Rock Hill
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Rock Hill clear sky chart from 2005-11-06 to 2025-11-18.

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% 6% 2 6% 4 4 4 4 6% 8% 43%	126914
	Night	14% 6% 2 6% 4 4 4 4 5% 7% 44%	123465
Transparency	Day	38% 6% 12% 20% 18% 6%	130573
	Night	38% 6% 11% 18% 21% 7%	127875
Seeing	Day	17% 10% 24% 24% 18% 7%	16695
	Night	16% 9% 23% 30% 19% 3	26744

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	15% 6% 2 4 2 2 3 2 3 5% 56%	4056
	11th to 20th	Day	12% 6% 2 4 3 3 3 2 3 4 58%	4060
	21st to end	Day	17% 8% 3 6% 4 3 3 3 4 4 44%	4466
February	1st to 10th	Day	15% 7% 2 6% 4 3 4 4 6% 7% 42%	1800
	11th to 20th	Day	22% 6% 2 4 3 3 3 3 4 4 45%	1933
	21st to end	Day	18% 8% 2 6% 4 4 4 3 4 5% 41%	1700
March	1st to 10th	Day	21% 7% 3 6% 4 3 4 3 4 5% 41%	4935
	11th to 20th	Day	10% 4 2 6% 4 3 4 3 4 6% 54%	4980
	21st to end	Day	17% 6% 3 6% 4 4 3 3 4 5% 47%	5465
April	1st to 10th	Day	12% 4 2 4 3 3 4 5% 6% 8% 49%	2200
	11th to 20th	Day	12% 5% 1 6% 4 5% 5% 3 5% 7% 48%	2361
	21st to end	Day	12% 4 2 5% 4 5% 4 3 5% 7% 49%	2548
May	1st to 10th	Day	10% 7% 3 7% 5% 5% 6% 4 8% 11% 35%	5862
	11th to 20th	Day	18% 8% 3 6% 5% 5% 5% 5% 6% 8% 31%	5861
	21st to end	Day	12% 6% 2 6% 4 4 5% 3 6% 8% 43%	6433
June	1st to 10th	Day	9% 5% 2 7% 6% 5% 6% 5% 7% 11% 38%	2600
	11th to 20th	Day	7% 5% 2 5% 5% 6% 5% 4 8% 11% 42%	2600
	21st to end	Day	6% 4 2 5% 6% 7% 6% 5% 8% 11% 41%	2600
July	1st to 10th	Day	8% 6% 3 8% 6% 6% 6% 5% 8% 10% 34%	5900
	11th to 20th	Day	6% 5% 2 7% 6% 6% 7% 6% 10% 13% 32%	5900
	21st to end	Day	8% 7% 3 7% 6% 6% 7% 6% 9% 12% 29%	6424
August	1st to 10th	Day	21% 8% 3 6% 5% 5% 6% 5% 7% 9% 24%	4234
	11th to 20th	Day	10% 6% 2 7% 5% 6% 6% 5% 7% 12% 34%	4174
	21st to end	Day	6% 3 2 5% 4 5% 4 4 7% 9% 51%	4378
September	1st to 10th	Day	7% 5% 2 6% 4 6% 5% 5% 7% 9% 44%	2215
	11th to 20th	Day	12% 7% 2 7% 4 4 6% 5% 6% 8% 40%	2200
	21st to end	Day	17% 6% 2 7% 4 4 4 3 5% 7% 40%	2200
October	1st to 10th	Day	14% 4 2 4 3 4 3 3 4 6% 53%	2081
	11th to 20th	Day	13% 5% 2 3 3 3 3 2 3 4 59%	2000
	21st to end	Day	13% 4 2 4 3 3 2 2 4 5% 57%	2200
November	1st to 10th	Day	13% 6% 2 4 4 3 3 2 4 5% 55%	2057
	11th to 20th	Day	13% 5% 2 3 3 2 3 3 4 7% 54%	1922
	21st to end	Day	15% 5% 1 5% 4 3 4 3 4 6% 50%	1800
December	1st to 10th	Day	14% 6% 3 5% 3 3 3 2 3 5% 52%	2930
	11th to 20th	Day	17% 8% 3 6% 4 4 3 2 5% 6% 41%	3373
	21st to end	Day	21% 6% 2 4 3 3 4 3 4 6% 45%	4466
January	1st to 10th	Night	14% 5% 2 5% 3 3 3 2 3 4 58%	5889
	11th to 20th	Night	11% 6% 2 4 3 3 3 3 4 5% 54%	5900
	21st to end	Night	17% 9% 2 5% 3 3 3 2 4 5% 47%	6490
February	1st to 10th	Night	15% 7% 2 5% 4 4 4 3 5% 5% 46%	2600
	11th to 20th	Night	21% 6% 2 5% 4 4 3 3 4 4 43%	2348
	21st to end	Night	16% 5% 2 4 5% 3 4 4 5% 6% 46%	1928
March	1st to 10th	Night	20% 7% 2 5% 4 3 3 3 4 6% 43%	4918
