WYA Eastern Lassen Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the WYA Eastern Lassen Site clear sky chart from 2005-11-06 to 2025-11-18.

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	2 5% 2 5% 4 4 4 3 5% 7% 59%	135666
	Night	3 6% 3 6% 4 4 4 3 5% 7% 56%	132039
Transparency	Day	24% 4 7% 20% 25% 19%	130601
	Night	27% 5% 9% 22% 24% 14%	128100
Seeing	Day	6% 9% 21% 28% 25% 11%	16228
	Night	6% 6% 20% 39% 25% 4	25455

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	2 9% 5% 12% 7% 7% 6% 5% 7% 9% 32%	3639
	11th to 20th	Day	3 7% 4 9% 6% 4 4 4 5% 7% 48%	3640
	21st to end	Day	3 11% 5% 10% 6% 4 4 3 4 7% 44%	4460
February	1st to 10th	Day	4 7% 4 9% 5% 5% 5% 5% 5% 9% 42%	1800
	11th to 20th	Day	2 6% 3 8% 6% 5% 6% 5% 6% 12% 42%	1898
	21st to end	Day	2 7% 3 7% 6% 6% 5% 5% 6% 8% 43%	1650
March	1st to 10th	Day	4 10% 5% 11% 6% 6% 6% 4 7% 8% 34%	4518
	11th to 20th	Day	1 6% 3 8% 4 5% 4 4 6% 10% 48%	4912
	21st to end	Day	2 9% 4 8% 5% 5% 5% 4 7% 9% 41%	5831
April	1st to 10th	Day	3 8% 3 8% 6% 5% 5% 5% 7% 8% 44%	2400
	11th to 20th	Day	4 6% 2 6% 4 5% 5% 4 6% 8% 51%	2369
	21st to end	Day	4 6% 2 5% 4 4 4 4 7% 9% 51%	2400
May	1st to 10th	Day	2 5% 3 5% 3 2 2 3 4 8% 64%	5911
	11th to 20th	Day	2 4 1 4 3 3 3 3 4 7% 67%	6277
	21st to end	Day	2 3 1 4 3 3 3 3 5% 6% 68%	6902
June	1st to 10th	Day	3 4 2 6% 5% 4 5% 5% 6% 8% 53%	2800
	11th to 20th	Day	3 3 1 4 3 3 3 3 4 6% 68%	2800
	21st to end	Day	2 2 1 3 3 3 3 3 3 4 74%	2800
July	1st to 10th	Day	1 1 1 2 2 2 3 3 84%	6319
	11th to 20th	Day	1 2 1 2 2 2 2 2 3 3 81%	6320
	21st to end	Day	2 2 1 4 4 4 4 4 6% 7% 63%	6880
August	1st to 10th	Day	2 2 1 2 2 2 3 3 4 6% 72%	5787
	11th to 20th	Day	1 1 2 2 2 2 2 3 4 82%	5418
	21st to end	Day	3 2 1 3 2 2 2 2 3 4 76%	5984
September	1st to 10th	Day	2 1 1 2 1 2 2 2 3 5% 80%	2400
	11th to 20th	Day	3 3 1 4 3 2 3 3 4 5% 70%	2377
	21st to end	Day	2 3 1 4 3 2 2 2 4 6% 72%	2200
October	1st to 10th	Day	3 3 1 2 3 3 3 2 4 6% 70%	3290
	11th to 20th	Day	5% 3 2 4 3 3 3 3 4 6% 65%	3261
	21st to end	Day	2 3 2 5% 3 4 4 4 6% 9% 57%	3411
November	1st to 10th	Day	5% 7% 2 6% 5% 3 5% 5% 6% 8% 48%	1854
	11th to 20th	Day	3 7% 3 7% 5% 5% 7% 6% 7% 8% 41%	1863
	21st to end	Day	2 6% 3 7% 5% 5% 5% 4 7% 9% 48%	1643
December	1st to 10th	Day	2 5% 2 6% 3 3 5% 4 6% 7% 56%	2620
	11th to 20th	Day	4 13% 5% 11% 7% 7% 6% 5% 7% 7% 26%	3028
	21st to end	Day	4 10% 5% 11% 7% 6% 6% 5% 6% 12% 29%	4004
January	1st to 10th	Night	2 8% 5% 11% 7% 7% 6% 5% 8% 8% 33%	6306
	11th to 20th	Night	2 8% 4 8% 5% 5% 4 4 5% 7% 48%	6320
	21st to end	Night	4 10% 4 9% 6% 4 4 3 5% 7% 45%	6512
February	1st to 10th	Night	3 8% 3 9% 5% 5% 5% 5% 7% 8% 44%	2600
	11th to 20th	Night	2 8% 3 9% 6% 5% 5% 5% 6% 9% 44%	2389
	21st to end	Night	2 8% 3 6% 6% 5% 5% 4 7% 9% 44%	1980
March	1st to 10th	Night	2 9% 5% 11% 6% 6% 6% 4 5% 7% 39%	5376
