Shelly Island Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Shelly 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	15% 6% 3 5% 5% 3 5% 5% 6% 8% 40%	74719
	Night	15% 6% 3 5% 5% 3 5% 5% 5% 7% 42%	75060
Transparency	Day	42% 6% 12% 23% 15% 3	74426
	Night	41% 6% 11% 23% 16% 3	74872
Seeing	Day	20% 14% 26% 22% 13% 5%	13761
	Night	19% 11% 29% 28% 12% 1	23274

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	13% 7% 3 6% 5% 3 5% 6% 5% 7% 40%	1517
	11th to 20th	Day	15% 8% 3 6% 6% 4 6% 6% 4 6% 35%	1605
	21st to end	Day	14% 9% 4 6% 6% 3 5% 5% 5% 7% 36%	1782
February	1st to 10th	Day	15% 9% 3 6% 5% 3 4 6% 5% 7% 35%	1620
	11th to 20th	Day	15% 7% 4 6% 5% 4 4 5% 5% 6% 40%	1627
	21st to end	Day	20% 8% 4 6% 5% 4 5% 4 4 6% 36%	1480
March	1st to 10th	Day	12% 6% 2 5% 5% 3 5% 4 5% 6% 46%	1898
	11th to 20th	Day	16% 6% 2 5% 5% 4 5% 4 6% 8% 37%	1980
	21st to end	Day	21% 6% 2 5% 5% 2 5% 4 4 5% 39%	2237
April	1st to 10th	Day	15% 6% 3 5% 5% 4 4 5% 6% 6% 40%	2160
	11th to 20th	Day	15% 6% 3 5% 4 3 4 4 5% 6% 43%	2134
	21st to end	Day	20% 5% 2 4 5% 3 4 5% 5% 7% 40%	2160
May	1st to 10th	Day	17% 7% 2 6% 4 3 5% 5% 6% 8% 37%	2323
	11th to 20th	Day	19% 7% 3 6% 5% 4 5% 5% 5% 7% 36%	2478
	21st to end	Day	15% 5% 2 4 6% 3 5% 5% 7% 8% 40%	2725
June	1st to 10th	Day	13% 4 3 5% 5% 3 6% 6% 6% 10% 40%	2520
	11th to 20th	Day	13% 5% 3 5% 5% 4 5% 5% 7% 10% 38%	2520
	21st to end	Day	10% 3 2 4 5% 4 6% 5% 6% 11% 44%	2520
July	1st to 10th	Day	9% 4 2 5% 5% 5% 6% 5% 7% 10% 41%	2520
	11th to 20th	Day	7% 4 2 5% 4 4 5% 6% 9% 10% 44%	2520
	21st to end	Day	9% 5% 3 6% 6% 4 6% 6% 7% 8% 38%	2584
August	1st to 10th	Day	11% 5% 2 4 6% 4 6% 6% 8% 8% 39%	2327
	11th to 20th	Day	12% 6% 3 5% 5% 3 5% 5% 7% 8% 40%	2210
	21st to end	Day	14% 5% 2 4 5% 3 5% 5% 6% 9% 43%	2376
September	1st to 10th	Day	17% 5% 2 4 6% 3 4 3 4 6% 45%	2160
	11th to 20th	Day	14% 4 2 4 3 3 3 4 5% 8% 49%	2160
	21st to end	Day	19% 7% 2 5% 4 2 3 4 5% 6% 41%	1908
October	1st to 10th	Day	17% 6% 2 4 5% 2 4 4 5% 6% 44%	1800
	11th to 20th	Day	17% 6% 2 5% 5% 2 3 4 4 7% 45%	1800
	21st to end	Day	17% 7% 2 4 4 3 4 4 5% 7% 43%	1980
November	1st to 10th	Day	13% 5% 2 5% 3 3 3 4 5% 7% 50%	1694
	11th to 20th	Day	18% 4 2 4 4 3 4 4 5% 6% 46%	1672
	21st to end	Day	17% 5% 3 5% 4 4 4 5% 5% 7% 42%	1520
December	1st to 10th	Day	19% 8% 2 6% 6% 3 5% 5% 6% 7% 34%	1520
	11th to 20th	Day	18% 6% 2 5% 6% 3 5% 5% 5% 8% 36%	1910
	21st to end	Day	18% 6% 4 7% 6% 4 6% 6% 6% 7% 31%	2772
January	1st to 10th	Night	14% 8% 3 5% 6% 3 4 4 6% 7% 41%	2649
	11th to 20th	Night	18% 7% 3 5% 5% 3 4 5% 5% 7% 37%	2441
	21st to end	Night	13% 7% 3 6% 5% 3 4 4 5% 8% 41%	2574
February	1st to 10th	Night	18% 7% 3 5% 5% 3 4 5% 6% 7% 38%	2340
	11th to 20th	Night	17% 7% 4 6% 4 3 5% 4 5% 7% 40%	2219
	21st to end	Night	17% 7% 3 7% 6% 3 5% 5% 4 6% 38%	1776
