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 2024-11-05.

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%	101136
	Night	13% 6% 2 6% 4 4 4 4 6% 8% 43%	97863
Transparency	Day	39% 6% 11% 19% 18% 6%	104785
	Night	39% 6% 11% 18% 19% 7%	102352
Seeing	Day	15% 9% 22% 25% 20% 9%	15531
	Night	15% 7% 23% 33% 19% 2	25618

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	15% 6% 2 5% 4 3 3 3 4 7% 48%	2836
	11th to 20th	Day	12% 5% 2 5% 3 3 4 3 4 6% 52%	2840
	21st to end	Day	15% 9% 3 7% 4 4 5% 3 4 4 44%	3124
February	1st to 10th	Day	16% 8% 2 6% 5% 3 4 4 6% 6% 41%	1710
	11th to 20th	Day	19% 6% 2 6% 3 3 3 3 5% 4 45%	1750
	21st to end	Day	18% 7% 2 6% 4 4 4 3 5% 7% 41%	1717
March	1st to 10th	Day	20% 10% 3 7% 4 3 3 3 4 6% 36%	3435
	11th to 20th	Day	9% 5% 2 4 4 3 4 3 5% 7% 53%	3480
	21st to end	Day	17% 6% 2 6% 4 4 4 3 5% 6% 44%	3815
April	1st to 10th	Day	9% 4 2 4 4 4 4 4 6% 7% 51%	2147
	11th to 20th	Day	9% 6% 2 6% 4 4 4 4 5% 7% 49%	2254
	21st to end	Day	10% 4 2 5% 4 4 5% 4 6% 7% 52%	2280
May	1st to 10th	Day	8% 6% 3 8% 5% 5% 5% 5% 7% 10% 38%	4051
	11th to 20th	Day	16% 8% 3 7% 4 4 4 4 6% 8% 37%	4081
	21st to end	Day	6% 4 2 6% 4 5% 5% 4 7% 10% 47%	4476
June	1st to 10th	Day	9% 7% 3 7% 5% 6% 6% 5% 8% 10% 36%	2470
	11th to 20th	Day	8% 5% 2 4 5% 6% 6% 5% 8% 12% 39%	2470
	21st to end	Day	7% 5% 2 6% 5% 6% 7% 6% 8% 11% 36%	2470
July	1st to 10th	Day	10% 6% 3 8% 7% 6% 7% 6% 8% 12% 28%	4120
	11th to 20th	Day	8% 6% 3 8% 6% 7% 7% 6% 9% 13% 28%	4120
	21st to end	Day	10% 7% 3 9% 6% 6% 7% 6% 8% 12% 25%	4466
August	1st to 10th	Day	22% 8% 3 8% 5% 6% 6% 6% 7% 8% 20%	4104
	11th to 20th	Day	10% 6% 3 8% 5% 4 5% 5% 7% 12% 34%	3930
	21st to end	Day	6% 4 2 7% 5% 4 6% 4 8% 11% 42%	4180
September	1st to 10th	Day	8% 6% 3 7% 6% 5% 5% 4 7% 9% 40%	2237
	11th to 20th	Day	13% 8% 3 7% 5% 5% 5% 3 6% 6% 40%	2090
	21st to end	Day	16% 7% 3 6% 5% 4 5% 3 6% 7% 38%	2090
October	1st to 10th	Day	15% 6% 2 5% 3 3 4 2 5% 7% 48%	1906
	11th to 20th	Day	10% 5% 2 6% 2 3 3 3 4 6% 57%	1900
	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% 56%	1898
	11th to 20th	Day	13% 6% 2 5% 3 3 4 2 3 7% 51%	1900
	21st to end	Day	14% 5% 2 5% 3 4 4 3 4 7% 50%	1712
December	1st to 10th	Day	16% 8% 3 7% 5% 4 4 4 5% 7% 38%	1710
	11th to 20th	Day	16% 4 2 4 4 3 2 2 3 6% 54%	2153
	21st to end	Day	16% 8% 3 6% 4 4 3 3 4 5% 43%	3124
January	1st to 10th	Night	15% 6% 2 5% 4 3 3 3 5% 6% 47%	4109
	11th to 20th	Night	11% 7% 3 7% 5% 4 5% 4 6% 6% 43%	4120
	21st to end	Night	15% 8% 3 6% 4 4 4 3 4 5% 44%	4532
February	1st to 10th	Night	16% 6% 3 5% 4 4 4 3 5% 7% 43%	2470
	11th to 20th	Night	17% 7% 2 6% 4 3 4 3 5% 6% 42%	2314
	21st to end	Night	14% 6% 3 6% 4 4 4 3 6% 7% 45%	1735
March	1st to 10th	Night	23% 10% 4 7% 4 4 3 3 4 6% 32%	3418
	11th to 20th	Night	12% 5% 2 6% 3 4 4 3 6% 6% 48%	3480
