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 2024-11-08.

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% 58%	105067
	Night	2 3 1 4 3 3 3 3 6% 8% 63%	102766
Transparency	Day	20% 7% 13% 25% 25% 9%	104820
	Night	18% 7% 13% 27% 26% 9%	102603
Seeing	Day	7% 11% 26% 31% 19% 6%	15350
	Night	3 6% 32% 39% 18% 2	25429

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	3 1 3 2 3 4 3 6% 8% 64%	2616
	11th to 20th	Day	2 1 4 2 3 3 3 5% 9% 67%	2860
	21st to end	Day	4 3 6% 4 4 4 3 6% 8% 59%	3146
February	1st to 10th	Day	3 1 5% 3 3 4 4 6% 8% 64%	1710
	11th to 20th	Day	1 2 1 3 2 3 4 4 6% 9% 67%	1797
	21st to end	Day	3 2 5% 3 3 4 4 6% 8% 62%	1570
March	1st to 10th	Day	4 2 3 3 3 4 2 5% 7% 67%	3282
	11th to 20th	Day	2 13% 3 5% 3 3 4 3 5% 7% 52%	3490
	21st to end	Day	1 4 3 6% 4 4 4 4 6% 8% 57%	3943
April	1st to 10th	Day	1 4 1 4 3 3 4 3 7% 9% 63%	2280
	11th to 20th	Day	2 5% 1 5% 3 4 4 4 6% 8% 59%	2251
	21st to end	Day	3 5% 1 4 4 4 4 4 7% 10% 54%	2280
May	1st to 10th	Day	4 6% 2 3 3 3 3 3 6% 7% 60%	3815
	11th to 20th	Day	3 5% 2 4 4 5% 6% 6% 7% 9% 49%	4287
	21st to end	Day	2 3 1 4 3 3 4 4 6% 7% 64%	4825
June	1st to 10th	Day	2 3 1 4 4 4 5% 5% 7% 7% 59%	2660
	11th to 20th	Day	2 2 1 3 2 4 4 4 7% 7% 66%	2660
	21st to end	Day	2 3 1 5% 4 5% 7% 5% 8% 9% 52%	2660
July	1st to 10th	Day	2 2 1 4 4 5% 5% 5% 7% 9% 56%	4430
	11th to 20th	Day	2 2 1 4 5% 6% 8% 8% 10% 12% 42%	4281
	21st to end	Day	3 3 1 5% 5% 6% 7% 6% 9% 9% 46%	4454
August	1st to 10th	Day	4 4 2 6% 6% 7% 8% 7% 10% 11% 35%	4092
	11th to 20th	Day	3 3 1 4 4 5% 6% 6% 10% 11% 47%	3756
	21st to end	Day	2 4 1 6% 4 5% 7% 6% 10% 10% 44%	4180
September	1st to 10th	Day	3 4 1 5% 4 5% 6% 4 8% 10% 50%	2280
	11th to 20th	Day	3 4 1 5% 3 3 4 4 7% 7% 59%	2280
	21st to end	Day	3 3 1 2 3 3 4 3 5% 6% 67%	2047
October	1st to 10th	Day	2 2 1 3 2 3 3 3 4 6% 71%	3170
	11th to 20th	Day	4 4 1 4 3 4 5% 4 5% 7% 60%	3150
	21st to end	Day	1 2 1 2 2 2 2 2 3 5% 77%	3438
November	1st to 10th	Day	1 2 1 2 1 2 2 2 5% 7% 75%	1808
	11th to 20th	Day	1 1 2 1 1 2 2 4 6% 79%	1710
	21st to end	Day	1 3 1 3 3 2 2 3 4 6% 73%	1619
December	1st to 10th	Day	2 1 4 2 3 4 2 5% 7% 71%	1520
	11th to 20th	Day	3 1 2 2 2 2 4 8% 10% 65%	1926
	21st to end	Day	1 2 1 4 2 3 3 3 6% 8% 67%	2794
January	1st to 10th	Night	3 2 7% 4 3 3 3 5% 8% 61%	4354
	11th to 20th	Night	2 1 2 1 1 2 3 5% 7% 75%	4110
	21st to end	Night	5% 2 6% 3 3 4 3 5% 8% 60%	4521
February	1st to 10th	Night	3 2 4 2 3 2 3 4 8% 68%	2470
	11th to 20th	Night	1 2 1 4 3 3 3 3 5% 7% 69%	2270
	21st to end	Night	3 1 3 3 3 3 2 4 8% 69%	1884
March	1st to 10th	Night	5% 2 3 2 2 3 3 4 7% 69%	3584
	11th to 20th	Night	2 12% 3 5% 3 3 3 3 4 7% 55%	3490
	21st to end	Night	1 4 2 5% 3 3 4 3 5% 7% 63%	3704
