Francis Marion University Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Francis Marion University Observatory clear sky chart from 2005-11-06 to 2025-10-19.

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% 41%	126543
	Night	12% 6% 2 6% 4 4 4 4 6% 7% 43%	123196
Transparency	Day	39% 6% 12% 19% 18% 6%	130148
	Night	38% 5% 11% 18% 20% 7%	127659
Seeing	Day	15% 9% 23% 25% 20% 9%	16830
	Night	15% 7% 24% 33% 19% 2	26899

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% 5% 2 5% 3 2 3 2 4 6% 55%	4056
	11th to 20th	Day	12% 5% 2 5% 3 3 3 2 3 5% 57%	4060
	21st to end	Day	13% 9% 3 7% 4 4 5% 3 4 5% 44%	4466
February	1st to 10th	Day	15% 7% 2 6% 5% 4 4 4 6% 6% 41%	1800
	11th to 20th	Day	20% 7% 2 6% 3 3 3 3 4 4 44%	1845
	21st to end	Day	17% 6% 2 6% 4 4 4 3 5% 7% 42%	1805
March	1st to 10th	Day	19% 9% 3 7% 4 4 4 3 4 6% 39%	4935
	11th to 20th	Day	10% 5% 2 4 4 3 4 3 5% 7% 54%	4980
	21st to end	Day	13% 5% 2 6% 4 4 4 3 5% 5% 49%	5465
April	1st to 10th	Day	9% 4 2 4 4 4 4 4 7% 7% 51%	2260
	11th to 20th	Day	9% 5% 2 6% 3 4 4 4 5% 7% 50%	2374
	21st to end	Day	9% 4 2 5% 4 4 5% 4 6% 7% 51%	2400
May	1st to 10th	Day	8% 7% 3 7% 5% 5% 5% 5% 7% 11% 37%	5817
	11th to 20th	Day	17% 8% 2 7% 5% 5% 5% 4 7% 9% 32%	5861
	21st to end	Day	10% 5% 2 6% 4 5% 5% 4 6% 9% 43%	6433
June	1st to 10th	Day	9% 7% 3 7% 5% 6% 6% 5% 7% 10% 36%	2600
	11th to 20th	Day	8% 5% 2 4 5% 6% 6% 5% 8% 12% 39%	2600
	21st to end	Day	7% 5% 2 6% 5% 6% 7% 6% 8% 12% 37%	2600
July	1st to 10th	Day	10% 7% 3 9% 7% 6% 7% 6% 7% 11% 26%	5900
	11th to 20th	Day	6% 5% 2 8% 5% 7% 7% 6% 10% 15% 29%	5900
	21st to end	Day	9% 7% 3 8% 6% 6% 7% 6% 8% 13% 27%	6424
August	1st to 10th	Day	23% 8% 3 8% 5% 6% 6% 6% 7% 8% 19%	4234
	11th to 20th	Day	10% 6% 3 8% 5% 4 5% 5% 7% 11% 34%	4057
	21st to end	Day	7% 4 2 7% 5% 4 6% 4 8% 11% 42%	4312
September	1st to 10th	Day	7% 6% 3 6% 6% 5% 5% 4 7% 9% 41%	2355
	11th to 20th	Day	13% 7% 3 7% 5% 5% 5% 3 6% 6% 41%	2200
	21st to end	Day	16% 7% 3 6% 5% 4 4 4 6% 7% 38%	2200
October	1st to 10th	Day	14% 6% 2 6% 4 3 4 2 5% 7% 47%	2006
	11th to 20th	Day	10% 4 2 6% 2 3 3 3 4 6% 57%	1980
	21st to end	Day	10% 4 2 4 3 3 3 3 4 6% 57%	2090
November	1st to 10th	Day	13% 5% 2 4 3 3 3 3 4 6% 55%	1957
	11th to 20th	Day	14% 6% 2 5% 3 3 4 2 3 7% 51%	2000
	21st to end	Day	13% 5% 2 5% 3 4 4 3 4 6% 51%	1802
December	1st to 10th	Day	10% 5% 2 5% 4 3 4 4 5% 5% 52%	2930
	11th to 20th	Day	16% 5% 2 6% 4 4 3 3 4 7% 46%	3373
	21st to end	Day	16% 8% 3 6% 4 4 3 3 4 6% 44%	4466
January	1st to 10th	Night	12% 5% 2 4 3 3 3 3 4 5% 56%	5889
	11th to 20th	Night	12% 6% 2 5% 4 4 4 4 5% 6% 49%	5900
	21st to end	Night	13% 8% 2 7% 4 3 3 3 4 6% 46%	6490
February	1st to 10th	Night	16% 6% 3 5% 4 4 4 3 5% 7% 44%	2600
	11th to 20th	Night	18% 7% 2 6% 4 3 4 3 5% 6% 41%	2439
	21st to end	Night	13% 6% 2 6% 4 3 4 3 5% 7% 47%	1823
