Plumas Lake Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Plumas Lake Observatory clear sky chart from 2005-11-06 to 2025-12-18.

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% 3 1 3 2 2 3 2 4 5% 67%	135844
	Night	9% 3 1 4 3 3 3 2 4 5% 62%	132604
Transparency	Day	21% 4 7% 19% 25% 24%	130771
	Night	25% 5% 8% 19% 23% 21%	128675
Seeing	Day	8% 5% 14% 24% 29% 20%	16608
	Night	10% 3 17% 38% 28% 3	25922

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% 7% 2 7% 5% 4 4 3 5% 6% 37%	3639
	11th to 20th	Day	16% 9% 2 6% 3 2 2 2 3 4 51%	4051
	21st to end	Day	19% 7% 3 6% 3 4 3 3 3 5% 43%	4510
February	1st to 10th	Day	19% 5% 2 5% 4 3 3 3 5% 5% 45%	1800
	11th to 20th	Day	15% 5% 2 5% 4 4 4 4 5% 7% 46%	1852
	21st to end	Day	14% 5% 2 4 4 3 3 3 5% 6% 50%	1650
March	1st to 10th	Day	15% 9% 3 8% 4 4 4 3 5% 6% 39%	4731
	11th to 20th	Day	15% 5% 2 6% 3 4 4 3 4 7% 48%	5000
	21st to end	Day	15% 5% 2 6% 4 4 5% 4 5% 8% 42%	5653
April	1st to 10th	Day	11% 6% 2 4 4 4 4 4 5% 8% 49%	2400
	11th to 20th	Day	9% 3 2 5% 4 3 3 3 5% 5% 59%	2369
	21st to end	Day	5% 4 1 3 3 3 3 2 5% 8% 64%	2400
May	1st to 10th	Day	4 2 1 2 1 1 2 1 3 5% 79%	5631
	11th to 20th	Day	2 2 1 2 1 2 2 1 3 5% 79%	6250
	21st to end	Day	2 1 1 1 1 2 2 3 5% 82%	6902
June	1st to 10th	Day	4 3 1 3 2 2 2 3 4 7% 68%	2800
	11th to 20th	Day	1 1 2 1 1 2 1 3 5% 83%	2800
	21st to end	Day	2 1 1 1 1 2 2 3 3 86%	2800
July	1st to 10th	Day	1 1 1 1 1 1 93%	6319
	11th to 20th	Day	1 1 1 1 1 1 1 2 2 89%	6278
	21st to end	Day	1 1 2 1 1 2 2 3 5% 83%	6402
August	1st to 10th	Day	1 1 1 1 1 2 1 3 4 84%	5777
	11th to 20th	Day	1 1 1 1 1 1 2 3 90%	5503
	21st to end	Day	1 1 1 1 1 1 1 2 4 86%	5984
September	1st to 10th	Day	1 1 1 1 2 1 2 4 87%	2400
	11th to 20th	Day	2 2 1 2 2 2 2 2 3 5% 77%	2400
	21st to end	Day	3 2 2 1 1 1 2 3 3 80%	2143
October	1st to 10th	Day	2 2 1 3 2 2 2 2 3 4 77%	3280
	11th to 20th	Day	4 3 1 3 2 2 2 2 4 5% 71%	3261
	21st to end	Day	5% 3 1 3 3 4 4 3 4 7% 63%	3558
November	1st to 10th	Day	10% 3 1 4 3 3 3 4 6% 8% 56%	1920
	11th to 20th	Day	13% 5% 1 6% 4 4 3 3 5% 8% 47%	1890
	21st to end	Day	11% 6% 2 6% 4 4 3 2 4 6% 53%	1703
December	1st to 10th	Day	9% 4 2 5% 3 5% 4 4 7% 8% 50%	2700
	11th to 20th	Day	26% 9% 4 9% 5% 5% 4 3 5% 6% 23%	3084
	21st to end	Day	26% 9% 3 7% 4 5% 5% 4 5% 7% 26%	4004
January	1st to 10th	Night	12% 5% 3 7% 5% 5% 4 4 5% 9% 43%	6306
	11th to 20th	Night	16% 5% 2 6% 3 2 3 2 3 4 53%	5924
	21st to end	Night	17% 7% 2 6% 4 3 3 3 4 5% 46%	6468
February	1st to 10th	Night	18% 4 1 4 3 4 3 3 4 6% 49%	2600
	11th to 20th	Night	12% 5% 2 5% 3 4 3 4 5% 8% 49%	2433
	21st to end	Night	14% 5% 1 4 3 3 4 4 4 5% 53%	1980
March	1st to 10th	Night	16% 7% 3 6% 4 4 4 3 4 7% 42%	5160
	11th to 20th	Night	17% 5% 1 4 2 3 3 3 4 6% 51%	5000