April	1st to 10th	Night	1 2 1 2 2 2 3 3 5% 8% 70%	1900
	11th to 20th	Night	3 5% 2 5% 3 3 4 3 5% 6% 62%	1877
	21st to end	Night	2 4 2 4 4 3 3 3 6% 7% 63%	1900
May	1st to 10th	Night	4 4 1 3 2 3 3 3 5% 7% 67%	3151
	11th to 20th	Night	7% 7% 2 4 4 3 4 4 6% 7% 54%	2614
	21st to end	Night	2 2 1 2 2 2 2 3 4 5% 75%	2762
June	1st to 10th	Night	2 1 1 3 3 3 3 3 5% 7% 68%	1520
	11th to 20th	Night	1 2 1 3 3 2 2 3 5% 7% 71%	1520
	21st to end	Night	3 2 1 3 4 4 5% 4 7% 9% 58%	1520
July	1st to 10th	Night	3 2 1 4 4 4 6% 5% 6% 7% 59%	2540
	11th to 20th	Night	2 2 1 5% 4 5% 6% 7% 11% 12% 46%	2685
	21st to end	Night	3 3 1 4 5% 5% 5% 5% 8% 11% 48%	3109
August	1st to 10th	Night	5% 5% 2 7% 6% 6% 7% 6% 9% 12% 35%	2847
	11th to 20th	Night	3 3 1 4 3 4 5% 5% 10% 15% 46%	3080
	21st to end	Night	3 4 2 6% 5% 6% 7% 6% 10% 13% 39%	3487
September	1st to 10th	Night	4 3 1 5% 4 5% 4 4 7% 8% 53%	1900
	11th to 20th	Night	5% 3 1 4 2 3 3 3 5% 7% 64%	1900
	21st to end	Night	5% 3 1 3 3 3 2 2 4 6% 68%	2127
October	1st to 10th	Night	2 2 1 2 2 2 2 3 4 6% 75%	3800
	11th to 20th	Night	4 3 1 3 3 3 4 3 5% 7% 64%	3778
	21st to end	Night	2 2 1 2 2 1 2 1 3 5% 78%	4120
November	1st to 10th	Night	1 2 1 1 2 3 3 4 7% 76%	2409
	11th to 20th	Night	1 1 2 2 1 2 2 4 5% 80%	2470
	21st to end	Night	1 4 1 3 2 2 2 3 4 6% 71%	2559
December	1st to 10th	Night	2 1 3 2 3 2 2 5% 7% 74%	2660
	11th to 20th	Night	1 3 2 4 2 3 3 3 6% 9% 64%	3271
	21st to end	Night	3 2 4 3 3 3 3 5% 7% 66%	4873

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	14% 6% 18% 35% 24% 3	2617
	11th to 20th	Day	13% 8% 17% 30% 30% 3	2860
	21st to end	Day	23% 7% 14% 32% 21% 3	3146
February	1st to 10th	Day	18% 6% 14% 32% 22% 8%	1710
	11th to 20th	Day	15% 6% 12% 33% 28% 7%	1807
	21st to end	Day	19% 5% 11% 24% 30% 11%	1570
March	1st to 10th	Day	15% 4 10% 24% 39% 7%	3282
	11th to 20th	Day	30% 5% 11% 34% 17% 3	3490
	21st to end	Day	22% 7% 12% 34% 22% 3	3943
April	1st to 10th	Day	18% 6% 11% 28% 26% 12%	2280
	11th to 20th	Day	21% 6% 9% 26% 27% 10%	2254
	21st to end	Day	22% 7% 13% 26% 25% 6%	2280
May	1st to 10th	Day	20% 7% 9% 21% 27% 16%	3815
	11th to 20th	Day	24% 10% 15% 25% 17% 9%	4291
	21st to end	Day	14% 8% 9% 19% 31% 18%	4825
June	1st to 10th	Day	20% 8% 13% 23% 23% 14%	2660
	11th to 20th	Day	14% 6% 9% 19% 25% 26%	2660
	21st to end	Day	22% 8% 12% 21% 22% 14%	2660
July	1st to 10th	Day	20% 9% 11% 17% 22% 21%	4361
	11th to 20th	Day	25% 11% 19% 27% 13% 5%	4206
	21st to end	Day	28% 10% 19% 21% 14% 7%	4314
August	1st to 10th	Day	36% 12% 22% 16% 10% 4	4078
	11th to 20th	Day	25% 11% 20% 17% 20% 6%	3792
	21st to end	Day	27% 12% 18% 19% 20% 4	4179
September	1st to 10th	Day	26% 7% 15% 21% 22% 9%	2280
	11th to 20th	Day	21% 6% 12% 20% 29% 12%	2280
	21st to end	Day	17% 4 8% 26% 36% 8%	2047
