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 2025-07-06.

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	13% 9% 3 7% 5% 4 4 4 6% 8% 38%	119704
	Night	14% 9% 3 7% 4 4 4 3 5% 6% 40%	119271
Transparency	Day	43% 6% 12% 23% 15% 2	123308
	Night	43% 5% 11% 23% 16% 2	123808
Seeing	Day	16% 10% 27% 25% 17% 6%	16526
	Night	17% 9% 29% 30% 13% 1	26374

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	13% 11% 7% 14% 6% 5% 4 4 5% 7% 24%	3617
	11th to 20th	Day	11% 11% 7% 14% 8% 8% 6% 5% 4 5% 22%	3620
	21st to end	Day	20% 17% 5% 10% 5% 4 3 3 3 4 26%	4325
February	1st to 10th	Day	15% 12% 5% 10% 5% 5% 3 4 5% 6% 29%	1919
	11th to 20th	Day	15% 11% 4 9% 6% 5% 4 3 6% 6% 30%	1948
	21st to end	Day	15% 13% 4 9% 5% 4 3 2 4 6% 34%	1650
March	1st to 10th	Day	17% 15% 3 7% 4 3 3 2 3 4 39%	4480
	11th to 20th	Day	10% 11% 3 5% 3 3 3 3 5% 8% 45%	4689
	21st to end	Day	16% 11% 3 9% 5% 5% 5% 4 5% 7% 30%	5967
April	1st to 10th	Day	15% 9% 3 7% 5% 4 5% 4 6% 9% 35%	2400
	11th to 20th	Day	15% 9% 3 7% 4 4 4 3 5% 6% 40%	2374
	21st to end	Day	19% 8% 3 7% 4 4 4 3 5% 6% 37%	2440
May	1st to 10th	Day	18% 9% 3 8% 5% 5% 4 4 6% 10% 29%	5862
	11th to 20th	Day	16% 8% 3 7% 5% 4 5% 3 5% 7% 37%	5898
	21st to end	Day	11% 7% 3 8% 5% 5% 5% 5% 7% 9% 36%	6935
June	1st to 10th	Day	9% 5% 2 5% 4 5% 5% 4 7% 9% 45%	2800
	11th to 20th	Day	8% 5% 2 6% 4 5% 5% 4 6% 8% 47%	2800
	21st to end	Day	7% 6% 2 6% 5% 4 5% 4 7% 10% 42%	2800
July	1st to 10th	Day	13% 5% 2 6% 4 4 5% 5% 7% 10% 40%	4511
	11th to 20th	Day	5% 5% 2 6% 5% 5% 5% 5% 7% 11% 44%	4440
	21st to end	Day	9% 5% 3 7% 5% 6% 6% 5% 8% 10% 38%	4813
August	1st to 10th	Day	9% 4 1 4 3 4 4 3 5% 9% 53%	4047
	11th to 20th	Day	9% 5% 2 6% 4 5% 5% 4 7% 10% 41%	3729
	21st to end	Day	5% 3 2 6% 4 4 4 4 6% 10% 51%	4180
September	1st to 10th	Day	12% 6% 2 6% 4 3 5% 4 5% 7% 45%	2280
	11th to 20th	Day	9% 4 2 5% 5% 5% 5% 4 6% 8% 48%	2152
	21st to end	Day	11% 6% 2 5% 3 3 4 3 5% 8% 49%	2090
October	1st to 10th	Day	11% 5% 2 5% 3 3 4 3 5% 7% 52%	2066
	11th to 20th	Day	15% 6% 3 6% 4 3 4 3 4 5% 47%	1900
	21st to end	Day	18% 8% 3 7% 3 3 3 2 4 6% 41%	1914
November	1st to 10th	Day	10% 6% 2 6% 4 4 2 3 5% 5% 53%	1762
	11th to 20th	Day	16% 10% 3 7% 4 3 3 2 5% 6% 41%	1800
	21st to end	Day	16% 11% 4 8% 4 4 4 3 4 5% 38%	1800
December	1st to 10th	Day	11% 10% 4 7% 4 4 3 3 5% 5% 44%	2706
	11th to 20th	Day	14% 14% 5% 11% 5% 5% 4 4 5% 6% 28%	3008
	21st to end	Day	28% 17% 6% 12% 4 4 4 2 3 5% 16%	3982
January	1st to 10th	Night	16% 14% 7% 12% 4 4 4 3 4 5% 29%	6348
	11th to 20th	Night	13% 17% 6% 10% 5% 5% 7% 4 3 3 26%	6360
	21st to end	Night	20% 12% 6% 8% 5% 4 4 3 4 5% 30%	6651
February	1st to 10th	Night	18% 12% 4 9% 5% 4 4 3 4 5% 31%	2479
	11th to 20th	Night	15% 9% 4 9% 5% 5% 3 3 5% 6% 36%	2338
	21st to end	Night	19% 9% 3 8% 4 4 3 3 4 5% 37%	1980
