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 2025-05-11.

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% 6% 2 6% 4 4 5% 4 6% 8% 45%	114333
	Night	11% 6% 2 6% 4 4 4 4 6% 8% 45%	114624
Transparency	Day	34% 9% 15% 20% 16% 6%	118009
	Night	35% 8% 13% 18% 19% 7%	119087
Seeing	Day	11% 4 16% 28% 29% 11%	16216
	Night	12% 5% 19% 30% 27% 7%	26380

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	10% 4 2 6% 3 4 4 3 5% 6% 53%	4056
	11th to 20th	Day	14% 6% 2 6% 3 4 3 3 5% 6% 48%	4060
	21st to end	Day	14% 10% 3 7% 3 3 3 2 4 6% 45%	4466
February	1st to 10th	Day	13% 7% 2 6% 5% 5% 4 4 5% 7% 42%	1800
	11th to 20th	Day	18% 7% 3 5% 3 3 3 4 4 6% 44%	1925
	21st to end	Day	12% 7% 2 7% 5% 4 3 3 6% 6% 47%	1802
March	1st to 10th	Day	19% 9% 3 7% 3 4 3 3 5% 6% 38%	4935
	11th to 20th	Day	7% 4 2 4 3 4 4 3 6% 8% 56%	4980
	21st to end	Day	12% 6% 2 5% 4 4 4 3 4 5% 50%	5465
April	1st to 10th	Day	7% 4 2 5% 4 4 5% 3 5% 8% 53%	2213
	11th to 20th	Day	9% 4 2 5% 3 3 4 4 6% 8% 53%	2374
	21st to end	Day	5% 4 1 4 4 3 4 4 5% 9% 58%	2400
May	1st to 10th	Day	6% 4 2 5% 4 5% 5% 5% 7% 10% 48%	4147
	11th to 20th	Day	13% 7% 2 6% 4 4 5% 4 6% 7% 43%	4081
	21st to end	Day	5% 3 2 4 3 4 4 4 7% 11% 53%	4476
June	1st to 10th	Day	8% 6% 2 7% 6% 5% 6% 6% 9% 10% 36%	2470
	11th to 20th	Day	7% 5% 2 4 5% 5% 7% 5% 7% 11% 42%	2470
	21st to end	Day	6% 5% 2 5% 5% 5% 7% 6% 10% 13% 36%	2470
July	1st to 10th	Day	11% 6% 2 7% 5% 6% 7% 6% 8% 11% 32%	4120
	11th to 20th	Day	5% 4 2 7% 5% 6% 6% 6% 9% 14% 37%	4120
	21st to end	Day	5% 5% 2 7% 6% 6% 7% 6% 9% 14% 34%	4466
August	1st to 10th	Day	19% 7% 3 7% 5% 5% 5% 5% 8% 10% 27%	4104
	11th to 20th	Day	5% 4 2 6% 5% 5% 6% 6% 8% 12% 40%	3925
	21st to end	Day	10% 6% 2 6% 4 4 5% 4 7% 9% 41%	4180
September	1st to 10th	Day	12% 5% 3 7% 5% 4 4 5% 6% 10% 38%	2157
	11th to 20th	Day	10% 7% 2 7% 5% 6% 6% 4 8% 9% 37%	2090
	21st to end	Day	13% 6% 3 6% 4 5% 5% 4 6% 8% 41%	2090
October	1st to 10th	Day	11% 6% 2 6% 5% 4 5% 4 6% 5% 47%	1940
	11th to 20th	Day	8% 3 1 4 2 3 3 3 4 6% 61%	1900
	21st to end	Day	9% 5% 2 6% 3 3 3 3 4 7% 56%	2090
November	1st to 10th	Day	11% 5% 1 6% 3 4 4 4 4 6% 52%	1957
	11th to 20th	Day	11% 5% 3 6% 4 4 3 3 4 7% 50%	2000
	21st to end	Day	11% 6% 2 5% 3 3 3 3 5% 6% 52%	1835
December	1st to 10th	Day	7% 5% 2 4 4 4 4 3 5% 6% 55%	2930
	11th to 20th	Day	13% 5% 3 7% 5% 3 3 3 5% 6% 47%	3373
	21st to end	Day	16% 9% 2 7% 4 3 4 3 5% 6% 40%	4466
January	1st to 10th	Night	9% 5% 2 4 4 4 4 3 4 6% 56%	5889
	11th to 20th	Night	13% 7% 3 7% 4 4 4 4 6% 7% 42%	5900
	21st to end	Night	12% 7% 3 7% 4 4 3 3 5% 6% 45%	6490
February	1st to 10th	Night	14% 6% 2 6% 5% 4 4 4 5% 6% 44%	2600
	11th to 20th	Night	16% 8% 3 5% 4 4 4 3 6% 6% 42%	2358
	21st to end	Night	12% 7% 3 7% 4 5% 4 4 5% 6% 43%	1825
March	1st to 10th	Night	15% 9% 3 7% 4 3 3 3 4 5% 42%	4918
