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 2025-02-25.

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	11% 4 1 3 2 2 2 2 3 5% 64%	112680
	Night	13% 4 1 3 2 2 3 2 4 5% 59%	114784
Transparency	Day	24% 3 6% 17% 24% 25%	112430
	Night	29% 3 7% 17% 22% 22%	114623
Seeing	Day	14% 8% 25% 33% 16% 5%	15445
	Night	15% 3 13% 34% 30% 6%	24876

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	22% 7% 1 3 2 3 3 3 4 6% 46%	3639
	11th to 20th	Day	18% 7% 2 5% 3 3 2 2 2 3 54%	3939
	21st to end	Day	21% 7% 2 4 2 2 3 2 3 5% 47%	4510
February	1st to 10th	Day	24% 7% 2 4 3 2 3 3 4 6% 44%	1800
	11th to 20th	Day	20% 7% 1 5% 4 3 3 3 4 6% 44%	1883
	21st to end	Day	21% 8% 2 4 3 2 3 3 4 6% 45%	1611
March	1st to 10th	Day	23% 16% 3 6% 4 3 4 3 4 5% 29%	3221
	11th to 20th	Day	16% 5% 1 3 2 2 2 2 4 6% 56%	3490
	21st to end	Day	21% 10% 3 5% 4 4 4 4 6% 7% 33%	3979
April	1st to 10th	Day	17% 7% 2 4 3 3 3 3 5% 6% 45%	2280
	11th to 20th	Day	15% 5% 1 4 3 3 2 4 5% 6% 53%	2249
	21st to end	Day	11% 4 1 3 3 2 3 2 5% 7% 59%	2280
May	1st to 10th	Day	8% 3 1 2 1 2 2 3 3 5% 70%	3916
	11th to 20th	Day	4 2 1 2 2 1 2 2 2 4 78%	4377
	21st to end	Day	5% 2 1 2 1 2 2 2 3 5% 73%	4813
June	1st to 10th	Day	7% 3 1 3 2 3 3 2 5% 7% 63%	2660
	11th to 20th	Day	2 2 1 2 1 2 1 2 3 4 80%	2660
	21st to end	Day	3 1 2 1 1 1 1 2 3 84%	2660
July	1st to 10th	Day	1 1 1 1 1 1 93%	4420
	11th to 20th	Day	2 1 1 1 1 1 2 1 2 3 85%	4420
	21st to end	Day	1 1 1 1 1 1 1 2 3 88%	4518
August	1st to 10th	Day	2 1 1 2 2 2 2 2 3 4 79%	4091
	11th to 20th	Day	1 1 1 1 1 1 2 2 90%	3782
	21st to end	Day	2 1 1 1 1 1 1 1 2 90%	4180
September	1st to 10th	Day	2 1 1 1 2 1 2 3 88%	2280
	11th to 20th	Day	4 2 1 3 2 2 2 2 3 5% 75%	2280
	21st to end	Day	4 2 1 2 1 2 2 1 2 4 79%	2053
October	1st to 10th	Day	4 2 1 3 2 2 2 2 4 5% 73%	3180
	11th to 20th	Day	7% 2 1 2 2 2 2 2 3 5% 72%	3161
	21st to end	Day	6% 3 1 4 3 3 3 3 5% 6% 63%	3448
November	1st to 10th	Day	13% 5% 1 4 3 3 3 2 4 7% 54%	1788
	11th to 20th	Day	16% 3 2 3 3 4 5% 3 5% 7% 50%	1800
	21st to end	Day	15% 4 2 5% 4 3 3 3 4 5% 53%	1660
December	1st to 10th	Day	11% 3 1 3 4 3 3 3 4 7% 58%	2620
	11th to 20th	Day	31% 6% 2 4 3 3 3 3 5% 7% 33%	3028
	21st to end	Day	30% 6% 2 5% 3 4 4 3 6% 8% 31%	4004
January	1st to 10th	Night	16% 8% 2 4 3 3 3 3 5% 7% 45%	6306
	11th to 20th	Night	18% 5% 1 4 3 3 3 2 3 4 54%	6030
	21st to end	Night	20% 6% 2 5% 3 4 3 3 4 4 46%	6468
February	1st to 10th	Night	23% 6% 1 3 3 3 3 2 3 5% 49%	2600
	11th to 20th	Night	18% 6% 2 5% 3 3 4 3 5% 5% 45%	2403
	21st to end	Night	19% 7% 2 4 3 2 4 3 4 6% 47%	1937
March	1st to 10th	Night	28% 16% 2 5% 3 3 3 2 3 4 31%	3650
	11th to 20th	Night	19% 6% 1 3 2 2 2 2 3 4 56%	3490
	21st to end	Night	25% 9% 2 5% 4 3 4 3 5% 5% 34%	3660
