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 2024-11-04.

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	13% 6% 2 6% 5% 5% 5% 4 6% 8% 40%	100902
	Night	14% 6% 2 6% 4 4 4 4 6% 7% 43%	98071
Transparency	Day	40% 6% 11% 22% 17% 4	104695
	Night	38% 5% 11% 22% 19% 5%	102416
Seeing	Day	16% 9% 22% 24% 19% 10%	14698
	Night	16% 7% 22% 31% 20% 3	24698

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% 10% 3 7% 4 5% 4 4 5% 5% 36%	2836
	11th to 20th	Day	20% 7% 3 9% 5% 3 4 3 4 5% 38%	2840
	21st to end	Day	24% 9% 2 5% 3 3 3 3 4 5% 38%	3316
February	1st to 10th	Day	18% 10% 2 6% 4 3 5% 3 4 5% 38%	1900
	11th to 20th	Day	21% 8% 2 7% 4 4 4 3 4 7% 35%	1848
	21st to end	Day	22% 9% 2 6% 4 4 3 3 4 6% 37%	1570
March	1st to 10th	Day	26% 11% 3 6% 3 4 4 3 5% 5% 30%	3120
	11th to 20th	Day	14% 7% 3 6% 4 3 3 3 4 6% 48%	3469
	21st to end	Day	16% 7% 2 8% 6% 4 4 4 6% 7% 37%	3815
April	1st to 10th	Day	11% 6% 2 6% 5% 4 6% 4 5% 8% 42%	2261
	11th to 20th	Day	14% 7% 2 6% 5% 3 4 4 5% 7% 44%	2254
	21st to end	Day	12% 5% 2 6% 5% 5% 5% 4 5% 8% 44%	2387
May	1st to 10th	Day	13% 7% 3 7% 5% 5% 5% 4 6% 8% 38%	4082
	11th to 20th	Day	15% 7% 3 7% 5% 5% 5% 4 6% 9% 35%	4081
	21st to end	Day	10% 5% 2 6% 4 5% 5% 4 7% 10% 40%	4476
June	1st to 10th	Day	10% 5% 2 7% 5% 6% 5% 6% 8% 10% 36%	2470
	11th to 20th	Day	4 3 1 7% 5% 5% 6% 6% 9% 11% 43%	2470
	21st to end	Day	7% 4 2 5% 6% 7% 5% 6% 8% 11% 39%	2470
July	1st to 10th	Day	7% 5% 2 7% 6% 7% 8% 6% 9% 12% 32%	4120
	11th to 20th	Day	10% 6% 2 6% 5% 5% 6% 5% 9% 11% 36%	4120
	21st to end	Day	14% 10% 4 10% 7% 8% 7% 6% 7% 9% 19%	4466
August	1st to 10th	Day	5% 3 2 6% 5% 6% 6% 5% 8% 11% 43%	4057
	11th to 20th	Day	7% 4 2 6% 5% 6% 6% 5% 8% 11% 41%	3729
	21st to end	Day	8% 5% 2 6% 5% 4 5% 4 5% 7% 50%	4180
September	1st to 10th	Day	11% 6% 2 6% 5% 5% 6% 5% 7% 8% 39%	2166
	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	11% 5% 2 4 4 3 3 3 3 5% 55%	1699
	11th to 20th	Day	13% 5% 2 6% 4 3 4 4 5% 6% 49%	1710
	21st to end	Day	18% 5% 1 6% 4 4 4 4 5% 7% 44%	1710
December	1st to 10th	Day	21% 8% 3 8% 5% 4 3 4 5% 5% 34%	1710
	11th to 20th	Day	15% 6% 3 7% 6% 5% 5% 4 4 6% 39%	2153
	21st to end	Day	19% 10% 3 7% 5% 4 4 3 5% 7% 33%	3124
January	1st to 10th	Night	18% 10% 3 6% 4 3 4 3 5% 7% 39%	4109
	11th to 20th	Night	19% 8% 3 7% 4 4 3 3 3 6% 41%	4120
	21st to end	Night	23% 12% 3 6% 4 3 5% 3 4 5% 34%	4338
February	1st to 10th	Night	16% 9% 3 7% 4 4 3 3 4 5% 42%	2280
	11th to 20th	Night	21% 8% 3 6% 5% 4 4 4 4 4 38%	2218
