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 2024-11-28.

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% 41%	101197
	Night	14% 6% 2 6% 4 4 4 3 6% 7% 43%	98293
Transparency	Day	39% 6% 11% 20% 18% 6%	105017
	Night	39% 5% 10% 20% 20% 7%	102612
Seeing	Day	18% 10% 22% 23% 18% 9%	14983
	Night	17% 10% 23% 27% 19% 3	25016

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	18% 10% 3 7% 4 4 4 3 5% 6% 37%	2836
	11th to 20th	Day	15% 6% 2 5% 4 4 5% 3 4 5% 49%	2840
	21st to end	Day	27% 7% 2 5% 3 3 2 1 3 4 42%	3192
February	1st to 10th	Day	18% 7% 2 7% 4 5% 4 5% 5% 6% 39%	1900
	11th to 20th	Day	23% 6% 2 5% 4 3 4 3 4 5% 42%	1848
	21st to end	Day	22% 7% 3 6% 4 4 3 2 5% 5% 40%	1570
March	1st to 10th	Day	25% 8% 3 6% 3 3 3 3 4 6% 36%	3241
	11th to 20th	Day	14% 5% 2 5% 4 4 4 3 5% 6% 49%	3480
	21st to end	Day	19% 5% 2 7% 5% 4 4 3 4 6% 41%	3815
April	1st to 10th	Day	12% 5% 2 5% 5% 5% 5% 5% 6% 6% 45%	2118
	11th to 20th	Day	12% 6% 2 6% 5% 5% 3 3 4 7% 47%	2254
	21st to end	Day	11% 4 1 5% 3 4 4 4 5% 8% 50%	2456
May	1st to 10th	Day	11% 7% 2 6% 5% 4 4 4 6% 8% 42%	4082
	11th to 20th	Day	17% 7% 2 6% 4 5% 5% 4 6% 8% 36%	4081
	21st to end	Day	8% 5% 2 6% 5% 5% 6% 4 7% 10% 43%	4476
June	1st to 10th	Day	10% 6% 2 8% 5% 5% 6% 5% 8% 10% 34%	2470
	11th to 20th	Day	6% 4 1 5% 4 5% 6% 6% 9% 11% 42%	2470
	21st to end	Day	8% 4 2 6% 5% 6% 6% 5% 10% 10% 39%	2470
July	1st to 10th	Day	8% 5% 2 7% 6% 6% 8% 6% 9% 12% 30%	4120
	11th to 20th	Day	10% 5% 2 6% 6% 7% 6% 5% 9% 12% 34%	4120
	21st to end	Day	12% 8% 3 10% 8% 8% 8% 6% 8% 9% 19%	4466
August	1st to 10th	Day	7% 6% 2 6% 5% 5% 6% 5% 8% 11% 38%	4104
	11th to 20th	Day	8% 5% 2 6% 5% 4 5% 5% 7% 12% 42%	3859
	21st to end	Day	8% 4 2 5% 4 4 5% 4 6% 8% 49%	4160
September	1st to 10th	Day	11% 7% 2 6% 5% 4 5% 4 8% 7% 41%	2090
	11th to 20th	Day	16% 7% 2 6% 4 4 4 3 7% 7% 40%	2090
	21st to end	Day	14% 5% 1 5% 4 4 4 4 5% 6% 48%	2090
October	1st to 10th	Day	13% 5% 2 4 2 3 3 2 4 5% 56%	2090
	11th to 20th	Day	12% 5% 2 4 2 2 2 3 5% 6% 58%	1964
	21st to end	Day	14% 4 2 4 3 3 3 3 4 5% 55%	2090
November	1st to 10th	Day	14% 5% 2 4 3 3 3 2 4 5% 55%	1795
	11th to 20th	Day	15% 5% 1 4 3 3 4 3 5% 6% 52%	1800
	21st to end	Day	14% 6% 2 5% 4 3 5% 4 4 7% 47%	1773
December	1st to 10th	Day	24% 7% 3 5% 3 4 4 3 5% 6% 38%	1710
	11th to 20th	Day	17% 6% 2 6% 5% 4 4 3 4 5% 44%	2153
	21st to end	Day	23% 7% 3 5% 3 4 4 3 6% 7% 36%	3124
January	1st to 10th	Night	18% 8% 2 5% 4 3 4 3 4 8% 41%	4109
	11th to 20th	Night	13% 8% 3 7% 4 3 4 3 5% 5% 44%	4120
	21st to end	Night	25% 8% 2 6% 4 3 3 2 3 4 39%	4460
February	1st to 10th	Night	16% 7% 2 5% 4 3 4 4 6% 6% 43%	2280
	11th to 20th	Night	21% 7% 2 7% 4 3 3 3 4 5% 42%	2218
	21st to end	Night	21% 6% 2 6% 4 4 4 4 5% 5% 39%	1884
March	1st to 10th	Night	25% 8% 2 6% 4 4 4 3 4 5% 36%	3610
