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 2024-01-12.

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	3 5% 2 5% 5% 3 4 5% 5% 8% 56%	74774
	Night	3 6% 3 5% 5% 3 4 4 6% 8% 53%	74965
Transparency	Day	26% 6% 9% 21% 22% 16%	74478
	Night	28% 6% 11% 22% 21% 13%	74782
Seeing	Day	6% 9% 21% 28% 25% 10%	13758
	Night	6% 6% 19% 39% 25% 5%	23130

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	3 10% 5% 9% 9% 5% 6% 5% 7% 12% 29%	1519
	11th to 20th	Day	4 6% 3 8% 7% 5% 6% 7% 6% 9% 39%	1451
	21st to end	Day	3 10% 3 8% 7% 3 5% 6% 7% 11% 36%	1760
February	1st to 10th	Day	4 6% 4 7% 6% 4 5% 6% 6% 9% 44%	1620
	11th to 20th	Day	2 5% 3 6% 7% 3 6% 7% 6% 12% 44%	1703
	21st to end	Day	3 6% 4 7% 8% 4 6% 6% 6% 8% 41%	1480
March	1st to 10th	Day	5% 9% 5% 8% 6% 4 5% 6% 7% 7% 37%	1800
	11th to 20th	Day	4 8% 4 7% 7% 4 6% 6% 7% 11% 38%	1944
	21st to end	Day	3 9% 4 7% 6% 4 6% 6% 7% 10% 39%	2316
April	1st to 10th	Day	3 7% 4 6% 7% 4 5% 6% 6% 8% 43%	2160
	11th to 20th	Day	4 5% 3 6% 5% 4 5% 5% 6% 8% 50%	2129
	21st to end	Day	4 5% 3 4 5% 3 4 5% 6% 9% 52%	2160
May	1st to 10th	Day	3 4 2 4 4 2 3 5% 5% 8% 59%	2345
	11th to 20th	Day	5% 6% 2 5% 5% 2 4 5% 5% 9% 53%	2477
	21st to end	Day	5% 7% 3 7% 7% 4 4 5% 6% 7% 46%	2723
June	1st to 10th	Day	4 4 2 5% 6% 3 5% 5% 5% 9% 52%	2520
	11th to 20th	Day	3 3 1 4 3 2 4 3 4 6% 67%	2520
	21st to end	Day	3 2 1 2 3 2 3 3 3 4 73%	2520
July	1st to 10th	Day	1 2 1 1 2 2 3 3 4 5% 76%	2520
	11th to 20th	Day	1 1 1 2 2 1 2 2 3 4 81%	2520
	21st to end	Day	2 2 1 3 3 2 3 4 4 5% 71%	2741
August	1st to 10th	Day	2 2 1 3 3 2 4 4 4 6% 69%	2338
	11th to 20th	Day	1 1 1 2 2 2 3 3 3 4 78%	2151
	21st to end	Day	1 1 1 1 2 1 2 2 3 4 83%	2376
September	1st to 10th	Day	2 1 1 2 1 1 2 3 2 4 82%	2160
	11th to 20th	Day	2 2 1 3 3 1 3 3 4 5% 71%	2140
	21st to end	Day	3 3 1 3 3 1 2 3 4 7% 71%	1980
October	1st to 10th	Day	5% 4 2 3 4 2 3 3 4 6% 64%	1810
	11th to 20th	Day	3 3 2 5% 4 2 4 3 4 7% 64%	1800
	21st to end	Day	4 4 2 4 5% 3 4 4 6% 7% 60%	1904
November	1st to 10th	Day	6% 7% 2 5% 5% 3 5% 6% 6% 8% 48%	1674
	11th to 20th	Day	4 6% 3 5% 6% 4 7% 8% 7% 8% 43%	1710
	21st to end	Day	2 5% 3 6% 6% 3 6% 5% 7% 10% 48%	1561
December	1st to 10th	Day	4 7% 4 8% 6% 3 6% 6% 6% 8% 43%	1520
	11th to 20th	Day	6% 10% 4 10% 8% 5% 7% 7% 7% 8% 28%	1928
	21st to end	Day	4 9% 3 9% 8% 4 6% 6% 6% 12% 32%	2794
January	1st to 10th	Night	5% 10% 4 8% 7% 4 6% 5% 8% 9% 35%	2646
	11th to 20th	Night	3 7% 4 7% 7% 4 5% 5% 7% 9% 43%	2548
	21st to end	Night	4 10% 4 7% 6% 3 4 5% 6% 9% 41%	2592
February	1st to 10th	Night	3 7% 3 8% 5% 3 5% 5% 6% 8% 46%	2340
	11th to 20th	Night	2 7% 3 7% 7% 3 5% 5% 6% 9% 45%	2144
	21st to end	Night	3 8% 5% 6% 7% 3 5% 6% 7% 9% 43%	1776
March	1st to 10th	Night	4 9% 5% 8% 7% 4 6% 5% 7% 9% 36%	2160
