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 2024-11-25.

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% 65%	105356
	Night	13% 4 1 3 2 2 3 2 4 5% 60%	102848
Transparency	Day	24% 3 7% 18% 24% 25%	105107
	Night	29% 3 7% 17% 21% 22%	102686
Seeing	Day	11% 7% 20% 31% 26% 6%	15209
	Night	13% 3 15% 36% 29% 4	24500

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	28% 9% 2 4 3 3 3 2 3 5% 39%	2539
	11th to 20th	Day	26% 10% 3 7% 4 4 3 2 3 5% 34%	2763
	21st to end	Day	28% 9% 3 5% 3 3 3 3 4 6% 33%	3146
February	1st to 10th	Day	22% 7% 2 5% 2 3 3 3 4 5% 45%	1710
	11th to 20th	Day	19% 7% 2 5% 3 3 4 3 4 7% 44%	1775
	21st to end	Day	21% 8% 2 3 3 3 3 3 4 6% 45%	1570
March	1st to 10th	Day	24% 14% 3 7% 4 4 4 3 4 5% 29%	3244
	11th to 20th	Day	17% 5% 1 3 2 2 2 2 4 6% 56%	3490
	21st to end	Day	21% 10% 3 5% 4 3 4 3 7% 7% 33%	3971
April	1st to 10th	Day	18% 6% 2 4 4 2 3 3 5% 6% 46%	2280
	11th to 20th	Day	15% 4 2 4 3 3 3 3 4 6% 54%	2249
	21st to end	Day	9% 4 2 3 2 3 3 2 5% 6% 59%	2280
May	1st to 10th	Day	9% 2 1 3 1 2 2 2 3 5% 70%	3939
	11th to 20th	Day	3 2 1 2 2 2 2 2 2 4 79%	4377
	21st to end	Day	4 2 1 2 2 2 2 2 4 5% 74%	4813
June	1st to 10th	Day	7% 4 1 3 2 2 3 3 4 7% 63%	2660
	11th to 20th	Day	2 2 1 2 1 2 2 1 3 4 80%	2660
	21st to end	Day	3 1 2 1 1 1 2 2 3 83%	2660
July	1st to 10th	Day	1 1 1 1 1 1 1 93%	4420
	11th to 20th	Day	2 1 1 1 1 1 1 2 3 86%	4420
	21st to end	Day	1 1 1 1 1 1 1 2 2 89%	4496
August	1st to 10th	Day	2 1 1 2 2 2 2 2 3 5% 78%	4091
	11th to 20th	Day	1 1 1 1 1 1 2 2 89%	3795
	21st to end	Day	2 1 1 1 1 1 1 1 2 90%	4180
September	1st to 10th	Day	2 1 1 1 1 2 4 88%	2280
	11th to 20th	Day	4 2 1 2 1 2 2 2 3 4 75%	2280
	21st to end	Day	4 2 1 2 1 1 2 1 3 3 80%	2049
October	1st to 10th	Day	3 2 1 3 2 2 2 2 4 5% 74%	3180
	11th to 20th	Day	7% 2 1 2 1 2 2 2 4 5% 71%	3161
	21st to end	Day	6% 3 1 4 3 3 4 3 5% 6% 62%	3448
November	1st to 10th	Day	14% 4 1 4 3 3 4 2 5% 6% 53%	1796
	11th to 20th	Day	16% 4 1 4 3 3 4 3 5% 6% 50%	1800
	21st to end	Day	15% 5% 1 4 3 3 3 3 4 6% 52%	1592
December	1st to 10th	Day	19% 4 1 4 3 3 4 3 4 7% 48%	1520
	11th to 20th	Day	32% 8% 3 5% 4 3 2 3 4 5% 31%	1928
	21st to end	Day	18% 7% 2 6% 4 5% 4 4 6% 7% 37%	2794
January	1st to 10th	Night	20% 9% 2 5% 3 3 3 3 5% 6% 40%	4406
	11th to 20th	Night	26% 6% 2 6% 4 4 4 3 4 5% 36%	4202
	21st to end	Night	28% 8% 3 6% 4 3 4 3 4 5% 31%	4521
February	1st to 10th	Night	21% 6% 1 3 3 3 3 3 4 5% 49%	2470
	11th to 20th	Night	17% 7% 1 5% 3 4 4 4 4 6% 45%	2291
	21st to end	Night	21% 6% 1 3 3 2 4 2 4 5% 48%	1884
March	1st to 10th	Night	29% 14% 3 5% 3 3 3 2 3 4 31%	3626
	11th to 20th	Night	19% 6% 1 2 2 2 2 2 3 5% 56%	3490
	21st to end	Night	25% 9% 3 5% 4 3 4 4 5% 6% 34%	3680
