Kiawah Island Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Kiawah Island Observatory clear sky chart from 2005-11-06 to 2024-03-01.

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	10% 5% 2 5% 5% 3 5% 5% 6% 8% 45%	78909
	Night	11% 6% 3 5% 5% 3 5% 5% 6% 8% 44%	80507
Transparency	Day	35% 9% 15% 20% 16% 5%	78632
	Night	37% 9% 13% 19% 17% 6%	80303
Seeing	Day	12% 4 16% 28% 29% 11%	14563
	Night	12% 5% 19% 30% 27% 7%	24751

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	11% 5% 2 6% 5% 3 6% 5% 6% 8% 44%	2836
	11th to 20th	Day	12% 5% 3 5% 4 3 4 4 5% 8% 46%	2840
	21st to end	Day	14% 10% 4 6% 5% 2 3 3 3 5% 45%	3124
February	1st to 10th	Day	14% 7% 3 6% 4 4 5% 5% 5% 7% 41%	1710
	11th to 20th	Day	18% 7% 3 4 4 2 4 4 4 6% 46%	1826
	21st to end	Day	12% 7% 3 6% 5% 3 4 4 5% 6% 45%	1714
March	1st to 10th	Day	11% 4 3 4 3 3 4 4 5% 7% 51%	1980
	11th to 20th	Day	11% 5% 2 4 4 3 5% 4 7% 7% 47%	1980
	21st to end	Day	11% 5% 2 4 4 3 5% 5% 5% 7% 48%	2178
April	1st to 10th	Day	7% 4 2 4 4 3 5% 5% 5% 8% 54%	1991
	11th to 20th	Day	9% 4 2 5% 4 3 4 4 6% 8% 51%	2134
	21st to end	Day	5% 3 1 3 4 2 4 5% 5% 9% 59%	2160
May	1st to 10th	Day	7% 3 2 4 4 3 5% 6% 6% 8% 53%	2304
	11th to 20th	Day	9% 5% 2 4 5% 3 5% 5% 6% 8% 48%	2301
	21st to end	Day	9% 5% 3 5% 5% 4 5% 6% 7% 10% 43%	2531
June	1st to 10th	Day	8% 6% 2 6% 6% 4 6% 7% 8% 11% 36%	2340
	11th to 20th	Day	7% 4 3 4 5% 4 7% 7% 7% 11% 42%	2340
	21st to end	Day	6% 4 2 5% 5% 4 7% 7% 10% 13% 37%	2340
July	1st to 10th	Day	8% 4 2 5% 5% 4 6% 7% 9% 12% 38%	2340
	11th to 20th	Day	7% 6% 2 6% 6% 4 7% 7% 8% 11% 37%	2340
	21st to end	Day	7% 5% 2 5% 5% 4 6% 7% 8% 13% 37%	2547
August	1st to 10th	Day	9% 4 2 5% 6% 4 6% 8% 10% 13% 32%	2327
	11th to 20th	Day	7% 5% 3 6% 7% 5% 7% 8% 8% 10% 34%	2240
	21st to end	Day	13% 7% 3 7% 6% 3 6% 6% 8% 9% 32%	2376
September	1st to 10th	Day	11% 5% 3 6% 6% 4 4 6% 7% 11% 39%	2044
	11th to 20th	Day	10% 7% 2 5% 6% 4 7% 5% 7% 9% 38%	1980
	21st to end	Day	13% 6% 3 5% 5% 3 5% 5% 6% 8% 40%	1980
October	1st to 10th	Day	11% 5% 2 5% 5% 3 5% 5% 5% 6% 48%	1838
	11th to 20th	Day	9% 3 2 4 3 2 4 3 4 6% 60%	1800
	21st to end	Day	10% 5% 2 5% 4 2 3 3 4 7% 55%	1980
November	1st to 10th	Day	10% 4 2 4 4 3 4 4 4 6% 54%	1857
	11th to 20th	Day	11% 5% 3 5% 5% 3 4 3 4 7% 50%	1900
	21st to end	Day	12% 5% 2 5% 4 2 4 4 4 7% 51%	1744
December	1st to 10th	Day	11% 7% 3 5% 5% 3 4 5% 5% 7% 45%	1710
	11th to 20th	Day	15% 4 2 4 3 2 3 4 4 7% 53%	2153
	21st to end	Day	16% 9% 2 5% 4 3 4 4 5% 5% 43%	3124
January	1st to 10th	Night	13% 6% 2 5% 5% 4 5% 5% 5% 8% 43%	4109
	11th to 20th	Night	10% 6% 3 6% 6% 3 5% 5% 6% 8% 42%	4120
	21st to end	Night	13% 8% 4 6% 5% 3 4 4 4 6% 44%	4532
February	1st to 10th	Night	15% 6% 2 5% 5% 3 4 5% 5% 6% 44%	2470
	11th to 20th	Night	16% 7% 3 4 4 3 4 4 6% 6% 43%	2237
	21st to end	Night	12% 7% 3 6% 4 3 5% 5% 5% 7% 43%	1737
