Lake Sky Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Lake Sky Observatory clear sky chart from 2005-11-06 to 2026-02-16.

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	17% 8% 2 6% 4 4 4 4 5% 7% 39%	130391
	Night	17% 8% 2 6% 4 4 4 3 5% 6% 40%	130416
Transparency	Day	29% 37% 7% 15% 10% 2	133911
	Night	29% 36% 7% 14% 12% 2	134957
Seeing	Day	25% 18% 32% 18% 6% 2	16593
	Night	23% 17% 34% 21% 5%	25654

By Month

Percentage Cloud Cover NE (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	11% 9% 3 8% 4 4 4 4 5% 5% 42%	4661
	11th to 20th	Day	14% 9% 3 6% 4 3 4 3 4 5% 45%	4650
	21st to end	Day	14% 10% 2 5% 3 3 3 3 5% 6% 46%	5124
February	1st to 10th	Day	17% 7% 3 6% 4 5% 5% 4 6% 7% 37%	1981
	11th to 20th	Day	13% 8% 3 6% 5% 4 5% 4 5% 7% 40%	1998
	21st to end	Day	19% 11% 3 6% 4 4 3 3 4 6% 38%	1650
March	1st to 10th	Day	21% 11% 2 6% 3 3 3 3 4 6% 37%	4476
	11th to 20th	Day	15% 7% 2 6% 5% 4 5% 4 5% 8% 40%	4722
	21st to end	Day	23% 11% 2 5% 4 3 4 3 6% 6% 33%	5850
April	1st to 10th	Day	19% 10% 3 6% 5% 4 4 3 5% 6% 37%	2400
	11th to 20th	Day	18% 6% 2 5% 5% 4 4 4 5% 8% 40%	2374
	21st to end	Day	21% 6% 2 7% 5% 4 4 4 5% 6% 37%	2580
May	1st to 10th	Day	29% 10% 3 7% 4 4 3 3 5% 7% 24%	5886
	11th to 20th	Day	21% 9% 3 8% 4 4 4 3 5% 6% 32%	5998
	21st to end	Day	20% 7% 3 6% 4 4 5% 4 6% 7% 34%	6949
June	1st to 10th	Day	18% 7% 3 6% 4 4 5% 4 6% 8% 35%	2800
	11th to 20th	Day	16% 7% 2 6% 5% 4 5% 5% 6% 8% 37%	2800
	21st to end	Day	16% 6% 2 7% 5% 5% 5% 4 6% 8% 36%	2800
July	1st to 10th	Day	16% 7% 2 7% 5% 6% 5% 5% 6% 8% 33%	6360
	11th to 20th	Day	12% 6% 3 7% 5% 5% 5% 5% 7% 10% 35%	6360
	21st to end	Day	17% 7% 2 5% 4 4 4 4 5% 7% 42%	6926
August	1st to 10th	Day	17% 7% 2 7% 5% 5% 6% 4 6% 9% 32%	4311
	11th to 20th	Day	13% 6% 2 7% 5% 5% 5% 4 6% 7% 40%	4029
	21st to end	Day	7% 4 2 6% 5% 5% 5% 4 6% 8% 46%	4312
September	1st to 10th	Day	14% 6% 2 5% 4 5% 4 3 5% 6% 46%	2400
	11th to 20th	Day	15% 5% 2 5% 3 3 4 3 4 6% 51%	2215
	21st to end	Day	18% 6% 2 5% 4 5% 4 3 4 7% 41%	2200
October	1st to 10th	Day	21% 6% 2 4 4 3 4 3 4 5% 43%	2200
	11th to 20th	Day	15% 7% 2 4 4 3 3 3 4 7% 46%	1954
	21st to end	Day	18% 7% 3 6% 4 4 3 3 5% 5% 42%	1980
November	1st to 10th	Day	13% 8% 2 6% 4 4 4 3 4 5% 49%	1851
	11th to 20th	Day	17% 7% 2 4 3 3 3 3 6% 5% 48%	1890
