Charleston
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Charleston clear sky chart from 2005-11-06 to 2026-10-29.

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	11% 5% 2 6% 4 4 5% 4 7% 9% 42%	149102
	Night	10% 6% 2 6% 4 4 4 4 6% 9% 44%	142997
Transparency	Day	25% 35% 13% 14% 10% 3	152721
	Night	25% 35% 11% 13% 12% 5%	147451
Seeing	Day	11% 5% 19% 28% 27% 10%	18288
	Night	12% 6% 20% 29% 26% 7%	28337

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	11% 5% 2 5% 3 3 4 3 5% 8% 51%	5258
	11th to 20th	Day	13% 5% 2 6% 4 3 3 3 5% 8% 49%	5280
	21st to end	Day	16% 8% 2 6% 4 4 4 3 5% 7% 42%	5780
February	1st to 10th	Day	13% 6% 2 6% 4 4 5% 4 5% 8% 42%	1890
	11th to 20th	Day	18% 7% 2 6% 3 3 4 3 4 5% 45%	2002
	21st to end	Day	13% 7% 3 6% 4 3 4 4 4 5% 46%	1899
March	1st to 10th	Day	21% 9% 3 7% 4 3 3 3 5% 5% 37%	5045
	11th to 20th	Day	8% 4 1 4 4 4 4 3 6% 7% 55%	5090
	21st to end	Day	13% 5% 2 6% 4 4 4 3 4 5% 50%	5586
April	1st to 10th	Day	7% 5% 1 5% 4 4 4 5% 5% 6% 53%	2337
	11th to 20th	Day	9% 3 2 5% 3 3 4 4 7% 7% 54%	2494
	21st to end	Day	6% 3 1 4 4 3 4 4 6% 9% 56%	2520
May	1st to 10th	Day	13% 6% 2 6% 4 4 5% 4 6% 10% 39%	7473
	11th to 20th	Day	12% 6% 2 5% 4 4 5% 4 7% 9% 42%	7626
	21st to end	Day	8% 4 2 7% 5% 5% 6% 5% 8% 11% 39%	8391
June	1st to 10th	Day	8% 6% 2 7% 5% 6% 8% 6% 8% 10% 33%	2730
	11th to 20th	Day	7% 4 1 6% 5% 6% 6% 6% 10% 12% 38%	2730
	21st to end	Day	6% 5% 2 6% 5% 6% 7% 7% 10% 13% 35%	2730
July	1st to 10th	Day	9% 5% 2 6% 5% 6% 7% 6% 9% 13% 32%	7680
	11th to 20th	Day	5% 4 2 7% 5% 6% 7% 6% 10% 14% 34%	7680
	21st to end	Day	5% 5% 2 7% 5% 6% 6% 6% 9% 15% 34%	8340
August	1st to 10th	Day	16% 7% 3 9% 6% 6% 6% 6% 9% 12% 20%	6014
	11th to 20th	Day	5% 4 2 6% 5% 6% 5% 5% 9% 14% 40%	5764
	21st to end	Day	11% 6% 3 8% 5% 5% 6% 5% 8% 10% 33%	6080
September	1st to 10th	Day	11% 6% 2 6% 5% 5% 5% 5% 7% 10% 37%	2410
	11th to 20th	Day	11% 6% 3 7% 4 5% 6% 5% 8% 9% 36%	2310
	21st to end	Day	13% 7% 3 7% 4 4 4 4 6% 8% 40%	2310
October	1st to 10th	Day	14% 7% 2 6% 3 5% 4 3 5% 5% 46%	2132
	11th to 20th	Day	9% 3 2 4 3 3 3 3 5% 7% 59%	2100
	21st to end	Day	9% 5% 2 5% 4 3 4 2 4 6% 57%	2280
November	1st to 10th	Day	10% 5% 2 4 4 3 4 3 5% 6% 54%	2057
	11th to 20th	Day	11% 6% 2 6% 4 4 3 3 5% 6% 50%	2100
	21st to end	Day	12% 5% 2 4 3 3 3 2 6% 7% 52%	1936
December	1st to 10th	Day	9% 5% 2 5% 3 4 5% 4 5% 6% 52%	3020
	11th to 20th	Day	14% 6% 2 6% 3 3 4 3 5% 6% 49%	3463
	21st to end	Day	17% 8% 3 6% 4 3 3 3 5% 6% 42%	4565
January	1st to 10th	Night	8% 5% 2 5% 4 4 4 4 5% 6% 53%	7635
	11th to 20th	Night	11% 6% 2 6% 4 4 4 3 5% 7% 47%	7680
	21st to end	Night	14% 7% 3 7% 5% 4 4 3 4 6% 43%	8401
February	1st to 10th	Night	14% 7% 3 6% 4 4 4 4 5% 6% 45%	2730
	11th to 20th	Night	16% 7% 3 6% 4 4 4 4 5% 6% 42%	2500
	21st to end	Night	12% 9% 3 7% 5% 4 4 3 5% 6% 42%	1904