	21st to end	Night	24% 7% 2 6% 4 4 4 4 4 6% 35%	1884
March	1st to 10th	Night	26% 8% 3 7% 4 4 4 3 4 6% 31%	3733
	11th to 20th	Night	11% 7% 2 5% 4 4 4 3 6% 7% 48%	3490
	21st to end	Night	15% 8% 3 8% 5% 4 5% 4 4 5% 39%	3817
April	1st to 10th	Night	11% 5% 2 5% 4 4 4 4 6% 7% 48%	1933
	11th to 20th	Night	13% 5% 2 4 4 3 3 3 7% 7% 50%	1875
	21st to end	Night	12% 5% 2 5% 4 3 4 4 5% 7% 49%	1791
May	1st to 10th	Night	13% 7% 3 7% 5% 5% 5% 4 6% 7% 38%	2813
	11th to 20th	Night	15% 6% 2 6% 5% 5% 4 4 6% 8% 38%	2812
	21st to end	Night	8% 5% 2 6% 4 5% 5% 5% 8% 10% 42%	3079
June	1st to 10th	Night	7% 6% 2 7% 7% 6% 5% 5% 7% 10% 39%	1710
	11th to 20th	Night	4 3 1 4 4 4 5% 4 8% 12% 51%	1710
	21st to end	Night	5% 3 2 5% 5% 5% 5% 5% 8% 11% 46%	1710
July	1st to 10th	Night	8% 6% 3 6% 6% 6% 7% 5% 9% 10% 34%	2840
	11th to 20th	Night	10% 5% 2 5% 4 4 5% 4 7% 12% 42%	2840
	21st to end	Night	12% 9% 3 9% 7% 7% 6% 5% 7% 10% 25%	3087
August	1st to 10th	Night	4 3 1 5% 4 5% 5% 4 8% 10% 50%	2854
	11th to 20th	Night	6% 4 2 6% 4 4 5% 4 6% 9% 50%	3100
	21st to end	Night	7% 3 1 4 4 3 4 4 6% 8% 55%	3476
September	1st to 10th	Night	10% 6% 2 6% 5% 5% 5% 4 7% 10% 42%	1995
	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% 5% 2 4 3 3 4 3 5% 6% 53%	2452
	11th to 20th	Night	12% 6% 2 5% 4 3 2 3 5% 7% 51%	2470
	21st to end	Night	16% 7% 2 6% 4 4 4 3 4 6% 44%	2470
December	1st to 10th	Night	20% 7% 3 6% 5% 4 4 4 5% 5% 36%	2470
	11th to 20th	Night	17% 5% 2 6% 4 3 4 3 5% 5% 46%	3042
	21st to end	Night	19% 7% 3 7% 4 5% 5% 4 8% 8% 30%	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	46% 4 7% 20% 21% 2	2837
	11th to 20th	Day	48% 4 8% 19% 20% 1	2840
	21st to end	Day	49% 3 7% 22% 17% 1	3316
February	1st to 10th	Day	48% 4 8% 20% 17% 3	1900
	11th to 20th	Day	51% 4 7% 18% 17% 4	1858
	21st to end	Day	49% 3 8% 15% 19% 6%	1570
March	1st to 10th	Day	54% 5% 9% 12% 13% 8%	3120
	11th to 20th	Day	38% 5% 7% 23% 22% 6%	3469
	21st to end	Day	47% 4 9% 25% 14% 2	3815
April	1st to 10th	Day	39% 6% 10% 23% 18% 4	2261
	11th to 20th	Day	39% 4 8% 19% 24% 6%	2254
	21st to end	Day	37% 5% 11% 28% 17% 2	2387
May	1st to 10th	Day	43% 6% 12% 22% 14% 2	4120
	11th to 20th	Day	44% 7% 14% 24% 11% 1	4085
	21st to end	Day	37% 8% 11% 23% 20% 1	4489
June	1st to 10th	Day	38% 9% 16% 22% 13% 2	2470
	11th to 20th	Day	30% 9% 17% 26% 16% 2	2470
	21st to end	Day	34% 11% 18% 19% 14% 4	2470
July	1st to 10th	Day	40% 8% 23% 20% 8% 1	4055
	11th to 20th	Day	39% 8% 23% 21% 6% 2	4051