	11th to 20th	Night	3 9% 4 7% 5% 3 5% 7% 7% 10% 43%	2012
	21st to end	Night	4 9% 4 5% 7% 4 5% 5% 5% 9% 43%	2039
April	1st to 10th	Night	3 7% 4 6% 5% 3 7% 5% 7% 9% 45%	1800
	11th to 20th	Night	3 7% 3 5% 5% 4 4 4 5% 7% 53%	1779
	21st to end	Night	4 5% 1 4 3 3 4 5% 6% 9% 56%	1800
May	1st to 10th	Night	6% 5% 2 4 4 2 3 4 5% 6% 59%	1610
	11th to 20th	Night	3 4 2 6% 5% 3 5% 5% 6% 8% 52%	1416
	21st to end	Night	5% 6% 2 5% 6% 3 5% 5% 6% 8% 48%	1552
June	1st to 10th	Night	4 6% 2 4 5% 2 4 4 6% 7% 56%	1440
	11th to 20th	Night	3 3 1 3 2 1 3 4 4 7% 69%	1440
	21st to end	Night	2 2 1 3 3 3 4 3 4 5% 71%	1440
July	1st to 10th	Night	1 1 1 2 2 2 3 2 4 6% 77%	1440
	11th to 20th	Night	1 1 1 1 2 1 1 3 3 4 82%	1440
	21st to end	Night	2 1 2 3 2 3 3 4 4 76%	1576
August	1st to 10th	Night	4 4 1 2 3 1 4 3 5% 6% 69%	1598
	11th to 20th	Night	2 2 1 3 2 2 2 2 4 4 76%	1750
	21st to end	Night	1 1 1 2 1 2 2 2 3 84%	1980
September	1st to 10th	Night	2 1 1 1 1 1 2 2 3 4 82%	1800
	11th to 20th	Night	2 3 1 2 2 1 2 3 3 6% 75%	1818
	21st to end	Night	2 2 1 3 2 2 2 3 4 6% 73%	1980
October	1st to 10th	Night	4 4 2 3 3 2 3 2 3 5% 69%	2147
	11th to 20th	Night	2 3 2 4 3 2 3 3 4 6% 68%	2160
	21st to end	Night	3 4 2 3 3 2 3 4 5% 6% 64%	2452
November	1st to 10th	Night	5% 5% 2 5% 6% 3 4 6% 8% 8% 47%	2405
	11th to 20th	Night	4 6% 3 6% 4 3 4 6% 8% 9% 49%	2470
	21st to end	Night	2 6% 3 6% 6% 3 5% 4 6% 9% 50%	2617
December	1st to 10th	Night	5% 7% 4 7% 6% 3 5% 5% 5% 9% 44%	2660
	11th to 20th	Night	6% 10% 5% 10% 7% 4 6% 6% 8% 10% 28%	3276
	21st to end	Night	4 9% 4 9% 9% 5% 6% 7% 6% 10% 32%	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	44% 10% 20% 21% 5% 1	1519
	11th to 20th	Day	37% 10% 15% 19% 16% 3	1451
	21st to end	Day	39% 8% 12% 25% 14% 2	1760
February	1st to 10th	Day	33% 8% 15% 27% 13% 5%	1620
	11th to 20th	Day	30% 7% 17% 25% 18% 3	1713
	21st to end	Day	36% 9% 14% 23% 15% 3	1480
March	1st to 10th	Day	40% 8% 13% 23% 13% 2	1800
	11th to 20th	Day	38% 7% 12% 23% 17% 3	1944
	21st to end	Day	37% 10% 19% 22% 10% 3	2316
April	1st to 10th	Day	34% 8% 14% 25% 17% 3	2160
	11th to 20th	Day	29% 7% 12% 29% 19% 4	2132
	21st to end	Day	27% 5% 15% 31% 20% 2	2160
May	1st to 10th	Day	21% 5% 10% 31% 29% 4	2345
	11th to 20th	Day	27% 5% 10% 28% 24% 6%	2480
	21st to end	Day	34% 6% 12% 27% 17% 4	2724
June	1st to 10th	Day	27% 6% 8% 24% 29% 6%	2520
	11th to 20th	Day	18% 4 7% 22% 35% 15%	2520
	21st to end	Day	14% 4 3 14% 35% 30%	2520
July	1st to 10th	Day	10% 3 3 11% 26% 47%	2451
	11th to 20th	Day	9% 2 1 9% 26% 53%	2445
	21st to end	Day	15% 3 1 8% 21% 51%	2589
August	1st to 10th	Day	17% 3 2 12% 27% 39%	2321
	11th to 20th	Day	10% 4 2 10% 30% 43%	2151
	21st to end	Day	8% 2 1 12% 33% 44%	2376
September	1st to 10th	Day	9% 2 1 11% 32% 46%	2160
	11th to 20th	Day	15% 3 5% 17% 32% 28%	2140
