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	11% 6% 2 6% 4 4 5% 4 6% 9% 42%	120152
	Night	11% 6% 2 6% 4 4 4 4 6% 8% 44%	118672
Transparency	Day	23% 41% 9% 13% 11% 4	123813
	Night	22% 40% 8% 12% 12% 5%	123149
Seeing	Day	11% 5% 20% 28% 27% 10%	16458
	Night	12% 6% 20% 30% 26% 7%	26565

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 4 3 3 4 3 5% 8% 52%	4056
	11th to 20th	Day	13% 6% 2 6% 4 3 3 3 4 7% 49%	4060
	21st to end	Day	15% 9% 2 6% 3 3 3 2 5% 7% 44%	4466
February	1st to 10th	Day	13% 6% 2 6% 5% 5% 5% 4 5% 8% 41%	1800
	11th to 20th	Day	18% 7% 2 6% 4 3 4 3 4 5% 44%	1904
	21st to end	Day	13% 6% 3 6% 4 3 4 4 4 5% 47%	1811
March	1st to 10th	Day	21% 9% 3 7% 4 3 3 3 5% 5% 37%	4935
	11th to 20th	Day	8% 4 1 4 4 4 4 3 5% 7% 55%	4980
	21st to end	Day	13% 5% 2 6% 4 4 4 3 4 5% 50%	5465
April	1st to 10th	Day	7% 5% 1 5% 4 4 4 5% 5% 6% 54%	2226
	11th to 20th	Day	9% 3 2 5% 3 3 4 4 7% 7% 53%	2374
	21st to end	Day	5% 3 1 4 3 4 4 4 6% 10% 56%	2400
May	1st to 10th	Day	9% 5% 2 6% 4 5% 5% 5% 7% 11% 41%	5761
	11th to 20th	Day	14% 7% 2 6% 4 4 5% 5% 7% 9% 37%	5861
	21st to end	Day	8% 4 2 6% 4 5% 5% 5% 8% 10% 45%	6433
June	1st to 10th	Day	8% 6% 2 7% 5% 6% 8% 6% 8% 10% 33%	2600
	11th to 20th	Day	7% 4 1 6% 4 6% 6% 6% 10% 12% 39%	2600
	21st to end	Day	6% 5% 2 6% 5% 6% 7% 7% 10% 13% 35%	2600
July	1st to 10th	Day	13% 6% 2 7% 6% 6% 7% 6% 8% 11% 28%	4185
	11th to 20th	Day	6% 4 3 8% 6% 6% 8% 6% 10% 13% 32%	4120
	21st to end	Day	6% 6% 2 8% 6% 7% 8% 7% 9% 13% 29%	4466
August	1st to 10th	Day	19% 6% 3 8% 6% 5% 5% 6% 8% 11% 23%	4104
	11th to 20th	Day	6% 5% 2 7% 6% 7% 6% 5% 9% 13% 34%	3916
	21st to end	Day	10% 6% 3 7% 5% 5% 6% 5% 7% 9% 38%	4180
September	1st to 10th	Day	12% 6% 2 6% 5% 5% 6% 5% 7% 10% 36%	2180
	11th to 20th	Day	12% 7% 3 8% 5% 5% 6% 5% 7% 8% 35%	2090
	21st to end	Day	13% 7% 3 6% 4 4 5% 4 6% 9% 39%	2090
October	1st to 10th	Day	12% 6% 2 7% 3 4 4 3 5% 5% 48%	1929
	11th to 20th	Day	8% 3 2 4 4 3 4 3 5% 7% 58%	1900
	21st to end	Day	9% 5% 2 5% 4 3 4 2 4 6% 56%	2090
November	1st to 10th	Day	10% 5% 2 4 4 3 4 3 5% 6% 53%	1957
	11th to 20th	Day	12% 7% 2 6% 4 4 3 3 5% 6% 49%	2000
	21st to end	Day	12% 5% 2 5% 3 3 4 2 6% 6% 52%	1844
December	1st to 10th	Day	8% 5% 2 5% 3 4 5% 4 5% 6% 53%	2930
	11th to 20th	Day	14% 6% 2 6% 3 3 3 3 5% 6% 49%	3373
	21st to end	Day	17% 9% 3 6% 4 3 3 3 5% 6% 41%	4466
January	1st to 10th	Night	9% 5% 1 4 3 3 3 3 5% 6% 56%	5889
	11th to 20th	Night	13% 7% 2 6% 5% 4 4 4 6% 8% 42%	5900
	21st to end	Night	12% 7% 3 7% 4 4 4 3 5% 7% 45%	6490
February	1st to 10th	Night	14% 7% 3 6% 4 4 4 4 5% 6% 44%	2600
	11th to 20th	Night	16% 7% 3 6% 4 4 4 3 5% 6% 42%	2378
	21st to end	Night	12% 8% 2 7% 5% 4 4 3 5% 6% 43%	1816
March	1st to 10th	Night	16% 10% 3 7% 4 3 3 3 4 5% 40%	4918
	11th to 20th	Night	9% 5% 2 5% 3 3 3 3 6% 9% 52%	4980
