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 2025-03-12.

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	19% 14% 4 10% 6% 5% 5% 4 5% 6% 23%	110991
	Night	15% 16% 5% 10% 6% 5% 5% 4 5% 5% 24%	116759
Transparency	Day	59% 8% 15% 12% 4 1	110681
	Night	59% 8% 12% 13% 7% 1	116718
Seeing	Day	23% 7% 16% 25% 22% 8%	18284
	Night	23% 6% 17% 28% 21% 4	23007

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	8% 19% 5% 8% 6% 5% 6% 5% 6% 7% 26%	1819
	11th to 20th	Day	11% 20% 5% 10% 5% 5% 5% 3 6% 6% 25%	2302
	21st to end	Day	14% 15% 5% 10% 6% 6% 4 3 4 4 29%	2992
February	1st to 10th	Day	10% 16% 5% 8% 4 3 4 3 5% 7% 36%	1355
	11th to 20th	Day	13% 24% 5% 10% 7% 5% 4 3 4 5% 19%	1552
	21st to end	Day	9% 16% 5% 10% 6% 6% 4 3 4 5% 33%	1425
March	1st to 10th	Day	7% 18% 6% 13% 6% 4 3 3 4 5% 29%	3167
	11th to 20th	Day	11% 12% 4 10% 5% 6% 6% 5% 6% 7% 29%	3284
	21st to end	Day	13% 27% 5% 9% 5% 4 4 3 4 5% 20%	4151
April	1st to 10th	Day	11% 19% 5% 9% 5% 5% 5% 5% 6% 6% 25%	2332
	11th to 20th	Day	8% 14% 4 10% 5% 5% 5% 5% 7% 6% 32%	2626
	21st to end	Day	12% 14% 4 9% 7% 6% 4 4 6% 7% 27%	2698
May	1st to 10th	Day	18% 23% 6% 12% 6% 5% 4 3 4 4 15%	5032
	11th to 20th	Day	10% 14% 5% 12% 7% 6% 5% 4 6% 6% 25%	5023
	21st to end	Day	22% 15% 4 10% 6% 6% 5% 3 5% 5% 20%	6055
June	1st to 10th	Day	18% 11% 3 9% 6% 6% 6% 5% 7% 7% 22%	3420
	11th to 20th	Day	19% 9% 3 9% 7% 6% 6% 5% 6% 7% 23%	3420
	21st to end	Day	19% 11% 4 9% 6% 5% 6% 5% 6% 8% 21%	3420
July	1st to 10th	Day	30% 11% 4 9% 6% 5% 5% 4 5% 6% 17%	5700
	11th to 20th	Day	29% 12% 3 10% 6% 5% 5% 4 4 6% 16%	5594
	21st to end	Day	28% 10% 3 7% 5% 4 4 4 5% 7% 22%	5543
August	1st to 10th	Day	31% 11% 4 10% 5% 4 4 4 5% 6% 16%	5046
	11th to 20th	Day	35% 9% 3 7% 5% 4 5% 3 4 6% 19%	4444
	21st to end	Day	26% 10% 4 8% 5% 5% 5% 4 7% 7% 19%	4884
September	1st to 10th	Day	34% 12% 3 9% 5% 5% 4 3 4 5% 15%	2356
	11th to 20th	Day	34% 13% 4 10% 5% 4 4 2 3 4 17%	2280
	21st to end	Day	25% 12% 4 9% 5% 5% 4 3 4 5% 24%	2022
October	1st to 10th	Day	18% 11% 4 10% 7% 7% 7% 5% 6% 7% 19%	3129
	11th to 20th	Day	14% 8% 4 7% 4 5% 4 4 5% 7% 38%	2803
	21st to end	Day	19% 12% 3 7% 4 4 5% 4 6% 6% 30%	2663
November	1st to 10th	Day	13% 19% 5% 11% 5% 4 5% 4 5% 4 26%	1356
	11th to 20th	Day	13% 13% 3 9% 6% 4 3 4 4 4 36%	1200
	21st to end	Day	13% 18% 6% 10% 5% 5% 4 3 4 5% 26%	1005
December	1st to 10th	Day	16% 18% 6% 13% 6% 6% 4 3 3 5% 21%	1374
	11th to 20th	Day	10% 22% 7% 12% 7% 6% 5% 4 4 5% 18%	1517
	21st to end	Day	6% 18% 6% 12% 7% 6% 4 4 5% 5% 27%	2002
January	1st to 10th	Night	8% 17% 5% 10% 6% 5% 6% 4 6% 6% 27%	8146
	11th to 20th	Night	12% 19% 5% 13% 6% 4 4 3 4 6% 24%	7623
	21st to end	Night	13% 16% 4 9% 6% 4 4 3 4 4 31%	7986
February	1st to 10th	Night	9% 16% 5% 9% 4 4 3 3 5% 5% 37%	3043
	11th to 20th	Night	16% 22% 6% 10% 5% 5% 4 3 4 5% 20%	2735
	21st to end	Night	10% 15% 5% 11% 7% 5% 5% 4 5% 4 29%	2204
