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 2025-07-21.

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% 10% 4 9% 6% 5% 5% 4 5% 7% 33%	119807
	Night	14% 11% 4 9% 5% 5% 4 4 5% 6% 32%	119501
Transparency	Day	48% 6% 11% 21% 12% 1	123392
	Night	50% 6% 11% 20% 11% 1	124057
Seeing	Day	18% 12% 30% 23% 13% 5%	16793
	Night	19% 9% 30% 29% 11% 1	26651

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	11% 19% 7% 16% 8% 6% 5% 3 5% 5% 16%	3617
	11th to 20th	Day	11% 15% 7% 18% 15% 10% 6% 3 4 3 6%	3620
	21st to end	Day	16% 20% 9% 16% 7% 5% 5% 3 3 4 13%	3982
February	1st to 10th	Day	12% 15% 5% 14% 10% 6% 5% 4 5% 5% 20%	1825
	11th to 20th	Day	12% 15% 6% 14% 9% 5% 4 3 3 5% 23%	1948
	21st to end	Day	15% 14% 6% 12% 7% 4 4 4 5% 7% 23%	1650
March	1st to 10th	Day	13% 10% 4 10% 6% 5% 5% 3 5% 6% 33%	4480
	11th to 20th	Day	11% 8% 4 9% 6% 5% 5% 4 5% 7% 36%	4882
	21st to end	Day	19% 12% 5% 8% 5% 4 4 3 4 5% 29%	5613
April	1st to 10th	Day	18% 14% 3 7% 5% 4 4 3 5% 6% 30%	2400
	11th to 20th	Day	16% 11% 3 6% 5% 4 4 3 5% 6% 36%	2457
	21st to end	Day	16% 8% 3 8% 5% 5% 5% 3 6% 7% 35%	2600
May	1st to 10th	Day	19% 8% 2 6% 4 4 5% 4 5% 7% 37%	5862
	11th to 20th	Day	13% 8% 3 7% 5% 4 5% 4 6% 8% 37%	5861
	21st to end	Day	14% 6% 3 7% 5% 4 5% 3 5% 7% 41%	6890
June	1st to 10th	Day	10% 7% 2 5% 4 5% 5% 4 7% 9% 43%	2800
	11th to 20th	Day	11% 6% 2 6% 4 5% 4 4 6% 8% 43%	2800
	21st to end	Day	10% 6% 2 7% 5% 5% 5% 4 7% 8% 42%	2800
July	1st to 10th	Day	10% 7% 3 8% 5% 5% 5% 5% 7% 9% 37%	4580
	11th to 20th	Day	6% 4 2 5% 4 4 5% 4 7% 11% 49%	4580
	21st to end	Day	7% 5% 2 6% 5% 5% 5% 4 7% 8% 45%	4814
August	1st to 10th	Day	11% 6% 2 6% 4 6% 6% 4 6% 8% 41%	4114
	11th to 20th	Day	10% 6% 2 5% 4 5% 5% 3 5% 8% 47%	4044
	21st to end	Day	7% 6% 2 7% 5% 5% 5% 4 6% 8% 43%	4288
September	1st to 10th	Day	10% 7% 3 6% 5% 4 4 4 6% 7% 43%	2233
	11th to 20th	Day	11% 5% 1 5% 4 4 5% 3 7% 8% 47%	2090
	21st to end	Day	12% 7% 3 7% 6% 5% 5% 3 5% 8% 41%	2090
October	1st to 10th	Day	16% 7% 3 7% 4 4 4 3 4 6% 42%	2090
	11th to 20th	Day	20% 10% 3 8% 5% 5% 5% 3 5% 7% 29%	1849
	21st to end	Day	21% 11% 3 9% 7% 5% 5% 3 5% 6% 25%	1881
November	1st to 10th	Day	12% 11% 3 7% 5% 4 4 4 5% 7% 37%	1762
	11th to 20th	Day	14% 16% 5% 10% 6% 5% 4 4 6% 6% 24%	1800
	21st to end	Day	20% 15% 6% 9% 5% 4 5% 3 4 6% 23%	1800
December	1st to 10th	Day	10% 19% 8% 14% 7% 5% 4 4 6% 6% 17%	2715
	11th to 20th	Day	15% 19% 11% 12% 8% 5% 5% 3 4 4 14%	3008
	21st to end	Day	23% 22% 7% 11% 5% 4 3 3 4 4 13%	3982
January	1st to 10th	Night	11% 16% 7% 17% 8% 6% 5% 4 5% 5% 17%	6348
	11th to 20th	Night	11% 16% 7% 19% 11% 9% 4 3 4 5% 11%	6360
	21st to end	Night	15% 17% 8% 12% 7% 6% 6% 4 5% 5% 15%	6996
February	1st to 10th	Night	13% 16% 5% 11% 9% 6% 5% 4 5% 5% 21%	2571
	11th to 20th	Night	12% 14% 6% 11% 8% 6% 5% 5% 5% 6% 24%	2338
