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 2024-01-15.

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	11% 6% 3 5% 6% 4 5% 5% 6% 8% 43%	74810
	Night	11% 5% 2 5% 5% 3 4 5% 6% 8% 46%	74988
Transparency	Day	38% 7% 11% 18% 19% 6%	74545
	Night	36% 7% 11% 18% 20% 8%	74772
Seeing	Day	13% 9% 20% 24% 22% 11%	13379
	Night	13% 7% 21% 31% 23% 5%	23411

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	17% 8% 2 4 3 2 3 4 5% 6% 43%	1715
	11th to 20th	Day	15% 7% 3 6% 4 3 4 3 4 6% 44%	1846
	21st to end	Day	16% 6% 4 5% 3 2 4 4 6% 5% 45%	1980
February	1st to 10th	Day	16% 7% 4 5% 6% 2 4 5% 5% 7% 39%	1800
	11th to 20th	Day	18% 7% 3 5% 5% 3 4 3 5% 5% 43%	1748
	21st to end	Day	18% 6% 3 6% 4 4 5% 4 5% 6% 39%	1480
March	1st to 10th	Day	13% 7% 2 5% 5% 3 4 5% 6% 6% 45%	1800
	11th to 20th	Day	15% 7% 2 6% 6% 3 5% 5% 5% 7% 39%	1865
	21st to end	Day	11% 6% 3 6% 5% 3 4 5% 6% 6% 45%	2340
April	1st to 10th	Day	11% 5% 3 4 5% 3 5% 6% 5% 7% 46%	2160
	11th to 20th	Day	12% 6% 2 5% 6% 3 5% 4 5% 7% 46%	2134
	21st to end	Day	7% 4 2 4 5% 3 5% 3 5% 8% 54%	2160
May	1st to 10th	Day	10% 4 2 4 5% 3 4 4 6% 7% 51%	2203
	11th to 20th	Day	6% 5% 2 6% 6% 4 6% 6% 7% 8% 43%	2301
	21st to end	Day	8% 5% 2 5% 6% 4 5% 7% 6% 8% 44%	2531
June	1st to 10th	Day	8% 6% 3 7% 7% 4 7% 6% 7% 10% 35%	2340
	11th to 20th	Day	6% 3 3 5% 6% 5% 7% 6% 8% 12% 40%	2340
	21st to end	Day	6% 5% 2 5% 7% 5% 6% 7% 8% 12% 36%	2340
July	1st to 10th	Day	8% 5% 3 6% 7% 5% 8% 9% 9% 11% 30%	2340
	11th to 20th	Day	7% 5% 3 6% 7% 5% 8% 8% 9% 10% 31%	2332
	21st to end	Day	7% 4 2 7% 7% 5% 7% 8% 8% 12% 33%	2350
August	1st to 10th	Day	4 4 2 6% 7% 4 8% 9% 9% 11% 36%	2148
	11th to 20th	Day	10% 5% 2 6% 8% 5% 7% 6% 8% 11% 32%	2125
	21st to end	Day	11% 6% 3 6% 7% 5% 7% 7% 6% 9% 34%	2376
September	1st to 10th	Day	10% 5% 3 7% 6% 4 6% 5% 6% 8% 40%	2139
	11th to 20th	Day	12% 6% 2 5% 6% 4 5% 5% 6% 7% 43%	1980
	21st to end	Day	9% 4 2 4 6% 3 4 4 5% 7% 51%	1980
October	1st to 10th	Day	8% 4 2 5% 4 3 4 4 5% 7% 54%	1980
	11th to 20th	Day	10% 4 2 4 4 2 4 4 5% 7% 53%	1978
	21st to end	Day	12% 5% 3 4 4 2 3 3 4 6% 54%	1920
November	1st to 10th	Day	10% 4 2 4 4 2 4 4 4 5% 57%	1672
	11th to 20th	Day	12% 7% 3 4 4 2 3 4 4 6% 53%	1710
	21st to end	Day	13% 5% 2 4 5% 3 4 4 5% 7% 48%	1710
December	1st to 10th	Day	15% 7% 3 5% 4 4 5% 4 5% 6% 44%	1710
	11th to 20th	Day	15% 9% 3 6% 5% 2 4 4 4 4 43%	2153
	21st to end	Day	15% 9% 3 6% 4 3 5% 4 4 6% 40%	3124
January	1st to 10th	Night	15% 7% 3 4 4 3 3 4 4 5% 48%	2447
	11th to 20th	Night	16% 7% 3 5% 5% 2 4 3 4 7% 44%	2220
	21st to end	Night	18% 7% 2 5% 4 2 3 3 5% 6% 44%	2376
February	1st to 10th	Night	14% 8% 3 5% 5% 3 4 4 5% 7% 42%	2160
	11th to 20th	Night	20% 7% 3 4 3 2 3 3 4 4 47%	2098
	21st to end	Night	17% 8% 4 6% 6% 3 4 3 5% 6% 40%	1776
March	1st to 10th	Night	13% 6% 2 5% 4 3 4 4 5% 6% 49%	2160