March	1st to 10th	Night	9% 4 2 5% 6% 3 4 5% 5% 7% 51%	1987
	11th to 20th	Night	12% 5% 2 5% 5% 3 5% 5% 6% 7% 46%	1980
	21st to end	Night	10% 6% 3 5% 5% 3 4 4 5% 8% 48%	2178
April	1st to 10th	Night	9% 4 3 5% 5% 3 3 5% 5% 7% 52%	1967
	11th to 20th	Night	11% 3 2 5% 5% 3 4 4 4 7% 52%	1775
	21st to end	Night	7% 4 3 4 3 2 4 4 6% 8% 56%	1800
May	1st to 10th	Night	6% 4 2 5% 4 3 4 5% 6% 7% 54%	1670
	11th to 20th	Night	7% 4 2 5% 4 3 4 6% 8% 10% 47%	1592
	21st to end	Night	8% 4 2 5% 5% 3 6% 5% 8% 10% 44%	1744
June	1st to 10th	Night	9% 6% 3 4 7% 3 7% 6% 9% 12% 35%	1620
	11th to 20th	Night	5% 4 3 5% 5% 4 6% 7% 8% 13% 38%	1620
	21st to end	Night	5% 4 3 5% 7% 4 7% 8% 9% 15% 33%	1620
July	1st to 10th	Night	7% 5% 3 5% 7% 3 6% 6% 9% 13% 36%	1620
	11th to 20th	Night	8% 5% 3 5% 6% 4 6% 6% 8% 13% 35%	1620
	21st to end	Night	7% 4 2 6% 6% 3 7% 7% 8% 13% 37%	1771
August	1st to 10th	Night	7% 6% 2 6% 6% 4 6% 8% 10% 14% 32%	1611
	11th to 20th	Night	8% 5% 3 7% 6% 5% 7% 8% 9% 13% 29%	1642
	21st to end	Night	11% 7% 3 7% 6% 5% 6% 7% 8% 11% 30%	1980
September	1st to 10th	Night	9% 5% 3 6% 6% 4 6% 6% 7% 11% 37%	1912
	11th to 20th	Night	11% 6% 2 6% 6% 5% 5% 6% 6% 9% 37%	1980
	21st to end	Night	13% 6% 3 6% 6% 3 5% 5% 6% 8% 38%	1980
October	1st to 10th	Night	11% 6% 3 5% 5% 3 3 4 5% 8% 47%	2120
	11th to 20th	Night	9% 3 1 4 4 3 4 4 6% 7% 56%	2160
	21st to end	Night	9% 4 1 5% 5% 3 3 4 4 5% 56%	2376
November	1st to 10th	Night	10% 4 2 4 4 2 3 4 5% 7% 55%	2221
	11th to 20th	Night	13% 7% 2 4 4 2 3 3 5% 6% 50%	2280
	21st to end	Night	11% 6% 3 5% 5% 3 4 4 5% 7% 47%	2432
December	1st to 10th	Night	13% 6% 3 6% 5% 3 5% 5% 6% 8% 41%	2470
	11th to 20th	Night	15% 7% 2 4 4 2 3 3 4 6% 48%	3042
	21st to end	Night	17% 8% 3 6% 6% 3 4 4 4 7% 40%	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	36% 6% 10% 20% 20% 8%	2837
	11th to 20th	Day	36% 5% 9% 17% 23% 10%	2840
	21st to end	Day	41% 7% 7% 16% 22% 8%	3124
February	1st to 10th	Day	42% 6% 11% 17% 19% 6%	1710
	11th to 20th	Day	40% 5% 9% 15% 23% 7%	1836
	21st to end	Day	38% 7% 10% 19% 16% 11%	1714
March	1st to 10th	Day	31% 6% 11% 21% 20% 11%	1980
	11th to 20th	Day	34% 8% 15% 19% 16% 7%	1980
	21st to end	Day	34% 8% 13% 21% 17% 6%	2178
April	1st to 10th	Day	29% 7% 15% 21% 19% 8%	1991
	11th to 20th	Day	30% 6% 12% 25% 18% 9%	2134
	21st to end	Day	22% 7% 14% 32% 20% 5%	2160
May	1st to 10th	Day	27% 8% 15% 27% 18% 5%	2304
	11th to 20th	Day	32% 9% 20% 24% 13% 2	2305
	21st to end	Day	33% 11% 20% 22% 13%	2531
June	1st to 10th	Day	37% 15% 20% 17% 10%	2340
	11th to 20th	Day	32% 12% 23% 26% 7%	2340
	21st to end	Day	32% 18% 25% 18% 7% 1	2340
July	1st to 10th	Day	34% 14% 25% 20% 6% 1	2275
	11th to 20th	Day	37% 16% 26% 15% 6%	2271
	21st to end	Day	32% 16% 28% 21% 2	2405
August	1st to 10th	Day	36% 16% 26% 18% 4	2314
	11th to 20th	Day	39% 16% 26% 16% 3	2240
	21st to end	Day	45% 15% 20% 16% 4 1	2376
