Lookout Mountain Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Lookout Mountain Observatory 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	2 3 1 4 3 4 5% 4 7% 8% 59%	135399
	Night	2 3 1 4 3 3 3 3 5% 8% 64%	132439
Transparency	Day	19% 7% 13% 26% 25% 10%	130349
	Night	17% 6% 12% 27% 27% 10%	128493
Seeing	Day	7% 11% 26% 31% 19% 6%	16739
	Night	3 6% 32% 40% 17% 2	26818

By Month

Percentage Cloud Cover SW (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	4 3 6% 2 3 3 3 5% 7% 65%	3758
	11th to 20th	Day	1 1 4 3 3 3 3 5% 8% 68%	4100
	21st to end	Day	4 2 5% 3 4 3 3 5% 7% 64%	4510
February	1st to 10th	Day	3 1 5% 2 3 4 4 6% 8% 65%	1800
	11th to 20th	Day	1 2 1 3 2 3 4 4 6% 9% 67%	1895
	21st to end	Day	2 2 5% 3 3 4 3 6% 8% 63%	1650
March	1st to 10th	Day	4 2 4 3 3 4 3 5% 7% 67%	4722
	11th to 20th	Day	1 9% 3 6% 4 4 5% 4 6% 9% 50%	5000
	21st to end	Day	1 3 2 5% 3 3 4 4 6% 9% 60%	5656
April	1st to 10th	Day	1 4 1 4 3 3 4 3 6% 9% 63%	2400
	11th to 20th	Day	2 5% 1 5% 3 4 4 4 6% 8% 59%	2371
	21st to end	Day	2 4 1 4 4 4 4 4 7% 10% 55%	2400
May	1st to 10th	Day	5% 6% 2 4 3 3 4 4 7% 9% 53%	5455
	11th to 20th	Day	2 4 1 4 3 4 5% 5% 7% 8% 57%	6155
	21st to end	Day	1 2 1 3 3 3 4 4 7% 8% 64%	6925
June	1st to 10th	Day	2 3 1 4 4 4 5% 5% 7% 8% 58%	2800
	11th to 20th	Day	1 2 1 3 2 3 4 4 7% 7% 66%	2800
	21st to end	Day	1 3 1 4 4 5% 7% 5% 8% 9% 53%	2800
July	1st to 10th	Day	2 2 1 3 3 4 5% 5% 7% 10% 58%	6339
	11th to 20th	Day	2 2 1 4 5% 6% 8% 8% 10% 12% 44%	6121
	21st to end	Day	2 3 1 5% 5% 7% 7% 6% 9% 9% 45%	6401
August	1st to 10th	Day	3 3 2 5% 5% 6% 7% 6% 9% 11% 42%	5779
	11th to 20th	Day	2 2 1 4 4 5% 6% 6% 9% 10% 51%	5409
	21st to end	Day	3 4 2 6% 4 5% 7% 6% 10% 10% 44%	5984
September	1st to 10th	Day	3 4 1 5% 4 4 6% 4 8% 10% 51%	2400
	11th to 20th	Day	3 4 2 5% 3 3 4 4 7% 7% 59%	2400
	21st to end	Day	3 3 1 2 3 3 4 3 5% 6% 66%	2155
October	1st to 10th	Day	2 2 1 3 2 2 3 3 4 6% 72%	3270
	11th to 20th	Day	4 4 1 4 3 4 5% 4 5% 7% 60%	3250
	21st to end	Day	1 2 1 2 2 2 2 2 3 5% 77%	3548
November	1st to 10th	Day	2 1 2 1 2 2 2 5% 7% 75%	1929
	11th to 20th	Day	1 1 2 1 1 2 2 4 6% 79%	1863
	21st to end	Day	1 3 1 3 3 2 2 3 4 6% 74%	1704
December	1st to 10th	Day	3 1 3 1 2 2 1 4 6% 78%	2620
	11th to 20th	Day	2 1 2 1 1 2 3 6% 10% 73%	3026
	21st to end	Day	2 1 3 2 3 3 3 6% 8% 69%	4004
January	1st to 10th	Night	4 3 8% 4 3 3 3 5% 8% 61%	6236
	11th to 20th	Night	2 1 2 2 2 2 2 5% 7% 76%	5880
	21st to end	Night	4 2 4 3 3 3 3 5% 7% 65%	6468
February	1st to 10th	Night	3 2 4 2 3 2 3 4 8% 68%	2600
	11th to 20th	Night	1 2 1 4 3 2 3 3 5% 8% 69%	2392
	21st to end	Night	3 1 3 3 3 2 2 4 8% 70%	1980
March	1st to 10th	Night	5% 2 4 2 2 3 3 4 6% 68%	5159
	11th to 20th	Night	1 9% 2 5% 3 3 4 3 4 6% 60%	5000
