Star Lake Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Star Lake Observatory clear sky chart from 2005-11-06 to 2026-01-17.

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	11% 6% 2 7% 5% 5% 5% 4 6% 8% 40%	126033
	Night	13% 7% 2 6% 4 4 4 3 5% 7% 44%	125699
Transparency	Day	39% 7% 11% 18% 17% 7%	129689
	Night	37% 6% 11% 18% 19% 9%	130105
Seeing	Day	14% 9% 21% 24% 22% 10%	16433
	Night	15% 8% 22% 29% 21% 5%	26175

By Month

Percentage Cloud Cover SE (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	17% 7% 3 7% 4 3 4 3 5% 6% 42%	4167
	11th to 20th	Day	12% 6% 3 6% 4 4 4 3 3 5% 51%	4134
	21st to end	Day	21% 11% 3 6% 3 4 4 3 4 5% 36%	4730
February	1st to 10th	Day	19% 8% 3 6% 4 4 4 3 5% 5% 39%	2000
	11th to 20th	Day	19% 8% 2 6% 4 4 3 3 5% 6% 39%	1948
	21st to end	Day	15% 10% 4 8% 5% 4 3 4 5% 6% 36%	1650
March	1st to 10th	Day	19% 10% 3 8% 4 4 5% 4 5% 6% 34%	4742
	11th to 20th	Day	14% 6% 2 7% 4 4 4 4 5% 5% 45%	4980
	21st to end	Day	13% 8% 3 9% 5% 5% 4 3 5% 5% 39%	5709
April	1st to 10th	Day	12% 7% 2 7% 6% 5% 6% 4 6% 6% 41%	2400
	11th to 20th	Day	16% 7% 2 5% 5% 5% 4 4 6% 6% 41%	2374
	21st to end	Day	9% 6% 2 5% 5% 4 5% 4 6% 8% 45%	2400
May	1st to 10th	Day	17% 7% 3 7% 5% 5% 5% 4 7% 8% 32%	5405
	11th to 20th	Day	11% 6% 2 7% 5% 4 5% 4 7% 9% 40%	5404
	21st to end	Day	7% 5% 2 6% 5% 5% 6% 4 8% 11% 41%	5932
June	1st to 10th	Day	8% 6% 2 8% 6% 7% 7% 5% 9% 10% 32%	2538
	11th to 20th	Day	5% 5% 2 7% 7% 6% 7% 6% 8% 10% 35%	2676
	21st to end	Day	7% 5% 2 8% 7% 6% 8% 6% 8% 11% 30%	2600
July	1st to 10th	Day	5% 5% 3 8% 7% 8% 8% 7% 9% 11% 30%	5900
	11th to 20th	Day	6% 4 2 7% 6% 7% 8% 7% 9% 12% 31%	5900
	21st to end	Day	7% 6% 3 9% 7% 7% 6% 6% 9% 12% 29%	6062
August	1st to 10th	Day	3 4 1 5% 4 5% 5% 4 7% 8% 56%	3905
	11th to 20th	Day	6% 4 2 7% 5% 6% 6% 5% 7% 10% 44%	3849
	21st to end	Day	11% 5% 2 8% 6% 6% 6% 5% 7% 8% 37%	4312
September	1st to 10th	Day	10% 6% 2 8% 6% 7% 6% 5% 6% 8% 36%	2400
	11th to 20th	Day	12% 6% 2 7% 5% 5% 6% 4 6% 7% 42%	2355
	21st to end	Day	10% 5% 2 6% 4 3 4 3 6% 8% 51%	2200
October	1st to 10th	Day	6% 5% 2 6% 4 4 4 3 5% 6% 55%	2075
	11th to 20th	Day	8% 4 2 6% 3 4 3 2 5% 7% 57%	2000
	21st to end	Day	10% 5% 2 4 3 3 3 3 4 6% 56%	2200
November	1st to 10th	Day	10% 6% 2 4 4 4 4 3 5% 5% 52%	2058
	11th to 20th	Day	11% 6% 2 6% 4 3 4 2 5% 6% 52%	1984
