Rock Hill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Rock Hill Observatory clear sky chart from 2005-11-06 to 2026-07-16.

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	10% 4 1 3 2 2 2 2 3 5% 66%	146783
	Night	13% 4 1 3 2 2 3 2 4 5% 60%	143343
Transparency	Day	24% 3 6% 18% 26% 24%	141719
	Night	29% 3 7% 18% 23% 21%	139394
Seeing	Day	13% 7% 25% 33% 16% 6%	17314
	Night	15% 3 13% 35% 29% 5%	26591

By Month

Percentage Cloud Cover SW (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	27% 7% 1 3 2 2 3 2 4 6% 42%	4722
	11th to 20th	Day	14% 5% 1 4 3 3 2 2 3 4 59%	5137
	21st to end	Day	17% 6% 2 5% 3 4 4 3 5% 7% 46%	5843
February	1st to 10th	Day	23% 6% 1 4 3 2 3 3 4 6% 45%	1890
	11th to 20th	Day	20% 8% 1 5% 4 4 3 3 4 6% 43%	1980
	21st to end	Day	21% 8% 2 4 3 2 3 3 4 6% 45%	1730
March	1st to 10th	Day	20% 14% 2 6% 4 3 3 3 4 6% 35%	4742
	11th to 20th	Day	20% 8% 1 4 2 3 2 2 4 5% 48%	5110
	21st to end	Day	23% 9% 2 5% 3 3 4 3 5% 7% 36%	5831
April	1st to 10th	Day	17% 7% 2 4 3 3 3 3 5% 6% 46%	2520
	11th to 20th	Day	15% 4 1 4 3 3 2 3 5% 6% 54%	2489
	21st to end	Day	11% 3 1 3 3 2 3 2 5% 6% 58%	2520
May	1st to 10th	Day	7% 4 1 3 2 2 2 2 3 4 69%	7275
	11th to 20th	Day	3 2 1 2 1 1 2 1 2 5% 79%	8160
	21st to end	Day	4 2 1 2 1 2 2 2 3 5% 75%	8992
June	1st to 10th	Day	6% 3 1 3 2 2 2 2 5% 7% 65%	2940
	11th to 20th	Day	2 2 1 2 1 2 1 2 3 4 81%	2940
	21st to end	Day	2 1 2 1 1 1 1 2 3 85%	2940
July	1st to 10th	Day	1 1 1 1 1 2 93%	6459
	11th to 20th	Day	1 1 1 1 1 1 1 2 2 89%	6391
	21st to end	Day	1 1 1 1 2 2 2 3 4 84%	6491
August	1st to 10th	Day	2 1 1 1 1 1 1 2 2 3 84%	5777
	11th to 20th	Day	1 1 1 1 1 1 2 2 91%	5448
	21st to end	Day	2 1 1 1 2 1 2 2 3 84%	5984
September	1st to 10th	Day	2 1 1 1 1 2 1 2 3 87%	2400
	11th to 20th	Day	4 2 1 3 2 2 2 2 3 5% 75%	2400
	21st to end	Day	5% 2 1 2 1 2 2 1 2 4 79%	2161
October	1st to 10th	Day	4 2 1 3 2 2 2 2 4 5% 73%	3280
	11th to 20th	Day	7% 2 1 2 2 2 2 2 3 5% 72%	3261
	21st to end	Day	6% 3 1 4 3 3 3 3 4 6% 63%	3558
November	1st to 10th	Day	13% 5% 1 4 3 3 3 2 4 7% 54%	1879
	11th to 20th	Day	17% 3 2 3 3 4 5% 3 5% 7% 48%	1890
