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 2025-07-21.

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	15% 9% 3 8% 5% 4 4 4 6% 7% 34%	120460
	Night	16% 10% 3 8% 5% 4 4 3 5% 6% 35%	118673
Transparency	Day	49% 2 9% 23% 14% 2	124161
	Night	50% 2 8% 22% 15% 2	123109
Seeing	Day	24% 11% 24% 22% 14% 5%	15967
	Night	23% 8% 22% 30% 16% 2	26051

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	16% 21% 7% 17% 7% 3 3 2 3 5% 17%	4056
	11th to 20th	Day	16% 19% 9% 11% 7% 4 3 2 3 3 24%	4060
	21st to end	Day	18% 15% 4 9% 5% 4 4 3 5% 5% 29%	4466
February	1st to 10th	Day	21% 14% 5% 8% 5% 4 3 3 4 5% 28%	1800
	11th to 20th	Day	24% 13% 4 10% 4 4 4 3 4 5% 26%	1884
	21st to end	Day	23% 13% 3 7% 4 4 4 2 4 5% 31%	1650
March	1st to 10th	Day	17% 13% 4 7% 4 4 5% 4 5% 5% 32%	4882
	11th to 20th	Day	14% 7% 2 6% 4 3 4 3 5% 7% 43%	4980
	21st to end	Day	18% 9% 3 8% 5% 4 5% 4 6% 7% 31%	5465
April	1st to 10th	Day	16% 10% 3 7% 5% 4 5% 3 5% 7% 35%	2200
	11th to 20th	Day	16% 7% 3 6% 5% 4 4 3 5% 6% 40%	2476
	21st to end	Day	18% 8% 2 7% 4 5% 5% 4 5% 7% 35%	2600
May	1st to 10th	Day	16% 9% 3 9% 6% 5% 6% 5% 7% 7% 26%	5862
	11th to 20th	Day	17% 9% 3 7% 5% 5% 5% 4 7% 8% 30%	5861
	21st to end	Day	15% 7% 3 6% 5% 6% 5% 4 7% 9% 32%	6433
June	1st to 10th	Day	12% 5% 3 6% 5% 4 5% 5% 7% 9% 39%	2600
	11th to 20th	Day	9% 5% 3 6% 5% 5% 5% 5% 7% 11% 38%	2600
	21st to end	Day	9% 5% 3 6% 5% 5% 5% 5% 8% 10% 39%	2600
July	1st to 10th	Day	9% 5% 2 6% 5% 5% 5% 4 7% 9% 43%	4250
	11th to 20th	Day	10% 6% 2 6% 5% 5% 5% 4 7% 10% 40%	4250
	21st to end	Day	17% 9% 3 9% 6% 6% 5% 4 6% 7% 28%	4466
August	1st to 10th	Day	11% 7% 3 7% 5% 6% 6% 5% 7% 10% 34%	4104
	11th to 20th	Day	12% 6% 2 8% 5% 5% 6% 5% 7% 9% 35%	4044
	21st to end	Day	6% 3 2 4 4 4 4 4 7% 9% 52%	4387
September	1st to 10th	Day	13% 5% 2 5% 4 4 5% 3 5% 7% 47%	2119
	11th to 20th	Day	11% 6% 2 6% 4 4 4 4 6% 7% 45%	2090
	21st to end	Day	18% 8% 3 6% 4 4 3 3 4 6% 41%	2090
October	1st to 10th	Day	16% 5% 2 5% 4 4 4 2 6% 9% 45%	1986
	11th to 20th	Day	16% 7% 2 5% 3 4 3 3 5% 6% 45%	1900
	21st to end	Day	21% 6% 2 7% 3 3 3 3 4 6% 41%	2090
November	1st to 10th	Day	13% 6% 2 6% 3 4 3 3 5% 5% 50%	1840
	11th to 20th	Day	15% 11% 3 8% 4 3 4 3 6% 7% 36%	1800
	21st to end	Day	17% 12% 2 7% 4 4 3 4 4 6% 36%	1800
December	1st to 10th	Day	20% 13% 3 13% 4 4 3 3 3 4 30%	2930
	11th to 20th	Day	24% 14% 4 9% 3 4 4 3 5% 5% 26%	3373
	21st to end	Day	17% 15% 5% 11% 6% 5% 5% 4 5% 6% 21%	4466
January	1st to 10th	Night	18% 18% 5% 16% 5% 4 3 3 4 4 21%	5889
	11th to 20th	Night	19% 14% 5% 14% 6% 4 3 2 3 4 24%	5900
	21st to end	Night	21% 11% 4 8% 5% 4 4 3 4 4 30%	6490
February	1st to 10th	Night	19% 14% 4 8% 6% 4 4 3 4 5% 28%	2600
	11th to 20th	Night	23% 14% 4 9% 6% 4 4 3 3 5% 26%	2397
	21st to end	Night	22% 14% 3 8% 5% 4 4 3 4 5% 28%	1980
