Central-Clemson
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Central-Clemson clear sky chart from 2005-11-06 to 2025-04-26.

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	14% 5% 2 5% 4 4 4 4 6% 8% 44%	114055
	Night	15% 5% 2 5% 4 4 4 4 5% 7% 45%	114566
Transparency	Day	37% 7% 11% 20% 19% 7%	117823
	Night	37% 6% 10% 18% 22% 7%	118937
Seeing	Day	16% 9% 22% 25% 18% 9%	15624
	Night	16% 6% 23% 31% 20% 4	25765

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% 4 2 4 3 3 3 3 4 6% 54%	4056
	11th to 20th	Day	13% 5% 2 4 3 3 2 2 4 3 60%	4060
	21st to end	Day	21% 6% 2 6% 3 2 3 2 4 7% 45%	4466
February	1st to 10th	Day	16% 6% 2 5% 4 4 4 4 5% 7% 43%	1940
	11th to 20th	Day	22% 6% 2 5% 4 3 3 3 4 6% 43%	1948
	21st to end	Day	20% 5% 2 6% 4 4 3 3 4 5% 43%	1650
March	1st to 10th	Day	22% 6% 2 6% 4 3 4 3 4 6% 40%	4802
	11th to 20th	Day	11% 4 2 5% 3 3 3 3 4 6% 55%	4980
	21st to end	Day	18% 5% 2 5% 3 4 4 3 4 5% 46%	5465
April	1st to 10th	Day	13% 4 1 5% 4 4 5% 4 6% 8% 47%	2200
	11th to 20th	Day	12% 5% 2 6% 4 4 4 3 5% 7% 49%	2365
	21st to end	Day	11% 5% 2 4 4 4 3 4 5% 8% 50%	2535
May	1st to 10th	Day	12% 7% 2 7% 4 4 5% 4 7% 8% 40%	4082
	11th to 20th	Day	17% 7% 2 7% 4 4 5% 4 5% 8% 38%	4081
	21st to end	Day	7% 4 2 5% 4 4 5% 5% 8% 11% 44%	4476
June	1st to 10th	Day	10% 5% 2 6% 6% 6% 5% 5% 10% 11% 33%	2470
	11th to 20th	Day	5% 4 2 6% 6% 6% 5% 6% 10% 12% 40%	2470
	21st to end	Day	7% 4 2 6% 5% 6% 6% 5% 7% 11% 40%	2470
July	1st to 10th	Day	7% 5% 2 7% 6% 6% 7% 6% 8% 11% 35%	4120
	11th to 20th	Day	8% 5% 2 6% 5% 6% 6% 5% 8% 11% 38%	4120
	21st to end	Day	9% 8% 4 10% 7% 8% 8% 7% 8% 10% 21%	4466
August	1st to 10th	Day	12% 6% 3 7% 5% 5% 6% 5% 8% 10% 33%	4104
	11th to 20th	Day	9% 4 2 6% 5% 5% 6% 5% 7% 10% 41%	4044
	21st to end	Day	7% 4 2 5% 4 4 4 4 6% 8% 52%	4103
September	1st to 10th	Day	8% 5% 2 6% 5% 5% 6% 5% 7% 9% 42%	2090
	11th to 20th	Day	15% 7% 2 6% 5% 5% 4 4 6% 9% 39%	2090
	21st to end	Day	15% 6% 2 5% 4 3 3 3 6% 7% 46%	2090
October	1st to 10th	Day	14% 5% 1 4 3 3 3 2 4 6% 56%	2081
	11th to 20th	Day	11% 4 2 4 3 3 2 3 4 6% 58%	1900
	21st to end	Day	13% 4 2 4 3 3 2 2 4 4 58%	2090
November	1st to 10th	Day	14% 5% 2 3 3 3 2 2 4 5% 55%	1872
	11th to 20th	Day	15% 4 1 4 2 4 4 2 5% 7% 51%	1800
	21st to end	Day	16% 4 2 4 3 3 3 3 6% 6% 50%	1800
December	1st to 10th	Day	20% 5% 2 4 2 3 2 3 4 6% 50%	2930
	11th to 20th	Day	20% 6% 2 5% 4 4 4 4 5% 4 43%	3373
	21st to end	Day	24% 4 2 3 2 3 3 2 4 6% 46%	4466
January	1st to 10th	Night	15% 3 1 4 3 3 3 3 3 5% 56%	5889
	11th to 20th	Night	13% 6% 2 5% 3 3 3 4 4 5% 53%	5900
	21st to end	Night	19% 6% 2 4 3 3 3 3 4 6% 48%	6490
February	1st to 10th	Night	16% 6% 2 5% 4 3 4 3 4 6% 48%	2458
	11th to 20th	Night	23% 5% 2 5% 4 3 3 3 4 4 44%	2338
	21st to end	Night	18% 6% 2 5% 3 4 4 3 5% 5% 45%	1980
March	1st to 10th	Night	20% 5% 2 6% 3 3 4 4 5% 7% 41%	5038
