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	12% 8% 3 8% 5% 4 4 4 6% 8% 38%	149311
	Night	14% 9% 3 7% 4 4 4 3 5% 7% 40%	143059
Transparency	Day	43% 6% 12% 23% 14% 2	152870
	Night	43% 5% 11% 23% 16% 2	147577
Seeing	Day	16% 10% 27% 25% 16% 6%	18370
	Night	17% 10% 29% 30% 13% 1	28108

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	12% 11% 6% 13% 6% 5% 4 4 5% 7% 28%	4691
	11th to 20th	Day	9% 10% 6% 14% 8% 7% 6% 5% 5% 5% 25%	4710
	21st to end	Day	17% 16% 5% 11% 6% 4 3 4 3 4 25%	5607
February	1st to 10th	Day	15% 12% 5% 10% 5% 5% 3 4 5% 6% 29%	2015
	11th to 20th	Day	15% 11% 4 9% 6% 5% 4 3 5% 6% 31%	2048
	21st to end	Day	15% 13% 4 9% 5% 4 3 2 4 6% 34%	1730
March	1st to 10th	Day	17% 15% 3 7% 4 3 3 2 3 4 39%	4580
	11th to 20th	Day	10% 11% 3 6% 4 3 4 3 5% 8% 45%	4792
	21st to end	Day	15% 11% 3 9% 5% 5% 5% 4 5% 7% 31%	6099
April	1st to 10th	Day	15% 9% 3 7% 5% 4 5% 4 5% 8% 35%	2520
	11th to 20th	Day	15% 8% 3 8% 4 4 4 4 5% 6% 39%	2494
	21st to end	Day	18% 8% 2 7% 4 4 4 3 5% 6% 38%	2562
May	1st to 10th	Day	17% 8% 3 8% 5% 5% 5% 4 6% 9% 31%	7616
	11th to 20th	Day	15% 7% 3 7% 5% 4 5% 3 5% 7% 39%	7677
	21st to end	Day	14% 8% 3 8% 5% 5% 6% 5% 7% 9% 31%	9047
June	1st to 10th	Day	9% 5% 2 5% 4 5% 5% 4 6% 9% 45%	2940
	11th to 20th	Day	8% 5% 2 6% 4 5% 5% 4 6% 8% 47%	2940
	21st to end	Day	8% 6% 2 6% 5% 4 5% 4 7% 10% 41%	2940
July	1st to 10th	Day	10% 5% 2 6% 4 5% 5% 4 7% 10% 43%	8280
	11th to 20th	Day	7% 6% 2 7% 5% 5% 5% 5% 7% 10% 42%	8280
	21st to end	Day	10% 6% 3 7% 5% 5% 5% 5% 8% 10% 37%	8990
August	1st to 10th	Day	11% 5% 2 5% 3 3 4 3 6% 9% 49%	5927
	11th to 20th	Day	9% 6% 3 7% 5% 6% 6% 5% 7% 11% 35%	5486
	21st to end	Day	4 3 2 4 4 3 4 3 6% 10% 57%	6080
September	1st to 10th	Day	11% 5% 2 5% 4 3 5% 4 5% 7% 47%	2520
	11th to 20th	Day	9% 4 2 5% 4 4 5% 4 6% 8% 49%	2378
	21st to end	Day	11% 5% 2 6% 4 4 4 4 5% 8% 48%	2310
October	1st to 10th	Day	10% 5% 2 5% 3 3 4 3 5% 7% 53%	2284
	11th to 20th	Day	15% 6% 2 6% 4 3 4 3 4 5% 48%	2100
	21st to end	Day	18% 8% 3 7% 4 3 4 2 4 6% 42%	2089
November	1st to 10th	Day	10% 6% 2 6% 4 4 2 3 4 5% 53%	1852
	11th to 20th	Day	16% 10% 3 7% 4 3 3 2 5% 6% 41%	1890
	21st to end	Day	17% 11% 4 8% 4 4 3 3 4 5% 37%	1890
December	1st to 10th	Day	11% 10% 4 7% 5% 4 3 3 5% 6% 44%	2789
	11th to 20th	Day	14% 14% 5% 11% 5% 4 4 4 5% 6% 28%	3088
	21st to end	Day	27% 17% 6% 12% 4 4 4 2 3 5% 16%	4070
January	1st to 10th	Night	14% 13% 6% 11% 4 4 4 3 4 5% 32%	8232
	11th to 20th	Night	11% 14% 5% 11% 5% 5% 6% 4 4 4 29%	8280
	21st to end	Night	17% 11% 5% 8% 5% 4 4 4 4 5% 31%	8605
February	1st to 10th	Night	18% 11% 4 9% 5% 4 4 3 4 5% 32%	2603
	11th to 20th	Night	16% 9% 4 8% 5% 4 3 3 5% 6% 37%	2458
	21st to end	Night	18% 9% 3 8% 4 4 4 3 4 6% 37%	2076
March	1st to 10th	Night	18% 15% 4 6% 4 3 3 2 4 5% 37%	5493
