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 2024-03-28.

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% 3 6% 6% 3 5% 5% 6% 8% 39%	79192
	Night	14% 6% 3 5% 5% 3 4 5% 5% 7% 43%	80818
Transparency	Day	41% 7% 11% 21% 17% 3	78915
	Night	40% 6% 11% 21% 18% 4	80614
Seeing	Day	16% 9% 22% 24% 19% 10%	13823
	Night	16% 7% 23% 31% 20% 3	23969

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% 9% 3 6% 5% 3 4 4 5% 5% 37%	2836
	11th to 20th	Day	20% 6% 3 8% 6% 2 4 3 4 5% 38%	2840
	21st to end	Day	24% 9% 3 4 4 2 3 4 4 5% 38%	3316
February	1st to 10th	Day	18% 10% 3 6% 5% 3 5% 4 4 6% 38%	1900
	11th to 20th	Day	22% 7% 3 6% 4 3 4 4 4 7% 35%	1848
	21st to end	Day	22% 8% 3 5% 5% 3 4 4 4 6% 38%	1570
March	1st to 10th	Day	17% 6% 3 4 4 3 5% 5% 5% 7% 41%	1900
	11th to 20th	Day	20% 7% 3 6% 5% 3 4 4 5% 6% 38%	2079
	21st to end	Day	15% 6% 3 7% 7% 4 6% 6% 6% 7% 35%	2255
April	1st to 10th	Day	11% 6% 2 5% 6% 3 6% 5% 5% 7% 43%	2142
	11th to 20th	Day	14% 6% 2 5% 5% 2 4 5% 5% 7% 45%	2134
	21st to end	Day	12% 4 2 5% 5% 4 5% 5% 5% 8% 44%	2261
May	1st to 10th	Day	12% 5% 3 5% 5% 4 4 5% 5% 8% 45%	2340
	11th to 20th	Day	11% 5% 3 6% 6% 4 6% 6% 6% 9% 38%	2301
	21st to end	Day	11% 4 2 5% 6% 3 7% 6% 7% 10% 39%	2531
June	1st to 10th	Day	10% 4 2 6% 6% 5% 6% 6% 7% 11% 37%	2340
	11th to 20th	Day	5% 3 2 6% 6% 3 7% 7% 9% 11% 43%	2340
	21st to end	Day	7% 4 2 4 7% 5% 7% 7% 8% 11% 39%	2340
July	1st to 10th	Day	8% 4 3 6% 8% 4 9% 7% 8% 10% 32%	2340
	11th to 20th	Day	11% 4 2 5% 6% 3 7% 8% 9% 12% 32%	2340
	21st to end	Day	6% 5% 2 7% 7% 6% 8% 9% 7% 12% 30%	2547
August	1st to 10th	Day	8% 5% 2 7% 8% 6% 8% 8% 9% 10% 29%	2308
	11th to 20th	Day	9% 5% 2 7% 7% 5% 7% 7% 7% 10% 33%	2125
	21st to end	Day	13% 6% 3 7% 7% 4 7% 6% 7% 8% 32%	2376
September	1st to 10th	Day	12% 5% 3 5% 6% 4 6% 6% 7% 8% 39%	2052
	11th to 20th	Day	15% 5% 3 5% 6% 3 5% 4 6% 6% 42%	1980
	21st to end	Day	13% 4 2 4 6% 3 4 4 5% 5% 50%	1980
October	1st to 10th	Day	12% 5% 2 4 5% 2 3 4 4 5% 54%	1980
	11th to 20th	Day	11% 4 2 4 5% 3 3 4 4 6% 54%	1926
	21st to end	Day	14% 5% 2 4 3 2 3 5% 5% 7% 51%	1886
November	1st to 10th	Day	11% 5% 3 4 4 2 4 4 3 6% 55%	1672
	11th to 20th	Day	13% 5% 2 5% 4 2 4 5% 5% 6% 49%	1710
	21st to end	Day	18% 5% 2 4 5% 3 4 5% 5% 7% 44%	1710
December	1st to 10th	Day	21% 8% 3 7% 6% 3 4 5% 5% 5% 34%	1710
	11th to 20th	Day	15% 6% 3 6% 7% 4 5% 5% 4 6% 39%	2153
	21st to end	Day	19% 9% 3 6% 6% 3 4 4 5% 7% 33%	3124
January	1st to 10th	Night	18% 9% 3 6% 4 2 4 4 4 7% 39%	4109
	11th to 20th	Night	19% 7% 3 6% 5% 3 3 4 3 6% 41%	4120
	21st to end	Night	23% 12% 3 5% 4 3 4 3 4 5% 35%	4338
February	1st to 10th	Night	16% 8% 3 6% 5% 3 4 3 4 6% 43%	2280
	11th to 20th	Night	22% 7% 3 5% 6% 3 4 4 4 4 38%	2218
	21st to end	Night	24% 7% 2 5% 4 3 4 5% 4 6% 35%	1884
March	1st to 10th	Night	17% 5% 3 5% 5% 2 4 4 5% 8% 43%	2280
	11th to 20th	Night	16% 7% 3 6% 5% 3 4 4 5% 7% 40%	2100
	21st to end	Night	14% 7% 4 6% 6% 4 5% 5% 4 5% 41%	2256
April	1st to 10th	Night	11% 4 2 4 4 2 5% 5% 6% 7% 49%	1832
	11th to 20th	Night	13% 5% 3 3 4 2 3 4 7% 7% 51%	1775
