Greer
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Greer clear sky chart from 2005-11-06 to 2026-07-31.

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	3 4 2 6% 4 4 4 4 5% 7% 57%	147041
	Night	1 3 1 4 3 3 4 4 6% 7% 64%	143559
Transparency	Day	25% 6% 8% 14% 27% 20%	141934
	Night	18% 5% 9% 16% 30% 22%	139673
Seeing	Day	12% 19% 26% 23% 15% 5%	16931
	Night	6% 14% 25% 30% 20% 5%	26973

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	2 1 6% 5% 5% 4 4 5% 7% 59%	5318
	11th to 20th	Day	1 1 3 2 2 2 2 4 5% 76%	5340
	21st to end	Day	1 6% 2 5% 3 3 3 3 6% 6% 60%	5844
February	1st to 10th	Day	1 4 1 6% 3 4 3 3 4 7% 63%	1890
	11th to 20th	Day	1 3 2 6% 4 5% 4 3 5% 7% 60%	1919
	21st to end	Day	3 2 5% 3 4 3 3 5% 8% 62%	1903
March	1st to 10th	Day	3 2 7% 4 3 3 3 5% 6% 63%	5064
	11th to 20th	Day	1 5% 3 7% 3 4 5% 4 7% 7% 54%	5110
	21st to end	Day	2 5% 3 5% 3 4 4 3 5% 7% 59%	5607
April	1st to 10th	Day	1 4 2 4 4 3 5% 3 5% 6% 64%	2386
	11th to 20th	Day	1 5% 2 4 3 3 3 3 5% 5% 66%	2491
	21st to end	Day	1 3 2 4 3 3 3 3 5% 7% 65%	2520
May	1st to 10th	Day	3 6% 2 5% 3 3 3 3 4 6% 63%	7450
	11th to 20th	Day	3 3 2 4 3 3 4 3 4 5% 67%	7596
	21st to end	Day	3 5% 2 4 3 3 3 2 3 4 68%	8370
June	1st to 10th	Day	4 3 1 4 3 3 4 4 5% 6% 64%	2730
	11th to 20th	Day	3 3 2 3 3 3 3 2 3 5% 69%	2730
	21st to end	Day	6% 5% 2 6% 5% 4 5% 4 6% 5% 51%	2730
July	1st to 10th	Day	5% 5% 2 6% 5% 5% 5% 4 6% 6% 51%	6009
	11th to 20th	Day	6% 5% 2 8% 8% 9% 8% 6% 9% 8% 31%	6010
	21st to end	Day	8% 7% 3 8% 7% 8% 7% 5% 7% 8% 31%	6532
August	1st to 10th	Day	6% 6% 3 9% 7% 7% 7% 6% 7% 7% 35%	5779
	11th to 20th	Day	7% 6% 3 8% 7% 7% 8% 6% 8% 8% 32%	5649
	21st to end	Day	7% 6% 3 8% 6% 7% 7% 6% 8% 9% 34%	5984
September	1st to 10th	Day	9% 7% 3 7% 5% 5% 5% 4 6% 7% 41%	2374
	11th to 20th	Day	8% 6% 2 6% 5% 5% 4 3 4 5% 52%	2200
	21st to end	Day	5% 4 1 6% 3 4 4 3 5% 6% 59%	2200
October	1st to 10th	Day	7% 5% 2 5% 3 3 3 2 4 5% 61%	3285
	11th to 20th	Day	3 3 1 3 2 2 3 3 5% 9% 66%	3250
	21st to end	Day	1 1 2 1 1 1 1 3 7% 79%	3548
November	1st to 10th	Day	1 2 1 3 3 2 2 3 5% 7% 71%	2059
	11th to 20th	Day	2 1 4 3 3 3 4 4 7% 68%	2100
	21st to end	Day	1 5% 2 5% 3 2 3 3 5% 8% 63%	1916
December	1st to 10th	Day	1 3 1 3 2 2 3 3 4 5% 72%	3040
	11th to 20th	Day	1 5% 2 5% 3 2 4 3 6% 10% 60%	3499
	21st to end	Day	3 1 5% 3 3 3 4 5% 7% 66%	4609
January	1st to 10th	Night	1 4 2 7% 4 5% 5% 4 6% 7% 56%	7605
	11th to 20th	Night	1 1 2 1 1 1 1 3 5% 84%	7650
	21st to end	Night	3 2 5% 4 4 6% 5% 6% 8% 57%	8370
February	1st to 10th	Night	1 2 1 3 2 3 5% 5% 7% 7% 65%	2730
	11th to 20th	Night	3 1 5% 4 4 4 4 5% 7% 61%	2583
	21st to end	Night	1 2 1 3 3 3 4 4 6% 9% 65%	1903
March	1st to 10th	Night	2 1 3 2 2 3 2 5% 9% 70%	5050
	11th to 20th	Night	1 3 1 5% 3 3 4 4 6% 8% 63%	5110
	21st to end	Night	2 1 3 2 3 3 4 6% 9% 65%	5610