	11th to 20th	Night	12% 7% 2 5% 4 3 3 3 5% 7% 48%	5417
	21st to end	Night	18% 9% 3 8% 6% 5% 5% 3 5% 6% 33%	5072
April	1st to 10th	Night	14% 7% 3 7% 5% 4 4 3 4 7% 42%	2100
	11th to 20th	Night	16% 8% 2 6% 4 3 3 3 4 7% 44%	2075
	21st to end	Night	17% 8% 3 6% 5% 4 5% 4 6% 5% 36%	2055
May	1st to 10th	Night	19% 8% 3 7% 5% 4 4 3 5% 8% 34%	5237
	11th to 20th	Night	14% 6% 2 6% 4 4 5% 4 6% 8% 41%	5181
	21st to end	Night	14% 7% 3 8% 5% 5% 5% 4 6% 8% 34%	5139
June	1st to 10th	Night	8% 6% 3 5% 4 4 4 4 5% 7% 49%	1680
	11th to 20th	Night	8% 4 2 7% 4 4 4 4 5% 7% 52%	1680
	21st to end	Night	8% 5% 2 6% 4 4 4 4 7% 10% 46%	1680
July	1st to 10th	Night	9% 5% 1 6% 4 4 5% 4 6% 9% 47%	4708
	11th to 20th	Night	6% 5% 2 5% 4 5% 5% 4 6% 10% 48%	4707
	21st to end	Night	8% 6% 3 8% 5% 4 4 5% 7% 11% 39%	5117
August	1st to 10th	Night	10% 5% 2 5% 3 4 4 3 5% 8% 51%	4220
	11th to 20th	Night	7% 5% 2 6% 5% 4 5% 5% 7% 9% 46%	4560
	21st to end	Night	4 2 1 4 3 3 4 4 6% 9% 60%	5046
September	1st to 10th	Night	10% 5% 2 4 3 3 3 3 5% 7% 55%	2100
	11th to 20th	Night	6% 5% 2 5% 4 4 3 4 6% 8% 55%	2221
	21st to end	Night	12% 5% 2 5% 4 3 5% 3 6% 6% 49%	2310
October	1st to 10th	Night	8% 4 2 5% 3 3 3 3 5% 6% 58%	2333
	11th to 20th	Night	14% 6% 2 4 3 3 3 3 4 7% 51%	2520
	21st to end	Night	18% 8% 2 5% 4 4 4 3 5% 5% 43%	2926
November	1st to 10th	Night	10% 6% 2 6% 4 4 4 3 5% 6% 52%	2666
	11th to 20th	Night	13% 8% 3 6% 4 3 3 3 5% 6% 44%	2730
	21st to end	Night	19% 10% 3 7% 4 3 3 2 4 5% 38%	2730
December	1st to 10th	Night	14% 10% 3 7% 4 3 3 3 4 5% 44%	4613
	11th to 20th	Night	16% 10% 4 9% 5% 4 4 3 4 6% 34%	5339
	21st to end	Night	28% 17% 5% 9% 4 4 4 3 3 5% 19%	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	54% 5% 12% 20% 8% 1	4692
	11th to 20th	Day	60% 5% 9% 13% 11% 2	4710
	21st to end	Day	63% 4 7% 17% 6% 2	5607
February	1st to 10th	Day	54% 5% 13% 18% 9% 1	2015
	11th to 20th	Day	53% 6% 11% 20% 10% 1	2058
	21st to end	Day	51% 5% 8% 23% 10% 2	1730
March	1st to 10th	Day	51% 3 6% 19% 18% 3	4580
	11th to 20th	Day	37% 5% 11% 30% 16% 1	4792
	21st to end	Day	50% 8% 12% 22% 7% 1	6101
April	1st to 10th	Day	45% 8% 12% 21% 14% 1	2520
	11th to 20th	Day	44% 6% 11% 24% 11% 3	2494
	21st to end	Day	46% 6% 12% 23% 10% 2	2562
May	1st to 10th	Day	47% 7% 14% 20% 12%	7651
	11th to 20th	Day	44% 6% 12% 22% 16% 1	7681
	21st to end	Day	46% 8% 12% 19% 13% 2	9061
June	1st to 10th	Day	34% 8% 15% 24% 16% 3	2940
	11th to 20th	Day	34% 7% 11% 23% 21% 4	2940
	21st to end	Day	36% 8% 15% 21% 16% 4	2940
July	1st to 10th	Day	35% 5% 14% 27% 16% 2	8211
	11th to 20th	Day	37% 6% 14% 26% 16% 1	8205
	21st to end	Day	40% 5% 19% 25% 10% 1	8794
August	1st to 10th	Day	32% 6% 15% 30% 17% 1	5912
	11th to 20th	Day	41% 7% 17% 23% 11% 1	5522
	21st to end	Day	23% 4 15% 38% 16% 3	6079
September	1st to 10th	Day	34% 7% 11% 25% 19% 4	2520
