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 2026-09-14.

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 6% 4 4 5% 4 6% 9% 44%	148643
	Night	10% 6% 2 6% 4 4 5% 4 6% 9% 44%	142454
Transparency	Day	34% 9% 18% 20% 14% 5%	152275
	Night	35% 8% 15% 19% 17% 6%	146894
Seeing	Day	11% 4 16% 28% 29% 11%	18090
	Night	12% 5% 19% 30% 27% 7%	28137

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	9% 4 3 7% 4 4 4 2 5% 6% 50%	5258
	11th to 20th	Day	14% 5% 2 6% 3 4 3 3 5% 7% 48%	5280
	21st to end	Day	14% 9% 3 7% 4 3 4 2 4 7% 43%	5780
February	1st to 10th	Day	13% 7% 2 6% 4 5% 4 3 5% 7% 43%	1890
	11th to 20th	Day	18% 7% 3 5% 3 3 3 4 4 6% 45%	2024
	21st to end	Day	12% 7% 2 7% 5% 4 3 3 5% 5% 46%	1890
March	1st to 10th	Day	19% 9% 3 7% 4 4 3 3 5% 6% 38%	5045
	11th to 20th	Day	7% 4 2 4 3 4 4 3 6% 8% 56%	5090
	21st to end	Day	12% 6% 2 5% 4 4 4 3 4 5% 50%	5586
April	1st to 10th	Day	7% 4 2 5% 4 4 5% 3 6% 8% 52%	2324
	11th to 20th	Day	8% 4 2 5% 3 3 4 4 6% 8% 54%	2494
	21st to end	Day	5% 4 1 4 4 3 4 4 5% 9% 57%	2520
May	1st to 10th	Day	12% 6% 2 6% 4 4 5% 4 6% 9% 41%	7497
	11th to 20th	Day	12% 6% 2 5% 4 4 4 4 7% 9% 44%	7626
	21st to end	Day	7% 4 2 6% 4 5% 6% 5% 7% 11% 42%	8391
June	1st to 10th	Day	8% 6% 2 7% 6% 5% 6% 5% 9% 10% 36%	2730
	11th to 20th	Day	6% 5% 2 5% 5% 5% 7% 5% 7% 11% 42%	2730
	21st to end	Day	5% 4 2 5% 5% 5% 6% 6% 10% 13% 38%	2730
July	1st to 10th	Day	7% 4 2 6% 5% 5% 7% 6% 9% 12% 37%	7680
	11th to 20th	Day	4 3 1 6% 5% 5% 5% 5% 9% 15% 42%	7680
	21st to end	Day	4 4 2 6% 5% 5% 6% 6% 8% 15% 40%	8340
August	1st to 10th	Day	16% 7% 3 9% 6% 6% 6% 5% 9% 12% 23%	6014
	11th to 20th	Day	4 3 2 5% 4 4 5% 5% 8% 13% 47%	5773
	21st to end	Day	10% 6% 2 7% 5% 5% 5% 5% 8% 10% 37%	6080
September	1st to 10th	Day	11% 5% 3 6% 5% 4 4 4 7% 11% 40%	2384
	11th to 20th	Day	9% 7% 2 6% 4 5% 6% 4 8% 9% 38%	2233
	21st to end	Day	14% 6% 3 6% 4 5% 5% 4 6% 8% 41%	2200
October	1st to 10th	Day	12% 6% 2 6% 5% 4 5% 4 6% 5% 46%	2042
	11th to 20th	Day	9% 3 1 5% 2 3 3 3 4 6% 60%	2000
	21st to end	Day	10% 6% 2 5% 3 3 3 3 4 6% 56%	2200
November	1st to 10th	Day	10% 5% 1 6% 3 4 4 4 4 6% 53%	2057
	11th to 20th	Day	10% 5% 2 6% 4 4 3 2 4 7% 52%	2100
	21st to end	Day	11% 6% 1 5% 3 3 4 3 5% 7% 52%	1927
December	1st to 10th	Day	8% 5% 2 4 4 4 4 3 5% 6% 54%	3020
	11th to 20th	Day	12% 5% 2 7% 5% 3 3 3 5% 6% 47%	3463
	21st to end	Day	16% 9% 2 6% 4 3 4 3 5% 6% 41%	4565
January	1st to 10th	Night	8% 5% 2 6% 4 4 4 3 5% 6% 53%	7635
	11th to 20th	Night	11% 6% 2 6% 4 4 4 4 5% 7% 47%	7680
	21st to end	Night	14% 8% 3 7% 4 4 3 3 5% 6% 44%	8401
February	1st to 10th	Night	14% 6% 2 6% 5% 4 4 3 5% 6% 45%	2730
	11th to 20th	Night	15% 8% 3 5% 4 4 4 3 6% 6% 42%	2479
	21st to end	Night	12% 7% 3 7% 4 5% 4 4 5% 6% 42%	1913
March	1st to 10th	Night	15% 9% 3 7% 4 3 3 3 4 5% 43%	5028
	11th to 20th	Night	9% 4 2 5% 3 3 4 4 6% 9% 51%	5090