	11th to 20th	Night	10% 4 2 5% 3 3 4 4 6% 9% 51%	4980
	21st to end	Night	12% 6% 2 5% 3 3 3 3 5% 7% 50%	5467
April	1st to 10th	Night	8% 4 2 5% 4 4 4 4 6% 7% 51%	2185
	11th to 20th	Night	11% 3 2 5% 4 4 4 3 5% 7% 53%	1975
	21st to end	Night	6% 4 2 4 3 4 4 4 7% 7% 54%	2000
May	1st to 10th	Night	5% 5% 2 5% 4 4 4 4 7% 11% 50%	2983
	11th to 20th	Night	12% 6% 2 5% 4 4 4 4 6% 8% 44%	2818
	21st to end	Night	6% 3 1 5% 4 4 5% 3 7% 9% 52%	3079
June	1st to 10th	Night	8% 7% 2 5% 5% 5% 6% 5% 9% 12% 35%	1710
	11th to 20th	Night	6% 5% 2 5% 5% 6% 6% 6% 8% 13% 38%	1710
	21st to end	Night	5% 5% 2 6% 6% 6% 7% 6% 9% 15% 33%	1710
July	1st to 10th	Night	9% 6% 3 7% 6% 5% 7% 6% 9% 12% 31%	2840
	11th to 20th	Night	5% 4 3 6% 6% 6% 6% 6% 10% 15% 33%	2840
	21st to end	Night	6% 5% 2 8% 6% 6% 8% 7% 10% 15% 27%	3087
August	1st to 10th	Night	17% 8% 2 7% 4 5% 5% 6% 8% 12% 25%	2825
	11th to 20th	Night	6% 4 2 6% 4 6% 6% 5% 8% 13% 38%	2871
	21st to end	Night	8% 6% 3 8% 4 5% 5% 4 7% 9% 43%	3476
September	1st to 10th	Night	10% 6% 3 7% 5% 5% 6% 5% 7% 11% 36%	2019
	11th to 20th	Night	12% 7% 2 7% 5% 6% 7% 4 6% 10% 36%	2090
	21st to end	Night	13% 6% 3 7% 4 4 5% 4 6% 8% 38%	2090
October	1st to 10th	Night	11% 6% 3 6% 5% 4 3 3 5% 8% 46%	2238
	11th to 20th	Night	8% 3 1 4 4 3 4 4 5% 6% 58%	2280
	21st to end	Night	9% 4 1 6% 4 3 3 3 4 5% 57%	2508
November	1st to 10th	Night	10% 5% 2 5% 4 3 3 3 5% 7% 53%	2341
	11th to 20th	Night	13% 7% 2 5% 4 3 4 3 5% 6% 50%	2400
	21st to end	Night	11% 6% 2 6% 4 4 4 4 6% 6% 48%	2560
December	1st to 10th	Night	9% 5% 3 5% 3 3 3 3 4 6% 56%	4250
	11th to 20th	Night	13% 8% 3 8% 6% 4 3 3 4 6% 43%	4822
	21st to end	Night	15% 7% 2 6% 5% 4 4 4 4 6% 43%	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	31% 5% 7% 18% 31% 8%	4057
	11th to 20th	Day	37% 4 7% 17% 27% 9%	4060
	21st to end	Day	40% 5% 7% 19% 20% 9%	4466
February	1st to 10th	Day	40% 6% 11% 18% 19% 6%	1800
	11th to 20th	Day	41% 5% 9% 15% 23% 7%	1935
	21st to end	Day	37% 6% 9% 19% 17% 10%	1802
March	1st to 10th	Day	45% 7% 8% 14% 17% 8%	4935
	11th to 20th	Day	25% 7% 12% 16% 21% 19%	4980
	21st to end	Day	36% 7% 9% 20% 18% 10%	5465
April	1st to 10th	Day	29% 7% 15% 22% 19% 9%	2213
	11th to 20th	Day	28% 6% 12% 25% 19% 9%	2374
	21st to end	Day	22% 7% 16% 31% 20% 5%	2400
May	1st to 10th	Day	28% 11% 20% 28% 10% 3	4184
	11th to 20th	Day	38% 8% 16% 26% 11% 1	4085
	21st to end	Day	23% 10% 21% 27% 18%	4489
June	1st to 10th	Day	36% 15% 19% 18% 10%	2470
	11th to 20th	Day	31% 12% 22% 27% 8%	2470
	21st to end	Day	32% 18% 24% 19% 7% 1	2470
July	1st to 10th	Day	41% 14% 21% 20% 4	4055
	11th to 20th	Day	33% 18% 36% 10% 3	4051
	21st to end	Day	36% 17% 30% 15% 2	4324
August	1st to 10th	Day	48% 12% 26% 11% 2	4091
	11th to 20th	Day	32% 14% 24% 23% 8%	3964
	21st to end	Day	37% 11% 19% 30% 3	4180
September	1st to 10th	Day	39% 13% 20% 15% 11% 3	2157
