Nashville
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Nashville clear sky chart from 2005-11-06 to 2025-11-03.

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	12% 7% 3 7% 5% 5% 5% 4 6% 8% 39%	126707
	Night	13% 7% 3 7% 4 4 4 3 5% 7% 42%	123365
Transparency	Day	41% 6% 10% 23% 16% 3	130385
	Night	40% 5% 10% 23% 18% 4	127758
Seeing	Day	17% 10% 23% 24% 18% 8%	16523
	Night	17% 8% 24% 30% 19% 3	26165

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	14% 11% 6% 12% 5% 5% 5% 3 4 5% 30%	3617
	11th to 20th	Day	16% 8% 3 11% 4 3 4 3 4 5% 40%	3773
	21st to end	Day	19% 9% 5% 6% 4 3 3 3 6% 6% 36%	4898
February	1st to 10th	Day	17% 12% 4 8% 4 4 4 3 5% 7% 34%	2000
	11th to 20th	Day	20% 10% 3 10% 5% 4 3 3 4 6% 32%	1948
	21st to end	Day	18% 10% 4 6% 4 4 3 3 5% 5% 38%	1650
March	1st to 10th	Day	16% 12% 4 8% 5% 4 4 3 4 5% 35%	4480
	11th to 20th	Day	12% 8% 3 6% 5% 4 4 3 4 5% 47%	4750
	21st to end	Day	11% 7% 3 7% 5% 4 5% 4 5% 6% 43%	5778
April	1st to 10th	Day	13% 6% 3 5% 4 4 5% 4 6% 7% 44%	2400
	11th to 20th	Day	13% 7% 3 7% 5% 3 4 4 5% 6% 43%	2374
	21st to end	Day	12% 7% 3 7% 5% 5% 5% 4 5% 7% 39%	2440
May	1st to 10th	Day	18% 9% 3 7% 4 5% 5% 5% 7% 10% 27%	5862
	11th to 20th	Day	14% 9% 3 9% 5% 5% 5% 4 6% 8% 31%	5861
	21st to end	Day	14% 8% 3 7% 4 4 5% 4 6% 7% 38%	6433
June	1st to 10th	Day	8% 6% 2 6% 5% 6% 6% 5% 7% 8% 40%	2612
	11th to 20th	Day	5% 4 2 6% 5% 6% 6% 5% 7% 11% 44%	2800
	21st to end	Day	4 4 2 5% 5% 5% 6% 6% 9% 12% 40%	2800
July	1st to 10th	Day	5% 5% 2 7% 6% 6% 7% 5% 8% 11% 38%	6360
	11th to 20th	Day	6% 5% 3 7% 6% 6% 7% 6% 9% 13% 32%	6232
	21st to end	Day	11% 7% 3 8% 6% 6% 7% 5% 8% 11% 28%	6424
August	1st to 10th	Day	4 3 1 6% 4 5% 6% 4 8% 10% 50%	4021
	11th to 20th	Day	6% 5% 2 7% 5% 6% 7% 5% 8% 11% 38%	3849
	21st to end	Day	6% 4 1 5% 4 5% 5% 4 6% 8% 52%	4312
September	1st to 10th	Day	10% 6% 2 6% 5% 5% 5% 5% 7% 8% 40%	2371
	11th to 20th	Day	12% 6% 2 5% 5% 5% 4 3 6% 8% 46%	2200
	21st to end	Day	11% 6% 2 5% 4 3 5% 4 5% 7% 48%	2200
October	1st to 10th	Day	10% 5% 1 5% 3 3 4 3 5% 6% 55%	2200
	11th to 20th	Day	11% 6% 2 5% 3 3 3 2 4 6% 54%	2155
	21st to end	Day	17% 5% 2 6% 4 4 4 3 4 5% 47%	1995
November	1st to 10th	Day	12% 5% 2 4 3 3 3 3 4 5% 54%	1780
	11th to 20th	Day	14% 7% 3 5% 3 4 4 3 5% 8% 47%	1800
	21st to end	Day	14% 8% 2 6% 4 3 5% 3 4 6% 45%	1800
December	1st to 10th	Day	15% 8% 2 6% 4 4 4 3 5% 5% 44%	2930
	11th to 20th	Day	20% 10% 4 8% 5% 4 4 3 4 6% 31%	3373
	21st to end	Day	17% 10% 4 8% 5% 5% 5% 4 6% 7% 27%	4229
January	1st to 10th	Night	15% 10% 4 11% 4 3 4 3 5% 6% 34%	6348
	11th to 20th	Night	17% 10% 3 8% 4 3 3 2 3 4 42%	6178
	21st to end	Night	18% 11% 4 7% 4 3 3 2 4 6% 39%	6053
February	1st to 10th	Night	20% 11% 3 8% 4 4 4 4 4 4 34%	2400
	11th to 20th	Night	22% 9% 4 8% 5% 3 3 2 4 5% 35%	2338
	21st to end	Night	22% 9% 3 7% 5% 4 3 3 5% 4 35%	1980