	21st to end	Night	12% 5% 2 5% 3 3 4 3 5% 7% 51%	5588
April	1st to 10th	Night	8% 5% 2 5% 4 4 4 4 6% 8% 50%	2294
	11th to 20th	Night	10% 3 2 5% 4 4 4 3 5% 7% 54%	2075
	21st to end	Night	6% 4 2 4 3 4 4 4 7% 7% 54%	2100
May	1st to 10th	Night	9% 7% 3 6% 4 4 5% 4 7% 9% 42%	5398
	11th to 20th	Night	11% 6% 2 5% 3 4 4 4 7% 10% 43%	5234
	21st to end	Night	9% 5% 2 6% 5% 5% 6% 5% 8% 10% 39%	5761
June	1st to 10th	Night	8% 7% 2 5% 6% 5% 6% 5% 9% 12% 35%	1890
	11th to 20th	Night	6% 5% 2 6% 5% 6% 6% 6% 9% 12% 37%	1890
	21st to end	Night	4 4 2 6% 5% 6% 7% 6% 9% 14% 36%	1890
July	1st to 10th	Night	6% 4 2 6% 5% 5% 7% 6% 10% 13% 34%	5278
	11th to 20th	Night	4 4 2 6% 5% 6% 6% 6% 9% 14% 37%	5277
	21st to end	Night	5% 4 2 7% 5% 6% 7% 6% 10% 15% 34%	5736
August	1st to 10th	Night	13% 7% 2 7% 5% 5% 6% 6% 9% 14% 24%	4135
	11th to 20th	Night	5% 3 2 5% 4 5% 6% 5% 9% 15% 41%	4212
	21st to end	Night	7% 6% 3 9% 5% 6% 6% 5% 8% 10% 36%	5045
September	1st to 10th	Night	9% 5% 3 6% 5% 5% 6% 5% 7% 11% 38%	2231
	11th to 20th	Night	11% 6% 2 6% 5% 6% 6% 5% 6% 10% 37%	2234
	21st to end	Night	13% 6% 3 7% 4 4 5% 4 6% 9% 39%	2200
October	1st to 10th	Night	12% 7% 3 6% 5% 4 4 3 5% 8% 44%	2356
	11th to 20th	Night	9% 3 1 4 3 3 3 4 5% 6% 58%	2400
	21st to end	Night	9% 4 1 6% 4 3 3 3 4 5% 57%	2640
November	1st to 10th	Night	10% 5% 2 5% 4 3 3 3 5% 7% 54%	2461
	11th to 20th	Night	12% 7% 2 5% 4 3 3 2 5% 6% 50%	2520
	21st to end	Night	11% 6% 2 6% 4 4 4 4 6% 6% 47%	2688
December	1st to 10th	Night	10% 6% 3 5% 3 3 3 3 4 6% 55%	4380
	11th to 20th	Night	13% 7% 3 8% 6% 4 3 3 4 6% 44%	4952
	21st to end	Night	15% 7% 2 6% 4 4 4 4 5% 6% 43%	6633

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	33% 5% 10% 21% 25% 6%	5259
	11th to 20th	Day	36% 4 6% 16% 26% 12%	5280
	21st to end	Day	40% 5% 9% 19% 18% 9%	5780
February	1st to 10th	Day	40% 6% 10% 18% 20% 6%	1890
	11th to 20th	Day	41% 5% 9% 17% 22% 7%	2034
	21st to end	Day	38% 6% 9% 19% 17% 11%	1890
March	1st to 10th	Day	45% 7% 9% 15% 17% 8%	5045
	11th to 20th	Day	25% 7% 12% 17% 21% 18%	5090
	21st to end	Day	35% 7% 10% 20% 18% 10%	5586
April	1st to 10th	Day	29% 8% 15% 22% 18% 8%	2324
	11th to 20th	Day	27% 6% 11% 26% 20% 9%	2494
	21st to end	Day	24% 7% 15% 31% 20% 4	2520
May	1st to 10th	Day	37% 9% 19% 24% 10% 2	7531
	11th to 20th	Day	34% 8% 20% 25% 12% 1	7630
	21st to end	Day	33% 12% 21% 19% 14%	8405
June	1st to 10th	Day	36% 15% 19% 18% 11% 1	2730
	11th to 20th	Day	32% 12% 24% 25% 7%	2730
	21st to end	Day	30% 17% 26% 20% 6% 1	2730
July	1st to 10th	Day	35% 15% 30% 18% 3	7615
	11th to 20th	Day	29% 17% 40% 12% 2	7611
	21st to end	Day	30% 14% 31% 21% 3	8156
August	1st to 10th	Day	51% 13% 24% 10% 1	6001
	11th to 20th	Day	25% 13% 29% 28% 6%	5812
	21st to end	Day	39% 12% 21% 24% 3	6080
September	1st to 10th	Day	36% 12% 21% 17% 11% 3	2384
