Chattanooga
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Chattanooga clear sky chart from 2005-11-06 to 2025-09-04.

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% 5% 5% 5% 4 6% 8% 40%	125790
	Night	14% 6% 2 6% 4 4 4 4 6% 7% 42%	122933
Transparency	Day	41% 5% 12% 23% 17% 3	129539
	Night	40% 5% 11% 23% 18% 4	127252
Seeing	Day	16% 9% 22% 24% 19% 10%	15787
	Night	16% 7% 23% 31% 20% 3	25757

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% 8% 4 6% 3 3 3 3 4 5% 43%	4056
	11th to 20th	Day	16% 5% 4 8% 4 3 3 3 4 5% 44%	4060
	21st to end	Day	21% 9% 3 6% 3 3 4 4 4 6% 38%	4742
February	1st to 10th	Day	18% 10% 2 7% 4 4 5% 3 5% 6% 38%	2000
	11th to 20th	Day	22% 8% 2 7% 4 4 4 3 4 6% 35%	1948
	21st to end	Day	21% 8% 3 6% 4 4 4 3 4 6% 39%	1650
March	1st to 10th	Day	19% 9% 3 6% 4 5% 5% 4 6% 5% 33%	4480
	11th to 20th	Day	12% 6% 2 5% 4 3 3 3 5% 6% 49%	4969
	21st to end	Day	14% 6% 2 7% 5% 4 4 3 5% 7% 42%	5465
April	1st to 10th	Day	12% 6% 2 6% 5% 4 6% 4 5% 8% 42%	2380
	11th to 20th	Day	13% 6% 2 6% 4 3 4 4 5% 7% 46%	2374
	21st to end	Day	12% 5% 2 6% 5% 5% 5% 4 5% 8% 42%	2513
May	1st to 10th	Day	17% 8% 3 7% 5% 4 5% 4 6% 9% 32%	5862
	11th to 20th	Day	17% 8% 3 8% 5% 6% 6% 4 7% 8% 29%	5861
	21st to end	Day	17% 7% 2 6% 4 4 4 4 6% 8% 38%	6433
June	1st to 10th	Day	10% 4 2 7% 5% 6% 5% 6% 8% 10% 37%	2600
	11th to 20th	Day	5% 4 2 7% 5% 5% 6% 6% 9% 11% 41%	2600
	21st to end	Day	7% 4 2 5% 6% 7% 5% 6% 8% 11% 39%	2600
July	1st to 10th	Day	6% 4 2 6% 5% 6% 7% 5% 8% 12% 39%	5900
	11th to 20th	Day	8% 5% 2 6% 6% 5% 7% 6% 9% 12% 34%	5900
	21st to end	Day	12% 8% 3 9% 6% 7% 7% 6% 7% 11% 23%	6424
August	1st to 10th	Day	5% 4 2 6% 4 6% 6% 5% 8% 11% 43%	4186
	11th to 20th	Day	7% 4 2 6% 5% 6% 6% 5% 8% 11% 41%	3849
	21st to end	Day	8% 5% 2 6% 5% 4 5% 4 5% 7% 50%	4312
September	1st to 10th	Day	11% 6% 2 6% 5% 5% 6% 5% 7% 8% 39%	2202
	11th to 20th	Day	15% 6% 3 6% 5% 5% 4 4 6% 6% 41%	2090
	21st to end	Day	14% 5% 1 6% 5% 4 3 3 6% 6% 48%	2090
October	1st to 10th	Day	11% 5% 2 5% 4 3 3 3 4 5% 54%	2090
	11th to 20th	Day	10% 4 2 5% 3 4 3 3 4 7% 55%	2033
	21st to end	Day	14% 6% 1 4 3 3 4 4 5% 7% 51%	1990
November	1st to 10th	Day	12% 6% 3 5% 4 3 3 3 3 5% 53%	1762
	11th to 20th	Day	13% 5% 2 6% 4 3 4 4 5% 6% 48%	1800
	21st to end	Day	17% 5% 2 6% 4 4 4 4 5% 7% 44%	1800
December	1st to 10th	Day	17% 7% 2 6% 4 4 3 3 4 5% 44%	2930
	11th to 20th	Day	19% 10% 3 8% 5% 5% 5% 3 4 5% 33%	3373
	21st to end	Day	20% 8% 3 6% 4 4 4 4 5% 8% 35%	4466
January	1st to 10th	Night	16% 9% 4 7% 3 3 4 3 4 6% 42%	5889
	11th to 20th	Night	17% 7% 2 7% 4 3 3 3 3 5% 45%	5900
	21st to end	Night	19% 10% 2 5% 4 3 5% 4 5% 6% 39%	6198
February	1st to 10th	Night	16% 9% 3 7% 4 4 3 3 5% 6% 42%	2400
	11th to 20th	Night	22% 8% 3 6% 5% 4 4 4 3 4 37%	2338
	21st to end	Night	23% 7% 2 5% 4 4 4 4 4 6% 37%	1980