September	1st to 10th	Day	38% 13% 20% 16% 11% 3	2044
	11th to 20th	Day	40% 12% 17% 18% 11% 2	1980
	21st to end	Day	41% 10% 15% 19% 13% 3	1980
October	1st to 10th	Day	36% 7% 11% 18% 21% 7%	1838
	11th to 20th	Day	26% 7% 12% 22% 26% 8%	1800
	21st to end	Day	31% 5% 9% 18% 27% 11%	1980
November	1st to 10th	Day	31% 6% 11% 17% 25% 11%	1857
	11th to 20th	Day	35% 5% 11% 17% 22% 10%	1900
	21st to end	Day	34% 6% 10% 23% 22% 6%	1744
December	1st to 10th	Day	39% 7% 13% 20% 15% 7%	1710
	11th to 20th	Day	34% 5% 8% 22% 25% 6%	2150
	21st to end	Day	42% 6% 8% 19% 21% 4	3124
January	1st to 10th	Night	38% 7% 8% 15% 22% 10%	4109
	11th to 20th	Night	38% 6% 9% 17% 25% 5%	4120
	21st to end	Night	40% 7% 8% 13% 23% 10%	4532
February	1st to 10th	Night	40% 5% 9% 21% 18% 8%	2470
	11th to 20th	Night	41% 6% 10% 14% 20% 10%	2238
	21st to end	Night	41% 6% 10% 18% 14% 11%	1737
March	1st to 10th	Night	31% 7% 12% 19% 19% 12%	1987
	11th to 20th	Night	36% 8% 13% 19% 14% 8%	1980
	21st to end	Night	35% 7% 15% 19% 17% 8%	2178
April	1st to 10th	Night	32% 6% 13% 20% 20% 9%	1967
	11th to 20th	Night	33% 5% 10% 22% 20% 10%	1775
	21st to end	Night	26% 7% 13% 26% 23% 5%	1800
May	1st to 10th	Night	27% 8% 15% 24% 22% 5%	1670
	11th to 20th	Night	29% 12% 21% 22% 16% 2	1593
	21st to end	Night	33% 11% 17% 26% 12% 1	1745
June	1st to 10th	Night	38% 13% 21% 19% 9%	1620
	11th to 20th	Night	35% 13% 21% 24% 7%	1620
	21st to end	Night	34% 17% 25% 20% 4	1620
July	1st to 10th	Night	34% 13% 26% 19% 6% 1	1575
	11th to 20th	Night	37% 15% 26% 16% 5% 1	1566
	21st to end	Night	32% 15% 27% 24% 2	1672
August	1st to 10th	Night	36% 17% 25% 18% 3	1600
	11th to 20th	Night	40% 18% 21% 18% 3	1642
	21st to end	Night	45% 16% 19% 17% 4	1980
September	1st to 10th	Night	38% 14% 19% 17% 11% 1	1912
	11th to 20th	Night	42% 12% 15% 18% 12% 1	1980
	21st to end	Night	41% 10% 15% 18% 13% 2	1980
October	1st to 10th	Night	37% 8% 11% 19% 19% 6%	2120
	11th to 20th	Night	27% 8% 9% 23% 25% 9%	2160
	21st to end	Night	30% 5% 9% 18% 25% 13%	2376
November	1st to 10th	Night	30% 6% 10% 19% 24% 12%	2221
	11th to 20th	Night	35% 6% 8% 18% 23% 10%	2280
	21st to end	Night	36% 6% 10% 18% 21% 9%	2432
December	1st to 10th	Night	41% 7% 14% 15% 16% 6%	2470
	11th to 20th	Night	38% 5% 8% 17% 26% 6%	3044
	21st to end	Night	44% 7% 7% 20% 18% 4	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	13% 5% 33% 41% 8%	270
	11th to 20th	Day	8% 6% 26% 42% 14% 3	270
	21st to end	Day	20% 8% 37% 26% 8%	297
February	1st to 10th	Day	17% 8% 23% 38% 14%	270
	11th to 20th	Day	22% 7% 26% 34% 11%	270
	21st to end	Day	12% 6% 34% 37% 11% 1	369
March	1st to 10th	Day	11% 6% 31% 33% 19% 1	420
	11th to 20th	Day	14% 5% 29% 37% 13% 2	420
	21st to end	Day	11% 6% 29% 33% 17% 2	462
April	1st to 10th	Day	10% 7% 27% 35% 20% 2	420
	11th to 20th	Day	9% 6% 24% 30% 23% 8%	412
