Great East Lake
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Great East Lake clear sky chart from 2005-11-06 to 2025-11-18.

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	18% 8% 2 6% 4 4 4 4 5% 7% 38%	126658
	Night	17% 8% 3 6% 4 4 4 4 5% 6% 39%	123723
Transparency	Day	30% 37% 7% 14% 10% 2	130165
	Night	30% 37% 6% 15% 11% 2	128303
Seeing	Day	24% 16% 32% 19% 8% 2	16267
	Night	22% 13% 32% 25% 8% 1	25086

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	13% 8% 2 6% 5% 5% 6% 4 7% 7% 39%	3597
	11th to 20th	Day	12% 8% 3 7% 4 4 3 3 4 6% 46%	3570
	21st to end	Day	13% 9% 2 6% 4 4 3 3 4 5% 46%	3960
February	1st to 10th	Day	15% 10% 3 6% 4 5% 4 4 6% 5% 37%	1787
	11th to 20th	Day	12% 7% 3 7% 4 5% 4 5% 6% 6% 40%	1948
	21st to end	Day	15% 11% 3 8% 5% 4 5% 3 5% 5% 36%	1650
March	1st to 10th	Day	20% 13% 2 6% 5% 4 4 3 4 5% 34%	4476
	11th to 20th	Day	16% 10% 3 6% 4 4 4 3 5% 6% 39%	4796
	21st to end	Day	25% 10% 2 5% 4 3 3 3 5% 6% 33%	5730
April	1st to 10th	Day	19% 12% 3 6% 4 3 3 3 5% 6% 37%	2400
	11th to 20th	Day	17% 7% 2 6% 5% 4 5% 4 6% 6% 38%	2428
	21st to end	Day	21% 8% 4 7% 5% 5% 4 3 4 5% 34%	2600
May	1st to 10th	Day	34% 11% 3 7% 4 4 4 3 4 5% 23%	5886
	11th to 20th	Day	19% 8% 3 8% 6% 5% 5% 4 5% 7% 29%	6040
	21st to end	Day	20% 6% 3 7% 4 5% 5% 4 6% 7% 32%	6949
June	1st to 10th	Day	22% 8% 2 6% 5% 4 4 4 5% 8% 32%	2800
	11th to 20th	Day	16% 6% 2 6% 5% 5% 4 4 6% 7% 38%	2800
	21st to end	Day	20% 7% 3 7% 5% 4 5% 4 5% 8% 33%	2800
July	1st to 10th	Day	17% 7% 2 7% 5% 5% 5% 5% 7% 9% 32%	6360
	11th to 20th	Day	11% 6% 2 6% 4 4 5% 4 7% 10% 39%	6360
	21st to end	Day	15% 6% 2 5% 4 4 4 4 6% 8% 42%	6926
August	1st to 10th	Day	14% 6% 3 7% 5% 5% 5% 4 7% 8% 36%	4316
	11th to 20th	Day	17% 8% 3 7% 5% 4 4 4 6% 8% 35%	4175
	21st to end	Day	10% 5% 1 6% 4 5% 5% 4 7% 8% 45%	4360
September	1st to 10th	Day	13% 5% 2 5% 3 4 5% 4 5% 8% 48%	2385
	11th to 20th	Day	14% 5% 2 5% 3 3 3 3 4 6% 54%	2200
	21st to end	Day	16% 6% 2 5% 3 4 4 3 5% 7% 46%	2200
October	1st to 10th	Day	20% 7% 2 6% 4 3 4 3 5% 5% 41%	2200
	11th to 20th	Day	16% 6% 1 5% 4 4 4 3 5% 7% 44%	1924
	21st to end	Day	23% 6% 2 6% 3 4 3 3 5% 4 40%	1980
November	1st to 10th	Day	15% 6% 2 4 3 3 4 3 4 7% 48%	1851
	11th to 20th	Day	18% 5% 2 4 3 2 4 4 5% 7% 46%	1863
	21st to end	Day	21% 8% 1 6% 3 3 4 3 5% 5% 41%	1795
December	1st to 10th	Day	15% 11% 2 5% 4 4 5% 4 6% 7% 37%	2600
	11th to 20th	Day	19% 7% 2 4 3 3 3 3 4 6% 45%	2986
	21st to end	Day	20% 10% 3 6% 3 3 4 3 5% 5% 36%	3960
January	1st to 10th	Night	13% 7% 2 6% 4 4 5% 4 5% 6% 43%	6349
	11th to 20th	Night	11% 9% 3 10% 6% 5% 4 3 5% 7% 36%	6315
	21st to end	Night	15% 8% 3 7% 4 4 4 4 5% 5% 41%	6996
February	1st to 10th	Night	14% 10% 3 7% 5% 4 4 3 4 6% 38%	2610
	11th to 20th	Night	12% 10% 2 5% 5% 5% 5% 4 5% 6% 41%	2338
	21st to end	Night	13% 9% 3 6% 6% 4 4 4 6% 7% 39%	1980