	21st to end	Night	13% 5% 2 5% 3 3 3 4 5% 7% 50%	5467
April	1st to 10th	Night	9% 5% 2 5% 4 3 4 4 6% 8% 50%	2171
	11th to 20th	Night	11% 4 1 5% 4 3 4 4 5% 7% 53%	1975
	21st to end	Night	6% 4 3 5% 3 4 4 4 6% 9% 53%	2000
May	1st to 10th	Night	5% 5% 2 6% 4 4 5% 4 7% 10% 47%	4175
	11th to 20th	Night	14% 6% 2 6% 4 5% 5% 4 7% 10% 38%	4032
	21st to end	Night	10% 4 2 6% 3 4 5% 5% 7% 10% 44%	4421
June	1st to 10th	Night	8% 6% 2 7% 5% 6% 5% 5% 8% 13% 35%	1800
	11th to 20th	Night	5% 5% 3 5% 6% 5% 5% 6% 8% 12% 39%	1800
	21st to end	Night	5% 5% 2 6% 5% 6% 6% 6% 10% 13% 36%	1800
July	1st to 10th	Night	10% 5% 3 8% 6% 6% 6% 6% 9% 11% 31%	2885
	11th to 20th	Night	6% 5% 2 8% 6% 6% 6% 5% 9% 13% 33%	2840
	21st to end	Night	6% 6% 2 7% 5% 6% 7% 6% 10% 15% 29%	3087
August	1st to 10th	Night	18% 7% 2 7% 5% 4 5% 5% 9% 13% 24%	2825
	11th to 20th	Night	7% 4 2 7% 5% 6% 7% 5% 8% 12% 36%	2879
	21st to end	Night	8% 5% 2 7% 5% 5% 4 4 6% 9% 44%	3476
September	1st to 10th	Night	10% 6% 2 7% 7% 5% 6% 4 7% 10% 36%	1995
	11th to 20th	Night	10% 7% 2 7% 5% 6% 5% 4 7% 9% 37%	2090
	21st to end	Night	13% 6% 3 7% 4 5% 3 4 8% 8% 38%	2090
October	1st to 10th	Night	12% 6% 2 5% 4 4 4 4 5% 8% 47%	2249
	11th to 20th	Night	9% 4 2 4 3 3 4 4 6% 7% 57%	2280
	21st to end	Night	9% 5% 2 5% 3 3 3 3 5% 5% 57%	2508
November	1st to 10th	Night	11% 5% 2 5% 3 3 3 3 5% 6% 55%	2341
	11th to 20th	Night	13% 7% 2 5% 3 3 3 2 5% 6% 50%	2400
	21st to end	Night	11% 6% 2 6% 4 4 4 4 6% 7% 47%	2553
December	1st to 10th	Night	9% 6% 2 5% 3 3 3 3 4 6% 55%	4250
	11th to 20th	Night	15% 8% 3 8% 5% 4 4 3 4 5% 43%	4822
	21st to end	Night	16% 7% 3 6% 4 4 4 3 5% 6% 43%	6490

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	23% 27% 4 13% 27% 6%	4057
	11th to 20th	Day	28% 27% 3 12% 22% 8%	4060
	21st to end	Day	29% 30% 6% 14% 15% 7%	4466
February	1st to 10th	Day	16% 62% 4 9% 8% 1	1800
	11th to 20th	Day	17% 61% 4 8% 8% 2	1914
	21st to end	Day	17% 61% 3 5% 9% 5%	1811
March	1st to 10th	Day	39% 30% 6% 9% 11% 5%	4935
	11th to 20th	Day	17% 30% 7% 11% 20% 16%	4980
	21st to end	Day	27% 30% 5% 15% 15% 8%	5465
April	1st to 10th	Day	13% 62% 6% 9% 6% 4	2226
	11th to 20th	Day	14% 62% 4 11% 5% 4	2374
	21st to end	Day	8% 62% 6% 14% 8% 1	2400
May	1st to 10th	Day	26% 34% 16% 18% 5% 1	5795
	11th to 20th	Day	31% 31% 15% 18% 5%	5865
	21st to end	Day	20% 32% 14% 18% 15%	6447
June	1st to 10th	Day	15% 63% 8% 9% 5%	2600
	11th to 20th	Day	10% 63% 12% 12% 4	2600
	21st to end	Day	13% 63% 12% 10% 2	2600
July	1st to 10th	Day	30% 40% 16% 13% 1	4120
	11th to 20th	Day	24% 44% 25% 6% 1	4051
	21st to end	Day	26% 43% 19% 9% 2	4324
August	1st to 10th	Day	38% 40% 18% 4	4091
	11th to 20th	Day	18% 41% 16% 17% 8%	3955
	21st to end	Day	19% 38% 13% 28% 2	4180
September	1st to 10th	Day	16% 62% 10% 7% 5% 1	2180
