Shelbyville
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Shelbyville clear sky chart from 2005-11-06 to 2026-05-02.

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% 9% 3 8% 5% 4 4 4 6% 7% 36%	131243
	Night	15% 10% 3 8% 5% 4 4 3 5% 7% 37%	131412
Transparency	Day	45% 6% 11% 23% 13% 2	134871
	Night	46% 5% 11% 22% 14% 2	135858
Seeing	Day	16% 9% 25% 24% 17% 8%	17471
	Night	18% 8% 28% 30% 15% 2	27367

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	15% 14% 7% 12% 6% 5% 4 3 5% 7% 22%	4691
	11th to 20th	Day	11% 13% 6% 14% 8% 7% 6% 3 4 4 24%	4710
	21st to end	Day	18% 14% 7% 12% 6% 6% 5% 3 4 5% 20%	5551
February	1st to 10th	Day	17% 15% 5% 9% 5% 5% 3 3 4 6% 26%	2084
	11th to 20th	Day	16% 12% 3 11% 6% 5% 4 3 5% 6% 28%	2048
	21st to end	Day	17% 12% 4 11% 4 3 4 3 4 6% 32%	1730
March	1st to 10th	Day	20% 14% 6% 8% 5% 3 3 2 4 4 32%	4580
	11th to 20th	Day	12% 9% 3 6% 4 4 5% 4 6% 7% 41%	4965
	21st to end	Day	16% 11% 4 8% 5% 4 5% 3 5% 7% 31%	5759
April	1st to 10th	Day	15% 11% 3 8% 5% 5% 4 3 5% 7% 33%	2520
	11th to 20th	Day	17% 8% 3 7% 5% 4 4 4 6% 6% 37%	2494
	21st to end	Day	19% 8% 3 7% 5% 4 4 4 6% 7% 35%	2715
May	1st to 10th	Day	19% 9% 3 8% 5% 5% 5% 4 6% 7% 29%	5875
	11th to 20th	Day	20% 8% 3 7% 4 4 4 3 6% 6% 35%	5861
	21st to end	Day	13% 6% 3 8% 5% 5% 5% 5% 7% 10% 33%	6433
June	1st to 10th	Day	11% 6% 2 6% 5% 5% 5% 4 6% 8% 43%	2795
	11th to 20th	Day	9% 6% 2 6% 4 5% 4 4 7% 8% 45%	2800
	21st to end	Day	9% 6% 2 6% 4 5% 5% 5% 7% 9% 41%	2800
July	1st to 10th	Day	11% 6% 2 6% 4 4 5% 4 6% 11% 42%	6360
	11th to 20th	Day	10% 5% 2 6% 4 5% 5% 5% 7% 11% 39%	6360
	21st to end	Day	11% 7% 3 7% 5% 6% 6% 5% 7% 9% 34%	6606
August	1st to 10th	Day	7% 5% 2 5% 4 4 5% 4 8% 10% 46%	4234
	11th to 20th	Day	9% 5% 2 6% 5% 4 5% 4 8% 11% 41%	4023
	21st to end	Day	5% 4 2 5% 4 4 4 4 6% 10% 54%	4312
September	1st to 10th	Day	11% 5% 2 5% 4 4 4 3 6% 7% 48%	2345
	11th to 20th	Day	9% 5% 1 5% 4 4 4 3 6% 8% 49%	2200
	21st to end	Day	14% 6% 2 6% 4 3 4 2 5% 7% 48%	2200
October	1st to 10th	Day	12% 5% 2 6% 4 4 4 3 4 7% 51%	2200
	11th to 20th	Day	12% 7% 2 6% 3 4 5% 4 5% 7% 46%	2060
	21st to end	Day	19% 9% 3 7% 4 4 4 3 5% 6% 36%	1980
November	1st to 10th	Day	8% 7% 2 6% 3 4 5% 3 4 7% 50%	1852
	11th to 20th	Day	15% 12% 3 7% 4 3 4 3 6% 6% 36%	1890
	21st to end	Day	19% 12% 4 8% 4 3 3 3 4 6% 33%	1890
December	1st to 10th	Day	16% 12% 4 8% 4 3 3 2 4 5% 39%	3020
	11th to 20th	Day	16% 15% 5% 11% 5% 5% 5% 3 4 5% 25%	3230
	21st to end	Day	25% 18% 6% 9% 5% 5% 4 3 5% 5% 16%	4070
January	1st to 10th	Night	16% 14% 6% 12% 6% 5% 4 3 5% 5% 23%	8232
	11th to 20th	Night	10% 17% 6% 14% 7% 6% 5% 4 4 5% 23%	8280
	21st to end	Night	18% 12% 4 8% 5% 5% 4 3 5% 6% 30%	8626
February	1st to 10th	Night	19% 12% 5% 9% 6% 4 4 3 5% 5% 28%	2531
	11th to 20th	Night	15% 11% 5% 9% 5% 5% 5% 3 5% 5% 32%	2458