March	1st to 10th	Night	16% 10% 3 7% 4 3 3 3 4 5% 41%	5028
	11th to 20th	Night	9% 5% 2 5% 3 3 3 3 6% 9% 52%	5090
	21st to end	Night	13% 5% 2 5% 3 3 3 4 6% 7% 50%	5588
April	1st to 10th	Night	9% 5% 2 5% 4 3 4 4 6% 8% 50%	2280
	11th to 20th	Night	11% 4 1 5% 4 3 4 4 5% 7% 53%	2075
	21st to end	Night	7% 4 2 5% 3 4 4 4 6% 9% 53%	2100
May	1st to 10th	Night	9% 6% 2 7% 4 4 5% 4 7% 9% 42%	5421
	11th to 20th	Night	12% 5% 2 6% 4 4 4 4 7% 9% 44%	5234
	21st to end	Night	9% 5% 2 7% 4 5% 6% 5% 8% 10% 38%	5761
June	1st to 10th	Night	8% 6% 2 7% 5% 6% 5% 5% 8% 12% 35%	1890
	11th to 20th	Night	5% 5% 3 5% 6% 5% 5% 6% 8% 12% 39%	1890
	21st to end	Night	4 5% 2 6% 5% 6% 6% 6% 10% 13% 37%	1890
July	1st to 10th	Night	7% 4 2 6% 5% 6% 7% 6% 10% 12% 35%	5278
	11th to 20th	Night	5% 5% 2 8% 5% 6% 6% 5% 8% 13% 37%	5277
	21st to end	Night	6% 5% 2 7% 5% 5% 7% 6% 9% 14% 34%	5736
August	1st to 10th	Night	14% 6% 3 7% 5% 5% 6% 5% 9% 15% 25%	4135
	11th to 20th	Night	5% 4 2 6% 5% 5% 6% 5% 9% 15% 39%	4234
	21st to end	Night	8% 6% 3 8% 6% 6% 5% 5% 7% 10% 38%	5045
September	1st to 10th	Night	9% 5% 2 7% 7% 5% 6% 4 7% 10% 38%	2205
	11th to 20th	Night	9% 7% 2 7% 5% 6% 5% 5% 7% 9% 38%	2310
	21st to end	Night	14% 6% 3 7% 4 4 3 4 7% 8% 39%	2310
October	1st to 10th	Night	14% 7% 2 6% 4 4 4 4 5% 7% 44%	2486
	11th to 20th	Night	9% 3 2 4 3 3 4 4 5% 7% 58%	2520
	21st to end	Night	9% 5% 2 5% 3 3 3 3 5% 5% 58%	2737
November	1st to 10th	Night	10% 5% 2 5% 3 3 3 3 5% 6% 55%	2461
	11th to 20th	Night	13% 6% 2 5% 3 3 3 2 5% 6% 51%	2520
	21st to end	Night	10% 5% 2 6% 4 4 4 4 6% 7% 46%	2681
December	1st to 10th	Night	10% 6% 2 5% 3 3 3 3 4 6% 54%	4380
	11th to 20th	Night	15% 7% 3 8% 5% 4 4 3 4 5% 43%	4952
	21st to end	Night	15% 7% 2 6% 4 3 4 3 5% 6% 44%	6633

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	27% 22% 7% 18% 22% 5%	5259
	11th to 20th	Day	29% 21% 3 13% 22% 11%	5280
	21st to end	Day	32% 24% 8% 15% 14% 7%	5780
February	1st to 10th	Day	17% 59% 4 10% 9% 2	1890
	11th to 20th	Day	18% 58% 5% 9% 9% 2	2012
	21st to end	Day	19% 59% 3 6% 8% 5%	1899
March	1st to 10th	Day	38% 30% 6% 9% 11% 5%	5045
	11th to 20th	Day	17% 29% 8% 11% 20% 15%	5090
	21st to end	Day	27% 30% 6% 15% 14% 8%	5586
April	1st to 10th	Day	14% 59% 6% 10% 6% 4	2337
	11th to 20th	Day	14% 59% 4 13% 7% 4	2494
	21st to end	Day	10% 59% 6% 15% 9% 2	2520
May	1st to 10th	Day	32% 28% 15% 18% 7% 1	7507
	11th to 20th	Day	29% 25% 15% 20% 10%	7630
	21st to end	Day	27% 29% 17% 15% 12%	8405
June	1st to 10th	Day	16% 60% 9% 9% 5% 1	2730
	11th to 20th	Day	12% 60% 12% 12% 4	2730
	21st to end	Day	13% 60% 14% 11% 2	2730
July	1st to 10th	Day	28% 28% 28% 15% 2	7615
	11th to 20th	Day	24% 31% 34% 10%	7611
	21st to end	Day	26% 27% 25% 18% 4	8156
August	1st to 10th	Day	44% 33% 18% 5%	6001
	11th to 20th	Day	16% 31% 24% 24% 5%	5803
	21st to end	Day	28% 31% 17% 22% 2	6080