March	1st to 10th	Night	20% 10% 3 6% 4 4 4 3 4 7% 35%	5373
	11th to 20th	Night	11% 6% 2 5% 4 4 4 4 6% 6% 49%	5196
	21st to end	Night	13% 8% 3 7% 5% 4 4 3 5% 7% 41%	5192
April	1st to 10th	Night	11% 6% 2 7% 4 4 4 4 6% 6% 45%	2000
	11th to 20th	Night	14% 7% 2 4 3 4 4 3 5% 7% 47%	1975
	21st to end	Night	12% 7% 3 6% 4 4 4 4 6% 8% 42%	1960
May	1st to 10th	Night	13% 9% 4 9% 6% 5% 5% 5% 6% 7% 32%	4033
	11th to 20th	Night	14% 9% 3 9% 6% 5% 5% 4 6% 8% 32%	4032
	21st to end	Night	12% 7% 2 8% 5% 5% 5% 4 6% 10% 37%	4421
June	1st to 10th	Night	8% 7% 2 7% 4 5% 4 5% 6% 8% 46%	1781
	11th to 20th	Night	3 4 1 4 4 4 4 4 9% 11% 51%	1600
	21st to end	Night	3 3 1 5% 4 4 6% 5% 8% 12% 49%	1600
July	1st to 10th	Night	7% 5% 2 6% 5% 5% 5% 5% 8% 12% 42%	3618
	11th to 20th	Night	5% 5% 2 6% 4 5% 6% 5% 8% 15% 40%	3735
	21st to end	Night	8% 7% 3 9% 5% 6% 6% 6% 8% 12% 30%	4427
August	1st to 10th	Night	4 3 2 5% 3 4 3 4 5% 8% 59%	3110
	11th to 20th	Night	4 4 2 5% 4 5% 6% 4 7% 12% 48%	3200
	21st to end	Night	5% 3 1 4 3 4 4 4 6% 9% 57%	3586
September	1st to 10th	Night	9% 6% 3 6% 4 4 4 4 5% 6% 50%	2007
	11th to 20th	Night	10% 5% 2 6% 3 5% 4 3 6% 7% 48%	2200
	21st to end	Night	11% 4 2 5% 4 2 4 3 6% 6% 52%	2200
October	1st to 10th	Night	9% 4 2 4 2 2 3 2 4 5% 62%	2200
	11th to 20th	Night	14% 5% 2 5% 3 3 3 3 4 5% 52%	2241
	21st to end	Night	16% 6% 2 5% 4 3 4 3 4 6% 49%	2844
November	1st to 10th	Night	12% 5% 2 5% 3 4 4 2 5% 5% 52%	2563
	11th to 20th	Night	13% 6% 2 6% 3 3 3 4 5% 6% 49%	2600
	21st to end	Night	16% 7% 3 5% 4 3 3 3 4 6% 46%	2600
December	1st to 10th	Night	17% 11% 3 8% 4 3 3 3 4 4 40%	4250
	11th to 20th	Night	22% 9% 3 6% 3 3 3 2 3 5% 40%	4822
	21st to end	Night	18% 11% 5% 10% 6% 5% 5% 4 5% 5% 28%	6702

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	54% 5% 10% 18% 11% 1	3618
	11th to 20th	Day	47% 4 5% 23% 20% 1	3773
	21st to end	Day	47% 4 7% 22% 16% 4	4898
February	1st to 10th	Day	52% 4 8% 22% 12% 2	2000
	11th to 20th	Day	55% 3 7% 19% 13% 3	1958
	21st to end	Day	48% 3 8% 18% 16% 6%	1650
March	1st to 10th	Day	51% 4 5% 15% 16% 9%	4480
	11th to 20th	Day	40% 4 7% 22% 21% 7%	4750
	21st to end	Day	42% 5% 8% 30% 15% 1	5780
April	1st to 10th	Day	38% 6% 11% 23% 20% 3	2400
	11th to 20th	Day	41% 5% 6% 22% 22% 5%	2374
	21st to end	Day	42% 5% 9% 26% 15% 2	2440
May	1st to 10th	Day	48% 9% 14% 18% 10% 1	5897
	11th to 20th	Day	50% 7% 13% 19% 10%	5865
	21st to end	Day	43% 7% 8% 22% 17% 2	6447
June	1st to 10th	Day	36% 9% 11% 23% 17% 3	2612
	11th to 20th	Day	31% 8% 16% 24% 19% 3	2800
	21st to end	Day	32% 9% 17% 22% 16% 5%	2800
July	1st to 10th	Day	36% 9% 16% 26% 12% 1	6291
	11th to 20th	Day	37% 11% 19% 25% 7% 1	6157
	21st to end	Day	46% 9% 19% 20% 6% 1	6240
August	1st to 10th	Day	27% 7% 14% 33% 18% 1	4008
	11th to 20th	Day	35% 8% 13% 27% 16% 2	3885