	21st to end	Night	20% 11% 5% 8% 5% 4 3 4 5% 6% 31%	2076
March	1st to 10th	Night	20% 13% 3 6% 4 3 3 2 4 5% 38%	5493
	11th to 20th	Night	13% 7% 3 7% 4 4 4 3 5% 7% 45%	5213
	21st to end	Night	17% 10% 3 7% 5% 4 4 4 6% 7% 34%	5453
April	1st to 10th	Night	15% 9% 2 7% 5% 4 4 3 6% 7% 38%	2100
	11th to 20th	Night	14% 8% 3 7% 5% 5% 3 4 5% 7% 40%	2075
	21st to end	Night	16% 8% 2 6% 4 4 4 4 6% 8% 37%	1903
May	1st to 10th	Night	20% 9% 3 8% 5% 4 5% 4 5% 7% 29%	4042
	11th to 20th	Night	16% 8% 2 6% 5% 4 4 4 6% 8% 37%	4032
	21st to end	Night	14% 7% 2 7% 5% 4 4 4 6% 8% 37%	4421
June	1st to 10th	Night	9% 6% 2 7% 5% 4 3 4 6% 8% 47%	1600
	11th to 20th	Night	9% 7% 2 5% 4 4 4 5% 6% 9% 45%	1600
	21st to end	Night	7% 4 1 5% 4 4 5% 4 7% 11% 48%	1600
July	1st to 10th	Night	9% 5% 2 6% 4 4 4 4 6% 8% 48%	3618
	11th to 20th	Night	11% 4 2 6% 5% 4 4 4 7% 9% 45%	3617
	21st to end	Night	10% 7% 2 7% 4 4 5% 4 8% 11% 39%	4248
August	1st to 10th	Night	8% 5% 2 7% 4 4 4 3 5% 8% 50%	2915
	11th to 20th	Night	7% 5% 2 7% 4 4 6% 4 6% 8% 47%	3022
	21st to end	Night	4 4 2 5% 4 4 4 4 6% 8% 56%	3586
September	1st to 10th	Night	10% 5% 2 6% 3 5% 5% 3 4 7% 50%	2033
	11th to 20th	Night	7% 4 1 5% 3 3 4 4 5% 7% 56%	2200
	21st to end	Night	14% 6% 2 6% 4 4 3 3 4 6% 49%	2200
October	1st to 10th	Night	9% 4 2 5% 4 3 3 3 5% 6% 57%	2200
	11th to 20th	Night	15% 7% 2 6% 3 4 4 3 5% 5% 47%	2335
	21st to end	Night	18% 7% 2 7% 4 4 4 3 5% 7% 40%	2860
November	1st to 10th	Night	12% 8% 3 5% 4 3 3 3 4 6% 48%	2666
	11th to 20th	Night	15% 9% 3 6% 4 4 4 4 5% 7% 40%	2730
	21st to end	Night	19% 12% 4 8% 5% 4 3 3 4 5% 32%	2730
December	1st to 10th	Night	17% 11% 3 8% 4 4 4 3 4 5% 38%	4380
	11th to 20th	Night	18% 14% 6% 10% 5% 4 4 3 4 5% 27%	5187
	21st to end	Night	27% 18% 5% 10% 5% 4 3 3 4 4 17%	7150

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	61% 4 11% 18% 5%	4692
	11th to 20th	Day	61% 5% 10% 14% 9% 2	4710
	21st to end	Day	66% 3 8% 14% 6% 2	5551
February	1st to 10th	Day	58% 4 10% 18% 9% 1	2084
	11th to 20th	Day	55% 5% 10% 20% 10% 1	2058
	21st to end	Day	53% 5% 9% 22% 9% 2	1730
March	1st to 10th	Day	56% 4 6% 19% 13% 2	4580
	11th to 20th	Day	41% 5% 11% 30% 13% 1	4965
	21st to end	Day	52% 6% 12% 20% 10% 1	5761
April	1st to 10th	Day	49% 6% 12% 20% 13% 1	2520
	11th to 20th	Day	46% 6% 12% 22% 12% 3	2494
	21st to end	Day	47% 5% 11% 23% 11% 2	2715
May	1st to 10th	Day	51% 8% 11% 20% 9%	5910
	11th to 20th	Day	48% 7% 10% 25% 10%	5865
	21st to end	Day	42% 9% 14% 21% 13% 1	6447
June	1st to 10th	Day	38% 7% 11% 24% 16% 3	2795
	11th to 20th	Day	36% 8% 11% 24% 19% 2	2800
	21st to end	Day	37% 9% 15% 20% 16% 4	2800
July	1st to 10th	Day	35% 7% 11% 25% 19% 1	6291
	11th to 20th	Day	36% 9% 15% 25% 13% 1	6285
	21st to end	Day	43% 10% 13% 19% 12% 2	6419
August	1st to 10th	Day	29% 9% 17% 31% 13% 1	4221
	11th to 20th	Day	33% 9% 14% 30% 14% 1	4062
