Sand Island Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sand Island Observatory clear sky chart from 2005-11-06 to 2026-02-16.

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% 5% 5% 5% 4 6% 8% 43%	130948
	Night	12% 6% 2 6% 4 4 4 4 6% 8% 44%	129913
Transparency	Day	37% 6% 12% 20% 19% 6%	134766
	Night	38% 5% 11% 19% 20% 8%	134157
Seeing	Day	13% 9% 21% 25% 22% 11%	16101
	Night	13% 7% 21% 31% 23% 5%	26118

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	15% 10% 3 7% 3 4 4 3 5% 5% 42%	5295
	11th to 20th	Day	9% 5% 4 5% 3 3 4 3 5% 6% 53%	5880
	21st to end	Day	21% 10% 3 8% 4 4 4 3 5% 5% 34%	6436
February	1st to 10th	Day	15% 8% 3 6% 5% 4 4 4 6% 6% 40%	2100
	11th to 20th	Day	18% 7% 2 6% 4 4 4 3 5% 4 42%	1998
	21st to end	Day	16% 7% 2 7% 4 5% 5% 2 5% 6% 40%	1650
March	1st to 10th	Day	17% 9% 3 7% 4 4 5% 4 6% 6% 36%	4480
	11th to 20th	Day	10% 5% 2 5% 4 4 4 4 5% 7% 49%	4697
	21st to end	Day	13% 7% 3 7% 5% 5% 5% 3 5% 5% 41%	5875
April	1st to 10th	Day	12% 6% 2 5% 5% 4 4 5% 5% 7% 45%	2400
	11th to 20th	Day	11% 6% 2 6% 5% 5% 4 3 5% 7% 46%	2374
	21st to end	Day	7% 4 2 5% 5% 5% 5% 2 5% 8% 53%	2400
May	1st to 10th	Day	12% 7% 2 7% 5% 5% 5% 3 6% 8% 40%	5532
	11th to 20th	Day	11% 6% 2 7% 5% 5% 5% 4 8% 10% 36%	5861
	21st to end	Day	8% 4 2 6% 5% 5% 5% 5% 7% 10% 44%	6433
June	1st to 10th	Day	8% 6% 3 8% 6% 6% 6% 5% 7% 10% 35%	2600
	11th to 20th	Day	5% 4 2 6% 6% 6% 6% 5% 8% 12% 39%	2600
	21st to end	Day	6% 5% 2 6% 6% 6% 6% 5% 9% 12% 36%	2600
July	1st to 10th	Day	5% 5% 2 7% 6% 6% 7% 7% 10% 14% 32%	5900
	11th to 20th	Day	5% 4 2 5% 5% 6% 7% 6% 10% 13% 38%	5864
	21st to end	Day	5% 5% 2 7% 6% 7% 8% 7% 10% 14% 28%	5922
August	1st to 10th	Day	3 3 1 5% 4 5% 6% 5% 7% 10% 52%	3905
	11th to 20th	Day	6% 4 2 5% 4 5% 5% 4 7% 10% 49%	3849
	21st to end	Day	6% 4 2 5% 5% 6% 6% 4 7% 10% 46%	4312
September	1st to 10th	Day	10% 5% 2 7% 6% 6% 5% 4 6% 8% 40%	2376
	11th to 20th	Day	12% 6% 2 6% 5% 4 5% 4 5% 7% 44%	2200
	21st to end	Day	8% 5% 2 5% 5% 4 4 3 5% 7% 52%	2200
October	1st to 10th	Day	8% 5% 2 5% 4 4 4 3 5% 6% 53%	2200
	11th to 20th	Day	9% 4 2 4 3 3 4 3 5% 6% 57%	2197
	21st to end	Day	12% 5% 2 5% 3 3 3 3 4 6% 55%	2132
November	1st to 10th	Day	11% 5% 2 5% 3 3 3 3 4 5% 55%	1852
	11th to 20th	Day	12% 7% 2 4 3 3 3 3 4 6% 54%	1890
	21st to end	Day	13% 6% 2 5% 4 4 3 3 5% 8% 48%	1890
December	1st to 10th	Day	16% 7% 2 5% 3 3 3 3 4 6% 46%	3020
	11th to 20th	Day	10% 8% 3 7% 4 4 3 3 5% 5% 49%	3463
	21st to end	Day	17% 8% 3 5% 3 4 4 2 4 5% 45%	4565
January	1st to 10th	Night	16% 8% 4 6% 4 4 4 3 4 5% 43%	7595
	11th to 20th	Night	14% 8% 3 7% 4 3 3 2 3 5% 48%	7080
	21st to end	Night	20% 10% 3 8% 4 4 3 3 4 5% 37%	7745
February	1st to 10th	Night	14% 8% 3 6% 4 4 4 3 5% 6% 43%	2520
	11th to 20th	Night	20% 9% 3 5% 3 3 3 2 4 4 45%	2399
	21st to end	Night	16% 8% 3 7% 4 4 3 3 5% 6% 42%	1980