	11th to 20th	Day	32% 6% 12% 23% 23% 3	2378
	21st to end	Day	34% 6% 10% 23% 25% 3	2310
October	1st to 10th	Day	22% 4 8% 24% 38% 4	3648
	11th to 20th	Day	28% 4 8% 24% 34% 3	3340
	21st to end	Day	38% 3 8% 24% 26% 1	3301
November	1st to 10th	Day	34% 4 13% 27% 18% 5%	1852
	11th to 20th	Day	45% 4 9% 22% 16% 5%	1890
	21st to end	Day	51% 3 9% 22% 14% 1	1890
December	1st to 10th	Day	43% 4 13% 28% 12%	2789
	11th to 20th	Day	57% 4 9% 23% 6% 1	3085
	21st to end	Day	71% 5% 10% 10% 3	4070
January	1st to 10th	Night	54% 4 12% 23% 7%	8233
	11th to 20th	Night	58% 4 8% 14% 13% 3	8280
	21st to end	Night	55% 3 9% 22% 9% 1	8605
February	1st to 10th	Night	54% 5% 12% 18% 10% 2	2603
	11th to 20th	Night	48% 4 11% 21% 13% 2	2459
	21st to end	Night	49% 5% 10% 22% 12% 2	2076
March	1st to 10th	Night	51% 4 7% 21% 13% 3	5493
	11th to 20th	Night	35% 6% 10% 28% 20% 2	5417
	21st to end	Night	49% 7% 10% 24% 8% 1	5072
April	1st to 10th	Night	42% 6% 11% 24% 15% 2	2100
	11th to 20th	Night	41% 5% 11% 22% 17% 4	2075
	21st to end	Night	46% 7% 11% 21% 12% 3	2055
May	1st to 10th	Night	48% 8% 12% 22% 10% 1	5264
	11th to 20th	Night	39% 6% 12% 23% 18% 1	5182
	21st to end	Night	45% 7% 13% 20% 14% 2	5149
June	1st to 10th	Night	34% 5% 11% 25% 20% 5%	1680
	11th to 20th	Night	32% 6% 11% 24% 23% 4	1680
	21st to end	Night	31% 7% 16% 23% 18% 5%	1680
July	1st to 10th	Night	34% 6% 11% 26% 21% 2	4670
	11th to 20th	Night	31% 6% 15% 26% 20% 2	4662
	21st to end	Night	37% 5% 18% 25% 12% 1	5002
August	1st to 10th	Night	31% 5% 13% 32% 17% 1	4211
	11th to 20th	Night	34% 5% 20% 26% 14% 1	4590
	21st to end	Night	21% 5% 15% 35% 20% 3	5046
September	1st to 10th	Night	29% 5% 10% 29% 22% 6%	2100
	11th to 20th	Night	27% 5% 12% 26% 25% 5%	2221
	21st to end	Night	33% 7% 10% 21% 25% 4	2310
October	1st to 10th	Night	19% 3 6% 24% 44% 3	3749
	11th to 20th	Night	24% 3 8% 26% 35% 4	4019
	21st to end	Night	34% 4 8% 24% 24% 6%	4664
November	1st to 10th	Night	34% 5% 10% 24% 22% 5%	2666
	11th to 20th	Night	39% 5% 10% 24% 19% 2	2730
	21st to end	Night	49% 4 10% 22% 14% 1	2730
December	1st to 10th	Night	44% 4 12% 26% 14%	4613
	11th to 20th	Night	52% 4 12% 21% 11%	5341
	21st to end	Night	69% 4 10% 11% 5% 1	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	24% 17% 33% 18% 7% 1	330
	11th to 20th	Day	26% 21% 35% 15% 3	330
	21st to end	Day	24% 20% 36% 15% 5%	363
February	1st to 10th	Day	24% 17% 37% 19% 3	330
	11th to 20th	Day	28% 19% 33% 15% 3 2	330
	21st to end	Day	27% 16% 38% 16% 3	273
March	1st to 10th	Day	25% 22% 34% 17% 2	330
	11th to 20th	Day	28% 21% 32% 16% 4	353
	21st to end	Day	30% 15% 36% 13% 5% 1	594
April	1st to 10th	Day	22% 16% 32% 22% 6% 1	540
	11th to 20th	Day	22% 17% 29% 19% 12% 2	532
	21st to end	Day	23% 13% 36% 19% 8% 1	540
May	1st to 10th	Day	23% 13% 31% 22% 10% 2	543
