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 2024-02-05.

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	12% 4 1 3 3 2 3 3 4 5% 61%	78332
	Night	15% 4 2 3 3 2 3 3 4 5% 56%	80554
Transparency	Day	27% 3 7% 19% 23% 20%	78044
	Night	31% 4 8% 18% 21% 18%	80363
Seeing	Day	14% 8% 24% 33% 16% 5%	14066
	Night	15% 3 13% 34% 30% 6%	23504

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% 9% 2 3 3 1 3 3 3 5% 40%	2539
	11th to 20th	Day	26% 9% 3 6% 5% 3 3 3 3 4 35%	2741
	21st to end	Day	27% 9% 2 5% 4 2 4 3 4 6% 34%	3146
February	1st to 10th	Day	22% 6% 2 3 3 1 3 3 4 6% 46%	1657
	11th to 20th	Day	18% 6% 2 4 3 2 3 4 4 6% 47%	1690
	21st to end	Day	22% 8% 2 3 3 1 3 3 4 5% 45%	1480
March	1st to 10th	Day	22% 9% 2 5% 5% 3 4 4 4 7% 36%	1844
	11th to 20th	Day	24% 6% 2 5% 3 2 3 3 5% 7% 39%	1980
	21st to end	Day	21% 7% 2 3 4 3 3 4 5% 7% 42%	2268
April	1st to 10th	Day	18% 6% 2 4 4 2 3 4 5% 7% 44%	2160
	11th to 20th	Day	16% 4 1 3 3 2 2 4 4 6% 53%	2129
	21st to end	Day	12% 3 1 3 3 1 3 3 5% 7% 59%	2160
May	1st to 10th	Day	8% 5% 1 3 2 2 3 4 4 6% 62%	2224
	11th to 20th	Day	6% 3 2 3 3 2 3 4 4 7% 64%	2477
	21st to end	Day	8% 3 2 3 3 2 4 4 5% 6% 61%	2723
June	1st to 10th	Day	7% 3 1 3 2 2 3 3 5% 8% 62%	2520
	11th to 20th	Day	3 2 1 1 2 2 1 2 3 4 80%	2520
	21st to end	Day	3 1 1 2 1 1 2 1 2 3 84%	2520
July	1st to 10th	Day	1 1 1 1 1 2 2 3 88%	2520
	11th to 20th	Day	1 1 1 1 1 1 2 2 3 86%	2520
	21st to end	Day	1 1 1 2 2 1 2 2 2 3 83%	2583
August	1st to 10th	Day	3 1 1 1 1 1 2 3 3 83%	2328
	11th to 20th	Day	1 1 1 1 1 2 3 3 86%	2156
	21st to end	Day	1 1 1 1 1 1 1 1 2 91%	2376
September	1st to 10th	Day	2 1 1 1 1 2 2 2 3 88%	2160
	11th to 20th	Day	4 2 1 2 2 1 2 3 3 5% 76%	2160
	21st to end	Day	5% 2 1 2 2 1 2 2 2 4 78%	1945
October	1st to 10th	Day	6% 2 2 3 2 2 3 2 4 5% 69%	1800
	11th to 20th	Day	8% 2 1 2 2 2 3 3 3 5% 70%	1800
	21st to end	Day	9% 3 1 3 3 2 3 3 4 5% 64%	1980
November	1st to 10th	Day	14% 5% 2 4 3 2 3 3 4 7% 53%	1697
	11th to 20th	Day	16% 2 1 3 4 3 5% 4 4 7% 52%	1710
	21st to end	Day	13% 4 2 4 4 3 3 4 4 6% 54%	1577
December	1st to 10th	Day	18% 4 2 3 4 2 3 4 4 7% 48%	1520
	11th to 20th	Day	31% 7% 3 5% 4 2 3 3 4 6% 31%	1928
	21st to end	Day	18% 7% 2 5% 5% 3 5% 4 6% 8% 38%	2794
January	1st to 10th	Night	20% 9% 2 4 3 2 3 4 5% 7% 41%	4406
	11th to 20th	Night	26% 7% 2 5% 5% 3 3 4 4 5% 36%	4221
	21st to end	Night	28% 7% 3 6% 5% 3 4 4 4 5% 32%	4521
February	1st to 10th	Night	21% 5% 1 2 4 2 3 3 3 5% 50%	2397
	11th to 20th	Night	17% 5% 2 4 3 3 4 4 5% 5% 47%	2156
	21st to end	Night	21% 6% 2 3 3 2 3 5% 4 5% 47%	1776
March	1st to 10th	Night	26% 8% 2 5% 4 2 3 4 3 5% 37%	2113
	11th to 20th	Night	21% 7% 2 3 4 2 3 4 5% 7% 42%	1980
	21st to end	Night	21% 5% 2 3 4 2 4 4 5% 5% 45%	2087