	21st to end	Night	16% 5% 2 5% 4 3 4 3 5% 7% 47%	3817
April	1st to 10th	Night	11% 6% 3 5% 4 3 3 4 6% 6% 49%	2031
	11th to 20th	Night	10% 5% 2 6% 4 4 4 4 5% 6% 51%	1875
	21st to end	Night	9% 4 2 5% 4 4 4 4 5% 7% 52%	1900
May	1st to 10th	Night	6% 6% 2 6% 5% 5% 5% 5% 8% 10% 43%	2859
	11th to 20th	Night	13% 8% 3 7% 4 4 4 4 5% 9% 39%	2812
	21st to end	Night	6% 5% 2 7% 4 5% 5% 5% 7% 9% 45%	3079
June	1st to 10th	Night	10% 7% 3 5% 6% 5% 5% 6% 8% 10% 35%	1710
	11th to 20th	Night	10% 5% 2 7% 6% 5% 4 5% 8% 11% 39%	1710
	21st to end	Night	7% 5% 2 6% 5% 5% 6% 6% 9% 12% 36%	1710
July	1st to 10th	Night	10% 7% 4 9% 7% 6% 6% 5% 9% 10% 28%	2840
	11th to 20th	Night	12% 7% 2 8% 6% 5% 6% 6% 8% 12% 28%	2840
	21st to end	Night	12% 9% 3 11% 7% 6% 6% 5% 8% 9% 24%	3087
August	1st to 10th	Night	22% 7% 2 7% 5% 5% 5% 5% 7% 10% 24%	2825
	11th to 20th	Night	9% 7% 3 9% 6% 6% 5% 4 7% 10% 34%	2867
	21st to end	Night	6% 4 2 5% 4 4 5% 5% 8% 11% 47%	3476
September	1st to 10th	Night	9% 5% 2 6% 5% 5% 7% 5% 7% 10% 40%	1938
	11th to 20th	Night	13% 7% 3 8% 6% 5% 5% 4 6% 6% 39%	2090
	21st to end	Night	16% 7% 3 7% 5% 4 4 3 5% 6% 41%	2090
October	1st to 10th	Night	13% 6% 2 5% 3 4 3 3 5% 6% 50%	2272
	11th to 20th	Night	10% 5% 2 5% 3 3 3 3 4 6% 57%	2280
	21st to end	Night	9% 6% 2 4 3 3 3 3 4 6% 57%	2508
November	1st to 10th	Night	11% 5% 2 5% 4 3 3 2 3 5% 56%	2280
	11th to 20th	Night	15% 7% 2 4 3 3 3 2 4 6% 51%	2280
	21st to end	Night	13% 6% 1 4 3 4 4 3 5% 7% 49%	2465
December	1st to 10th	Night	17% 8% 2 6% 3 4 3 4 5% 6% 43%	2470
	11th to 20th	Night	17% 5% 2 5% 3 3 3 3 4 5% 51%	3042
	21st to end	Night	17% 7% 3 5% 4 3 3 3 5% 7% 43%	4532

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	37% 3 7% 22% 23% 7%	2837
	11th to 20th	Day	34% 3 6% 20% 27% 9%	2840
	21st to end	Day	45% 4 5% 15% 22% 10%	3124
February	1st to 10th	Day	43% 4 9% 18% 21% 4	1710
	11th to 20th	Day	43% 3 6% 18% 24% 6%	1759
	21st to end	Day	43% 5% 8% 16% 19% 9%	1717
March	1st to 10th	Day	51% 4 7% 15% 15% 8%	3435
	11th to 20th	Day	29% 5% 9% 18% 28% 11%	3480
	21st to end	Day	41% 6% 7% 19% 21% 6%	3815
April	1st to 10th	Day	30% 6% 11% 19% 27% 7%	2147
	11th to 20th	Day	33% 5% 11% 20% 21% 11%	2254
	21st to end	Day	30% 6% 12% 24% 23% 5%	2280
May	1st to 10th	Day	39% 8% 16% 23% 13% 2	4088
	11th to 20th	Day	45% 6% 10% 22% 15% 1	4085
	21st to end	Day	29% 9% 17% 20% 21% 3	4489
June	1st to 10th	Day	40% 10% 14% 23% 11% 1	2470
	11th to 20th	Day	35% 11% 17% 23% 13% 2	2470
	21st to end	Day	38% 10% 17% 21% 12% 1	2470
July	1st to 10th	Day	47% 9% 17% 19% 6% 2	4055
	11th to 20th	Day	45% 11% 26% 13% 4 1	4051
	21st to end	Day	50% 12% 17% 15% 6% 1	4324
August	1st to 10th	Day	60% 8% 17% 11% 4	4091
	11th to 20th	Day	41% 8% 16% 22% 12%	3969
	21st to end	Day	34% 8% 13% 32% 12% 1	4180
September	1st to 10th	Day	38% 11% 14% 15% 15% 6%	2237