	11th to 20th	Day	20% 9% 27% 25% 15% 4	579
	21st to end	Day	12% 6% 32% 26% 17% 7%	844
June	1st to 10th	Day	11% 6% 27% 29% 20% 7%	780
	11th to 20th	Day	10% 7% 25% 33% 18% 6%	780
	21st to end	Day	10% 6% 30% 28% 20% 6%	780
July	1st to 10th	Day	11% 2 15% 29% 25% 18%	771
	11th to 20th	Day	7% 3 18% 34% 25% 13%	768
	21st to end	Day	11% 2 16% 33% 29% 8%	849
August	1st to 10th	Day	9% 4 15% 32% 30% 12%	631
	11th to 20th	Day	12% 2 19% 29% 25% 13%	559
	21st to end	Day	5% 2 14% 33% 32% 14%	627
September	1st to 10th	Day	10% 4 18% 30% 31% 8%	570
	11th to 20th	Day	8% 2 14% 32% 31% 13%	570
	21st to end	Day	13% 7% 23% 24% 22% 11%	570
October	1st to 10th	Day	10% 5% 26% 31% 20% 7%	544
	11th to 20th	Day	15% 14% 31% 26% 13% 2	360
	21st to end	Day	26% 17% 30% 19% 7% 1	387
November	1st to 10th	Day	13% 15% 33% 23% 13% 4	330
	11th to 20th	Day	19% 18% 33% 20% 8% 1	330
	21st to end	Day	27% 12% 31% 25% 5% 1	330
December	1st to 10th	Day	16% 18% 41% 17% 5% 2	330
	11th to 20th	Day	22% 18% 35% 19% 7%	330
	21st to end	Day	30% 17% 34% 14% 5%	363
January	1st to 10th	Night	20% 16% 37% 23% 4	1050
	11th to 20th	Night	23% 17% 35% 21% 4	1050
	21st to end	Night	21% 16% 37% 24% 2	1155
February	1st to 10th	Night	24% 14% 36% 24% 2	923
	11th to 20th	Night	21% 17% 36% 24% 3	823
	21st to end	Night	25% 14% 37% 22% 1	692
March	1st to 10th	Night	23% 15% 34% 25% 4	840
	11th to 20th	Night	21% 13% 36% 24% 5%	813
	21st to end	Night	23% 12% 33% 27% 5%	693
April	1st to 10th	Night	19% 13% 37% 26% 5%	630
	11th to 20th	Night	21% 12% 32% 25% 9%	623
	21st to end	Night	21% 9% 36% 27% 7% 1	630
May	1st to 10th	Night	18% 7% 35% 32% 8%	630
	11th to 20th	Night	16% 7% 28% 32% 16% 1	621
	21st to end	Night	12% 5% 23% 35% 22% 2	683
June	1st to 10th	Night	11% 4 24% 40% 21%	630
	11th to 20th	Night	11% 3 23% 44% 19%	630
	21st to end	Night	10% 2 23% 38% 25%	630
July	1st to 10th	Night	10% 2 11% 33% 37% 7%	615
	11th to 20th	Night	5% 3 15% 43% 30% 5%	612
	21st to end	Night	8% 2 14% 42% 30% 3	690
August	1st to 10th	Night	7% 3 12% 41% 33% 4	627
	11th to 20th	Night	7% 3 15% 40% 31% 4	624
	21st to end	Night	5% 1 15% 35% 39% 4	693
September	1st to 10th	Night	12% 3 14% 36% 31% 4	630
	11th to 20th	Night	6% 3 15% 41% 32% 4	751
	21st to end	Night	14% 5% 21% 30% 25% 4	840
October	1st to 10th	Night	10% 5% 24% 38% 19% 4	840
	11th to 20th	Night	18% 12% 28% 28% 14%	840
	21st to end	Night	22% 12% 32% 27% 7% 1	913
November	1st to 10th	Night	13% 11% 35% 30% 10%	820
	11th to 20th	Night	17% 13% 34% 28% 7% 1	840
	21st to end	Night	26% 13% 29% 24% 7%	840
December	1st to 10th	Night	23% 12% 37% 23% 5% 1	982
	11th to 20th	Night	24% 11% 35% 25% 6%	1050
	21st to end	Night	30% 16% 32% 19% 3	1155

Page generated at 2026-10-26 03:37:46. Version ac07

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