	11th to 20th	Night	1 7% 3 10% 5% 4 5% 4 5% 7% 48%	5071
	21st to end	Night	2 8% 5% 10% 7% 6% 6% 4 6% 9% 38%	5143
April	1st to 10th	Night	2 8% 3 6% 4 5% 6% 4 7% 9% 46%	2000
	11th to 20th	Night	2 7% 3 6% 4 4 4 3 5% 7% 54%	1979
	21st to end	Night	4 6% 1 4 3 4 4 4 6% 8% 56%	2000
May	1st to 10th	Night	3 4 2 5% 3 3 3 3 4 4 66%	4034
	11th to 20th	Night	2 4 2 5% 3 3 3 2 4 5% 67%	3616
	21st to end	Night	2 3 1 3 2 2 3 2 4 6% 73%	3972
June	1st to 10th	Night	3 6% 1 5% 4 4 3 4 6% 7% 56%	1600
	11th to 20th	Night	3 3 1 3 1 2 3 3 4 6% 71%	1600
	21st to end	Night	2 2 1 3 3 3 3 2 4 4 73%	1600
July	1st to 10th	Night	1 2 1 1 1 1 3 3 85%	3640
	11th to 20th	Night	2 2 1 3 2 1 2 2 2 3 80%	3640
	21st to end	Night	2 2 1 4 3 3 4 3 5% 6% 66%	3970
August	1st to 10th	Night	2 3 1 2 2 1 2 2 5% 6% 74%	4022
	11th to 20th	Night	1 1 2 1 1 1 1 2 3 86%	4435
	21st to end	Night	2 3 1 3 2 2 3 2 3 3 75%	5015
September	1st to 10th	Night	2 2 1 2 1 2 2 2 3 4 80%	2000
	11th to 20th	Night	2 3 1 3 2 2 2 3 3 6% 73%	2021
	21st to end	Night	2 2 1 3 2 2 2 2 4 6% 75%	2200
October	1st to 10th	Night	2 3 1 3 3 2 2 1 3 4 76%	3907
	11th to 20th	Night	1 3 1 4 2 2 2 3 4 5% 71%	3897
	21st to end	Night	3 4 2 5% 3 4 3 3 5% 7% 59%	4395
November	1st to 10th	Night	5% 5% 2 6% 5% 3 4 5% 8% 8% 48%	2665
	11th to 20th	Night	4 7% 2 7% 4 4 4 5% 8% 8% 46%	2691
	21st to end	Night	2 6% 3 7% 5% 5% 4 3 6% 8% 50%	2755
December	1st to 10th	Night	3 5% 2 6% 3 3 4 3 4 7% 60%	4560
	11th to 20th	Night	4 11% 5% 13% 7% 6% 6% 5% 8% 10% 25%	5176
	21st to end	Night	4 10% 6% 11% 7% 6% 6% 5% 6% 9% 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	41% 10% 15% 22% 11%	3639
	11th to 20th	Day	35% 8% 11% 21% 22% 3	3640
	21st to end	Day	40% 7% 7% 15% 21% 10%	4460
February	1st to 10th	Day	34% 8% 13% 26% 13% 5%	1800
	11th to 20th	Day	32% 7% 16% 25% 17% 3	1908
	21st to end	Day	33% 9% 15% 23% 15% 5%	1650
March	1st to 10th	Day	41% 8% 9% 20% 18% 4	4518
	11th to 20th	Day	27% 4 12% 35% 22% 1	4912
	21st to end	Day	33% 8% 15% 25% 17% 2	5831
April	1st to 10th	Day	33% 7% 14% 27% 16% 2	2400
	11th to 20th	Day	28% 6% 12% 29% 20% 4	2372
	21st to end	Day	27% 5% 15% 31% 20% 2	2400
May	1st to 10th	Day	20% 3 8% 27% 38% 4	5911
	11th to 20th	Day	19% 2 7% 23% 43% 7%	6280
	21st to end	Day	16% 3 7% 33% 34% 7%	6904
June	1st to 10th	Day	26% 6% 8% 25% 30% 5%	2800
	11th to 20th	Day	17% 4 6% 21% 36% 17%	2800
	21st to end	Day	14% 3 3 14% 35% 32%	2800
July	1st to 10th	Day	7% 1 1 8% 19% 63%	6251
	11th to 20th	Day	10% 1 1 4 16% 69%	6245
	21st to end	Day	20% 2 1 16% 28% 35%	6681
August	1st to 10th	Day	17% 2 1 8% 21% 51%	4215
	11th to 20th	Day	10% 2 1 7% 33% 47%	3922
	21st to end	Day	10% 1 1 11% 39% 38%	4311
September	1st to 10th	Day	9% 1 1 12% 32% 44%	2400
	11th to 20th	Day	17% 3 5% 18% 32% 26%	2377
