Greenville
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Greenville clear sky chart from 2005-11-06 to 2025-07-06.

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% 6% 2 6% 5% 4 5% 4 6% 8% 42%	119367
	Night	13% 6% 2 6% 4 4 4 4 5% 7% 44%	119400
Transparency	Day	40% 5% 11% 21% 18% 5%	123009
	Night	39% 5% 11% 20% 20% 6%	123895
Seeing	Day	16% 8% 23% 25% 19% 9%	16236
	Night	16% 6% 24% 35% 17% 2	26134

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	13% 6% 2 6% 3 3 3 2 3 4 57%	4056
	11th to 20th	Day	13% 5% 2 5% 4 3 3 2 3 4 56%	4060
	21st to end	Day	11% 10% 3 8% 4 5% 4 3 4 5% 43%	4466
February	1st to 10th	Day	15% 7% 2 6% 5% 4 4 3 5% 7% 43%	1800
	11th to 20th	Day	20% 7% 2 6% 4 3 3 2 4 5% 45%	1873
	21st to end	Day	15% 6% 3 6% 4 5% 4 3 6% 6% 42%	1679
March	1st to 10th	Day	20% 8% 2 6% 3 4 3 4 5% 6% 39%	4935
	11th to 20th	Day	10% 5% 2 5% 4 3 4 3 4 6% 54%	4980
	21st to end	Day	13% 5% 2 5% 3 3 4 3 5% 5% 51%	5498
April	1st to 10th	Day	11% 5% 2 5% 3 3 4 3 5% 7% 52%	2400
	11th to 20th	Day	9% 5% 2 5% 5% 3 4 3 6% 8% 51%	2374
	21st to end	Day	10% 4 2 6% 5% 4 4 4 7% 7% 48%	2400
May	1st to 10th	Day	8% 6% 3 7% 5% 6% 6% 5% 8% 10% 35%	5448
	11th to 20th	Day	16% 9% 3 8% 5% 5% 5% 5% 6% 9% 29%	5861
	21st to end	Day	12% 5% 2 6% 5% 4 5% 4 6% 9% 43%	6615
June	1st to 10th	Day	9% 6% 2 7% 5% 5% 6% 5% 6% 10% 38%	2800
	11th to 20th	Day	9% 4 2 5% 5% 5% 6% 5% 7% 9% 43%	2800
	21st to end	Day	7% 4 2 6% 6% 5% 7% 5% 8% 11% 38%	2771
July	1st to 10th	Day	6% 5% 2 7% 6% 6% 7% 7% 9% 13% 33%	4185
	11th to 20th	Day	10% 7% 4 9% 7% 6% 6% 5% 8% 11% 27%	4120
	21st to end	Day	10% 8% 4 9% 7% 6% 6% 6% 8% 10% 27%	4466
August	1st to 10th	Day	20% 11% 4 9% 6% 5% 5% 5% 7% 9% 20%	4104
	11th to 20th	Day	9% 7% 3 8% 6% 5% 6% 5% 7% 9% 35%	3741
	21st to end	Day	6% 5% 2 7% 4 5% 6% 4 7% 11% 43%	4180
September	1st to 10th	Day	11% 5% 2 6% 4 5% 5% 5% 7% 8% 39%	2280
	11th to 20th	Day	13% 7% 3 7% 5% 5% 5% 3 6% 7% 40%	2280
	21st to end	Day	18% 8% 3 7% 5% 5% 4 4 5% 7% 35%	1991
October	1st to 10th	Day	14% 6% 2 5% 3 4 4 2 5% 7% 47%	1900
	11th to 20th	Day	13% 6% 2 5% 3 3 4 3 4 5% 54%	1900
	21st to end	Day	11% 5% 2 6% 4 3 3 2 4 5% 54%	2090
November	1st to 10th	Day	14% 5% 2 6% 4 3 4 3 4 6% 50%	1957
	11th to 20th	Day	15% 6% 1 5% 3 3 3 3 5% 5% 50%	1870
	21st to end	Day	13% 5% 2 4 3 3 3 3 5% 7% 52%	1725
December	1st to 10th	Day	11% 5% 2 5% 4 4 4 3 5% 5% 52%	2600
	11th to 20th	Day	14% 7% 3 7% 4 4 4 3 5% 5% 44%	2990
	21st to end	Day	13% 6% 2 6% 4 4 4 3 5% 7% 47%	4172
January	1st to 10th	Night	11% 7% 2 5% 3 3 3 3 4 6% 54%	5889
	11th to 20th	Night	14% 6% 2 6% 4 3 3 3 4 4 52%	5900
	21st to end	Night	13% 8% 2 7% 4 4 3 3 4 6% 46%	6490
February	1st to 10th	Night	18% 6% 2 5% 4 3 3 3 5% 7% 43%	2600
	11th to 20th	Night	19% 7% 3 6% 3 3 3 4 4 6% 41%	2411
	21st to end	Night	16% 5% 2 5% 3 3 4 3 5% 5% 48%	1949