	21st to end	Day	14% 4 2 4 3 3 3 3 4 5% 55%	1743
December	1st to 10th	Day	10% 3 1 3 3 3 3 3 5% 7% 59%	2700
	11th to 20th	Day	31% 6% 2 4 3 3 3 3 5% 7% 33%	3108
	21st to end	Day	30% 6% 2 5% 3 4 4 3 6% 7% 31%	4092
January	1st to 10th	Night	24% 8% 2 4 3 3 3 3 5% 7% 40%	8172
	11th to 20th	Night	14% 4 1 4 2 2 2 2 4 5% 60%	7839
	21st to end	Night	17% 5% 2 6% 4 4 4 3 4 5% 45%	8371
February	1st to 10th	Night	22% 6% 1 3 3 2 3 2 3 5% 49%	2730
	11th to 20th	Night	19% 6% 2 5% 3 3 4 3 5% 5% 44%	2526
	21st to end	Night	19% 7% 2 4 3 2 3 3 4 5% 47%	2076
March	1st to 10th	Night	24% 13% 2 5% 3 3 3 2 3 6% 36%	5368
	11th to 20th	Night	24% 9% 1 3 2 2 2 2 3 4 48%	5110
	21st to end	Night	24% 9% 2 5% 3 3 4 3 5% 6% 36%	5372
April	1st to 10th	Night	16% 6% 2 4 3 3 3 3 4 6% 48%	2100
	11th to 20th	Night	15% 4 2 4 3 3 3 3 4 6% 53%	2079
	21st to end	Night	10% 4 1 3 3 2 3 3 5% 6% 61%	2100
May	1st to 10th	Night	6% 4 1 3 3 3 3 2 3 4 67%	5670
	11th to 20th	Night	3 1 1 2 2 2 2 2 3 4 78%	4700
	21st to end	Night	4 1 1 2 2 2 2 2 3 5% 77%	5181
June	1st to 10th	Night	7% 3 1 2 3 3 2 3 5% 6% 66%	1680
	11th to 20th	Night	1 2 1 2 2 1 2 1 3 4 80%	1680
	21st to end	Night	2 2 1 2 2 2 2 1 2 3 82%	1680
July	1st to 10th	Night	1 1 1 1 1 1 2 93%	3720
	11th to 20th	Night	2 1 1 1 1 1 1 1 3 87%	3680
	21st to end	Night	1 1 1 2 2 2 2 3 4 4 78%	4373
August	1st to 10th	Night	2 2 1 1 1 2 1 2 3 4 81%	4035
	11th to 20th	Night	1 1 1 1 2 3 91%	4416
	21st to end	Night	3 1 1 2 1 1 2 2 2 3 82%	5015
September	1st to 10th	Night	2 1 1 1 1 1 1 2 4 87%	2000
	11th to 20th	Night	4 2 1 2 2 1 2 1 3 4 77%	2000
	21st to end	Night	3 1 2 1 2 2 2 3 5% 78%	2233
October	1st to 10th	Night	4 2 1 2 3 2 2 2 2 3 77%	3920
	11th to 20th	Night	5% 1 1 2 1 2 2 2 3 4 78%	3897
	21st to end	Night	10% 3 1 3 2 3 3 2 4 6% 63%	4253
November	1st to 10th	Night	16% 5% 1 3 2 2 2 3 4 7% 54%	2640
	11th to 20th	Night	13% 5% 2 4 2 3 3 3 5% 6% 53%	2730
	21st to end	Night	14% 4 1 4 3 3 3 2 4 5% 56%	2875
December	1st to 10th	Night	12% 3 1 2 2 2 3 3 4 6% 62%	4700
	11th to 20th	Night	30% 7% 2 4 3 3 4 3 6% 8% 30%	5316
	21st to end	Night	31% 7% 3 6% 4 4 4 3 4 5% 31%	7106