April	1st to 10th	Night	17% 5% 3 5% 4 3 4 3 4 6% 47%	1800
	11th to 20th	Night	15% 3 2 4 4 2 3 3 4 7% 54%	1779
	21st to end	Night	10% 3 2 3 3 2 3 3 4 6% 62%	1800
May	1st to 10th	Night	10% 4 1 2 3 2 3 4 4 5% 62%	1729
	11th to 20th	Night	8% 2 1 3 2 2 3 4 5% 8% 62%	1416
	21st to end	Night	8% 2 2 3 3 2 3 4 5% 6% 63%	1552
June	1st to 10th	Night	7% 3 1 2 3 2 3 3 4 7% 66%	1440
	11th to 20th	Night	2 2 1 2 3 1 2 1 3 5% 78%	1440
	21st to end	Night	2 2 1 2 2 1 3 2 2 3 81%	1440
July	1st to 10th	Night	1 1 1 1 1 2 1 2 3 88%	1440
	11th to 20th	Night	1 1 1 1 1 1 1 1 1 3 88%	1440
	21st to end	Night	1 2 1 2 2 1 2 3 3 4 80%	1730
August	1st to 10th	Night	3 2 1 2 2 2 2 2 3 3 78%	1611
	11th to 20th	Night	1 1 1 1 1 1 2 2 3 4 83%	1744
	21st to end	Night	1 1 1 1 2 2 2 3 86%	1980
September	1st to 10th	Night	2 1 1 1 1 1 1 2 3 87%	1800
	11th to 20th	Night	4 2 1 1 2 1 2 2 3 4 79%	1800
	21st to end	Night	3 1 1 2 2 2 3 3 5% 77%	2009
October	1st to 10th	Night	7% 2 1 2 3 2 2 3 2 4 72%	2160
	11th to 20th	Night	8% 2 1 2 2 1 3 3 3 5% 71%	2160
	21st to end	Night	9% 3 1 2 2 2 2 2 4 5% 68%	2376
November	1st to 10th	Night	16% 4 1 3 2 2 2 4 4 6% 54%	2382
	11th to 20th	Night	12% 4 2 4 3 2 3 4 5% 6% 56%	2470
	21st to end	Night	14% 3 2 3 4 2 3 3 4 5% 56%	2601
December	1st to 10th	Night	22% 5% 1 2 3 2 3 3 4 6% 50%	2660
	11th to 20th	Night	34% 6% 2 4 3 2 3 3 5% 6% 33%	3276
	21st to end	Night	21% 6% 3 6% 5% 3 5% 4 5% 6% 35%	4862

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	49% 4 6% 24% 14% 3	2539
	11th to 20th	Day	55% 4 7% 15% 11% 9%	2741
	21st to end	Day	53% 3 9% 17% 12% 6%	3146
February	1st to 10th	Day	41% 3 8% 19% 18% 11%	1657
	11th to 20th	Day	38% 4 8% 19% 23% 7%	1699
	21st to end	Day	42% 3 8% 23% 16% 8%	1480
March	1st to 10th	Day	50% 4 9% 17% 16% 5%	1844
	11th to 20th	Day	45% 3 9% 22% 16% 5%	1980
	21st to end	Day	43% 6% 13% 20% 15% 4	2268
April	1st to 10th	Day	40% 4 12% 23% 17% 4	2160
	11th to 20th	Day	32% 5% 12% 25% 20% 6%	2132
	21st to end	Day	25% 4 13% 29% 25% 4	2160
May	1st to 10th	Day	23% 3 10% 27% 29% 8%	2224
	11th to 20th	Day	22% 4 9% 30% 25% 10%	2480
	21st to end	Day	24% 4 13% 31% 22% 6%	2724
June	1st to 10th	Day	22% 3 9% 26% 28% 12%	2520
	11th to 20th	Day	11% 2 7% 21% 34% 25%	2520
	21st to end	Day	9% 1 2 14% 37% 37%	2520
July	1st to 10th	Day	5% 2 2 9% 26% 56%	2451
	11th to 20th	Day	7% 1 3 12% 24% 53%	2445
	21st to end	Day	9% 2 2 7% 25% 55%	2439
August	1st to 10th	Day	9% 2 1 12% 29% 47%	2312
	11th to 20th	Day	6% 2 2 9% 34% 48%	2156
	21st to end	Day	5% 1 10% 36% 47%	2376
September	1st to 10th	Day	5% 1 1 9% 27% 56%	2160
	11th to 20th	Day	14% 2 4 21% 35% 25%	2160
	21st to end	Day	13% 2 4 19% 34% 29%	1945
October	1st to 10th	Day	18% 3 5% 20% 28% 26%	1800