March	1st to 10th	Night	18% 15% 4 6% 4 3 3 2 4 5% 37%	5373
	11th to 20th	Night	12% 7% 2 5% 4 3 3 3 5% 7% 48%	5299
	21st to end	Night	18% 9% 3 8% 5% 5% 5% 3 5% 6% 33%	4962
April	1st to 10th	Night	15% 7% 2 6% 5% 4 4 3 4 7% 43%	2000
	11th to 20th	Night	17% 8% 2 6% 4 3 3 3 4 7% 43%	1975
	21st to end	Night	17% 8% 3 6% 5% 5% 6% 4 6% 5% 36%	1957
May	1st to 10th	Night	18% 9% 3 7% 5% 4 4 3 5% 8% 34%	4033
	11th to 20th	Night	15% 7% 2 7% 4 4 5% 4 6% 9% 38%	3992
	21st to end	Night	13% 7% 3 7% 5% 5% 5% 3 6% 8% 39%	3942
June	1st to 10th	Night	8% 6% 3 5% 4 4 4 4 5% 7% 50%	1600
	11th to 20th	Night	8% 4 2 7% 4 4 4 4 6% 7% 52%	1600
	21st to end	Night	7% 5% 2 6% 5% 4 4 4 7% 10% 46%	1600
July	1st to 10th	Night	11% 5% 1 5% 4 3 5% 3 5% 10% 47%	2570
	11th to 20th	Night	4 4 1 4 3 3 4 4 7% 10% 56%	2530
	21st to end	Night	8% 6% 3 8% 5% 4 4 4 8% 11% 40%	2750
August	1st to 10th	Night	7% 4 2 4 3 4 4 4 6% 9% 54%	2877
	11th to 20th	Night	7% 5% 2 6% 5% 4 4 4 6% 8% 50%	3100
	21st to end	Night	5% 3 1 4 3 4 4 4 5% 9% 57%	3476
September	1st to 10th	Night	11% 5% 2 4 3 3 4 3 5% 7% 54%	1900
	11th to 20th	Night	6% 5% 2 5% 4 4 3 3 6% 8% 55%	2009
	21st to end	Night	11% 5% 2 4 4 3 5% 3 6% 7% 51%	2090
October	1st to 10th	Night	9% 4 2 5% 4 3 3 3 5% 6% 57%	2111
	11th to 20th	Night	14% 6% 2 4 3 3 3 3 3 6% 52%	2280
	21st to end	Night	18% 8% 2 5% 4 4 4 3 5% 6% 42%	2682
November	1st to 10th	Night	10% 6% 2 6% 4 4 3 3 5% 5% 52%	2536
	11th to 20th	Night	13% 8% 2 7% 4 3 3 3 5% 6% 44%	2600
	21st to end	Night	19% 10% 3 7% 4 3 3 2 4 6% 38%	2600
December	1st to 10th	Night	13% 10% 3 7% 4 3 3 3 4 5% 45%	4476
	11th to 20th	Night	16% 10% 4 9% 5% 4 4 3 5% 6% 34%	5199
	21st to end	Night	28% 17% 6% 9% 4 4 4 3 3 5% 18%	6996

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	58% 6% 9% 19% 7% 1	3618
	11th to 20th	Day	63% 6% 9% 11% 8% 2	3620
	21st to end	Day	63% 4 7% 17% 7% 2	4325
February	1st to 10th	Day	54% 5% 13% 18% 10% 1	1919
	11th to 20th	Day	53% 6% 11% 20% 10% 1	1958
	21st to end	Day	51% 5% 8% 23% 10% 3	1650
March	1st to 10th	Day	51% 4 6% 19% 18% 3	4480
	11th to 20th	Day	36% 6% 11% 31% 16% 1	4689
	21st to end	Day	50% 8% 11% 22% 7% 1	5969
April	1st to 10th	Day	45% 8% 12% 21% 14% 1	2400
	11th to 20th	Day	44% 6% 10% 25% 11% 3	2374
	21st to end	Day	47% 6% 13% 22% 10% 3	2440
May	1st to 10th	Day	48% 9% 14% 19% 10% 1	5897
	11th to 20th	Day	45% 7% 12% 24% 12% 1	5902
	21st to end	Day	40% 9% 13% 21% 15% 2	6949
June	1st to 10th	Day	34% 8% 14% 25% 16% 3	2800
	11th to 20th	Day	34% 8% 11% 23% 21% 4	2800
	21st to end	Day	35% 8% 16% 21% 17% 4	2800
July	1st to 10th	Day	38% 6% 11% 28% 15% 2	4442
	11th to 20th	Day	31% 7% 15% 28% 17% 1	4365
	21st to end	Day	38% 7% 19% 21% 14% 1	4662
August	1st to 10th	Day	27% 6% 15% 35% 15% 1	4032
	11th to 20th	Day	36% 7% 15% 25% 16% 1	3765
	21st to end	Day	28% 5% 17% 31% 14% 4	4179