	21st to end	Night	1 3 1 4 3 2 3 3 5% 7% 69%	5309
April	1st to 10th	Night	1 2 1 3 2 2 3 3 5% 8% 70%	2000
	11th to 20th	Night	2 5% 2 5% 3 3 4 3 5% 6% 62%	1977
	21st to end	Night	2 4 2 4 4 3 3 3 6% 7% 64%	2000
May	1st to 10th	Night	5% 6% 2 5% 3 3 3 4 6% 7% 56%	4508
	11th to 20th	Night	5% 5% 1 3 3 2 3 3 5% 7% 62%	3756
	21st to end	Night	2 2 1 3 2 3 3 3 5% 6% 72%	3972
June	1st to 10th	Night	2 2 1 3 3 3 3 3 6% 7% 67%	1600
	11th to 20th	Night	1 2 1 3 2 2 2 4 5% 7% 72%	1600
	21st to end	Night	3 2 1 3 4 4 5% 5% 7% 9% 57%	1600
July	1st to 10th	Night	2 2 1 4 4 4 5% 4 6% 8% 60%	3640
	11th to 20th	Night	2 2 1 4 4 5% 6% 6% 11% 13% 46%	3841
	21st to end	Night	3 3 1 5% 4 5% 6% 5% 9% 12% 47%	4472
August	1st to 10th	Night	3 4 2 5% 4 5% 5% 5% 8% 11% 48%	4033
	11th to 20th	Night	2 2 1 4 3 4 5% 5% 10% 14% 48%	4437
	21st to end	Night	4 5% 2 7% 5% 6% 6% 6% 10% 13% 37%	4993
September	1st to 10th	Night	4 3 2 5% 4 5% 4 4 7% 9% 53%	2000
	11th to 20th	Night	5% 3 1 4 2 3 4 3 5% 7% 63%	2000
	21st to end	Night	4 3 1 3 3 3 3 2 4 7% 67%	2239
October	1st to 10th	Night	2 2 1 2 2 2 2 2 4 6% 75%	3920
	11th to 20th	Night	5% 3 1 3 3 3 4 3 5% 7% 64%	3898
	21st to end	Night	2 2 1 2 2 1 2 1 3 5% 78%	4252
November	1st to 10th	Night	1 2 2 1 1 3 3 4 7% 76%	2566
	11th to 20th	Night	1 1 2 2 1 2 2 4 6% 80%	2692
	21st to end	Night	1 4 1 3 2 2 2 3 4 6% 72%	2694
December	1st to 10th	Night	2 1 3 1 2 2 2 3 4 81%	4570
	11th to 20th	Night	2 1 3 2 2 2 2 4 7% 75%	5181
	21st to end	Night	2 1 4 3 3 3 3 5% 7% 68%	6974

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	18% 4 13% 35% 25% 5%	3759
	11th to 20th	Day	12% 7% 13% 22% 35% 10%	4100
	21st to end	Day	19% 6% 10% 30% 27% 7%	4510
February	1st to 10th	Day	17% 6% 14% 31% 23% 9%	1800
	11th to 20th	Day	15% 6% 12% 33% 28% 7%	1905
	21st to end	Day	18% 5% 11% 23% 32% 12%	1650
March	1st to 10th	Day	15% 4 9% 19% 39% 15%	4722
	11th to 20th	Day	27% 5% 9% 30% 23% 6%	5000
	21st to end	Day	18% 6% 12% 42% 18% 3	5656
April	1st to 10th	Day	18% 6% 11% 28% 26% 11%	2400
	11th to 20th	Day	20% 6% 9% 26% 28% 10%	2374
	21st to end	Day	21% 7% 12% 27% 26% 7%	2400
May	1st to 10th	Day	24% 8% 12% 24% 21% 11%	5455
	11th to 20th	Day	19% 8% 12% 24% 25% 12%	6159
	21st to end	Day	14% 8% 10% 23% 31% 15%	6926
June	1st to 10th	Day	20% 8% 13% 24% 22% 13%	2800
	11th to 20th	Day	14% 6% 9% 19% 25% 26%	2800
	21st to end	Day	22% 8% 11% 21% 22% 17%	2800
July	1st to 10th	Day	18% 8% 11% 15% 22% 26%	6271
	11th to 20th	Day	23% 12% 23% 26% 10% 6%	6046
	21st to end	Day	28% 11% 19% 21% 15% 6%	6217
August	1st to 10th	Day	36% 12% 22% 16% 10% 4	4208
	11th to 20th	Day	25% 11% 20% 18% 20% 6%	3913
	21st to end	Day	27% 12% 18% 20% 20% 3	4311
September	1st to 10th	Day	25% 8% 15% 22% 21% 9%	2400
	11th to 20th	Day	21% 6% 12% 20% 29% 11%	2400