	11th to 20th	Night	13% 6% 3 6% 5% 3 5% 5% 5% 7% 42%	2093
	21st to end	Night	12% 5% 2 5% 4 2 4 4 5% 7% 49%	2030
April	1st to 10th	Night	10% 6% 3 4 5% 3 4 5% 6% 8% 47%	1800
	11th to 20th	Night	12% 5% 2 4 4 3 4 4 7% 8% 47%	1775
	21st to end	Night	7% 3 1 4 4 2 4 4 5% 8% 57%	1800
May	1st to 10th	Night	8% 4 2 3 4 3 3 4 5% 7% 56%	1756
	11th to 20th	Night	7% 5% 2 5% 5% 4 6% 6% 7% 9% 44%	1592
	21st to end	Night	6% 4 2 4 6% 5% 6% 5% 8% 11% 44%	1744
June	1st to 10th	Night	6% 5% 3 6% 6% 4 6% 7% 8% 12% 36%	1620
	11th to 20th	Night	5% 2 2 4 5% 3 5% 6% 10% 12% 45%	1620
	21st to end	Night	5% 5% 3 7% 7% 3 6% 5% 9% 11% 38%	1620
July	1st to 10th	Night	5% 5% 3 5% 6% 4 6% 9% 9% 14% 34%	1620
	11th to 20th	Night	5% 3 2 6% 7% 5% 6% 7% 9% 12% 38%	1624
	21st to end	Night	3 5% 2 6% 7% 4 6% 8% 10% 11% 39%	1968
August	1st to 10th	Night	3 4 3 5% 6% 5% 7% 8% 10% 13% 36%	1790
	11th to 20th	Night	7% 5% 2 6% 6% 6% 7% 8% 8% 13% 34%	1776
	21st to end	Night	9% 7% 4 6% 6% 5% 6% 6% 6% 9% 38%	1980
September	1st to 10th	Night	8% 6% 3 6% 6% 4 5% 6% 7% 9% 40%	1801
	11th to 20th	Night	11% 5% 2 5% 4 4 5% 6% 7% 9% 44%	1980
	21st to end	Night	7% 4 2 6% 6% 3 4 5% 6% 8% 50%	1980
October	1st to 10th	Night	7% 3 2 4 5% 2 4 4 6% 7% 55%	1980
	11th to 20th	Night	10% 4 2 4 4 2 3 4 5% 7% 58%	1980
	21st to end	Night	11% 5% 2 5% 4 3 3 2 3 5% 57%	2432
November	1st to 10th	Night	9% 4 2 4 3 3 3 4 4 5% 59%	2406
	11th to 20th	Night	13% 4 2 4 3 2 3 3 4 6% 56%	2470
	21st to end	Night	13% 6% 2 5% 4 3 4 4 4 7% 48%	2470
December	1st to 10th	Night	15% 6% 2 5% 4 2 4 4 5% 6% 47%	2470
	11th to 20th	Night	16% 7% 3 5% 4 2 3 3 4 5% 47%	3042
	21st to end	Night	15% 8% 3 5% 5% 3 4 5% 5% 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	41% 5% 6% 17% 26% 6%	1716
	11th to 20th	Day	42% 4 7% 15% 24% 8%	1846
	21st to end	Day	40% 4 6% 17% 25% 8%	1980
February	1st to 10th	Day	44% 5% 7% 17% 20% 7%	1800
	11th to 20th	Day	45% 3 5% 17% 22% 8%	1758
	21st to end	Day	46% 5% 8% 14% 17% 11%	1480
March	1st to 10th	Day	38% 5% 7% 18% 22% 10%	1800
	11th to 20th	Day	45% 6% 10% 14% 18% 7%	1865
	21st to end	Day	39% 6% 9% 18% 22% 6%	2340
April	1st to 10th	Day	37% 6% 10% 20% 20% 8%	2160
	11th to 20th	Day	38% 5% 9% 19% 21% 8%	2134
	21st to end	Day	29% 7% 10% 21% 29% 4	2160
May	1st to 10th	Day	32% 5% 10% 22% 25% 6%	2203
	11th to 20th	Day	35% 9% 15% 22% 16% 2	2305
	21st to end	Day	34% 9% 11% 22% 21% 3	2531
June	1st to 10th	Day	41% 10% 12% 23% 11% 2	2340
	11th to 20th	Day	33% 11% 17% 24% 13% 3	2340
	21st to end	Day	38% 10% 18% 20% 11% 2	2340
July	1st to 10th	Day	43% 14% 20% 15% 8% 1	2275
	11th to 20th	Day	41% 15% 19% 16% 7% 1	2263
	21st to end	Day	39% 14% 20% 20% 6% 1	2219
August	1st to 10th	Day	36% 14% 22% 21% 8%	2136
	11th to 20th	Day	42% 14% 17% 16% 9% 3	2125
	21st to end	Day	45% 11% 15% 16% 11% 3	2376