March	1st to 10th	Night	23% 8% 3 7% 4 4 4 4 4 7% 32%	5373
	11th to 20th	Night	11% 6% 2 5% 4 4 4 3 6% 7% 49%	4990
	21st to end	Night	15% 8% 3 7% 5% 4 5% 3 4 5% 41%	5467
April	1st to 10th	Night	11% 5% 2 5% 4 4 4 4 6% 7% 47%	2035
	11th to 20th	Night	12% 5% 2 4 4 3 3 3 6% 6% 51%	1975
	21st to end	Night	12% 6% 2 5% 3 3 4 4 5% 7% 48%	1885
May	1st to 10th	Night	14% 8% 3 8% 5% 6% 6% 4 7% 8% 32%	4033
	11th to 20th	Night	20% 8% 3 8% 6% 5% 5% 4 6% 7% 29%	4032
	21st to end	Night	13% 6% 2 7% 4 5% 5% 4 7% 9% 38%	4421
June	1st to 10th	Night	6% 6% 2 7% 7% 6% 5% 5% 7% 10% 40%	1800
	11th to 20th	Night	5% 3 2 4 4 4 5% 4 8% 12% 50%	1800
	21st to end	Night	5% 3 2 5% 5% 5% 5% 5% 8% 11% 46%	1800
July	1st to 10th	Night	7% 5% 3 6% 6% 6% 6% 5% 9% 10% 37%	4058
	11th to 20th	Night	8% 5% 2 6% 5% 5% 5% 5% 8% 14% 38%	4057
	21st to end	Night	10% 7% 3 8% 6% 6% 6% 5% 8% 11% 29%	4427
August	1st to 10th	Night	4 3 1 5% 4 5% 5% 4 8% 10% 49%	2945
	11th to 20th	Night	7% 5% 2 6% 4 4 5% 4 6% 9% 50%	3200
	21st to end	Night	7% 3 1 4 4 3 4 4 6% 8% 55%	3586
September	1st to 10th	Night	10% 6% 2 7% 5% 5% 5% 4 6% 10% 41%	2025
	11th to 20th	Night	14% 5% 2 6% 5% 5% 4 3 5% 8% 43%	2090
	21st to end	Night	12% 6% 2 5% 3 3 3 3 5% 8% 50%	2090
October	1st to 10th	Night	11% 3 1 4 4 3 2 3 3 7% 58%	2090
	11th to 20th	Night	12% 5% 1 5% 5% 4 4 3 4 5% 54%	2146
	21st to end	Night	14% 5% 2 5% 3 2 3 3 4 5% 54%	2605
November	1st to 10th	Night	13% 6% 2 4 3 3 4 3 5% 6% 51%	2536
	11th to 20th	Night	13% 6% 2 5% 4 3 2 3 5% 7% 50%	2600
	21st to end	Night	16% 7% 2 6% 4 4 4 3 4 6% 45%	2600
December	1st to 10th	Night	17% 8% 2 6% 4 3 4 3 5% 5% 43%	4250
	11th to 20th	Night	23% 7% 2 6% 4 3 4 3 4 5% 39%	4822
	21st to end	Night	20% 7% 2 7% 4 5% 5% 3 6% 7% 34%	6490

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% 3 7% 25% 20% 1	4057
	11th to 20th	Day	42% 4 6% 22% 24% 2	4060
	21st to end	Day	48% 3 7% 22% 18% 2	4742
February	1st to 10th	Day	47% 4 8% 21% 16% 3	2000
	11th to 20th	Day	51% 4 7% 18% 17% 3	1958
	21st to end	Day	48% 3 8% 16% 20% 6%	1650
March	1st to 10th	Day	50% 4 7% 14% 18% 8%	4480
	11th to 20th	Day	36% 4 5% 21% 23% 11%	4969
	21st to end	Day	43% 3 8% 29% 16% 2	5465
April	1st to 10th	Day	39% 6% 10% 23% 18% 4	2380
	11th to 20th	Day	38% 4 8% 21% 24% 6%	2374
	21st to end	Day	39% 5% 11% 27% 16% 2	2513
May	1st to 10th	Day	48% 5% 14% 21% 11% 1	5897
	11th to 20th	Day	51% 6% 15% 19% 8% 1	5865
	21st to end	Day	43% 6% 8% 22% 18% 3	6447
June	1st to 10th	Day	38% 8% 16% 23% 13% 2	2600
	11th to 20th	Day	31% 10% 17% 26% 15% 2	2600
	21st to end	Day	33% 11% 18% 20% 14% 4	2600
July	1st to 10th	Day	36% 7% 20% 24% 12% 1	5835
	11th to 20th	Day	38% 7% 30% 19% 4 1	5831
	21st to end	Day	54% 7% 19% 16% 4	6240
August	1st to 10th	Day	32% 7% 14% 36% 11% 1	4173
	11th to 20th	Day	34% 7% 17% 27% 15% 1	3885
	21st to end	Day	32% 6% 11% 31% 19% 2	4311