March	1st to 10th	Night	19% 8% 3 6% 4 3 4 3 4 5% 41%	4918
	11th to 20th	Night	14% 5% 2 5% 3 3 3 3 5% 7% 50%	4980
	21st to end	Night	13% 5% 2 5% 4 4 4 3 5% 6% 50%	5432
April	1st to 10th	Night	12% 6% 2 6% 4 3 4 4 6% 7% 47%	2000
	11th to 20th	Night	10% 5% 2 5% 4 4 3 4 5% 8% 49%	1975
	21st to end	Night	10% 5% 2 5% 3 4 3 4 6% 8% 50%	2000
May	1st to 10th	Night	9% 8% 3 8% 6% 6% 6% 4 6% 9% 34%	4476
	11th to 20th	Night	17% 8% 3 7% 5% 5% 5% 4 5% 7% 32%	4032
	21st to end	Night	14% 7% 3 6% 5% 4 5% 3 6% 9% 40%	4220
June	1st to 10th	Night	9% 5% 2 6% 5% 5% 5% 4 9% 10% 41%	1600
	11th to 20th	Night	10% 6% 2 7% 5% 6% 6% 5% 7% 10% 37%	1600
	21st to end	Night	7% 5% 2 6% 6% 5% 4 7% 8% 12% 38%	1621
July	1st to 10th	Night	7% 5% 3 7% 5% 5% 5% 5% 9% 14% 36%	2885
	11th to 20th	Night	11% 7% 3 8% 5% 5% 6% 4 9% 12% 30%	2840
	21st to end	Night	15% 9% 4 10% 5% 5% 6% 5% 6% 7% 29%	3087
August	1st to 10th	Night	18% 9% 4 9% 5% 5% 6% 4 7% 9% 25%	2825
	11th to 20th	Night	9% 7% 4 9% 6% 5% 6% 5% 8% 10% 32%	3054
	21st to end	Night	6% 5% 2 6% 5% 6% 6% 5% 8% 10% 41%	3476
September	1st to 10th	Night	9% 5% 2 7% 6% 5% 4 4 6% 9% 43%	1900
	11th to 20th	Night	14% 7% 3 7% 5% 3 4 4 4 8% 41%	1900
	21st to end	Night	18% 7% 4 9% 5% 4 5% 4 5% 6% 34%	2184
October	1st to 10th	Night	13% 6% 2 5% 4 3 4 3 5% 6% 49%	2280
	11th to 20th	Night	12% 6% 2 5% 3 3 3 3 4 5% 54%	2280
	21st to end	Night	10% 5% 2 5% 4 3 3 3 4 6% 56%	2508
November	1st to 10th	Night	14% 7% 2 4 4 3 4 3 4 5% 52%	2341
	11th to 20th	Night	15% 7% 2 5% 4 3 3 3 5% 5% 49%	2526
	21st to end	Night	12% 5% 2 5% 3 3 3 3 4 7% 51%	2670
December	1st to 10th	Night	10% 5% 2 5% 3 3 3 3 4 6% 55%	4580
	11th to 20th	Night	16% 7% 3 7% 4 4 4 3 5% 6% 40%	5205
	21st to end	Night	13% 6% 2 5% 3 3 3 3 5% 7% 50%	6766

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	35% 1 4 20% 30% 9%	4057
	11th to 20th	Day	34% 2 4 16% 34% 10%	4060
	21st to end	Day	45% 3 5% 18% 21% 9%	4466
February	1st to 10th	Day	42% 3 10% 22% 20% 4	1800
	11th to 20th	Day	45% 3 6% 18% 23% 6%	1882
	21st to end	Day	43% 4 9% 19% 20% 6%	1679
March	1st to 10th	Day	47% 2 7% 16% 18% 9%	4935
	11th to 20th	Day	33% 2 7% 17% 26% 15%	4980
	21st to end	Day	36% 4 9% 23% 22% 6%	5498
April	1st to 10th	Day	32% 4 9% 23% 24% 7%	2400
	11th to 20th	Day	32% 4 11% 24% 22% 7%	2374
	21st to end	Day	35% 4 14% 24% 20% 4	2400
May	1st to 10th	Day	42% 6% 18% 22% 10% 1	5480
	11th to 20th	Day	51% 5% 13% 20% 10% 1	5865
	21st to end	Day	38% 5% 12% 27% 16% 2	6630
June	1st to 10th	Day	40% 7% 14% 21% 17% 2	2800
	11th to 20th	Day	36% 7% 16% 24% 15% 2	2800
	21st to end	Day	36% 9% 18% 23% 13% 2	2771
July	1st to 10th	Day	39% 10% 18% 21% 10% 1	4120
	11th to 20th	Day	49% 10% 20% 16% 5% 1	4051
	21st to end	Day	49% 9% 14% 18% 10%	4324
August	1st to 10th	Day	59% 6% 18% 12% 5%	4091
	11th to 20th	Day	42% 7% 14% 25% 11% 1	3777
