Lake Lanier Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Lake Lanier 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	14% 6% 2 6% 4 4 5% 4 6% 8% 42%	127194
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 43%	124505
Transparency	Day	39% 5% 11% 20% 18% 6%	130970
	Night	39% 5% 10% 20% 20% 6%	128798
Seeing	Day	18% 10% 23% 23% 18% 8%	16468
	Night	17% 10% 23% 27% 19% 3	26531

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% 9% 3 6% 3 3 3 2 4 4 46%	4146
	11th to 20th	Day	13% 6% 2 4 3 3 4 2 3 5% 54%	4114
	21st to end	Day	22% 8% 3 6% 3 3 2 2 4 5% 41%	4576
February	1st to 10th	Day	17% 7% 2 6% 4 5% 3 5% 5% 6% 40%	2000
	11th to 20th	Day	24% 6% 2 5% 4 3 4 3 4 5% 42%	1948
	21st to end	Day	21% 7% 3 6% 4 4 3 3 5% 5% 42%	1650
March	1st to 10th	Day	22% 8% 3 6% 4 4 3 4 4 6% 36%	4657
	11th to 20th	Day	11% 5% 2 5% 4 4 3 3 4 6% 52%	4980
	21st to end	Day	17% 5% 2 6% 5% 4 4 3 4 5% 45%	5465
April	1st to 10th	Day	12% 5% 1 5% 5% 5% 5% 5% 6% 7% 44%	2230
	11th to 20th	Day	12% 5% 2 6% 5% 5% 3 3 4 6% 48%	2374
	21st to end	Day	12% 4 1 5% 3 4 4 4 5% 8% 49%	2585
May	1st to 10th	Day	14% 7% 3 6% 5% 4 5% 4 7% 8% 37%	5862
	11th to 20th	Day	19% 7% 2 8% 5% 6% 5% 5% 7% 8% 29%	5861
	21st to end	Day	14% 6% 2 7% 4 4 5% 4 6% 8% 40%	6433
June	1st to 10th	Day	9% 6% 2 8% 6% 6% 6% 5% 8% 10% 34%	2600
	11th to 20th	Day	6% 4 1 5% 5% 5% 7% 6% 9% 11% 41%	2600
	21st to end	Day	7% 4 2 6% 4 6% 6% 5% 10% 10% 40%	2600
July	1st to 10th	Day	7% 4 2 6% 5% 6% 7% 6% 9% 12% 37%	5900
	11th to 20th	Day	7% 4 2 6% 5% 6% 7% 5% 9% 14% 35%	5900
	21st to end	Day	10% 7% 3 9% 7% 7% 7% 6% 8% 10% 27%	6424
August	1st to 10th	Day	8% 6% 2 6% 5% 5% 6% 5% 8% 11% 37%	4234
	11th to 20th	Day	8% 5% 2 6% 5% 4 5% 4 7% 11% 42%	3983
	21st to end	Day	8% 4 2 5% 4 4 5% 4 6% 8% 49%	4291
September	1st to 10th	Day	11% 6% 2 6% 4 4 5% 5% 8% 8% 41%	2200
	11th to 20th	Day	15% 7% 2 6% 4 4 4 3 7% 7% 41%	2200
	21st to end	Day	14% 5% 2 5% 4 4 4 4 5% 6% 47%	2200
October	1st to 10th	Day	13% 5% 2 4 2 3 3 3 5% 5% 55%	2200
	11th to 20th	Day	11% 5% 2 4 2 2 2 3 5% 6% 59%	2067
	21st to end	Day	14% 5% 2 4 3 3 3 3 4 5% 55%	2200
November	1st to 10th	Day	14% 5% 1 4 3 3 3 2 4 5% 56%	1886
	11th to 20th	Day	14% 5% 1 3 3 3 4 3 5% 6% 53%	1890
	21st to end	Day	14% 6% 2 5% 4 3 5% 4 4 7% 47%	1890
December	1st to 10th	Day	21% 5% 2 4 3 3 4 3 5% 6% 45%	3020
	11th to 20th	Day	18% 7% 2 7% 5% 4 4 3 4 5% 42%	3463
	21st to end	Day	23% 6% 2 4 3 3 3 3 6% 7% 40%	4565
January	1st to 10th	Night	16% 6% 2 5% 4 3 3 3 4 6% 48%	6019
	11th to 20th	Night	14% 7% 2 6% 4 3 3 3 4 5% 48%	5979
	21st to end	Night	19% 7% 2 6% 4 3 3 2 4 5% 44%	6376
February	1st to 10th	Night	16% 7% 2 5% 4 3 4 4 6% 6% 43%	2400
	11th to 20th	Night	22% 7% 2 6% 4 3 3 3 4 5% 41%	2338
	21st to end	Night	20% 6% 2 6% 4 4 4 4 5% 6% 41%	1980
March	1st to 10th	Night	22% 8% 2 6% 4 4 4 4 4 6% 35%	5194
