30% 3 12% 31% 20% 3	840
	21st to end	Night	28% 4 19% 31% 17% 2	924

Page generated at 2026-01-13 22:14:28. Version ac07

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