Charleston Lake
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Charleston Lake clear sky chart from 2005-11-06 to 2026-07-01.

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	17% 11% 4 8% 5% 4 4 4 5% 7% 32%	138331
	Night	17% 12% 4 9% 5% 5% 4 3 5% 6% 30%	133991
Transparency	Day	51% 6% 12% 21% 10% 1	141824
	Night	53% 6% 11% 19% 10% 1	138587
Seeing	Day	23% 16% 31% 19% 8% 2	16579
	Night	23% 14% 30% 24% 9% 1	25722

By Month

Percentage Cloud Cover NE (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% 17% 6% 14% 6% 4 4 3 5% 5% 22%	4661
	11th to 20th	Day	13% 20% 9% 16% 6% 5% 4 3 3 4 17%	4650
	21st to end	Day	11% 16% 7% 16% 9% 7% 5% 3 3 4 18%	5500
February	1st to 10th	Day	16% 17% 4 13% 6% 5% 4 3 5% 5% 22%	1890
	11th to 20th	Day	14% 16% 6% 10% 7% 4 4 4 5% 5% 25%	1854
	21st to end	Day	19% 15% 5% 12% 7% 5% 4 3 4 3 22%	1730
March	1st to 10th	Day	20% 13% 4 10% 6% 4 4 3 4 5% 28%	4622
	11th to 20th	Day	11% 13% 5% 9% 6% 5% 5% 3 6% 6% 31%	5090
	21st to end	Day	20% 11% 3 8% 4 4 3 3 3 5% 36%	5864
April	1st to 10th	Day	21% 14% 3 8% 5% 4 3 2 4 5% 31%	2520
	11th to 20th	Day	23% 10% 3 6% 4 4 4 3 3 5% 36%	2494
	21st to end	Day	21% 11% 4 8% 4 4 4 4 5% 5% 32%	2612
May	1st to 10th	Day	24% 10% 3 8% 5% 5% 5% 3 5% 6% 26%	8237
	11th to 20th	Day	17% 9% 3 8% 5% 4 5% 3 5% 7% 35%	8221
	21st to end	Day	20% 9% 2 6% 4 4 4 4 6% 7% 33%	9060
June	1st to 10th	Day	19% 8% 3 6% 5% 5% 5% 3 6% 8% 33%	2940
	11th to 20th	Day	16% 6% 2 6% 5% 5% 4 4 6% 8% 37%	2940
	21st to end	Day	17% 8% 3 7% 4 4 5% 5% 6% 7% 35%	2940
July	1st to 10th	Day	12% 6% 2 7% 5% 5% 5% 5% 7% 10% 37%	6360
	11th to 20th	Day	11% 6% 2 6% 4 5% 5% 4 6% 9% 42%	6360
	21st to end	Day	11% 5% 2 5% 4 4 5% 4 7% 9% 44%	6926
August	1st to 10th	Day	15% 7% 3 7% 4 4 5% 4 5% 7% 38%	4405
	11th to 20th	Day	17% 6% 2 6% 5% 4 5% 4 6% 10% 36%	4111
	21st to end	Day	16% 6% 2 6% 4 4 4 4 6% 10% 39%	4312
September	1st to 10th	Day	13% 6% 2 6% 4 4 4 4 7% 7% 43%	2400
	11th to 20th	Day	11% 6% 2 6% 4 4 4 3 5% 7% 48%	2400
	21st to end	Day	17% 7% 3 6% 4 4 5% 4 5% 6% 39%	2130
October	1st to 10th	Day	20% 8% 3 7% 3 4 3 4 4 7% 37%	2000
	11th to 20th	Day	20% 9% 3 8% 5% 5% 4 4 5% 7% 30%	2000
	21st to end	Day	25% 10% 3 7% 5% 4 5% 4 4 5% 27%	2070
November	1st to 10th	Day	16% 10% 3 8% 5% 5% 4 4 6% 7% 31%	1851
	11th to 20th	Day	15% 14% 4 10% 5% 5% 4 3 4 5% 31%	1707