	11th to 20th	Day	19% 3 6% 20% 25% 27%	1800
	21st to end	Day	24% 3 5% 19% 25% 23%	1980
November	1st to 10th	Day	32% 4 7% 21% 21% 15%	1697
	11th to 20th	Day	33% 5% 9% 21% 23% 10%	1710
	21st to end	Day	32% 4 8% 19% 24% 13%	1577
December	1st to 10th	Day	38% 3 8% 21% 20% 11%	1520
	11th to 20th	Day	57% 3 6% 14% 15% 6%	1928
	21st to end	Day	44% 5% 14% 20% 13% 4	2794
January	1st to 10th	Night	43% 5% 13% 22% 12% 4	4406
	11th to 20th	Night	51% 5% 11% 11% 12% 10%	4221
	21st to end	Night	55% 4 9% 13% 12% 7%	4521
February	1st to 10th	Night	40% 2 7% 19% 19% 14%	2397
	11th to 20th	Night	38% 5% 6% 21% 21% 9%	2158
	21st to end	Night	39% 3 7% 24% 19% 8%	1776
March	1st to 10th	Night	51% 4 8% 17% 14% 6%	2113
	11th to 20th	Night	42% 3 9% 20% 19% 7%	1980
	21st to end	Night	41% 6% 10% 22% 15% 6%	2087
April	1st to 10th	Night	40% 5% 12% 17% 20% 6%	1800
	11th to 20th	Night	33% 4 11% 24% 18% 10%	1779
	21st to end	Night	25% 5% 12% 32% 22% 6%	1800
May	1st to 10th	Night	25% 4 10% 21% 28% 11%	1729
	11th to 20th	Night	21% 4 11% 31% 22% 11%	1416
	21st to end	Night	21% 4 15% 29% 25% 6%	1552
June	1st to 10th	Night	20% 5% 11% 25% 27% 12%	1440
	11th to 20th	Night	11% 3 6% 22% 32% 27%	1440
	21st to end	Night	10% 2 3 12% 34% 39%	1440
July	1st to 10th	Night	5% 2 2 11% 28% 53%	1400
	11th to 20th	Night	7% 1 2 12% 21% 57%	1392
	21st to end	Night	10% 2 3 8% 21% 57%	1634
August	1st to 10th	Night	13% 2 1 14% 23% 46%	1602
	11th to 20th	Night	7% 2 2 11% 31% 48%	1744
	21st to end	Night	8% 1 1 10% 28% 52%	1980
September	1st to 10th	Night	6% 2 2 7% 29% 54%	1800
	11th to 20th	Night	12% 2 5% 17% 41% 24%	1800
	21st to end	Night	12% 2 4 19% 31% 33%	2009
October	1st to 10th	Night	19% 3 6% 17% 27% 29%	2160
	11th to 20th	Night	17% 3 4 18% 26% 30%	2160
	21st to end	Night	19% 3 5% 20% 27% 25%	2376
November	1st to 10th	Night	31% 4 8% 21% 18% 18%	2382
	11th to 20th	Night	29% 4 9% 23% 25% 10%	2470
	21st to end	Night	32% 3 6% 21% 22% 16%	2601
December	1st to 10th	Night	37% 3 9% 19% 18% 15%	2660
	11th to 20th	Night	53% 3 6% 14% 16% 7%	3276
	21st to end	Night	50% 6% 9% 17% 12% 5%	4862

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	44% 2 23% 17% 10% 3	180
	11th to 20th	Day	25% 8% 22% 27% 14% 4	303
	21st to end	Day	27% 6% 22% 29% 13% 4	407
February	1st to 10th	Day	28% 9% 24% 25% 12% 2	353
	11th to 20th	Day	25% 13% 26% 25% 9% 2	388
	21st to end	Day	27% 12% 28% 23% 9% 1	346
March	1st to 10th	Day	34% 13% 25% 21% 6% 2	420
	11th to 20th	Day	31% 12% 25% 18% 11% 3	420
	21st to end	Day	25% 16% 30% 21% 8% 1	462
April	1st to 10th	Day	24% 13% 30% 21% 10% 2	420
	11th to 20th	Day	20% 17% 29% 23% 9% 2	412
	21st to end	Day	15% 12% 27% 28% 16% 3	420
May	1st to 10th	Day	12% 14% 34% 26% 10% 4	423