	11th to 20th	Night	11% 6% 2 4 3 3 4 4 7% 9% 49%	3480
	21st to end	Night	17% 7% 2 6% 4 4 3 3 5% 6% 42%	3817
April	1st to 10th	Night	11% 5% 2 5% 4 4 4 3 6% 7% 49%	2076
	11th to 20th	Night	14% 5% 1 6% 3 3 3 4 5% 7% 48%	1875
	21st to end	Night	10% 5% 2 5% 4 4 3 3 5% 7% 51%	1722
May	1st to 10th	Night	11% 5% 3 8% 5% 6% 6% 4 6% 8% 39%	2813
	11th to 20th	Night	15% 7% 2 7% 5% 4 5% 4 6% 7% 39%	2812
	21st to end	Night	8% 5% 2 5% 4 4 5% 5% 7% 11% 44%	3079
June	1st to 10th	Night	8% 6% 2 7% 6% 7% 5% 4 9% 10% 35%	1710
	11th to 20th	Night	5% 5% 2 5% 5% 5% 5% 5% 8% 12% 43%	1710
	21st to end	Night	7% 4 3 5% 5% 5% 5% 5% 8% 10% 43%	1710
July	1st to 10th	Night	11% 7% 3 9% 6% 6% 6% 5% 7% 9% 31%	2840
	11th to 20th	Night	7% 5% 2 7% 6% 6% 6% 5% 10% 12% 33%	2840
	21st to end	Night	13% 9% 3 8% 7% 7% 7% 5% 9% 10% 21%	3087
August	1st to 10th	Night	9% 6% 2 6% 5% 5% 5% 5% 8% 13% 37%	2825
	11th to 20th	Night	10% 6% 2 7% 5% 4 5% 3 5% 8% 43%	2966
	21st to end	Night	9% 5% 2 6% 4 3 3 3 6% 8% 51%	3476
September	1st to 10th	Night	10% 6% 3 7% 6% 5% 4 4 6% 8% 42%	2076
	11th to 20th	Night	15% 6% 2 7% 4 4 5% 3 6% 7% 42%	2090
	21st to end	Night	14% 5% 2 5% 4 4 4 4 5% 6% 47%	2090
October	1st to 10th	Night	12% 5% 1 4 3 3 3 3 4 6% 56%	2090
	11th to 20th	Night	11% 5% 2 5% 3 3 3 2 4 5% 57%	2214
	21st to end	Night	13% 3 2 5% 3 3 4 3 4 5% 56%	2508
November	1st to 10th	Night	14% 6% 1 4 3 3 3 2 4 5% 55%	2500
	11th to 20th	Night	14% 5% 2 4 3 3 3 3 5% 6% 53%	2600
	21st to end	Night	16% 5% 2 4 4 3 4 3 5% 7% 46%	2562
December	1st to 10th	Night	20% 7% 2 6% 4 3 4 3 4 6% 39%	2470
	11th to 20th	Night	17% 6% 2 5% 3 3 3 3 5% 5% 48%	3042
	21st to end	Night	21% 7% 3 7% 4 4 4 4 5% 6% 34%	4532

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	49% 3 5% 16% 20% 6%	2837
	11th to 20th	Day	38% 3 7% 20% 24% 7%	2840
	21st to end	Day	48% 2 3 18% 21% 7%	3192
February	1st to 10th	Day	45% 4 7% 18% 20% 5%	1900
	11th to 20th	Day	45% 3 6% 20% 20% 6%	1858
	21st to end	Day	48% 3 7% 14% 19% 9%	1570
March	1st to 10th	Day	51% 3 9% 14% 13% 10%	3241
	11th to 20th	Day	37% 4 7% 17% 25% 11%	3480
	21st to end	Day	45% 4 7% 21% 20% 4	3815
April	1st to 10th	Day	38% 5% 9% 22% 21% 5%	2118
	11th to 20th	Day	38% 4 7% 19% 23% 9%	2254
	21st to end	Day	31% 5% 11% 27% 21% 4	2456
May	1st to 10th	Day	39% 5% 15% 21% 18% 2	4120
	11th to 20th	Day	46% 5% 13% 23% 12% 1	4085
	21st to end	Day	35% 7% 15% 22% 20% 1	4489
June	1st to 10th	Day	41% 9% 18% 21% 9% 2	2470
	11th to 20th	Day	31% 9% 21% 26% 12% 1	2470
	21st to end	Day	36% 9% 17% 22% 13% 3	2470
July	1st to 10th	Day	42% 11% 22% 20% 5% 1	4055
	11th to 20th	Day	40% 9% 24% 19% 5% 2	4051
	21st to end	Day	57% 11% 15% 12% 5%	4324
August	1st to 10th	Day	36% 9% 13% 33% 8%	4091
	11th to 20th	Day	34% 7% 19% 24% 14% 3	3898
	21st to end	Day	32% 7% 14% 24% 21% 3	4160
September	1st to 10th	Day	40% 7% 14% 16% 18% 4	2090