March	1st to 10th	Night	18% 9% 3 7% 4 4 3 3 4 5% 39%	4918
	11th to 20th	Night	11% 5% 2 5% 3 4 4 3 5% 8% 52%	4980
	21st to end	Night	12% 4 2 5% 4 4 3 3 5% 7% 51%	5467
April	1st to 10th	Night	12% 6% 3 5% 4 3 4 4 6% 6% 48%	2138
	11th to 20th	Night	10% 5% 2 6% 3 4 4 4 5% 6% 53%	1975
	21st to end	Night	8% 4 2 5% 4 4 4 4 5% 7% 52%	2000
May	1st to 10th	Night	7% 6% 2 6% 5% 5% 5% 5% 7% 11% 42%	4107
	11th to 20th	Night	16% 7% 3 6% 4 4 4 4 6% 10% 36%	4032
	21st to end	Night	11% 6% 2 7% 4 4 5% 4 7% 9% 41%	4421
June	1st to 10th	Night	10% 6% 3 5% 6% 5% 5% 6% 9% 10% 35%	1800
	11th to 20th	Night	10% 5% 2 7% 6% 5% 4 5% 8% 11% 38%	1800
	21st to end	Night	6% 5% 2 6% 5% 5% 6% 7% 9% 13% 37%	1800
July	1st to 10th	Night	10% 7% 3 10% 7% 7% 7% 5% 9% 10% 25%	4058
	11th to 20th	Night	10% 6% 2 8% 6% 6% 7% 6% 9% 14% 26%	4057
	21st to end	Night	11% 9% 4 11% 7% 6% 6% 5% 8% 9% 24%	4427
August	1st to 10th	Night	22% 8% 2 7% 5% 5% 5% 5% 7% 10% 24%	2915
	11th to 20th	Night	9% 8% 3 9% 6% 6% 5% 4 7% 10% 34%	2959
	21st to end	Night	6% 4 2 5% 4 4 5% 5% 8% 11% 46%	3586
September	1st to 10th	Night	9% 5% 2 6% 5% 5% 6% 5% 7% 10% 41%	2040
	11th to 20th	Night	12% 7% 3 8% 5% 5% 5% 4 6% 6% 40%	2200
	21st to end	Night	16% 7% 3 7% 5% 4 4 3 5% 6% 40%	2200
October	1st to 10th	Night	12% 6% 2 5% 3 4 3 3 6% 6% 50%	2392
	11th to 20th	Night	11% 5% 1 4 3 3 3 3 4 6% 56%	2377
	21st to end	Night	9% 6% 2 4 3 3 3 3 4 6% 57%	2508
November	1st to 10th	Night	11% 6% 3 5% 4 4 3 2 3 5% 55%	2341
	11th to 20th	Night	15% 7% 2 4 3 3 3 2 4 6% 51%	2400
	21st to end	Night	13% 6% 1 4 3 4 4 4 5% 7% 50%	2595
December	1st to 10th	Night	12% 6% 2 5% 3 3 3 3 4 5% 54%	4250
	11th to 20th	Night	14% 7% 4 7% 5% 4 4 3 4 5% 43%	4822
	21st to end	Night	16% 7% 3 6% 4 3 4 3 5% 6% 44%	6490

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	33% 3 5% 18% 33% 8%	4057
	11th to 20th	Day	32% 3 4 17% 35% 10%	4060
	21st to end	Day	44% 3 5% 15% 22% 11%	4466
February	1st to 10th	Day	43% 4 9% 19% 20% 5%	1800
	11th to 20th	Day	44% 3 6% 17% 24% 6%	1854
	21st to end	Day	42% 4 8% 17% 20% 9%	1805
March	1st to 10th	Day	49% 3 6% 12% 22% 9%	4935
	11th to 20th	Day	31% 4 8% 16% 27% 15%	4980
	21st to end	Day	38% 4 8% 18% 21% 10%	5465
April	1st to 10th	Day	30% 6% 11% 20% 26% 7%	2260
	11th to 20th	Day	32% 4 10% 21% 21% 11%	2374
	21st to end	Day	30% 6% 12% 24% 22% 5%	2400
May	1st to 10th	Day	40% 6% 16% 26% 10% 1	5851
	11th to 20th	Day	48% 6% 14% 21% 11% 1	5865
	21st to end	Day	36% 7% 14% 20% 20% 3	6447
June	1st to 10th	Day	40% 9% 15% 23% 11% 1	2600
	11th to 20th	Day	35% 10% 18% 22% 12% 2	2600
	21st to end	Day	37% 10% 18% 22% 11% 1	2600
July	1st to 10th	Day	50% 8% 19% 17% 4 1	5835
	11th to 20th	Day	42% 11% 32% 11% 3 1	5831
	21st to end	Day	47% 10% 16% 18% 8% 1	6240
August	1st to 10th	Day	60% 8% 17% 11% 4	4221
	11th to 20th	Day	42% 8% 16% 22% 12%	4096