	21st to end	Day	35% 5% 15% 29% 14% 2	4180
September	1st to 10th	Day	40% 9% 14% 17% 17% 3	2280
	11th to 20th	Day	42% 6% 11% 19% 20% 3	2280
	21st to end	Day	48% 4 9% 21% 14% 3	1991
October	1st to 10th	Day	33% 4 9% 25% 25% 5%	3160
	11th to 20th	Day	24% 2 4 20% 27% 22%	3140
	21st to end	Day	26% 2 5% 24% 30% 13%	3428
November	1st to 10th	Day	37% 3 7% 22% 26% 6%	1957
	11th to 20th	Day	36% 3 8% 22% 24% 7%	1870
	21st to end	Day	34% 3 7% 26% 24% 6%	1725
December	1st to 10th	Day	35% 3 8% 23% 24% 7%	2600
	11th to 20th	Day	43% 4 10% 21% 17% 6%	2986
	21st to end	Day	38% 3 11% 26% 21% 2	4172
January	1st to 10th	Night	33% 2 5% 18% 32% 9%	5889
	11th to 20th	Night	37% 3 4 16% 32% 7%	5900
	21st to end	Night	42% 3 6% 18% 25% 6%	6490
February	1st to 10th	Night	42% 3 8% 22% 19% 6%	2600
	11th to 20th	Night	45% 3 9% 16% 20% 7%	2413
	21st to end	Night	38% 3 8% 21% 21% 9%	1949
March	1st to 10th	Night	47% 3 7% 14% 20% 9%	4918
	11th to 20th	Night	34% 4 9% 13% 24% 17%	4980
	21st to end	Night	36% 3 9% 25% 20% 6%	5432
April	1st to 10th	Night	36% 5% 10% 18% 23% 7%	2000
	11th to 20th	Night	34% 4 10% 24% 21% 6%	1975
	21st to end	Night	33% 4 13% 29% 17% 4	2000
May	1st to 10th	Night	45% 6% 17% 20% 10% 1	4506
	11th to 20th	Night	51% 6% 10% 21% 11% 1	4033
	21st to end	Night	42% 6% 14% 24% 13% 1	4227
June	1st to 10th	Night	35% 8% 18% 22% 15% 2	1600
	11th to 20th	Night	39% 8% 16% 22% 12% 2	1600
	21st to end	Night	36% 9% 18% 23% 12% 1	1621
July	1st to 10th	Night	37% 11% 22% 19% 10% 1	2840
	11th to 20th	Night	44% 11% 25% 15% 4 1	2786
	21st to end	Night	53% 7% 13% 16% 10% 1	2988
August	1st to 10th	Night	55% 7% 20% 15% 4	2814
	11th to 20th	Night	44% 9% 19% 19% 8% 1	3084
	21st to end	Night	35% 7% 15% 29% 14% 2	3476
September	1st to 10th	Night	38% 7% 17% 17% 17% 4	1900
	11th to 20th	Night	41% 7% 11% 21% 17% 3	1900
	21st to end	Night	50% 5% 10% 17% 15% 2	2184
October	1st to 10th	Night	38% 4 7% 20% 26% 4	3800
	11th to 20th	Night	24% 3 5% 18% 25% 26%	3779
	21st to end	Night	23% 2 7% 28% 26% 13%	4120
November	1st to 10th	Night	36% 3 6% 22% 27% 5%	2341
	11th to 20th	Night	38% 3 8% 16% 26% 9%	2526
	21st to end	Night	33% 3 8% 24% 25% 6%	2670
December	1st to 10th	Night	33% 2 8% 16% 31% 10%	4580
	11th to 20th	Night	46% 4 10% 16% 18% 7%	5208
	21st to end	Night	35% 4 11% 27% 21% 2	6766

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	23% 20% 37% 14% 6%	300
	11th to 20th	Day	18% 23% 34% 19% 7%	300
	21st to end	Day	21% 17% 39% 18% 5% 1	330
February	1st to 10th	Day	18% 13% 43% 20% 5%	300
	11th to 20th	Day	30% 13% 37% 16% 4	421
	21st to end	Day	21% 12% 36% 23% 6% 1	414
March	1st to 10th	Day	20% 15% 34% 25% 6% 1	500
	11th to 20th	Day	19% 17% 34% 21% 7% 1	500
	21st to end	Day	19% 12% 35% 24% 8% 2	550
April	1st to 10th	Day	18% 17% 30% 22% 10% 3	500
	11th to 20th	Day	12% 13% 30% 28% 12% 5%	492