	21st to end	Day	59% 9% 13% 13% 5%	4324
August	1st to 10th	Day	31% 7% 14% 36% 11% 1	4044
	11th to 20th	Day	33% 7% 17% 27% 15% 2	3765
	21st to end	Day	32% 6% 11% 30% 19% 2	4179
September	1st to 10th	Day	39% 8% 13% 17% 16% 6%	2166
	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	33% 4 8% 20% 29% 7%	1699
	11th to 20th	Day	36% 4 6% 23% 26% 6%	1710
	21st to end	Day	40% 4 7% 24% 21% 4	1710
December	1st to 10th	Day	52% 4 6% 21% 14% 3	1710
	11th to 20th	Day	47% 5% 7% 20% 19% 3	2150
	21st to end	Day	48% 5% 9% 22% 12% 2	3124
January	1st to 10th	Night	46% 5% 7% 22% 19% 2	4109
	11th to 20th	Night	45% 3 8% 21% 18% 5%	4120
	21st to end	Night	52% 4 6% 18% 17% 2	4338
February	1st to 10th	Night	44% 3 10% 21% 19% 4	2280
	11th to 20th	Night	51% 4 7% 17% 16% 6%	2219
	21st to end	Night	50% 4 8% 17% 17% 5%	1884
March	1st to 10th	Night	53% 5% 9% 13% 15% 5%	3733
	11th to 20th	Night	35% 4 8% 23% 19% 11%	3490
	21st to end	Night	46% 5% 7% 22% 18% 2	3817
April	1st to 10th	Night	34% 5% 11% 24% 22% 4	1933
	11th to 20th	Night	33% 4 8% 24% 22% 9%	1875
	21st to end	Night	34% 6% 12% 27% 19% 2	1791
May	1st to 10th	Night	42% 8% 13% 21% 14% 2	2840
	11th to 20th	Night	42% 8% 14% 24% 13%	2813
	21st to end	Night	32% 10% 15% 25% 17% 1	3087
June	1st to 10th	Night	37% 9% 15% 22% 14% 2	1710
	11th to 20th	Night	24% 7% 21% 29% 17% 2	1710
	21st to end	Night	30% 9% 20% 21% 15% 5%	1710
July	1st to 10th	Night	42% 8% 20% 20% 8% 1	2795
	11th to 20th	Night	35% 7% 25% 24% 7% 3	2786
	21st to end	Night	53% 10% 15% 15% 6% 1	2988
August	1st to 10th	Night	28% 7% 14% 37% 14% 1	2843
	11th to 20th	Night	29% 6% 21% 28% 15% 1	3130
	21st to end	Night	27% 6% 14% 29% 22% 1	3476
September	1st to 10th	Night	37% 7% 17% 19% 13% 5%	1995
	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	33% 4 7% 22% 25% 9%	2452
	11th to 20th	Night	34% 3 6% 22% 26% 8%	2470
	21st to end	Night	42% 4 6% 23% 22% 4	2470
December	1st to 10th	Night	49% 5% 8% 19% 16% 3	2470
	11th to 20th	Night	39% 4 8% 20% 25% 3	3044
	21st to end	Night	48% 6% 10% 21% 15% 2	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% 24% 26% 20% 4	270
	11th to 20th	Day	25% 26% 27% 14% 7% 1	270
	21st to end	Day	23% 18% 42% 14% 3	297
February	1st to 10th	Day	26% 19% 33% 17% 5%	270
	11th to 20th	Day	37% 18% 30% 12% 2 1	270
	21st to end	Day	36% 21% 31% 10% 2	225
March	1st to 10th	Day	25% 22% 32% 16% 4 1	270
	11th to 20th	Day	22% 18% 30% 22% 6%	449
	21st to end	Day	20% 17% 35% 21% 8% 1	506
April	1st to 10th	Day	18% 16% 29% 22% 12% 2	460