March	1st to 10th	Night	20% 9% 3 7% 4 4 4 4 5% 6% 37%	5373
	11th to 20th	Night	13% 6% 2 5% 4 3 4 3 5% 6% 48%	5261
	21st to end	Night	13% 6% 2 7% 4 4 4 3 5% 6% 46%	5082
April	1st to 10th	Night	11% 6% 2 5% 5% 3 4 4 6% 8% 46%	2000
	11th to 20th	Night	12% 5% 2 5% 3 4 4 4 6% 7% 48%	1975
	21st to end	Night	7% 4 1 5% 4 3 4 3 6% 8% 55%	2000
May	1st to 10th	Night	12% 7% 3 7% 5% 5% 4 4 6% 8% 40%	4362
	11th to 20th	Night	13% 7% 3 6% 5% 5% 5% 4 7% 8% 38%	4032
	21st to end	Night	7% 5% 2 5% 4 5% 5% 5% 7% 12% 41%	4421
June	1st to 10th	Night	6% 6% 2 6% 6% 6% 6% 6% 9% 12% 36%	1800
	11th to 20th	Night	5% 4 2 4 5% 5% 5% 6% 10% 12% 43%	1800
	21st to end	Night	5% 5% 3 8% 7% 5% 6% 5% 8% 11% 38%	1800
July	1st to 10th	Night	6% 6% 2 7% 5% 6% 7% 6% 9% 14% 31%	4058
	11th to 20th	Night	5% 4 2 6% 6% 6% 6% 6% 11% 16% 33%	4075
	21st to end	Night	5% 7% 3 9% 8% 6% 8% 6% 10% 11% 27%	4929
August	1st to 10th	Night	2 3 2 5% 4 5% 5% 5% 8% 13% 48%	3244
	11th to 20th	Night	5% 4 2 5% 4 6% 6% 5% 8% 11% 46%	3200
	21st to end	Night	5% 5% 2 6% 5% 5% 6% 5% 8% 9% 44%	3586
September	1st to 10th	Night	9% 6% 3 7% 5% 5% 5% 5% 7% 9% 40%	2002
	11th to 20th	Night	11% 5% 2 5% 3 5% 5% 4 7% 8% 44%	2200
	21st to end	Night	7% 5% 2 7% 4 4 4 3 6% 8% 50%	2200
October	1st to 10th	Night	8% 4 2 5% 4 3 4 3 6% 7% 53%	2200
	11th to 20th	Night	9% 4 2 4 3 2 3 3 4 6% 61%	2200
	21st to end	Night	11% 5% 2 5% 3 3 3 2 4 5% 58%	2703
November	1st to 10th	Night	9% 5% 2 5% 3 3 4 3 4 5% 58%	2666
	11th to 20th	Night	12% 5% 2 4 3 3 3 3 4 6% 56%	2730
	21st to end	Night	12% 7% 2 6% 3 3 5% 3 4 7% 49%	2730
December	1st to 10th	Night	15% 7% 2 5% 3 3 3 3 5% 5% 50%	4380
	11th to 20th	Night	14% 7% 2 6% 3 3 3 3 4 6% 50%	4952
	21st to end	Night	17% 8% 2 6% 4 3 3 3 4 6% 45%	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	45% 2 6% 25% 18% 3	5296
	11th to 20th	Day	33% 3 6% 21% 29% 8%	5880
	21st to end	Day	53% 2 6% 12% 16% 11%	6436
February	1st to 10th	Day	43% 4 7% 16% 22% 8%	2100
	11th to 20th	Day	45% 3 5% 16% 23% 8%	2008
	21st to end	Day	45% 5% 7% 16% 16% 11%	1650
March	1st to 10th	Day	47% 4 7% 14% 20% 7%	4480
	11th to 20th	Day	35% 4 5% 18% 19% 19%	4697
	21st to end	Day	44% 4 9% 15% 20% 8%	5877
April	1st to 10th	Day	38% 5% 10% 20% 19% 9%	2400
	11th to 20th	Day	37% 5% 9% 18% 22% 9%	2374
	21st to end	Day	30% 6% 11% 21% 27% 5%	2400
May	1st to 10th	Day	42% 6% 11% 24% 15% 2	5567
	11th to 20th	Day	41% 6% 19% 22% 12% 1	5865
	21st to end	Day	33% 7% 15% 26% 18% 1	6447
June	1st to 10th	Day	41% 10% 12% 23% 12% 2	2600
	11th to 20th	Day	34% 10% 17% 23% 14% 3	2600
	21st to end	Day	37% 9% 18% 21% 11% 2	2600
July	1st to 10th	Day	38% 9% 29% 17% 7%	5835
	11th to 20th	Day	34% 10% 31% 22% 3	5795
	21st to end	Day	42% 12% 22% 22% 2	5752
August	1st to 10th	Day	25% 8% 14% 30% 22% 1	3893
	11th to 20th	Day	30% 8% 15% 31% 15% 2	3885
	21st to end	Day	33% 7% 20% 22% 16% 2	4311
