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 2024-01-31.

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	18% 9% 4 7% 6% 3 5% 4 5% 6% 33%	74880
	Night	18% 10% 4 7% 6% 3 4 4 5% 6% 32%	75244
Transparency	Day	51% 6% 12% 20% 10% 1	74585
	Night	53% 6% 11% 19% 10% 1	75073
Seeing	Day	23% 16% 30% 20% 9% 2	13457
	Night	23% 13% 30% 24% 9% 1	22747

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	19% 16% 5% 9% 8% 3 5% 4 4 4 22%	1517
	11th to 20th	Day	20% 16% 7% 8% 6% 3 5% 5% 4 4 22%	1520
	21st to end	Day	14% 16% 6% 10% 9% 5% 5% 5% 4 5% 21%	1781
February	1st to 10th	Day	17% 16% 5% 12% 6% 4 5% 4 5% 5% 20%	1620
	11th to 20th	Day	15% 15% 6% 9% 7% 4 5% 4 5% 5% 26%	1582
	21st to end	Day	21% 14% 5% 10% 8% 4 4 4 4 3 23%	1480
March	1st to 10th	Day	16% 11% 6% 10% 9% 4 4 4 4 5% 28%	1814
	11th to 20th	Day	18% 12% 5% 7% 8% 4 4 3 4 5% 29%	1980
	21st to end	Day	22% 10% 3 6% 5% 3 3 3 3 4 38%	2283
April	1st to 10th	Day	21% 11% 4 7% 5% 3 4 3 4 5% 33%	2160
	11th to 20th	Day	22% 8% 3 5% 5% 2 4 4 3 5% 38%	2134
	21st to end	Day	21% 10% 4 7% 5% 3 4 5% 5% 5% 31%	2237
May	1st to 10th	Day	19% 8% 3 6% 6% 3 5% 4 5% 6% 33%	2520
	11th to 20th	Day	17% 7% 3 5% 5% 3 5% 4 6% 7% 39%	2478
	21st to end	Day	15% 6% 2 5% 5% 3 5% 5% 6% 8% 39%	2725
June	1st to 10th	Day	20% 7% 3 5% 6% 4 5% 5% 6% 8% 32%	2520
	11th to 20th	Day	17% 6% 2 5% 5% 3 5% 5% 6% 8% 39%	2520
	21st to end	Day	16% 8% 3 6% 5% 3 5% 6% 6% 8% 35%	2520
July	1st to 10th	Day	12% 6% 2 5% 5% 2 4 5% 7% 8% 43%	2520
	11th to 20th	Day	13% 5% 2 5% 5% 4 5% 6% 6% 9% 39%	2520
	21st to end	Day	11% 4 3 5% 5% 3 5% 6% 8% 10% 39%	2744
August	1st to 10th	Day	11% 6% 3 7% 5% 3 6% 5% 6% 7% 41%	2423
	11th to 20th	Day	11% 5% 2 5% 6% 4 6% 5% 6% 11% 38%	2270
	21st to end	Day	13% 6% 2 6% 5% 3 5% 5% 5% 10% 40%	2376
September	1st to 10th	Day	14% 5% 2 5% 6% 3 4 5% 7% 7% 42%	2160
	11th to 20th	Day	13% 6% 3 5% 4 3 3 4 5% 7% 47%	2160
	21st to end	Day	17% 7% 3 5% 5% 3 5% 5% 5% 6% 39%	1918
October	1st to 10th	Day	22% 8% 3 6% 4 3 3 5% 4 7% 36%	1800
	11th to 20th	Day	22% 9% 4 7% 6% 4 5% 4 5% 7% 29%	1800
	21st to end	Day	27% 10% 4 6% 7% 3 5% 5% 4 5% 26%	1864
November	1st to 10th	Day	15% 9% 3 7% 7% 5% 4 6% 6% 8% 32%	1671
	11th to 20th	Day	17% 11% 4 9% 6% 4 4 4 4 5% 31%	1545
	21st to end	Day	22% 13% 4 8% 7% 3 5% 4 5% 6% 21%	1520
December	1st to 10th	Day	25% 16% 6% 9% 6% 3 4 4 5% 6% 18%	1520
	11th to 20th	Day	22% 15% 6% 10% 6% 3 4 3 4 5% 22%	1906
	21st to end	Day	27% 16% 6% 9% 6% 3 4 3 3 3 19%	2772
January	1st to 10th	Night	19% 16% 6% 11% 7% 4 5% 4 5% 4 19%	2649
	11th to 20th	Night	19% 15% 7% 9% 8% 4 5% 4 5% 4 20%	2660
	21st to end	Night	15% 14% 7% 10% 6% 4 4 5% 4 5% 25%	2802
February	1st to 10th	Night	19% 13% 6% 11% 8% 3 6% 5% 5% 5% 19%	2340
	11th to 20th	Night	15% 13% 5% 9% 7% 4 4 4 5% 6% 28%	2264
	21st to end	Night	19% 12% 5% 9% 7% 3 5% 4 4 5% 27%	1776