April	1st to 10th	Night	16% 6% 2 5% 4 4 3 2 4 5% 48%	1900
	11th to 20th	Night	15% 4 2 5% 3 3 2 3 3 6% 54%	1879
	21st to end	Night	10% 4 1 3 2 2 3 3 4 6% 62%	1900
May	1st to 10th	Night	7% 5% 1 3 2 3 3 2 3 5% 67%	3039
	11th to 20th	Night	4 1 1 2 1 1 2 2 3 5% 77%	2516
	21st to end	Night	5% 2 1 2 2 2 2 2 3 4 76%	2762
June	1st to 10th	Night	7% 4 1 2 2 3 2 3 5% 6% 65%	1520
	11th to 20th	Night	1 2 1 3 2 1 2 1 3 5% 79%	1520
	21st to end	Night	2 2 1 2 2 2 2 1 2 3 81%	1520
July	1st to 10th	Night	1 1 1 1 1 2 93%	2540
	11th to 20th	Night	3 2 1 2 1 2 2 1 2 3 83%	2540
	21st to end	Night	1 1 1 1 1 2 2 3 3 85%	3038
August	1st to 10th	Night	3 2 1 2 2 2 2 2 4 5% 74%	2848
	11th to 20th	Night	1 1 1 1 1 2 3 89%	3064
	21st to end	Night	3 1 2 1 1 1 1 1 2 87%	3498
September	1st to 10th	Night	2 1 1 1 1 1 1 2 3 87%	1900
	11th to 20th	Night	4 2 1 2 2 1 2 1 3 4 77%	1900
	21st to end	Night	3 1 2 1 2 2 2 3 5% 78%	2121
October	1st to 10th	Night	4 2 1 2 3 2 2 2 2 3 77%	3800
	11th to 20th	Night	4 1 1 2 1 2 2 2 2 4 78%	3777
	21st to end	Night	10% 3 1 3 2 3 3 2 4 6% 63%	4121
November	1st to 10th	Night	16% 5% 1 3 2 2 2 3 4 7% 54%	2511
	11th to 20th	Night	12% 5% 2 4 2 3 3 3 5% 6% 55%	2600
	21st to end	Night	15% 4 2 4 3 3 3 2 4 5% 55%	2738
December	1st to 10th	Night	13% 3 1 2 2 2 3 3 4 6% 62%	4560
	11th to 20th	Night	30% 7% 2 4 3 3 4 3 6% 8% 29%	5176
	21st to end	Night	30% 7% 3 6% 4 4 4 3 4 5% 31%	6952

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	41% 3 12% 20% 15% 8%	3639
	11th to 20th	Day	39% 3 5% 14% 24% 16%	3939
	21st to end	Day	42% 2 7% 13% 19% 17%	4510
February	1st to 10th	Day	43% 3 8% 18% 18% 10%	1800
	11th to 20th	Day	42% 4 8% 19% 21% 6%	1892
	21st to end	Day	42% 3 9% 23% 16% 7%	1611
March	1st to 10th	Day	57% 6% 8% 15% 11% 4	3221
	11th to 20th	Day	32% 3 9% 31% 15% 10%	3490
	21st to end	Day	48% 7% 13% 20% 9% 2	3979
April	1st to 10th	Day	39% 4 12% 24% 17% 4	2280
	11th to 20th	Day	31% 5% 13% 26% 19% 6%	2252
	21st to end	Day	24% 4 14% 29% 25% 4	2280
May	1st to 10th	Day	18% 3 9% 29% 24% 16%	3916
	11th to 20th	Day	12% 2 6% 24% 43% 12%	4380
	21st to end	Day	15% 3 11% 36% 29% 6%	4814
June	1st to 10th	Day	21% 3 9% 26% 29% 11%	2660
	11th to 20th	Day	10% 2 7% 20% 36% 24%	2660
	21st to end	Day	9% 1 2 14% 36% 39%	2660
July	1st to 10th	Day	3 1 1 5% 15% 75%	4351
	11th to 20th	Day	8% 1 2 7% 16% 67%	4345
	21st to end	Day	6% 1 1 4 25% 63%	4375
August	1st to 10th	Day	11% 2 1 8% 22% 57%	4076
	11th to 20th	Day	4 1 1 6% 38% 50%	3819
	21st to end	Day	6% 2 8% 40% 44%	4179
September	1st to 10th	Day	5% 1 1 8% 27% 57%	2280
	11th to 20th	Day	14% 2 4 21% 36% 23%	2280
	21st to end	Day	12% 2 3 18% 33% 31%	2053
October	1st to 10th	Day	14% 2 3 12% 25% 44%	3180