	21st to end	Night	12% 5% 2 4 4 2 5% 4 5% 7% 50%	1697
May	1st to 10th	Night	11% 5% 2 3 4 4 5% 4 6% 8% 48%	1620
	11th to 20th	Night	10% 5% 2 6% 6% 4 6% 5% 6% 8% 43%	1592
	21st to end	Night	9% 4 2 4 5% 4 5% 6% 8% 10% 43%	1744
June	1st to 10th	Night	6% 5% 2 6% 8% 4 5% 6% 7% 10% 41%	1620
	11th to 20th	Night	4 2 2 3 4 3 5% 5% 8% 13% 52%	1620
	21st to end	Night	5% 3 2 3 6% 4 5% 6% 7% 12% 47%	1620
July	1st to 10th	Night	8% 5% 3 5% 6% 4 7% 7% 8% 10% 36%	1620
	11th to 20th	Night	9% 5% 3 5% 4 3 5% 6% 7% 13% 41%	1620
	21st to end	Night	5% 5% 2 5% 6% 4 7% 7% 9% 12% 38%	1771
August	1st to 10th	Night	6% 4 2 5% 5% 4 6% 6% 9% 11% 42%	1626
	11th to 20th	Night	10% 5% 4 5% 5% 4 6% 6% 7% 9% 39%	1776
	21st to end	Night	11% 5% 2 5% 6% 4 5% 6% 7% 10% 39%	1980
September	1st to 10th	Night	9% 5% 2 5% 6% 4 5% 5% 6% 10% 41%	1890
	11th to 20th	Night	13% 5% 2 5% 6% 3 4 5% 5% 8% 44%	1980
	21st to end	Night	11% 5% 2 4 4 2 4 4 5% 8% 52%	1980
October	1st to 10th	Night	12% 3 1 3 4 2 3 3 3 7% 58%	1980
	11th to 20th	Night	12% 5% 2 5% 5% 3 4 4 4 5% 52%	2033
	21st to end	Night	15% 4 2 4 3 2 3 4 4 5% 53%	2467
November	1st to 10th	Night	13% 5% 2 4 3 2 4 4 4 6% 53%	2406
	11th to 20th	Night	12% 6% 3 4 4 2 3 3 5% 7% 51%	2470
	21st to end	Night	17% 6% 3 5% 5% 2 4 4 4 6% 45%	2470
December	1st to 10th	Night	20% 7% 3 6% 6% 3 4 5% 5% 5% 36%	2470
	11th to 20th	Night	17% 5% 2 5% 5% 2 3 4 5% 6% 46%	3042
	21st to end	Night	19% 7% 3 6% 5% 4 5% 5% 7% 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	43% 5% 10% 17% 20% 7%	1900
	11th to 20th	Day	48% 5% 9% 18% 16% 4	2079
	21st to end	Day	48% 6% 10% 23% 12% 2	2255
April	1st to 10th	Day	38% 6% 11% 23% 18% 4	2142
	11th to 20th	Day	39% 4 8% 20% 23% 6%	2134
	21st to end	Day	37% 5% 11% 27% 17% 2	2261
May	1st to 10th	Day	36% 6% 13% 26% 15% 3	2340
	11th to 20th	Day	41% 7% 15% 22% 13% 1	2305
	21st to end	Day	37% 10% 13% 24% 15% 2	2531
June	1st to 10th	Day	38% 9% 16% 22% 14% 1	2340
	11th to 20th	Day	30% 10% 17% 26% 15% 2	2340
	21st to end	Day	34% 12% 18% 19% 13% 4	2340
July	1st to 10th	Day	42% 12% 16% 18% 10% 2	2275
	11th to 20th	Day	40% 12% 19% 16% 9% 4	2271
	21st to end	Day	41% 12% 18% 19% 9% 1	2405
August	1st to 10th	Day	42% 11% 19% 19% 7% 1	2295
	11th to 20th	Day	43% 10% 15% 19% 12% 1	2125
	21st to end	Day	45% 10% 14% 16% 12% 3	2376
September	1st to 10th	Day	39% 8% 13% 17% 16% 6%	2052
	11th to 20th	Day	40% 7% 11% 20% 19% 3	1980
	21st to end	Day	35% 5% 9% 21% 22% 7%	1980
October	1st to 10th	Day	33% 5% 8% 21% 25% 8%	1980
	11th to 20th	Day	32% 5% 8% 23% 24% 8%	1926
	21st to end	Day	34% 4 7% 24% 27% 5%	1886
November	1st to 10th	Day	33% 4 7% 20% 29% 7%	1672
	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	41% 4 9% 17% 20% 8%	2280
	11th to 20th	Night	44% 5% 9% 21% 17% 4	2100
	21st to end	Night	45% 4 9% 24% 16% 2	2256
April	1st to 10th	Night	33% 5% 11% 24% 22% 4	1832
	11th to 20th	Night	32% 4 8% 25% 22% 9%	1775
	21st to end	Night	33% 6% 12% 27% 20% 2	1697
May	1st to 10th	Night	34% 6% 13% 23% 22% 3	1620
	11th to 20th	Night	37% 9% 17% 23% 14% 1	1593
	21st to end	Night	35% 9% 14% 24% 16% 2	1745