	11th to 20th	Day	44% 6% 9% 16% 20% 5%	2090
	21st to end	Day	43% 6% 10% 20% 17% 5%	2090
October	1st to 10th	Day	37% 4 6% 19% 27% 6%	3166
	11th to 20th	Day	19% 3 5% 17% 28% 27%	3140
	21st to end	Day	26% 3 5% 20% 30% 17%	3428
November	1st to 10th	Day	31% 3 8% 19% 28% 10%	1898
	11th to 20th	Day	36% 4 7% 18% 26% 10%	1900
	21st to end	Day	36% 4 8% 20% 23% 9%	1712
December	1st to 10th	Day	45% 5% 10% 18% 17% 5%	1710
	11th to 20th	Day	35% 3 5% 24% 25% 8%	2150
	21st to end	Day	43% 4 6% 20% 23% 3	3124
January	1st to 10th	Night	38% 4 7% 19% 24% 8%	4109
	11th to 20th	Night	40% 4 8% 18% 24% 6%	4120
	21st to end	Night	42% 5% 7% 13% 25% 8%	4532
February	1st to 10th	Night	42% 3 8% 18% 20% 8%	2470
	11th to 20th	Night	44% 4 7% 16% 21% 9%	2316
	21st to end	Night	38% 5% 10% 17% 20% 10%	1735
March	1st to 10th	Night	54% 4 9% 12% 14% 7%	3418
	11th to 20th	Night	35% 5% 9% 13% 23% 16%	3480
	21st to end	Night	38% 5% 9% 22% 20% 7%	3817
April	1st to 10th	Night	34% 5% 10% 19% 22% 10%	2031
	11th to 20th	Night	33% 6% 10% 20% 21% 11%	1875
	21st to end	Night	29% 7% 12% 24% 23% 5%	1900
May	1st to 10th	Night	33% 10% 16% 23% 15% 2	2886
	11th to 20th	Night	41% 9% 12% 21% 16% 1	2813
	21st to end	Night	30% 10% 17% 25% 17% 2	3087
June	1st to 10th	Night	39% 12% 15% 22% 10% 2	1710
	11th to 20th	Night	37% 9% 18% 22% 11% 2	1710
	21st to end	Night	35% 12% 21% 21% 9% 2	1710
July	1st to 10th	Night	49% 10% 18% 17% 5% 2	2795
	11th to 20th	Night	46% 12% 27% 10% 4 1	2786
	21st to end	Night	55% 10% 14% 16% 6%	2988
August	1st to 10th	Night	53% 9% 22% 13% 3 1	2814
	11th to 20th	Night	44% 8% 19% 21% 7%	2894
	21st to end	Night	30% 7% 15% 35% 12% 1	3476
September	1st to 10th	Night	37% 9% 18% 17% 14% 5%	1938
	11th to 20th	Night	45% 7% 12% 15% 17% 4	2090
	21st to end	Night	45% 6% 11% 20% 15% 4	2090
October	1st to 10th	Night	36% 5% 7% 19% 26% 6%	3792
	11th to 20th	Night	20% 3 5% 14% 27% 31%	3779
	21st to end	Night	25% 3 6% 23% 26% 17%	4120
November	1st to 10th	Night	33% 3 8% 16% 26% 14%	2280
	11th to 20th	Night	37% 4 7% 15% 25% 12%	2280
	21st to end	Night	35% 4 8% 21% 22% 10%	2465
December	1st to 10th	Night	43% 5% 11% 15% 20% 7%	2470
	11th to 20th	Night	38% 4 7% 18% 26% 8%	3044
	21st to end	Night	41% 4 7% 19% 25% 3	4532

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% 19% 36% 21% 4	270
	11th to 20th	Day	16% 19% 34% 24% 7%	270
	21st to end	Day	23% 19% 35% 19% 4 1	297
February	1st to 10th	Day	20% 16% 37% 21% 5% 1	270
	11th to 20th	Day	32% 17% 35% 14% 3	270
	21st to end	Day	19% 20% 31% 23% 7% 1	372
March	1st to 10th	Day	18% 17% 32% 23% 9% 1	460
	11th to 20th	Day	20% 14% 40% 20% 6% 1	460
	21st to end	Day	17% 17% 34% 20% 11% 1	506
April	1st to 10th	Day	16% 18% 32% 21% 10% 3	460
	11th to 20th	Day	11% 15% 26% 22% 18% 7%	452
	21st to end	Day	13% 10% 26% 22% 21% 8%	460