	11th to 20th	Day	41% 12% 17% 18% 11% 2	2090
	21st to end	Day	39% 9% 15% 20% 13% 3	2090
October	1st to 10th	Day	41% 6% 8% 13% 25% 7%	3226
	11th to 20th	Day	16% 4 7% 14% 31% 27%	3140
	21st to end	Day	22% 6% 8% 18% 38% 7%	3428
November	1st to 10th	Day	32% 6% 11% 17% 24% 10%	1957
	11th to 20th	Day	35% 5% 10% 17% 23% 10%	2000
	21st to end	Day	34% 6% 10% 21% 22% 7%	1835
December	1st to 10th	Day	30% 6% 10% 18% 21% 14%	2930
	11th to 20th	Day	37% 7% 10% 21% 21% 4	3370
	21st to end	Day	43% 6% 7% 22% 19% 3	4466
January	1st to 10th	Night	29% 5% 7% 14% 36% 9%	5889
	11th to 20th	Night	40% 5% 7% 16% 26% 6%	5900
	21st to end	Night	38% 6% 9% 13% 24% 11%	6490
February	1st to 10th	Night	40% 5% 9% 21% 18% 7%	2600
	11th to 20th	Night	41% 6% 10% 15% 19% 9%	2359
	21st to end	Night	41% 5% 10% 18% 15% 12%	1825
March	1st to 10th	Night	44% 6% 9% 14% 18% 10%	4918
	11th to 20th	Night	29% 6% 10% 15% 25% 16%	4980
	21st to end	Night	33% 6% 11% 18% 20% 11%	5467
April	1st to 10th	Night	31% 7% 13% 21% 19% 9%	2185
	11th to 20th	Night	31% 5% 10% 23% 21% 11%	1975
	21st to end	Night	26% 8% 13% 25% 22% 5%	2000
May	1st to 10th	Night	24% 11% 20% 29% 13% 3	3011
	11th to 20th	Night	35% 11% 15% 24% 14% 1	2819
	21st to end	Night	25% 10% 17% 29% 17% 1	3087
June	1st to 10th	Night	37% 14% 21% 19% 9%	1710
	11th to 20th	Night	34% 13% 21% 25% 7%	1710
	21st to end	Night	34% 17% 25% 20% 4	1710
July	1st to 10th	Night	41% 12% 21% 21% 4 1	2795
	11th to 20th	Night	36% 17% 34% 10% 3	2786
	21st to end	Night	40% 16% 25% 16% 1	2988
August	1st to 10th	Night	48% 14% 25% 11% 2	2814
	11th to 20th	Night	32% 15% 24% 23% 6%	2898
	21st to end	Night	36% 11% 17% 33% 3	3476
September	1st to 10th	Night	39% 14% 19% 16% 11% 1	2019
	11th to 20th	Night	42% 13% 15% 18% 11% 1	2090
	21st to end	Night	41% 10% 15% 19% 13% 2	2090
October	1st to 10th	Night	37% 6% 10% 15% 23% 8%	3732
	11th to 20th	Night	16% 5% 5% 14% 28% 31%	3779
	21st to end	Night	23% 6% 7% 19% 31% 14%	4120
November	1st to 10th	Night	31% 6% 11% 18% 23% 11%	2341
	11th to 20th	Night	35% 6% 8% 18% 24% 10%	2400
	21st to end	Night	36% 6% 10% 18% 22% 9%	2560
December	1st to 10th	Night	30% 5% 9% 17% 26% 13%	4250
	11th to 20th	Night	42% 6% 11% 14% 20% 7%	4824
	21st to end	Night	41% 7% 8% 22% 20% 3	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	12% 5% 33% 42% 7%	300
	11th to 20th	Day	10% 6% 26% 41% 14% 3	300
	21st to end	Day	19% 8% 37% 27% 9%	330
February	1st to 10th	Day	16% 8% 23% 39% 14%	300
	11th to 20th	Day	24% 7% 26% 33% 10%	300
	21st to end	Day	10% 6% 34% 38% 11%	401
March	1st to 10th	Day	13% 7% 30% 30% 18% 1	500
	11th to 20th	Day	13% 4 29% 39% 14% 2	500
	21st to end	Day	12% 6% 30% 33% 17% 2	550
April	1st to 10th	Day	9% 8% 26% 38% 18% 2	500
	11th to 20th	Day	8% 5% 23% 31% 25% 7%	492
	21st to end	Day	4 3 16% 35% 32% 9%	500