	11th to 20th	Night	10% 4 1 4 3 3 4 3 5% 7% 54%	4980
	21st to end	Night	15% 5% 2 5% 4 3 4 4 5% 5% 48%	5467
April	1st to 10th	Night	13% 5% 2 5% 4 4 4 3 5% 7% 49%	2200
	11th to 20th	Night	11% 4 2 5% 4 3 3 3 5% 7% 52%	1986
	21st to end	Night	10% 5% 1 6% 3 4 4 3 5% 7% 50%	1868
May	1st to 10th	Night	12% 7% 2 7% 4 5% 4 4 5% 9% 42%	4033
	11th to 20th	Night	19% 7% 3 6% 4 4 5% 4 7% 8% 33%	4032
	21st to end	Night	12% 6% 3 6% 4 4 5% 4 6% 10% 40%	4421
June	1st to 10th	Night	10% 6% 3 7% 5% 6% 7% 4 8% 10% 35%	1800
	11th to 20th	Night	11% 5% 2 6% 5% 5% 6% 5% 7% 11% 36%	1800
	21st to end	Night	7% 6% 2 7% 6% 5% 6% 6% 8% 12% 36%	1800
July	1st to 10th	Night	10% 8% 3 8% 6% 6% 7% 5% 8% 10% 27%	4058
	11th to 20th	Night	7% 6% 2 8% 5% 7% 8% 7% 12% 13% 25%	4057
	21st to end	Night	11% 9% 3 10% 7% 7% 7% 5% 8% 10% 23%	4427
August	1st to 10th	Night	23% 8% 3 7% 5% 4 5% 4 7% 10% 23%	2915
	11th to 20th	Night	10% 7% 4 8% 4 5% 6% 5% 9% 10% 32%	2875
	21st to end	Night	6% 4 2 5% 5% 5% 5% 5% 6% 9% 48%	3484
September	1st to 10th	Night	9% 4 2 6% 5% 5% 5% 4 7% 10% 43%	2180
	11th to 20th	Night	13% 6% 3 7% 5% 4 5% 4 5% 8% 40%	2200
	21st to end	Night	17% 7% 2 6% 5% 4 4 4 5% 7% 39%	2200
October	1st to 10th	Night	13% 5% 2 4 3 4 3 3 5% 6% 52%	2317
	11th to 20th	Night	13% 6% 2 4 3 3 3 3 4 6% 55%	2400
	21st to end	Night	13% 5% 1 5% 3 3 3 2 3 5% 57%	2640
November	1st to 10th	Night	14% 6% 2 4 3 2 3 3 3 5% 54%	2461
	11th to 20th	Night	12% 6% 2 5% 4 3 4 3 3 5% 54%	2629
	21st to end	Night	13% 5% 2 4 3 4 4 4 5% 7% 50%	2600
December	1st to 10th	Night	15% 6% 2 5% 3 3 3 3 4 5% 52%	4250
	11th to 20th	Night	18% 7% 3 6% 4 3 3 3 4 5% 43%	4822
	21st to end	Night	20% 5% 2 5% 3 3 3 3 4 5% 46%	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	33% 3 5% 20% 32% 7%	4057
	11th to 20th	Day	32% 3 5% 15% 37% 10%	4060
	21st to end	Day	43% 4 4 17% 23% 9%	4466
February	1st to 10th	Day	40% 4 10% 21% 19% 5%	1800
	11th to 20th	Day	43% 3 7% 18% 23% 6%	1943
	21st to end	Day	45% 4 7% 16% 21% 8%	1700
March	1st to 10th	Day	45% 4 6% 14% 20% 11%	4935
	11th to 20th	Day	31% 4 7% 14% 25% 19%	4980
	21st to end	Day	41% 4 6% 21% 23% 6%	5465
April	1st to 10th	Day	32% 4 11% 22% 25% 6%	2200
	11th to 20th	Day	36% 5% 9% 20% 22% 9%	2361
	21st to end	Day	34% 6% 12% 22% 21% 4	2548
May	1st to 10th	Day	40% 9% 18% 21% 11% 1	5897
	11th to 20th	Day	49% 7% 14% 18% 11% 1	5865
	21st to end	Day	38% 6% 14% 21% 18% 3	6447
June	1st to 10th	Day	38% 8% 15% 25% 12% 2	2600
	11th to 20th	Day	34% 9% 19% 23% 15% 1	2600
	21st to end	Day	33% 10% 17% 22% 15% 2	2600
July	1st to 10th	Day	39% 11% 17% 23% 8% 1	5835
	11th to 20th	Day	36% 14% 30% 16% 3	5831
	21st to end	Day	43% 12% 17% 22% 6% 1	6240
August	1st to 10th	Day	52% 9% 18% 16% 4	4221
	11th to 20th	Day	39% 10% 17% 20% 12% 1	4213
	21st to end	Day	27% 8% 16% 31% 16% 2	4378