	21st to end	Day	26% 7% 14% 31% 18% 4	4311
September	1st to 10th	Day	35% 6% 11% 25% 17% 4	2345
	11th to 20th	Day	31% 6% 11% 26% 21% 4	2200
	21st to end	Day	37% 5% 10% 22% 25% 2	2200
October	1st to 10th	Day	23% 4 8% 31% 33% 2	3590
	11th to 20th	Day	30% 6% 9% 27% 26% 2	3341
	21st to end	Day	39% 4 10% 23% 23% 2	3180
November	1st to 10th	Day	35% 5% 11% 29% 17% 4	1852
	11th to 20th	Day	47% 4 10% 21% 14% 4	1890
	21st to end	Day	53% 4 8% 23% 11% 1	1890
December	1st to 10th	Day	49% 3 10% 27% 12%	3020
	11th to 20th	Day	61% 5% 8% 17% 8%	3227
	21st to end	Day	69% 5% 10% 10% 5%	4070
January	1st to 10th	Night	61% 5% 11% 17% 7%	8233
	11th to 20th	Night	63% 4 8% 12% 11% 2	8280
	21st to end	Night	54% 4 10% 22% 9% 1	8626
February	1st to 10th	Night	58% 4 11% 18% 8% 1	2531
	11th to 20th	Night	52% 4 12% 19% 12% 1	2459
	21st to end	Night	54% 5% 10% 18% 10% 2	2076
March	1st to 10th	Night	50% 4 6% 26% 11% 3	5493
	11th to 20th	Night	38% 5% 11% 27% 18% 1	5213
	21st to end	Night	49% 5% 9% 27% 8% 1	5453
April	1st to 10th	Night	44% 5% 11% 25% 13% 2	2100
	11th to 20th	Night	44% 5% 11% 20% 17% 5%	2075
	21st to end	Night	43% 7% 11% 24% 13% 3	1903
May	1st to 10th	Night	52% 8% 10% 18% 8% 3	4069
	11th to 20th	Night	44% 7% 12% 25% 11% 1	4033
	21st to end	Night	41% 9% 15% 20% 11% 3	4429
June	1st to 10th	Night	34% 7% 12% 23% 21% 3	1600
	11th to 20th	Night	33% 8% 10% 23% 22% 4	1600
	21st to end	Night	28% 9% 16% 23% 20% 4	1600
July	1st to 10th	Night	34% 7% 10% 26% 19% 5%	3580
	11th to 20th	Night	33% 9% 16% 26% 15% 1	3572
	21st to end	Night	37% 9% 14% 25% 13% 3	4123
August	1st to 10th	Night	31% 7% 14% 34% 12% 1	2904
	11th to 20th	Night	32% 8% 12% 28% 18% 1	3049
	21st to end	Night	24% 7% 13% 29% 23% 4	3586
September	1st to 10th	Night	32% 7% 11% 25% 19% 7%	2033
	11th to 20th	Night	26% 6% 10% 27% 27% 4	2200
	21st to end	Night	36% 6% 10% 21% 24% 3	2200
October	1st to 10th	Night	20% 4 7% 24% 43% 3	3590
	11th to 20th	Night	28% 4 8% 24% 34% 2	3793
	21st to end	Night	35% 4 8% 23% 25% 4	4610
November	1st to 10th	Night	38% 5% 10% 23% 22% 3	2666
	11th to 20th	Night	43% 4 10% 22% 17% 3	2730
	21st to end	Night	54% 5% 10% 20% 11% 1	2730
December	1st to 10th	Night	48% 4 11% 25% 11%	4380
	11th to 20th	Night	59% 4 10% 16% 10% 1	5189
	21st to end	Night	71% 5% 9% 11% 4	7150

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	20% 17% 32% 24% 6% 1	330
	11th to 20th	Day	24% 20% 32% 19% 5%	330
	21st to end	Day	27% 13% 34% 21% 4	363
February	1st to 10th	Day	26% 13% 33% 22% 5%	330
	11th to 20th	Day	23% 16% 37% 20% 5%	330
	21st to end	Day	24% 12% 42% 18% 3	273
March	1st to 10th	Day	21% 20% 38% 17% 4	330
	11th to 20th	Day	25% 14% 37% 17% 7% 1	480
	21st to end	Day	26% 15% 32% 19% 6% 1	594
April	1st to 10th	Day	19% 16% 33% 23% 8% 2	540
	11th to 20th	Day	19% 15% 30% 23% 10% 3	532
	21st to end	Day	20% 13% 34% 21% 9% 2	540