April	1st to 10th	Night	15% 7% 2 5% 3 3 2 3 4 6% 48%	1900
	11th to 20th	Night	14% 4 2 4 3 2 3 3 4 7% 54%	1879
	21st to end	Night	9% 3 1 2 2 3 3 3 5% 6% 63%	1900
May	1st to 10th	Night	7% 4 1 3 2 2 3 3 4 5% 66%	3018
	11th to 20th	Night	4 1 1 2 1 1 2 3 3 5% 77%	2516
	21st to end	Night	4 2 1 2 2 2 2 2 3 5% 76%	2762
June	1st to 10th	Night	7% 3 1 3 2 3 3 3 4 5% 66%	1520
	11th to 20th	Night	2 1 1 2 2 1 2 2 3 5% 79%	1520
	21st to end	Night	3 2 2 2 2 2 2 2 2 82%	1520
July	1st to 10th	Night	1 1 1 1 1 1 1 1 93%	2540
	11th to 20th	Night	2 1 1 2 1 1 2 1 2 3 83%	2540
	21st to end	Night	1 1 1 1 1 1 2 3 3 85%	3063
August	1st to 10th	Night	3 2 1 2 2 2 2 2 4 4 74%	2848
	11th to 20th	Night	1 1 1 1 1 1 2 2 90%	3052
	21st to end	Night	3 1 1 1 1 1 1 1 2 87%	3498
September	1st to 10th	Night	1 1 1 1 1 1 1 1 3 88%	1900
	11th to 20th	Night	4 2 1 2 1 2 2 2 3 5% 76%	1900
	21st to end	Night	3 1 1 2 1 2 2 1 3 5% 79%	2127
October	1st to 10th	Night	4 2 1 2 2 2 2 2 2 3 77%	3800
	11th to 20th	Night	4 1 1 2 2 1 2 2 3 4 78%	3777
	21st to end	Night	9% 2 1 3 3 2 3 3 5% 6% 62%	4121
November	1st to 10th	Night	16% 5% 1 4 3 2 2 3 5% 6% 54%	2502
	11th to 20th	Night	13% 4 1 4 3 2 3 3 5% 7% 54%	2600
	21st to end	Night	15% 4 2 4 3 3 2 3 3 5% 54%	2677
December	1st to 10th	Night	22% 4 1 3 3 2 3 2 4 6% 49%	2660
	11th to 20th	Night	35% 6% 2 4 2 3 3 2 5% 6% 32%	3276
	21st to end	Night	22% 7% 3 7% 5% 5% 4 3 5% 6% 35%	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	50% 3 7% 24% 13% 3	2539
	11th to 20th	Day	56% 4 7% 15% 11% 8%	2763
	21st to end	Day	53% 4 9% 17% 12% 5%	3146
February	1st to 10th	Day	42% 3 9% 17% 18% 10%	1710
	11th to 20th	Day	41% 4 9% 17% 22% 7%	1784
	21st to end	Day	41% 4 10% 22% 16% 7%	1570
March	1st to 10th	Day	57% 5% 9% 14% 11% 4	3244
	11th to 20th	Day	32% 2 9% 30% 16% 10%	3490
	21st to end	Day	48% 7% 13% 19% 10% 2	3971
April	1st to 10th	Day	38% 5% 11% 25% 17% 4	2280
	11th to 20th	Day	31% 5% 13% 24% 21% 6%	2252
	21st to end	Day	25% 4 13% 29% 25% 4	2280
May	1st to 10th	Day	17% 4 9% 29% 24% 17%	3939
	11th to 20th	Day	12% 3 6% 24% 43% 12%	4380
	21st to end	Day	14% 4 12% 36% 29% 6%	4814
June	1st to 10th	Day	21% 3 11% 24% 28% 13%	2660
	11th to 20th	Day	10% 2 8% 20% 36% 25%	2660
	21st to end	Day	9% 2 3 13% 36% 38%	2660
July	1st to 10th	Day	3 1 1 5% 15% 74%	4351
	11th to 20th	Day	7% 1 3 7% 16% 66%	4345
	21st to end	Day	5% 1 1 4 26% 63%	4354
August	1st to 10th	Day	12% 1 2 7% 23% 54%	4076
	11th to 20th	Day	5% 1 1 7% 40% 47%	3832
	21st to end	Day	6% 1 2 9% 41% 42%	4179
September	1st to 10th	Day	5% 1 2 8% 27% 58%	2280
	11th to 20th	Day	14% 2 5% 20% 36% 23%	2280
	21st to end	Day	12% 2 4 18% 32% 32%	2049
October	1st to 10th	Day	15% 2 4 13% 24% 43%	3180