	21st to end	Day	17% 8% 3 5% 4 4 4 3 5% 6% 42%	1890
December	1st to 10th	Day	14% 9% 2 5% 4 3 4 3 5% 6% 46%	2700
	11th to 20th	Day	19% 7% 2 4 3 3 3 3 4 6% 46%	3066
	21st to end	Day	20% 10% 3 6% 4 3 3 3 4 6% 38%	4048
January	1st to 10th	Night	15% 8% 2 6% 3 3 4 3 5% 7% 44%	8234
	11th to 20th	Night	15% 10% 3 8% 4 4 4 3 4 5% 39%	8235
	21st to end	Night	14% 9% 3 6% 4 4 4 3 4 5% 43%	9057
February	1st to 10th	Night	15% 9% 3 6% 4 4 4 4 5% 6% 39%	2633
	11th to 20th	Night	14% 9% 3 7% 5% 4 4 4 6% 6% 40%	2399
	21st to end	Night	17% 9% 2 7% 5% 4 3 3 4 6% 40%	1980
March	1st to 10th	Night	22% 10% 1 4 4 3 4 3 5% 6% 38%	5364
	11th to 20th	Night	16% 8% 3 6% 4 4 5% 3 5% 7% 39%	5237
	21st to end	Night	23% 10% 2 5% 4 4 4 4 5% 5% 34%	5066
April	1st to 10th	Night	18% 9% 2 7% 5% 5% 4 4 6% 5% 35%	2000
	11th to 20th	Night	19% 8% 3 7% 4 4 4 3 5% 6% 38%	1975
	21st to end	Night	21% 8% 3 5% 4 3 4 3 5% 6% 40%	1819
May	1st to 10th	Night	30% 10% 2 6% 4 4 4 3 4 7% 26%	4054
	11th to 20th	Night	19% 8% 3 8% 6% 5% 5% 4 5% 7% 29%	3923
	21st to end	Night	21% 8% 3 7% 4 4 5% 3 6% 7% 32%	3926
June	1st to 10th	Night	16% 9% 2 6% 5% 4 4 3 5% 7% 38%	1600
	11th to 20th	Night	18% 8% 2 5% 6% 4 5% 3 5% 7% 37%	1600
	21st to end	Night	14% 6% 2 6% 4 4 3 4 6% 8% 42%	1600
July	1st to 10th	Night	17% 7% 3 6% 5% 4 4 4 6% 10% 35%	3600
	11th to 20th	Night	17% 8% 3 6% 4 4 4 3 6% 8% 38%	3600
	21st to end	Night	14% 8% 2 7% 5% 4 5% 3 4 5% 44%	3926
August	1st to 10th	Night	22% 7% 3 8% 5% 5% 5% 4 5% 6% 31%	2806
	11th to 20th	Night	12% 8% 3 8% 5% 6% 6% 5% 6% 7% 35%	3001
	21st to end	Night	9% 4 2 4 3 3 3 3 5% 7% 57%	3586
September	1st to 10th	Night	13% 7% 3 7% 4 4 4 4 5% 6% 44%	2000
	11th to 20th	Night	12% 8% 2 4 3 3 3 3 4 6% 52%	2185
	21st to end	Night	17% 8% 3 6% 4 3 4 3 5% 5% 42%	2200
October	1st to 10th	Night	19% 7% 2 5% 3 3 3 3 5% 7% 43%	2200
	11th to 20th	Night	15% 6% 2 4 4 3 4 3 5% 6% 48%	2440
	21st to end	Night	20% 9% 2 6% 4 3 4 3 4 5% 40%	2860
November	1st to 10th	Night	13% 6% 2 5% 4 4 3 3 6% 5% 48%	2667
	11th to 20th	Night	16% 7% 2 5% 3 3 4 3 4 5% 47%	2730
	21st to end	Night	16% 8% 2 4 4 4 5% 3 5% 6% 45%	2730
December	1st to 10th	Night	16% 9% 2 6% 4 3 3 2 4 5% 46%	4685
	11th to 20th	Night	23% 8% 3 5% 3 3 4 3 4 5% 40%	5348
	21st to end	Night	17% 11% 3 7% 4 3 3 3 4 5% 41%	7150