	21st to end	Day	28% 6% 9% 31% 23% 2	4311
September	1st to 10th	Day	39% 8% 11% 21% 18% 4	2371
	11th to 20th	Day	37% 6% 9% 24% 22% 2	2200
	21st to end	Day	34% 5% 10% 25% 22% 5%	2200
October	1st to 10th	Day	22% 4 5% 19% 42% 9%	3590
	11th to 20th	Day	23% 4 7% 25% 29% 12%	3486
	21st to end	Day	31% 3 6% 24% 29% 6%	3195
November	1st to 10th	Day	33% 3 8% 24% 26% 5%	1780
	11th to 20th	Day	37% 3 7% 24% 23% 6%	1800
	21st to end	Day	41% 3 7% 26% 19% 4	1800
December	1st to 10th	Day	42% 4 6% 28% 20% 1	2930
	11th to 20th	Day	54% 4 8% 20% 12% 2	3370
	21st to end	Day	53% 6% 10% 23% 7% 1	4229
January	1st to 10th	Night	49% 5% 9% 21% 14% 1	6348
	11th to 20th	Night	48% 3 4 20% 20% 4	6178
	21st to end	Night	49% 3 5% 22% 16% 6%	6053
February	1st to 10th	Night	53% 4 9% 18% 15% 1	2400
	11th to 20th	Night	53% 2 6% 19% 16% 4	2339
	21st to end	Night	52% 4 7% 16% 17% 5%	1980
March	1st to 10th	Night	50% 4 5% 16% 18% 6%	5373
	11th to 20th	Night	35% 4 7% 21% 20% 13%	5196
	21st to end	Night	43% 4 8% 27% 16% 2	5192
April	1st to 10th	Night	38% 5% 10% 25% 21% 3	2000
	11th to 20th	Night	37% 4 8% 22% 22% 8%	1975
	21st to end	Night	39% 5% 10% 26% 18% 2	1960
May	1st to 10th	Night	48% 8% 12% 21% 10% 1	4060
	11th to 20th	Night	48% 9% 14% 17% 12%	4033
	21st to end	Night	41% 8% 12% 23% 14% 1	4429
June	1st to 10th	Night	33% 8% 14% 26% 17% 3	1781
	11th to 20th	Night	24% 6% 17% 31% 19% 3	1600
	21st to end	Night	25% 9% 17% 30% 16% 4	1600
July	1st to 10th	Night	32% 9% 18% 27% 13% 2	3580
	11th to 20th	Night	30% 10% 24% 27% 7% 2	3690
	21st to end	Night	44% 9% 20% 20% 6% 1	4297
August	1st to 10th	Night	22% 6% 10% 38% 23% 1	3099
	11th to 20th	Night	26% 7% 20% 32% 14% 1	3230
	21st to end	Night	23% 6% 12% 33% 23% 3	3586
September	1st to 10th	Night	34% 6% 11% 24% 20% 5%	2007
	11th to 20th	Night	35% 6% 10% 22% 26% 2	2200
	21st to end	Night	31% 6% 10% 22% 25% 7%	2200
October	1st to 10th	Night	20% 4 5% 19% 40% 11%	3590
	11th to 20th	Night	21% 3 6% 23% 33% 14%	3649
	21st to end	Night	26% 3 7% 24% 31% 10%	4594
November	1st to 10th	Night	35% 4 7% 23% 24% 7%	2563
	11th to 20th	Night	35% 4 6% 22% 27% 7%	2600
	21st to end	Night	40% 4 7% 25% 20% 5%	2600
December	1st to 10th	Night	48% 4 6% 18% 23% 2	4250
	11th to 20th	Night	48% 3 5% 23% 19% 2	4824
	21st to end	Night	56% 5% 8% 21% 9% 1	6702

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	24% 19% 36% 13% 7% 1	300
	11th to 20th	Day	31% 21% 30% 13% 4 1	300
	21st to end	Day	22% 21% 39% 14% 4	330
February	1st to 10th	Day	26% 20% 33% 17% 3 1	300
	11th to 20th	Day	39% 20% 29% 10% 2 1	300
	21st to end	Day	34% 17% 35% 12% 2	249
March	1st to 10th	Day	26% 20% 33% 16% 4 1	300
	11th to 20th	Day	24% 21% 33% 15% 6% 1	400
	21st to end	Day	23% 20% 34% 16% 7% 1	550
April	1st to 10th	Day	21% 18% 27% 25% 7% 2	500
	11th to 20th	Day	22% 13% 30% 25% 9% 2	492
	21st to end	Day	17% 16% 33% 20% 11% 3	500