September	1st to 10th	Day	34% 8% 15% 20% 19% 4	2215
	11th to 20th	Day	39% 8% 11% 18% 20% 3	2200
	21st to end	Day	44% 6% 9% 20% 16% 5%	2200
October	1st to 10th	Day	30% 5% 7% 20% 29% 10%	3393
	11th to 20th	Day	20% 2 6% 18% 29% 25%	3240
	21st to end	Day	26% 3 5% 24% 26% 16%	3538
November	1st to 10th	Day	33% 3 8% 18% 28% 9%	2057
	11th to 20th	Day	31% 3 8% 22% 28% 8%	1922
	21st to end	Day	35% 4 8% 21% 24% 7%	1800
December	1st to 10th	Day	36% 3 6% 19% 28% 9%	2930
	11th to 20th	Day	45% 4 7% 18% 21% 6%	3370
	21st to end	Day	40% 5% 8% 25% 20% 3	4466
January	1st to 10th	Night	31% 3 5% 17% 36% 8%	5889
	11th to 20th	Night	32% 4 6% 15% 35% 8%	5900
	21st to end	Night	41% 3 4 17% 28% 8%	6490
February	1st to 10th	Night	40% 4 9% 21% 21% 6%	2600
	11th to 20th	Night	45% 2 7% 17% 22% 7%	2349
	21st to end	Night	39% 3 10% 17% 23% 8%	1928
March	1st to 10th	Night	43% 5% 7% 12% 26% 7%	4918
	11th to 20th	Night	29% 4 7% 14% 28% 18%	4980
	21st to end	Night	38% 4 7% 21% 21% 8%	5467
April	1st to 10th	Night	35% 4 10% 19% 24% 7%	2200
	11th to 20th	Night	31% 4 9% 23% 23% 9%	1986
	21st to end	Night	33% 5% 11% 26% 20% 4	1868
May	1st to 10th	Night	40% 6% 16% 22% 14% 2	4060
	11th to 20th	Night	47% 8% 15% 18% 11% 1	4033
	21st to end	Night	37% 8% 13% 25% 15% 2	4429
June	1st to 10th	Night	41% 10% 15% 20% 12% 2	1800
	11th to 20th	Night	39% 10% 17% 21% 12% 2	1800
	21st to end	Night	37% 12% 17% 19% 12% 3	1800
July	1st to 10th	Night	47% 12% 18% 15% 7% 1	4015
	11th to 20th	Night	42% 18% 25% 11% 3 1	4006
	21st to end	Night	51% 12% 16% 16% 5%	4297
August	1st to 10th	Night	54% 8% 19% 14% 4	2904
	11th to 20th	Night	41% 12% 23% 16% 8% 1	2902
	21st to end	Night	30% 8% 13% 26% 20% 3	3484
September	1st to 10th	Night	34% 9% 15% 21% 16% 5%	2180
	11th to 20th	Night	41% 8% 13% 16% 20% 2	2200
	21st to end	Night	44% 7% 11% 18% 16% 5%	2200
October	1st to 10th	Night	33% 5% 10% 19% 26% 6%	3785
	11th to 20th	Night	22% 3 5% 18% 27% 24%	3899
	21st to end	Night	28% 3 6% 21% 26% 17%	4252
November	1st to 10th	Night	34% 3 7% 18% 27% 11%	2461
	11th to 20th	Night	34% 4 6% 18% 27% 12%	2629
	21st to end	Night	34% 4 6% 22% 27% 7%	2600
December	1st to 10th	Night	35% 3 6% 19% 29% 8%	4250
	11th to 20th	Night	44% 4 10% 15% 18% 9%	4824
	21st to end	Night	40% 3 10% 23% 22% 2	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	21% 22% 39% 17% 2	300
	11th to 20th	Day	26% 21% 32% 16% 4	300
	21st to end	Day	22% 23% 39% 12% 3	330
February	1st to 10th	Day	21% 20% 40% 13% 5%	300
	11th to 20th	Day	37% 20% 33% 8% 1	300
	21st to end	Day	26% 20% 34% 16% 3	299
March	1st to 10th	Day	24% 17% 32% 19% 7% 1	500
	11th to 20th	Day	22% 15% 39% 18% 5% 1	500
	21st to end	Day	23% 18% 35% 18% 5% 1	550
April	1st to 10th	Day	19% 18% 34% 19% 9% 1	500
	11th to 20th	Day	16% 15% 34% 21% 11% 3	492
	21st to end	Day	17% 8% 30% 27% 15% 4	500