	21st to end	Night	16% 14% 4 12% 5% 6% 5% 4 4 6% 25%	1980
March	1st to 10th	Night	14% 9% 3 8% 5% 4 4 3 5% 6% 38%	5373
	11th to 20th	Night	11% 8% 3 6% 4 4 4 3 6% 9% 42%	5077
	21st to end	Night	17% 11% 5% 8% 4 3 4 3 5% 7% 33%	5358
April	1st to 10th	Night	15% 11% 3 7% 5% 4 4 4 5% 6% 36%	2000
	11th to 20th	Night	19% 8% 2 7% 4 4 4 3 4 6% 38%	1892
	21st to end	Night	17% 8% 3 5% 5% 4 4 3 5% 8% 38%	1800
May	1st to 10th	Night	16% 8% 3 8% 4 5% 4 3 5% 6% 36%	4033
	11th to 20th	Night	16% 8% 3 7% 4 3 4 3 4 7% 42%	4032
	21st to end	Night	16% 7% 2 6% 4 3 4 3 5% 6% 44%	3968
June	1st to 10th	Night	10% 6% 2 5% 5% 4 4 4 6% 8% 46%	1600
	11th to 20th	Night	11% 6% 2 6% 4 4 4 4 7% 8% 44%	1600
	21st to end	Night	8% 6% 2 5% 5% 4 4 4 6% 8% 47%	1600
July	1st to 10th	Night	9% 6% 3 7% 5% 5% 5% 4 7% 8% 42%	2610
	11th to 20th	Night	6% 4 2 4 4 3 4 4 6% 7% 56%	2610
	21st to end	Night	6% 4 2 6% 4 5% 6% 3 6% 8% 48%	2750
August	1st to 10th	Night	10% 5% 2 5% 4 3 4 3 5% 8% 50%	2814
	11th to 20th	Night	10% 4 2 5% 4 3 5% 4 5% 8% 50%	2785
	21st to end	Night	9% 6% 2 6% 5% 5% 4 4 5% 8% 46%	3366
September	1st to 10th	Night	9% 5% 1 5% 4 4 4 4 5% 7% 52%	1926
	11th to 20th	Night	11% 5% 2 4 3 3 4 3 5% 6% 54%	2090
	21st to end	Night	13% 8% 3 6% 4 4 4 4 5% 7% 43%	2090
October	1st to 10th	Night	16% 5% 2 6% 3 3 4 3 4 6% 49%	2090
	11th to 20th	Night	18% 11% 3 7% 5% 5% 3 3 5% 7% 34%	2326
	21st to end	Night	21% 10% 3 7% 5% 3 5% 4 4 6% 32%	2717
November	1st to 10th	Night	14% 8% 3 7% 4 4 4 3 6% 6% 40%	2536
	11th to 20th	Night	14% 14% 3 10% 6% 6% 5% 5% 7% 6% 25%	2600
	21st to end	Night	17% 15% 4 9% 5% 5% 4 4 5% 6% 27%	2600
December	1st to 10th	Night	11% 17% 6% 13% 7% 6% 5% 4 5% 6% 19%	4470
	11th to 20th	Night	16% 21% 6% 12% 8% 5% 5% 3 4 5% 14%	5199
	21st to end	Night	25% 21% 5% 10% 5% 4 3 3 4 5% 15%	6996

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	69% 7% 10% 12% 2	3618
	11th to 20th	Day	81% 5% 7% 5% 2	3620
	21st to end	Day	76% 5% 9% 7% 3	3982
February	1st to 10th	Day	66% 6% 8% 14% 5%	1825
	11th to 20th	Day	65% 4 8% 14% 8%	1958
	21st to end	Day	61% 6% 11% 15% 7%	1650
March	1st to 10th	Day	51% 5% 13% 19% 10% 2	4480
	11th to 20th	Day	46% 6% 15% 21% 11% 1	4882
	21st to end	Day	57% 4 10% 16% 11% 1	5615
April	1st to 10th	Day	55% 5% 10% 19% 12% 1	2400
	11th to 20th	Day	49% 5% 9% 22% 13% 2	2457
	21st to end	Day	48% 6% 11% 20% 12% 2	2600
May	1st to 10th	Day	46% 6% 11% 19% 17% 2	5897
	11th to 20th	Day	43% 7% 11% 23% 15% 2	5865
	21st to end	Day	40% 7% 9% 24% 19% 2	6904
June	1st to 10th	Day	36% 7% 12% 29% 15% 2	2800
	11th to 20th	Day	37% 6% 14% 23% 19% 1	2800
	21st to end	Day	37% 8% 13% 22% 17% 3	2800
July	1st to 10th	Day	41% 7% 13% 20% 15% 3	4511
	11th to 20th	Day	27% 7% 15% 32% 16% 1	4505
	21st to end	Day	33% 8% 13% 23% 22% 2	4663
August	1st to 10th	Day	38% 8% 11% 26% 16% 1	4100
	11th to 20th	Day	34% 7% 11% 29% 18% 1	4083