June	1st to 10th	Night	36% 9% 15% 23% 14% 3	1620
	11th to 20th	Night	23% 8% 22% 30% 15% 2	1620
	21st to end	Night	29% 10% 21% 21% 15% 5%	1620
July	1st to 10th	Night	39% 9% 18% 19% 13% 2	1575
	11th to 20th	Night	33% 11% 22% 17% 12% 5%	1566
	21st to end	Night	33% 11% 22% 22% 10% 2	1672
August	1st to 10th	Night	33% 12% 20% 25% 9% 1	1615
	11th to 20th	Night	37% 10% 20% 18% 13% 2	1776
	21st to end	Night	39% 10% 17% 19% 13% 2	1980
September	1st to 10th	Night	37% 8% 18% 19% 13% 4	1890
	11th to 20th	Night	36% 9% 12% 19% 21% 2	1980
	21st to end	Night	32% 6% 10% 22% 23% 7%	1980
October	1st to 10th	Night	29% 4 8% 24% 26% 9%	1980
	11th to 20th	Night	35% 4 8% 21% 22% 9%	2033
	21st to end	Night	33% 3 5% 24% 27% 9%	2467
November	1st to 10th	Night	32% 4 7% 22% 25% 10%	2406
	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	21% 17% 33% 21% 8% 1	490
April	1st to 10th	Day	17% 16% 28% 24% 13% 2	420
	11th to 20th	Day	17% 13% 32% 24% 13% 1	412
	21st to end	Day	17% 12% 28% 27% 13% 4	420
May	1st to 10th	Day	13% 12% 30% 24% 15% 6%	423
	11th to 20th	Day	13% 5% 23% 26% 22% 11%	447
	21st to end	Day	15% 4 16% 25% 24% 16%	485
June	1st to 10th	Day	15% 3 15% 27% 26% 15%	450
	11th to 20th	Day	6% 3 13% 29% 27% 22%	450
	21st to end	Day	9% 2 18% 28% 26% 18%	450
July	1st to 10th	Day	9% 2 9% 24% 30% 25%	445
	11th to 20th	Day	12% 2 9% 21% 32% 25%	444
	21st to end	Day	8% 2 9% 23% 32% 27%	488
August	1st to 10th	Day	12% 2 11% 23% 32% 20%	446
	11th to 20th	Day	12% 3 10% 28% 33% 14%	439
	21st to end	Day	14% 2 12% 22% 31% 20%	495
September	1st to 10th	Day	11% 2 10% 25% 35% 17%	450
	11th to 20th	Day	14% 3 12% 23% 32% 16%	450
	21st to end	Day	14% 3 17% 26% 33% 8%	450
October	1st to 10th	Day	15% 4 16% 30% 23% 12%	450
	11th to 20th	Day	10% 10% 25% 32% 20% 2	450
	21st to end	Day	18% 9% 22% 31% 14% 5%	401
November	1st to 10th	Day	19% 9% 27% 26% 14% 4	270
	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% 11% 33% 29% 9%	821
April	1st to 10th	Night	14% 10% 31% 33% 13%	572
	11th to 20th	Night	16% 8% 32% 32% 11% 1	533
	21st to end	Night	15% 6% 25% 41% 13% 1	540
May	1st to 10th	Night	12% 6% 26% 35% 20% 2	540
	11th to 20th	Night	12% 4 22% 33% 25% 4	531
	21st to end	Night	11% 1 11% 35% 35% 5%	584
June	1st to 10th	Night	7% 2 14% 36% 35% 6%	540
	11th to 20th	Night	4 1 11% 35% 38% 11%	540
	21st to end	Night	7% 1 9% 34% 40% 9%	540
July	1st to 10th	Night	11% 1 9% 26% 42% 11%	525
	11th to 20th	Night	9% 1 8% 26% 41% 14%	522
	21st to end	Night	8% 1 8% 25% 47% 11%	591
August	1st to 10th	Night	8% 1 8% 24% 49% 10%	537
	11th to 20th	Night	12% 2 8% 30% 43% 6%	534
	21st to end	Night	13% 1 9% 24% 44% 9%	594
September	1st to 10th	Night	10% 2 8% 31% 40% 8%	630
	11th to 20th	Night	15% 3 13% 26% 38% 5%	720
	21st to end	Night	14% 2 13% 32% 35% 3	720
October	1st to 10th	Night	13% 3 17% 30% 29% 8%	720
	11th to 20th	Night	14% 6% 24% 34% 21% 1	720
	21st to end	Night	17% 6% 19% 39% 16% 2	792
November	1st to 10th	Night	15% 7% 19% 36% 21% 2	740
	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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