March	1st to 10th	Night	20% 11% 3 6% 4 4 3 3 4 6% 37%	4970
	11th to 20th	Night	14% 9% 2 6% 3 3 3 3 6% 9% 42%	4980
	21st to end	Night	16% 10% 3 8% 6% 5% 5% 4 5% 6% 33%	5467
April	1st to 10th	Night	17% 11% 2 7% 5% 4 4 3 6% 7% 36%	2200
	11th to 20th	Night	14% 7% 3 6% 5% 4 5% 4 6% 8% 38%	1887
	21st to end	Night	16% 6% 3 8% 5% 4 4 4 5% 6% 39%	1800
May	1st to 10th	Night	21% 10% 3 9% 5% 5% 4 4 6% 8% 25%	4033
	11th to 20th	Night	16% 8% 3 8% 5% 4 6% 4 6% 8% 31%	4032
	21st to end	Night	18% 9% 4 8% 5% 4 4 4 6% 8% 31%	4421
June	1st to 10th	Night	10% 6% 2 6% 4 4 5% 5% 7% 9% 42%	1800
	11th to 20th	Night	8% 5% 2 8% 6% 5% 5% 5% 6% 8% 42%	1800
	21st to end	Night	10% 6% 2 5% 5% 6% 4 4 6% 10% 45%	1800
July	1st to 10th	Night	8% 6% 2 7% 5% 4 5% 5% 7% 11% 39%	2930
	11th to 20th	Night	9% 6% 2 6% 5% 5% 4 4 6% 9% 43%	2930
	21st to end	Night	13% 9% 3 8% 5% 5% 5% 3 6% 8% 33%	3087
August	1st to 10th	Night	12% 7% 3 8% 6% 5% 5% 5% 7% 8% 34%	2825
	11th to 20th	Night	14% 6% 3 7% 5% 4 5% 4 5% 7% 41%	2785
	21st to end	Night	6% 4 2 4 4 4 5% 4 6% 9% 55%	3235
September	1st to 10th	Night	11% 6% 2 6% 4 5% 5% 3 5% 8% 47%	2057
	11th to 20th	Night	11% 5% 2 5% 4 4 4 3 6% 7% 50%	2090
	21st to end	Night	14% 8% 2 6% 4 5% 3 4 6% 7% 42%	2090
October	1st to 10th	Night	13% 4 2 5% 4 3 4 4 6% 6% 50%	2193
	11th to 20th	Night	15% 7% 3 6% 3 4 4 3 4 6% 45%	2280
	21st to end	Night	21% 7% 2 6% 4 4 4 3 4 5% 42%	2508
November	1st to 10th	Night	13% 6% 2 6% 4 3 4 4 4 6% 48%	2455
	11th to 20th	Night	19% 10% 2 5% 4 4 4 3 6% 6% 38%	2600
	21st to end	Night	18% 12% 3 6% 4 4 4 3 4 5% 37%	2600
December	1st to 10th	Night	18% 13% 3 11% 5% 4 4 3 4 5% 30%	4250
	11th to 20th	Night	21% 15% 6% 9% 3 4 3 3 4 5% 28%	4822
	21st to end	Night	16% 17% 5% 9% 6% 6% 5% 3 5% 6% 22%	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	74% 1 8% 12% 5%	4057
	11th to 20th	Day	69% 1 4 14% 10% 1	4060
	21st to end	Day	60% 1 7% 23% 8% 1	4466
February	1st to 10th	Day	61% 3 8% 19% 10%	1800
	11th to 20th	Day	62% 3 6% 17% 11% 2	1894
	21st to end	Day	57% 2 7% 20% 12% 2	1650
March	1st to 10th	Day	56% 1 5% 21% 12% 4	4882
	11th to 20th	Day	43% 1 6% 26% 22% 3	4980
	21st to end	Day	53% 2 6% 23% 14% 2	5465
April	1st to 10th	Day	50% 2 9% 21% 14% 4	2200
	11th to 20th	Day	45% 2 8% 23% 18% 4	2476
	21st to end	Day	50% 3 10% 23% 12% 3	2600
May	1st to 10th	Day	57% 2 10% 20% 10% 2	5897
	11th to 20th	Day	53% 3 9% 23% 10% 1	5865
	21st to end	Day	48% 3 12% 22% 12% 3	6447
June	1st to 10th	Day	41% 4 13% 25% 17% 1	2600
	11th to 20th	Day	39% 5% 15% 23% 16% 1	2600
	21st to end	Day	38% 6% 15% 25% 14% 2	2600
July	1st to 10th	Day	39% 4 14% 26% 14% 3	4185
	11th to 20th	Day	40% 3 11% 32% 13% 1	4181
	21st to end	Day	56% 4 10% 16% 14% 1	4324
August	1st to 10th	Day	45% 4 14% 27% 10%	4091
	11th to 20th	Day	45% 3 8% 28% 15% 1	4083
	21st to end	Day	29% 3 14% 33% 20% 2	4387