	21st to end	Day	35% 8% 13% 29% 14% 1	4287
September	1st to 10th	Day	38% 7% 13% 26% 14% 2	2233
	11th to 20th	Day	33% 7% 13% 29% 16% 2	2090
	21st to end	Day	42% 5% 12% 24% 15%	2090
October	1st to 10th	Day	30% 4 8% 35% 21% 2	3480
	11th to 20th	Day	42% 5% 11% 28% 14%	3052
	21st to end	Day	50% 6% 12% 23% 8% 1	3081
November	1st to 10th	Day	46% 6% 13% 23% 11% 1	1762
	11th to 20th	Day	59% 6% 12% 17% 6% 1	1800
	21st to end	Day	62% 5% 14% 14% 5%	1800
December	1st to 10th	Day	66% 9% 15% 9% 2	2715
	11th to 20th	Day	74% 5% 8% 11% 1	3005
	21st to end	Day	75% 5% 6% 11% 4	3982
January	1st to 10th	Night	66% 7% 9% 12% 5% 1	6348
	11th to 20th	Night	75% 5% 8% 6% 5%	6360
	21st to end	Night	69% 7% 11% 10% 3	6996
February	1st to 10th	Night	64% 7% 10% 14% 6%	2571
	11th to 20th	Night	60% 6% 9% 17% 8% 1	2339
	21st to end	Night	60% 5% 11% 16% 7% 1	1980
March	1st to 10th	Night	46% 5% 14% 25% 10% 1	5373
	11th to 20th	Night	38% 5% 16% 26% 14% 2	5077
	21st to end	Night	51% 5% 11% 20% 11% 1	5358
April	1st to 10th	Night	48% 6% 9% 22% 14% 1	2000
	11th to 20th	Night	47% 4 9% 20% 17% 3	1892
	21st to end	Night	45% 5% 13% 20% 14% 3	1800
May	1st to 10th	Night	47% 6% 12% 15% 16% 3	4060
	11th to 20th	Night	43% 5% 9% 23% 17% 3	4033
	21st to end	Night	41% 5% 8% 24% 19% 3	3976
June	1st to 10th	Night	36% 6% 10% 28% 17% 3	1600
	11th to 20th	Night	36% 6% 13% 24% 19% 1	1600
	21st to end	Night	33% 7% 14% 28% 16% 2	1600
July	1st to 10th	Night	36% 7% 12% 26% 12% 7%	2570
	11th to 20th	Night	26% 7% 13% 30% 23% 1	2562
	21st to end	Night	32% 8% 15% 21% 22% 2	2662
August	1st to 10th	Night	32% 5% 13% 30% 19% 1	2804
	11th to 20th	Night	31% 7% 11% 34% 16% 1	2812
	21st to end	Night	34% 8% 11% 30% 16% 2	3366
September	1st to 10th	Night	32% 6% 12% 30% 18% 2	1926
	11th to 20th	Night	29% 6% 11% 34% 19% 1	2090
	21st to end	Night	40% 6% 12% 27% 14% 1	2090
October	1st to 10th	Night	27% 4 10% 36% 21% 1	3480
	11th to 20th	Night	42% 5% 10% 24% 18%	3862
	21st to end	Night	43% 4 10% 29% 13%	4467
November	1st to 10th	Night	44% 6% 10% 25% 14% 1	2536
	11th to 20th	Night	56% 6% 13% 17% 7% 1	2600
	21st to end	Night	58% 6% 14% 18% 4	2600
December	1st to 10th	Night	63% 9% 15% 11% 2	4470
	11th to 20th	Night	71% 5% 10% 10% 3	5201
	21st to end	Night	72% 5% 9% 9% 5%	6996

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	20% 18% 36% 18% 6% 2	300
	11th to 20th	Day	22% 18% 41% 15% 4 1	300
	21st to end	Day	28% 19% 30% 16% 7%	330
February	1st to 10th	Day	22% 23% 35% 16% 2 2	300
	11th to 20th	Day	22% 25% 37% 14% 2	300
	21st to end	Day	27% 27% 35% 9% 3	249
March	1st to 10th	Day	23% 20% 42% 11% 4	300
	11th to 20th	Day	22% 15% 34% 19% 9% 1	454
	21st to end	Day	27% 13% 32% 22% 4 2	550
April	1st to 10th	Day	26% 17% 31% 19% 6% 1	500
	11th to 20th	Day	26% 22% 28% 15% 6% 4	492
	21st to end	Day	21% 12% 32% 22% 10% 2	500
