Lalen Lake Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Lalen Lake Observatory clear sky chart from 2005-11-06 to 2026-07-16.

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	19% 15% 5% 10% 6% 5% 5% 4 5% 6% 21%	142869
	Night	15% 16% 5% 10% 6% 5% 5% 4 5% 6% 23%	138678
Transparency	Day	61% 8% 14% 12% 4 1	142512
	Night	60% 8% 11% 13% 7% 1	138620
Seeing	Day	23% 7% 16% 25% 21% 8%	20271
	Night	23% 6% 17% 28% 21% 4	24556

By Month

Percentage Cloud Cover NW (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	9% 19% 4 8% 5% 6% 5% 4 5% 8% 27%	2361
	11th to 20th	Day	13% 20% 6% 11% 5% 5% 4 3 5% 7% 22%	3004
	21st to end	Day	13% 15% 5% 9% 6% 5% 4 3 5% 7% 25%	3871
February	1st to 10th	Day	10% 16% 5% 8% 5% 3 4 3 5% 7% 35%	1423
	11th to 20th	Day	14% 24% 5% 10% 7% 5% 4 3 4 5% 19%	1632
	21st to end	Day	9% 16% 5% 10% 5% 5% 4 3 4 5% 34%	1494
March	1st to 10th	Day	6% 18% 7% 15% 7% 5% 4 3 4 4 26%	4499
	11th to 20th	Day	8% 16% 5% 12% 6% 6% 6% 4 6% 6% 24%	4776
	21st to end	Day	9% 20% 4 9% 5% 5% 5% 4 5% 7% 27%	6086
April	1st to 10th	Day	10% 19% 5% 9% 5% 5% 5% 5% 6% 6% 26%	2577
	11th to 20th	Day	9% 16% 4 10% 6% 5% 5% 4 6% 6% 30%	2906
	21st to end	Day	13% 15% 4 9% 7% 5% 4 4 6% 6% 25%	2982
May	1st to 10th	Day	19% 24% 5% 12% 6% 5% 4 3 4 5% 13%	9355
	11th to 20th	Day	15% 18% 6% 13% 7% 6% 5% 3 5% 5% 17%	9381
	21st to end	Day	18% 17% 5% 12% 7% 6% 5% 4 5% 5% 16%	11314
June	1st to 10th	Day	18% 11% 3 10% 6% 6% 6% 5% 6% 7% 22%	3780
	11th to 20th	Day	17% 9% 3 9% 7% 6% 6% 5% 6% 7% 25%	3780
	21st to end	Day	19% 11% 4 9% 6% 5% 6% 5% 6% 8% 20%	3780
July	1st to 10th	Day	27% 11% 4 9% 6% 5% 5% 4 5% 6% 18%	8340
	11th to 20th	Day	24% 12% 4 11% 6% 5% 6% 4 5% 7% 16%	8106
	21st to end	Day	28% 10% 4 8% 5% 4 4 4 5% 7% 21%	7957
August	1st to 10th	Day	30% 11% 4 10% 5% 5% 4 4 5% 6% 17%	5206
	11th to 20th	Day	34% 9% 3 7% 5% 4 5% 3 4 6% 19%	4587
	21st to end	Day	26% 10% 3 8% 5% 5% 5% 4 6% 7% 19%	5038
September	1st to 10th	Day	35% 12% 3 9% 5% 5% 4 3 4 5% 15%	2479
	11th to 20th	Day	33% 13% 4 10% 5% 4 4 2 3 4 17%	2400
	21st to end	Day	25% 12% 4 9% 5% 5% 4 3 4 5% 23%	2128
October	1st to 10th	Day	18% 11% 4 10% 7% 7% 7% 5% 6% 7% 19%	3228
	11th to 20th	Day	14% 9% 4 7% 4 5% 4 4 5% 7% 37%	2892
	21st to end	Day	19% 13% 3 7% 4 4 5% 4 6% 6% 29%	2748
November	1st to 10th	Day	12% 19% 5% 11% 5% 4 6% 4 5% 4 26%	1425
	11th to 20th	Day	13% 13% 3 10% 6% 4 3 4 5% 5% 35%	1260