	21st to end	Day	22% 14% 4 9% 6% 4 4 3 5% 6% 22%	1680
December	1st to 10th	Day	21% 18% 5% 11% 6% 5% 5% 3 5% 5% 17%	2680
	11th to 20th	Day	19% 19% 6% 11% 4 4 4 2 4 4 22%	3066
	21st to end	Day	22% 17% 5% 12% 5% 4 4 3 3 4 23%	4048
January	1st to 10th	Night	16% 18% 7% 13% 6% 5% 4 3 4 4 21%	8234
	11th to 20th	Night	14% 19% 7% 15% 7% 6% 5% 3 4 4 16%	8235
	21st to end	Night	15% 14% 7% 11% 7% 7% 5% 3 5% 5% 20%	8668
February	1st to 10th	Night	17% 14% 5% 11% 7% 5% 5% 4 5% 4 22%	2730
	11th to 20th	Night	15% 14% 5% 11% 6% 4 4 3 5% 6% 27%	2652
	21st to end	Night	17% 15% 5% 10% 7% 5% 4 3 4 5% 25%	2076
March	1st to 10th	Night	21% 13% 3 9% 5% 5% 3 3 4 5% 29%	5423
	11th to 20th	Night	15% 14% 4 7% 5% 4 4 4 6% 8% 31%	5089
	21st to end	Night	21% 12% 3 9% 5% 4 3 3 4 5% 32%	5309
April	1st to 10th	Night	21% 11% 3 7% 4 3 4 3 4 6% 32%	2100
	11th to 20th	Night	22% 9% 4 6% 4 4 3 3 4 6% 35%	2075
	21st to end	Night	22% 9% 3 7% 5% 4 3 3 4 6% 34%	2023
May	1st to 10th	Night	24% 11% 3 8% 4 5% 4 4 6% 7% 24%	4660
	11th to 20th	Night	16% 10% 3 7% 5% 5% 4 3 5% 7% 35%	4639
	21st to end	Night	19% 7% 2 6% 4 4 5% 4 6% 8% 35%	5114
June	1st to 10th	Night	16% 8% 3 7% 6% 5% 4 4 6% 8% 33%	1680
	11th to 20th	Night	14% 5% 2 6% 4 5% 4 4 6% 8% 41%	1680
	21st to end	Night	17% 7% 3 6% 4 5% 5% 4 5% 8% 35%	1680
July	1st to 10th	Night	13% 6% 2 7% 5% 5% 5% 5% 6% 9% 37%	3600
	11th to 20th	Night	13% 6% 3 6% 5% 4 4 3 5% 7% 44%	3600
	21st to end	Night	9% 5% 2 6% 4 4 5% 4 6% 8% 48%	3926
August	1st to 10th	Night	11% 8% 3 7% 4 4 4 4 6% 8% 41%	2740
	11th to 20th	Night	14% 7% 2 6% 4 4 3 4 6% 7% 42%	2903
	21st to end	Night	10% 5% 2 7% 5% 5% 6% 5% 7% 9% 39%	3586
September	1st to 10th	Night	12% 6% 2 7% 4 4 4 3 5% 7% 46%	2000
	11th to 20th	Night	13% 5% 2 4 3 4 3 4 6% 7% 50%	2000
	21st to end	Night	16% 6% 2 7% 4 4 4 4 5% 7% 41%	2265
October	1st to 10th	Night	18% 7% 3 6% 4 4 4 4 5% 7% 37%	2400
	11th to 20th	Night	20% 8% 3 8% 5% 5% 5% 3 5% 6% 33%	2400
	21st to end	Night	24% 9% 3 8% 5% 4 4 4 4 5% 30%	2770
November	1st to 10th	Night	19% 11% 3 7% 4 4 4 3 5% 6% 35%	2667
	11th to 20th	Night	17% 11% 4 8% 4 5% 4 4 5% 6% 33%	2909
	21st to end	Night	21% 14% 3 9% 5% 5% 5% 4 4 6% 23%	2940
December	1st to 10th	Night	19% 17% 5% 12% 6% 5% 5% 3 5% 6% 16%	4720
	11th to 20th	Night	23% 15% 5% 9% 5% 3 3 3 4 4 27%	5348
	21st to end	Night	28% 14% 4 12% 4 4 4 3 5% 4 19%	7150