March	1st to 10th	Night	16% 14% 5% 8% 6% 4 4 4 4 5% 30%	2132
	11th to 20th	Night	18% 13% 5% 7% 5% 2 3 4 4 5% 33%	1979
	21st to end	Night	20% 10% 4 7% 5% 3 3 3 3 5% 38%	2072
April	1st to 10th	Night	22% 9% 4 7% 5% 2 4 4 4 6% 34%	1800
	11th to 20th	Night	21% 6% 4 5% 5% 3 4 3 5% 7% 36%	1775
	21st to end	Night	22% 9% 3 7% 6% 3 3 4 5% 6% 32%	1735
May	1st to 10th	Night	23% 7% 2 5% 5% 3 3 5% 5% 8% 34%	1440
	11th to 20th	Night	13% 6% 3 7% 6% 4 5% 5% 5% 6% 42%	1415
	21st to end	Night	12% 5% 2 6% 5% 4 5% 4 6% 8% 43%	1550
June	1st to 10th	Night	18% 7% 3 6% 6% 3 4 5% 6% 8% 33%	1440
	11th to 20th	Night	15% 5% 2 5% 5% 4 4 5% 6% 8% 42%	1440
	21st to end	Night	16% 7% 3 5% 5% 4 6% 5% 5% 9% 36%	1440
July	1st to 10th	Night	11% 5% 2 4 4 3 5% 5% 5% 8% 47%	1440
	11th to 20th	Night	11% 5% 3 5% 5% 3 5% 6% 6% 10% 42%	1440
	21st to end	Night	10% 5% 2 5% 5% 3 4 5% 7% 8% 45%	1574
August	1st to 10th	Night	10% 6% 2 5% 6% 3 5% 5% 6% 8% 43%	1511
	11th to 20th	Night	12% 7% 2 6% 6% 3 4 4 6% 9% 42%	1611
	21st to end	Night	11% 5% 2 5% 5% 3 5% 5% 7% 8% 43%	1980
September	1st to 10th	Night	13% 6% 2 6% 5% 3 4 4 5% 7% 45%	1800
	11th to 20th	Night	14% 5% 2 4 4 3 3 4 6% 7% 48%	1800
	21st to end	Night	16% 5% 3 5% 5% 3 4 4 5% 7% 42%	2037
October	1st to 10th	Night	19% 7% 3 6% 5% 3 5% 5% 5% 7% 34%	2160
	11th to 20th	Night	21% 7% 3 7% 6% 3 5% 5% 5% 6% 31%	2160
	21st to end	Night	25% 9% 3 7% 6% 3 4 4 4 6% 28%	2492
November	1st to 10th	Night	19% 10% 3 6% 5% 2 4 4 5% 7% 36%	2407
	11th to 20th	Night	18% 10% 4 6% 6% 3 5% 5% 5% 6% 32%	2631
	21st to end	Night	22% 13% 4 8% 6% 3 5% 6% 4 6% 24%	2660
December	1st to 10th	Night	22% 15% 6% 9% 7% 3 5% 4 4 6% 19%	2660
	11th to 20th	Night	22% 14% 5% 8% 6% 3 4 4 4 5% 27%	3288
	21st to end	Night	32% 13% 5% 11% 5% 3 4 4 4 4 16%	4884

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	66% 6% 9% 14% 5% 1	1518
	11th to 20th	Day	66% 5% 9% 10% 9% 1	1520
	21st to end	Day	64% 6% 10% 11% 7% 1	1781
February	1st to 10th	Day	66% 4 9% 15% 5% 1	1620
	11th to 20th	Day	60% 5% 10% 16% 9% 1	1591
	21st to end	Day	66% 4 8% 12% 9% 1	1480
March	1st to 10th	Day	60% 4 10% 16% 9% 2	1817
	11th to 20th	Day	57% 5% 11% 14% 12% 2	1980
	21st to end	Day	52% 4 7% 18% 14% 5%	2283
April	1st to 10th	Day	54% 4 7% 18% 13% 3	2160
	11th to 20th	Day	49% 4 7% 21% 16% 2	2134
	21st to end	Day	54% 4 9% 16% 13% 4	2237
May	1st to 10th	Day	50% 5% 10% 17% 16% 2	2520
	11th to 20th	Day	44% 6% 9% 24% 15% 2	2482
	21st to end	Day	42% 7% 16% 21% 11% 3	2725
June	1st to 10th	Day	50% 7% 10% 20% 12% 1	2520
	11th to 20th	Day	42% 6% 11% 26% 13% 2	2520
	21st to end	Day	46% 7% 15% 20% 11% 1	2520
July	1st to 10th	Day	36% 7% 12% 29% 15% 1	2450
	11th to 20th	Day	38% 8% 17% 27% 10%	2445
	21st to end	Day	38% 9% 19% 23% 11%	2591
August	1st to 10th	Day	39% 8% 17% 26% 10% 1	2409