September	1st to 10th	Day	37% 4 10% 23% 21% 5%	2119
	11th to 20th	Day	38% 3 9% 26% 22% 2	2090
	21st to end	Day	46% 3 9% 21% 19% 2	2090
October	1st to 10th	Day	32% 2 6% 24% 33% 3	3298
	11th to 20th	Day	32% 1 10% 27% 23% 6%	3140
	21st to end	Day	32% 1 9% 31% 21% 6%	3428
November	1st to 10th	Day	37% 2 7% 28% 21% 4	1840
	11th to 20th	Day	49% 2 6% 21% 20% 3	1800
	21st to end	Day	51% 2 10% 21% 16% 1	1800
December	1st to 10th	Day	62% 1 5% 24% 7% 1	2930
	11th to 20th	Day	63% 1 8% 18% 10%	3370
	21st to end	Day	66% 2 8% 20% 5%	4466
January	1st to 10th	Night	71% 1 7% 15% 6% 1	5889
	11th to 20th	Night	67% 1 4 15% 10% 2	5900
	21st to end	Night	59% 1 6% 22% 10% 1	6490
February	1st to 10th	Night	60% 2 8% 18% 12%	2600
	11th to 20th	Night	63% 2 6% 16% 12% 1	2398
	21st to end	Night	60% 2 8% 16% 11% 3	1980
March	1st to 10th	Night	52% 1 3 25% 16% 3	4970
	11th to 20th	Night	41% 2 5% 26% 23% 2	4980
	21st to end	Night	53% 1 6% 22% 16% 2	5467
April	1st to 10th	Night	49% 3 8% 21% 17% 3	2200
	11th to 20th	Night	44% 3 9% 23% 15% 6%	1887
	21st to end	Night	47% 3 9% 21% 19% 2	1800
May	1st to 10th	Night	58% 3 11% 16% 10% 2	4060
	11th to 20th	Night	51% 5% 12% 21% 9% 1	4033
	21st to end	Night	53% 3 11% 20% 10% 3	4429
June	1st to 10th	Night	37% 5% 14% 24% 18% 2	1800
	11th to 20th	Night	40% 5% 15% 19% 19% 2	1800
	21st to end	Night	36% 6% 14% 25% 17% 3	1800
July	1st to 10th	Night	40% 4 15% 22% 15% 3	2885
	11th to 20th	Night	38% 4 12% 31% 13% 2	2876
	21st to end	Night	49% 3 11% 20% 14% 2	2988
August	1st to 10th	Night	47% 4 14% 25% 10% 1	2814
	11th to 20th	Night	44% 3 9% 25% 19%	2812
	21st to end	Night	28% 3 11% 35% 22% 2	3235
September	1st to 10th	Night	36% 4 11% 22% 19% 8%	2057
	11th to 20th	Night	34% 3 10% 24% 25% 4	2090
	21st to end	Night	42% 3 9% 25% 20% 1	2090
October	1st to 10th	Night	31% 2 7% 26% 29% 5%	3661
	11th to 20th	Night	34% 2 6% 26% 23% 10%	3779
	21st to end	Night	32% 1 6% 28% 24% 8%	4120
November	1st to 10th	Night	39% 2 8% 22% 23% 5%	2455
	11th to 20th	Night	48% 2 6% 20% 19% 4	2600
	21st to end	Night	51% 2 8% 22% 17% 2	2600
December	1st to 10th	Night	60% 1 9% 22% 8% 1	4250
	11th to 20th	Night	62% 1 9% 19% 8% 1	4824
	21st to end	Night	65% 2 8% 18% 8%	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	33% 21% 32% 10% 4 1	300
	11th to 20th	Day	41% 22% 26% 9% 2	300
	21st to end	Day	34% 22% 30% 10% 3	330
February	1st to 10th	Day	36% 21% 28% 12% 4	300
	11th to 20th	Day	50% 20% 23% 6% 2	300
	21st to end	Day	40% 18% 31% 10% 1	249
March	1st to 10th	Day	32% 18% 27% 17% 6%	471
	11th to 20th	Day	29% 20% 32% 14% 4 1	500
	21st to end	Day	34% 15% 29% 18% 4	550
April	1st to 10th	Day	30% 17% 28% 18% 7%	500
	11th to 20th	Day	26% 17% 30% 15% 9% 3	492
	21st to end	Day	28% 13% 29% 19% 10% 2	500
May	1st to 10th	Day	28% 14% 30% 17% 7% 4	503