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	63% 7% 10% 15% 4 1	4662
	11th to 20th	Day	71% 5% 8% 8% 7% 1	4680
	21st to end	Day	68% 6% 8% 9% 9% 1	5500
February	1st to 10th	Day	64% 4 9% 16% 5% 1	1890
	11th to 20th	Day	61% 5% 10% 16% 8% 1	1863
	21st to end	Day	67% 4 8% 12% 8% 1	1730
March	1st to 10th	Day	58% 5% 9% 14% 11% 2	4625
	11th to 20th	Day	52% 6% 17% 13% 11% 1	5090
	21st to end	Day	51% 4 6% 20% 16% 3	5864
April	1st to 10th	Day	56% 4 8% 18% 13% 2	2520
	11th to 20th	Day	52% 5% 7% 19% 16% 2	2494
	21st to end	Day	54% 4 9% 16% 14% 3	2612
May	1st to 10th	Day	56% 7% 9% 18% 9% 1	8249
	11th to 20th	Day	48% 7% 12% 22% 11% 1	8225
	21st to end	Day	48% 6% 11% 24% 9% 1	9061
June	1st to 10th	Day	49% 7% 10% 21% 12% 1	2940
	11th to 20th	Day	43% 7% 11% 26% 11% 2	2940
	21st to end	Day	47% 7% 14% 21% 11% 1	2940
July	1st to 10th	Day	40% 7% 14% 27% 11%	6290
	11th to 20th	Day	36% 7% 15% 27% 14%	6285
	21st to end	Day	34% 8% 14% 35% 9%	6728
August	1st to 10th	Day	44% 6% 15% 24% 10% 1	4391
	11th to 20th	Day	42% 7% 15% 26% 9% 1	4150
	21st to end	Day	39% 6% 16% 27% 11%	4312
September	1st to 10th	Day	37% 7% 16% 28% 12% 1	2400
	11th to 20th	Day	35% 6% 13% 32% 15%	2400
	21st to end	Day	45% 7% 13% 25% 11%	2130
October	1st to 10th	Day	43% 6% 13% 28% 9%	3260
	11th to 20th	Day	47% 6% 15% 24% 7% 1	3240
	21st to end	Day	48% 6% 14% 24% 7%	3321
November	1st to 10th	Day	49% 8% 13% 21% 8% 1	1851
	11th to 20th	Day	55% 7% 11% 17% 9% 1	1707
	21st to end	Day	63% 8% 12% 11% 7%	1680
December	1st to 10th	Day	69% 6% 13% 10% 2	2680
	11th to 20th	Day	66% 6% 10% 13% 5%	3066
	21st to end	Day	66% 5% 8% 16% 5%	4048
January	1st to 10th	Night	66% 6% 12% 12% 5%	8234
	11th to 20th	Night	70% 7% 7% 10% 6%	8280
	21st to end	Night	64% 6% 7% 12% 10% 1	8668
February	1st to 10th	Night	65% 5% 9% 14% 7% 1	2730
	11th to 20th	Night	58% 5% 11% 19% 7% 1	2654
	21st to end	Night	61% 5% 9% 16% 9% 1	2076
March	1st to 10th	Night	58% 4 8% 17% 10% 3	5433
	11th to 20th	Night	50% 7% 17% 16% 8% 3	5090
	21st to end	Night	54% 4 7% 21% 13% 2	5309
April	1st to 10th	Night	52% 5% 9% 19% 14% 1	2100
	11th to 20th	Night	51% 5% 9% 18% 16% 1	2075
	21st to end	Night	52% 4 9% 17% 15% 3	2023
May	1st to 10th	Night	57% 7% 10% 15% 9% 2	4666
	11th to 20th	Night	47% 7% 14% 20% 12% 1	4640
	21st to end	Night	45% 7% 12% 21% 15% 1	5115
June	1st to 10th	Night	47% 7% 13% 19% 13% 1	1680
	11th to 20th	Night	40% 8% 13% 22% 16% 2	1680
	21st to end	Night	46% 8% 12% 21% 13% 1	1680
July	1st to 10th	Night	41% 7% 16% 21% 15% 1	3560
	11th to 20th	Night	38% 6% 13% 31% 11% 1	3552
	21st to end	Night	34% 8% 13% 31% 13% 1	3809
August	1st to 10th	Night	39% 8% 15% 25% 12% 1	2730
	11th to 20th	Night	40% 7% 14% 29% 9% 1	2930
	21st to end	Night	36% 9% 17% 27% 11% 1	3586
September	1st to 10th	Night	37% 7% 13% 29% 14% 1	2000
	11th to 20th	Night	32% 7% 14% 32% 14% 1	2000
	21st to end	Night	42% 6% 12% 27% 13% 1	2265
October	1st to 10th	Night	40% 6% 12% 27% 13% 1	3920
	11th to 20th	Night	43% 6% 15% 26% 9%	3899
	21st to end	Night	48% 6% 15% 23% 8%	4469
November	1st to 10th	Night	49% 6% 13% 22% 9% 1	2667
	11th to 20th	Night	50% 6% 13% 19% 10% 1	2909
	21st to end	Night	61% 7% 11% 15% 5%	2940
December	1st to 10th	Night	68% 7% 11% 11% 3	4720
	11th to 20th	Night	62% 6% 9% 14% 9%	5348
	21st to end	Night	67% 5% 7% 11% 8%	7150