	11th to 20th	Night	9% 4 1 4 3 3 4 3 6% 8% 54%	4980
	21st to end	Night	17% 5% 2 5% 4 4 4 3 5% 6% 47%	5467
April	1st to 10th	Night	13% 4 2 5% 4 3 4 3 4 7% 50%	2200
	11th to 20th	Night	13% 4 2 5% 3 3 3 3 5% 7% 51%	1999
	21st to end	Night	12% 5% 1 5% 5% 3 5% 3 5% 7% 50%	1761
May	1st to 10th	Night	13% 7% 3 7% 4 5% 5% 4 6% 9% 39%	2813
	11th to 20th	Night	19% 6% 2 7% 4 4 4 4 5% 7% 38%	2812
	21st to end	Night	9% 5% 2 6% 5% 5% 5% 4 8% 10% 44%	3079
June	1st to 10th	Night	9% 6% 2 7% 6% 6% 6% 5% 9% 11% 34%	1710
	11th to 20th	Night	8% 5% 2 6% 5% 5% 7% 5% 7% 11% 40%	1710
	21st to end	Night	8% 5% 3 6% 4 5% 7% 6% 8% 11% 38%	1710
July	1st to 10th	Night	11% 8% 3 8% 7% 6% 7% 6% 8% 9% 28%	2840
	11th to 20th	Night	9% 5% 2 6% 6% 6% 6% 6% 11% 12% 31%	2840
	21st to end	Night	14% 9% 4 10% 7% 6% 6% 6% 8% 9% 21%	3087
August	1st to 10th	Night	12% 7% 3 8% 6% 6% 6% 5% 8% 11% 30%	2825
	11th to 20th	Night	13% 7% 3 9% 6% 6% 7% 5% 7% 9% 28%	2785
	21st to end	Night	9% 6% 2 6% 5% 5% 5% 4 6% 8% 44%	3517
September	1st to 10th	Night	10% 6% 2 8% 5% 5% 5% 4 7% 9% 39%	2090
	11th to 20th	Night	17% 7% 3 7% 4 5% 6% 4 5% 8% 35%	2090
	21st to end	Night	15% 5% 2 7% 4 4 4 3 4 6% 45%	2090
October	1st to 10th	Night	13% 4 2 6% 3 3 4 3 5% 5% 52%	2097
	11th to 20th	Night	12% 4 2 5% 3 4 3 3 5% 5% 54%	2280
	21st to end	Night	13% 3 2 3 3 2 3 3 4 5% 58%	2508
November	1st to 10th	Night	15% 5% 2 5% 3 3 3 2 3 5% 56%	2421
	11th to 20th	Night	17% 7% 2 4 2 2 3 2 4 6% 52%	2600
	21st to end	Night	17% 4 2 3 3 4 3 4 5% 6% 49%	2600
December	1st to 10th	Night	18% 5% 2 4 3 3 3 2 4 4 52%	4250
	11th to 20th	Night	21% 6% 2 6% 4 3 3 3 3 4 44%	4822
	21st to end	Night	22% 5% 2 4 3 3 4 3 4 6% 44%	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	32% 4 5% 22% 31% 7%	4057
	11th to 20th	Day	30% 3 4 15% 37% 10%	4060
	21st to end	Day	42% 2 4 18% 23% 11%	4466
February	1st to 10th	Day	40% 4 9% 21% 20% 6%	1940
	11th to 20th	Day	43% 4 6% 19% 22% 6%	1958
	21st to end	Day	43% 4 8% 14% 22% 9%	1650
March	1st to 10th	Day	45% 4 6% 14% 18% 12%	4802
	11th to 20th	Day	31% 4 6% 13% 26% 20%	4980
	21st to end	Day	41% 4 6% 22% 21% 6%	5465
April	1st to 10th	Day	34% 6% 11% 20% 23% 6%	2200
	11th to 20th	Day	35% 6% 9% 21% 21% 9%	2365
	21st to end	Day	31% 6% 12% 27% 20% 4	2535
May	1st to 10th	Day	39% 9% 14% 20% 15% 4	4120
	11th to 20th	Day	42% 9% 12% 22% 14% 1	4085
	21st to end	Day	29% 11% 16% 21% 20% 2	4489
June	1st to 10th	Day	39% 12% 18% 21% 8% 2	2470
	11th to 20th	Day	32% 12% 18% 24% 12% 2	2470
	21st to end	Day	35% 11% 18% 19% 13% 4	2470
July	1st to 10th	Day	38% 14% 19% 21% 6% 1	4055
	11th to 20th	Day	35% 13% 25% 20% 5% 2	4051
	21st to end	Day	53% 14% 16% 11% 5%	4324
August	1st to 10th	Day	43% 10% 16% 26% 6%	4091
	11th to 20th	Day	35% 10% 21% 18% 14% 2	4083
	21st to end	Day	28% 8% 15% 25% 20% 3	4103