October	1st to 10th	Day	13% 5% 7% 21% 36% 18%	3170
	11th to 20th	Day	21% 6% 7% 29% 30% 8%	3150
	21st to end	Day	11% 3 6% 32% 42% 6%	3438
November	1st to 10th	Day	10% 3 10% 30% 37% 10%	1808
	11th to 20th	Day	8% 3 11% 38% 34% 6%	1710
	21st to end	Day	13% 4 10% 32% 31% 10%	1619
December	1st to 10th	Day	14% 5% 17% 35% 20% 8%	1520
	11th to 20th	Day	13% 6% 19% 36% 23% 3	1924
	21st to end	Day	15% 7% 14% 25% 30% 9%	2794
January	1st to 10th	Night	18% 6% 14% 34% 26% 2	4354
	11th to 20th	Night	9% 6% 15% 34% 32% 5%	4110
	21st to end	Night	22% 8% 13% 35% 19% 3	4521
February	1st to 10th	Night	16% 4 14% 33% 25% 8%	2470
	11th to 20th	Night	16% 5% 10% 34% 28% 7%	2270
	21st to end	Night	14% 5% 10% 25% 36% 11%	1884
March	1st to 10th	Night	14% 5% 8% 25% 43% 5%	3584
	11th to 20th	Night	28% 7% 10% 33% 18% 4	3490
	21st to end	Night	19% 6% 13% 35% 25% 3	3704
April	1st to 10th	Night	14% 6% 10% 32% 28% 11%	1900
	11th to 20th	Night	21% 6% 10% 24% 28% 11%	1877
	21st to end	Night	19% 7% 11% 28% 27% 7%	1900
May	1st to 10th	Night	17% 5% 10% 18% 31% 19%	3151
	11th to 20th	Night	24% 10% 15% 26% 17% 9%	2615
	21st to end	Night	11% 6% 8% 21% 37% 17%	2762
June	1st to 10th	Night	15% 7% 14% 22% 24% 17%	1520
	11th to 20th	Night	12% 5% 8% 20% 26% 29%	1520
	21st to end	Night	21% 7% 13% 18% 23% 17%	1520
July	1st to 10th	Night	21% 10% 12% 18% 12% 27%	2500
	11th to 20th	Night	22% 11% 27% 27% 8% 5%	2637
	21st to end	Night	24% 13% 24% 22% 10% 8%	3011
August	1st to 10th	Night	37% 13% 24% 11% 9% 6%	2838
	11th to 20th	Night	24% 11% 28% 13% 16% 8%	3110
	21st to end	Night	30% 13% 20% 19% 15% 3	3487
September	1st to 10th	Night	25% 9% 17% 19% 20% 10%	1900
	11th to 20th	Night	20% 6% 13% 20% 27% 14%	1900
	21st to end	Night	18% 6% 7% 23% 36% 9%	2127
October	1st to 10th	Night	12% 5% 8% 21% 35% 20%	3800
	11th to 20th	Night	18% 6% 8% 28% 31% 9%	3778
	21st to end	Night	11% 2 5% 32% 41% 9%	4120
November	1st to 10th	Night	9% 4 8% 33% 37% 9%	2409
	11th to 20th	Night	8% 4 8% 37% 36% 8%	2470
	21st to end	Night	16% 4 8% 30% 34% 9%	2559
December	1st to 10th	Night	12% 4 16% 36% 25% 7%	2660
	11th to 20th	Night	16% 6% 16% 31% 28% 3	3272
	21st to end	Night	17% 8% 14% 26% 29% 7%	4873

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% 36% 17% 1	270
	11th to 20th	Day	7% 11% 34% 33% 13%	270
	21st to end	Day	2 3 37% 40% 15% 2	297
February	1st to 10th	Day	5% 16% 38% 34% 7% 1	270
	11th to 20th	Day	7% 12% 40% 32% 10%	270
	21st to end	Day	12% 19% 37% 28% 3 1	225
March	1st to 10th	Day	10% 22% 34% 26% 8% 1	357
	11th to 20th	Day	10% 18% 32% 25% 12% 2	460
	21st to end	Day	14% 28% 32% 18% 7% 1	506
April	1st to 10th	Day	13% 25% 31% 21% 8% 2	460
	11th to 20th	Day	14% 29% 31% 18% 6% 2	452
	21st to end	Day	11% 25% 32% 23% 8% 1	460