	21st to end	Day	12% 9% 29% 25% 20% 5%	500
May	1st to 10th	Day	13% 10% 34% 23% 14% 6%	503
	11th to 20th	Day	18% 5% 20% 28% 21% 9%	527
	21st to end	Day	16% 3 17% 26% 21% 15%	573
June	1st to 10th	Day	12% 3 17% 32% 25% 12%	530
	11th to 20th	Day	12% 3 16% 24% 27% 18%	530
	21st to end	Day	9% 2 13% 26% 33% 17%	530
July	1st to 10th	Day	10% 2 6% 25% 37% 21%	505
	11th to 20th	Day	11% 2 5% 27% 35% 20%	484
	21st to end	Day	10% 2 9% 29% 37% 14%	532
August	1st to 10th	Day	16% 4 10% 23% 27% 19%	486
	11th to 20th	Day	13% 2 8% 23% 32% 21%	479
	21st to end	Day	10% 3 7% 27% 27% 26%	539
September	1st to 10th	Day	13% 3 11% 26% 25% 23%	490
	11th to 20th	Day	17% 3 10% 27% 28% 14%	490
	21st to end	Day	23% 2 17% 24% 24% 10%	490
October	1st to 10th	Day	18% 2 17% 25% 25% 13%	490
	11th to 20th	Day	14% 7% 22% 30% 21% 6%	490
	21st to end	Day	14% 5% 24% 27% 22% 8%	539
November	1st to 10th	Day	21% 5% 24% 31% 15% 4	495
	11th to 20th	Day	18% 8% 31% 29% 11% 2	435
	21st to end	Day	17% 10% 26% 26% 19% 2	325
December	1st to 10th	Day	19% 11% 42% 20% 8% 1	200
	11th to 20th	Day	17% 8% 39% 24% 11% 3	200
	21st to end	Day	19% 11% 37% 20% 10% 2	267
January	1st to 10th	Night	19% 12% 36% 27% 6% 1	800
	11th to 20th	Night	18% 11% 31% 34% 6%	800
	21st to end	Night	19% 9% 38% 30% 4	880
February	1st to 10th	Night	22% 9% 35% 30% 5%	800
	11th to 20th	Night	24% 13% 38% 24% 2	783
	21st to end	Night	19% 12% 35% 30% 4	660
March	1st to 10th	Night	17% 10% 35% 31% 6%	800
	11th to 20th	Night	20% 10% 37% 26% 7% 1	800
	21st to end	Night	18% 9% 37% 30% 7%	880
April	1st to 10th	Night	16% 11% 35% 33% 5%	800
	11th to 20th	Night	13% 9% 35% 32% 11% 1	791
	21st to end	Night	13% 6% 32% 41% 9% 1	800
May	1st to 10th	Night	13% 8% 31% 37% 10% 1	799
	11th to 20th	Night	16% 4 22% 41% 16% 1	591
	21st to end	Night	11% 2 15% 41% 30% 1	565
June	1st to 10th	Night	10% 2 18% 48% 21% 2	400
	11th to 20th	Night	11% 3 13% 46% 24% 3	400
	21st to end	Night	7% 3 12% 47% 29% 2	421
July	1st to 10th	Night	8% 2 7% 39% 38% 5%	570
	11th to 20th	Night	11% 1 4 36% 42% 5%	552
	21st to end	Night	8% 2 10% 42% 33% 5%	624
August	1st to 10th	Night	14% 2 5% 40% 35% 4	567
	11th to 20th	Night	13% 2 5% 36% 37% 6%	719
	21st to end	Night	13% 2 7% 31% 41% 6%	836
September	1st to 10th	Night	12% 2 10% 33% 37% 7%	760
	11th to 20th	Night	17% 3 10% 35% 32% 4	760
	21st to end	Night	22% 3 11% 34% 27% 2	760
October	1st to 10th	Night	17% 3 15% 33% 27% 5%	760
	11th to 20th	Night	15% 5% 21% 37% 19% 2	760
	21st to end	Night	13% 4 23% 35% 21% 3	836
November	1st to 10th	Night	17% 4 21% 39% 17% 3	780
	11th to 20th	Night	19% 8% 24% 36% 13% 1	800
	21st to end	Night	15% 7% 27% 35% 15% 1	800
December	1st to 10th	Night	19% 8% 29% 37% 7%	800
	11th to 20th	Night	18% 7% 27% 39% 9%	800
	21st to end	Night	17% 8% 30% 36% 9%	880

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