	21st to end	Day	34% 8% 13% 31% 12% 1	4312
September	1st to 10th	Day	37% 11% 14% 17% 15% 5%	2355
	11th to 20th	Day	43% 6% 9% 17% 20% 4	2200
	21st to end	Day	44% 6% 10% 20% 16% 5%	2200
October	1st to 10th	Day	37% 4 6% 19% 27% 6%	3266
	11th to 20th	Day	20% 3 5% 17% 29% 27%	3220
	21st to end	Day	26% 3 5% 20% 30% 17%	3428
November	1st to 10th	Day	32% 3 8% 19% 28% 10%	1957
	11th to 20th	Day	36% 4 6% 19% 26% 9%	2000
	21st to end	Day	35% 4 8% 20% 24% 9%	1802
December	1st to 10th	Day	34% 4 7% 16% 27% 11%	2930
	11th to 20th	Day	41% 2 10% 22% 19% 5%	3370
	21st to end	Day	42% 3 8% 21% 23% 2	4466
January	1st to 10th	Night	31% 3 5% 16% 37% 7%	5889
	11th to 20th	Night	36% 3 6% 15% 32% 7%	5900
	21st to end	Night	41% 4 6% 13% 27% 10%	6490
February	1st to 10th	Night	41% 3 8% 20% 20% 8%	2600
	11th to 20th	Night	45% 4 7% 16% 20% 8%	2441
	21st to end	Night	37% 5% 9% 16% 22% 11%	1823
March	1st to 10th	Night	48% 4 6% 11% 24% 7%	4918
	11th to 20th	Night	32% 4 8% 11% 27% 19%	4980
	21st to end	Night	35% 4 9% 21% 21% 10%	5467
April	1st to 10th	Night	35% 5% 9% 20% 21% 10%	2138
	11th to 20th	Night	32% 5% 9% 21% 21% 11%	1975
	21st to end	Night	30% 6% 13% 25% 22% 5%	2000
May	1st to 10th	Night	35% 8% 17% 24% 14% 2	4134
	11th to 20th	Night	44% 7% 18% 19% 12%	4033
	21st to end	Night	37% 7% 15% 22% 17% 2	4429
June	1st to 10th	Night	39% 12% 16% 22% 10% 1	1800
	11th to 20th	Night	37% 10% 20% 21% 11% 2	1800
	21st to end	Night	34% 12% 22% 22% 9% 2	1800
July	1st to 10th	Night	52% 8% 22% 14% 3 1	4015
	11th to 20th	Night	45% 12% 32% 8% 3	4006
	21st to end	Night	54% 9% 14% 18% 4	4297
August	1st to 10th	Night	54% 9% 22% 12% 2 1	2904
	11th to 20th	Night	44% 8% 19% 21% 7%	2986
	21st to end	Night	30% 7% 15% 35% 12% 1	3586
September	1st to 10th	Night	37% 9% 17% 18% 14% 5%	2040
	11th to 20th	Night	44% 7% 12% 17% 17% 4	2200
	21st to end	Night	45% 6% 12% 20% 14% 4	2200
October	1st to 10th	Night	36% 5% 8% 19% 26% 6%	3912
	11th to 20th	Night	21% 3 5% 14% 27% 31%	3876
	21st to end	Night	25% 3 6% 23% 26% 17%	4120
November	1st to 10th	Night	33% 3 8% 16% 26% 13%	2341
	11th to 20th	Night	37% 4 7% 15% 26% 12%	2400
	21st to end	Night	35% 4 8% 21% 23% 9%	2595
December	1st to 10th	Night	34% 3 7% 14% 30% 11%	4250
	11th to 20th	Night	44% 3 10% 16% 19% 8%	4824
	21st to end	Night	41% 4 7% 22% 24% 2	6490

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	21% 21% 35% 19% 4	300
	11th to 20th	Day	17% 20% 34% 23% 6%	300
	21st to end	Day	23% 20% 35% 18% 3 1	330
February	1st to 10th	Day	19% 16% 39% 20% 5% 1	300
	11th to 20th	Day	35% 17% 34% 12% 2	300
	21st to end	Day	18% 19% 31% 24% 7% 1	404
March	1st to 10th	Day	21% 17% 31% 23% 8% 1	500
	11th to 20th	Day	20% 14% 40% 20% 5% 1	500
	21st to end	Day	17% 17% 35% 20% 10% 1	550
April	1st to 10th	Day	16% 18% 33% 21% 10% 3	500
	11th to 20th	Day	11% 15% 28% 22% 17% 6%	492