	11th to 20th	Day	20% 60% 5% 9% 5% 1	2090
	21st to end	Day	17% 60% 9% 9% 4	2090
October	1st to 10th	Day	28% 36% 5% 9% 18% 3	3202
	11th to 20th	Day	7% 36% 3 9% 24% 21%	3140
	21st to end	Day	14% 36% 5% 13% 28% 3	3428
November	1st to 10th	Day	18% 55% 5% 9% 10% 3	1957
	11th to 20th	Day	18% 56% 4 8% 10% 4	2000
	21st to end	Day	14% 56% 4 11% 12% 3	1844
December	1st to 10th	Day	20% 35% 5% 12% 16% 12%	2930
	11th to 20th	Day	27% 33% 5% 14% 18% 3	3370
	21st to end	Day	34% 28% 6% 18% 14% 1	4466
January	1st to 10th	Night	21% 27% 4 10% 31% 7%	5889
	11th to 20th	Night	31% 27% 4 13% 21% 4	5900
	21st to end	Night	28% 29% 6% 8% 20% 9%	6490
February	1st to 10th	Night	15% 61% 3 10% 8% 2	2600
	11th to 20th	Night	18% 60% 4 7% 8% 3	2379
	21st to end	Night	19% 61% 3 4 8% 5%	1816
March	1st to 10th	Night	36% 30% 6% 9% 13% 5%	4918
	11th to 20th	Night	20% 29% 6% 10% 21% 14%	4980
	21st to end	Night	24% 30% 7% 15% 15% 9%	5467
April	1st to 10th	Night	13% 62% 6% 8% 7% 4	2171
	11th to 20th	Night	13% 62% 4 10% 7% 4	1975
	21st to end	Night	11% 62% 5% 13% 7% 2	2000
May	1st to 10th	Night	23% 34% 16% 22% 5% 1	4204
	11th to 20th	Night	30% 32% 16% 15% 6%	4033
	21st to end	Night	24% 32% 12% 19% 13% 1	4429
June	1st to 10th	Night	14% 64% 10% 9% 3	1800
	11th to 20th	Night	11% 64% 10% 10% 4	1800
	21st to end	Night	13% 63% 12% 11% 2	1800
July	1st to 10th	Night	28% 41% 14% 16% 1	2840
	11th to 20th	Night	24% 44% 26% 5% 1	2786
	21st to end	Night	26% 44% 18% 11% 2	2988
August	1st to 10th	Night	35% 42% 17% 5%	2814
	11th to 20th	Night	19% 41% 18% 17% 5%	2906
	21st to end	Night	17% 38% 11% 31% 2	3476
September	1st to 10th	Night	16% 62% 9% 7% 5% 1	1995
	11th to 20th	Night	19% 60% 7% 8% 4 1	2090
	21st to end	Night	18% 60% 8% 10% 4	2090
October	1st to 10th	Night	24% 36% 7% 11% 17% 5%	3756
	11th to 20th	Night	6% 36% 3 8% 20% 26%	3779
	21st to end	Night	14% 37% 4 14% 23% 9%	4120
November	1st to 10th	Night	18% 55% 6% 8% 10% 3	2341
	11th to 20th	Night	17% 56% 4 8% 11% 5%	2400
	21st to end	Night	14% 57% 5% 9% 12% 4	2553
December	1st to 10th	Night	19% 35% 4 13% 21% 10%	4250
	11th to 20th	Night	31% 32% 7% 9% 15% 6%	4824
	21st to end	Night	31% 28% 7% 19% 14% 1	6490

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% 7% 42% 35% 6%	300
	11th to 20th	Day	9% 8% 36% 32% 13% 2	300
	21st to end	Day	19% 10% 38% 27% 5% 1	330
February	1st to 10th	Day	14% 8% 33% 31% 12%	300
	11th to 20th	Day	26% 9% 32% 27% 6%	300
	21st to end	Day	10% 10% 38% 33% 8% 2	410
March	1st to 10th	Day	14% 9% 34% 27% 15%	500
	11th to 20th	Day	13% 7% 34% 34% 11% 2	500
	21st to end	Day	12% 8% 36% 28% 14% 2	550
April	1st to 10th	Day	11% 10% 30% 32% 15% 2	500
	11th to 20th	Day	7% 8% 28% 28% 22% 7%	492
	21st to end	Day	5% 4 23% 30% 30% 9%	500
May	1st to 10th	Day	5% 6% 28% 33% 22% 7%	503
	11th to 20th	Day	11% 2 18% 31% 28% 10%	527