	11th to 20th	Day	40% 8% 17% 24% 11% 1	2270
	21st to end	Day	39% 7% 14% 28% 12%	2376
September	1st to 10th	Day	39% 7% 16% 25% 12% 1	2160
	11th to 20th	Day	36% 6% 12% 30% 15%	2160
	21st to end	Day	44% 7% 13% 26% 10%	1918
October	1st to 10th	Day	49% 5% 12% 22% 11% 1	1800
	11th to 20th	Day	55% 7% 14% 17% 6% 1	1800
	21st to end	Day	61% 5% 9% 16% 8% 1	1864
November	1st to 10th	Day	48% 8% 14% 21% 8% 1	1671
	11th to 20th	Day	55% 7% 11% 17% 9% 1	1545
	21st to end	Day	63% 8% 12% 11% 7%	1520
December	1st to 10th	Day	68% 6% 12% 11% 3	1520
	11th to 20th	Day	66% 6% 13% 11% 3	1906
	21st to end	Day	71% 6% 7% 14% 3	2772
January	1st to 10th	Night	68% 6% 10% 11% 5%	2649
	11th to 20th	Night	66% 7% 9% 10% 7% 1	2660
	21st to end	Night	60% 6% 9% 15% 8% 1	2802
February	1st to 10th	Night	67% 5% 9% 12% 6% 1	2340
	11th to 20th	Night	58% 5% 11% 19% 8%	2266
	21st to end	Night	60% 5% 9% 16% 10% 1	1776
March	1st to 10th	Night	57% 4 11% 16% 10% 1	2142
	11th to 20th	Night	54% 5% 9% 18% 12% 1	1980
	21st to end	Night	51% 4 6% 19% 16% 4	2072
April	1st to 10th	Night	51% 4 8% 20% 15% 2	1800
	11th to 20th	Night	49% 6% 10% 18% 17% 2	1775
	21st to end	Night	53% 4 10% 16% 14% 3	1735
May	1st to 10th	Night	48% 6% 10% 17% 15% 5%	1440
	11th to 20th	Night	41% 6% 10% 23% 16% 3	1416
	21st to end	Night	37% 7% 15% 20% 17% 3	1551
June	1st to 10th	Night	48% 7% 14% 20% 11% 1	1440
	11th to 20th	Night	39% 7% 13% 21% 17% 2	1440
	21st to end	Night	43% 9% 13% 21% 13% 1	1440
July	1st to 10th	Night	34% 8% 12% 27% 17% 3	1400
	11th to 20th	Night	35% 8% 15% 30% 13%	1392
	21st to end	Night	35% 9% 16% 27% 12% 1	1486
August	1st to 10th	Night	37% 9% 17% 23% 14%	1501
	11th to 20th	Night	39% 7% 14% 26% 13% 1	1611
	21st to end	Night	36% 8% 15% 28% 13%	1980
September	1st to 10th	Night	38% 7% 14% 27% 14% 1	1800
	11th to 20th	Night	34% 7% 14% 30% 13% 1	1800
	21st to end	Night	41% 6% 11% 29% 11% 1	2037
October	1st to 10th	Night	48% 8% 11% 20% 12% 1	2160
	11th to 20th	Night	52% 7% 13% 19% 9% 1	2160
	21st to end	Night	58% 6% 13% 14% 9%	2492
November	1st to 10th	Night	48% 6% 14% 21% 10% 1	2407
	11th to 20th	Night	50% 7% 13% 19% 10% 2	2631
	21st to end	Night	61% 8% 11% 15% 5%	2660
December	1st to 10th	Night	67% 7% 10% 13% 4	2660
	11th to 20th	Night	61% 6% 11% 15% 6% 1	3288
	21st to end	Night	71% 7% 9% 11% 2	4884

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	25% 23% 34% 11% 6% 1	180
	11th to 20th	Day	33% 19% 31% 15% 1 1	180
	21st to end	Day	24% 21% 33% 18% 4	251
February	1st to 10th	Day	35% 26% 33% 5%	240
	11th to 20th	Day	32% 24% 37% 6% 1	249
	21st to end	Day	34% 19% 33% 12% 2	346
March	1st to 10th	Day	23% 22% 32% 19% 4	420
	11th to 20th	Day	28% 20% 33% 15% 4	420
	21st to end	Day	34% 20% 24% 14% 7% 1	462
April	1st to 10th	Day	31% 17% 31% 15% 6%	420
	11th to 20th	Day	29% 19% 27% 14% 7% 4	412