March	1st to 10th	Night	23% 10% 2 6% 4 4 4 3 4 5% 34%	5364
	11th to 20th	Night	14% 9% 3 7% 5% 5% 5% 4 6% 7% 36%	5149
	21st to end	Night	25% 8% 3 5% 3 4 3 4 5% 7% 33%	5199
April	1st to 10th	Night	19% 9% 3 6% 5% 4 4 3 4 7% 36%	2000
	11th to 20th	Night	18% 8% 3 7% 4 3 4 4 5% 6% 39%	1919
	21st to end	Night	21% 8% 3 6% 4 3 4 3 5% 7% 37%	1800
May	1st to 10th	Night	34% 9% 3 7% 4 3 4 3 4 6% 22%	4054
	11th to 20th	Night	14% 9% 4 10% 6% 6% 6% 5% 7% 7% 28%	3896
	21st to end	Night	20% 7% 3 7% 4 4 4 4 6% 7% 34%	3926
June	1st to 10th	Night	20% 8% 2 7% 4 4 4 4 6% 7% 34%	1600
	11th to 20th	Night	18% 7% 2 6% 4 4 3 4 6% 7% 39%	1600
	21st to end	Night	18% 7% 2 6% 4 5% 4 3 4 8% 39%	1600
July	1st to 10th	Night	13% 6% 2 7% 5% 5% 5% 4 7% 9% 38%	3600
	11th to 20th	Night	12% 8% 3 7% 5% 5% 4 4 6% 8% 39%	3600
	21st to end	Night	11% 5% 2 5% 4 4 4 3 6% 7% 50%	3926
August	1st to 10th	Night	21% 7% 3 7% 5% 4 5% 4 5% 6% 34%	2802
	11th to 20th	Night	16% 8% 3 6% 5% 4 5% 3 5% 8% 37%	2872
	21st to end	Night	10% 5% 1 6% 4 4 4 4 6% 8% 49%	3535
September	1st to 10th	Night	13% 5% 2 5% 3 3 4 4 5% 6% 52%	2013
	11th to 20th	Night	13% 5% 2 4 3 2 3 3 5% 6% 53%	2200
	21st to end	Night	15% 6% 2 6% 3 3 3 3 5% 6% 47%	2200
October	1st to 10th	Night	18% 7% 2 7% 4 4 4 4 5% 6% 41%	2200
	11th to 20th	Night	16% 6% 2 5% 4 4 4 4 5% 7% 43%	2472
	21st to end	Night	23% 8% 2 5% 4 4 4 3 4 6% 37%	2860
November	1st to 10th	Night	15% 6% 2 6% 3 5% 3 3 5% 6% 45%	2667
	11th to 20th	Night	17% 7% 2 5% 4 3 3 4 5% 6% 45%	2692
	21st to end	Night	17% 7% 2 4 4 3 4 4 5% 7% 42%	2605
December	1st to 10th	Night	15% 12% 3 6% 4 3 3 4 4 7% 40%	4580
	11th to 20th	Night	23% 7% 2 5% 4 3 3 3 4 5% 41%	5208
	21st to end	Night	20% 10% 3 8% 4 3 3 3 4 5% 37%	6996

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	33% 26% 5% 22% 13% 1	3598
	11th to 20th	Day	29% 26% 9% 21% 12% 3	3600
	21st to end	Day	31% 27% 5% 16% 16% 4	3960
February	1st to 10th	Day	19% 61% 5% 8% 5% 2	1787
	11th to 20th	Day	19% 60% 3 7% 9% 2	1957
	21st to end	Day	18% 61% 3 9% 8% 2	1650
March	1st to 10th	Day	42% 28% 4 8% 9% 9%	4479
	11th to 20th	Day	35% 27% 10% 14% 10% 3	4796
	21st to end	Day	41% 27% 3 11% 14% 5%	5730
April	1st to 10th	Day	21% 61% 2 7% 7% 2	2400
	11th to 20th	Day	18% 61% 3 9% 7% 1	2428
	21st to end	Day	21% 61% 3 7% 8% 1	2600
May	1st to 10th	Day	51% 28% 7% 9% 4	5897
	11th to 20th	Day	42% 29% 11% 13% 4 1	6044
	21st to end	Day	39% 29% 9% 14% 9%	6949
June	1st to 10th	Day	18% 61% 3 10% 7%	2800
	11th to 20th	Day	16% 61% 5% 9% 8% 1	2800
	21st to end	Day	18% 60% 3 11% 8% 1	2800
July	1st to 10th	Day	36% 28% 9% 16% 10%	6290
	11th to 20th	Day	29% 28% 11% 18% 13% 1	6285
	21st to end	Day	31% 25% 8% 21% 14% 1	6728
August	1st to 10th	Day	30% 37% 9% 16% 7%	4302
	11th to 20th	Day	33% 37% 9% 16% 5%	4214
	21st to end	Day	20% 38% 9% 18% 14% 1	4360