	21st to end	Day	13% 18% 6% 10% 6% 5% 4 3 4 5% 26%	1055
December	1st to 10th	Day	16% 18% 6% 13% 6% 5% 4 3 3 5% 21%	1416
	11th to 20th	Day	10% 22% 7% 11% 7% 6% 5% 4 4 5% 20%	1557
	21st to end	Day	5% 18% 6% 12% 7% 6% 5% 4 5% 5% 28%	2046
January	1st to 10th	Night	8% 17% 4 10% 6% 5% 5% 4 5% 6% 30%	10564
	11th to 20th	Night	14% 20% 5% 12% 6% 5% 4 3 4 5% 21%	9938
	21st to end	Night	12% 16% 4 9% 6% 5% 5% 4 6% 7% 28%	10342
February	1st to 10th	Night	9% 16% 5% 9% 4 4 3 3 6% 5% 36%	3195
	11th to 20th	Night	15% 22% 6% 10% 6% 5% 4 3 4 5% 20%	2875
	21st to end	Night	10% 14% 5% 11% 7% 5% 5% 4 5% 4 31%	2311
March	1st to 10th	Night	6% 18% 6% 14% 6% 4 5% 3 4 5% 29%	5575
	11th to 20th	Night	8% 18% 6% 12% 5% 5% 5% 4 6% 6% 25%	5399
	21st to end	Night	9% 20% 4 9% 4 5% 5% 4 6% 7% 27%	5087
April	1st to 10th	Night	10% 20% 6% 11% 7% 6% 5% 4 5% 4 23%	2040
	11th to 20th	Night	11% 18% 3 8% 6% 5% 4 3 5% 7% 29%	1663
	21st to end	Night	14% 15% 4 10% 6% 5% 4 4 4 5% 29%	1638
May	1st to 10th	Night	20% 22% 6% 11% 7% 5% 6% 4 4 4 12%	3593
	11th to 20th	Night	18% 20% 6% 12% 7% 5% 5% 3 4 5% 15%	3510
	21st to end	Night	22% 16% 6% 13% 7% 6% 5% 3 4 4 14%	2875
June	1st to 10th	Night	17% 13% 6% 11% 6% 6% 5% 4 5% 5% 22%	840
	11th to 20th	Night	17% 12% 5% 9% 4 4 6% 3 7% 9% 25%	840
	21st to end	Night	22% 10% 6% 9% 5% 5% 6% 5% 5% 8% 18%	840
July	1st to 10th	Night	33% 11% 4 10% 7% 6% 5% 4 5% 4 9%	1860
	11th to 20th	Night	30% 12% 4 10% 5% 5% 5% 4 5% 6% 15%	1978
	21st to end	Night	30% 8% 3 10% 5% 4 4 4 6% 6% 19%	2911
August	1st to 10th	Night	34% 10% 4 7% 4 3 4 3 5% 7% 19%	1954
	11th to 20th	Night	34% 8% 3 7% 4 4 5% 4 5% 5% 21%	2469
	21st to end	Night	32% 12% 3 9% 6% 5% 5% 3 4 5% 15%	2871
September	1st to 10th	Night	37% 13% 3 9% 5% 4 4 3 3 4 15%	1919
	11th to 20th	Night	36% 13% 3 9% 4 3 3 3 4 4 17%	2000
	21st to end	Night	28% 10% 3 9% 5% 6% 4 4 4 5% 22%	2270
October	1st to 10th	Night	21% 12% 4 12% 6% 6% 6% 5% 5% 5% 17%	3953
	11th to 20th	Night	14% 10% 4 9% 6% 5% 5% 4 6% 6% 33%	4256
	21st to end	Night	18% 14% 5% 9% 5% 4 4 3 5% 6% 27%	5053
November	1st to 10th	Night	14% 18% 5% 10% 5% 5% 3 3 5% 5% 26%	3091
	11th to 20th	Night	12% 13% 3 8% 5% 5% 4 4 5% 5% 35%	3360
	21st to end	Night	13% 18% 5% 11% 6% 5% 4 3 4 5% 26%	3565
December	1st to 10th	Night	15% 18% 6% 13% 7% 6% 5% 4 4 5% 17%	5989
	11th to 20th	Night	9% 23% 6% 12% 6% 5% 5% 4 5% 6% 20%	6880
	21st to end	Night	6% 16% 6% 13% 7% 6% 6% 5% 7% 7% 21%	9174