	21st to end	Day	5% 3 18% 35% 29% 9%	420
May	1st to 10th	Day	5% 2 23% 40% 22% 8%	423
	11th to 20th	Day	10% 2 11% 28% 36% 12%	447
	21st to end	Day	12% 2 7% 25% 38% 16%	485
June	1st to 10th	Day	10% 1 9% 29% 40% 12%	450
	11th to 20th	Day	9% 2 8% 21% 40% 21%	450
	21st to end	Day	6% 1 6% 25% 43% 19%	450
July	1st to 10th	Day	7% 2 4 15% 45% 27%	445
	11th to 20th	Day	7% 2 3 12% 46% 28%	444
	21st to end	Day	7% 2 4 18% 52% 17%	488
August	1st to 10th	Day	12% 1 4 18% 43% 23%	446
	11th to 20th	Day	8% 1 4 13% 45% 30%	439
	21st to end	Day	14% 2 6% 16% 38% 24%	495
September	1st to 10th	Day	11% 1 6% 20% 41% 21%	450
	11th to 20th	Day	14% 2 10% 22% 41% 11%	450
	21st to end	Day	15% 1 7% 24% 36% 17%	450
October	1st to 10th	Day	13% 5% 14% 31% 26% 10%	450
	11th to 20th	Day	10% 2 14% 31% 32% 11%	450
	21st to end	Day	12% 5% 16% 29% 32% 7%	495
November	1st to 10th	Day	14% 5% 17% 32% 24% 8%	455
	11th to 20th	Day	15% 6% 25% 34% 17% 2	460
	21st to end	Day	12% 6% 22% 35% 24% 3	304
December	1st to 10th	Day	17% 4 23% 34% 21% 1	270
	11th to 20th	Day	17% 5% 26% 34% 14% 4	270
	21st to end	Day	20% 6% 22% 31% 19% 2	297
January	1st to 10th	Night	12% 10% 33% 34% 10% 1	760
	11th to 20th	Night	13% 10% 30% 32% 15% 1	760
	21st to end	Night	17% 9% 36% 32% 6%	836
February	1st to 10th	Night	17% 8% 28% 34% 13%	760
	11th to 20th	Night	17% 10% 28% 36% 8%	743
	21st to end	Night	15% 6% 28% 39% 11% 1	628
March	1st to 10th	Night	11% 6% 30% 38% 15% 1	723
	11th to 20th	Night	13% 6% 26% 38% 15% 1	720
	21st to end	Night	11% 7% 27% 34% 19% 2	792
April	1st to 10th	Night	9% 8% 27% 33% 21% 3	720
	11th to 20th	Night	12% 5% 21% 35% 23% 5%	711
	21st to end	Night	8% 4 21% 35% 27% 4	720
May	1st to 10th	Night	7% 3 21% 37% 24% 7%	590
	11th to 20th	Night	8% 3 16% 32% 31% 10%	531
	21st to end	Night	10% 9% 23% 41% 17%	584
June	1st to 10th	Night	11% 2 8% 30% 39% 11%	540
	11th to 20th	Night	7% 2 6% 23% 41% 20%	540
	21st to end	Night	6% 2 7% 27% 42% 16%	540
July	1st to 10th	Night	9% 1 5% 21% 48% 15%	525
	11th to 20th	Night	10% 1 3 15% 52% 19%	522
	21st to end	Night	6% 1 4 26% 52% 12%	591
August	1st to 10th	Night	10% 2 4 21% 47% 16%	537
	11th to 20th	Night	8% 3 4 19% 47% 18%	578
	21st to end	Night	14% 1 7% 16% 41% 21%	792
September	1st to 10th	Night	10% 2 8% 20% 44% 16%	720
	11th to 20th	Night	13% 3 8% 21% 45% 10%	720
	21st to end	Night	17% 2 10% 21% 40% 11%	720
October	1st to 10th	Night	14% 3 14% 31% 29% 9%	720
	11th to 20th	Night	10% 3 17% 31% 30% 9%	720
	21st to end	Night	10% 5% 20% 29% 29% 7%	792
November	1st to 10th	Night	13% 4 20% 31% 27% 4	740
	11th to 20th	Night	16% 7% 24% 31% 20% 3	760
	21st to end	Night	13% 7% 23% 33% 21% 3	760
December	1st to 10th	Night	15% 8% 25% 34% 15% 3	760
	11th to 20th	Night	19% 7% 25% 29% 17% 3	760
	21st to end	Night	20% 8% 27% 31% 14% 1	836

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