September	1st to 10th	Day	40% 9% 12% 16% 16% 6%	2139
	11th to 20th	Day	40% 7% 12% 17% 18% 5%	1980
	21st to end	Day	33% 6% 10% 20% 23% 8%	1980
October	1st to 10th	Day	30% 5% 11% 17% 25% 13%	1980
	11th to 20th	Day	30% 4 7% 20% 27% 11%	1978
	21st to end	Day	33% 3 5% 17% 28% 14%	1920
November	1st to 10th	Day	29% 5% 6% 19% 29% 12%	1672
	11th to 20th	Day	33% 4 7% 17% 27% 12%	1710
	21st to end	Day	36% 4 9% 17% 24% 9%	1710
December	1st to 10th	Day	42% 5% 8% 19% 20% 6%	1710
	11th to 20th	Day	45% 4 6% 14% 26% 5%	2150
	21st to end	Day	45% 4 7% 20% 20% 4	3124
January	1st to 10th	Night	39% 4 7% 16% 26% 7%	2447
	11th to 20th	Night	42% 4 6% 17% 23% 7%	2220
	21st to end	Night	43% 3 6% 13% 26% 9%	2376
February	1st to 10th	Night	43% 5% 7% 16% 19% 10%	2160
	11th to 20th	Night	42% 4 5% 15% 22% 11%	2099
	21st to end	Night	45% 5% 7% 15% 15% 12%	1776
March	1st to 10th	Night	36% 4 8% 16% 22% 13%	2160
	11th to 20th	Night	41% 6% 10% 16% 18% 9%	2093
	21st to end	Night	34% 6% 9% 19% 24% 8%	2030
April	1st to 10th	Night	34% 6% 13% 17% 20% 12%	1800
	11th to 20th	Night	35% 5% 9% 18% 23% 10%	1775
	21st to end	Night	26% 6% 10% 24% 27% 8%	1800
May	1st to 10th	Night	27% 6% 11% 22% 26% 8%	1756
	11th to 20th	Night	34% 9% 16% 20% 17% 4	1593
	21st to end	Night	31% 10% 14% 19% 21% 4	1745
June	1st to 10th	Night	37% 11% 18% 18% 15% 1	1620
	11th to 20th	Night	26% 11% 20% 25% 15% 3	1620
	21st to end	Night	36% 10% 21% 17% 13% 4	1620
July	1st to 10th	Night	35% 15% 25% 15% 8% 2	1575
	11th to 20th	Night	33% 15% 28% 17% 6% 1	1570
	21st to end	Night	31% 15% 25% 22% 7%	1858
August	1st to 10th	Night	31% 16% 23% 22% 7% 1	1778
	11th to 20th	Night	38% 15% 18% 15% 9% 4	1776
	21st to end	Night	40% 13% 15% 15% 13% 4	1980
September	1st to 10th	Night	38% 11% 13% 16% 15% 7%	1801
	11th to 20th	Night	35% 11% 12% 17% 18% 7%	1980
	21st to end	Night	32% 6% 10% 21% 21% 10%	1980
October	1st to 10th	Night	27% 6% 10% 17% 23% 16%	1980
	11th to 20th	Night	27% 5% 8% 19% 28% 14%	1980
	21st to end	Night	32% 2 6% 16% 26% 18%	2432
November	1st to 10th	Night	28% 5% 7% 16% 27% 16%	2406
	11th to 20th	Night	31% 4 5% 19% 28% 14%	2470
	21st to end	Night	37% 4 7% 19% 21% 12%	2470
December	1st to 10th	Night	37% 5% 10% 20% 23% 5%	2470
	11th to 20th	Night	41% 3 6% 13% 30% 7%	3044
	21st to end	Night	42% 4 9% 17% 24% 3	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	21% 26% 34% 16% 3 1	270
	11th to 20th	Day	19% 20% 32% 16% 11% 2	254
	21st to end	Day	17% 28% 33% 19% 2 1	264
February	1st to 10th	Day	23% 23% 32% 18% 3	240
	11th to 20th	Day	29% 20% 35% 14% 2	240
	21st to end	Day	19% 23% 41% 15% 3	198
March	1st to 10th	Day	19% 22% 38% 15% 6%	240
	11th to 20th	Day	19% 19% 31% 21% 9% 1	305
	21st to end	Day	19% 17% 30% 24% 10%	462
April	1st to 10th	Day	16% 15% 28% 26% 13% 2	420
	11th to 20th	Day	14% 13% 30% 25% 15% 2	412