	21st to end	Day	18% 5% 8% 26% 36% 8%	2155
October	1st to 10th	Day	13% 5% 7% 21% 36% 18%	3270
	11th to 20th	Day	20% 6% 7% 29% 30% 8%	3250
	21st to end	Day	11% 3 6% 32% 42% 7%	3548
November	1st to 10th	Day	10% 3 10% 31% 36% 9%	1929
	11th to 20th	Day	8% 3 10% 39% 33% 7%	1863
	21st to end	Day	13% 4 10% 32% 31% 9%	1704
December	1st to 10th	Day	10% 4 11% 43% 25% 8%	2620
	11th to 20th	Day	8% 4 17% 43% 25% 2	3024
	21st to end	Day	14% 6% 12% 32% 29% 8%	4004
January	1st to 10th	Night	20% 6% 10% 35% 24% 5%	6236
	11th to 20th	Night	8% 5% 11% 26% 34% 15%	5880
	21st to end	Night	18% 6% 11% 29% 25% 12%	6468
February	1st to 10th	Night	16% 4 14% 31% 26% 9%	2600
	11th to 20th	Night	15% 5% 10% 34% 28% 7%	2392
	21st to end	Night	14% 5% 9% 24% 36% 12%	1980
March	1st to 10th	Night	16% 5% 6% 19% 43% 11%	5159
	11th to 20th	Night	23% 6% 8% 25% 29% 9%	5000
	21st to end	Night	15% 5% 12% 43% 21% 3	5309
April	1st to 10th	Night	14% 5% 10% 33% 27% 11%	2000
	11th to 20th	Night	20% 6% 10% 24% 29% 11%	1977
	21st to end	Night	18% 7% 11% 28% 28% 8%	2000
May	1st to 10th	Night	25% 7% 12% 20% 22% 13%	4505
	11th to 20th	Night	18% 7% 12% 25% 24% 14%	3757
	21st to end	Night	12% 6% 13% 19% 33% 17%	3972
June	1st to 10th	Night	15% 7% 15% 22% 24% 16%	1600
	11th to 20th	Night	12% 5% 8% 20% 25% 30%	1600
	21st to end	Night	21% 8% 13% 18% 23% 19%	1600
July	1st to 10th	Night	19% 9% 14% 15% 14% 29%	3600
	11th to 20th	Night	22% 13% 30% 24% 6% 6%	3793
	21st to end	Night	25% 12% 25% 19% 11% 8%	4344
August	1st to 10th	Night	36% 13% 24% 12% 9% 6%	2928
	11th to 20th	Night	23% 11% 28% 13% 16% 8%	3209
	21st to end	Night	30% 13% 19% 19% 14% 3	3597
September	1st to 10th	Night	25% 9% 16% 20% 20% 10%	2000
	11th to 20th	Night	20% 6% 12% 19% 28% 14%	2000
	21st to end	Night	18% 6% 7% 24% 36% 9%	2239
October	1st to 10th	Night	12% 5% 8% 21% 35% 20%	3920
	11th to 20th	Night	18% 6% 8% 28% 31% 9%	3898
	21st to end	Night	11% 2 5% 33% 41% 9%	4252
November	1st to 10th	Night	9% 3 8% 33% 38% 8%	2566
	11th to 20th	Night	7% 4 8% 37% 36% 8%	2692
	21st to end	Night	15% 3 8% 30% 35% 9%	2694
December	1st to 10th	Night	10% 3 10% 46% 27% 5%	4570
	11th to 20th	Night	11% 4 12% 38% 33% 2	5182
	21st to end	Night	15% 6% 13% 29% 31% 5%	6974

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	4 9% 33% 37% 16% 1	300
	11th to 20th	Day	7% 11% 33% 36% 13%	300
	21st to end	Day	2 3 38% 41% 14% 2	330
February	1st to 10th	Day	7% 16% 38% 33% 6% 1	300
	11th to 20th	Day	8% 11% 40% 31% 9%	300
	21st to end	Day	11% 17% 37% 30% 4 2	249
March	1st to 10th	Day	10% 20% 34% 27% 8% 1	392
	11th to 20th	Day	11% 19% 32% 25% 11% 2	500
	21st to end	Day	14% 27% 32% 19% 7% 1	550
April	1st to 10th	Day	13% 24% 31% 22% 9% 2	500
	11th to 20th	Day	14% 29% 30% 18% 6% 1	492