	11th to 20th	Day	17% 2 4 19% 37% 21%	3161
	21st to end	Day	23% 3 6% 20% 31% 17%	3448
November	1st to 10th	Day	31% 5% 7% 21% 22% 14%	1796
	11th to 20th	Day	34% 4 11% 21% 22% 8%	1800
	21st to end	Day	34% 4 9% 19% 22% 12%	1592
December	1st to 10th	Day	37% 4 8% 20% 20% 11%	1520
	11th to 20th	Day	57% 3 7% 14% 13% 6%	1928
	21st to end	Day	45% 5% 14% 20% 12% 4	2794
January	1st to 10th	Night	44% 4 14% 22% 11% 4	4406
	11th to 20th	Night	52% 4 12% 11% 12% 10%	4202
	21st to end	Night	56% 3 10% 12% 12% 6%	4521
February	1st to 10th	Night	39% 3 7% 19% 19% 13%	2470
	11th to 20th	Night	39% 5% 7% 20% 20% 9%	2293
	21st to end	Night	38% 5% 9% 23% 18% 7%	1884
March	1st to 10th	Night	59% 5% 8% 14% 10% 5%	3626
	11th to 20th	Night	33% 3 7% 26% 20% 11%	3490
	21st to end	Night	50% 6% 12% 19% 10% 3	3680
April	1st to 10th	Night	38% 5% 12% 19% 21% 6%	1900
	11th to 20th	Night	32% 5% 12% 24% 18% 9%	1879
	21st to end	Night	23% 5% 12% 30% 23% 6%	1900
May	1st to 10th	Night	19% 6% 12% 20% 25% 18%	3018
	11th to 20th	Night	12% 3 7% 23% 39% 16%	2516
	21st to end	Night	12% 3 12% 37% 28% 7%	2762
June	1st to 10th	Night	21% 5% 11% 26% 26% 12%	1520
	11th to 20th	Night	11% 2 8% 20% 33% 26%	1520
	21st to end	Night	11% 3 3 12% 31% 41%	1520
July	1st to 10th	Night	3 1 1 6% 15% 73%	2500
	11th to 20th	Night	11% 1 2 8% 14% 66%	2492
	21st to end	Night	6% 2 2 5% 23% 63%	2967
August	1st to 10th	Night	15% 1 1 9% 21% 53%	2839
	11th to 20th	Night	4 1 1 9% 34% 50%	3081
	21st to end	Night	8% 1 2 12% 30% 47%	3498
September	1st to 10th	Night	6% 2 2 7% 29% 55%	1900
	11th to 20th	Night	13% 2 6% 17% 39% 24%	1900
	21st to end	Night	11% 2 5% 17% 30% 35%	2127
October	1st to 10th	Night	15% 2 4 11% 19% 50%	3800
	11th to 20th	Night	13% 2 5% 24% 33% 23%	3777
	21st to end	Night	23% 2 5% 18% 32% 20%	4121
November	1st to 10th	Night	31% 4 9% 21% 18% 17%	2502
	11th to 20th	Night	29% 4 12% 23% 22% 10%	2600
	21st to end	Night	34% 3 7% 19% 21% 16%	2677
December	1st to 10th	Night	37% 4 9% 19% 18% 14%	2660
	11th to 20th	Night	53% 4 6% 14% 17% 6%	3276
	21st to end	Night	51% 5% 11% 16% 12% 5%	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	40% 6% 21% 18% 13% 2	180
	11th to 20th	Day	25% 7% 18% 28% 18% 4	312
	21st to end	Day	22% 8% 21% 25% 19% 6%	407
February	1st to 10th	Day	25% 9% 18% 27% 18% 4	370
	11th to 20th	Day	21% 14% 23% 26% 13% 3	419
	21st to end	Day	19% 12% 24% 29% 14% 1	382
March	1st to 10th	Day	27% 16% 26% 18% 10% 2	460
	11th to 20th	Day	25% 11% 23% 20% 17% 4	460
	21st to end	Day	21% 13% 32% 22% 11% 1	506
April	1st to 10th	Day	19% 12% 28% 24% 16% 2	460
	11th to 20th	Day	17% 13% 26% 24% 16% 4	452
	21st to end	Day	7% 12% 25% 29% 22% 5%	460
May	1st to 10th	Day	9% 12% 31% 26% 18% 4	463
	11th to 20th	Day	8% 8% 26% 26% 26% 6%	488