	11th to 20th	Night	10% 5% 1 4 3 3 4 4 6% 8% 52%	4980
	21st to end	Night	15% 7% 2 6% 4 4 3 3 5% 6% 45%	5467
April	1st to 10th	Night	11% 5% 2 5% 4 4 4 3 6% 7% 49%	2185
	11th to 20th	Night	13% 5% 1 6% 3 3 3 3 5% 7% 50%	1975
	21st to end	Night	10% 5% 2 5% 4 4 3 3 5% 7% 50%	1812
May	1st to 10th	Night	11% 6% 3 8% 5% 6% 5% 4 7% 9% 35%	4033
	11th to 20th	Night	20% 7% 2 7% 5% 5% 5% 5% 6% 7% 31%	4032
	21st to end	Night	13% 6% 2 6% 4 4 5% 4 7% 9% 41%	4421
June	1st to 10th	Night	8% 6% 2 7% 6% 7% 5% 4 9% 10% 36%	1800
	11th to 20th	Night	5% 5% 2 6% 5% 5% 5% 5% 8% 12% 42%	1800
	21st to end	Night	6% 4 3 5% 5% 5% 5% 5% 8% 10% 43%	1800
July	1st to 10th	Night	9% 6% 3 8% 6% 6% 6% 5% 8% 10% 34%	4058
	11th to 20th	Night	7% 5% 2 8% 6% 6% 7% 6% 10% 12% 33%	4057
	21st to end	Night	12% 8% 3 8% 6% 6% 6% 5% 8% 9% 29%	4427
August	1st to 10th	Night	10% 6% 2 6% 5% 5% 5% 5% 8% 13% 36%	2915
	11th to 20th	Night	10% 6% 2 8% 6% 4 5% 3 5% 8% 42%	3062
	21st to end	Night	9% 5% 2 6% 4 4 3 3 6% 8% 50%	3586
September	1st to 10th	Night	9% 5% 3 7% 6% 5% 5% 4 7% 8% 40%	2185
	11th to 20th	Night	14% 6% 2 7% 4 4 5% 3 6% 8% 42%	2200
	21st to end	Night	14% 6% 2 5% 4 4 4 4 5% 6% 47%	2200
October	1st to 10th	Night	12% 5% 2 5% 3 3 3 3 4 6% 55%	2200
	11th to 20th	Night	11% 5% 2 5% 3 3 3 2 4 5% 58%	2331
	21st to end	Night	14% 3 2 5% 3 3 4 3 4 5% 56%	2640
November	1st to 10th	Night	14% 6% 1 4 3 3 3 2 4 5% 55%	2628
	11th to 20th	Night	14% 5% 2 4 3 3 3 3 5% 6% 53%	2730
	21st to end	Night	16% 5% 2 4 4 3 4 3 5% 7% 46%	2730
December	1st to 10th	Night	18% 6% 2 5% 3 2 3 2 4 5% 48%	4380
	11th to 20th	Night	20% 6% 2 5% 3 3 4 3 5% 5% 45%	4952
	21st to end	Night	22% 6% 2 6% 4 3 3 3 4 5% 41%	6633

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% 2 4 21% 23% 7%	4147
	11th to 20th	Day	34% 3 5% 19% 28% 10%	4114
	21st to end	Day	48% 2 3 17% 21% 8%	4576
February	1st to 10th	Day	44% 4 7% 20% 20% 5%	2000
	11th to 20th	Day	45% 3 6% 20% 20% 6%	1958
	21st to end	Day	47% 2 6% 15% 21% 9%	1650
March	1st to 10th	Day	51% 3 6% 13% 18% 9%	4657
	11th to 20th	Day	34% 3 6% 15% 24% 18%	4980
	21st to end	Day	43% 3 6% 23% 20% 5%	5465
April	1st to 10th	Day	38% 5% 9% 22% 21% 5%	2230
	11th to 20th	Day	37% 4 7% 20% 24% 8%	2374
	21st to end	Day	33% 5% 12% 27% 20% 4	2585
May	1st to 10th	Day	44% 5% 15% 20% 14% 2	5897
	11th to 20th	Day	51% 6% 15% 18% 9% 1	5865
	21st to end	Day	42% 6% 11% 21% 18% 2	6447
June	1st to 10th	Day	41% 9% 18% 21% 9% 2	2600
	11th to 20th	Day	32% 9% 21% 26% 12% 1	2600
	21st to end	Day	35% 9% 17% 22% 13% 3	2600
July	1st to 10th	Day	36% 10% 20% 26% 7% 1	5835
	11th to 20th	Day	36% 11% 30% 18% 4 1	5831
	21st to end	Day	49% 10% 18% 18% 4	6240
August	1st to 10th	Day	38% 9% 13% 32% 8%	4221
	11th to 20th	Day	34% 7% 19% 24% 13% 3	4022
	21st to end	Day	32% 6% 14% 23% 21% 3	4291
September	1st to 10th	Day	39% 7% 15% 17% 19% 4	2200