	21st to end	Day	15% 3 4 23% 31% 25%	2200
October	1st to 10th	Day	17% 2 4 15% 34% 29%	3290
	11th to 20th	Day	20% 3 3 26% 38% 10%	3261
	21st to end	Day	24% 3 7% 31% 27% 9%	3411
November	1st to 10th	Day	29% 6% 12% 26% 21% 6%	1854
	11th to 20th	Day	33% 9% 14% 25% 16% 3	1863
	21st to end	Day	29% 6% 12% 25% 23% 5%	1643
December	1st to 10th	Day	27% 4 7% 24% 29% 8%	2620
	11th to 20th	Day	50% 7% 13% 19% 10% 1	3028
	21st to end	Day	40% 8% 19% 22% 10% 1	4004
January	1st to 10th	Night	40% 9% 16% 24% 11% 1	6306
	11th to 20th	Night	34% 6% 9% 22% 21% 7%	6320
	21st to end	Night	38% 6% 8% 13% 26% 8%	6512
February	1st to 10th	Night	33% 7% 14% 24% 18% 4	2600
	11th to 20th	Night	33% 7% 14% 24% 19% 3	2390
	21st to end	Night	34% 6% 14% 25% 16% 5%	1980
March	1st to 10th	Night	40% 6% 10% 19% 19% 5%	5376
	11th to 20th	Night	28% 4 13% 34% 20% 1	5071
	21st to end	Night	35% 8% 18% 27% 11% 1	5143
April	1st to 10th	Night	31% 7% 14% 29% 14% 4	2000
	11th to 20th	Night	29% 6% 14% 25% 21% 5%	1979
	21st to end	Night	25% 5% 15% 34% 19% 3	2000
May	1st to 10th	Night	20% 5% 7% 28% 33% 7%	4031
	11th to 20th	Night	20% 2 6% 29% 36% 6%	3616
	21st to end	Night	14% 3 9% 31% 40% 3	3972
June	1st to 10th	Night	25% 5% 11% 31% 24% 4	1600
	11th to 20th	Night	14% 4 8% 23% 38% 12%	1600
	21st to end	Night	16% 3 4 17% 30% 30%	1600
July	1st to 10th	Night	8% 1 1 11% 20% 59%	3600
	11th to 20th	Night	13% 1 1 4 19% 62%	3592
	21st to end	Night	20% 1 2 18% 28% 30%	3857
August	1st to 10th	Night	16% 2 2 9% 25% 47%	2917
	11th to 20th	Night	9% 1 1 7% 37% 44%	3211
	21st to end	Night	10% 1 1 14% 41% 32%	3608
September	1st to 10th	Night	11% 2 2 10% 29% 47%	2000
	11th to 20th	Night	16% 2 5% 19% 31% 28%	2021
	21st to end	Night	12% 3 3 23% 32% 26%	2200
October	1st to 10th	Night	16% 1 4 14% 35% 30%	3907
	11th to 20th	Night	16% 2 5% 31% 35% 11%	3897
	21st to end	Night	22% 2 7% 31% 28% 10%	4395
November	1st to 10th	Night	28% 7% 12% 26% 20% 7%	2665
	11th to 20th	Night	30% 7% 16% 24% 20% 2	2691
	21st to end	Night	29% 5% 14% 25% 20% 6%	2755
December	1st to 10th	Night	25% 4 8% 22% 31% 11%	4560
	11th to 20th	Night	44% 7% 11% 21% 15% 1	5176
	21st to end	Night	44% 8% 16% 19% 12% 1	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	12% 17% 22% 27% 20% 5%	200
	11th to 20th	Day	5% 11% 24% 27% 25% 10%	200
	21st to end	Day	8% 6% 19% 32% 24% 11%	416
February	1st to 10th	Day	8% 12% 22% 26% 25% 8%	400
	11th to 20th	Day	7% 16% 23% 28% 21% 6%	470
	21st to end	Day	4 14% 23% 33% 20% 6%	414
March	1st to 10th	Day	10% 13% 28% 27% 17% 5%	500
	11th to 20th	Day	9% 10% 24% 29% 21% 8%	500
	21st to end	Day	9% 13% 26% 28% 19% 5%	550
April	1st to 10th	Day	10% 10% 24% 29% 22% 6%	500
	11th to 20th	Day	8% 14% 21% 28% 20% 9%	492
	21st to end	Day	8% 12% 19% 26% 23% 12%	500
May	1st to 10th	Day	7% 11% 21% 31% 19% 10%	503