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	35% 21% 6% 21% 15% 2	4662
	11th to 20th	Day	33% 20% 6% 18% 16% 7%	4680
	21st to end	Day	33% 22% 7% 19% 19% 1	5124
February	1st to 10th	Day	20% 58% 6% 9% 6% 1	1981
	11th to 20th	Day	19% 59% 2 8% 10% 1	2007
	21st to end	Day	16% 62% 5% 10% 6% 1	1650
March	1st to 10th	Day	39% 28% 6% 9% 9% 9%	4479
	11th to 20th	Day	29% 30% 10% 22% 8% 1	4722
	21st to end	Day	40% 30% 3 11% 11% 5%	5850
April	1st to 10th	Day	22% 62% 3 6% 6% 2	2400
	11th to 20th	Day	16% 62% 4 8% 9% 1	2374
	21st to end	Day	20% 62% 2 8% 6% 2	2580
May	1st to 10th	Day	46% 31% 11% 8% 5%	5897
	11th to 20th	Day	42% 29% 11% 11% 6%	6002
	21st to end	Day	38% 30% 6% 16% 10% 1	6949
June	1st to 10th	Day	17% 62% 4 10% 6%	2800
	11th to 20th	Day	16% 62% 6% 9% 6% 1	2800
	21st to end	Day	16% 61% 4 10% 9% 1	2800
July	1st to 10th	Day	37% 27% 8% 16% 11% 1	6290
	11th to 20th	Day	34% 26% 13% 18% 8% 1	6285
	21st to end	Day	32% 25% 7% 21% 13% 1	6728
August	1st to 10th	Day	34% 36% 11% 14% 5% 1	4297
	11th to 20th	Day	27% 37% 10% 21% 5%	4068
	21st to end	Day	20% 36% 9% 22% 12% 1	4312
September	1st to 10th	Day	16% 57% 5% 12% 8% 1	2400
	11th to 20th	Day	17% 57% 3 10% 11% 2	2215
	21st to end	Day	20% 57% 5% 9% 8% 1	2200
October	1st to 10th	Day	26% 36% 4 15% 16% 3	3590
	11th to 20th	Day	19% 36% 3 16% 23% 3	3144
	21st to end	Day	19% 36% 8% 22% 14% 2	3180
November	1st to 10th	Day	20% 54% 6% 10% 9% 1	1851
	11th to 20th	Day	17% 54% 4 10% 13% 2	1890
	21st to end	Day	18% 55% 5% 14% 6% 2	1890
December	1st to 10th	Day	24% 35% 10% 21% 9%	2700
	11th to 20th	Day	28% 30% 10% 12% 15% 5%	3066
	21st to end	Day	37% 26% 3 16% 13% 5%	4048
January	1st to 10th	Night	32% 20% 7% 26% 13% 2	8234
	11th to 20th	Night	37% 21% 8% 16% 13% 5%	8280
	21st to end	Night	36% 22% 6% 14% 17% 3	9057
February	1st to 10th	Night	18% 60% 5% 9% 7% 1	2633
	11th to 20th	Night	18% 59% 2 8% 11% 2	2401
	21st to end	Night	18% 62% 3 8% 7% 2	1980
March	1st to 10th	Night	37% 29% 6% 10% 12% 6%	5374
	11th to 20th	Night	32% 30% 13% 14% 9% 2	5238
	21st to end	Night	40% 29% 3 8% 17% 4	5066
April	1st to 10th	Night	22% 61% 3 7% 5% 2	2000
	11th to 20th	Night	17% 63% 5% 7% 8% 1	1975
	21st to end	Night	18% 62% 4 8% 7% 1	1819
May	1st to 10th	Night	44% 31% 11% 8% 6%	4060
	11th to 20th	Night	42% 32% 10% 10% 5% 1	3924
	21st to end	Night	40% 30% 8% 15% 7%	3927
June	1st to 10th	Night	16% 63% 4 9% 8% 1	1600
	11th to 20th	Night	16% 62% 6% 10% 6%	1600
	21st to end	Night	15% 60% 4 12% 8% 1	1600
July	1st to 10th	Night	34% 27% 9% 18% 11%	3560
	11th to 20th	Night	34% 26% 15% 15% 9% 1	3552
	21st to end	Night	32% 25% 8% 21% 13% 1	3809
August	1st to 10th	Night	38% 38% 7% 11% 5% 1	2796
	11th to 20th	Night	32% 38% 8% 17% 5%	3028
	21st to end	Night	14% 36% 11% 23% 16% 1	3586
September	1st to 10th	Night	16% 58% 4 13% 8% 1	2000
	11th to 20th	Night	16% 57% 3 10% 12% 1	2185
	21st to end	Night	21% 57% 4 10% 8% 1	2200
October	1st to 10th	Night	25% 36% 8% 16% 12% 2	3590
	11th to 20th	Night	12% 36% 5% 18% 26% 3	3963
	21st to end	Night	20% 35% 7% 23% 14% 2	4610
November	1st to 10th	Night	18% 55% 4 11% 10% 2	2667
	11th to 20th	Night	17% 54% 4 10% 12% 3	2730
	21st to end	Night	17% 54% 7% 15% 7% 1	2730
December	1st to 10th	Night	27% 34% 7% 22% 8%	4685
	11th to 20th	Night	34% 31% 8% 9% 16% 2	5348
	21st to end	Night	35% 27% 4 16% 15% 4	7150