March	1st to 10th	Night	6% 18% 6% 13% 5% 3 4 3 5% 5% 31%	3907
	11th to 20th	Night	10% 13% 5% 11% 5% 5% 5% 4 6% 6% 31%	3699
	21st to end	Night	13% 26% 4 9% 4 4 4 3 5% 5% 24%	3480
April	1st to 10th	Night	10% 20% 6% 11% 7% 6% 5% 4 4 4 23%	1847
	11th to 20th	Night	10% 17% 3 8% 6% 5% 4 4 5% 7% 32%	1503
	21st to end	Night	13% 14% 4 9% 7% 5% 4 4 5% 5% 31%	1482
May	1st to 10th	Night	19% 20% 6% 12% 7% 5% 5% 4 5% 5% 13%	1937
	11th to 20th	Night	12% 15% 6% 11% 7% 6% 5% 5% 5% 7% 22%	1882
	21st to end	Night	24% 12% 4 10% 6% 6% 5% 4 4 5% 20%	1525
June	1st to 10th	Night	17% 13% 6% 11% 6% 6% 5% 4 5% 5% 21%	760
	11th to 20th	Night	18% 12% 5% 9% 4 4 5% 3 7% 9% 24%	760
	21st to end	Night	22% 10% 6% 9% 5% 5% 6% 5% 5% 8% 19%	760
July	1st to 10th	Night	35% 10% 4 9% 7% 5% 5% 4 6% 5% 12%	1270
	11th to 20th	Night	34% 12% 3 6% 4 5% 5% 4 5% 6% 16%	1366
	21st to end	Night	31% 8% 3 9% 5% 3 3 3 6% 6% 22%	2022
August	1st to 10th	Night	34% 10% 4 7% 4 3 4 3 5% 7% 19%	1894
	11th to 20th	Night	35% 8% 3 7% 4 5% 4 4 5% 5% 22%	2392
	21st to end	Night	31% 12% 3 9% 6% 6% 5% 3 4 5% 15%	2783
September	1st to 10th	Night	36% 13% 3 9% 5% 4 4 3 4 4 16%	1822
	11th to 20th	Night	36% 13% 3 9% 4 3 3 3 4 4 18%	1900
	21st to end	Night	28% 10% 3 9% 5% 5% 4 4 4 5% 23%	2156
October	1st to 10th	Night	21% 12% 4 12% 6% 6% 6% 5% 5% 5% 17%	3832
	11th to 20th	Night	13% 10% 4 9% 6% 5% 5% 4 6% 6% 33%	4125
	21st to end	Night	18% 14% 5% 9% 5% 4 4 3 5% 6% 28%	4896
November	1st to 10th	Night	15% 18% 5% 10% 5% 5% 3 3 5% 5% 25%	2940
	11th to 20th	Night	12% 13% 3 8% 5% 4 4 4 5% 5% 36%	3200
	21st to end	Night	13% 18% 5% 11% 6% 5% 4 3 4 5% 26%	3395
December	1st to 10th	Night	15% 18% 6% 13% 7% 6% 5% 3 4 5% 17%	5811
	11th to 20th	Night	9% 24% 7% 12% 6% 5% 5% 4 5% 6% 18%	6700
	21st to end	Night	6% 16% 6% 13% 7% 6% 6% 5% 7% 7% 20%	8976

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% 12% 10% 11% 12% 2	1819
	11th to 20th	Day	55% 10% 11% 15% 7% 2	2317
	21st to end	Day	57% 5% 6% 16% 13% 3	2992
February	1st to 10th	Day	48% 7% 11% 17% 13% 4	1355
	11th to 20th	Day	66% 7% 8% 11% 8% 1	1559
	21st to end	Day	53% 7% 9% 20% 10% 1	1425
March	1st to 10th	Day	55% 6% 8% 13% 13% 5%	3167
	11th to 20th	Day	46% 11% 12% 15% 14% 2	3284
	21st to end	Day	64% 7% 13% 8% 6% 2	4151
April	1st to 10th	Day	57% 7% 15% 15% 6% 1	2332
	11th to 20th	Day	47% 8% 15% 21% 8%	2626
	21st to end	Day	52% 8% 13% 18% 7% 1	2698
May	1st to 10th	Day	72% 6% 10% 10% 2	5032
	11th to 20th	Day	53% 7% 16% 20% 4	5026
	21st to end	Day	62% 8% 13% 14% 2	6056
June	1st to 10th	Day	56% 11% 20% 12% 2	3420
	11th to 20th	Day	56% 11% 20% 10% 3	3420
	21st to end	Day	56% 12% 23% 8%	3420
July	1st to 10th	Day	66% 10% 18% 6% 1	5611
	11th to 20th	Day	69% 9% 18% 4	5495
	21st to end	Day	58% 10% 22% 10%	5368
August	1st to 10th	Day	68% 9% 17% 6%	5027
	11th to 20th	Day	64% 8% 16% 10% 1	4488