May	1st to 10th	Day	20% 13% 28% 24% 10% 3	507
	11th to 20th	Day	21% 8% 25% 24% 15% 7%	527
	21st to end	Day	12% 6% 25% 25% 18% 13%	573
June	1st to 10th	Day	11% 7% 23% 30% 21% 8%	725
	11th to 20th	Day	11% 4 21% 31% 24% 9%	730
	21st to end	Day	11% 4 23% 28% 24% 10%	730
July	1st to 10th	Day	11% 3 13% 26% 26% 21%	721
	11th to 20th	Day	8% 4 16% 31% 28% 14%	718
	21st to end	Day	10% 2 12% 28% 31% 15%	654
August	1st to 10th	Day	7% 3 13% 25% 32% 20%	526
	11th to 20th	Day	13% 3 13% 26% 29% 15%	519
	21st to end	Day	8% 3 11% 25% 36% 18%	583
September	1st to 10th	Day	8% 4 16% 26% 34% 12%	530
	11th to 20th	Day	8% 2 11% 27% 30% 21%	530
	21st to end	Day	16% 7% 20% 21% 23% 13%	530
October	1st to 10th	Day	10% 5% 21% 33% 23% 9%	530
	11th to 20th	Day	10% 12% 30% 28% 16% 3	490
	21st to end	Day	27% 10% 36% 20% 6% 1	363
November	1st to 10th	Day	14% 13% 32% 26% 12% 4	330
	11th to 20th	Day	19% 18% 30% 19% 12% 2	330
	21st to end	Day	24% 12% 34% 24% 6% 1	330
December	1st to 10th	Day	18% 17% 37% 20% 6% 2	330
	11th to 20th	Day	20% 14% 39% 22% 5% 1	330
	21st to end	Day	28% 15% 33% 18% 5%	363
January	1st to 10th	Night	22% 12% 34% 27% 5%	1050
	11th to 20th	Night	24% 12% 38% 23% 4	1050
	21st to end	Night	22% 12% 35% 27% 3	1006
February	1st to 10th	Night	25% 9% 36% 27% 3	840
	11th to 20th	Night	22% 12% 38% 26% 2	823
	21st to end	Night	24% 10% 40% 24% 2	692
March	1st to 10th	Night	22% 12% 35% 26% 5%	840
	11th to 20th	Night	20% 12% 36% 26% 6%	840
	21st to end	Night	23% 8% 34% 29% 5% 1	866
April	1st to 10th	Night	17% 11% 38% 27% 7%	630
	11th to 20th	Night	16% 10% 34% 30% 10%	623
	21st to end	Night	21% 5% 37% 27% 9%	630
May	1st to 10th	Night	21% 6% 32% 31% 10% 1	603
	11th to 20th	Night	16% 7% 24% 32% 19% 2	591
	21st to end	Night	9% 4 24% 36% 26% 2	650
June	1st to 10th	Night	10% 3 23% 42% 21% 1	600
	11th to 20th	Night	10% 4 18% 42% 24% 2	600
	21st to end	Night	7% 2 19% 40% 31% 2	600
July	1st to 10th	Night	9% 2 10% 35% 36% 8%	585
	11th to 20th	Night	8% 4 11% 38% 34% 5%	582
	21st to end	Night	9% 3 12% 39% 34% 4	657
August	1st to 10th	Night	7% 2 11% 37% 39% 4	597
	11th to 20th	Night	7% 2 10% 40% 36% 5%	594
	21st to end	Night	5% 2 10% 37% 41% 5%	660
September	1st to 10th	Night	12% 3 10% 36% 35% 5%	633
	11th to 20th	Night	8% 3 12% 35% 37% 5%	800
	21st to end	Night	15% 5% 18% 34% 25% 3	800
October	1st to 10th	Night	11% 5% 20% 35% 25% 4	800
	11th to 20th	Night	17% 8% 28% 30% 16% 2	800
	21st to end	Night	21% 9% 32% 30% 8%	880
November	1st to 10th	Night	15% 9% 32% 29% 14% 1	820
	11th to 20th	Night	18% 12% 33% 26% 10% 1	840
	21st to end	Night	24% 11% 28% 30% 6%	840
December	1st to 10th	Night	23% 10% 38% 24% 5%	840
	11th to 20th	Night	25% 7% 35% 26% 6% 1	950
	21st to end	Night	31% 12% 32% 23% 3	1155

Page generated at 2026-04-29 02:21:42. Version ac07

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