	21st to end	Day	14% 7% 2 5% 4 3 4 3 5% 6% 46%	1890
December	1st to 10th	Day	16% 8% 3 6% 3 4 5% 3 5% 5% 41%	3020
	11th to 20th	Day	11% 8% 3 7% 5% 4 4 4 5% 5% 44%	3463
	21st to end	Day	16% 7% 2 6% 3 3 4 3 4 6% 45%	4671
January	1st to 10th	Night	20% 7% 2 5% 3 3 3 2 3 5% 47%	6018
	11th to 20th	Night	18% 8% 4 5% 3 3 3 2 3 4 48%	5979
	21st to end	Night	25% 9% 2 5% 3 3 3 3 4 5% 39%	6218
February	1st to 10th	Night	17% 8% 3 5% 4 4 4 4 4 6% 41%	2400
	11th to 20th	Night	21% 9% 3 6% 3 3 3 2 5% 5% 41%	2338
	21st to end	Night	19% 8% 3 7% 4 4 4 3 5% 5% 38%	1980
March	1st to 10th	Night	18% 11% 3 7% 3 4 4 3 5% 6% 37%	5153
	11th to 20th	Night	14% 7% 2 5% 4 3 4 3 5% 6% 46%	4980
	21st to end	Night	18% 7% 2 5% 3 4 4 3 5% 5% 43%	5206
April	1st to 10th	Night	15% 7% 2 5% 5% 4 3 3 5% 6% 44%	2000
	11th to 20th	Night	13% 6% 2 5% 4 4 4 4 5% 6% 47%	1975
	21st to end	Night	9% 5% 2 4 4 3 4 3 6% 7% 53%	2000
May	1st to 10th	Night	17% 8% 2 7% 4 4 4 3 5% 7% 39%	4490
	11th to 20th	Night	14% 6% 2 6% 4 4 4 3 6% 9% 42%	4489
	21st to end	Night	8% 6% 2 7% 5% 5% 5% 4 7% 11% 39%	4922
June	1st to 10th	Night	7% 5% 2 7% 5% 5% 7% 5% 7% 10% 40%	1826
	11th to 20th	Night	4 5% 2 5% 4 5% 5% 6% 8% 9% 48%	1692
	21st to end	Night	5% 5% 2 7% 5% 5% 6% 6% 9% 12% 38%	1800
July	1st to 10th	Night	4 5% 3 7% 6% 6% 7% 6% 9% 13% 35%	4078
	11th to 20th	Night	4 5% 2 6% 5% 6% 7% 6% 10% 13% 35%	4077
	21st to end	Night	6% 6% 3 8% 7% 7% 8% 5% 9% 11% 30%	4792
August	1st to 10th	Night	2 2 1 4 3 4 4 4 6% 11% 59%	3244
	11th to 20th	Night	4 3 2 5% 5% 6% 6% 6% 9% 12% 44%	3200
	21st to end	Night	7% 6% 3 8% 5% 5% 6% 5% 7% 8% 41%	3586
September	1st to 10th	Night	8% 5% 2 7% 5% 5% 5% 5% 8% 10% 42%	2000
	11th to 20th	Night	9% 6% 2 6% 4 4 5% 5% 7% 9% 44%	2025
	21st to end	Night	7% 5% 2 6% 4 4 4 3 5% 8% 52%	2200
October	1st to 10th	Night	7% 4 2 5% 4 3 4 3 4 6% 58%	2320
	11th to 20th	Night	8% 4 2 4 2 3 4 3 4 6% 60%	2400
	21st to end	Night	11% 6% 2 5% 3 3 3 3 4 5% 57%	2640
November	1st to 10th	Night	12% 6% 2 4 3 3 3 2 5% 5% 56%	2460
	11th to 20th	Night	11% 6% 2 4 3 3 3 3 5% 6% 55%	2631
	21st to end	Night	14% 6% 2 5% 4 4 4 3 5% 5% 48%	2730
December	1st to 10th	Night	20% 7% 2 5% 4 4 3 3 4 4 45%	4380
	11th to 20th	Night	14% 10% 3 6% 4 4 4 2 4 5% 45%	4952
	21st to end	Night	21% 8% 3 6% 3 3 3 2 3 4 45%	6518