March	1st to 10th	Night	14% 5% 3 4 5% 3 4 4 5% 7% 46%	2059
	11th to 20th	Night	17% 6% 3 5% 5% 3 4 4 4 5% 44%	1980
	21st to end	Night	18% 6% 3 6% 5% 3 4 4 5% 7% 40%	2118
April	1st to 10th	Night	17% 7% 3 6% 5% 3 4 4 5% 8% 37%	1800
	11th to 20th	Night	14% 5% 2 5% 4 3 5% 5% 6% 9% 42%	1775
	21st to end	Night	19% 5% 3 4 5% 1 4 4 5% 6% 43%	1800
May	1st to 10th	Night	19% 6% 3 7% 5% 3 5% 4 4 6% 38%	1652
	11th to 20th	Night	20% 7% 2 5% 5% 3 4 4 5% 8% 37%	1415
	21st to end	Night	12% 6% 3 5% 5% 4 4 5% 6% 9% 41%	1550
June	1st to 10th	Night	13% 4 2 5% 4 3 5% 5% 6% 6% 46%	1440
	11th to 20th	Night	13% 5% 1 5% 5% 5% 4 5% 8% 10% 40%	1440
	21st to end	Night	9% 3 3 5% 5% 3 5% 6% 7% 10% 44%	1440
July	1st to 10th	Night	9% 7% 2 6% 6% 4 6% 5% 6% 10% 38%	1440
	11th to 20th	Night	6% 4 1 3 6% 3 6% 7% 6% 12% 46%	1440
	21st to end	Night	10% 4 2 5% 5% 4 6% 6% 7% 10% 43%	1730
August	1st to 10th	Night	10% 4 3 5% 5% 4 6% 6% 6% 10% 42%	1611
	11th to 20th	Night	11% 5% 2 7% 7% 4 6% 6% 5% 7% 40%	1671
	21st to end	Night	12% 5% 2 5% 6% 3 5% 5% 6% 7% 44%	1980
September	1st to 10th	Night	15% 5% 3 5% 4 3 4 5% 5% 6% 46%	1800
	11th to 20th	Night	13% 5% 2 4 3 3 3 4 4 7% 53%	1800
	21st to end	Night	18% 5% 3 4 4 3 5% 5% 5% 6% 41%	2048
October	1st to 10th	Night	20% 5% 3 5% 5% 2 3 3 4 5% 45%	2160
	11th to 20th	Night	17% 5% 2 5% 4 3 3 4 5% 6% 47%	2160
	21st to end	Night	18% 5% 2 4 4 2 3 4 5% 7% 46%	2376
November	1st to 10th	Night	11% 5% 2 4 4 3 4 4 5% 6% 52%	2384
	11th to 20th	Night	15% 6% 3 5% 4 2 3 4 5% 6% 46%	2503
	21st to end	Night	15% 4 2 6% 5% 3 5% 5% 5% 7% 44%	2660
December	1st to 10th	Night	18% 7% 3 5% 5% 3 5% 5% 5% 6% 37%	2660
	11th to 20th	Night	18% 5% 2 5% 5% 3 5% 5% 5% 7% 39%	3285
	21st to end	Night	17% 7% 3 6% 6% 4 6% 6% 6% 8% 31%	4884

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	43% 5% 13% 23% 14% 3	1518
	11th to 20th	Day	50% 5% 13% 19% 12% 2	1605
	21st to end	Day	48% 5% 10% 22% 12% 2	1782
February	1st to 10th	Day	48% 5% 12% 23% 10% 2	1620
	11th to 20th	Day	45% 6% 9% 26% 12% 2	1636
	21st to end	Day	51% 4 8% 22% 11% 4	1480
March	1st to 10th	Day	40% 4 13% 26% 15% 2	1898
	11th to 20th	Day	44% 5% 11% 19% 16% 5%	1980
	21st to end	Day	47% 5% 8% 20% 17% 3	2237
April	1st to 10th	Day	44% 5% 11% 20% 16% 4	2160
	11th to 20th	Day	40% 5% 10% 23% 17% 5%	2134
	21st to end	Day	44% 5% 8% 23% 16% 4	2160
May	1st to 10th	Day	45% 6% 10% 22% 15% 3	2323
	11th to 20th	Day	49% 5% 10% 20% 13% 3	2482
	21st to end	Day	41% 7% 15% 22% 13% 2	2725
June	1st to 10th	Day	40% 7% 12% 24% 16% 2	2520
	11th to 20th	Day	41% 8% 13% 23% 13% 3	2520
	21st to end	Day	34% 8% 13% 28% 16% 1	2520
July	1st to 10th	Day	37% 10% 12% 21% 17% 3	2450
	11th to 20th	Day	32% 7% 14% 27% 18% 3	2445
	21st to end	Day	41% 8% 11% 21% 17% 2	2439
August	1st to 10th	Day	39% 8% 13% 24% 14% 1	2314
	11th to 20th	Day	39% 7% 12% 23% 17% 2	2210