May	1st to 10th	Day	11% 14% 29% 21% 17% 8%	463
	11th to 20th	Day	13% 7% 19% 28% 24% 9%	487
	21st to end	Day	14% 5% 15% 25% 25% 16%	529
June	1st to 10th	Day	13% 3 14% 32% 26% 13%	490
	11th to 20th	Day	11% 3 14% 30% 22% 19%	490
	21st to end	Day	10% 3 12% 31% 29% 16%	490
July	1st to 10th	Day	10% 2 6% 27% 35% 20%	485
	11th to 20th	Day	8% 3 7% 26% 38% 18%	484
	21st to end	Day	6% 2 12% 29% 36% 15%	532
August	1st to 10th	Day	14% 3 12% 23% 28% 19%	486
	11th to 20th	Day	12% 3 8% 26% 31% 20%	479
	21st to end	Day	11% 3 12% 21% 32% 21%	539
September	1st to 10th	Day	8% 4 13% 28% 28% 19%	490
	11th to 20th	Day	17% 4 12% 26% 30% 11%	490
	21st to end	Day	20% 3 14% 30% 23% 9%	490
October	1st to 10th	Day	17% 5% 20% 28% 20% 11%	490
	11th to 20th	Day	13% 7% 23% 25% 25% 7%	490
	21st to end	Day	11% 6% 24% 28% 24% 6%	539
November	1st to 10th	Day	17% 7% 24% 30% 16% 6%	472
	11th to 20th	Day	16% 9% 36% 26% 12% 2	460
	21st to end	Day	13% 12% 31% 27% 14% 3	272
December	1st to 10th	Day	21% 13% 34% 23% 10%	270
	11th to 20th	Day	20% 13% 32% 22% 11% 1	270
	21st to end	Day	24% 13% 33% 18% 9% 3	297
January	1st to 10th	Night	19% 12% 35% 27% 7%	760
	11th to 20th	Night	17% 11% 31% 32% 9%	760
	21st to end	Night	18% 12% 35% 31% 4	836
February	1st to 10th	Night	21% 12% 30% 32% 6%	760
	11th to 20th	Night	22% 12% 35% 29% 2	743
	21st to end	Night	17% 14% 36% 28% 5%	628
March	1st to 10th	Night	17% 10% 34% 32% 7% 1	760
	11th to 20th	Night	18% 11% 34% 30% 7%	760
	21st to end	Night	15% 11% 35% 31% 8%	836
April	1st to 10th	Night	15% 14% 32% 31% 8%	760
	11th to 20th	Night	13% 11% 29% 32% 14%	751
	21st to end	Night	11% 9% 31% 38% 11%	760
May	1st to 10th	Night	10% 7% 32% 38% 13%	603
	11th to 20th	Night	12% 6% 24% 40% 16% 2	561
	21st to end	Night	10% 2 15% 38% 31% 4	617
June	1st to 10th	Night	12% 2 16% 45% 21% 4	570
	11th to 20th	Night	12% 4 14% 36% 27% 6%	570
	21st to end	Night	8% 3 13% 35% 36% 5%	570
July	1st to 10th	Night	8% 3 9% 37% 36% 7%	555
	11th to 20th	Night	11% 1 8% 34% 40% 6%	552
	21st to end	Night	8% 1 8% 42% 35% 5%	624
August	1st to 10th	Night	13% 2 7% 32% 37% 8%	567
	11th to 20th	Night	12% 1 8% 34% 40% 6%	607
	21st to end	Night	13% 1 8% 31% 40% 7%	836
September	1st to 10th	Night	10% 2 9% 32% 41% 6%	760
	11th to 20th	Night	16% 2 12% 33% 31% 6%	760
	21st to end	Night	20% 4 13% 32% 28% 3	760
October	1st to 10th	Night	15% 3 19% 31% 27% 4	760
	11th to 20th	Night	13% 5% 24% 33% 23% 2	760
	21st to end	Night	12% 6% 22% 36% 21% 3	836
November	1st to 10th	Night	14% 6% 24% 34% 19% 3	760
	11th to 20th	Night	18% 9% 26% 33% 13% 1	760
	21st to end	Night	16% 6% 27% 32% 17% 1	760
December	1st to 10th	Night	20% 10% 27% 33% 9% 1	760
	11th to 20th	Night	18% 8% 27% 34% 12% 1	760
	21st to end	Night	20% 12% 26% 32% 9% 1	836

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