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	45% 3 11% 21% 14% 6%	4723
	11th to 20th	Day	32% 2 4 12% 25% 25%	5137
	21st to end	Day	39% 2 7% 18% 18% 15%	5843
February	1st to 10th	Day	42% 3 7% 18% 19% 11%	1890
	11th to 20th	Day	43% 4 8% 20% 20% 6%	1989
	21st to end	Day	41% 3 9% 22% 16% 8%	1730
March	1st to 10th	Day	50% 5% 7% 18% 17% 4	4742
	11th to 20th	Day	40% 3 11% 27% 12% 8%	5110
	21st to end	Day	47% 6% 13% 22% 11% 2	5831
April	1st to 10th	Day	38% 4 11% 25% 17% 4	2520
	11th to 20th	Day	31% 4 13% 26% 19% 6%	2492
	21st to end	Day	25% 4 13% 30% 24% 3	2520
May	1st to 10th	Day	20% 3 7% 25% 32% 14%	7275
	11th to 20th	Day	11% 2 5% 25% 43% 14%	8163
	21st to end	Day	13% 3 7% 27% 43% 7%	8994
June	1st to 10th	Day	20% 3 9% 25% 31% 12%	2940
	11th to 20th	Day	10% 2 6% 19% 36% 27%	2940
	21st to end	Day	8% 1 2 13% 38% 38%	2940
July	1st to 10th	Day	3 1 1 6% 19% 70%	6391
	11th to 20th	Day	6% 1 1 5% 11% 76%	6316
	21st to end	Day	8% 1 2 14% 27% 48%	6303
August	1st to 10th	Day	11% 1 1 8% 23% 57%	4206
	11th to 20th	Day	4 1 1 6% 38% 50%	3941
	21st to end	Day	6% 1 2 8% 40% 43%	4311
September	1st to 10th	Day	6% 1 1 10% 27% 55%	2400
	11th to 20th	Day	14% 2 4 21% 36% 23%	2400
	21st to end	Day	13% 2 4 17% 34% 31%	2161
October	1st to 10th	Day	15% 2 3 12% 25% 43%	3280
	11th to 20th	Day	17% 2 4 17% 40% 20%	3261
	21st to end	Day	23% 3 5% 20% 33% 16%	3558
November	1st to 10th	Day	30% 4 8% 21% 23% 14%	1879
	11th to 20th	Day	36% 5% 9% 20% 21% 9%	1890
	21st to end	Day	32% 4 8% 19% 24% 14%	1743
December	1st to 10th	Day	27% 2 6% 23% 28% 13%	2700
	11th to 20th	Day	53% 2 7% 16% 15% 7%	3108
	21st to end	Day	52% 4 12% 17% 10% 3	4092
January	1st to 10th	Night	45% 4 14% 18% 13% 7%	8172
	11th to 20th	Night	29% 3 6% 12% 23% 26%	7839
	21st to end	Night	42% 3 6% 15% 19% 16%	8371
February	1st to 10th	Night	40% 2 6% 19% 19% 13%	2730
	11th to 20th	Night	41% 5% 6% 21% 19% 8%	2528
	21st to end	Night	39% 3 8% 24% 18% 8%	2076
March	1st to 10th	Night	51% 4 6% 19% 13% 6%	5368
	11th to 20th	Night	42% 3 9% 22% 16% 8%	5110
	21st to end	Night	47% 6% 11% 21% 12% 3	5372
April	1st to 10th	Night	38% 5% 11% 20% 20% 6%	2100
	11th to 20th	Night	33% 4 12% 24% 17% 10%	2079
	21st to end	Night	26% 4 11% 32% 22% 5%	2100
May	1st to 10th	Night	22% 3 11% 24% 25% 15%	5668
	11th to 20th	Night	12% 2 5% 26% 39% 15%	4700
	21st to end	Night	12% 2 7% 29% 43% 6%	5182
June	1st to 10th	Night	20% 5% 10% 24% 28% 12%	1680
	11th to 20th	Night	10% 2 6% 20% 34% 29%	1680
	21st to end	Night	10% 2 3 11% 35% 39%	1680
July	1st to 10th	Night	3 1 1 6% 25% 65%	3680
	11th to 20th	Night	8% 1 1 5% 10% 75%	3632
	21st to end	Night	11% 2 2 13% 23% 49%	4249
August	1st to 10th	Night	13% 1 1 8% 24% 52%	2929
	11th to 20th	Night	4 1 1 8% 37% 49%	3191
	21st to end	Night	9% 1 1 12% 32% 45%	3608
September	1st to 10th	Night	6% 2 2 8% 30% 52%	2000
	11th to 20th	Night	13% 2 5% 18% 40% 22%	2000
	21st to end	Night	12% 2 4 18% 31% 34%	2233
October	1st to 10th	Night	15% 2 3 11% 21% 48%	3920
	11th to 20th	Night	13% 2 4 24% 34% 22%	3897
	21st to end	Night	23% 3 4 19% 32% 19%	4253
November	1st to 10th	Night	30% 4 9% 22% 19% 17%	2640
	11th to 20th	Night	31% 4 10% 22% 23% 10%	2730
	21st to end	Night	31% 3 7% 21% 22% 16%	2875
December	1st to 10th	Night	25% 2 7% 24% 24% 18%	4700
	11th to 20th	Night	52% 3 7% 14% 17% 7%	5316
	21st to end	Night	57% 5% 9% 15% 9% 4	7106