September	1st to 10th	Day	17% 56% 11% 9% 6% 1	2410
	11th to 20th	Day	20% 56% 6% 12% 5% 1	2310
	21st to end	Day	20% 55% 10% 10% 6%	2310
October	1st to 10th	Day	31% 34% 5% 9% 17% 3	3405
	11th to 20th	Day	8% 34% 3 9% 25% 21%	3340
	21st to end	Day	14% 35% 5% 13% 29% 4	3618
November	1st to 10th	Day	18% 53% 5% 9% 12% 4	2057
	11th to 20th	Day	17% 53% 4 10% 11% 5%	2100
	21st to end	Day	14% 54% 5% 11% 12% 3	1936
December	1st to 10th	Day	21% 34% 5% 12% 16% 11%	3020
	11th to 20th	Day	27% 32% 5% 14% 18% 3	3460
	21st to end	Day	33% 27% 6% 19% 14% 1	4565
January	1st to 10th	Night	24% 22% 7% 15% 25% 6%	7636
	11th to 20th	Night	30% 21% 4 12% 24% 8%	7680
	21st to end	Night	34% 23% 6% 10% 18% 8%	8401
February	1st to 10th	Night	16% 58% 3 11% 9% 3	2730
	11th to 20th	Night	19% 58% 4 8% 9% 2	2501
	21st to end	Night	21% 58% 3 4 8% 6%	1904
March	1st to 10th	Night	36% 30% 6% 10% 13% 5%	5028
	11th to 20th	Night	20% 28% 6% 11% 21% 14%	5090
	21st to end	Night	24% 29% 7% 16% 15% 9%	5588
April	1st to 10th	Night	13% 59% 7% 10% 7% 4	2280
	11th to 20th	Night	14% 59% 4 12% 8% 4	2075
	21st to end	Night	12% 60% 5% 13% 8% 2	2100
May	1st to 10th	Night	31% 27% 15% 20% 6% 1	5450
	11th to 20th	Night	29% 26% 16% 18% 10% 2	5235
	21st to end	Night	30% 28% 14% 17% 10%	5771
June	1st to 10th	Night	15% 61% 10% 9% 4 1	1890
	11th to 20th	Night	13% 62% 11% 11% 3	1890
	21st to end	Night	13% 60% 13% 11% 2	1890
July	1st to 10th	Night	28% 28% 25% 17% 1	5235
	11th to 20th	Night	28% 29% 33% 9% 1	5226
	21st to end	Night	28% 27% 23% 18% 4	5606
August	1st to 10th	Night	37% 34% 22% 6%	4124
	11th to 20th	Night	19% 32% 22% 24% 4	4261
	21st to end	Night	27% 31% 14% 26% 2	5045
September	1st to 10th	Night	17% 56% 10% 11% 6% 1	2205
	11th to 20th	Night	20% 56% 8% 11% 5% 1	2310
	21st to end	Night	19% 55% 9% 12% 5%	2310
October	1st to 10th	Night	27% 34% 7% 11% 16% 5%	3993
	11th to 20th	Night	7% 34% 3 8% 21% 26%	4019
	21st to end	Night	14% 35% 4 14% 23% 10%	4349
November	1st to 10th	Night	18% 53% 5% 9% 11% 4	2461
	11th to 20th	Night	17% 53% 4 9% 11% 5%	2520
	21st to end	Night	15% 54% 5% 9% 12% 5%	2681
December	1st to 10th	Night	20% 34% 4 13% 20% 9%	4380
	11th to 20th	Night	31% 32% 7% 9% 15% 6%	4954
	21st to end	Night	31% 28% 7% 19% 14% 1	6633

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	10% 6% 38% 37% 8%	330
	11th to 20th	Day	9% 8% 35% 33% 12% 2	330
	21st to end	Day	19% 10% 39% 28% 5% 1	363
February	1st to 10th	Day	14% 9% 34% 32% 11%	330
	11th to 20th	Day	25% 8% 33% 27% 6%	330
	21st to end	Day	12% 10% 38% 31% 8% 2	442
March	1st to 10th	Day	13% 9% 33% 28% 16% 2	540
	11th to 20th	Day	13% 7% 33% 34% 11% 1	540
	21st to end	Day	12% 8% 35% 28% 15% 3	594
April	1st to 10th	Day	11% 9% 28% 32% 16% 3	540
	11th to 20th	Day	7% 7% 26% 28% 22% 9%	532
	21st to end	Day	6% 4 21% 30% 30% 9%	540