	21st to end	Day	62% 10% 17% 9% 1	4883
September	1st to 10th	Day	69% 9% 12% 8% 2	2356
	11th to 20th	Day	70% 7% 13% 9% 1	2280
	21st to end	Day	62% 7% 15% 14% 2	2022
October	1st to 10th	Day	61% 7% 16% 14% 2	3129
	11th to 20th	Day	44% 6% 19% 19% 11% 1	2803
	21st to end	Day	50% 9% 15% 21% 4 1	2663
November	1st to 10th	Day	58% 7% 12% 14% 7% 1	1356
	11th to 20th	Day	50% 7% 11% 23% 8% 1	1200
	21st to end	Day	60% 7% 9% 12% 11% 2	1005
December	1st to 10th	Day	67% 7% 8% 14% 5%	1374
	11th to 20th	Day	65% 7% 13% 12% 2	1520
	21st to end	Day	54% 10% 12% 14% 9% 1	2002
January	1st to 10th	Night	54% 11% 10% 11% 11% 4	8146
	11th to 20th	Night	59% 9% 9% 14% 6% 2	7671
	21st to end	Night	53% 7% 7% 14% 16% 4	7986
February	1st to 10th	Night	48% 7% 8% 19% 12% 5%	3043
	11th to 20th	Night	65% 6% 10% 11% 8% 1	2739
	21st to end	Night	55% 7% 9% 16% 11% 1	2204
March	1st to 10th	Night	54% 6% 7% 16% 10% 7%	3907
	11th to 20th	Night	48% 11% 10% 14% 16% 2	3701
	21st to end	Night	58% 9% 14% 10% 7% 2	3480
April	1st to 10th	Night	62% 7% 11% 14% 4 1	1847
	11th to 20th	Night	51% 8% 14% 20% 7%	1503
	21st to end	Night	54% 6% 14% 18% 7%	1482
May	1st to 10th	Night	70% 8% 11% 10% 1	1937
	11th to 20th	Night	59% 8% 14% 16% 3	1883
	21st to end	Night	64% 9% 12% 13% 2	1525
June	1st to 10th	Night	62% 11% 14% 12% 1	760
	11th to 20th	Night	56% 13% 20% 9% 3	760
	21st to end	Night	62% 12% 20% 6%	760
July	1st to 10th	Night	71% 9% 14% 5%	1250
	11th to 20th	Night	68% 10% 18% 4	1342
	21st to end	Night	62% 8% 23% 7%	1958
August	1st to 10th	Night	65% 8% 20% 7%	1888
	11th to 20th	Night	63% 9% 15% 10% 2	2414
	21st to end	Night	71% 8% 13% 7% 2	2783
September	1st to 10th	Night	73% 9% 9% 7% 2	1822
	11th to 20th	Night	69% 7% 12% 10% 1	1900
	21st to end	Night	63% 7% 15% 13% 3	2156
October	1st to 10th	Night	66% 6% 13% 12% 2	3832
	11th to 20th	Night	48% 7% 17% 16% 11% 1	4125
	21st to end	Night	57% 5% 12% 20% 6%	4896
November	1st to 10th	Night	59% 7% 11% 15% 6% 1	2940
	11th to 20th	Night	48% 7% 12% 24% 10%	3200
	21st to end	Night	59% 7% 10% 11% 11% 2	3395
December	1st to 10th	Night	68% 7% 8% 12% 5%	5811
	11th to 20th	Night	66% 7% 13% 11% 3	6696
	21st to end	Night	57% 10% 14% 12% 5% 1	8976

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% 10% 18% 25% 25% 12%	100
	11th to 20th	Day	22% 9% 13% 27% 18% 11%	272
	21st to end	Day	23% 8% 16% 22% 22% 8%	330
February	1st to 10th	Day	13% 8% 16% 30% 17% 16%	300
	11th to 20th	Day	27% 7% 20% 26% 16% 5%	392
	21st to end	Day	17% 8% 25% 28% 13% 9%	331
March	1st to 10th	Day	17% 8% 21% 28% 18% 8%	400
	11th to 20th	Day	13% 7% 21% 29% 20% 10%	433
	21st to end	Day	15% 8% 19% 27% 25% 7%	616
April	1st to 10th	Day	15% 10% 23% 28% 20% 5%	574
	11th to 20th	Day	14% 8% 15% 29% 25% 8%	640
	21st to end	Day	17% 7% 16% 26% 26% 8%	650
May	1st to 10th	Day	21% 10% 20% 24% 21% 4	653