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	52% 10% 8% 10% 19% 2	2361
	11th to 20th	Day	58% 10% 10% 14% 7% 2	3019
	21st to end	Day	55% 9% 6% 17% 10% 2	3872
February	1st to 10th	Day	49% 7% 11% 17% 12% 4	1423
	11th to 20th	Day	65% 7% 8% 11% 8% 1	1639
	21st to end	Day	52% 6% 9% 20% 12% 1	1494
March	1st to 10th	Day	59% 5% 6% 12% 14% 3	4499
	11th to 20th	Day	53% 9% 10% 14% 13% 1	4776
	21st to end	Day	54% 7% 13% 17% 8% 1	6086
April	1st to 10th	Day	56% 7% 15% 16% 5% 1	2577
	11th to 20th	Day	50% 8% 15% 19% 8%	2906
	21st to end	Day	55% 8% 13% 17% 7% 1	2982
May	1st to 10th	Day	72% 6% 8% 12% 2	9355
	11th to 20th	Day	65% 6% 12% 15% 3	9384
	21st to end	Day	66% 7% 14% 12% 1	11315
June	1st to 10th	Day	56% 10% 19% 13% 2	3780
	11th to 20th	Day	53% 11% 22% 11% 3	3780
	21st to end	Day	58% 12% 22% 8%	3780
July	1st to 10th	Day	64% 10% 18% 7%	8251
	11th to 20th	Day	67% 10% 19% 5%	8007
	21st to end	Day	60% 9% 22% 9%	7734
August	1st to 10th	Day	67% 9% 18% 6%	5187
	11th to 20th	Day	64% 8% 16% 10% 1	4631
	21st to end	Day	62% 10% 17% 9% 1	5037
September	1st to 10th	Day	70% 9% 12% 7% 2	2479
	11th to 20th	Day	71% 7% 13% 9% 1	2400
	21st to end	Day	62% 7% 14% 15% 2	2128
October	1st to 10th	Day	62% 7% 16% 14% 2	3228
	11th to 20th	Day	45% 6% 18% 19% 11% 1	2892
	21st to end	Day	51% 9% 15% 20% 4 1	2748
November	1st to 10th	Day	58% 7% 12% 14% 7% 1	1425
	11th to 20th	Day	50% 7% 11% 23% 9% 1	1260
	21st to end	Day	60% 6% 8% 13% 11% 2	1055
December	1st to 10th	Day	66% 7% 8% 14% 5%	1416
	11th to 20th	Day	64% 7% 13% 12% 3 1	1560
	21st to end	Day	54% 10% 12% 14% 9% 2	2046
January	1st to 10th	Night	52% 9% 8% 10% 17% 4	10564
	11th to 20th	Night	63% 9% 8% 12% 6% 1	9986
	21st to end	Night	52% 10% 8% 15% 12% 3	10342
February	1st to 10th	Night	48% 7% 9% 20% 12% 5%	3195
	11th to 20th	Night	65% 6% 9% 10% 9% 1	2879
	21st to end	Night	54% 7% 10% 16% 12% 1	2311
March	1st to 10th	Night	57% 5% 7% 15% 11% 5%	5575
	11th to 20th	Night	55% 9% 8% 14% 13% 1	5401
	21st to end	Night	53% 8% 12% 19% 7% 1	5087
April	1st to 10th	Night	62% 7% 11% 15% 4 1	2040
	11th to 20th	Night	53% 8% 13% 19% 7%	1663
	21st to end	Night	56% 5% 14% 17% 7%	1638
May	1st to 10th	Night	73% 7% 8% 10% 2	3590
	11th to 20th	Night	70% 7% 10% 10% 3	3511
	21st to end	Night	71% 7% 11% 10% 1	2879
June	1st to 10th	Night	62% 10% 14% 13%	840
	11th to 20th	Night	55% 12% 21% 9% 3	840
	21st to end	Night	63% 12% 20% 6%	840
July	1st to 10th	Night	75% 8% 12% 4	1840
	11th to 20th	Night	70% 9% 16% 4	1954
	21st to end	Night	64% 7% 22% 7%	2829
August	1st to 10th	Night	65% 8% 20% 7%	1948
	11th to 20th	Night	63% 9% 15% 10% 2	2491
	21st to end	Night	71% 8% 13% 7% 2	2871
September	1st to 10th	Night	74% 8% 9% 7% 2	1919
	11th to 20th	Night	70% 7% 12% 10% 1	2000
	21st to end	Night	63% 7% 14% 12% 3	2270
October	1st to 10th	Night	66% 6% 14% 12% 2	3953
	11th to 20th	Night	49% 7% 17% 15% 11% 1	4256
	21st to end	Night	58% 5% 12% 20% 5%	5053
November	1st to 10th	Night	59% 7% 11% 15% 6% 1	3091
	11th to 20th	Night	48% 8% 11% 23% 9%	3360
	21st to end	Night	59% 7% 10% 12% 10% 2	3565
December	1st to 10th	Night	67% 7% 8% 12% 5% 1	5989
	11th to 20th	Night	64% 7% 13% 11% 4 1	6876
	21st to end	Night	57% 10% 14% 12% 6% 2	9174