September	1st to 10th	Day	13% 57% 6% 15% 8% 1	2385
	11th to 20th	Day	16% 56% 2 13% 11% 2	2200
	21st to end	Day	17% 56% 2 15% 8% 1	2200
October	1st to 10th	Day	30% 36% 5% 15% 13% 2	3590
	11th to 20th	Day	19% 35% 6% 21% 17% 2	3101
	21st to end	Day	21% 35% 6% 21% 15% 2	3180
November	1st to 10th	Day	19% 53% 6% 12% 9% 1	1851
	11th to 20th	Day	18% 54% 3 9% 14% 3	1863
	21st to end	Day	19% 56% 6% 9% 9% 1	1795
December	1st to 10th	Day	27% 37% 8% 21% 6%	2600
	11th to 20th	Day	28% 31% 7% 12% 17% 5%	2986
	21st to end	Day	38% 25% 5% 14% 12% 6%	3960
January	1st to 10th	Night	31% 25% 7% 26% 9% 2	6349
	11th to 20th	Night	35% 26% 9% 16% 11% 2	6360
	21st to end	Night	34% 27% 6% 14% 13% 6%	6996
February	1st to 10th	Night	19% 61% 5% 7% 6% 2	2610
	11th to 20th	Night	17% 60% 3 8% 10% 2	2340
	21st to end	Night	17% 61% 3 10% 6% 3	1980
March	1st to 10th	Night	42% 28% 6% 7% 11% 6%	5374
	11th to 20th	Night	35% 28% 12% 12% 11% 2	5150
	21st to end	Night	40% 28% 2 9% 17% 4	5199
April	1st to 10th	Night	21% 61% 2 6% 8% 2	2000
	11th to 20th	Night	20% 62% 3 8% 8% 1	1919
	21st to end	Night	18% 61% 4 7% 9% 1	1800
May	1st to 10th	Night	51% 28% 8% 9% 4	4060
	11th to 20th	Night	41% 29% 10% 15% 5%	3897
	21st to end	Night	38% 28% 11% 15% 6% 1	3927
June	1st to 10th	Night	19% 61% 3 10% 6%	1600
	11th to 20th	Night	16% 62% 5% 10% 6% 1	1600
	21st to end	Night	17% 60% 3 13% 5% 1	1600
July	1st to 10th	Night	31% 28% 12% 20% 10%	3560
	11th to 20th	Night	33% 28% 10% 15% 14%	3552
	21st to end	Night	26% 26% 10% 28% 11%	3809
August	1st to 10th	Night	36% 37% 8% 13% 6%	2792
	11th to 20th	Night	32% 38% 10% 13% 7%	2899
	21st to end	Night	18% 37% 11% 20% 13% 1	3535
September	1st to 10th	Night	14% 57% 5% 14% 9% 1	2013
	11th to 20th	Night	16% 56% 3 13% 10% 2	2200
	21st to end	Night	16% 56% 4 15% 8% 1	2200
October	1st to 10th	Night	29% 36% 7% 16% 10% 1	3590
	11th to 20th	Night	16% 35% 6% 19% 20% 3	4034
	21st to end	Night	23% 35% 5% 22% 13% 2	4610
November	1st to 10th	Night	20% 52% 4 14% 8% 2	2667
	11th to 20th	Night	18% 54% 3 10% 13% 2	2692
	21st to end	Night	17% 56% 7% 11% 8% 1	2605
December	1st to 10th	Night	29% 35% 4 23% 8%	4580
	11th to 20th	Night	34% 31% 6% 9% 17% 3	5208
	21st to end	Night	39% 25% 4 16% 13% 3	6996

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	27% 23% 32% 15% 3 1	200
	11th to 20th	Day	28% 20% 35% 13% 3 2	200
	21st to end	Day	21% 21% 41% 12% 4	220
February	1st to 10th	Day	28% 24% 32% 13% 2	357
	11th to 20th	Day	27% 24% 33% 13% 3	496
	21st to end	Day	29% 19% 31% 17% 4	414
March	1st to 10th	Day	29% 21% 33% 14% 3	500
	11th to 20th	Day	25% 27% 31% 12% 4 1	500
	21st to end	Day	30% 20% 30% 11% 6% 2	550
April	1st to 10th	Day	34% 21% 27% 10% 6% 1	500
	11th to 20th	Day	30% 26% 26% 11% 5% 1	492
	21st to end	Day	31% 19% 31% 12% 6% 1	500