	21st to end	Day	9% 2 12% 32% 29% 15%	573
June	1st to 10th	Day	9% 2 12% 34% 32% 11%	530
	11th to 20th	Day	7% 3 10% 26% 33% 21%	530
	21st to end	Day	6% 2 8% 27% 40% 18%	530
July	1st to 10th	Day	8% 1 4 20% 47% 21%	505
	11th to 20th	Day	7% 1 4 17% 49% 22%	484
	21st to end	Day	6% 1 5% 28% 44% 16%	532
August	1st to 10th	Day	11% 2 7% 21% 42% 17%	486
	11th to 20th	Day	6% 2 5% 20% 45% 23%	479
	21st to end	Day	14% 2 6% 19% 39% 20%	539
September	1st to 10th	Day	11% 5% 5% 22% 41% 17%	490
	11th to 20th	Day	16% 4 10% 26% 35% 11%	490
	21st to end	Day	14% 2 9% 24% 37% 14%	490
October	1st to 10th	Day	17% 3 18% 31% 21% 10%	490
	11th to 20th	Day	8% 4 18% 33% 27% 11%	490
	21st to end	Day	9% 5% 17% 31% 32% 6%	539
November	1st to 10th	Day	16% 4 23% 27% 23% 7%	495
	11th to 20th	Day	16% 5% 27% 32% 17% 2	500
	21st to end	Day	13% 9% 23% 33% 21% 2	344
December	1st to 10th	Day	14% 7% 26% 38% 15%	300
	11th to 20th	Day	14% 8% 32% 29% 14% 3	300
	21st to end	Day	21% 8% 26% 30% 13% 2	330
January	1st to 10th	Night	12% 12% 34% 33% 9% 1	800
	11th to 20th	Night	14% 10% 31% 31% 13% 1	800
	21st to end	Night	16% 9% 37% 32% 5%	880
February	1st to 10th	Night	15% 9% 29% 34% 12%	800
	11th to 20th	Night	17% 11% 31% 33% 8%	783
	21st to end	Night	15% 8% 31% 36% 10% 1	660
March	1st to 10th	Night	11% 7% 33% 35% 14%	800
	11th to 20th	Night	12% 9% 28% 37% 13% 2	800
	21st to end	Night	11% 7% 32% 32% 17% 2	880
April	1st to 10th	Night	9% 10% 28% 32% 20% 2	800
	11th to 20th	Night	11% 7% 24% 31% 22% 5%	791
	21st to end	Night	7% 5% 22% 34% 29% 4	800
May	1st to 10th	Night	7% 5% 21% 36% 25% 7%	666
	11th to 20th	Night	8% 4 19% 29% 30% 9%	591
	21st to end	Night	9% 1 12% 26% 36% 16%	650
June	1st to 10th	Night	8% 4 11% 31% 38% 9%	600
	11th to 20th	Night	6% 2 8% 22% 42% 20%	600
	21st to end	Night	6% 2 8% 27% 42% 16%	600
July	1st to 10th	Night	9% 1 5% 19% 49% 16%	570
	11th to 20th	Night	10% 1 3 17% 50% 20%	552
	21st to end	Night	6% 1 4 25% 51% 13%	624
August	1st to 10th	Night	10% 2 3 22% 47% 16%	567
	11th to 20th	Night	8% 2 6% 21% 45% 18%	619
	21st to end	Night	13% 1 6% 20% 40% 19%	836
September	1st to 10th	Night	11% 2 8% 23% 42% 14%	760
	11th to 20th	Night	13% 2 7% 26% 41% 11%	760
	21st to end	Night	16% 2 10% 23% 37% 12%	760
October	1st to 10th	Night	15% 3 15% 32% 26% 9%	760
	11th to 20th	Night	9% 4 22% 31% 28% 7%	760
	21st to end	Night	10% 6% 19% 31% 28% 6%	836
November	1st to 10th	Night	11% 6% 22% 32% 25% 4	780
	11th to 20th	Night	16% 8% 25% 32% 18% 3	800
	21st to end	Night	12% 9% 24% 34% 17% 4	800
December	1st to 10th	Night	15% 8% 27% 35% 14% 2	800
	11th to 20th	Night	18% 8% 27% 28% 17% 2	800
	21st to end	Night	18% 9% 30% 30% 13% 1	880

Page generated at 2025-07-03 01:30:13. Version ac07

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