	21st to end	Day	6% 7% 20% 31% 27% 9%	529
June	1st to 10th	Day	4 8% 20% 32% 28% 7%	490
	11th to 20th	Day	3 8% 22% 37% 25% 5%	490
	21st to end	Day	2 2 19% 39% 30% 7%	490
July	1st to 10th	Day	1 1 14% 42% 35% 7%	486
	11th to 20th	Day	1 1 10% 42% 39% 7%	484
	21st to end	Day	1 11% 41% 38% 8%	531
August	1st to 10th	Day	2 2 12% 42% 35% 8%	487
	11th to 20th	Day	1 1 17% 36% 36% 9%	477
	21st to end	Day	1 1 12% 35% 43% 7%	539
September	1st to 10th	Day	2 2 12% 43% 32% 9%	490
	11th to 20th	Day	5% 5% 22% 33% 29% 6%	490
	21st to end	Day	3 5% 21% 33% 31% 7%	490
October	1st to 10th	Day	3 8% 22% 28% 29% 9%	490
	11th to 20th	Day	6% 7% 17% 30% 31% 9%	490
	21st to end	Day	10% 7% 19% 29% 26% 8%	539
November	1st to 10th	Day	15% 6% 18% 25% 30% 7%	332
	11th to 20th	Day	15% 4 17% 31% 26% 8%	300
	21st to end	Day	15% 5% 18% 29% 25% 8%	208
December	1st to 10th	Day	22% 8% 16% 26% 21% 8%	180
	11th to 20th	Day	22% 6% 22% 23% 17% 10%	180
	21st to end	Day	23% 9% 19% 27% 19% 3	198
January	1st to 10th	Night	24% 5% 16% 28% 22% 6%	760
	11th to 20th	Night	19% 4 16% 38% 19% 3	760
	21st to end	Night	23% 2 16% 30% 24% 5%	836
February	1st to 10th	Night	22% 4 16% 34% 21% 3	760
	11th to 20th	Night	19% 6% 18% 34% 20% 4	744
	21st to end	Night	22% 5% 21% 36% 16% 2	628
March	1st to 10th	Night	25% 7% 19% 28% 18% 3	760
	11th to 20th	Night	22% 6% 16% 28% 25% 3	760
	21st to end	Night	20% 5% 21% 32% 20% 1	836
April	1st to 10th	Night	16% 5% 19% 38% 20% 2	760
	11th to 20th	Night	13% 6% 21% 33% 23% 4	751
	21st to end	Night	9% 5% 18% 37% 27% 4	760
May	1st to 10th	Night	8% 5% 24% 35% 25% 3	676
	11th to 20th	Night	7% 3 17% 37% 35% 2	374
	21st to end	Night	8% 3 14% 36% 38% 1	411
June	1st to 10th	Night	7% 1 18% 36% 36% 2	380
	11th to 20th	Night	2 1 16% 46% 33% 2	380
	21st to end	Night	3 1 10% 47% 37% 3	380
July	1st to 10th	Night	6% 45% 45% 3	370
	11th to 20th	Night	2 6% 50% 40% 2	368
	21st to end	Night	1 6% 37% 50% 6%	591
August	1st to 10th	Night	3 6% 42% 44% 5%	567
	11th to 20th	Night	1 1 6% 45% 42% 6%	702
	21st to end	Night	1 1 5% 41% 49% 3	836
September	1st to 10th	Night	2 7% 39% 47% 6%	760
	11th to 20th	Night	4 2 11% 36% 40% 7%	760
	21st to end	Night	3 2 15% 37% 35% 9%	760
October	1st to 10th	Night	6% 3 12% 41% 32% 6%	760
	11th to 20th	Night	7% 4 11% 36% 37% 5%	760
	21st to end	Night	8% 3 15% 34% 28% 11%	836
November	1st to 10th	Night	14% 4 14% 30% 30% 7%	780
	11th to 20th	Night	13% 3 16% 38% 26% 5%	800
	21st to end	Night	15% 3 16% 37% 24% 5%	778
December	1st to 10th	Night	22% 4 14% 34% 21% 6%	760
	11th to 20th	Night	30% 3 13% 31% 20% 3	760
	21st to end	Night	25% 4 19% 32% 19% 2	836

Page generated at 2024-11-22 05:02:24. Version ac07

Copyright © 2024 Attilla Danko.