	11th to 20th	Day	18% 9% 17% 27% 21% 8%	675
	21st to end	Day	23% 8% 17% 28% 19% 5%	851
June	1st to 10th	Day	19% 5% 14% 27% 23% 11%	870
	11th to 20th	Day	19% 4 16% 28% 23% 10%	870
	21st to end	Day	18% 6% 14% 26% 26% 10%	870
July	1st to 10th	Day	22% 5% 14% 27% 25% 7%	856
	11th to 20th	Day	24% 4 12% 29% 25% 7%	852
	21st to end	Day	28% 3 11% 24% 25% 9%	770
August	1st to 10th	Day	30% 5% 16% 22% 20% 8%	676
	11th to 20th	Day	29% 7% 15% 22% 20% 7%	665
	21st to end	Day	31% 7% 12% 19% 22% 8%	748
September	1st to 10th	Day	32% 7% 13% 21% 21% 6%	603
	11th to 20th	Day	40% 6% 11% 16% 20% 8%	580
	21st to end	Day	28% 5% 15% 19% 21% 12%	395
October	1st to 10th	Day	33% 5% 16% 23% 16% 8%	390
	11th to 20th	Day	24% 8% 12% 26% 21% 8%	390
	21st to end	Day	25% 4 14% 28% 19% 10%	429
November	1st to 10th	Day	23% 7% 19% 20% 19% 12%	388
	11th to 20th	Day	17% 5% 15% 28% 23% 12%	300
	21st to end	Day	22% 6% 15% 25% 26% 7%	105
December	1st to 10th	Day	25% 9% 12% 27% 23% 4	100
	11th to 20th	Day	23% 6% 23% 29% 12% 7%	100
	21st to end	Day	22% 7% 24% 22% 21% 5%	110
January	1st to 10th	Night	14% 6% 18% 31% 25% 6%	1000
	11th to 20th	Night	21% 8% 19% 30% 17% 5%	977
	21st to end	Night	23% 7% 18% 31% 18% 4	990
February	1st to 10th	Night	14% 8% 19% 34% 20% 4	900
	11th to 20th	Night	26% 7% 22% 27% 16% 1	884
	21st to end	Night	17% 7% 23% 35% 14% 5%	637
March	1st to 10th	Night	15% 9% 21% 30% 21% 3	700
	11th to 20th	Night	14% 6% 21% 32% 23% 5%	665
	21st to end	Night	14% 8% 18% 27% 27% 5%	633
April	1st to 10th	Night	18% 8% 22% 24% 23% 5%	570
	11th to 20th	Night	18% 5% 15% 27% 30% 5%	563
	21st to end	Night	16% 5% 15% 30% 30% 4	570
May	1st to 10th	Night	18% 7% 16% 27% 28% 3	404
	11th to 20th	Night	20% 3 15% 33% 27% 2	374
	21st to end	Night	20% 6% 15% 33% 26%	215
June	1st to 10th	Night	22% 7% 13% 24% 33% 2	190
	11th to 20th	Night	19% 3 15% 32% 29% 2	190
	21st to end	Night	18% 8% 10% 33% 27% 4	190
July	1st to 10th	Night	32% 5% 8% 24% 29% 3	185
	11th to 20th	Night	34% 5% 6% 23% 29% 3	241
	21st to end	Night	35% 6% 6% 20% 29% 5%	416
August	1st to 10th	Night	34% 4 11% 26% 22% 4	378
	11th to 20th	Night	39% 4 14% 21% 19% 3	507
	21st to end	Night	36% 5% 10% 21% 23% 5%	627
September	1st to 10th	Night	39% 6% 11% 22% 17% 5%	570
	11th to 20th	Night	41% 5% 7% 18% 23% 6%	570
	21st to end	Night	32% 5% 16% 18% 22% 7%	607
October	1st to 10th	Night	34% 7% 13% 23% 19% 4	689
	11th to 20th	Night	24% 5% 14% 27% 24% 6%	860
	21st to end	Night	25% 6% 16% 27% 23% 3	946
November	1st to 10th	Night	23% 7% 20% 26% 18% 5%	880
	11th to 20th	Night	16% 7% 18% 32% 21% 5%	900
	21st to end	Night	24% 6% 19% 26% 20% 5%	900
December	1st to 10th	Night	27% 7% 20% 26% 16% 4	979
	11th to 20th	Night	16% 7% 22% 32% 16% 6%	1000
	21st to end	Night	20% 7% 23% 27% 17% 6%	1100

Page generated at 2025-03-09 04:08:47. Version ac07

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