	11th to 20th	Day	39% 12% 17% 20% 11% 2	2233
	21st to end	Day	40% 9% 15% 20% 13% 3	2200
October	1st to 10th	Day	41% 6% 8% 13% 25% 6%	3328
	11th to 20th	Day	16% 5% 7% 14% 31% 27%	3240
	21st to end	Day	22% 6% 8% 18% 38% 7%	3538
November	1st to 10th	Day	32% 6% 11% 16% 25% 11%	2057
	11th to 20th	Day	33% 5% 10% 19% 23% 10%	2100
	21st to end	Day	33% 6% 11% 21% 22% 7%	1927
December	1st to 10th	Day	31% 6% 10% 18% 21% 14%	3020
	11th to 20th	Day	36% 7% 10% 21% 22% 5%	3460
	21st to end	Day	42% 6% 7% 23% 19% 3	4565
January	1st to 10th	Night	31% 6% 9% 18% 29% 7%	7636
	11th to 20th	Night	36% 4 7% 15% 28% 10%	7680
	21st to end	Night	41% 5% 9% 14% 21% 10%	8401
February	1st to 10th	Night	40% 5% 8% 21% 19% 7%	2730
	11th to 20th	Night	41% 6% 10% 15% 19% 9%	2480
	21st to end	Night	42% 5% 9% 17% 15% 12%	1913
March	1st to 10th	Night	43% 6% 9% 14% 18% 10%	5028
	11th to 20th	Night	28% 6% 10% 15% 25% 16%	5090
	21st to end	Night	33% 6% 11% 19% 20% 11%	5588
April	1st to 10th	Night	31% 7% 13% 22% 18% 8%	2294
	11th to 20th	Night	30% 5% 9% 24% 21% 11%	2075
	21st to end	Night	26% 8% 13% 25% 23% 5%	2100
May	1st to 10th	Night	36% 9% 18% 25% 10% 2	5426
	11th to 20th	Night	34% 9% 20% 23% 12% 2	5235
	21st to end	Night	36% 12% 18% 22% 12% 1	5771
June	1st to 10th	Night	38% 14% 20% 18% 10% 1	1890
	11th to 20th	Night	36% 13% 21% 24% 6%	1890
	21st to end	Night	33% 17% 26% 20% 5%	1890
July	1st to 10th	Night	35% 14% 29% 19% 3	5235
	11th to 20th	Night	34% 15% 37% 12% 2	5226
	21st to end	Night	34% 14% 28% 21% 3	5606
August	1st to 10th	Night	46% 16% 27% 10% 1	4124
	11th to 20th	Night	27% 15% 27% 26% 5%	4239
	21st to end	Night	40% 12% 19% 27% 2	5045
September	1st to 10th	Night	37% 13% 19% 18% 11% 1	2231
	11th to 20th	Night	40% 13% 15% 19% 11% 1	2234
	21st to end	Night	41% 10% 16% 20% 12% 2	2200
October	1st to 10th	Night	38% 6% 10% 15% 23% 8%	3850
	11th to 20th	Night	17% 5% 5% 14% 29% 30%	3899
	21st to end	Night	23% 6% 7% 18% 31% 14%	4252
November	1st to 10th	Night	30% 6% 11% 18% 23% 12%	2461
	11th to 20th	Night	34% 6% 8% 18% 24% 10%	2520
	21st to end	Night	36% 6% 10% 17% 21% 9%	2688
December	1st to 10th	Night	32% 5% 9% 17% 25% 12%	4380
	11th to 20th	Night	42% 6% 10% 15% 20% 7%	4954
	21st to end	Night	40% 7% 8% 22% 20% 3	6633

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% 31% 43% 9%	330
	11th to 20th	Day	10% 7% 27% 39% 14% 3	330
	21st to end	Day	19% 9% 36% 28% 8%	363
February	1st to 10th	Day	15% 8% 26% 38% 13%	330
	11th to 20th	Day	22% 7% 28% 33% 10%	330
	21st to end	Day	12% 7% 33% 36% 11%	433
March	1st to 10th	Day	12% 6% 28% 30% 19% 3	540
	11th to 20th	Day	12% 5% 28% 39% 14% 2	540
	21st to end	Day	11% 6% 29% 33% 18% 3	594
April	1st to 10th	Day	10% 8% 24% 37% 19% 3	540
	11th to 20th	Day	8% 5% 23% 31% 25% 8%	532
	21st to end	Day	5% 4 16% 36% 31% 9%	540
May	1st to 10th	Day	7% 3 22% 38% 21% 8%	543
