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 2005-11-05 to 2024-10-14.

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	20% 14% 4 9% 6% 5% 5% 4 5% 6% 22%	103043
	Night	17% 15% 5% 10% 5% 5% 4 4 5% 6% 25%	95644
Transparency	Day	60% 9% 15% 12% 4 1	102733
	Night	58% 9% 12% 13% 7% 2	95603
Seeing	Day	23% 7% 16% 25% 22% 8%	17982
	Night	23% 6% 17% 27% 21% 5%	22234

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	6% 16% 4 8% 6% 6% 7% 6% 8% 7% 26%	1269
	11th to 20th	Day	11% 15% 4 8% 4 4 4 3 6% 7% 34%	1587
	21st to end	Day	10% 15% 5% 9% 5% 7% 4 3 5% 6% 30%	2090
February	1st to 10th	Day	10% 17% 5% 8% 5% 3 4 3 4 6% 35%	1287
	11th to 20th	Day	14% 24% 5% 10% 7% 5% 4 4 4 4 19%	1472
	21st to end	Day	9% 17% 5% 9% 5% 6% 4 3 5% 4 34%	1356
March	1st to 10th	Day	7% 18% 6% 14% 6% 4 3 3 4 5% 30%	3069
	11th to 20th	Day	11% 12% 4 10% 5% 6% 6% 5% 6% 7% 29%	3271
	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	28% 14% 4 8% 5% 4 4 4 5% 5% 19%	1881
	11th to 20th	Day	22% 12% 5% 9% 4 6% 4 3 4 7% 24%	1631
	21st to end	Day	17% 14% 4 9% 4 4 5% 4 6% 6% 28%	1540
November	1st to 10th	Day	13% 19% 5% 10% 5% 4 5% 4 5% 3 28%	1287
	11th to 20th	Day	14% 13% 3 9% 5% 4 3 4 4 4 36%	1140
	21st to end	Day	14% 18% 5% 10% 6% 5% 4 3 4 5% 26%	955
December	1st to 10th	Day	19% 19% 5% 9% 6% 5% 4 3 4 5% 22%	798
	11th to 20th	Day	8% 24% 8% 11% 6% 5% 4 4 5% 6% 20%	967
	21st to end	Day	6% 17% 5% 10% 7% 6% 5% 4 6% 6% 26%	1397
January	1st to 10th	Night	7% 15% 5% 10% 7% 6% 6% 5% 7% 6% 27%	5686
	11th to 20th	Night	13% 15% 4 9% 5% 3 3 3 4 7% 33%	5322
	21st to end	Night	10% 14% 4 10% 6% 4 4 3 5% 6% 34%	5577
February	1st to 10th	Night	9% 17% 5% 9% 4 4 3 2 5% 4 36%	2891
	11th to 20th	Night	16% 22% 6% 10% 6% 5% 4 3 4 5% 19%	2595
	21st to end	Night	10% 15% 5% 11% 7% 5% 5% 4 5% 4 30%	2098
March	1st to 10th	Night	7% 17% 6% 12% 5% 3 4 3 4 5% 32%	3785
	11th to 20th	Night	10% 13% 5% 11% 5% 5% 5% 4 6% 6% 31%	3684
	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	31% 13% 4 8% 5% 4 4 3 4 5% 21%	2299
	11th to 20th	Night	22% 12% 5% 9% 5% 5% 4 3 5% 6% 25%	2407
	21st to end	Night	19% 14% 5% 9% 5% 4 4 4 5% 6% 25%	2816
November	1st to 10th	Night	15% 18% 5% 9% 5% 5% 3 3 5% 5% 26%	2789
	11th to 20th	Night	12% 13% 3 8% 5% 4 4 4 5% 5% 37%	3040
	21st to end	Night	13% 18% 5% 11% 6% 5% 4 3 4 4 26%	3225
December	1st to 10th	Night	20% 19% 4 11% 6% 5% 4 3 5% 5% 20%	3379
	11th to 20th	Night	10% 24% 7% 12% 6% 5% 4 3 5% 5% 19%	4240
	21st to end	Night	8% 17% 5% 11% 7% 6% 6% 5% 8% 8% 19%	6270