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	43% 4 4 19% 25% 5%	4168
	11th to 20th	Day	37% 4 3 16% 31% 9%	4134
	21st to end	Day	52% 3 6% 10% 16% 13%	4730
February	1st to 10th	Day	47% 4 7% 14% 19% 10%	2000
	11th to 20th	Day	45% 4 6% 15% 21% 9%	1958
	21st to end	Day	48% 5% 6% 15% 16% 10%	1650
March	1st to 10th	Day	49% 5% 6% 14% 17% 9%	4742
	11th to 20th	Day	41% 5% 6% 13% 18% 17%	4980
	21st to end	Day	46% 5% 9% 13% 18% 8%	5711
April	1st to 10th	Day	41% 8% 8% 17% 18% 9%	2400
	11th to 20th	Day	43% 5% 7% 17% 20% 8%	2374
	21st to end	Day	35% 8% 11% 21% 22% 4	2400
May	1st to 10th	Day	46% 8% 12% 21% 11% 2	5440
	11th to 20th	Day	38% 7% 16% 21% 17% 2	5408
	21st to end	Day	33% 9% 14% 28% 14% 2	5945
June	1st to 10th	Day	42% 10% 14% 21% 11% 1	2538
	11th to 20th	Day	38% 11% 16% 20% 13% 2	2676
	21st to end	Day	42% 12% 16% 21% 8% 1	2600
July	1st to 10th	Day	42% 13% 22% 16% 7%	5836
	11th to 20th	Day	39% 13% 31% 15% 1 1	5831
	21st to end	Day	44% 12% 17% 22% 4	5889
August	1st to 10th	Day	24% 8% 13% 29% 25% 1	3893
	11th to 20th	Day	33% 10% 18% 26% 11% 1	3885
	21st to end	Day	43% 9% 14% 16% 15% 2	4311
September	1st to 10th	Day	43% 10% 13% 16% 13% 4	2400
	11th to 20th	Day	40% 8% 10% 19% 17% 6%	2355
	21st to end	Day	31% 6% 11% 23% 20% 9%	2200
October	1st to 10th	Day	25% 5% 6% 11% 22% 31%	3374
	11th to 20th	Day	18% 2 6% 15% 28% 31%	3240
	21st to end	Day	22% 3 4 13% 38% 19%	3538
November	1st to 10th	Day	33% 5% 7% 17% 26% 13%	2058
	11th to 20th	Day	33% 4 7% 15% 29% 13%	1984
	21st to end	Day	39% 4 7% 17% 22% 11%	1890
December	1st to 10th	Day	44% 4 7% 15% 23% 7%	3020
	11th to 20th	Day	41% 5% 9% 13% 22% 10%	3460
	21st to end	Day	41% 5% 6% 20% 26% 3	4671
January	1st to 10th	Night	41% 3 6% 18% 26% 6%	6018
	11th to 20th	Night	41% 3 4 14% 27% 11%	5979
	21st to end	Night	50% 3 5% 12% 20% 10%	6218
February	1st to 10th	Night	44% 4 7% 15% 20% 10%	2400
	11th to 20th	Night	47% 4 4 14% 20% 11%	2339
	21st to end	Night	48% 6% 6% 14% 13% 12%	1980
March	1st to 10th	Night	48% 5% 7% 11% 21% 8%	5153
	11th to 20th	Night	39% 5% 6% 15% 19% 16%	4980
	21st to end	Night	42% 5% 9% 19% 17% 8%	5206
April	1st to 10th	Night	41% 6% 8% 16% 18% 11%	2000
	11th to 20th	Night	37% 5% 10% 19% 19% 10%	1975
	21st to end	Night	29% 6% 10% 22% 25% 7%	2000
May	1st to 10th	Night	44% 7% 12% 19% 15% 3	4520
	11th to 20th	Night	38% 6% 17% 23% 15% 1	4490
	21st to end	Night	35% 8% 18% 26% 12% 1	4931
June	1st to 10th	Night	36% 9% 17% 22% 13% 1	1826
	11th to 20th	Night	29% 9% 19% 26% 13% 3	1692
	21st to end	Night	33% 13% 21% 21% 11% 1	1800
July	1st to 10th	Night	36% 15% 27% 16% 6% 1	4035
	11th to 20th	Night	34% 16% 35% 12% 1 1	4026
	21st to end	Night	43% 15% 18% 20% 3	4652
August	1st to 10th	Night	18% 9% 19% 28% 25% 1	3232
	11th to 20th	Night	28% 13% 22% 28% 9% 1	3230
	21st to end	Night	38% 11% 15% 17% 15% 4	3586
September	1st to 10th	Night	35% 10% 18% 18% 13% 6%	2000
	11th to 20th	Night	33% 10% 16% 18% 16% 8%	2025
	21st to end	Night	30% 6% 12% 23% 19% 10%	2200
October	1st to 10th	Night	24% 5% 7% 12% 23% 30%	3788
	11th to 20th	Night	17% 3 4 19% 25% 32%	3899
	21st to end	Night	21% 3 5% 12% 37% 23%	4252
November	1st to 10th	Night	31% 4 7% 18% 24% 16%	2460
	11th to 20th	Night	30% 4 7% 15% 28% 15%	2631
	21st to end	Night	38% 4 6% 19% 21% 12%	2730
December	1st to 10th	Night	44% 4 5% 17% 26% 5%	4380
	11th to 20th	Night	42% 4 8% 15% 22% 9%	4954
	21st to end	Night	44% 3 5% 18% 22% 7%	6518