	11th to 20th	Day	11% 4 13% 29% 33% 11%	567
	21st to end	Day	11% 2 9% 26% 37% 15%	617
June	1st to 10th	Day	9% 2 10% 29% 39% 12%	570
	11th to 20th	Day	8% 3 8% 23% 40% 20%	570
	21st to end	Day	5% 1 5% 24% 45% 19%	570
July	1st to 10th	Day	6% 2 5% 15% 46% 27%	565
	11th to 20th	Day	6% 2 3 10% 48% 30%	564
	21st to end	Day	6% 2 3 18% 52% 19%	620
August	1st to 10th	Day	14% 3 5% 16% 42% 21%	566
	11th to 20th	Day	6% 1 4 12% 44% 32%	559
	21st to end	Day	14% 3 6% 18% 36% 22%	627
September	1st to 10th	Day	11% 2 7% 22% 40% 18%	570
	11th to 20th	Day	13% 3 10% 22% 39% 13%	542
	21st to end	Day	16% 2 7% 24% 36% 15%	530
October	1st to 10th	Day	15% 5% 14% 31% 27% 9%	530
	11th to 20th	Day	10% 2 15% 33% 32% 9%	530
	21st to end	Day	11% 5% 16% 30% 31% 7%	583
November	1st to 10th	Day	13% 5% 17% 32% 25% 8%	535
	11th to 20th	Day	13% 6% 26% 34% 18% 3	540
	21st to end	Day	11% 5% 23% 36% 23% 2	367
December	1st to 10th	Day	19% 3 23% 37% 18% 1	330
	11th to 20th	Day	15% 6% 27% 33% 16% 4	330
	21st to end	Day	18% 6% 21% 33% 19% 2	363
January	1st to 10th	Night	12% 10% 33% 35% 10% 1	840
	11th to 20th	Night	13% 9% 32% 31% 14% 1	840
	21st to end	Night	17% 9% 36% 32% 6%	924
February	1st to 10th	Night	16% 7% 28% 35% 13%	840
	11th to 20th	Night	17% 9% 29% 36% 8%	823
	21st to end	Night	15% 7% 27% 40% 10% 1	692
March	1st to 10th	Night	11% 6% 29% 36% 17% 1	840
	11th to 20th	Night	12% 8% 26% 40% 14% 1	840
	21st to end	Night	10% 8% 26% 35% 19% 2	924
April	1st to 10th	Night	9% 8% 26% 33% 21% 3	840
	11th to 20th	Night	11% 5% 20% 35% 24% 5%	831
	21st to end	Night	8% 4 19% 35% 30% 5%	840
May	1st to 10th	Night	8% 3 21% 36% 24% 8%	688
	11th to 20th	Night	8% 4 17% 33% 30% 8%	621
	21st to end	Night	10% 1 10% 24% 41% 15%	683
June	1st to 10th	Night	10% 3 8% 30% 38% 11%	630
	11th to 20th	Night	7% 2 7% 24% 42% 18%	630
	21st to end	Night	5% 2 6% 26% 45% 16%	630
July	1st to 10th	Night	8% 2 5% 20% 48% 16%	615
	11th to 20th	Night	9% 1 3 13% 52% 22%	612
	21st to end	Night	6% 1 4 25% 51% 13%	690
August	1st to 10th	Night	11% 3 5% 20% 46% 16%	627
	11th to 20th	Night	7% 3 4 18% 46% 21%	676
	21st to end	Night	13% 2 7% 18% 39% 20%	924
September	1st to 10th	Night	10% 2 7% 22% 43% 16%	840
	11th to 20th	Night	13% 3 7% 23% 43% 11%	813
	21st to end	Night	17% 2 9% 21% 40% 11%	800
October	1st to 10th	Night	15% 3 14% 31% 29% 8%	800
	11th to 20th	Night	11% 3 17% 32% 29% 8%	800
	21st to end	Night	10% 5% 21% 29% 30% 6%	880
November	1st to 10th	Night	12% 5% 19% 33% 27% 4	820
	11th to 20th	Night	15% 7% 23% 32% 20% 3	840
	21st to end	Night	12% 8% 23% 33% 21% 3	840
December	1st to 10th	Night	16% 8% 26% 34% 14% 2	840
	11th to 20th	Night	17% 8% 25% 30% 17% 3	840
	21st to end	Night	19% 8% 26% 33% 15% 1	924

Page generated at 2026-09-11 04:11:57. Version ac07

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