September	1st to 10th	Day	36% 11% 14% 18% 19% 3	2090
	11th to 20th	Day	41% 8% 12% 18% 18% 2	2090
	21st to end	Day	38% 6% 12% 20% 19% 6%	2090
October	1st to 10th	Day	27% 4 7% 17% 34% 10%	3458
	11th to 20th	Day	20% 3 6% 16% 29% 27%	3140
	21st to end	Day	28% 2 5% 22% 28% 15%	3428
November	1st to 10th	Day	32% 3 9% 18% 28% 10%	1872
	11th to 20th	Day	34% 3 7% 21% 26% 9%	1800
	21st to end	Day	35% 4 8% 21% 25% 7%	1800
December	1st to 10th	Day	37% 3 5% 17% 27% 10%	2930
	11th to 20th	Day	44% 5% 7% 17% 18% 9%	3370
	21st to end	Day	40% 3 8% 26% 20% 2	4466
January	1st to 10th	Night	30% 4 6% 19% 31% 10%	5889
	11th to 20th	Night	33% 4 5% 15% 33% 9%	5900
	21st to end	Night	38% 3 5% 17% 29% 9%	6490
February	1st to 10th	Night	39% 4 8% 21% 21% 7%	2458
	11th to 20th	Night	44% 4 5% 17% 23% 6%	2339
	21st to end	Night	41% 4 8% 17% 21% 9%	1980
March	1st to 10th	Night	41% 5% 8% 12% 27% 7%	5038
	11th to 20th	Night	27% 5% 7% 13% 29% 18%	4980
	21st to end	Night	39% 4 6% 20% 21% 10%	5467
April	1st to 10th	Night	35% 5% 9% 19% 25% 7%	2200
	11th to 20th	Night	33% 5% 7% 22% 24% 10%	1999
	21st to end	Night	34% 6% 10% 25% 21% 4	1761
May	1st to 10th	Night	40% 10% 12% 21% 15% 2	2840
	11th to 20th	Night	43% 8% 14% 19% 15% 1	2813
	21st to end	Night	30% 10% 15% 26% 17% 2	3087
June	1st to 10th	Night	39% 14% 15% 19% 11% 2	1710
	11th to 20th	Night	35% 10% 17% 24% 11% 2	1710
	21st to end	Night	35% 14% 17% 16% 13% 4	1710
July	1st to 10th	Night	49% 13% 17% 15% 6% 1	2795
	11th to 20th	Night	38% 17% 23% 18% 4 1	2786
	21st to end	Night	54% 15% 15% 12% 4 1	2988
August	1st to 10th	Night	46% 10% 21% 18% 5%	2814
	11th to 20th	Night	49% 13% 16% 14% 8% 1	2812
	21st to end	Night	36% 9% 13% 20% 19% 2	3517
September	1st to 10th	Night	39% 10% 15% 18% 15% 4	2090
	11th to 20th	Night	45% 9% 13% 17% 15% 2	2090
	21st to end	Night	40% 6% 12% 18% 18% 6%	2090
October	1st to 10th	Night	29% 5% 9% 14% 34% 9%	3500
	11th to 20th	Night	20% 4 6% 17% 28% 26%	3779
	21st to end	Night	27% 3 5% 20% 26% 18%	4120
November	1st to 10th	Night	34% 4 7% 19% 25% 11%	2421
	11th to 20th	Night	35% 3 5% 17% 28% 11%	2600
	21st to end	Night	35% 5% 7% 19% 26% 7%	2600
December	1st to 10th	Night	36% 3 5% 18% 30% 8%	4250
	11th to 20th	Night	44% 4 8% 16% 20% 8%	4824
	21st to end	Night	42% 4 8% 24% 19% 3	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	22% 16% 36% 22% 4	300
	11th to 20th	Day	24% 19% 28% 22% 5% 1	300
	21st to end	Day	22% 18% 40% 16% 4 1	330
February	1st to 10th	Day	20% 16% 35% 24% 5%	300
	11th to 20th	Day	36% 16% 31% 15% 2	300
	21st to end	Day	26% 23% 34% 15% 2	249
March	1st to 10th	Day	22% 15% 28% 27% 8% 1	440
	11th to 20th	Day	19% 12% 40% 22% 6% 1	500
	21st to end	Day	23% 15% 31% 22% 8% 1	550
April	1st to 10th	Day	19% 15% 28% 25% 12% 1	500
	11th to 20th	Day	13% 12% 34% 26% 13% 3	492
	21st to end	Day	17% 9% 24% 26% 16% 7%	480