	11th to 20th	Day	18% 14% 33% 23% 12% 1	452
	21st to end	Day	16% 12% 30% 27% 12% 3	460
May	1st to 10th	Day	13% 12% 31% 24% 15% 5%	463
	11th to 20th	Day	13% 6% 22% 27% 22% 10%	487
	21st to end	Day	14% 5% 19% 26% 22% 15%	529
June	1st to 10th	Day	15% 4 16% 27% 25% 14%	490
	11th to 20th	Day	5% 2 13% 30% 29% 21%	490
	21st to end	Day	9% 1 17% 28% 26% 18%	490
July	1st to 10th	Day	9% 1 10% 26% 30% 25%	485
	11th to 20th	Day	12% 2 8% 21% 32% 25%	484
	21st to end	Day	10% 2 8% 24% 31% 25%	532
August	1st to 10th	Day	11% 2 11% 24% 32% 20%	486
	11th to 20th	Day	11% 3 11% 28% 33% 14%	479
	21st to end	Day	13% 2 11% 20% 31% 23%	539
September	1st to 10th	Day	11% 2 10% 26% 35% 16%	490
	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	19% 8% 27% 27% 15% 4	279
	11th to 20th	Day	14% 14% 32% 31% 8%	270
	21st to end	Day	13% 13% 38% 24% 10% 2	270
December	1st to 10th	Day	31% 13% 28% 21% 5% 1	270
	11th to 20th	Day	20% 16% 37% 19% 7% 1	270
	21st to end	Day	28% 19% 31% 17% 5%	297
January	1st to 10th	Night	24% 13% 35% 24% 5%	760
	11th to 20th	Night	22% 13% 32% 27% 6%	760
	21st to end	Night	21% 12% 33% 30% 3	836
February	1st to 10th	Night	20% 12% 32% 32% 5%	760
	11th to 20th	Night	26% 13% 34% 25% 2	743
	21st to end	Night	27% 15% 35% 21% 2	628
March	1st to 10th	Night	19% 12% 32% 32% 5%	760
	11th to 20th	Night	18% 13% 32% 29% 8% 1	760
	21st to end	Night	18% 10% 33% 30% 9%	836
April	1st to 10th	Night	14% 11% 31% 32% 12%	603
	11th to 20th	Night	16% 8% 32% 33% 10% 1	563
	21st to end	Night	15% 6% 26% 40% 12% 1	570
May	1st to 10th	Night	12% 6% 24% 35% 20% 2	570
	11th to 20th	Night	13% 4 22% 33% 24% 4	561
	21st to end	Night	11% 2 12% 36% 34% 5%	617
June	1st to 10th	Night	8% 2 14% 36% 34% 6%	570
	11th to 20th	Night	4 1 11% 35% 39% 11%	570
	21st to end	Night	7% 1 8% 34% 41% 9%	570
July	1st to 10th	Night	11% 1 9% 26% 42% 11%	555
	11th to 20th	Night	9% 1 7% 26% 43% 14%	552
	21st to end	Night	9% 1 8% 27% 46% 10%	624
August	1st to 10th	Night	8% 1 7% 25% 50% 10%	567
	11th to 20th	Night	12% 2 8% 31% 41% 5%	564
	21st to end	Night	12% 1 9% 24% 45% 10%	627
September	1st to 10th	Night	11% 2 9% 31% 39% 8%	665
	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	755
	11th to 20th	Night	14% 7% 33% 33% 11% 1	760
	21st to end	Night	19% 9% 26% 34% 11% 1	760
December	1st to 10th	Night	24% 11% 28% 29% 8%	760
	11th to 20th	Night	19% 9% 29% 33% 9% 1	760
	21st to end	Night	25% 10% 28% 30% 6%	836

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