	11th to 20th	Day	11% 14% 26% 30% 14% 5%	448
	21st to end	Day	11% 11% 22% 32% 17% 7%	485
June	1st to 10th	Day	8% 9% 25% 37% 14% 7%	450
	11th to 20th	Day	6% 10% 28% 37% 13% 5%	450
	21st to end	Day	3 4 27% 41% 18% 8%	450
July	1st to 10th	Day	1 2 22% 52% 16% 7%	446
	11th to 20th	Day	2 21% 47% 21% 8%	444
	21st to end	Day	1 2 20% 50% 21% 7%	487
August	1st to 10th	Day	3 3 19% 46% 23% 6%	447
	11th to 20th	Day	2 2 24% 43% 22% 7%	437
	21st to end	Day	1 2 18% 51% 23% 5%	495
September	1st to 10th	Day	3 2 22% 46% 22% 4	450
	11th to 20th	Day	8% 4 27% 36% 21% 4	450
	21st to end	Day	5% 6% 28% 40% 17% 4	450
October	1st to 10th	Day	6% 9% 26% 31% 20% 7%	450
	11th to 20th	Day	8% 6% 21% 38% 19% 8%	450
	21st to end	Day	13% 7% 23% 30% 22% 6%	495
November	1st to 10th	Day	21% 4 19% 24% 27% 5%	293
	11th to 20th	Day	13% 3 20% 31% 24% 9%	270
	21st to end	Day	14% 7% 18% 31% 22% 9%	207
December	1st to 10th	Day	24% 6% 19% 24% 18% 8%	180
	11th to 20th	Day	23% 4 26% 21% 17% 9%	180
	21st to end	Day	28% 5% 22% 24% 17% 5%	198
January	1st to 10th	Night	28% 3 16% 27% 22% 5%	760
	11th to 20th	Night	21% 4 14% 39% 18% 3	760
	21st to end	Night	24% 3 16% 28% 23% 5%	836
February	1st to 10th	Night	24% 3 16% 32% 21% 3	738
	11th to 20th	Night	20% 6% 15% 33% 21% 4	704
	21st to end	Night	24% 4 19% 36% 14% 2	592
March	1st to 10th	Night	29% 6% 14% 30% 18% 3	720
	11th to 20th	Night	24% 4 14% 28% 25% 4	720
	21st to end	Night	23% 4 19% 33% 19% 2	792
April	1st to 10th	Night	20% 3 15% 39% 20% 3	720
	11th to 20th	Night	16% 5% 16% 36% 21% 5%	711
	21st to end	Night	12% 5% 15% 35% 27% 7%	720
May	1st to 10th	Night	11% 3 21% 35% 26% 4	649
	11th to 20th	Night	8% 3 12% 34% 36% 6%	354
	21st to end	Night	9% 3 12% 31% 41% 5%	389
June	1st to 10th	Night	8% 3 15% 29% 41% 5%	360
	11th to 20th	Night	2 3 13% 39% 37% 6%	360
	21st to end	Night	2 1 10% 34% 45% 8%	360
July	1st to 10th	Night	1 3 35% 55% 7%	350
	11th to 20th	Night	1 3 35% 53% 8%	348
	21st to end	Night	2 3 28% 53% 14%	548
August	1st to 10th	Night	3 4 33% 50% 9%	537
	11th to 20th	Night	1 4 40% 44% 11%	676
	21st to end	Night	1 3 34% 54% 6%	792
September	1st to 10th	Night	2 1 6% 36% 48% 7%	720
	11th to 20th	Night	5% 1 8% 35% 41% 11%	720
	21st to end	Night	4 1 15% 35% 34% 11%	720
October	1st to 10th	Night	8% 3 11% 40% 32% 7%	720
	11th to 20th	Night	9% 2 11% 34% 38% 6%	720
	21st to end	Night	10% 3 15% 30% 31% 12%	792
November	1st to 10th	Night	18% 3 12% 30% 29% 9%	740
	11th to 20th	Night	14% 2 13% 40% 25% 6%	760
	21st to end	Night	15% 4 14% 38% 23% 7%	760
December	1st to 10th	Night	25% 3 14% 33% 21% 5%	760
	11th to 20th	Night	31% 3 13% 32% 19% 3	760
	21st to end	Night	26% 5% 17% 32% 18% 2	836

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