	11th to 20th	Day	17% 2 4 17% 40% 20%	3161
	21st to end	Day	23% 3 5% 20% 33% 17%	3448
November	1st to 10th	Day	31% 4 7% 21% 22% 15%	1788
	11th to 20th	Day	33% 5% 9% 21% 22% 10%	1800
	21st to end	Day	33% 4 8% 19% 23% 14%	1660
December	1st to 10th	Day	28% 2 6% 22% 29% 13%	2620
	11th to 20th	Day	53% 2 7% 16% 15% 7%	3028
	21st to end	Day	52% 4 12% 18% 10% 3	4004
January	1st to 10th	Night	38% 4 16% 20% 14% 8%	6306
	11th to 20th	Night	36% 4 8% 13% 23% 16%	6030
	21st to end	Night	42% 3 7% 10% 20% 18%	6468
February	1st to 10th	Night	41% 2 7% 18% 19% 13%	2600
	11th to 20th	Night	40% 5% 6% 21% 20% 8%	2405
	21st to end	Night	39% 4 8% 25% 18% 7%	1937
March	1st to 10th	Night	58% 5% 8% 15% 10% 5%	3650
	11th to 20th	Night	34% 3 7% 25% 20% 11%	3490
	21st to end	Night	49% 7% 10% 21% 9% 4	3660
April	1st to 10th	Night	39% 5% 12% 19% 20% 6%	1900
	11th to 20th	Night	33% 4 12% 25% 17% 9%	1879
	21st to end	Night	24% 5% 12% 32% 22% 6%	1900
May	1st to 10th	Night	20% 5% 12% 21% 24% 17%	3039
	11th to 20th	Night	12% 2 7% 24% 39% 16%	2516
	21st to end	Night	12% 3 11% 38% 29% 6%	2762
June	1st to 10th	Night	20% 5% 11% 25% 27% 12%	1520
	11th to 20th	Night	11% 2 6% 21% 33% 26%	1520
	21st to end	Night	10% 2 3 11% 33% 40%	1520
July	1st to 10th	Night	3 1 1 6% 15% 73%	2500
	11th to 20th	Night	10% 2 1 7% 14% 66%	2492
	21st to end	Night	6% 1 1 5% 22% 64%	2943
August	1st to 10th	Night	14% 1 1 8% 23% 53%	2839
	11th to 20th	Night	4 1 1 8% 37% 49%	3093
	21st to end	Night	9% 1 1 12% 32% 46%	3498
September	1st to 10th	Night	6% 2 2 7% 30% 54%	1900
	11th to 20th	Night	13% 2 4 17% 41% 23%	1900
	21st to end	Night	11% 2 3 18% 30% 35%	2121
October	1st to 10th	Night	15% 2 3 10% 21% 49%	3800
	11th to 20th	Night	12% 2 5% 23% 34% 23%	3777
	21st to end	Night	23% 3 4 19% 32% 20%	4121
November	1st to 10th	Night	30% 4 8% 22% 18% 17%	2511
	11th to 20th	Night	29% 4 10% 23% 24% 10%	2600
	21st to end	Night	32% 3 6% 20% 21% 17%	2738
December	1st to 10th	Night	25% 2 7% 23% 25% 18%	4560
	11th to 20th	Night	53% 3 7% 14% 18% 6%	5176
	21st to end	Night	57% 5% 9% 16% 9% 4	6952

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% 2 25% 19% 11% 3	200
	11th to 20th	Day	23% 8% 24% 28% 13% 4	330
	21st to end	Day	25% 6% 22% 29% 13% 4	440
February	1st to 10th	Day	30% 9% 24% 24% 11% 2	400
	11th to 20th	Day	26% 12% 28% 25% 8% 2	461
	21st to end	Day	27% 11% 28% 23% 10% 2	399
March	1st to 10th	Day	35% 12% 24% 21% 6% 2	460
	11th to 20th	Day	30% 12% 25% 18% 12% 3	460
	21st to end	Day	25% 15% 31% 21% 7% 1	506
April	1st to 10th	Day	22% 14% 30% 21% 11% 2	460
	11th to 20th	Day	19% 16% 28% 23% 11% 2	452
	21st to end	Day	13% 12% 28% 28% 17% 3	460
May	1st to 10th	Day	11% 14% 35% 25% 10% 4	463