	21st to end	Day	37% 7% 12% 24% 16% 4	2376
September	1st to 10th	Day	40% 4 12% 20% 19% 4	2160
	11th to 20th	Day	33% 4 11% 26% 22% 4	2160
	21st to end	Day	44% 5% 8% 19% 21% 3	1908
October	1st to 10th	Day	41% 5% 11% 25% 16% 2	1800
	11th to 20th	Day	40% 4 10% 25% 19% 1	1800
	21st to end	Day	42% 4 9% 27% 17% 2	1980
November	1st to 10th	Day	34% 4 13% 25% 20% 4	1694
	11th to 20th	Day	40% 5% 11% 23% 19% 3	1672
	21st to end	Day	42% 5% 12% 30% 9% 2	1520
December	1st to 10th	Day	49% 6% 13% 20% 11% 1	1520
	11th to 20th	Day	46% 4 13% 29% 8% 1	1906
	21st to end	Day	51% 7% 14% 18% 8% 1	2772
January	1st to 10th	Night	42% 5% 11% 25% 13% 3	2649
	11th to 20th	Night	47% 5% 14% 22% 11% 1	2441
	21st to end	Night	42% 4 10% 27% 14% 3	2574
February	1st to 10th	Night	46% 5% 13% 24% 11% 1	2340
	11th to 20th	Night	45% 4 10% 22% 17% 3	2221
	21st to end	Night	47% 4 10% 20% 15% 4	1776
March	1st to 10th	Night	38% 5% 11% 26% 18% 2	2059
	11th to 20th	Night	43% 5% 10% 22% 17% 4	1980
	21st to end	Night	45% 5% 8% 20% 18% 4	2118
April	1st to 10th	Night	47% 4 9% 20% 16% 4	1800
	11th to 20th	Night	39% 6% 10% 22% 19% 4	1775
	21st to end	Night	42% 5% 10% 22% 17% 6%	1800
May	1st to 10th	Night	47% 6% 9% 21% 14% 4	1652
	11th to 20th	Night	44% 6% 11% 18% 18% 2	1416
	21st to end	Night	40% 7% 14% 22% 14% 3	1551
June	1st to 10th	Night	37% 8% 10% 24% 19% 2	1440
	11th to 20th	Night	38% 10% 13% 20% 17% 2	1440
	21st to end	Night	34% 9% 12% 27% 16% 2	1440
July	1st to 10th	Night	41% 9% 13% 19% 14% 3	1400
	11th to 20th	Night	30% 9% 14% 26% 17% 2	1392
	21st to end	Night	33% 9% 16% 22% 17% 2	1634
August	1st to 10th	Night	36% 8% 15% 26% 15% 1	1600
	11th to 20th	Night	41% 8% 12% 20% 18% 1	1671
	21st to end	Night	37% 7% 13% 22% 18% 3	1980
September	1st to 10th	Night	39% 5% 11% 21% 20% 4	1800
	11th to 20th	Night	32% 4 9% 27% 24% 4	1800
	21st to end	Night	43% 5% 8% 20% 21% 3	2048
October	1st to 10th	Night	43% 4 10% 25% 16% 2	2160
	11th to 20th	Night	38% 5% 10% 28% 17% 2	2160
	21st to end	Night	38% 4 9% 28% 18% 2	2376
November	1st to 10th	Night	33% 4 11% 27% 21% 4	2384
	11th to 20th	Night	39% 4 9% 25% 18% 5%	2503
	21st to end	Night	39% 5% 12% 27% 15% 2	2660
December	1st to 10th	Night	47% 5% 13% 22% 13%	2660
	11th to 20th	Night	44% 5% 12% 23% 14% 2	3288
	21st to end	Night	48% 8% 14% 21% 8%	4884

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	27% 27% 29% 14% 1 2	180
	11th to 20th	Day	30% 25% 28% 16% 1	229
	21st to end	Day	23% 28% 32% 12% 4 1	264
February	1st to 10th	Day	29% 27% 29% 12% 3	240
	11th to 20th	Day	29% 26% 37% 6% 1	294
	21st to end	Day	33% 22% 26% 18% 1	346
March	1st to 10th	Day	23% 23% 30% 19% 5%	420
	11th to 20th	Day	23% 24% 33% 15% 4 1	420
	21st to end	Day	27% 21% 32% 14% 5% 2	462
April	1st to 10th	Day	22% 24% 30% 15% 7% 1	420
	11th to 20th	Day	21% 22% 26% 16% 11% 4	412