September	1st to 10th	Day	36% 7% 11% 23% 19% 4	2280
	11th to 20th	Day	32% 6% 12% 22% 23% 4	2152
	21st to end	Day	33% 6% 10% 25% 24% 3	2090
October	1st to 10th	Day	21% 4 9% 24% 38% 3	3430
	11th to 20th	Day	28% 4 8% 24% 34% 2	3140
	21st to end	Day	38% 3 8% 24% 26% 1	3126
November	1st to 10th	Day	34% 4 14% 26% 18% 5%	1762
	11th to 20th	Day	44% 4 9% 22% 16% 5%	1800
	21st to end	Day	50% 4 10% 22% 14% 1	1800
December	1st to 10th	Day	42% 4 13% 28% 12%	2706
	11th to 20th	Day	57% 4 9% 23% 6% 1	3005
	21st to end	Day	72% 5% 10% 10% 3	3982
January	1st to 10th	Night	57% 5% 10% 21% 8%	6348
	11th to 20th	Night	63% 4 8% 12% 10% 3	6360
	21st to end	Night	58% 3 8% 20% 10% 1	6651
February	1st to 10th	Night	55% 5% 12% 17% 10% 2	2479
	11th to 20th	Night	49% 4 11% 20% 13% 2	2339
	21st to end	Night	49% 5% 10% 22% 13% 2	1980
March	1st to 10th	Night	51% 4 7% 21% 13% 3	5373
	11th to 20th	Night	34% 6% 9% 28% 20% 2	5299
	21st to end	Night	49% 7% 10% 24% 8% 1	4962
April	1st to 10th	Night	42% 6% 11% 24% 16% 2	2000
	11th to 20th	Night	41% 5% 10% 23% 17% 4	1975
	21st to end	Night	46% 8% 11% 20% 12% 3	1957
May	1st to 10th	Night	47% 9% 13% 21% 9% 2	4060
	11th to 20th	Night	41% 8% 11% 25% 14% 1	3993
	21st to end	Night	41% 7% 14% 21% 14% 3	3950
June	1st to 10th	Night	34% 5% 11% 25% 20% 4	1600
	11th to 20th	Night	31% 7% 11% 24% 23% 5%	1600
	21st to end	Night	30% 8% 16% 23% 19% 5%	1600
July	1st to 10th	Night	34% 6% 11% 26% 18% 4	2530
	11th to 20th	Night	22% 6% 13% 32% 24% 2	2482
	21st to end	Night	36% 5% 19% 20% 17% 3	2662
August	1st to 10th	Night	25% 7% 15% 36% 16% 1	2868
	11th to 20th	Night	32% 5% 14% 28% 20% 1	3130
	21st to end	Night	24% 6% 14% 32% 19% 4	3476
September	1st to 10th	Night	30% 5% 11% 27% 21% 7%	1900
	11th to 20th	Night	27% 5% 10% 27% 26% 5%	2009
	21st to end	Night	31% 7% 10% 23% 25% 4	2090
October	1st to 10th	Night	19% 4 6% 25% 43% 3	3527
	11th to 20th	Night	24% 3 8% 27% 36% 4	3779
	21st to end	Night	34% 4 9% 24% 24% 5%	4420
November	1st to 10th	Night	34% 5% 10% 24% 22% 5%	2536
	11th to 20th	Night	39% 5% 10% 24% 20% 3	2600
	21st to end	Night	48% 4 10% 22% 15% 1	2600
December	1st to 10th	Night	43% 4 12% 27% 14% 1	4476
	11th to 20th	Night	52% 4 12% 20% 11%	5201
	21st to end	Night	70% 4 10% 11% 4 1	6996

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	23% 17% 33% 19% 7% 1	300
	11th to 20th	Day	24% 19% 37% 15% 3	300
	21st to end	Day	23% 19% 36% 16% 5%	330
February	1st to 10th	Day	24% 17% 36% 19% 3	300
	11th to 20th	Day	27% 19% 32% 17% 3 2	300
	21st to end	Day	28% 14% 38% 16% 3	249
March	1st to 10th	Day	22% 21% 35% 19% 3	300
	11th to 20th	Day	27% 20% 32% 16% 4	322
	21st to end	Day	31% 15% 34% 14% 5% 1	550
April	1st to 10th	Day	21% 15% 33% 23% 7% 1	500
	11th to 20th	Day	22% 16% 28% 20% 12% 2	492
	21st to end	Day	24% 12% 37% 18% 8% 2	500