April	1st to 10th	Night	1 2 1 3 4 3 4 4 6% 7% 66%	2232
	11th to 20th	Night	3 1 3 3 3 2 3 4 6% 71%	2077
	21st to end	Night	1 2 1 1 2 2 2 2 4 8% 75%	2100
May	1st to 10th	Night	2 3 1 4 3 2 3 2 4 5% 70%	5492
	11th to 20th	Night	1 2 1 2 2 2 3 2 4 4 77%	5296
	21st to end	Night	1 1 1 2 2 2 2 2 3 4 80%	5837
June	1st to 10th	Night	1 3 1 3 3 3 3 3 4 7% 68%	1890
	11th to 20th	Night	2 2 1 2 2 2 2 2 3 5% 78%	1890
	21st to end	Night	2 3 2 4 4 4 5% 3 5% 6% 61%	1890
July	1st to 10th	Night	2 3 1 5% 4 4 4 4 6% 7% 60%	4190
	11th to 20th	Night	4 4 3 7% 6% 6% 7% 7% 9% 11% 36%	4190
	21st to end	Night	3 4 2 6% 5% 6% 7% 6% 9% 13% 39%	4562
August	1st to 10th	Night	2 3 2 5% 4 5% 6% 6% 8% 11% 48%	4033
	11th to 20th	Night	3 4 2 5% 5% 5% 7% 6% 10% 12% 41%	4199
	21st to end	Night	4 5% 2 6% 6% 6% 6% 6% 10% 12% 38%	4993
September	1st to 10th	Night	5% 5% 2 5% 4 4 5% 4 7% 8% 50%	2021
	11th to 20th	Night	4 3 1 5% 3 3 3 3 5% 6% 66%	2200
	21st to end	Night	2 3 1 3 2 2 3 3 5% 6% 70%	2200
October	1st to 10th	Night	2 2 1 3 3 3 3 3 4 5% 72%	3905
	11th to 20th	Night	1 2 1 2 2 2 2 2 4 6% 77%	3898
	21st to end	Night	1 1 2 1 2 2 2 4 5% 79%	4252
November	1st to 10th	Night	1 2 1 2 2 2 3 3 5% 7% 73%	2460
	11th to 20th	Night	1 1 1 4 2 3 2 3 5% 5% 72%	2520
	21st to end	Night	1 3 1 3 3 3 3 3 7% 7% 64%	2699
December	1st to 10th	Night	1 2 1 3 2 2 3 3 4 7% 72%	4370
	11th to 20th	Night	4 2 4 3 3 4 4 5% 8% 63%	4941
	21st to end	Night	2 1 4 3 4 4 3 5% 6% 67%	6611

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	21% 6% 7% 17% 32% 17%	5321
	11th to 20th	Day	11% 5% 7% 14% 27% 38%	5340
	21st to end	Day	22% 5% 6% 9% 28% 30%	5844
February	1st to 10th	Day	21% 2 9% 17% 25% 25%	1890
	11th to 20th	Day	22% 5% 6% 21% 27% 19%	1928
	21st to end	Day	19% 5% 8% 17% 27% 25%	1903
March	1st to 10th	Day	19% 5% 6% 10% 30% 30%	5064
	11th to 20th	Day	23% 8% 9% 12% 33% 15%	5110
	21st to end	Day	23% 4 9% 17% 38% 10%	5607
April	1st to 10th	Day	20% 4 7% 17% 35% 17%	2386
	11th to 20th	Day	20% 3 6% 14% 33% 24%	2494
	21st to end	Day	19% 4 6% 17% 38% 16%	2520
May	1st to 10th	Day	20% 6% 5% 16% 36% 17%	7450
	11th to 20th	Day	18% 3 7% 11% 36% 25%	7599
	21st to end	Day	19% 3 3 9% 26% 39%	8371
June	1st to 10th	Day	19% 5% 7% 13% 30% 26%	2730
	11th to 20th	Day	17% 4 4 9% 24% 41%	2730
	21st to end	Day	31% 6% 8% 10% 25% 20%	2730
July	1st to 10th	Day	31% 8% 7% 11% 15% 29%	5946
	11th to 20th	Day	42% 13% 14% 18% 8% 5%	5941
	21st to end	Day	44% 11% 18% 13% 12% 3	6348
August	1st to 10th	Day	47% 11% 19% 12% 8% 3	4208
	11th to 20th	Day	44% 11% 13% 19% 11% 2	4084
	21st to end	Day	43% 10% 14% 17% 14% 3	4311
September	1st to 10th	Day	40% 7% 10% 15% 18% 10%	2374
	11th to 20th	Day	34% 5% 7% 14% 29% 12%	2200
	21st to end	Day	24% 6% 9% 16% 29% 15%	2200
October	1st to 10th	Day	25% 6% 5% 12% 24% 28%	3285