	21st to end	Night	16% 6% 2 5% 4 4 4 4 6% 8% 42%	5320
April	1st to 10th	Night	8% 6% 2 5% 4 4 4 4 5% 6% 54%	2000
	11th to 20th	Night	8% 4 1 3 4 2 3 3 4 7% 60%	1979
	21st to end	Night	5% 3 1 3 3 2 3 3 5% 7% 67%	2000
May	1st to 10th	Night	3 2 1 2 1 2 2 2 3 5% 77%	4343
	11th to 20th	Night	2 1 2 2 2 2 2 4 6% 77%	3644
	21st to end	Night	1 1 1 1 1 1 1 3 4 84%	3972
June	1st to 10th	Night	4 2 1 2 2 2 3 2 4 7% 70%	1600
	11th to 20th	Night	1 1 1 2 1 2 2 2 5% 82%	1600
	21st to end	Night	1 1 1 2 1 2 1 2 2 3 84%	1600
July	1st to 10th	Night	1 1 1 1 1 1 1 92%	3640
	11th to 20th	Night	1 1 1 1 1 1 1 1 2 90%	3679
	21st to end	Night	1 1 1 1 1 1 2 2 3 5% 82%	4472
August	1st to 10th	Night	2 1 1 1 1 1 1 1 3 4 83%	4035
	11th to 20th	Night	1 1 1 1 2 3 91%	4345
	21st to end	Night	1 1 2 1 1 2 1 2 3 84%	5015
September	1st to 10th	Night	1 1 1 1 1 1 1 1 4 88%	2000
	11th to 20th	Night	3 2 1 2 1 2 2 2 3 4 77%	2000
	21st to end	Night	2 1 2 1 1 2 2 3 5% 81%	2252
October	1st to 10th	Night	3 2 1 2 2 2 2 2 3 3 79%	3920
	11th to 20th	Night	3 2 1 2 1 2 2 2 4 5% 77%	3897
	21st to end	Night	7% 2 1 3 3 3 3 3 4 7% 66%	4253
November	1st to 10th	Night	10% 3 2 4 3 3 3 3 5% 7% 58%	2597
	11th to 20th	Night	11% 4 2 4 3 3 4 3 5% 7% 54%	2730
	21st to end	Night	11% 4 2 5% 3 3 3 3 3 6% 56%	2914
December	1st to 10th	Night	10% 3 1 4 3 3 4 3 5% 7% 57%	4700
	11th to 20th	Night	25% 7% 3 7% 5% 5% 5% 4 6% 9% 25%	5274
	21st to end	Night	25% 7% 3 7% 5% 5% 5% 3 5% 5% 29%	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	45% 8% 14% 18% 11% 5%	3639
	11th to 20th	Day	39% 4 6% 18% 18% 14%	4051
	21st to end	Day	43% 6% 7% 14% 20% 10%	4510
February	1st to 10th	Day	41% 4 10% 18% 19% 8%	1800
	11th to 20th	Day	36% 6% 11% 21% 19% 7%	1861
	21st to end	Day	35% 6% 10% 23% 20% 7%	1650
March	1st to 10th	Day	44% 8% 9% 20% 17% 3	4731
	11th to 20th	Day	36% 6% 11% 25% 14% 8%	5000
	21st to end	Day	38% 9% 15% 24% 13% 2	5653
April	1st to 10th	Day	34% 5% 10% 25% 19% 7%	2400
	11th to 20th	Day	27% 5% 12% 25% 22% 9%	2372
	21st to end	Day	19% 5% 14% 29% 27% 7%	2400
May	1st to 10th	Day	11% 3 7% 25% 32% 22%	5631
	11th to 20th	Day	10% 3 7% 25% 40% 16%	6253
	21st to end	Day	7% 3 9% 31% 39% 12%	6904
June	1st to 10th	Day	17% 4 10% 27% 30% 14%	2800
	11th to 20th	Day	8% 2 8% 20% 34% 29%	2800
	21st to end	Day	6% 2 3 13% 36% 39%	2800
July	1st to 10th	Day	3 1 1 7% 20% 68%	6251
	11th to 20th	Day	5% 1 2 5% 14% 74%	6203
	21st to end	Day	7% 2 3 17% 26% 45%	6217
August	1st to 10th	Day	9% 2 3 8% 26% 53%	4206
	11th to 20th	Day	4 1 1 9% 41% 44%	3985
	21st to end	Day	6% 1 2 12% 39% 40%	4311
September	1st to 10th	Day	5% 1 3 10% 29% 52%	2400
	11th to 20th	Day	12% 3 6% 21% 37% 20%	2400
	21st to end	Day	11% 2 5% 18% 33% 30%	2143
October	1st to 10th	Day	12% 2 5% 12% 26% 43%	3280