	11th to 20th	Day	10% 13% 25% 30% 16% 5%	488
	21st to end	Day	10% 10% 23% 33% 17% 8%	529
June	1st to 10th	Day	8% 9% 25% 37% 15% 7%	490
	11th to 20th	Day	6% 10% 28% 38% 13% 5%	490
	21st to end	Day	3 4 28% 41% 17% 8%	490
July	1st to 10th	Day	1 2 22% 51% 17% 8%	486
	11th to 20th	Day	1 1 21% 46% 21% 8%	484
	21st to end	Day	1 2 21% 49% 20% 7%	531
August	1st to 10th	Day	4 3 19% 45% 23% 7%	487
	11th to 20th	Day	2 2 26% 41% 22% 6%	477
	21st to end	Day	1 3 18% 50% 23% 6%	539
September	1st to 10th	Day	3 2 24% 45% 22% 4	490
	11th to 20th	Day	8% 5% 27% 36% 20% 4	490
	21st to end	Day	5% 6% 28% 40% 17% 5%	490
October	1st to 10th	Day	6% 9% 26% 32% 21% 7%	490
	11th to 20th	Day	8% 6% 22% 38% 19% 8%	490
	21st to end	Day	12% 7% 24% 30% 22% 5%	539
November	1st to 10th	Day	19% 5% 20% 25% 25% 6%	324
	11th to 20th	Day	15% 4 21% 31% 22% 8%	300
	21st to end	Day	17% 7% 17% 30% 20% 9%	230
December	1st to 10th	Day	22% 5% 19% 26% 19% 10%	200
	11th to 20th	Day	25% 4 24% 22% 17% 9%	200
	21st to end	Day	30% 5% 21% 24% 15% 4	220
January	1st to 10th	Night	27% 3 17% 27% 22% 5%	800
	11th to 20th	Night	20% 4 15% 39% 18% 3	800
	21st to end	Night	23% 3 16% 30% 23% 5%	880
February	1st to 10th	Night	26% 3 17% 32% 20% 3	800
	11th to 20th	Night	21% 5% 17% 33% 20% 4	784
	21st to end	Night	23% 4 20% 37% 13% 2	646
March	1st to 10th	Night	30% 6% 15% 30% 17% 3	760
	11th to 20th	Night	24% 5% 15% 28% 25% 4	760
	21st to end	Night	24% 4 19% 33% 18% 2	836
April	1st to 10th	Night	19% 4 15% 40% 20% 3	760
	11th to 20th	Night	17% 5% 16% 36% 22% 5%	751
	21st to end	Night	11% 5% 15% 35% 27% 6%	760
May	1st to 10th	Night	11% 4 22% 35% 25% 4	684
	11th to 20th	Night	8% 3 12% 33% 38% 5%	374
	21st to end	Night	9% 2 12% 33% 40% 4	411
June	1st to 10th	Night	7% 3 16% 29% 41% 5%	380
	11th to 20th	Night	2 3 14% 40% 36% 6%	380
	21st to end	Night	2 1 9% 36% 44% 7%	380
July	1st to 10th	Night	1 3 35% 55% 6%	370
	11th to 20th	Night	1 4 35% 52% 8%	368
	21st to end	Night	2 3 29% 53% 13%	579
August	1st to 10th	Night	3 4 34% 50% 9%	567
	11th to 20th	Night	1 4 42% 42% 10%	714
	21st to end	Night	2 1 4 34% 54% 6%	836
September	1st to 10th	Night	2 1 6% 38% 47% 7%	760
	11th to 20th	Night	5% 1 8% 36% 40% 10%	760
	21st to end	Night	4 1 14% 36% 34% 10%	760
October	1st to 10th	Night	7% 2 11% 40% 32% 7%	760
	11th to 20th	Night	9% 3 11% 34% 38% 6%	760
	21st to end	Night	11% 3 15% 31% 30% 11%	836
November	1st to 10th	Night	18% 3 12% 30% 29% 8%	780
	11th to 20th	Night	14% 2 15% 39% 24% 5%	800
	21st to end	Night	16% 4 15% 37% 22% 6%	800
December	1st to 10th	Night	24% 3 15% 32% 21% 5%	800
	11th to 20th	Night	32% 3 12% 32% 19% 3	800
	21st to end	Night	28% 5% 17% 32% 17% 2	880

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