May	1st to 10th	Day	6% 6% 28% 33% 21% 7%	543
	11th to 20th	Day	12% 2 17% 30% 29% 10%	567
	21st to end	Day	9% 2 11% 31% 31% 15%	617
June	1st to 10th	Day	9% 2 12% 32% 32% 12%	570
	11th to 20th	Day	7% 2 11% 27% 33% 20%	570
	21st to end	Day	5% 2 8% 27% 41% 18%	570
July	1st to 10th	Day	7% 1 4 18% 48% 22%	565
	11th to 20th	Day	6% 1 4 14% 49% 26%	564
	21st to end	Day	5% 1 5% 26% 45% 18%	620
August	1st to 10th	Day	13% 2 7% 19% 41% 19%	566
	11th to 20th	Day	5% 2 5% 18% 43% 27%	559
	21st to end	Day	15% 1 6% 20% 38% 19%	627
September	1st to 10th	Day	10% 5% 6% 22% 42% 16%	570
	11th to 20th	Day	14% 3 9% 26% 35% 13%	570
	21st to end	Day	14% 2 9% 23% 38% 14%	570
October	1st to 10th	Day	20% 3 18% 29% 21% 10%	570
	11th to 20th	Day	9% 4 17% 31% 28% 11%	570
	21st to end	Day	10% 5% 18% 31% 30% 6%	615
November	1st to 10th	Day	15% 4 22% 28% 24% 7%	535
	11th to 20th	Day	15% 5% 27% 33% 18% 2	540
	21st to end	Day	13% 8% 24% 33% 21% 2	376
December	1st to 10th	Day	17% 7% 25% 38% 14%	330
	11th to 20th	Day	14% 7% 31% 29% 16% 3	330
	21st to end	Day	19% 7% 25% 32% 15% 2	363
January	1st to 10th	Night	12% 11% 33% 34% 9%	840
	11th to 20th	Night	14% 11% 32% 30% 12% 1	840
	21st to end	Night	16% 10% 37% 31% 5%	924
February	1st to 10th	Night	15% 9% 30% 35% 12%	840
	11th to 20th	Night	17% 11% 30% 35% 7%	823
	21st to end	Night	16% 9% 31% 35% 9% 1	692
March	1st to 10th	Night	11% 7% 31% 34% 15% 1	840
	11th to 20th	Night	11% 9% 27% 37% 14% 2	840
	21st to end	Night	10% 6% 31% 32% 18% 2	924
April	1st to 10th	Night	10% 10% 27% 31% 20% 3	840
	11th to 20th	Night	11% 7% 23% 30% 23% 6%	831
	21st to end	Night	7% 5% 21% 34% 29% 4	840
May	1st to 10th	Night	8% 4 21% 36% 24% 7%	699
	11th to 20th	Night	9% 4 19% 29% 31% 9%	621
	21st to end	Night	8% 1 11% 25% 38% 16%	683
June	1st to 10th	Night	9% 3 11% 30% 38% 9%	630
	11th to 20th	Night	6% 3 9% 22% 42% 19%	630
	21st to end	Night	5% 2 8% 27% 44% 15%	630
July	1st to 10th	Night	9% 1 5% 19% 50% 17%	615
	11th to 20th	Night	9% 1 2 16% 50% 22%	612
	21st to end	Night	6% 1 3 25% 50% 14%	690
August	1st to 10th	Night	11% 2 4 21% 46% 16%	627
	11th to 20th	Night	8% 2 6% 19% 44% 22%	685
	21st to end	Night	13% 2 6% 21% 40% 18%	924
September	1st to 10th	Night	10% 2 8% 24% 43% 14%	840
	11th to 20th	Night	12% 2 7% 27% 41% 11%	840
	21st to end	Night	16% 2 10% 23% 37% 13%	840
October	1st to 10th	Night	17% 3 15% 31% 25% 9%	840
	11th to 20th	Night	10% 4 21% 31% 28% 7%	840
	21st to end	Night	10% 6% 20% 30% 27% 6%	913
November	1st to 10th	Night	11% 6% 22% 34% 25% 4	820
	11th to 20th	Night	15% 8% 24% 33% 18% 2	840
	21st to end	Night	12% 9% 24% 35% 17% 3	840
December	1st to 10th	Night	16% 8% 27% 34% 14% 1	840
	11th to 20th	Night	17% 8% 27% 28% 18% 2	840
	21st to end	Night	17% 8% 29% 31% 15% 1	924

Page generated at 2026-10-26 04:37:51. Version ac07

Copyright © 2026 Attilla Danko.