	11th to 20th	Day	42% 7% 12% 18% 20% 2	2090
	21st to end	Day	37% 5% 11% 18% 22% 7%	2090
October	1st to 10th	Day	25% 3 6% 14% 39% 12%	3480
	11th to 20th	Day	20% 3 5% 15% 29% 29%	3243
	21st to end	Day	29% 3 4 22% 28% 13%	3428
November	1st to 10th	Day	34% 3 7% 15% 31% 10%	1795
	11th to 20th	Day	34% 3 6% 20% 28% 10%	1800
	21st to end	Day	38% 4 7% 18% 26% 7%	1773
December	1st to 10th	Day	48% 3 7% 18% 17% 6%	1710
	11th to 20th	Day	43% 3 6% 17% 24% 8%	2150
	21st to end	Day	48% 4 7% 19% 19% 3	3124
January	1st to 10th	Night	43% 4 5% 16% 26% 6%	4109
	11th to 20th	Night	41% 3 6% 21% 19% 9%	4120
	21st to end	Night	51% 2 4 14% 24% 6%	4460
February	1st to 10th	Night	41% 4 8% 18% 21% 7%	2280
	11th to 20th	Night	47% 3 5% 18% 20% 8%	2219
	21st to end	Night	47% 3 7% 16% 19% 9%	1884
March	1st to 10th	Night	51% 3 8% 17% 15% 7%	3610
	11th to 20th	Night	32% 4 8% 17% 28% 11%	3480
	21st to end	Night	43% 4 6% 20% 19% 8%	3817
April	1st to 10th	Night	34% 5% 10% 20% 24% 6%	2076
	11th to 20th	Night	35% 4 7% 21% 23% 10%	1875
	21st to end	Night	32% 5% 11% 28% 20% 4	1722
May	1st to 10th	Night	40% 7% 14% 21% 16% 2	2840
	11th to 20th	Night	43% 6% 14% 24% 13% 1	2813
	21st to end	Night	32% 6% 15% 27% 18% 2	3087
June	1st to 10th	Night	40% 9% 17% 20% 11% 2	1710
	11th to 20th	Night	31% 8% 21% 25% 14% 1	1710
	21st to end	Night	33% 10% 18% 22% 14% 4	1710
July	1st to 10th	Night	47% 9% 19% 18% 6% 1	2795
	11th to 20th	Night	38% 13% 23% 19% 5% 1	2786
	21st to end	Night	55% 12% 15% 13% 4 1	2988
August	1st to 10th	Night	36% 9% 19% 29% 7%	2814
	11th to 20th	Night	38% 7% 18% 21% 15% 1	2993
	21st to end	Night	31% 6% 14% 23% 24% 2	3476
September	1st to 10th	Night	39% 7% 14% 18% 17% 4	2076
	11th to 20th	Night	40% 6% 12% 20% 19% 2	2090
	21st to end	Night	36% 6% 11% 19% 20% 8%	2090
October	1st to 10th	Night	26% 4 8% 15% 36% 11%	3480
	11th to 20th	Night	19% 2 5% 18% 32% 24%	3674
	21st to end	Night	25% 3 4 24% 28% 17%	4120
November	1st to 10th	Night	32% 4 7% 20% 26% 12%	2500
	11th to 20th	Night	34% 2 6% 18% 28% 12%	2600
	21st to end	Night	38% 3 6% 20% 25% 7%	2562
December	1st to 10th	Night	46% 4 7% 19% 19% 5%	2470
	11th to 20th	Night	38% 3 9% 17% 27% 6%	3044
	21st to end	Night	50% 4 6% 21% 16% 3	4532

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% 39% 10% 1	270
	11th to 20th	Day	30% 20% 28% 17% 4	270
	21st to end	Day	26% 27% 35% 10% 1 1	297
February	1st to 10th	Day	27% 19% 34% 16% 3	270
	11th to 20th	Day	37% 24% 27% 10% 1	270
	21st to end	Day	37% 19% 35% 8% 1	225
March	1st to 10th	Day	26% 15% 32% 19% 7% 1	339
	11th to 20th	Day	22% 22% 32% 18% 5% 1	460
	21st to end	Day	22% 16% 36% 18% 7% 1	506
April	1st to 10th	Day	23% 15% 28% 22% 10% 2	460
	11th to 20th	Day	14% 17% 35% 19% 13% 1	452
	21st to end	Day	18% 10% 27% 27% 13% 4	460