May	1st to 10th	Day	22% 12% 27% 23% 12% 4	503
	11th to 20th	Day	17% 8% 27% 25% 15% 7%	527
	21st to end	Day	12% 6% 22% 28% 20% 12%	573
June	1st to 10th	Day	11% 6% 23% 24% 23% 15%	542
	11th to 20th	Day	7% 4 19% 28% 29% 12%	730
	21st to end	Day	6% 3 19% 34% 23% 14%	730
July	1st to 10th	Day	9% 2 14% 31% 28% 16%	721
	11th to 20th	Day	8% 3 11% 28% 34% 15%	668
	21st to end	Day	9% 3 12% 27% 30% 20%	576
August	1st to 10th	Day	11% 2 12% 28% 33% 14%	526
	11th to 20th	Day	11% 3 13% 32% 31% 10%	519
	21st to end	Day	11% 2 12% 23% 33% 18%	583
September	1st to 10th	Day	11% 2 14% 29% 31% 13%	530
	11th to 20th	Day	14% 3 9% 25% 32% 16%	530
	21st to end	Day	16% 6% 16% 27% 27% 8%	530
October	1st to 10th	Day	14% 4 18% 29% 26% 8%	530
	11th to 20th	Day	14% 12% 28% 27% 15% 4	485
	21st to end	Day	24% 12% 28% 28% 7% 1	363
November	1st to 10th	Day	18% 12% 29% 26% 12% 3	306
	11th to 20th	Day	17% 16% 37% 24% 6%	300
	21st to end	Day	21% 15% 34% 20% 9% 2	300
December	1st to 10th	Day	27% 18% 32% 17% 6%	300
	11th to 20th	Day	22% 18% 31% 18% 10% 1	300
	21st to end	Day	26% 19% 32% 18% 5%	330
January	1st to 10th	Night	22% 16% 30% 27% 5%	1000
	11th to 20th	Night	27% 15% 30% 23% 5%	922
	21st to end	Night	21% 13% 36% 27% 3	880
February	1st to 10th	Night	24% 13% 33% 25% 4	800
	11th to 20th	Night	29% 15% 30% 23% 2	783
	21st to end	Night	29% 16% 32% 22% 2	660
March	1st to 10th	Night	23% 15% 30% 26% 7%	800
	11th to 20th	Night	21% 11% 33% 28% 7% 1	800
	21st to end	Night	19% 13% 33% 27% 8%	760
April	1st to 10th	Night	16% 11% 34% 30% 8% 1	600
	11th to 20th	Night	17% 12% 34% 27% 9% 1	593
	21st to end	Night	16% 10% 32% 29% 11% 1	600
May	1st to 10th	Night	15% 7% 33% 28% 17% 1	600
	11th to 20th	Night	14% 5% 28% 32% 19% 3	591
	21st to end	Night	8% 3 20% 36% 28% 4	650
June	1st to 10th	Night	12% 3 16% 36% 30% 4	600
	11th to 20th	Night	5% 2 14% 40% 34% 6%	600
	21st to end	Night	5% 1 14% 37% 35% 7%	600
July	1st to 10th	Night	8% 1 11% 32% 36% 11%	585
	11th to 20th	Night	8% 1 10% 29% 42% 10%	582
	21st to end	Night	8% 2 9% 32% 42% 8%	657
August	1st to 10th	Night	7% 2 8% 32% 45% 6%	597
	11th to 20th	Night	6% 2 10% 37% 38% 6%	594
	21st to end	Night	9% 2 8% 31% 42% 8%	660
September	1st to 10th	Night	11% 2 11% 30% 41% 5%	607
	11th to 20th	Night	12% 2 12% 31% 38% 6%	800
	21st to end	Night	13% 2 17% 31% 34% 4	800
October	1st to 10th	Night	11% 5% 17% 31% 30% 6%	800
	11th to 20th	Night	17% 7% 23% 33% 20% 1	800
	21st to end	Night	18% 6% 28% 29% 17% 1	880
November	1st to 10th	Night	15% 8% 20% 36% 18% 3	789
	11th to 20th	Night	15% 10% 31% 31% 13%	800
	21st to end	Night	19% 11% 26% 31% 12% 1	800
December	1st to 10th	Night	28% 10% 27% 28% 6% 1	800
	11th to 20th	Night	23% 9% 30% 30% 8% 1	800
	21st to end	Night	27% 12% 29% 27% 5%	975

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