	11th to 20th	Day	16% 4 5% 25% 31% 19%	3261
	21st to end	Day	21% 4 9% 22% 31% 14%	3558
November	1st to 10th	Day	25% 6% 11% 26% 23% 10%	1920
	11th to 20th	Day	35% 6% 12% 22% 18% 6%	1890
	21st to end	Day	32% 6% 11% 22% 19% 10%	1703
December	1st to 10th	Day	28% 6% 13% 26% 18% 8%	2700
	11th to 20th	Day	59% 6% 10% 12% 11% 2	3084
	21st to end	Day	54% 10% 12% 16% 7% 2	4004
January	1st to 10th	Night	36% 9% 17% 22% 11% 5%	6306
	11th to 20th	Night	36% 6% 8% 18% 20% 13%	5924
	21st to end	Night	40% 5% 9% 10% 24% 12%	6468
February	1st to 10th	Night	37% 4 9% 20% 20% 10%	2600
	11th to 20th	Night	33% 6% 10% 22% 21% 8%	2435
	21st to end	Night	32% 5% 9% 24% 21% 9%	1980
March	1st to 10th	Night	42% 8% 9% 18% 17% 5%	5160
	11th to 20th	Night	34% 6% 12% 22% 16% 9%	5000
	21st to end	Night	39% 8% 14% 23% 13% 3	5320
April	1st to 10th	Night	30% 6% 12% 21% 23% 7%	2000
	11th to 20th	Night	25% 5% 12% 27% 19% 12%	1979
	21st to end	Night	19% 5% 13% 29% 26% 8%	2000
May	1st to 10th	Night	11% 5% 10% 23% 28% 22%	4340
	11th to 20th	Night	10% 3 7% 28% 38% 14%	3644
	21st to end	Night	7% 2 8% 31% 38% 14%	3972
June	1st to 10th	Night	16% 5% 11% 27% 30% 12%	1600
	11th to 20th	Night	8% 2 6% 19% 36% 29%	1600
	21st to end	Night	8% 2 3 11% 33% 43%	1600
July	1st to 10th	Night	3 1 1 7% 21% 66%	3600
	11th to 20th	Night	6% 1 1 5% 12% 75%	3631
	21st to end	Night	8% 2 3 17% 26% 45%	4345
August	1st to 10th	Night	12% 2 2 8% 25% 51%	2929
	11th to 20th	Night	4 1 2 8% 40% 46%	3145
	21st to end	Night	7% 1 2 9% 35% 46%	3608
September	1st to 10th	Night	6% 2 2 8% 30% 53%	2000
	11th to 20th	Night	12% 2 6% 19% 37% 24%	2000
	21st to end	Night	9% 2 5% 17% 31% 36%	2252
October	1st to 10th	Night	13% 2 4 11% 19% 52%	3920
	11th to 20th	Night	11% 3 7% 27% 30% 22%	3897
	21st to end	Night	19% 3 6% 21% 30% 20%	4253
November	1st to 10th	Night	26% 5% 10% 23% 21% 15%	2597
	11th to 20th	Night	28% 6% 12% 25% 21% 8%	2730
	21st to end	Night	30% 5% 7% 23% 23% 12%	2914
December	1st to 10th	Night	25% 5% 11% 27% 21% 11%	4700
	11th to 20th	Night	52% 7% 10% 16% 12% 3	5274
	21st to end	Night	54% 8% 13% 15% 8% 2	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	36% 5% 15% 24% 19% 2	200
	11th to 20th	Day	20% 8% 17% 22% 21% 12%	377
	21st to end	Day	17% 5% 17% 24% 21% 15%	440
February	1st to 10th	Day	21% 8% 16% 19% 23% 13%	400
	11th to 20th	Day	18% 9% 22% 22% 19% 10%	445
	21st to end	Day	12% 12% 18% 28% 22% 8%	414
March	1st to 10th	Day	17% 11% 26% 25% 14% 7%	500
	11th to 20th	Day	19% 12% 19% 21% 17% 12%	500
	21st to end	Day	16% 12% 24% 22% 18% 8%	550
April	1st to 10th	Day	15% 7% 20% 26% 19% 13%	500
	11th to 20th	Day	10% 7% 22% 28% 18% 14%	492
	21st to end	Day	4 6% 19% 28% 25% 18%	500
May	1st to 10th	Day	6% 7% 23% 27% 23% 14%	503