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	29% 27% 27% 15% 2	220
	11th to 20th	Day	34% 30% 27% 6% 3	220
	21st to end	Day	31% 27% 31% 8% 2	242
February	1st to 10th	Day	34% 24% 31% 9% 1	439
	11th to 20th	Day	33% 30% 28% 8%	516
	21st to end	Day	33% 25% 30% 9% 3	414
March	1st to 10th	Day	34% 23% 31% 10% 2	500
	11th to 20th	Day	31% 25% 29% 11% 3	500
	21st to end	Day	32% 23% 31% 9% 3 2	550
April	1st to 10th	Day	36% 24% 26% 10% 4 1	500
	11th to 20th	Day	31% 22% 33% 11% 3 1	492
	21st to end	Day	30% 19% 33% 14% 2 2	500
May	1st to 10th	Day	33% 21% 28% 13% 4 1	503
	11th to 20th	Day	24% 18% 40% 13% 5% 1	527
	21st to end	Day	21% 14% 35% 21% 6% 1	573
June	1st to 10th	Day	20% 15% 35% 21% 7% 2	530
	11th to 20th	Day	18% 14% 39% 22% 6% 1	530
	21st to end	Day	19% 15% 34% 23% 7% 2	530
July	1st to 10th	Day	17% 10% 33% 26% 12% 2	525
	11th to 20th	Day	15% 9% 32% 29% 13% 2	524
	21st to end	Day	15% 9% 39% 28% 8% 1	576
August	1st to 10th	Day	12% 10% 34% 31% 11% 2	526
	11th to 20th	Day	16% 9% 30% 31% 11% 3	519
	21st to end	Day	12% 11% 36% 24% 13% 4	583
September	1st to 10th	Day	19% 11% 30% 21% 15% 4	530
	11th to 20th	Day	18% 11% 29% 22% 13% 8%	530
	21st to end	Day	23% 11% 33% 21% 8% 4	530
October	1st to 10th	Day	25% 15% 25% 22% 8% 4	530
	11th to 20th	Day	22% 18% 32% 18% 7% 2	485
	21st to end	Day	27% 18% 31% 17% 6% 1	462
November	1st to 10th	Day	23% 19% 33% 15% 8% 1	425
	11th to 20th	Day	29% 23% 29% 15% 3	430
	21st to end	Day	30% 22% 30% 14% 4	430
December	1st to 10th	Day	30% 25% 28% 13% 4	240
	11th to 20th	Day	41% 24% 28% 5% 3	220
	21st to end	Day	30% 24% 31% 10% 4 1	242
January	1st to 10th	Night	25% 23% 39% 12% 2	840
	11th to 20th	Night	31% 21% 33% 14% 1	840
	21st to end	Night	23% 25% 33% 17% 2	924
February	1st to 10th	Night	25% 20% 37% 16% 2	840
	11th to 20th	Night	26% 27% 35% 11% 1	804
	21st to end	Night	30% 22% 33% 14% 2	660
March	1st to 10th	Night	27% 24% 32% 15% 3	800
	11th to 20th	Night	22% 25% 36% 14% 2	800
	21st to end	Night	29% 21% 33% 15% 3	880
April	1st to 10th	Night	26% 20% 36% 16% 2	800
	11th to 20th	Night	25% 22% 34% 17% 3	791
	21st to end	Night	24% 21% 37% 16% 2	619
May	1st to 10th	Night	32% 14% 33% 18% 4	600
	11th to 20th	Night	21% 17% 40% 18% 4	547
	21st to end	Night	20% 9% 41% 25% 4	433
June	1st to 10th	Night	18% 9% 40% 27% 7%	400
	11th to 20th	Night	19% 9% 41% 26% 5% 1	400
	21st to end	Night	14% 8% 39% 34% 6%	400
July	1st to 10th	Night	14% 6% 34% 36% 9% 1	390
	11th to 20th	Night	16% 4 30% 38% 12%	388
	21st to end	Night	14% 5% 38% 37% 6%	438
August	1st to 10th	Night	13% 5% 37% 35% 9%	527
	11th to 20th	Night	15% 6% 31% 36% 12%	669
	21st to end	Night	15% 6% 30% 34% 14% 2	880
September	1st to 10th	Night	17% 7% 34% 31% 10% 1	800
	11th to 20th	Night	16% 9% 28% 29% 17% 2	800
	21st to end	Night	22% 9% 30% 27% 10% 2	800
October	1st to 10th	Night	24% 11% 29% 24% 11% 2	800
	11th to 20th	Night	20% 14% 32% 22% 10% 1	800
	21st to end	Night	27% 17% 32% 18% 6%	880
November	1st to 10th	Night	19% 17% 34% 23% 7% 1	820
	11th to 20th	Night	24% 21% 33% 15% 5% 1	840
	21st to end	Night	26% 19% 34% 16% 4	840
December	1st to 10th	Night	27% 20% 34% 15% 4	840
	11th to 20th	Night	28% 21% 32% 17% 2	840
	21st to end	Night	27% 20% 36% 15% 2	924

Page generated at 2026-02-13 01:42:13. Version ac07

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