	21st to end	Day	15% 3 4 24% 32% 23%	1980
October	1st to 10th	Day	23% 3 7% 23% 28% 17%	1810
	11th to 20th	Day	20% 4 6% 25% 32% 13%	1800
	21st to end	Day	22% 5% 10% 27% 23% 12%	1904
November	1st to 10th	Day	31% 7% 12% 23% 21% 6%	1674
	11th to 20th	Day	31% 10% 13% 25% 17% 3	1710
	21st to end	Day	29% 6% 13% 26% 22% 4	1561
December	1st to 10th	Day	38% 8% 12% 23% 16% 3	1520
	11th to 20th	Day	49% 8% 12% 19% 10% 1	1928
	21st to end	Day	42% 8% 18% 21% 12% 1	2794
January	1st to 10th	Night	41% 8% 18% 24% 9% 1	2646
	11th to 20th	Night	35% 7% 15% 23% 15% 6%	2548
	21st to end	Night	38% 6% 11% 24% 17% 4	2592
February	1st to 10th	Night	31% 7% 14% 25% 19% 3	2340
	11th to 20th	Night	32% 7% 15% 23% 19% 3	2145
	21st to end	Night	35% 7% 15% 25% 15% 4	1776
March	1st to 10th	Night	40% 8% 14% 23% 12% 2	2160
	11th to 20th	Night	33% 7% 15% 25% 17% 3	2012
	21st to end	Night	35% 9% 17% 25% 13% 2	2039
April	1st to 10th	Night	33% 8% 14% 27% 14% 4	1800
	11th to 20th	Night	30% 7% 14% 24% 21% 5%	1779
	21st to end	Night	24% 5% 16% 33% 19% 3	1800
May	1st to 10th	Night	25% 5% 9% 31% 28% 3	1610
	11th to 20th	Night	29% 6% 12% 30% 18% 6%	1416
	21st to end	Night	32% 5% 16% 28% 16% 3	1552
June	1st to 10th	Night	26% 5% 11% 30% 23% 5%	1440
	11th to 20th	Night	15% 4 9% 23% 38% 10%	1440
	21st to end	Night	17% 3 5% 17% 30% 29%	1440
July	1st to 10th	Night	11% 3 4 14% 27% 42%	1400
	11th to 20th	Night	8% 2 2 10% 29% 50%	1392
	21st to end	Night	13% 3 3 8% 25% 49%	1488
August	1st to 10th	Night	17% 3 3 14% 29% 34%	1590
	11th to 20th	Night	14% 2 1 12% 31% 40%	1750
	21st to end	Night	8% 1 2 14% 34% 41%	1980
September	1st to 10th	Night	10% 2 1 9% 29% 49%	1800
	11th to 20th	Night	13% 2 5% 19% 30% 30%	1818
	21st to end	Night	13% 3 4 23% 32% 24%	1980
October	1st to 10th	Night	21% 2 7% 21% 31% 18%	2147
	11th to 20th	Night	17% 4 7% 25% 29% 18%	2160
	21st to end	Night	19% 4 12% 28% 25% 13%	2452
November	1st to 10th	Night	29% 8% 12% 24% 20% 7%	2405
	11th to 20th	Night	29% 8% 16% 24% 21% 3	2470
	21st to end	Night	29% 5% 14% 25% 20% 6%	2617
December	1st to 10th	Night	35% 7% 12% 23% 19% 4	2660
	11th to 20th	Night	46% 8% 11% 19% 14% 2	3276
	21st to end	Night	45% 8% 14% 17% 15% 1	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	12% 18% 19% 26% 21% 5%	180
	11th to 20th	Day	5% 11% 23% 27% 25% 9%	163
	21st to end	Day	8% 7% 19% 29% 25% 12%	352
February	1st to 10th	Day	5% 11% 22% 28% 26% 8%	340
	11th to 20th	Day	8% 14% 21% 29% 22% 6%	395
	21st to end	Day	4 14% 25% 33% 18% 5%	346
March	1st to 10th	Day	10% 13% 25% 30% 17% 5%	420
	11th to 20th	Day	9% 8% 22% 32% 23% 7%	420
	21st to end	Day	9% 14% 25% 28% 20% 5%	462
April	1st to 10th	Day	10% 10% 23% 29% 22% 5%	420
	11th to 20th	Day	8% 14% 22% 27% 20% 8%	412
	21st to end	Day	7% 14% 19% 26% 23% 11%	420
May	1st to 10th	Day	8% 10% 21% 32% 19% 10%	423