	21st to end	Day	12% 10% 26% 23% 21% 8%	500
May	1st to 10th	Day	11% 14% 30% 22% 16% 7%	503
	11th to 20th	Day	14% 8% 20% 27% 22% 9%	527
	21st to end	Day	16% 6% 16% 24% 23% 15%	573
June	1st to 10th	Day	13% 3 16% 31% 24% 12%	530
	11th to 20th	Day	11% 4 14% 32% 22% 18%	530
	21st to end	Day	9% 2 11% 32% 29% 16%	530
July	1st to 10th	Day	10% 2 7% 28% 34% 19%	525
	11th to 20th	Day	8% 2 6% 26% 39% 19%	524
	21st to end	Day	6% 3 11% 28% 37% 16%	576
August	1st to 10th	Day	17% 3 13% 22% 26% 18%	526
	11th to 20th	Day	13% 3 8% 26% 31% 20%	519
	21st to end	Day	11% 3 12% 23% 31% 20%	583
September	1st to 10th	Day	8% 5% 14% 29% 27% 18%	530
	11th to 20th	Day	15% 5% 14% 26% 29% 11%	530
	21st to end	Day	20% 3 15% 30% 23% 9%	530
October	1st to 10th	Day	17% 5% 20% 29% 19% 10%	530
	11th to 20th	Day	14% 6% 24% 25% 24% 7%	522
	21st to end	Day	11% 6% 24% 28% 24% 6%	539
November	1st to 10th	Day	17% 7% 24% 30% 16% 6%	495
	11th to 20th	Day	17% 11% 34% 25% 12% 2	500
	21st to end	Day	15% 12% 31% 25% 14% 3	302
December	1st to 10th	Day	20% 15% 35% 21% 9%	300
	11th to 20th	Day	20% 12% 34% 23% 10% 2	300
	21st to end	Day	24% 13% 34% 17% 8% 3	330
January	1st to 10th	Night	19% 12% 36% 27% 7%	800
	11th to 20th	Night	17% 11% 32% 31% 9%	800
	21st to end	Night	18% 12% 35% 31% 4	880
February	1st to 10th	Night	20% 12% 30% 32% 6%	800
	11th to 20th	Night	23% 12% 35% 28% 2	783
	21st to end	Night	16% 14% 36% 28% 5%	660
March	1st to 10th	Night	17% 12% 33% 32% 6% 1	800
	11th to 20th	Night	17% 12% 35% 29% 7%	800
	21st to end	Night	15% 11% 37% 30% 7%	880
April	1st to 10th	Night	15% 15% 32% 30% 8%	800
	11th to 20th	Night	12% 11% 30% 33% 13%	791
	21st to end	Night	10% 9% 31% 39% 11%	800
May	1st to 10th	Night	10% 7% 32% 38% 13%	635
	11th to 20th	Night	13% 6% 24% 40% 15% 2	591
	21st to end	Night	11% 2 16% 38% 30% 4	650
June	1st to 10th	Night	13% 3 17% 44% 20% 4	600
	11th to 20th	Night	12% 3 15% 38% 26% 6%	600
	21st to end	Night	8% 3 13% 36% 36% 5%	600
July	1st to 10th	Night	8% 3 9% 37% 36% 7%	585
	11th to 20th	Night	11% 1 7% 35% 39% 7%	582
	21st to end	Night	8% 1 8% 41% 36% 5%	657
August	1st to 10th	Night	15% 2 7% 33% 36% 7%	597
	11th to 20th	Night	12% 1 7% 34% 39% 7%	639
	21st to end	Night	13% 1 8% 32% 38% 7%	880
September	1st to 10th	Night	10% 2 11% 32% 39% 5%	800
	11th to 20th	Night	15% 2 12% 34% 31% 6%	800
	21st to end	Night	20% 4 13% 33% 27% 3	800
October	1st to 10th	Night	15% 3 20% 32% 26% 4	800
	11th to 20th	Night	13% 5% 24% 33% 23% 2	793
	21st to end	Night	12% 6% 22% 36% 21% 3	836
November	1st to 10th	Night	14% 6% 24% 35% 18% 3	780
	11th to 20th	Night	18% 9% 27% 33% 13% 1	800
	21st to end	Night	15% 7% 28% 32% 17% 1	800
December	1st to 10th	Night	20% 11% 27% 33% 9% 1	800
	11th to 20th	Night	18% 8% 27% 34% 12% 1	800
	21st to end	Night	21% 11% 26% 33% 9% 1	880

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