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	47% 16% 13% 13% 9% 2	1269
	11th to 20th	Day	44% 12% 10% 20% 11% 3	1602
	21st to end	Day	52% 7% 8% 18% 10% 4	2090
February	1st to 10th	Day	50% 6% 10% 17% 13% 4	1287
	11th to 20th	Day	65% 8% 8% 11% 7% 1	1479
	21st to end	Day	52% 7% 9% 20% 11% 1	1356
March	1st to 10th	Day	55% 7% 8% 13% 13% 5%	3069
	11th to 20th	Day	46% 11% 12% 15% 14% 2	3271
	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	65% 7% 14% 11% 2	1881
	11th to 20th	Day	59% 8% 15% 14% 3 1	1631
	21st to end	Day	53% 9% 15% 14% 8% 1	1540
November	1st to 10th	Day	57% 8% 12% 14% 7% 1	1287
	11th to 20th	Day	49% 7% 12% 23% 9% 1	1140
	21st to end	Day	60% 6% 9% 12% 11% 2	955
December	1st to 10th	Day	64% 8% 11% 10% 8%	798
	11th to 20th	Day	63% 9% 12% 12% 3 1	970
	21st to end	Day	51% 12% 13% 9% 13% 2	1397
January	1st to 10th	Night	51% 14% 12% 12% 8% 2	5686
	11th to 20th	Night	47% 12% 10% 19% 9% 2	5370
	21st to end	Night	47% 8% 8% 18% 14% 5%	5577
February	1st to 10th	Night	50% 6% 7% 18% 13% 5%	2891
	11th to 20th	Night	65% 6% 10% 11% 7% 1	2599
	21st to end	Night	54% 7% 10% 16% 11% 1	2098
March	1st to 10th	Night	53% 6% 7% 16% 10% 7%	3785
	11th to 20th	Night	48% 11% 10% 13% 16% 2	3686
	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% 3	2299
	11th to 20th	Night	60% 8% 16% 10% 6% 1	2407
	21st to end	Night	59% 7% 12% 14% 8%	2816
November	1st to 10th	Night	58% 8% 11% 15% 6% 1	2789
	11th to 20th	Night	48% 7% 11% 25% 10%	3040
	21st to end	Night	60% 7% 9% 11% 10% 2	3225
December	1st to 10th	Night	65% 8% 9% 10% 8%	3379
	11th to 20th	Night	65% 8% 9% 13% 4	4236
	21st to end	Night	55% 12% 14% 10% 7% 2	6270

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	9% 9% 19% 26% 27% 11%	90
	11th to 20th	Day	22% 9% 13% 25% 20% 11%	253
	21st to end	Day	23% 7% 16% 22% 23% 9%	308
February	1st to 10th	Day	14% 8% 17% 31% 15% 14%	280
	11th to 20th	Day	27% 7% 21% 25% 14% 6%	362
	21st to end	Day	17% 8% 26% 28% 12% 8%	307
March	1st to 10th	Day	18% 7% 21% 28% 18% 9%	370
	11th to 20th	Day	14% 7% 21% 29% 20% 10%	429
	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	25% 8% 12% 26% 19% 8%	370
	21st to end	Day	24% 4 14% 26% 20% 11%	396
November	1st to 10th	Day	23% 8% 19% 18% 19% 12%	359
	11th to 20th	Day	18% 5% 15% 28% 23% 12%	280
	21st to end	Day	23% 6% 16% 22% 25% 7%	95
December	1st to 10th	Day	24% 9% 11% 28% 24% 3	90
	11th to 20th	Day	22% 7% 23% 29% 11% 8%	90
	21st to end	Day	22% 7% 23% 22% 21% 4	99
January	1st to 10th	Night	14% 6% 18% 32% 25% 6%	940
	11th to 20th	Night	21% 8% 18% 31% 18% 5%	920
	21st to end	Night	22% 6% 18% 31% 19% 4	935
February	1st to 10th	Night	15% 9% 20% 34% 18% 4	850
	11th to 20th	Night	27% 7% 23% 26% 16% 1	834
	21st to end	Night	17% 6% 24% 34% 14% 5%	603
March	1st to 10th	Night	16% 8% 21% 30% 21% 3	660
	11th to 20th	Night	14% 6% 21% 32% 22% 5%	660
	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	25% 5% 15% 26% 23% 6%	827
	21st to end	Night	25% 5% 16% 26% 24% 3	891
November	1st to 10th	Night	23% 7% 20% 27% 18% 5%	830
	11th to 20th	Night	17% 7% 19% 31% 21% 4	850
	21st to end	Night	25% 7% 20% 24% 20% 5%	850
December	1st to 10th	Night	28% 7% 20% 25% 16% 4	921
	11th to 20th	Night	16% 7% 22% 31% 17% 6%	940
	21st to end	Night	20% 8% 23% 26% 17% 6%	1034

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