September	1st to 10th	Day	39% 8% 13% 17% 16% 6%	2202
	11th to 20th	Day	41% 7% 10% 20% 19% 2	2090
	21st to end	Day	36% 5% 10% 21% 21% 7%	2090
October	1st to 10th	Day	24% 3 5% 20% 36% 12%	3480
	11th to 20th	Day	22% 3 5% 23% 33% 15%	3364
	21st to end	Day	29% 2 7% 26% 28% 8%	3253
November	1st to 10th	Day	35% 3 7% 20% 28% 7%	1762
	11th to 20th	Day	36% 4 5% 24% 26% 6%	1800
	21st to end	Day	40% 4 7% 24% 21% 4	1800
December	1st to 10th	Day	43% 2 5% 25% 21% 3	2930
	11th to 20th	Day	55% 3 6% 19% 13% 3	3370
	21st to end	Day	47% 4 12% 26% 10% 2	4466
January	1st to 10th	Night	44% 3 8% 24% 19% 2	5889
	11th to 20th	Night	42% 3 6% 21% 23% 5%	5900
	21st to end	Night	46% 3 5% 22% 18% 6%	6198
February	1st to 10th	Night	44% 3 9% 21% 18% 3	2400
	11th to 20th	Night	51% 4 7% 16% 15% 6%	2339
	21st to end	Night	48% 4 7% 18% 18% 5%	1980
March	1st to 10th	Night	52% 3 6% 16% 18% 4	5373
	11th to 20th	Night	35% 3 6% 20% 22% 14%	4990
	21st to end	Night	45% 3 6% 25% 17% 3	5467
April	1st to 10th	Night	34% 5% 11% 23% 22% 4	2035
	11th to 20th	Night	32% 4 8% 25% 22% 9%	1975
	21st to end	Night	34% 6% 12% 27% 19% 2	1885
May	1st to 10th	Night	47% 6% 16% 19% 11% 1	4060
	11th to 20th	Night	52% 6% 15% 18% 9%	4033
	21st to end	Night	40% 7% 13% 24% 14% 1	4429
June	1st to 10th	Night	36% 9% 15% 23% 14% 2	1800
	11th to 20th	Night	25% 7% 21% 28% 17% 2	1800
	21st to end	Night	30% 10% 20% 21% 15% 4	1800
July	1st to 10th	Night	40% 8% 19% 23% 9% 1	4015
	11th to 20th	Night	36% 6% 30% 21% 5% 2	4006
	21st to end	Night	48% 8% 22% 17% 5% 1	4297
August	1st to 10th	Night	29% 7% 14% 36% 13% 1	2934
	11th to 20th	Night	30% 6% 21% 28% 14% 1	3230
	21st to end	Night	28% 6% 13% 29% 22% 1	3586
September	1st to 10th	Night	38% 7% 18% 19% 13% 5%	2025
	11th to 20th	Night	38% 8% 11% 19% 21% 2	2090
	21st to end	Night	34% 6% 10% 21% 22% 7%	2090
October	1st to 10th	Night	20% 3 5% 25% 30% 16%	3480
	11th to 20th	Night	21% 2 5% 20% 35% 17%	3554
	21st to end	Night	25% 2 5% 28% 30% 10%	4292
November	1st to 10th	Night	34% 4 7% 22% 24% 9%	2536
	11th to 20th	Night	35% 3 6% 22% 27% 8%	2600
	21st to end	Night	41% 4 6% 23% 22% 4	2600
December	1st to 10th	Night	45% 4 5% 21% 24% 2	4250
	11th to 20th	Night	47% 3 6% 22% 19% 2	4824
	21st to end	Night	47% 4 10% 25% 13% 1	6490

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% 24% 28% 18% 3	300
	11th to 20th	Day	24% 25% 28% 16% 7% 1	300
	21st to end	Day	22% 16% 42% 16% 3	330
February	1st to 10th	Day	23% 20% 34% 18% 6%	300
	11th to 20th	Day	38% 18% 30% 12% 2 1	300
	21st to end	Day	33% 20% 32% 12% 3	249
March	1st to 10th	Day	25% 23% 32% 15% 4 1	300
	11th to 20th	Day	22% 19% 30% 23% 6%	489
	21st to end	Day	19% 17% 36% 21% 7% 1	550
April	1st to 10th	Day	19% 16% 30% 22% 12% 2	500
	11th to 20th	Day	17% 14% 33% 24% 12% 1	492
	21st to end	Day	16% 11% 30% 27% 12% 3	500
May	1st to 10th	Day	15% 12% 31% 23% 14% 5%	503