	11th to 20th	Day	40% 7% 13% 19% 19% 2	2200
	21st to end	Day	37% 5% 12% 18% 21% 7%	2200
October	1st to 10th	Day	26% 3 6% 15% 38% 12%	3590
	11th to 20th	Day	20% 3 5% 15% 29% 28%	3346
	21st to end	Day	30% 3 4 22% 29% 13%	3538
November	1st to 10th	Day	33% 3 7% 16% 31% 10%	1886
	11th to 20th	Day	33% 3 6% 21% 27% 10%	1890
	21st to end	Day	37% 4 7% 18% 27% 7%	1890
December	1st to 10th	Day	41% 2 5% 18% 26% 7%	3020
	11th to 20th	Day	47% 3 6% 17% 20% 7%	3460
	21st to end	Day	44% 3 7% 26% 17% 2	4565
January	1st to 10th	Night	37% 3 6% 19% 29% 7%	6019
	11th to 20th	Night	39% 2 5% 19% 26% 9%	5979
	21st to end	Night	44% 2 3 16% 27% 7%	6376
February	1st to 10th	Night	41% 4 8% 19% 21% 7%	2400
	11th to 20th	Night	47% 3 5% 18% 20% 7%	2339
	21st to end	Night	45% 3 6% 16% 20% 9%	1980
March	1st to 10th	Night	50% 3 6% 15% 20% 7%	5194
	11th to 20th	Night	30% 4 7% 15% 28% 16%	4980
	21st to end	Night	41% 3 6% 22% 20% 8%	5467
April	1st to 10th	Night	34% 5% 11% 20% 24% 6%	2185
	11th to 20th	Night	34% 4 7% 23% 23% 10%	1975
	21st to end	Night	34% 5% 11% 27% 20% 4	1812
May	1st to 10th	Night	43% 6% 17% 20% 12% 1	4060
	11th to 20th	Night	51% 6% 16% 18% 9% 1	4033
	21st to end	Night	39% 5% 13% 25% 16% 2	4429
June	1st to 10th	Night	40% 9% 17% 21% 12% 2	1800
	11th to 20th	Night	32% 8% 21% 25% 13% 1	1800
	21st to end	Night	32% 11% 18% 22% 14% 4	1800
July	1st to 10th	Night	44% 9% 18% 23% 6%	4015
	11th to 20th	Night	39% 13% 26% 17% 4 1	4006
	21st to end	Night	49% 10% 18% 17% 4 1	4297
August	1st to 10th	Night	38% 8% 19% 28% 7%	2904
	11th to 20th	Night	39% 7% 18% 21% 15% 1	3089
	21st to end	Night	32% 6% 14% 23% 23% 2	3586
September	1st to 10th	Night	40% 7% 14% 18% 17% 4	2185
	11th to 20th	Night	39% 6% 13% 21% 19% 2	2200
	21st to end	Night	37% 6% 12% 19% 19% 8%	2200
October	1st to 10th	Night	27% 4 8% 15% 35% 11%	3590
	11th to 20th	Night	19% 2 5% 18% 32% 24%	3791
	21st to end	Night	25% 3 4 23% 28% 17%	4252
November	1st to 10th	Night	32% 4 7% 19% 26% 12%	2628
	11th to 20th	Night	33% 2 6% 19% 28% 11%	2730
	21st to end	Night	38% 4 6% 20% 25% 7%	2730
December	1st to 10th	Night	40% 2 5% 18% 29% 6%	4380
	11th to 20th	Night	42% 4 9% 17% 23% 6%	4954
	21st to end	Night	45% 3 6% 28% 16% 2	6633

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	28% 22% 38% 10% 2	330
	11th to 20th	Day	28% 22% 29% 17% 4	318
	21st to end	Day	26% 26% 35% 11% 2 1	330
February	1st to 10th	Day	24% 18% 34% 19% 4	300
	11th to 20th	Day	38% 23% 27% 10% 1	300
	21st to end	Day	33% 18% 35% 10% 3	249
March	1st to 10th	Day	25% 17% 31% 18% 8% 1	373
	11th to 20th	Day	21% 22% 32% 19% 5% 1	500
	21st to end	Day	22% 17% 35% 19% 7% 1	550
April	1st to 10th	Day	23% 14% 30% 22% 10% 2	500
	11th to 20th	Day	14% 17% 36% 20% 13% 1	492
	21st to end	Day	19% 9% 26% 27% 14% 4	500
May	1st to 10th	Day	17% 13% 30% 21% 13% 6%	503