	11th to 20th	Day	10% 13% 21% 23% 20% 12%	528
	21st to end	Day	9% 8% 20% 24% 23% 16%	573
June	1st to 10th	Day	6% 10% 19% 25% 26% 14%	530
	11th to 20th	Day	8% 12% 27% 27% 20% 7%	530
	21st to end	Day	3 7% 19% 29% 29% 14%	530
July	1st to 10th	Day	2 5% 19% 30% 33% 10%	526
	11th to 20th	Day	2 3 20% 31% 31% 12%	524
	21st to end	Day	3 4 17% 30% 31% 15%	575
August	1st to 10th	Day	3 3 17% 31% 30% 16%	527
	11th to 20th	Day	2 5% 17% 28% 32% 16%	517
	21st to end	Day	3 5% 19% 31% 27% 15%	583
September	1st to 10th	Day	3 3 17% 31% 31% 14%	530
	11th to 20th	Day	6% 7% 19% 25% 30% 13%	530
	21st to end	Day	5% 7% 19% 32% 28% 9%	530
October	1st to 10th	Day	6% 10% 23% 25% 27% 9%	530
	11th to 20th	Day	8% 13% 17% 27% 27% 9%	530
	21st to end	Day	8% 13% 23% 28% 21% 7%	498
November	1st to 10th	Day	11% 9% 22% 25% 24% 9%	330
	11th to 20th	Day	4 12% 25% 25% 24% 11%	321
	21st to end	Day	8% 13% 17% 26% 25% 11%	221
December	1st to 10th	Day	9% 11% 18% 23% 24% 17%	200
	11th to 20th	Day	8% 6% 26% 25% 23% 13%	200
	21st to end	Day	13% 9% 19% 28% 25% 7%	220
January	1st to 10th	Night	9% 8% 22% 33% 22% 5%	800
	11th to 20th	Night	5% 9% 21% 37% 23% 5%	800
	21st to end	Night	10% 4 19% 38% 24% 6%	880
February	1st to 10th	Night	8% 9% 22% 37% 22% 2	800
	11th to 20th	Night	5% 8% 23% 40% 21% 3	784
	21st to end	Night	5% 11% 23% 41% 19% 2	660
March	1st to 10th	Night	7% 9% 27% 36% 17% 4	800
	11th to 20th	Night	7% 9% 22% 34% 24% 4	800
	21st to end	Night	8% 11% 26% 34% 18% 2	880
April	1st to 10th	Night	6% 9% 24% 38% 21% 2	800
	11th to 20th	Night	6% 10% 26% 33% 21% 4	791
	21st to end	Night	9% 6% 18% 38% 23% 5%	800
May	1st to 10th	Night	8% 8% 26% 35% 21% 2	586
	11th to 20th	Night	6% 5% 24% 35% 25% 6%	394
	21st to end	Night	6% 6% 19% 35% 30% 3	433
June	1st to 10th	Night	6% 5% 20% 40% 27% 3	400
	11th to 20th	Night	4 4 28% 42% 21% 2	400
	21st to end	Night	3 2 18% 50% 26% 2	400
July	1st to 10th	Night	2 1 18% 53% 25% 1	390
	11th to 20th	Night	2 1 17% 59% 20% 1	388
	21st to end	Night	3 1 11% 53% 30% 2	438
August	1st to 10th	Night	6% 12% 48% 30% 4	584
	11th to 20th	Night	4 1 11% 45% 34% 4	759
	21st to end	Night	2 1 13% 46% 35% 3	880
September	1st to 10th	Night	3 1 12% 42% 37% 5%	800
	11th to 20th	Night	3 3 14% 36% 36% 8%	800
	21st to end	Night	3 4 15% 35% 33% 10%	800
October	1st to 10th	Night	6% 4 17% 38% 29% 7%	800
	11th to 20th	Night	4 7% 16% 37% 31% 6%	800
	21st to end	Night	6% 6% 18% 35% 27% 9%	880
November	1st to 10th	Night	6% 6% 22% 34% 26% 6%	820
	11th to 20th	Night	8% 7% 21% 40% 19% 4	828
	21st to end	Night	5% 11% 19% 36% 25% 5%	800
December	1st to 10th	Night	8% 8% 20% 39% 21% 5%	800
	11th to 20th	Night	9% 7% 22% 35% 23% 4	800
	21st to end	Night	11% 10% 25% 38% 14% 2	880

Page generated at 2025-11-14 21:55:58. Version ac07

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