	11th to 20th	Day	13% 6% 24% 27% 21% 9%	527
	21st to end	Day	15% 5% 21% 25% 21% 14%	573
June	1st to 10th	Day	14% 4 16% 27% 26% 14%	530
	11th to 20th	Day	6% 2 13% 29% 28% 21%	530
	21st to end	Day	9% 1 15% 26% 28% 20%	530
July	1st to 10th	Day	9% 1 9% 26% 31% 24%	525
	11th to 20th	Day	11% 2 8% 21% 33% 25%	524
	21st to end	Day	10% 2 8% 23% 30% 27%	576
August	1st to 10th	Day	12% 2 11% 23% 32% 20%	526
	11th to 20th	Day	12% 3 10% 27% 33% 16%	519
	21st to end	Day	13% 2 11% 22% 32% 21%	583
September	1st to 10th	Day	11% 2 10% 25% 35% 16%	502
	11th to 20th	Day	15% 3 11% 25% 32% 15%	490
	21st to end	Day	17% 2 16% 25% 32% 7%	490
October	1st to 10th	Day	14% 3 17% 30% 23% 12%	490
	11th to 20th	Day	10% 10% 25% 32% 21% 3	490
	21st to end	Day	17% 8% 22% 31% 17% 5%	439
November	1st to 10th	Day	20% 8% 28% 26% 15% 4	300
	11th to 20th	Day	15% 14% 31% 30% 9%	300
	21st to end	Day	12% 14% 38% 24% 10% 2	300
December	1st to 10th	Day	29% 14% 30% 21% 5% 1	300
	11th to 20th	Day	21% 15% 36% 19% 7% 1	300
	21st to end	Day	27% 19% 29% 18% 6%	330
January	1st to 10th	Night	24% 13% 35% 24% 4	800
	11th to 20th	Night	22% 13% 33% 28% 5%	800
	21st to end	Night	21% 12% 34% 31% 3	880
February	1st to 10th	Night	20% 12% 31% 32% 5%	800
	11th to 20th	Night	27% 13% 34% 24% 2	783
	21st to end	Night	26% 14% 34% 23% 2	660
March	1st to 10th	Night	19% 12% 32% 32% 5%	800
	11th to 20th	Night	18% 13% 32% 29% 8% 1	800
	21st to end	Night	18% 10% 33% 30% 8%	880
April	1st to 10th	Night	14% 11% 32% 32% 11%	635
	11th to 20th	Night	16% 8% 31% 34% 10% 1	593
	21st to end	Night	16% 6% 25% 40% 13% 1	600
May	1st to 10th	Night	13% 6% 24% 37% 19% 2	600
	11th to 20th	Night	14% 4 22% 34% 23% 4	591
	21st to end	Night	13% 2 14% 34% 32% 5%	650
June	1st to 10th	Night	8% 2 14% 37% 33% 6%	600
	11th to 20th	Night	5% 2 11% 35% 38% 11%	600
	21st to end	Night	7% 1 8% 32% 42% 10%	600
July	1st to 10th	Night	11% 1 9% 26% 43% 11%	585
	11th to 20th	Night	9% 1 7% 25% 43% 14%	582
	21st to end	Night	9% 1 7% 26% 46% 11%	657
August	1st to 10th	Night	9% 1 7% 25% 49% 9%	597
	11th to 20th	Night	12% 2 8% 30% 42% 7%	594
	21st to end	Night	12% 1 9% 26% 43% 9%	660
September	1st to 10th	Night	11% 2 9% 32% 39% 8%	674
	11th to 20th	Night	15% 3 12% 26% 38% 5%	760
	21st to end	Night	16% 2 13% 31% 35% 3	760
October	1st to 10th	Night	13% 3 17% 31% 29% 7%	760
	11th to 20th	Night	14% 6% 24% 35% 21% 1	760
	21st to end	Night	17% 6% 19% 39% 18% 2	836
November	1st to 10th	Night	16% 7% 20% 35% 20% 2	780
	11th to 20th	Night	15% 7% 33% 33% 12% 1	800
	21st to end	Night	19% 9% 28% 34% 11% 1	800
December	1st to 10th	Night	24% 12% 28% 29% 8%	800
	11th to 20th	Night	20% 9% 28% 33% 9% 1	800
	21st to end	Night	25% 10% 28% 31% 7%	880

Page generated at 2025-09-01 01:51:06. Version ac07

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