	11th to 20th	Day	15% 4 3 20% 38% 21%	3250
	21st to end	Day	9% 1 4 13% 35% 37%	3548
November	1st to 10th	Day	13% 3 5% 23% 35% 20%	2059
	11th to 20th	Day	15% 4 6% 16% 33% 26%	2100
	21st to end	Day	21% 4 8% 14% 31% 22%	1916
December	1st to 10th	Day	14% 5% 5% 16% 38% 22%	3040
	11th to 20th	Day	21% 4 5% 14% 35% 20%	3498
	21st to end	Day	18% 5% 8% 19% 30% 20%	4609
January	1st to 10th	Night	23% 5% 9% 15% 33% 14%	7605
	11th to 20th	Night	7% 3 6% 14% 31% 39%	7650
	21st to end	Night	23% 5% 6% 9% 27% 30%	8370
February	1st to 10th	Night	17% 4 10% 18% 26% 25%	2730
	11th to 20th	Night	20% 6% 9% 19% 27% 19%	2584
	21st to end	Night	14% 5% 10% 17% 28% 26%	1903
March	1st to 10th	Night	11% 3 7% 11% 43% 25%	5050
	11th to 20th	Night	13% 8% 11% 15% 35% 18%	5110
	21st to end	Night	14% 5% 10% 24% 35% 11%	5610
April	1st to 10th	Night	17% 3 7% 23% 35% 14%	2232
	11th to 20th	Night	15% 3 6% 18% 35% 22%	2077
	21st to end	Night	10% 3 6% 25% 41% 16%	2100
May	1st to 10th	Night	16% 4 5% 20% 43% 12%	5490
	11th to 20th	Night	11% 3 5% 16% 41% 24%	5297
	21st to end	Night	10% 2 2 12% 33% 41%	5838
June	1st to 10th	Night	15% 5% 5% 14% 33% 27%	1890
	11th to 20th	Night	11% 2 3 12% 33% 39%	1890
	21st to end	Night	22% 5% 10% 14% 26% 23%	1890
July	1st to 10th	Night	22% 7% 10% 14% 17% 31%	4145
	11th to 20th	Night	36% 10% 22% 17% 10% 5%	4136
	21st to end	Night	33% 12% 29% 11% 13% 3	4434
August	1st to 10th	Night	33% 13% 27% 13% 10% 4	2928
	11th to 20th	Night	31% 11% 22% 19% 14% 3	3040
	21st to end	Night	32% 11% 18% 21% 16% 3	3597
September	1st to 10th	Night	29% 7% 15% 18% 20% 10%	2021
	11th to 20th	Night	20% 5% 9% 19% 32% 15%	2200
	21st to end	Night	15% 4 9% 21% 34% 17%	2200
October	1st to 10th	Night	15% 5% 5% 14% 31% 29%	3905
	11th to 20th	Night	10% 4 4 22% 35% 24%	3898
	21st to end	Night	9% 2 4 13% 35% 36%	4252
November	1st to 10th	Night	12% 3 6% 21% 37% 22%	2460
	11th to 20th	Night	13% 5% 6% 18% 30% 29%	2520
	21st to end	Night	17% 5% 9% 16% 29% 23%	2699
December	1st to 10th	Night	13% 4 6% 21% 32% 24%	4370
	11th to 20th	Night	21% 4 7% 13% 32% 24%	4941
	21st to end	Night	18% 5% 9% 18% 24% 27%	6611

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	10% 28% 35% 18% 7% 2	330
	11th to 20th	Day	14% 25% 28% 17% 13% 4	330
	21st to end	Day	10% 23% 28% 24% 13% 3	363
February	1st to 10th	Day	10% 25% 32% 19% 12% 3	330
	11th to 20th	Day	22% 29% 27% 17% 5%	394
	21st to end	Day	18% 33% 29% 14% 5% 1	446
March	1st to 10th	Day	18% 34% 27% 15% 4 1	540
	11th to 20th	Day	19% 27% 26% 19% 8% 2	540
	21st to end	Day	20% 30% 29% 15% 4 2	594
April	1st to 10th	Day	11% 29% 34% 17% 6% 3	540
	11th to 20th	Day	19% 28% 34% 13% 6%	532
	21st to end	Day	16% 29% 34% 14% 6% 1	540
May	1st to 10th	Day	13% 23% 38% 19% 7% 1	543
	11th to 20th	Day	10% 23% 35% 19% 10% 2	568