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	10% 9% 16% 28% 24% 13%	110
	11th to 20th	Day	24% 9% 12% 26% 19% 10%	291
	21st to end	Day	24% 8% 16% 21% 22% 10%	352
February	1st to 10th	Day	13% 7% 17% 30% 17% 16%	320
	11th to 20th	Day	27% 7% 21% 26% 15% 5%	422
	21st to end	Day	16% 8% 25% 29% 13% 8%	355
March	1st to 10th	Day	17% 7% 20% 27% 20% 9%	430
	11th to 20th	Day	13% 7% 21% 29% 21% 11%	495
	21st to end	Day	13% 7% 19% 27% 25% 9%	704
April	1st to 10th	Day	14% 10% 24% 27% 18% 7%	657
	11th to 20th	Day	16% 9% 17% 27% 24% 7%	740
	21st to end	Day	20% 9% 17% 24% 24% 7%	750
May	1st to 10th	Day	23% 11% 19% 24% 20% 4	753
	11th to 20th	Day	20% 9% 17% 26% 21% 8%	775
	21st to end	Day	22% 9% 16% 28% 20% 6%	973
June	1st to 10th	Day	20% 5% 15% 27% 23% 10%	990
	11th to 20th	Day	17% 5% 15% 27% 24% 12%	990
	21st to end	Day	18% 6% 13% 27% 25% 11%	990
July	1st to 10th	Day	21% 5% 13% 27% 27% 8%	976
	11th to 20th	Day	25% 3 12% 29% 24% 7%	943
	21st to end	Day	29% 4 11% 24% 24% 9%	826
August	1st to 10th	Day	29% 5% 15% 22% 20% 8%	726
	11th to 20th	Day	28% 8% 15% 22% 20% 7%	715
	21st to end	Day	32% 7% 12% 20% 21% 8%	803
September	1st to 10th	Day	34% 7% 14% 21% 19% 5%	645
	11th to 20th	Day	40% 6% 10% 17% 19% 8%	620
	21st to end	Day	27% 5% 15% 19% 22% 12%	425
October	1st to 10th	Day	33% 4 16% 23% 15% 7%	420
	11th to 20th	Day	25% 8% 12% 26% 21% 8%	420
	21st to end	Day	24% 5% 14% 28% 19% 10%	462
November	1st to 10th	Day	22% 7% 18% 22% 19% 11%	417
	11th to 20th	Day	17% 4 15% 28% 24% 11%	320
	21st to end	Day	21% 6% 16% 23% 24% 10%	115
December	1st to 10th	Day	23% 11% 12% 27% 24% 4	110
	11th to 20th	Day	21% 5% 24% 30% 14% 6%	110
	21st to end	Day	21% 7% 24% 24% 20% 5%	121
January	1st to 10th	Night	14% 6% 18% 32% 25% 6%	1060
	11th to 20th	Night	22% 8% 19% 30% 17% 5%	1035
	21st to end	Night	23% 7% 18% 30% 18% 5%	1045
February	1st to 10th	Night	14% 8% 19% 34% 20% 4	950
	11th to 20th	Night	26% 7% 22% 28% 16% 1	934
	21st to end	Night	16% 8% 23% 35% 14% 5%	672
March	1st to 10th	Night	16% 9% 21% 30% 21% 4	740
	11th to 20th	Night	13% 6% 20% 32% 23% 6%	740
	21st to end	Night	13% 8% 18% 28% 28% 5%	700
April	1st to 10th	Night	18% 9% 20% 26% 22% 6%	630
	11th to 20th	Night	20% 6% 15% 26% 28% 5%	623
	21st to end	Night	17% 6% 17% 29% 28% 3	630
May	1st to 10th	Night	20% 7% 16% 26% 27% 4	446
	11th to 20th	Night	21% 5% 15% 32% 25% 2	414
	21st to end	Night	19% 5% 15% 33% 27%	238
June	1st to 10th	Night	21% 7% 14% 24% 32% 1	210
	11th to 20th	Night	18% 2 14% 32% 31% 2	210
	21st to end	Night	19% 8% 10% 30% 30% 3	210
July	1st to 10th	Night	31% 5% 8% 24% 30% 2	205
	11th to 20th	Night	34% 4 6% 24% 30% 3	259
	21st to end	Night	35% 5% 7% 20% 28% 5%	438
August	1st to 10th	Night	34% 4 11% 25% 22% 4	398
	11th to 20th	Night	38% 4 14% 22% 19% 2	534
	21st to end	Night	37% 5% 10% 21% 23% 5%	660
September	1st to 10th	Night	40% 6% 11% 22% 17% 4	600
	11th to 20th	Night	40% 6% 7% 18% 23% 6%	600
	21st to end	Night	32% 5% 15% 18% 23% 7%	641
October	1st to 10th	Night	34% 7% 14% 23% 18% 4	730
	11th to 20th	Night	24% 5% 15% 27% 23% 5%	910
	21st to end	Night	25% 6% 16% 26% 23% 4	1001
November	1st to 10th	Night	23% 7% 19% 27% 19% 5%	930
	11th to 20th	Night	16% 7% 19% 32% 22% 5%	950
	21st to end	Night	23% 6% 19% 26% 21% 5%	950
December	1st to 10th	Night	27% 7% 21% 25% 16% 4	1037
	11th to 20th	Night	15% 6% 23% 33% 16% 6%	1060
	21st to end	Night	19% 7% 24% 28% 17% 5%	1166

Page generated at 2026-07-12 23:14:50. Version ac07

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