September	1st to 10th	Day	40% 9% 13% 17% 16% 6%	2376
	11th to 20th	Day	39% 7% 11% 19% 19% 5%	2200
	21st to end	Day	32% 5% 10% 22% 23% 8%	2200
October	1st to 10th	Day	27% 3 6% 12% 29% 23%	3590
	11th to 20th	Day	18% 3 4 16% 28% 31%	3553
	21st to end	Day	23% 2 5% 16% 42% 13%	3422
November	1st to 10th	Day	31% 4 6% 18% 28% 13%	1852
	11th to 20th	Day	32% 3 7% 18% 27% 12%	1890
	21st to end	Day	36% 4 9% 18% 24% 9%	1890
December	1st to 10th	Day	41% 3 5% 19% 28% 4	3020
	11th to 20th	Day	38% 3 9% 21% 23% 6%	3460
	21st to end	Day	42% 3 5% 22% 24% 3	4565
January	1st to 10th	Night	45% 2 8% 19% 23% 4	7596
	11th to 20th	Night	41% 2 4 18% 26% 9%	7080
	21st to end	Night	51% 2 5% 15% 19% 8%	7745
February	1st to 10th	Night	42% 5% 6% 17% 21% 10%	2520
	11th to 20th	Night	44% 4 5% 16% 20% 11%	2400
	21st to end	Night	43% 5% 7% 17% 16% 12%	1980
March	1st to 10th	Night	49% 4 6% 14% 19% 9%	5373
	11th to 20th	Night	36% 4 8% 14% 21% 17%	5261
	21st to end	Night	39% 3 9% 19% 18% 11%	5082
April	1st to 10th	Night	35% 5% 13% 16% 19% 12%	2000
	11th to 20th	Night	34% 5% 9% 18% 23% 12%	1975
	21st to end	Night	27% 6% 11% 23% 25% 8%	2000
May	1st to 10th	Night	40% 7% 14% 18% 18% 3	4392
	11th to 20th	Night	42% 5% 17% 22% 12% 2	4033
	21st to end	Night	33% 7% 16% 25% 17% 2	4429
June	1st to 10th	Night	36% 11% 17% 18% 16% 2	1800
	11th to 20th	Night	29% 11% 19% 24% 15% 3	1800
	21st to end	Night	37% 9% 20% 18% 13% 4	1800
July	1st to 10th	Night	41% 10% 28% 16% 5% 1	4015
	11th to 20th	Night	36% 11% 32% 18% 2	4024
	21st to end	Night	47% 12% 19% 18% 3	4785
August	1st to 10th	Night	24% 10% 14% 32% 19% 1	3232
	11th to 20th	Night	30% 9% 18% 28% 12% 3	3230
	21st to end	Night	33% 8% 17% 20% 19% 3	3586
September	1st to 10th	Night	39% 10% 14% 16% 15% 6%	2002
	11th to 20th	Night	35% 10% 11% 18% 19% 7%	2200
	21st to end	Night	32% 6% 10% 22% 21% 9%	2200
October	1st to 10th	Night	31% 4 6% 11% 21% 26%	3590
	11th to 20th	Night	16% 3 4 16% 27% 34%	3571
	21st to end	Night	22% 1 3 17% 36% 20%	4363
November	1st to 10th	Night	29% 5% 7% 16% 27% 16%	2666
	11th to 20th	Night	30% 4 5% 19% 29% 13%	2730
	21st to end	Night	36% 3 7% 20% 21% 12%	2730
December	1st to 10th	Night	37% 3 6% 22% 26% 6%	4380
	11th to 20th	Night	38% 2 8% 20% 26% 6%	4954
	21st to end	Night	41% 3 7% 20% 27% 2	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	21% 24% 34% 17% 3 1	330
	11th to 20th	Day	17% 20% 36% 16% 9% 2	330
	21st to end	Day	21% 24% 33% 19% 2 1	363
February	1st to 10th	Day	19% 21% 36% 18% 6%	330
	11th to 20th	Day	29% 20% 33% 15% 2	315
	21st to end	Day	20% 22% 38% 16% 3	249
March	1st to 10th	Day	21% 22% 38% 14% 5%	300
	11th to 20th	Day	18% 18% 33% 22% 8% 1	373
	21st to end	Day	20% 16% 31% 25% 9%	550
April	1st to 10th	Day	18% 15% 31% 23% 12% 2	500
	11th to 20th	Day	14% 13% 31% 26% 13% 2	492
	21st to end	Day	9% 12% 25% 28% 22% 4	500