May	1st to 10th	Day	10% 25% 34% 22% 9% 1	463
	11th to 20th	Day	9% 11% 28% 30% 17% 6%	613
	21st to end	Day	8% 13% 27% 33% 16% 4	734
June	1st to 10th	Day	4 9% 24% 33% 25% 6%	680
	11th to 20th	Day	3 10% 29% 34% 18% 6%	680
	21st to end	Day	3 7% 24% 32% 28% 6%	680
July	1st to 10th	Day	4 5% 15% 34% 34% 9%	671
	11th to 20th	Day	4 3 11% 31% 36% 15%	584
	21st to end	Day	6% 4 11% 34% 28% 18%	531
August	1st to 10th	Day	5% 5% 14% 31% 31% 14%	487
	11th to 20th	Day	6% 4 17% 35% 26% 12%	477
	21st to end	Day	5% 5% 17% 30% 30% 12%	539
September	1st to 10th	Day	4 5% 17% 32% 30% 12%	490
	11th to 20th	Day	4 8% 26% 34% 21% 6%	490
	21st to end	Day	7% 6% 19% 32% 26% 10%	336
October	1st to 10th	Day	7% 8% 18% 30% 23% 14%	300
	11th to 20th	Day	5% 10% 25% 30% 22% 8%	300
	21st to end	Day	8% 10% 27% 32% 18% 5%	330
November	1st to 10th	Day	7% 12% 28% 33% 18% 2	291
	11th to 20th	Day	3 7% 30% 36% 21% 2	270
	21st to end	Day	4 6% 31% 37% 18% 3	270
December	1st to 10th	Day	3 10% 38% 33% 12% 3	270
	11th to 20th	Day	4 4 38% 37% 14% 3	270
	21st to end	Day	6% 11% 39% 32% 11% 1	297
January	1st to 10th	Night	1 6% 37% 41% 14% 1	913
	11th to 20th	Night	2 8% 40% 37% 12% 1	760
	21st to end	Night	2 3 42% 41% 11% 1	836
February	1st to 10th	Night	2 6% 48% 36% 7% 1	760
	11th to 20th	Night	2 7% 48% 36% 7%	744
	21st to end	Night	1 9% 53% 33% 4	628
March	1st to 10th	Night	3 12% 43% 34% 8%	760
	11th to 20th	Night	4 6% 38% 41% 11% 1	760
	21st to end	Night	3 12% 45% 36% 5%	763
April	1st to 10th	Night	4 15% 46% 30% 5%	570
	11th to 20th	Night	6% 13% 39% 34% 7%	563
	21st to end	Night	5% 11% 39% 36% 8% 1	570
May	1st to 10th	Night	7% 8% 35% 39% 11% 1	570
	11th to 20th	Night	8% 6% 24% 44% 16% 1	561
	21st to end	Night	5% 8% 30% 40% 16% 1	617
June	1st to 10th	Night	2 4 22% 46% 24% 1	570
	11th to 20th	Night	1 5% 28% 47% 19% 1	570
	21st to end	Night	3 3 19% 44% 30% 1	570
July	1st to 10th	Night	3 3 12% 41% 37% 4	555
	11th to 20th	Night	3 2 6% 34% 49% 6%	552
	21st to end	Night	6% 2 5% 28% 52% 7%	624
August	1st to 10th	Night	4 3 10% 32% 46% 5%	567
	11th to 20th	Night	5% 2 9% 43% 38% 3	564
	21st to end	Night	5% 2 8% 44% 36% 5%	627
September	1st to 10th	Night	5% 2 12% 45% 31% 5%	570
	11th to 20th	Night	5% 2 18% 47% 25% 3	570
	21st to end	Night	5% 2 21% 44% 25% 2	645
October	1st to 10th	Night	4 2 26% 47% 19% 2	760
	11th to 20th	Night	2 4 37% 38% 17% 2	760
	21st to end	Night	3 6% 32% 38% 18% 2	836
November	1st to 10th	Night	2 5% 35% 44% 14%	869
	11th to 20th	Night	1 7% 35% 42% 13% 1	950
	21st to end	Night	3 7% 35% 40% 15% 1	950
December	1st to 10th	Night	1 7% 41% 40% 11% 1	950
	11th to 20th	Night	3 7% 38% 39% 12% 1	950
	21st to end	Night	2 10% 39% 40% 9%	1045

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