	11th to 20th	Day	23% 10% 26% 24% 14% 4	527
	21st to end	Day	17% 8% 24% 25% 19% 8%	573
June	1st to 10th	Day	17% 6% 24% 27% 18% 8%	530
	11th to 20th	Day	13% 6% 21% 32% 21% 7%	530
	21st to end	Day	12% 6% 28% 27% 19% 9%	530
July	1st to 10th	Day	16% 3 14% 30% 21% 16%	525
	11th to 20th	Day	11% 2 20% 35% 21% 11%	524
	21st to end	Day	17% 2 18% 29% 23% 11%	532
August	1st to 10th	Day	17% 3 15% 29% 24% 12%	486
	11th to 20th	Day	17% 3 15% 28% 27% 10%	479
	21st to end	Day	12% 2 15% 31% 27% 12%	539
September	1st to 10th	Day	14% 4 14% 29% 27% 12%	490
	11th to 20th	Day	14% 1 14% 30% 30% 11%	490
	21st to end	Day	24% 6% 23% 25% 17% 6%	490
October	1st to 10th	Day	19% 5% 23% 25% 23% 5%	490
	11th to 20th	Day	20% 10% 26% 25% 14% 4	490
	21st to end	Day	26% 11% 27% 23% 10% 3	539
November	1st to 10th	Day	20% 12% 29% 26% 11% 2	378
	11th to 20th	Day	29% 16% 27% 20% 6% 1	300
	21st to end	Day	27% 18% 33% 20% 3	300
December	1st to 10th	Day	35% 16% 29% 15% 4 2	300
	11th to 20th	Day	33% 15% 31% 17% 4 1	300
	21st to end	Day	34% 16% 29% 15% 6%	330
January	1st to 10th	Night	32% 15% 27% 21% 5%	800
	11th to 20th	Night	35% 15% 25% 21% 4	800
	21st to end	Night	30% 16% 29% 22% 3	880
February	1st to 10th	Night	28% 12% 30% 28% 3	800
	11th to 20th	Night	35% 17% 28% 19% 2	783
	21st to end	Night	33% 15% 29% 22% 2	660
March	1st to 10th	Night	29% 15% 26% 27% 4	800
	11th to 20th	Night	28% 16% 28% 22% 7%	800
	21st to end	Night	25% 12% 29% 27% 6%	880
April	1st to 10th	Night	25% 15% 29% 25% 6%	800
	11th to 20th	Night	20% 11% 28% 28% 12% 1	703
	21st to end	Night	21% 7% 31% 33% 7% 1	600
May	1st to 10th	Night	23% 7% 27% 31% 12% 1	600
	11th to 20th	Night	20% 6% 22% 35% 17% 1	591
	21st to end	Night	17% 3 19% 34% 25% 3	650
June	1st to 10th	Night	13% 2 18% 40% 25% 3	600
	11th to 20th	Night	11% 2 15% 40% 29% 4	600
	21st to end	Night	12% 3 14% 39% 29% 3	600
July	1st to 10th	Night	15% 2 11% 35% 31% 6%	585
	11th to 20th	Night	10% 1 10% 36% 37% 6%	582
	21st to end	Night	14% 2 8% 36% 35% 5%	624
August	1st to 10th	Night	14% 9% 37% 37% 4	567
	11th to 20th	Night	15% 2 6% 36% 35% 5%	564
	21st to end	Night	12% 1 9% 36% 36% 7%	686
September	1st to 10th	Night	14% 2 11% 33% 34% 6%	760
	11th to 20th	Night	14% 2 14% 31% 34% 5%	760
	21st to end	Night	19% 3 18% 32% 25% 3	760
October	1st to 10th	Night	16% 4 19% 33% 25% 3	760
	11th to 20th	Night	21% 7% 22% 32% 17% 1	760
	21st to end	Night	25% 7% 20% 33% 13% 1	836
November	1st to 10th	Night	18% 8% 24% 34% 16% 1	780
	11th to 20th	Night	26% 12% 23% 28% 10% 1	800
	21st to end	Night	26% 10% 23% 31% 9% 1	800
December	1st to 10th	Night	32% 11% 26% 26% 5%	800
	11th to 20th	Night	28% 14% 25% 28% 5%	800
	21st to end	Night	31% 12% 24% 29% 4	880

Page generated at 2025-07-18 01:10:04. Version ac07

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