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	24% 21% 35% 16% 3 1	330
	11th to 20th	Day	24% 22% 31% 17% 6% 1	318
	21st to end	Day	25% 23% 37% 11% 3	330
February	1st to 10th	Day	28% 21% 31% 15% 5% 1	300
	11th to 20th	Day	34% 19% 28% 15% 3	300
	21st to end	Day	22% 31% 34% 11% 2	249
March	1st to 10th	Day	26% 20% 33% 18% 2	300
	11th to 20th	Day	25% 22% 34% 16% 3	300
	21st to end	Day	20% 19% 32% 22% 7%	435
April	1st to 10th	Day	20% 16% 33% 22% 9%	500
	11th to 20th	Day	23% 14% 28% 23% 10% 1	492
	21st to end	Day	11% 12% 29% 30% 15% 3	500
May	1st to 10th	Day	15% 10% 29% 26% 16% 3	503
	11th to 20th	Day	14% 6% 24% 30% 17% 8%	527
	21st to end	Day	7% 4 22% 31% 25% 11%	573
June	1st to 10th	Day	12% 4 14% 29% 28% 13%	610
	11th to 20th	Day	7% 3 15% 31% 33% 10%	730
	21st to end	Day	9% 3 12% 26% 31% 19%	730
July	1st to 10th	Day	7% 2 9% 23% 42% 17%	721
	11th to 20th	Day	8% 1 6% 23% 37% 24%	718
	21st to end	Day	10% 2 7% 25% 36% 21%	638
August	1st to 10th	Day	4 2 6% 23% 37% 27%	526
	11th to 20th	Day	11% 2 5% 16% 38% 28%	519
	21st to end	Day	13% 4 7% 23% 30% 24%	583
September	1st to 10th	Day	10% 2 11% 27% 34% 17%	530
	11th to 20th	Day	11% 4 10% 25% 37% 13%	530
	21st to end	Day	11% 3 13% 32% 32% 9%	530
October	1st to 10th	Day	7% 7% 22% 35% 21% 8%	405
	11th to 20th	Day	9% 6% 26% 32% 21% 7%	330
	21st to end	Day	18% 10% 22% 29% 16% 5%	363
November	1st to 10th	Day	17% 8% 28% 30% 15% 2	330
	11th to 20th	Day	13% 13% 35% 25% 13% 1	330
	21st to end	Day	16% 20% 34% 18% 8% 3	330
December	1st to 10th	Day	24% 13% 33% 22% 7% 1	330
	11th to 20th	Day	20% 15% 39% 20% 5% 1	330
	21st to end	Day	24% 18% 28% 21% 8% 1	363
January	1st to 10th	Night	24% 14% 31% 24% 6%	1050
	11th to 20th	Night	23% 14% 31% 25% 6%	1031
	21st to end	Night	26% 13% 32% 25% 4	984
February	1st to 10th	Night	21% 14% 36% 24% 5%	800
	11th to 20th	Night	30% 14% 26% 25% 4	783
	21st to end	Night	24% 18% 36% 20% 3	660
March	1st to 10th	Night	20% 15% 34% 26% 6%	710
	11th to 20th	Night	18% 13% 34% 28% 7% 1	600
	21st to end	Night	16% 11% 35% 29% 8%	660
April	1st to 10th	Night	18% 11% 35% 26% 9% 1	600
	11th to 20th	Night	17% 12% 32% 27% 13% 1	593
	21st to end	Night	11% 9% 29% 36% 16% 1	600
May	1st to 10th	Night	13% 5% 27% 38% 15% 2	600
	11th to 20th	Night	10% 5% 23% 38% 21% 3	591
	21st to end	Night	7% 3 20% 36% 29% 6%	650
June	1st to 10th	Night	8% 3 12% 38% 33% 6%	600
	11th to 20th	Night	5% 2 11% 31% 43% 9%	600
	21st to end	Night	7% 2 8% 27% 42% 14%	600
July	1st to 10th	Night	6% 2 4 26% 46% 16%	585
	11th to 20th	Night	5% 2 3 21% 48% 20%	582
	21st to end	Night	6% 1 6% 25% 47% 16%	657
August	1st to 10th	Night	3 1 3 21% 50% 22%	597
	11th to 20th	Night	6% 1 5% 19% 50% 20%	594
	21st to end	Night	11% 3 4 24% 39% 19%	660
September	1st to 10th	Night	9% 2 8% 27% 43% 12%	600
	11th to 20th	Night	10% 4 8% 29% 41% 8%	625
	21st to end	Night	10% 3 13% 33% 37% 5%	800
October	1st to 10th	Night	9% 3 18% 36% 31% 3	800
	11th to 20th	Night	9% 5% 23% 35% 24% 4	800
	21st to end	Night	14% 8% 20% 36% 20% 2	880
November	1st to 10th	Night	13% 8% 22% 34% 21% 3	820
	11th to 20th	Night	15% 8% 28% 34% 14% 1	840
	21st to end	Night	18% 10% 28% 30% 14% 1	840
December	1st to 10th	Night	23% 11% 28% 28% 10%	840
	11th to 20th	Night	22% 8% 31% 29% 9%	840
	21st to end	Night	27% 12% 25% 28% 7%	1103

Page generated at 2026-01-14 00:14:30. Version ac07

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