	11th to 20th	Day	11% 13% 23% 24% 19% 11%	448
	21st to end	Day	11% 9% 20% 23% 21% 16%	485
June	1st to 10th	Day	7% 12% 21% 24% 23% 14%	450
	11th to 20th	Day	8% 12% 25% 27% 20% 6%	450
	21st to end	Day	3 8% 17% 31% 28% 13%	450
July	1st to 10th	Day	2 6% 19% 30% 33% 10%	446
	11th to 20th	Day	1 4 21% 31% 31% 12%	444
	21st to end	Day	2 4 17% 32% 31% 13%	487
August	1st to 10th	Day	4 4 17% 30% 31% 15%	447
	11th to 20th	Day	2 5% 16% 28% 33% 17%	437
	21st to end	Day	2 5% 20% 33% 27% 14%	495
September	1st to 10th	Day	3 3 17% 31% 32% 13%	450
	11th to 20th	Day	5% 7% 22% 26% 29% 12%	450
	21st to end	Day	5% 8% 21% 33% 26% 7%	450
October	1st to 10th	Day	6% 10% 22% 26% 27% 10%	450
	11th to 20th	Day	8% 12% 17% 27% 27% 9%	450
	21st to end	Day	7% 13% 23% 29% 21% 7%	419
November	1st to 10th	Day	13% 9% 20% 25% 24% 10%	270
	11th to 20th	Day	3 10% 23% 25% 27% 12%	270
	21st to end	Day	6% 13% 17% 29% 25% 11%	199
December	1st to 10th	Day	9% 12% 19% 22% 24% 13%	180
	11th to 20th	Day	7% 5% 27% 24% 23% 13%	180
	21st to end	Day	11% 8% 19% 27% 27% 8%	198
January	1st to 10th	Night	9% 8% 21% 33% 23% 6%	760
	11th to 20th	Night	5% 9% 20% 38% 23% 5%	728
	21st to end	Night	10% 5% 19% 37% 23% 6%	792
February	1st to 10th	Night	6% 8% 23% 38% 23% 3	720
	11th to 20th	Night	6% 8% 20% 41% 22% 4	704
	21st to end	Night	5% 11% 23% 40% 18% 3	592
March	1st to 10th	Night	7% 9% 27% 36% 17% 4	720
	11th to 20th	Night	7% 8% 20% 37% 24% 4	720
	21st to end	Night	8% 11% 26% 34% 18% 3	792
April	1st to 10th	Night	6% 10% 24% 38% 20% 2	720
	11th to 20th	Night	6% 11% 25% 33% 21% 5%	711
	21st to end	Night	9% 7% 19% 37% 23% 5%	720
May	1st to 10th	Night	8% 8% 25% 36% 21% 2	530
	11th to 20th	Night	5% 5% 23% 35% 25% 6%	354
	21st to end	Night	7% 6% 18% 35% 31% 3	389
June	1st to 10th	Night	6% 5% 20% 39% 27% 3	360
	11th to 20th	Night	4 4 28% 40% 21% 2	360
	21st to end	Night	3 2 18% 49% 26% 2	360
July	1st to 10th	Night	2 1 17% 52% 27% 1	350
	11th to 20th	Night	1 1 18% 58% 20% 1	348
	21st to end	Night	3 1 10% 53% 31% 2	394
August	1st to 10th	Night	6% 12% 48% 29% 5%	524
	11th to 20th	Night	4 1 9% 45% 36% 4	682
	21st to end	Night	2 1 12% 47% 35% 4	792
September	1st to 10th	Night	3 1 11% 41% 38% 6%	720
	11th to 20th	Night	3 3 13% 35% 36% 9%	720
	21st to end	Night	4 4 15% 34% 32% 11%	720
October	1st to 10th	Night	6% 4 17% 37% 29% 7%	720
	11th to 20th	Night	4 7% 15% 35% 32% 6%	720
	21st to end	Night	6% 6% 18% 34% 28% 10%	792
November	1st to 10th	Night	6% 6% 21% 35% 26% 6%	740
	11th to 20th	Night	7% 6% 20% 42% 20% 5%	760
	21st to end	Night	5% 10% 18% 36% 25% 5%	760
December	1st to 10th	Night	8% 9% 20% 39% 21% 4	760
	11th to 20th	Night	9% 6% 22% 36% 23% 4	760
	21st to end	Night	10% 9% 25% 39% 15% 2	836

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