May	1st to 10th	Day	13% 10% 30% 25% 15% 6%	463
	11th to 20th	Day	17% 6% 17% 30% 20% 10%	487
	21st to end	Day	15% 6% 20% 24% 22% 14%	529
June	1st to 10th	Day	13% 6% 15% 29% 24% 13%	490
	11th to 20th	Day	7% 6% 14% 27% 26% 21%	490
	21st to end	Day	10% 3 14% 31% 23% 19%	490
July	1st to 10th	Day	9% 3 10% 23% 30% 26%	485
	11th to 20th	Day	6% 3 8% 25% 36% 23%	484
	21st to end	Day	8% 2 8% 28% 34% 20%	532
August	1st to 10th	Day	15% 4 13% 24% 25% 20%	486
	11th to 20th	Day	13% 1 8% 28% 33% 17%	479
	21st to end	Day	12% 3 10% 24% 28% 23%	539
September	1st to 10th	Day	9% 2 13% 28% 30% 17%	490
	11th to 20th	Day	18% 3 14% 21% 30% 15%	490
	21st to end	Day	19% 5% 14% 29% 24% 9%	490
October	1st to 10th	Day	18% 4 16% 30% 21% 11%	490
	11th to 20th	Day	12% 7% 20% 33% 23% 5%	490
	21st to end	Day	18% 6% 20% 31% 20% 4	539
November	1st to 10th	Day	20% 8% 25% 27% 16% 4	410
	11th to 20th	Day	17% 12% 32% 28% 10% 1	300
	21st to end	Day	12% 12% 35% 27% 12% 2	300
December	1st to 10th	Day	27% 14% 27% 25% 6%	300
	11th to 20th	Day	23% 12% 32% 25% 7% 1	300
	21st to end	Day	31% 15% 26% 17% 8% 2	330
January	1st to 10th	Night	21% 10% 34% 29% 6%	800
	11th to 20th	Night	21% 12% 32% 29% 5%	800
	21st to end	Night	19% 11% 39% 28% 3	880
February	1st to 10th	Night	18% 9% 31% 38% 4	800
	11th to 20th	Night	26% 8% 35% 28% 3	783
	21st to end	Night	19% 13% 34% 31% 3	660
March	1st to 10th	Night	19% 9% 33% 34% 6%	800
	11th to 20th	Night	16% 9% 36% 30% 8% 1	800
	21st to end	Night	18% 10% 31% 32% 9%	880
April	1st to 10th	Night	15% 12% 29% 32% 11%	800
	11th to 20th	Night	14% 9% 27% 34% 15% 1	685
	21st to end	Night	13% 5% 24% 36% 20% 2	587
May	1st to 10th	Night	13% 5% 24% 36% 21% 1	570
	11th to 20th	Night	16% 5% 19% 32% 25% 3	561
	21st to end	Night	14% 2 14% 29% 35% 5%	617
June	1st to 10th	Night	12% 3 11% 34% 34% 6%	570
	11th to 20th	Night	10% 2 14% 25% 35% 14%	570
	21st to end	Night	9% 2 9% 35% 38% 7%	570
July	1st to 10th	Night	13% 2 8% 27% 38% 12%	555
	11th to 20th	Night	12% 1 5% 24% 43% 14%	552
	21st to end	Night	9% 2 8% 27% 45% 10%	624
August	1st to 10th	Night	14% 2 9% 25% 37% 12%	567
	11th to 20th	Night	14% 2 9% 27% 38% 9%	564
	21st to end	Night	14% 3 7% 25% 41% 10%	674
September	1st to 10th	Night	10% 3 12% 28% 37% 10%	760
	11th to 20th	Night	18% 3 14% 26% 33% 6%	760
	21st to end	Night	16% 4 15% 33% 30% 3	760
October	1st to 10th	Night	14% 5% 18% 27% 29% 6%	760
	11th to 20th	Night	14% 6% 22% 34% 21% 3	760
	21st to end	Night	15% 5% 20% 35% 22% 3	836
November	1st to 10th	Night	16% 5% 22% 35% 19% 3	780
	11th to 20th	Night	18% 8% 29% 32% 13% 1	800
	21st to end	Night	18% 7% 23% 37% 14% 1	800
December	1st to 10th	Night	23% 8% 27% 32% 9% 1	800
	11th to 20th	Night	20% 6% 30% 33% 11% 1	800
	21st to end	Night	26% 9% 26% 32% 8%	880

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