May	1st to 10th	Day	36% 16% 29% 14% 3 2	503
	11th to 20th	Day	23% 18% 35% 17% 5% 2	527
	21st to end	Day	23% 16% 32% 22% 6% 1	573
June	1st to 10th	Day	22% 14% 37% 19% 8% 1	530
	11th to 20th	Day	19% 17% 35% 22% 6% 2	530
	21st to end	Day	23% 15% 34% 18% 9% 2	530
July	1st to 10th	Day	20% 7% 34% 27% 9% 3	525
	11th to 20th	Day	16% 7% 35% 28% 12% 3	524
	21st to end	Day	16% 8% 36% 29% 8% 2	576
August	1st to 10th	Day	13% 10% 28% 30% 16% 3	526
	11th to 20th	Day	16% 8% 31% 24% 16% 5%	519
	21st to end	Day	13% 10% 31% 28% 14% 4	583
September	1st to 10th	Day	16% 11% 28% 25% 13% 7%	530
	11th to 20th	Day	18% 11% 28% 19% 15% 10%	530
	21st to end	Day	16% 12% 34% 23% 9% 5%	530
October	1st to 10th	Day	25% 12% 24% 22% 13% 4	530
	11th to 20th	Day	21% 15% 30% 21% 11% 2	449
	21st to end	Day	31% 12% 31% 19% 6%	462
November	1st to 10th	Day	23% 17% 33% 20% 5% 1	425
	11th to 20th	Day	29% 17% 29% 19% 4 1	421
	21st to end	Day	31% 20% 27% 17% 5% 1	395
December	1st to 10th	Day	25% 24% 30% 15% 7%	200
	11th to 20th	Day	32% 20% 36% 9% 4	200
	21st to end	Day	30% 16% 34% 12% 8%	220
January	1st to 10th	Night	21% 19% 38% 20% 2	800
	11th to 20th	Night	26% 17% 33% 21% 3	800
	21st to end	Night	18% 20% 38% 20% 4 1	880
February	1st to 10th	Night	23% 19% 37% 20% 2	800
	11th to 20th	Night	22% 19% 38% 18% 3	783
	21st to end	Night	22% 16% 35% 23% 4	660
March	1st to 10th	Night	27% 15% 37% 18% 2	800
	11th to 20th	Night	23% 17% 34% 22% 4 1	800
	21st to end	Night	28% 15% 32% 19% 6%	880
April	1st to 10th	Night	27% 16% 34% 20% 4	800
	11th to 20th	Night	24% 16% 35% 19% 6%	735
	21st to end	Night	25% 17% 32% 22% 4	600
May	1st to 10th	Night	28% 11% 31% 25% 4 1	600
	11th to 20th	Night	21% 15% 35% 23% 7% 1	533
	21st to end	Night	22% 8% 28% 33% 8% 1	433
June	1st to 10th	Night	23% 7% 33% 28% 10%	400
	11th to 20th	Night	20% 9% 31% 27% 13% 1	400
	21st to end	Night	22% 5% 31% 35% 9%	400
July	1st to 10th	Night	17% 2 28% 38% 14%	390
	11th to 20th	Night	14% 5% 23% 40% 18% 1	388
	21st to end	Night	14% 4 27% 42% 13% 1	438
August	1st to 10th	Night	16% 5% 22% 41% 15%	523
	11th to 20th	Night	16% 6% 26% 33% 17% 1	592
	21st to end	Night	15% 6% 26% 35% 16% 2	842
September	1st to 10th	Night	17% 4 32% 31% 16% 1	800
	11th to 20th	Night	17% 7% 25% 28% 20% 3	800
	21st to end	Night	18% 8% 30% 28% 15% 2	800
October	1st to 10th	Night	22% 11% 24% 28% 14% 2	800
	11th to 20th	Night	19% 12% 33% 25% 11% 1	800
	21st to end	Night	28% 13% 28% 25% 5%	880
November	1st to 10th	Night	20% 15% 33% 24% 7% 1	820
	11th to 20th	Night	23% 17% 31% 22% 5% 1	829
	21st to end	Night	23% 20% 30% 22% 5%	800
December	1st to 10th	Night	26% 13% 36% 20% 5%	800
	11th to 20th	Night	25% 17% 31% 22% 4	800
	21st to end	Night	26% 15% 33% 23% 3	880

Page generated at 2025-11-15 00:56:01. Version ac07

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