May	1st to 10th	Day	11% 15% 34% 22% 13% 4	503
	11th to 20th	Day	19% 5% 23% 27% 20% 7%	527
	21st to end	Day	14% 7% 21% 24% 24% 10%	573
June	1st to 10th	Day	10% 4 18% 33% 25% 9%	530
	11th to 20th	Day	9% 5% 19% 27% 27% 13%	530
	21st to end	Day	8% 2 15% 35% 29% 11%	530
July	1st to 10th	Day	9% 2 10% 29% 30% 21%	525
	11th to 20th	Day	8% 3 7% 30% 35% 17%	524
	21st to end	Day	7% 2 11% 30% 36% 13%	576
August	1st to 10th	Day	18% 4 12% 30% 20% 16%	526
	11th to 20th	Day	15% 3 11% 27% 26% 18%	519
	21st to end	Day	12% 3 11% 31% 27% 17%	583
September	1st to 10th	Day	9% 4 13% 33% 28% 13%	530
	11th to 20th	Day	16% 4 15% 25% 26% 14%	530
	21st to end	Day	20% 7% 17% 27% 24% 5%	530
October	1st to 10th	Day	15% 4 24% 25% 24% 8%	530
	11th to 20th	Day	12% 9% 25% 27% 23% 5%	530
	21st to end	Day	17% 9% 25% 26% 18% 5%	583
November	1st to 10th	Day	19% 8% 27% 28% 15% 3	535
	11th to 20th	Day	19% 14% 35% 24% 7% 2	380
	21st to end	Day	17% 11% 35% 24% 11% 2	300
December	1st to 10th	Day	27% 15% 38% 15% 5%	300
	11th to 20th	Day	22% 12% 35% 22% 9% 1	300
	21st to end	Day	27% 14% 35% 15% 7% 2	330
January	1st to 10th	Night	21% 19% 29% 25% 6%	800
	11th to 20th	Night	20% 15% 31% 30% 5%	800
	21st to end	Night	20% 15% 35% 27% 4	880
February	1st to 10th	Night	19% 12% 36% 29% 4	800
	11th to 20th	Night	27% 14% 37% 21% 1	783
	21st to end	Night	20% 17% 35% 23% 4	660
March	1st to 10th	Night	19% 13% 33% 29% 6%	800
	11th to 20th	Night	19% 15% 33% 25% 7% 1	800
	21st to end	Night	20% 13% 34% 26% 7%	880
April	1st to 10th	Night	18% 16% 32% 25% 9%	800
	11th to 20th	Night	14% 14% 28% 30% 13% 1	791
	21st to end	Night	13% 7% 29% 35% 15%	668
May	1st to 10th	Night	11% 9% 29% 36% 14%	600
	11th to 20th	Night	14% 7% 23% 36% 20% 1	591
	21st to end	Night	14% 4 14% 36% 30% 2	650
June	1st to 10th	Night	11% 4 17% 39% 25% 5%	600
	11th to 20th	Night	14% 3 15% 30% 30% 9%	600
	21st to end	Night	9% 3 11% 38% 34% 6%	600
July	1st to 10th	Night	11% 2 11% 32% 36% 8%	585
	11th to 20th	Night	9% 2 6% 34% 40% 9%	582
	21st to end	Night	8% 3 10% 30% 42% 7%	657
August	1st to 10th	Night	16% 2 8% 30% 35% 9%	597
	11th to 20th	Night	13% 4 9% 29% 39% 7%	594
	21st to end	Night	11% 2 10% 29% 42% 7%	817
September	1st to 10th	Night	10% 3 11% 32% 38% 6%	800
	11th to 20th	Night	15% 3 11% 28% 37% 6%	800
	21st to end	Night	20% 5% 16% 32% 25% 3	800
October	1st to 10th	Night	15% 5% 17% 32% 26% 6%	800
	11th to 20th	Night	17% 7% 22% 31% 21% 2	800
	21st to end	Night	16% 9% 22% 31% 20% 3	880
November	1st to 10th	Night	16% 7% 25% 33% 17% 3	820
	11th to 20th	Night	15% 11% 29% 33% 11% 1	829
	21st to end	Night	15% 11% 27% 33% 14% 1	800
December	1st to 10th	Night	21% 12% 26% 34% 7% 1	800
	11th to 20th	Night	20% 11% 25% 33% 11% 1	800
	21st to end	Night	25% 13% 22% 31% 9%	880

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