May	1st to 10th	Day	6% 3 22% 38% 23% 9%	503
	11th to 20th	Day	10% 3 13% 29% 34% 11%	487
	21st to end	Day	11% 2 8% 26% 38% 16%	529
June	1st to 10th	Day	9% 1 9% 29% 40% 12%	490
	11th to 20th	Day	8% 2 8% 22% 39% 20%	490
	21st to end	Day	6% 1 6% 25% 43% 19%	490
July	1st to 10th	Day	7% 2 5% 15% 45% 27%	485
	11th to 20th	Day	7% 2 3 11% 47% 29%	484
	21st to end	Day	7% 2 4 17% 53% 17%	532
August	1st to 10th	Day	13% 1 4 17% 43% 22%	486
	11th to 20th	Day	7% 1 5% 13% 45% 29%	479
	21st to end	Day	14% 3 6% 16% 37% 24%	539
September	1st to 10th	Day	13% 2 6% 20% 38% 20%	490
	11th to 20th	Day	14% 3 10% 21% 40% 11%	490
	21st to end	Day	15% 2 7% 24% 36% 16%	490
October	1st to 10th	Day	14% 5% 14% 30% 27% 10%	490
	11th to 20th	Day	9% 2 15% 33% 31% 10%	490
	21st to end	Day	11% 5% 14% 29% 33% 8%	539
November	1st to 10th	Day	14% 5% 17% 31% 25% 8%	495
	11th to 20th	Day	14% 6% 26% 33% 17% 3	500
	21st to end	Day	12% 6% 22% 34% 23% 3	335
December	1st to 10th	Day	16% 3 24% 37% 19% 1	300
	11th to 20th	Day	16% 6% 26% 35% 14% 4	300
	21st to end	Day	20% 7% 22% 32% 18% 2	330
January	1st to 10th	Night	12% 10% 34% 35% 9% 1	800
	11th to 20th	Night	13% 9% 31% 32% 14% 1	800
	21st to end	Night	17% 9% 35% 33% 6%	880
February	1st to 10th	Night	17% 7% 27% 35% 14%	800
	11th to 20th	Night	18% 9% 29% 35% 8%	783
	21st to end	Night	14% 6% 27% 41% 11% 1	660
March	1st to 10th	Night	11% 6% 30% 37% 15% 1	800
	11th to 20th	Night	12% 7% 26% 39% 15% 1	800
	21st to end	Night	11% 8% 27% 35% 18% 2	880
April	1st to 10th	Night	9% 8% 27% 34% 21% 2	800
	11th to 20th	Night	11% 6% 21% 35% 23% 5%	791
	21st to end	Night	8% 4 20% 35% 29% 5%	800
May	1st to 10th	Night	7% 3 21% 36% 25% 8%	655
	11th to 20th	Night	9% 4 17% 32% 29% 9%	563
	21st to end	Night	10% 1 9% 24% 41% 16%	617
June	1st to 10th	Night	10% 2 8% 31% 38% 11%	570
	11th to 20th	Night	7% 2 8% 23% 41% 19%	570
	21st to end	Night	6% 2 7% 27% 43% 15%	570
July	1st to 10th	Night	9% 2 5% 20% 48% 16%	555
	11th to 20th	Night	10% 1 3 14% 53% 20%	552
	21st to end	Night	6% 1 4 25% 52% 12%	624
August	1st to 10th	Night	11% 2 4 20% 46% 16%	567
	11th to 20th	Night	8% 3 5% 20% 47% 18%	611
	21st to end	Night	13% 1 7% 17% 39% 22%	836
September	1st to 10th	Night	11% 2 8% 21% 42% 16%	760
	11th to 20th	Night	14% 3 8% 22% 43% 10%	760
	21st to end	Night	16% 2 10% 21% 40% 11%	760
October	1st to 10th	Night	14% 3 14% 31% 29% 8%	760
	11th to 20th	Night	10% 3 18% 32% 29% 8%	760
	21st to end	Night	10% 5% 19% 29% 31% 6%	836
November	1st to 10th	Night	13% 4 20% 31% 28% 4	780
	11th to 20th	Night	16% 7% 24% 32% 20% 3	800
	21st to end	Night	13% 8% 23% 33% 21% 3	800
December	1st to 10th	Night	15% 8% 26% 35% 14% 3	800
	11th to 20th	Night	18% 8% 26% 29% 17% 3	800
	21st to end	Night	20% 8% 27% 32% 14% 1	880

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