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	42% 3 25% 18% 10% 3	220
	11th to 20th	Day	21% 8% 22% 28% 16% 5%	357
	21st to end	Day	23% 6% 22% 31% 14% 4	473
February	1st to 10th	Day	28% 9% 25% 24% 12% 3	430
	11th to 20th	Day	28% 12% 26% 24% 8% 1	498
	21st to end	Day	26% 11% 27% 26% 9% 2	446
March	1st to 10th	Day	32% 13% 25% 21% 6% 3	540
	11th to 20th	Day	29% 11% 25% 18% 13% 3	540
	21st to end	Day	24% 14% 30% 22% 8% 2	594
April	1st to 10th	Day	22% 13% 29% 22% 12% 3	540
	11th to 20th	Day	18% 15% 27% 25% 12% 3	532
	21st to end	Day	14% 11% 28% 28% 16% 3	540
May	1st to 10th	Day	10% 13% 34% 27% 10% 5%	543
	11th to 20th	Day	10% 13% 27% 29% 16% 5%	568
	21st to end	Day	9% 9% 24% 33% 17% 8%	617
June	1st to 10th	Day	6% 8% 27% 36% 15% 8%	570
	11th to 20th	Day	6% 9% 29% 38% 13% 6%	570
	21st to end	Day	3 6% 26% 39% 18% 8%	570
July	1st to 10th	Day	1 3 24% 47% 16% 8%	566
	11th to 20th	Day	2 1 22% 46% 21% 8%	545
	21st to end	Day	1 2 21% 49% 20% 8%	575
August	1st to 10th	Day	3 3 18% 45% 23% 7%	527
	11th to 20th	Day	2 2 25% 42% 22% 7%	517
	21st to end	Day	1 2 17% 50% 23% 6%	583
September	1st to 10th	Day	3 2 25% 44% 22% 4	530
	11th to 20th	Day	8% 5% 26% 36% 20% 5%	530
	21st to end	Day	5% 6% 27% 40% 17% 5%	530
October	1st to 10th	Day	6% 9% 25% 32% 20% 8%	530
	11th to 20th	Day	8% 6% 22% 37% 19% 8%	530
	21st to end	Day	13% 7% 24% 29% 22% 6%	583
November	1st to 10th	Day	19% 4 20% 25% 26% 7%	355
	11th to 20th	Day	18% 3 21% 29% 21% 8%	330
	21st to end	Day	15% 6% 17% 30% 21% 10%	253
December	1st to 10th	Day	20% 5% 19% 27% 19% 11%	220
	11th to 20th	Day	25% 4 22% 21% 18% 10%	220
	21st to end	Day	31% 5% 21% 24% 16% 4	242
January	1st to 10th	Night	28% 3 17% 27% 21% 5%	840
	11th to 20th	Night	19% 4 15% 39% 20% 3	840
	21st to end	Night	23% 2 16% 31% 23% 5%	924
February	1st to 10th	Night	25% 3 17% 33% 20% 3	840
	11th to 20th	Night	23% 5% 17% 33% 19% 4	824
	21st to end	Night	23% 4 21% 37% 13% 2	692
March	1st to 10th	Night	28% 6% 16% 31% 17% 3	840
	11th to 20th	Night	25% 5% 14% 29% 24% 3	840
	21st to end	Night	23% 4 18% 35% 18% 2	924
April	1st to 10th	Night	19% 4 15% 39% 20% 3	840
	11th to 20th	Night	17% 5% 16% 36% 22% 5%	831
	21st to end	Night	12% 5% 15% 36% 27% 6%	840
May	1st to 10th	Night	11% 4 21% 35% 26% 4	756
	11th to 20th	Night	8% 4 13% 35% 35% 5%	414
	21st to end	Night	8% 2 12% 34% 40% 4	455
June	1st to 10th	Night	7% 2 15% 32% 40% 4	420
	11th to 20th	Night	2 2 13% 41% 36% 5%	420
	21st to end	Night	2 1 9% 38% 43% 7%	420
July	1st to 10th	Night	1 3 37% 54% 6%	410
	11th to 20th	Night	2 4 36% 52% 7%	398
	21st to end	Night	2 3 29% 53% 12%	610
August	1st to 10th	Night	3 4 34% 50% 9%	597
	11th to 20th	Night	1 4 42% 42% 10%	752
	21st to end	Night	2 1 4 35% 53% 6%	880
September	1st to 10th	Night	2 1 6% 39% 45% 7%	800
	11th to 20th	Night	5% 1 8% 36% 40% 10%	800
	21st to end	Night	4 1 14% 37% 35% 10%	800
October	1st to 10th	Night	8% 2 11% 41% 32% 7%	800
	11th to 20th	Night	9% 3 11% 34% 37% 6%	800
	21st to end	Night	11% 3 14% 31% 30% 10%	880
November	1st to 10th	Night	17% 3 12% 30% 30% 8%	820
	11th to 20th	Night	16% 2 15% 39% 24% 5%	840
	21st to end	Night	15% 4 15% 38% 22% 6%	840
December	1st to 10th	Night	23% 2 15% 33% 22% 5%	840
	11th to 20th	Night	31% 3 12% 32% 19% 3	840
	21st to end	Night	29% 4 17% 31% 17% 2	924

Page generated at 2026-07-13 00:14:49. Version ac07

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