	21st to end	Day	10% 13% 26% 27% 20% 3	420
May	1st to 10th	Day	9% 12% 23% 29% 23% 4	423
	11th to 20th	Day	7% 4 21% 31% 23% 14%	447
	21st to end	Day	13% 2 16% 29% 24% 16%	485
June	1st to 10th	Day	10% 5% 12% 33% 26% 14%	450
	11th to 20th	Day	9% 3 12% 25% 33% 18%	450
	21st to end	Day	7% 2 13% 28% 30% 20%	450
July	1st to 10th	Day	7% 1 7% 24% 36% 24%	445
	11th to 20th	Day	6% 1 5% 18% 40% 30%	444
	21st to end	Day	5% 1 7% 22% 40% 26%	488
August	1st to 10th	Day	7% 1 8% 18% 34% 33%	446
	11th to 20th	Day	9% 1 5% 20% 36% 29%	439
	21st to end	Day	13% 2 7% 20% 32% 25%	495
September	1st to 10th	Day	8% 2 10% 24% 35% 20%	450
	11th to 20th	Day	14% 2 9% 23% 38% 14%	450
	21st to end	Day	10% 2 12% 31% 31% 14%	450
October	1st to 10th	Day	11% 4 20% 34% 23% 9%	450
	11th to 20th	Day	11% 6% 19% 30% 30% 4	448
	21st to end	Day	20% 13% 20% 29% 14% 4	297
November	1st to 10th	Day	19% 7% 25% 32% 13% 4	270
	11th to 20th	Day	16% 14% 32% 26% 10% 3	270
	21st to end	Day	12% 16% 32% 27% 10% 3	270
December	1st to 10th	Day	23% 14% 34% 19% 8% 1	270
	11th to 20th	Day	26% 13% 34% 20% 7%	270
	21st to end	Day	23% 17% 31% 21% 7% 1	297
January	1st to 10th	Night	19% 16% 32% 29% 5%	760
	11th to 20th	Night	19% 15% 26% 32% 8%	740
	21st to end	Night	21% 12% 34% 29% 4	792
February	1st to 10th	Night	18% 13% 32% 32% 5%	720
	11th to 20th	Night	24% 15% 28% 28% 6%	703
	21st to end	Night	19% 16% 35% 26% 4	592
March	1st to 10th	Night	15% 14% 33% 30% 8%	720
	11th to 20th	Night	15% 13% 34% 27% 10%	720
	21st to end	Night	15% 11% 32% 30% 11%	644
April	1st to 10th	Night	12% 10% 32% 31% 13%	540
	11th to 20th	Night	14% 9% 30% 29% 17% 1	533
	21st to end	Night	8% 7% 27% 40% 17% 1	540
May	1st to 10th	Night	10% 4 26% 35% 21% 4	540
	11th to 20th	Night	9% 2 20% 38% 27% 5%	531
	21st to end	Night	9% 1 11% 36% 36% 7%	584
June	1st to 10th	Night	9% 1 8% 38% 40% 5%	540
	11th to 20th	Night	6% 1 7% 29% 45% 12%	540
	21st to end	Night	6% 2 9% 28% 42% 12%	540
July	1st to 10th	Night	6% 4 25% 46% 19%	525
	11th to 20th	Night	6% 5% 19% 48% 21%	522
	21st to end	Night	4 1 6% 25% 52% 13%	591
August	1st to 10th	Night	5% 4 22% 50% 20%	537
	11th to 20th	Night	8% 1 5% 25% 48% 14%	534
	21st to end	Night	11% 2 7% 24% 40% 17%	594
September	1st to 10th	Night	11% 1 8% 23% 43% 14%	541
	11th to 20th	Night	13% 1 10% 32% 37% 6%	720
	21st to end	Night	9% 2 13% 31% 40% 6%	720
October	1st to 10th	Night	9% 1 23% 32% 30% 5%	720
	11th to 20th	Night	12% 4 21% 33% 28% 3	720
	21st to end	Night	14% 8% 19% 35% 23% 2	792
November	1st to 10th	Night	10% 7% 21% 37% 21% 3	740
	11th to 20th	Night	14% 9% 24% 37% 15% 1	760
	21st to end	Night	14% 9% 26% 36% 13% 1	760
December	1st to 10th	Night	18% 11% 29% 29% 13% 1	760
	11th to 20th	Night	20% 9% 31% 30% 10%	760
	21st to end	Night	21% 12% 29% 30% 8%	836

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