May	1st to 10th	Day	16% 13% 30% 22% 14% 6%	463
	11th to 20th	Day	16% 6% 24% 27% 16% 10%	487
	21st to end	Day	15% 7% 18% 28% 20% 13%	529
June	1st to 10th	Day	13% 4 19% 31% 23% 11%	490
	11th to 20th	Day	7% 5% 15% 29% 27% 18%	490
	21st to end	Day	11% 3 16% 30% 24% 15%	490
July	1st to 10th	Day	11% 4 9% 25% 27% 23%	485
	11th to 20th	Day	9% 3 10% 22% 35% 21%	484
	21st to end	Day	8% 2 9% 25% 35% 21%	532
August	1st to 10th	Day	11% 5% 13% 25% 26% 20%	486
	11th to 20th	Day	13% 4 11% 25% 32% 15%	479
	21st to end	Day	17% 3 8% 23% 28% 22%	539
September	1st to 10th	Day	11% 5% 12% 29% 30% 14%	490
	11th to 20th	Day	19% 4 14% 23% 29% 10%	490
	21st to end	Day	15% 4 19% 26% 25% 11%	490
October	1st to 10th	Day	17% 5% 19% 30% 20% 10%	490
	11th to 20th	Day	13% 10% 23% 27% 22% 4	490
	21st to end	Day	18% 8% 24% 27% 19% 3	539
November	1st to 10th	Day	23% 9% 26% 26% 14% 2	333
	11th to 20th	Day	19% 19% 31% 23% 7% 1	300
	21st to end	Day	15% 18% 35% 23% 9% 1	291
December	1st to 10th	Day	33% 19% 29% 15% 4	270
	11th to 20th	Day	25% 17% 34% 20% 4	270
	21st to end	Day	33% 16% 29% 17% 4 1	297
January	1st to 10th	Night	27% 19% 30% 20% 5%	760
	11th to 20th	Night	24% 18% 29% 23% 6%	760
	21st to end	Night	23% 20% 34% 21% 2	836
February	1st to 10th	Night	21% 16% 33% 25% 6%	760
	11th to 20th	Night	25% 20% 31% 22% 3	743
	21st to end	Night	25% 18% 35% 19% 3	628
March	1st to 10th	Night	21% 16% 34% 24% 4	760
	11th to 20th	Night	20% 13% 34% 24% 9%	760
	21st to end	Night	20% 15% 28% 29% 8% 1	836
April	1st to 10th	Night	16% 15% 32% 27% 10%	746
	11th to 20th	Night	16% 14% 27% 29% 13% 1	563
	21st to end	Night	14% 7% 29% 31% 19% 1	570
May	1st to 10th	Night	13% 9% 26% 30% 21% 1	570
	11th to 20th	Night	12% 7% 22% 30% 23% 5%	561
	21st to end	Night	15% 3 14% 30% 32% 6%	617
June	1st to 10th	Night	11% 2 15% 37% 29% 6%	570
	11th to 20th	Night	6% 3 16% 29% 32% 14%	570
	21st to end	Night	7% 2 12% 38% 32% 8%	570
July	1st to 10th	Night	10% 3 8% 25% 42% 12%	555
	11th to 20th	Night	10% 3 7% 22% 47% 12%	552
	21st to end	Night	10% 2 7% 31% 40% 11%	624
August	1st to 10th	Night	14% 3 7% 27% 38% 11%	567
	11th to 20th	Night	15% 2 9% 27% 40% 7%	564
	21st to end	Night	15% 3 10% 26% 36% 10%	627
September	1st to 10th	Night	11% 3 12% 29% 35% 10%	746
	11th to 20th	Night	17% 3 14% 27% 34% 4	760
	21st to end	Night	16% 4 15% 33% 28% 3	760
October	1st to 10th	Night	14% 4 21% 31% 24% 6%	760
	11th to 20th	Night	13% 8% 25% 31% 20% 3	760
	21st to end	Night	14% 10% 22% 32% 20% 2	836
November	1st to 10th	Night	17% 9% 23% 30% 19% 2	780
	11th to 20th	Night	17% 13% 30% 27% 13% 1	800
	21st to end	Night	20% 14% 27% 27% 12% 1	789
December	1st to 10th	Night	24% 18% 28% 23% 7%	760
	11th to 20th	Night	24% 13% 29% 25% 9% 1	760
	21st to end	Night	31% 13% 27% 24% 5%	836

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