May	1st to 10th	Day	22% 11% 31% 23% 11% 2	503
	11th to 20th	Day	20% 8% 27% 26% 16% 4	538
	21st to end	Day	11% 6% 32% 26% 17% 7%	789
June	1st to 10th	Day	11% 6% 25% 29% 21% 7%	730
	11th to 20th	Day	10% 6% 24% 35% 20% 6%	730
	21st to end	Day	8% 6% 30% 28% 21% 7%	730
July	1st to 10th	Day	11% 2 15% 27% 25% 20%	697
	11th to 20th	Day	7% 3 18% 34% 26% 13%	668
	21st to end	Day	10% 2 15% 33% 31% 9%	739
August	1st to 10th	Day	8% 3 14% 31% 32% 12%	545
	11th to 20th	Day	13% 2 19% 27% 25% 14%	479
	21st to end	Day	6% 2 12% 32% 33% 15%	539
September	1st to 10th	Day	11% 4 17% 28% 31% 9%	490
	11th to 20th	Day	8% 2 13% 29% 33% 14%	490
	21st to end	Day	13% 7% 23% 24% 22% 11%	490
October	1st to 10th	Day	11% 5% 26% 29% 21% 8%	466
	11th to 20th	Day	14% 16% 34% 22% 13% 1	300
	21st to end	Day	27% 18% 30% 19% 5%	330
November	1st to 10th	Day	12% 13% 32% 25% 14% 4	300
	11th to 20th	Day	19% 17% 33% 21% 8% 1	300
	21st to end	Day	25% 12% 31% 25% 6% 1	300
December	1st to 10th	Day	16% 18% 40% 18% 6% 3	300
	11th to 20th	Day	22% 16% 35% 19% 8%	300
	21st to end	Day	30% 17% 32% 15% 6%	330
January	1st to 10th	Night	20% 16% 38% 23% 4	1000
	11th to 20th	Night	23% 16% 35% 22% 4	1000
	21st to end	Night	21% 15% 37% 25% 2	1100
February	1st to 10th	Night	25% 14% 36% 23% 2	879
	11th to 20th	Night	21% 17% 36% 24% 3	783
	21st to end	Night	25% 14% 37% 23% 1	660
March	1st to 10th	Night	21% 14% 35% 26% 4	800
	11th to 20th	Night	20% 12% 36% 25% 5% 1	774
	21st to end	Night	24% 11% 32% 28% 5%	660
April	1st to 10th	Night	19% 12% 36% 28% 6%	600
	11th to 20th	Night	21% 12% 31% 26% 10%	593
	21st to end	Night	22% 8% 36% 27% 7% 1	600
May	1st to 10th	Night	18% 7% 35% 33% 8% 1	600
	11th to 20th	Night	17% 6% 27% 33% 17% 1	591
	21st to end	Night	11% 5% 23% 35% 23% 2	650
June	1st to 10th	Night	11% 4 23% 40% 21%	600
	11th to 20th	Night	10% 2 23% 45% 20% 1	600
	21st to end	Night	9% 2 23% 39% 27% 1	600
July	1st to 10th	Night	10% 1 12% 32% 38% 7%	570
	11th to 20th	Night	5% 3 15% 43% 30% 5%	552
	21st to end	Night	8% 2 13% 43% 31% 3	624
August	1st to 10th	Night	7% 2 11% 42% 33% 4	567
	11th to 20th	Night	6% 3 14% 39% 33% 4	564
	21st to end	Night	6% 2 14% 34% 41% 4	627
September	1st to 10th	Night	13% 3 13% 36% 31% 4	570
	11th to 20th	Night	7% 3 14% 40% 33% 4	679
	21st to end	Night	14% 5% 21% 30% 26% 4	760
October	1st to 10th	Night	10% 5% 23% 38% 20% 5%	760
	11th to 20th	Night	18% 11% 28% 28% 14% 1	760
	21st to end	Night	21% 12% 34% 26% 6%	836
November	1st to 10th	Night	13% 10% 36% 30% 11% 1	780
	11th to 20th	Night	18% 12% 33% 29% 7% 1	800
	21st to end	Night	25% 13% 30% 25% 7%	800
December	1st to 10th	Night	22% 11% 37% 23% 5% 1	935
	11th to 20th	Night	24% 9% 35% 26% 6%	1000
	21st to end	Night	30% 16% 31% 20% 3	1100

Page generated at 2025-07-03 00:30:17. Version ac07

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