	11th to 20th	Day	17% 6% 26% 26% 15% 10%	527
	21st to end	Day	16% 8% 18% 27% 19% 12%	573
June	1st to 10th	Day	13% 4 19% 30% 24% 10%	530
	11th to 20th	Day	7% 5% 15% 29% 27% 17%	530
	21st to end	Day	10% 3 15% 29% 27% 16%	530
July	1st to 10th	Day	10% 4 8% 26% 29% 23%	525
	11th to 20th	Day	8% 3 9% 21% 36% 23%	524
	21st to end	Day	7% 2 9% 24% 36% 22%	576
August	1st to 10th	Day	13% 5% 12% 25% 25% 19%	526
	11th to 20th	Day	13% 4 10% 24% 33% 16%	519
	21st to end	Day	17% 3 8% 23% 27% 21%	583
September	1st to 10th	Day	10% 5% 12% 31% 29% 13%	530
	11th to 20th	Day	18% 4 13% 23% 31% 11%	530
	21st to end	Day	15% 4 18% 26% 25% 11%	530
October	1st to 10th	Day	16% 5% 18% 32% 19% 9%	530
	11th to 20th	Day	13% 10% 23% 26% 24% 5%	530
	21st to end	Day	19% 8% 26% 27% 17% 3	583
November	1st to 10th	Day	21% 10% 26% 26% 15% 2	364
	11th to 20th	Day	17% 17% 33% 25% 7% 1	330
	21st to end	Day	15% 18% 37% 21% 8% 1	330
December	1st to 10th	Day	32% 18% 32% 15% 3	330
	11th to 20th	Day	25% 15% 32% 22% 5% 1	330
	21st to end	Day	31% 15% 30% 18% 4 1	363
January	1st to 10th	Night	27% 19% 30% 20% 4	840
	11th to 20th	Night	23% 19% 30% 23% 5%	825
	21st to end	Night	22% 20% 34% 22% 2	880
February	1st to 10th	Night	20% 15% 34% 26% 6%	800
	11th to 20th	Night	26% 20% 30% 21% 3	783
	21st to end	Night	24% 18% 35% 21% 3	660
March	1st to 10th	Night	22% 16% 34% 24% 4	800
	11th to 20th	Night	19% 14% 34% 25% 8%	800
	21st to end	Night	19% 14% 28% 30% 8% 1	880
April	1st to 10th	Night	16% 15% 32% 28% 9%	785
	11th to 20th	Night	15% 14% 27% 30% 13% 1	593
	21st to end	Night	14% 7% 28% 31% 19% 1	600
May	1st to 10th	Night	13% 9% 26% 31% 20% 1	600
	11th to 20th	Night	14% 7% 23% 30% 22% 4	591
	21st to end	Night	16% 3 15% 29% 30% 6%	650
June	1st to 10th	Night	11% 2 15% 38% 29% 6%	600
	11th to 20th	Night	6% 3 15% 29% 32% 14%	600
	21st to end	Night	7% 2 12% 37% 33% 9%	600
July	1st to 10th	Night	10% 3 8% 25% 43% 12%	585
	11th to 20th	Night	10% 3 7% 21% 48% 12%	582
	21st to end	Night	10% 2 7% 30% 40% 11%	657
August	1st to 10th	Night	16% 3 7% 28% 37% 11%	597
	11th to 20th	Night	15% 2 9% 26% 40% 8%	594
	21st to end	Night	16% 3 11% 26% 35% 10%	660
September	1st to 10th	Night	11% 4 12% 30% 34% 9%	785
	11th to 20th	Night	16% 3 14% 28% 35% 4	800
	21st to end	Night	16% 5% 15% 34% 28% 3	800
October	1st to 10th	Night	14% 4 21% 32% 23% 6%	800
	11th to 20th	Night	13% 8% 25% 31% 21% 3	800
	21st to end	Night	15% 10% 23% 31% 19% 2	880
November	1st to 10th	Night	17% 9% 23% 30% 19% 2	820
	11th to 20th	Night	17% 13% 30% 27% 12% 1	840
	21st to end	Night	20% 15% 28% 26% 11% 1	840
December	1st to 10th	Night	24% 17% 29% 23% 7%	840
	11th to 20th	Night	24% 13% 28% 25% 9%	840
	21st to end	Night	30% 13% 26% 26% 5%	924

Page generated at 2026-01-14 01:14:31. Version ac07

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