May	1st to 10th	Day	9% 11% 24% 30% 22% 4	503
	11th to 20th	Day	9% 5% 24% 31% 20% 12%	527
	21st to end	Day	12% 3 18% 30% 23% 14%	573
June	1st to 10th	Day	10% 4 15% 32% 26% 12%	530
	11th to 20th	Day	8% 3 12% 26% 33% 19%	530
	21st to end	Day	7% 2 11% 26% 31% 23%	530
July	1st to 10th	Day	6% 1 6% 23% 37% 26%	525
	11th to 20th	Day	6% 1 4 16% 40% 32%	524
	21st to end	Day	5% 1 7% 21% 40% 26%	576
August	1st to 10th	Day	7% 1 8% 20% 34% 30%	526
	11th to 20th	Day	8% 1 5% 20% 36% 30%	519
	21st to end	Day	12% 2 8% 21% 31% 25%	583
September	1st to 10th	Day	9% 2 10% 27% 34% 18%	530
	11th to 20th	Day	14% 2 9% 23% 38% 15%	530
	21st to end	Day	9% 3 13% 32% 29% 15%	530
October	1st to 10th	Day	11% 4 20% 36% 21% 8%	530
	11th to 20th	Day	9% 6% 19% 33% 29% 4	527
	21st to end	Day	19% 12% 19% 29% 15% 6%	363
November	1st to 10th	Day	22% 7% 24% 31% 12% 5%	330
	11th to 20th	Day	15% 12% 29% 29% 12% 3	330
	21st to end	Day	12% 16% 34% 25% 11% 2	330
December	1st to 10th	Day	26% 13% 34% 19% 7% 1	330
	11th to 20th	Day	23% 11% 35% 23% 8% 1	330
	21st to end	Day	23% 15% 30% 21% 9% 2	363
January	1st to 10th	Night	19% 15% 32% 30% 4	840
	11th to 20th	Night	19% 16% 26% 32% 7%	840
	21st to end	Night	22% 13% 33% 29% 4	924
February	1st to 10th	Night	18% 12% 32% 32% 6%	840
	11th to 20th	Night	26% 14% 28% 27% 5%	804
	21st to end	Night	19% 16% 36% 25% 3	660
March	1st to 10th	Night	16% 15% 32% 29% 7%	800
	11th to 20th	Night	15% 14% 33% 28% 9%	800
	21st to end	Night	15% 11% 32% 31% 10%	715
April	1st to 10th	Night	13% 11% 33% 31% 12%	600
	11th to 20th	Night	13% 9% 30% 31% 16% 1	593
	21st to end	Night	8% 6% 26% 41% 18% 2	600
May	1st to 10th	Night	10% 4 26% 36% 20% 4	600
	11th to 20th	Night	10% 2 21% 37% 26% 4	591
	21st to end	Night	9% 1 13% 38% 33% 6%	650
June	1st to 10th	Night	8% 1 9% 39% 38% 5%	600
	11th to 20th	Night	5% 2 7% 29% 46% 12%	600
	21st to end	Night	7% 2 8% 28% 44% 12%	600
July	1st to 10th	Night	6% 1 4 25% 45% 19%	585
	11th to 20th	Night	6% 5% 18% 49% 21%	582
	21st to end	Night	4 1 5% 24% 53% 13%	657
August	1st to 10th	Night	6% 3 22% 50% 19%	597
	11th to 20th	Night	8% 1 5% 23% 47% 16%	594
	21st to end	Night	11% 2 6% 25% 39% 17%	660
September	1st to 10th	Night	11% 1 9% 25% 40% 13%	602
	11th to 20th	Night	14% 1 10% 32% 37% 5%	800
	21st to end	Night	9% 2 12% 32% 39% 6%	800
October	1st to 10th	Night	10% 1 22% 33% 29% 5%	800
	11th to 20th	Night	11% 4 21% 34% 28% 3	800
	21st to end	Night	13% 7% 18% 36% 23% 2	880
November	1st to 10th	Night	10% 7% 21% 38% 21% 3	820
	11th to 20th	Night	14% 8% 23% 38% 16% 1	840
	21st to end	Night	14% 9% 27% 37% 13% 1	840
December	1st to 10th	Night	19% 10% 28% 29% 12% 1	840
	11th to 20th	Night	20% 9% 30% 31% 10%	840
	21st to end	Night	21% 12% 28% 30% 9%	924

Page generated at 2026-02-13 00:42:15. Version ac07

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