	21st to end	Day	11% 24% 32% 23% 9% 1	500
May	1st to 10th	Day	9% 24% 32% 23% 10% 1	503
	11th to 20th	Day	8% 12% 28% 29% 16% 5%	660
	21st to end	Day	7% 13% 26% 32% 17% 4	789
June	1st to 10th	Day	4 8% 24% 33% 24% 6%	730
	11th to 20th	Day	3 10% 29% 34% 19% 6%	730
	21st to end	Day	3 8% 24% 32% 27% 6%	730
July	1st to 10th	Day	4 4 15% 34% 33% 9%	721
	11th to 20th	Day	4 3 11% 30% 37% 15%	629
	21st to end	Day	5% 4 11% 33% 29% 17%	575
August	1st to 10th	Day	5% 5% 15% 31% 31% 13%	527
	11th to 20th	Day	6% 4 16% 34% 26% 13%	517
	21st to end	Day	5% 5% 17% 30% 31% 11%	583
September	1st to 10th	Day	4 5% 17% 34% 29% 12%	530
	11th to 20th	Day	4 8% 27% 35% 21% 6%	530
	21st to end	Day	7% 6% 19% 33% 25% 10%	368
October	1st to 10th	Day	6% 8% 18% 31% 24% 13%	330
	11th to 20th	Day	6% 10% 27% 29% 20% 8%	330
	21st to end	Day	8% 9% 27% 32% 18% 5%	363
November	1st to 10th	Day	6% 10% 29% 35% 18% 2	330
	11th to 20th	Day	3 7% 29% 37% 21% 3	321
	21st to end	Day	5% 6% 32% 36% 18% 4	300
December	1st to 10th	Day	3 9% 37% 32% 14% 5%	300
	11th to 20th	Day	4 4 37% 39% 14% 2	300
	21st to end	Day	5% 10% 38% 33% 11% 2	330
January	1st to 10th	Night	1 6% 38% 41% 13% 1	961
	11th to 20th	Night	2 7% 40% 38% 12% 1	800
	21st to end	Night	1 3 43% 41% 10% 1	880
February	1st to 10th	Night	2 6% 50% 35% 7% 1	800
	11th to 20th	Night	2 7% 49% 35% 7%	784
	21st to end	Night	1 9% 52% 34% 3	660
March	1st to 10th	Night	3 12% 43% 35% 8%	800
	11th to 20th	Night	4 6% 40% 39% 10% 1	800
	21st to end	Night	2 11% 45% 36% 5%	803
April	1st to 10th	Night	3 15% 46% 30% 5%	600
	11th to 20th	Night	6% 13% 41% 34% 7%	593
	21st to end	Night	5% 11% 39% 36% 9% 1	600
May	1st to 10th	Night	7% 8% 34% 39% 11% 1	600
	11th to 20th	Night	8% 6% 26% 43% 16% 1	591
	21st to end	Night	5% 8% 30% 40% 16% 1	650
June	1st to 10th	Night	3 3 22% 47% 24% 1	600
	11th to 20th	Night	1 5% 28% 47% 19% 1	600
	21st to end	Night	3 3 20% 45% 29% 1	600
July	1st to 10th	Night	3 3 12% 42% 36% 4	585
	11th to 20th	Night	3 2 6% 35% 49% 5%	582
	21st to end	Night	5% 2 5% 29% 52% 7%	657
August	1st to 10th	Night	4 3 11% 33% 45% 5%	597
	11th to 20th	Night	5% 2 9% 44% 37% 3	594
	21st to end	Night	6% 2 8% 44% 36% 4	660
September	1st to 10th	Night	5% 2 12% 45% 31% 5%	600
	11th to 20th	Night	5% 2 18% 48% 24% 3	600
	21st to end	Night	5% 2 21% 44% 25% 2	679
October	1st to 10th	Night	4 2 27% 47% 18% 2	800
	11th to 20th	Night	2 4 38% 38% 16% 2	800
	21st to end	Night	3 6% 32% 40% 17% 2	880
November	1st to 10th	Night	2 5% 35% 44% 13%	926
	11th to 20th	Night	1 6% 35% 43% 13% 1	1036
	21st to end	Night	3 7% 35% 40% 14% 1	1000
December	1st to 10th	Night	1 6% 40% 40% 12% 1	1000
	11th to 20th	Night	3 7% 40% 38% 12% 1	1000
	21st to end	Night	2 10% 39% 39% 10%	1100

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