	21st to end	Day	28% 13% 32% 17% 8% 1	420
May	1st to 10th	Day	27% 16% 28% 18% 9% 3	423
	11th to 20th	Day	23% 15% 28% 20% 10% 4	447
	21st to end	Day	20% 8% 21% 26% 17% 8%	485
June	1st to 10th	Day	15% 12% 28% 23% 16% 6%	450
	11th to 20th	Day	17% 9% 27% 25% 18% 4	450
	21st to end	Day	12% 7% 29% 28% 19% 6%	450
July	1st to 10th	Day	12% 4 21% 28% 23% 11%	445
	11th to 20th	Day	6% 5% 20% 34% 25% 10%	444
	21st to end	Day	12% 4 25% 33% 20% 5%	488
August	1st to 10th	Day	12% 3 20% 28% 26% 11%	446
	11th to 20th	Day	15% 6% 17% 29% 22% 11%	439
	21st to end	Day	13% 4 20% 31% 20% 12%	495
September	1st to 10th	Day	19% 7% 19% 21% 22% 12%	450
	11th to 20th	Day	16% 5% 19% 30% 20% 11%	450
	21st to end	Day	24% 10% 22% 25% 14% 5%	450
October	1st to 10th	Day	22% 7% 21% 25% 18% 7%	450
	11th to 20th	Day	20% 16% 23% 26% 12% 2	450
	21st to end	Day	20% 14% 34% 22% 8% 3	495
November	1st to 10th	Day	18% 12% 30% 24% 14% 2	377
	11th to 20th	Day	22% 18% 28% 20% 10% 1	332
	21st to end	Day	24% 20% 32% 17% 6% 1	180
December	1st to 10th	Day	26% 24% 31% 12% 4 2	180
	11th to 20th	Day	27% 23% 31% 15% 4 1	180
	21st to end	Day	26% 22% 27% 17% 7% 1	198
January	1st to 10th	Night	19% 17% 40% 20% 3	760
	11th to 20th	Night	25% 16% 29% 25% 4	737
	21st to end	Night	18% 18% 39% 23% 3	792
February	1st to 10th	Night	23% 18% 31% 26% 2	720
	11th to 20th	Night	24% 20% 38% 17% 1	703
	21st to end	Night	23% 20% 36% 17% 4	592
March	1st to 10th	Night	20% 17% 35% 24% 4	720
	11th to 20th	Night	22% 18% 33% 21% 5% 1	720
	21st to end	Night	22% 12% 39% 21% 6%	792
April	1st to 10th	Night	22% 18% 35% 20% 5%	720
	11th to 20th	Night	18% 12% 35% 26% 9%	711
	21st to end	Night	23% 10% 32% 25% 8%	720
May	1st to 10th	Night	21% 9% 30% 28% 11%	572
	11th to 20th	Night	22% 8% 31% 29% 11%	354
	21st to end	Night	16% 4 20% 39% 20% 1	389
June	1st to 10th	Night	14% 3 25% 37% 20% 1	360
	11th to 20th	Night	13% 6% 26% 37% 18% 1	360
	21st to end	Night	9% 2 24% 48% 16%	360
July	1st to 10th	Night	14% 2 19% 39% 24% 3	350
	11th to 20th	Night	6% 2 14% 44% 30% 2	348
	21st to end	Night	11% 3 15% 43% 26% 2	550
August	1st to 10th	Night	12% 2 12% 43% 28% 2	537
	11th to 20th	Night	13% 4 11% 39% 29% 4	607
	21st to end	Night	14% 3 16% 35% 29% 3	792
September	1st to 10th	Night	16% 3 22% 31% 24% 3	720
	11th to 20th	Night	13% 4 18% 35% 26% 4	720
	21st to end	Night	20% 5% 21% 31% 21% 2	720
October	1st to 10th	Night	21% 6% 22% 29% 19% 3	720
	11th to 20th	Night	19% 10% 27% 31% 12% 1	720
	21st to end	Night	20% 11% 30% 29% 9%	792
November	1st to 10th	Night	14% 9% 34% 30% 12% 1	740
	11th to 20th	Night	18% 16% 30% 25% 9% 1	760
	21st to end	Night	17% 16% 34% 26% 7%	760
December	1st to 10th	Night	23% 15% 35% 21% 6%	760
	11th to 20th	Night	18% 14% 38% 26% 3	760
	21st to end	Night	21% 15% 32% 30% 2	836

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