	21st to end	Day	11% 23% 35% 20% 10% 1	617
June	1st to 10th	Day	5% 15% 35% 31% 12% 2	570
	11th to 20th	Day	6% 16% 32% 29% 13% 4	570
	21st to end	Day	9% 12% 28% 32% 15% 4	570
July	1st to 10th	Day	11% 5% 17% 34% 25% 8%	566
	11th to 20th	Day	11% 3 10% 27% 34% 15%	564
	21st to end	Day	13% 3 7% 25% 35% 16%	616
August	1st to 10th	Day	10% 4 10% 26% 34% 16%	527
	11th to 20th	Day	14% 3 9% 29% 32% 12%	517
	21st to end	Day	14% 4 11% 28% 30% 14%	583
September	1st to 10th	Day	12% 6% 14% 34% 26% 8%	530
	11th to 20th	Day	9% 11% 27% 28% 19% 6%	530
	21st to end	Day	7% 15% 25% 30% 15% 7%	530
October	1st to 10th	Day	8% 20% 26% 23% 17% 6%	345
	11th to 20th	Day	12% 17% 23% 26% 15% 6%	330
	21st to end	Day	11% 14% 26% 27% 17% 6%	363
November	1st to 10th	Day	8% 21% 30% 25% 11% 5%	330
	11th to 20th	Day	8% 16% 27% 27% 17% 5%	330
	21st to end	Day	12% 25% 28% 22% 10% 4	330
December	1st to 10th	Day	9% 23% 25% 25% 15% 3	330
	11th to 20th	Day	7% 27% 26% 25% 11% 3	330
	21st to end	Day	18% 24% 29% 20% 8% 2	363
January	1st to 10th	Night	5% 19% 25% 31% 14% 6%	840
	11th to 20th	Night	5% 15% 31% 29% 15% 4	840
	21st to end	Night	5% 20% 28% 28% 15% 5%	924
February	1st to 10th	Night	5% 21% 28% 30% 13% 4	840
	11th to 20th	Night	8% 26% 31% 23% 10% 2	824
	21st to end	Night	9% 25% 32% 24% 9% 2	692
March	1st to 10th	Night	9% 28% 29% 24% 9% 1	840
	11th to 20th	Night	8% 20% 33% 23% 13% 3	840
	21st to end	Night	8% 22% 31% 26% 12% 2	924
April	1st to 10th	Night	6% 23% 34% 25% 10% 2	762
	11th to 20th	Night	8% 25% 36% 22% 8% 1	623
	21st to end	Night	5% 22% 32% 25% 13% 3	630
May	1st to 10th	Night	5% 15% 37% 29% 12% 2	630
	11th to 20th	Night	5% 11% 36% 30% 15% 3	621
	21st to end	Night	4 15% 34% 30% 14% 2	683
June	1st to 10th	Night	3 8% 33% 38% 17% 2	630
	11th to 20th	Night	3 7% 34% 39% 15% 1	630
	21st to end	Night	4 4 25% 38% 25% 4	630
July	1st to 10th	Night	5% 1 13% 37% 36% 7%	615
	11th to 20th	Night	5% 1 9% 32% 41% 12%	612
	21st to end	Night	7% 2 7% 28% 44% 12%	687
August	1st to 10th	Night	5% 2 6% 25% 47% 14%	597
	11th to 20th	Night	7% 2 7% 28% 44% 12%	594
	21st to end	Night	5% 1 8% 29% 45% 11%	660
September	1st to 10th	Night	6% 2 13% 35% 37% 7%	621
	11th to 20th	Night	4 4 23% 33% 30% 7%	800
	21st to end	Night	4 10% 16% 36% 28% 7%	800
October	1st to 10th	Night	4 14% 28% 32% 18% 4	800
	11th to 20th	Night	5% 11% 24% 37% 19% 5%	800
	21st to end	Night	5% 11% 24% 31% 22% 7%	880
November	1st to 10th	Night	5% 13% 23% 35% 17% 7%	820
	11th to 20th	Night	3 16% 25% 29% 22% 5%	840
	21st to end	Night	7% 20% 25% 25% 17% 6%	840
December	1st to 10th	Night	8% 18% 20% 32% 19% 4	840
	11th to 20th	Night	5% 20% 29% 27% 15% 3	840
	21st to end	Night	8% 22% 27% 28% 10% 4	924

Page generated at 2026-07-28 00:25:22. Version ac07

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