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	24% 21% 38% 11% 5%	220
	11th to 20th	Day	35% 20% 30% 13% 1	220
	21st to end	Day	24% 22% 34% 16% 4	311
February	1st to 10th	Day	29% 24% 34% 12% 2	330
	11th to 20th	Day	31% 24% 35% 9% 1	341
	21st to end	Day	33% 19% 36% 11% 2	446
March	1st to 10th	Day	26% 23% 30% 18% 4	540
	11th to 20th	Day	29% 20% 32% 14% 4	540
	21st to end	Day	35% 21% 25% 13% 6% 1	594
April	1st to 10th	Day	32% 19% 30% 14% 5%	540
	11th to 20th	Day	32% 21% 25% 13% 6% 3	532
	21st to end	Day	29% 19% 32% 12% 6% 2	540
May	1st to 10th	Day	24% 19% 29% 21% 6% 1	543
	11th to 20th	Day	18% 17% 37% 19% 8% 2	567
	21st to end	Day	20% 16% 31% 22% 10% 2	617
June	1st to 10th	Day	22% 15% 32% 21% 8% 1	570
	11th to 20th	Day	17% 14% 37% 21% 9% 2	570
	21st to end	Day	19% 14% 33% 22% 9% 3	570
July	1st to 10th	Day	14% 10% 29% 30% 15% 2	525
	11th to 20th	Day	14% 12% 30% 27% 15% 3	524
	21st to end	Day	13% 10% 34% 30% 11% 3	576
August	1st to 10th	Day	14% 7% 30% 29% 15% 4	526
	11th to 20th	Day	14% 6% 27% 29% 17% 6%	519
	21st to end	Day	14% 10% 34% 22% 13% 6%	583
September	1st to 10th	Day	15% 12% 31% 24% 11% 7%	530
	11th to 20th	Day	11% 8% 25% 26% 19% 11%	530
	21st to end	Day	19% 12% 29% 23% 15% 3	530
October	1st to 10th	Day	25% 14% 26% 18% 11% 6%	530
	11th to 20th	Day	25% 21% 29% 18% 5% 2	530
	21st to end	Day	32% 17% 27% 18% 5%	511
November	1st to 10th	Day	28% 20% 30% 16% 6%	425
	11th to 20th	Day	26% 18% 36% 12% 6% 1	247
	21st to end	Day	34% 19% 34% 11% 2	220
December	1st to 10th	Day	33% 25% 24% 14% 5%	220
	11th to 20th	Day	32% 23% 35% 6% 4	220
	21st to end	Day	36% 19% 27% 14% 4	242
January	1st to 10th	Night	30% 18% 31% 19% 3	840
	11th to 20th	Night	29% 18% 30% 19% 4	840
	21st to end	Night	22% 16% 34% 22% 6%	924
February	1st to 10th	Night	25% 19% 33% 20% 3	840
	11th to 20th	Night	23% 19% 37% 17% 4	823
	21st to end	Night	27% 19% 31% 20% 4	692
March	1st to 10th	Night	28% 14% 34% 21% 3	840
	11th to 20th	Night	27% 16% 31% 21% 4	840
	21st to end	Night	26% 15% 32% 19% 7%	924
April	1st to 10th	Night	27% 16% 33% 21% 3	840
	11th to 20th	Night	25% 19% 31% 18% 7%	831
	21st to end	Night	27% 11% 31% 23% 8% 1	763
May	1st to 10th	Night	26% 10% 29% 26% 9% 1	420
	11th to 20th	Night	15% 10% 38% 30% 7%	414
	21st to end	Night	14% 9% 38% 28% 11% 1	455
June	1st to 10th	Night	18% 8% 30% 34% 9% 1	420
	11th to 20th	Night	15% 12% 30% 32% 11% 1	420
	21st to end	Night	18% 8% 26% 35% 13%	420
July	1st to 10th	Night	11% 5% 31% 38% 16%	390
	11th to 20th	Night	13% 4 27% 37% 18% 1	388
	21st to end	Night	11% 5% 29% 42% 12% 1	438
August	1st to 10th	Night	13% 4 25% 37% 22%	487
	11th to 20th	Night	13% 6% 20% 34% 24% 3	609
	21st to end	Night	12% 7% 28% 36% 15% 2	880
September	1st to 10th	Night	14% 8% 29% 31% 16% 2	800
	11th to 20th	Night	15% 7% 21% 29% 24% 5%	800
	21st to end	Night	18% 10% 25% 27% 18% 2	800
October	1st to 10th	Night	22% 12% 27% 20% 17% 1	800
	11th to 20th	Night	23% 14% 30% 25% 8% 1	800
	21st to end	Night	29% 14% 28% 24% 5%	880
November	1st to 10th	Night	27% 15% 29% 21% 7%	820
	11th to 20th	Night	24% 18% 31% 18% 9%	840
	21st to end	Night	33% 18% 27% 16% 6%	840
December	1st to 10th	Night	32% 18% 29% 18% 4	840
	11th to 20th	Night	27% 21% 32% 17% 3 1	840
	21st to end	Night	34% 16% 30% 17% 2	924

Page generated at 2026-06-28 02:10:44. Version ac07

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