May	1st to 10th	Day	23% 17% 27% 22% 8% 3	503
	11th to 20th	Day	18% 10% 30% 26% 11% 6%	527
	21st to end	Day	14% 10% 28% 25% 18% 5%	770
June	1st to 10th	Day	11% 8% 36% 28% 12% 4	730
	11th to 20th	Day	13% 10% 30% 29% 12% 5%	730
	21st to end	Day	12% 8% 31% 28% 16% 4	730
July	1st to 10th	Day	8% 6% 24% 29% 22% 11%	721
	11th to 20th	Day	10% 7% 26% 31% 18% 9%	718
	21st to end	Day	9% 5% 28% 29% 21% 8%	740
August	1st to 10th	Day	10% 4 23% 25% 24% 14%	496
	11th to 20th	Day	12% 3 19% 27% 26% 13%	479
	21st to end	Day	11% 6% 20% 29% 23% 11%	539
September	1st to 10th	Day	13% 7% 21% 26% 21% 12%	490
	11th to 20th	Day	9% 5% 19% 27% 27% 13%	490
	21st to end	Day	18% 11% 21% 27% 16% 7%	490
October	1st to 10th	Day	19% 10% 27% 24% 15% 5%	490
	11th to 20th	Day	23% 14% 31% 23% 7% 1	415
	21st to end	Day	29% 14% 36% 17% 4 1	330
November	1st to 10th	Day	23% 18% 32% 18% 8% 1	300
	11th to 20th	Day	22% 22% 39% 13% 4 1	300
	21st to end	Day	27% 16% 40% 14% 3	300
December	1st to 10th	Day	23% 24% 32% 16% 4 1	300
	11th to 20th	Day	23% 19% 41% 13% 3 1	300
	21st to end	Day	28% 22% 33% 13% 2 2	330
January	1st to 10th	Night	21% 16% 34% 23% 7% 1	1000
	11th to 20th	Night	22% 14% 36% 22% 6%	1000
	21st to end	Night	22% 10% 32% 30% 6%	1100
February	1st to 10th	Night	22% 14% 35% 26% 3	809
	11th to 20th	Night	20% 13% 38% 25% 5%	783
	21st to end	Night	23% 15% 39% 19% 3	660
March	1st to 10th	Night	19% 12% 37% 25% 7% 1	800
	11th to 20th	Night	18% 13% 37% 27% 5% 1	800
	21st to end	Night	23% 10% 33% 26% 7%	835
April	1st to 10th	Night	18% 13% 36% 27% 5% 1	600
	11th to 20th	Night	24% 10% 33% 26% 7% 1	593
	21st to end	Night	22% 9% 35% 26% 7% 1	600
May	1st to 10th	Night	19% 7% 34% 32% 8% 1	600
	11th to 20th	Night	19% 7% 32% 29% 12% 1	591
	21st to end	Night	15% 6% 25% 34% 18% 1	650
June	1st to 10th	Night	12% 6% 29% 38% 15% 1	600
	11th to 20th	Night	13% 6% 27% 36% 18%	600
	21st to end	Night	10% 5% 25% 41% 18% 1	600
July	1st to 10th	Night	9% 2 19% 41% 26% 3	585
	11th to 20th	Night	7% 3 22% 41% 24% 3	582
	21st to end	Night	8% 2 21% 43% 25% 2	624
August	1st to 10th	Night	8% 2 19% 43% 26% 2	567
	11th to 20th	Night	9% 2 18% 41% 26% 4	564
	21st to end	Night	11% 3 19% 37% 27% 3	627
September	1st to 10th	Night	11% 3 22% 35% 24% 4	596
	11th to 20th	Night	12% 4 19% 36% 26% 3	760
	21st to end	Night	16% 7% 23% 33% 17% 5%	760
October	1st to 10th	Night	18% 6% 24% 34% 15% 2	760
	11th to 20th	Night	23% 11% 28% 29% 9% 1	760
	21st to end	Night	27% 11% 33% 22% 7%	836
November	1st to 10th	Night	21% 10% 32% 27% 9%	780
	11th to 20th	Night	21% 15% 36% 22% 6% 1	800
	21st to end	Night	25% 11% 34% 26% 4	800
December	1st to 10th	Night	23% 14% 34% 24% 5%	929
	11th to 20th	Night	25% 14% 36% 21% 4 1	1000
	21st to end	Night	31% 11% 35% 20% 2	1100

Page generated at 2025-07-18 01:10:04. Version ac07

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