	11th to 20th	Day	6% 5% 21% 23% 23% 22%	528
	21st to end	Day	4 4 15% 26% 27% 25%	573
June	1st to 10th	Day	4 3 14% 22% 36% 21%	530
	11th to 20th	Day	2 5% 14% 29% 34% 17%	530
	21st to end	Day	1 2 11% 24% 37% 26%	530
July	1st to 10th	Day	1 7% 27% 44% 21%	526
	11th to 20th	Day	1 5% 22% 44% 28%	524
	21st to end	Day	1 3 26% 41% 29%	575
August	1st to 10th	Day	2 1 4 26% 40% 28%	527
	11th to 20th	Day	1 1 5% 22% 38% 32%	517
	21st to end	Day	1 5% 19% 37% 37%	583
September	1st to 10th	Day	1 1 8% 21% 41% 29%	530
	11th to 20th	Day	2 4 11% 24% 30% 29%	530
	21st to end	Day	2 2 14% 25% 29% 27%	530
October	1st to 10th	Day	5% 5% 14% 21% 27% 28%	530
	11th to 20th	Day	5% 5% 14% 19% 28% 28%	530
	21st to end	Day	8% 7% 15% 20% 31% 19%	583
November	1st to 10th	Day	8% 8% 13% 23% 25% 22%	396
	11th to 20th	Day	14% 6% 12% 26% 29% 13%	330
	21st to end	Day	14% 3 14% 23% 33% 14%	231
December	1st to 10th	Day	19% 5% 13% 19% 27% 17%	220
	11th to 20th	Day	19% 4 14% 23% 26% 14%	214
	21st to end	Day	23% 10% 18% 20% 24% 4	220
January	1st to 10th	Night	19% 5% 17% 32% 22% 4	800
	11th to 20th	Night	17% 4 14% 39% 22% 4	800
	21st to end	Night	17% 4 15% 34% 26% 4	880
February	1st to 10th	Night	19% 5% 16% 34% 24% 2	800
	11th to 20th	Night	15% 5% 23% 34% 21% 3	784
	21st to end	Night	15% 7% 21% 37% 18% 2	660
March	1st to 10th	Night	17% 5% 22% 32% 21% 3	800
	11th to 20th	Night	17% 6% 19% 31% 25% 2	800
	21st to end	Night	16% 5% 24% 34% 20% 1	880
April	1st to 10th	Night	11% 4 24% 36% 24% 2	800
	11th to 20th	Night	9% 6% 24% 33% 25% 3	791
	21st to end	Night	7% 4 20% 41% 26% 3	800
May	1st to 10th	Night	6% 4 27% 38% 23% 3	713
	11th to 20th	Night	3 2 26% 43% 25% 1	409
	21st to end	Night	4 3 22% 43% 27%	433
June	1st to 10th	Night	6% 1 26% 42% 24% 1	400
	11th to 20th	Night	1 3 26% 47% 23% 1	400
	21st to end	Night	2 1 22% 44% 31% 1	400
July	1st to 10th	Night	1 17% 56% 26% 1	390
	11th to 20th	Night	1 1 16% 58% 24%	401
	21st to end	Night	1 1 13% 49% 34% 1	657
August	1st to 10th	Night	3 3 18% 46% 30% 2	597
	11th to 20th	Night	1 1 12% 47% 37% 3	719
	21st to end	Night	1 1 8% 45% 44% 2	880
September	1st to 10th	Night	1 1 9% 39% 46% 4	800
	11th to 20th	Night	3 2 14% 37% 39% 5%	800
	21st to end	Night	2 2 16% 35% 39% 7%	800
October	1st to 10th	Night	6% 3 13% 37% 35% 7%	800
	11th to 20th	Night	5% 3 14% 37% 36% 5%	800
	21st to end	Night	6% 4 13% 36% 32% 9%	880
November	1st to 10th	Night	11% 3 15% 31% 32% 7%	820
	11th to 20th	Night	11% 3 16% 39% 27% 4	840
	21st to end	Night	13% 4 15% 36% 29% 4	840
December	1st to 10th	Night	17% 3 17% 31% 24% 8%	840
	11th to 20th	Night	25% 3 14% 29% 25% 3	828
	21st to end	Night	22% 6% 19% 31% 20% 2	880

Page generated at 2025-12-14 22:13:57. Version ac07

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