	21st to end	Day	29% 18% 33% 12% 6% 2	420
May	1st to 10th	Day	22% 18% 31% 22% 6% 1	423
	11th to 20th	Day	19% 15% 36% 20% 8% 2	447
	21st to end	Day	18% 15% 31% 22% 12% 2	485
June	1st to 10th	Day	23% 14% 32% 21% 8% 2	450
	11th to 20th	Day	17% 12% 34% 24% 11% 3	450
	21st to end	Day	19% 13% 31% 23% 11% 3	450
July	1st to 10th	Day	14% 9% 26% 31% 17% 2	445
	11th to 20th	Day	15% 10% 29% 27% 16% 3	444
	21st to end	Day	13% 9% 33% 31% 11% 2	488
August	1st to 10th	Day	13% 8% 30% 30% 16% 4	446
	11th to 20th	Day	13% 6% 27% 30% 18% 6%	439
	21st to end	Day	13% 10% 34% 23% 13% 7%	495
September	1st to 10th	Day	14% 11% 32% 24% 11% 7%	450
	11th to 20th	Day	13% 9% 27% 24% 17% 10%	450
	21st to end	Day	19% 12% 29% 22% 15% 3	450
October	1st to 10th	Day	28% 14% 25% 17% 11% 6%	450
	11th to 20th	Day	26% 20% 27% 20% 6% 1	450
	21st to end	Day	35% 16% 27% 17% 5%	437
November	1st to 10th	Day	25% 21% 30% 17% 7%	365
	11th to 20th	Day	26% 18% 36% 13% 5% 1	205
	21st to end	Day	29% 21% 34% 13% 3	180
December	1st to 10th	Day	34% 24% 22% 14% 5%	180
	11th to 20th	Day	30% 25% 34% 8% 3	180
	21st to end	Day	36% 21% 27% 13% 3	198
January	1st to 10th	Night	31% 18% 29% 19% 3	760
	11th to 20th	Night	28% 18% 31% 19% 4	760
	21st to end	Night	22% 16% 34% 23% 6%	835
February	1st to 10th	Night	27% 20% 34% 17% 3	720
	11th to 20th	Night	22% 19% 37% 18% 4	703
	21st to end	Night	26% 18% 32% 20% 4	592
March	1st to 10th	Night	26% 14% 34% 22% 4	720
	11th to 20th	Night	27% 16% 32% 21% 4 1	720
	21st to end	Night	25% 14% 31% 21% 8%	792
April	1st to 10th	Night	26% 16% 34% 21% 3	720
	11th to 20th	Night	23% 18% 32% 18% 7% 1	711
	21st to end	Night	27% 11% 30% 23% 9% 1	655
May	1st to 10th	Night	26% 9% 27% 28% 9% 1	360
	11th to 20th	Night	14% 9% 38% 31% 8%	354
	21st to end	Night	12% 9% 37% 29% 12% 1	389
June	1st to 10th	Night	19% 8% 30% 33% 9% 1	360
	11th to 20th	Night	15% 11% 28% 33% 12% 1	360
	21st to end	Night	18% 7% 25% 36% 14%	360
July	1st to 10th	Night	10% 5% 30% 39% 16%	350
	11th to 20th	Night	14% 3 26% 36% 19% 1	348
	21st to end	Night	11% 5% 30% 41% 12% 1	394
August	1st to 10th	Night	13% 4 25% 36% 22%	437
	11th to 20th	Night	13% 5% 19% 36% 23% 3	547
	21st to end	Night	13% 7% 28% 35% 16% 2	792
September	1st to 10th	Night	15% 7% 29% 31% 16% 2	720
	11th to 20th	Night	16% 7% 22% 29% 21% 5%	720
	21st to end	Night	18% 10% 25% 26% 19% 2	720
October	1st to 10th	Night	23% 12% 28% 19% 16% 2	720
	11th to 20th	Night	24% 14% 29% 25% 7% 1	720
	21st to end	Night	30% 14% 27% 24% 4	792
November	1st to 10th	Night	26% 14% 30% 22% 7%	740
	11th to 20th	Night	25% 19% 30% 18% 9%	760
	21st to end	Night	33% 16% 27% 17% 6%	760
December	1st to 10th	Night	32% 18% 28% 18% 4	760
	11th to 20th	Night	27% 20% 32% 17% 3 1	760
	21st to end	Night	34% 16% 31% 17% 2	836

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