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 2024-11-30.

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% 2 6% 4 4 5% 4 6% 7% 40%	101553
	Night	16% 7% 2 6% 4 4 4 4 5% 7% 40%	98011
Transparency	Day	42% 6% 11% 24% 15% 2	105188
	Night	42% 6% 11% 24% 16% 2	102515
Seeing	Day	20% 14% 26% 22% 13% 5%	14970
	Night	19% 11% 29% 29% 12% 1	24378

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% 7% 2 7% 5% 4 4 4 4 6% 42%	2517
	11th to 20th	Day	13% 12% 4 10% 5% 5% 5% 4 5% 6% 31%	2750
	21st to end	Day	19% 10% 3 8% 6% 4 4 4 5% 5% 32%	3124
February	1st to 10th	Day	14% 9% 3 7% 5% 4 5% 5% 6% 7% 36%	1710
	11th to 20th	Day	14% 7% 3 7% 5% 5% 4 4 5% 5% 41%	1720
	21st to end	Day	19% 9% 4 6% 4 5% 4 3 4 6% 36%	1570
March	1st to 10th	Day	23% 6% 2 5% 4 4 4 4 5% 6% 36%	3301
	11th to 20th	Day	10% 5% 2 6% 5% 5% 4 4 6% 8% 44%	3480
	21st to end	Day	21% 8% 2 5% 3 3 4 3 4 5% 43%	3929
April	1st to 10th	Day	16% 7% 3 6% 5% 5% 5% 4 6% 7% 38%	2280
	11th to 20th	Day	14% 7% 2 6% 5% 4 4 3 5% 6% 43%	2254
	21st to end	Day	19% 6% 2 5% 4 4 4 4 5% 7% 40%	2280
May	1st to 10th	Day	28% 6% 2 6% 3 5% 4 3 5% 7% 31%	4066
	11th to 20th	Day	23% 8% 3 8% 5% 5% 4 3 5% 6% 29%	4398
	21st to end	Day	12% 4 2 5% 4 5% 4 4 7% 9% 44%	4823
June	1st to 10th	Day	12% 5% 3 6% 4 5% 5% 4 6% 9% 40%	2660
	11th to 20th	Day	13% 5% 3 5% 4 5% 4 4 7% 10% 38%	2660
	21st to end	Day	10% 4 2 5% 5% 5% 6% 4 6% 10% 44%	2660
July	1st to 10th	Day	6% 4 2 7% 5% 7% 6% 5% 8% 11% 39%	4440
	11th to 20th	Day	8% 6% 2 6% 4 5% 5% 5% 8% 10% 41%	4440
	21st to end	Day	10% 7% 3 8% 5% 5% 6% 5% 6% 8% 37%	4531
August	1st to 10th	Day	18% 5% 2 6% 5% 5% 6% 4 7% 9% 33%	4104
	11th to 20th	Day	13% 5% 2 5% 4 4 5% 4 7% 8% 42%	3867
	21st to end	Day	13% 5% 2 4 3 3 4 3 4 7% 50%	4180
September	1st to 10th	Day	16% 6% 1 5% 5% 3 4 3 4 6% 47%	2280
	11th to 20th	Day	13% 5% 1 5% 3 4 3 3 5% 8% 50%	2280
	21st to end	Day	22% 7% 2 6% 4 3 4 4 4 6% 39%	2014
October	1st to 10th	Day	17% 6% 2 5% 4 3 3 4 5% 7% 44%	1900
	11th to 20th	Day	16% 6% 2 6% 3 3 2 3 5% 7% 46%	1900
	21st to end	Day	16% 7% 2 5% 4 4 4 3 5% 7% 44%	2090
November	1st to 10th	Day	13% 5% 2 5% 3 3 3 3 5% 7% 51%	1785
	11th to 20th	Day	17% 5% 1 5% 4 4 4 3 6% 6% 46%	1760
	21st to end	Day	16% 7% 2 5% 4 4 4 4 5% 7% 42%	1598
December	1st to 10th	Day	19% 8% 2 7% 5% 5% 4 4 6% 7% 33%	1520
	11th to 20th	Day	18% 6% 2 6% 5% 3 5% 4 5% 8% 36%	1910
	21st to end	Day	18% 7% 3 8% 5% 5% 6% 4 6% 6% 31%	2772
January	1st to 10th	Night	14% 8% 3 8% 5% 5% 4 3 6% 6% 38%	4429
	11th to 20th	Night	18% 8% 3 6% 4 4 4 3 5% 7% 38%	4207
	21st to end	Night	23% 9% 2 7% 4 4 4 2 5% 6% 34%	4532
February	1st to 10th	Night	17% 8% 3 6% 5% 4 4 3 6% 6% 39%	2470
	11th to 20th	Night	17% 8% 3 6% 4 4 4 3 5% 7% 40%	2346
	21st to end	Night	17% 7% 3 8% 5% 4 5% 4 4 6% 38%	1884
March	1st to 10th	Night	22% 9% 3 5% 4 3 3 3 4 6% 38%	3549
	11th to 20th	Night	11% 6% 2 6% 4 4 4 3 5% 6% 48%	3480
	21st to end	Night	15% 8% 2 6% 4 4 4 2 4 5% 46%	3702
April	1st to 10th	Night	17% 8% 3 7% 5% 4 4 4 5% 7% 37%	1900
	11th to 20th	Night	15% 6% 2 6% 4 4 4 4 6% 8% 42%	1875
	21st to end	Night	19% 5% 2 5% 4 3 4 4 5% 6% 43%	1900
May	1st to 10th	Night	26% 7% 2 8% 4 4 4 3 4 6% 31%	2858
	11th to 20th	Night	26% 8% 3 7% 4 4 4 4 5% 7% 28%	2495
	21st to end	Night	12% 6% 2 6% 5% 4 4 5% 6% 10% 42%	2732
June	1st to 10th	Night	13% 5% 2 5% 4 4 5% 4 6% 6% 46%	1520
	11th to 20th	Night	12% 5% 1 5% 5% 5% 4 4 8% 10% 41%	1520
	21st to end	Night	9% 4 3 6% 5% 4 5% 5% 7% 10% 43%	1520
July	1st to 10th	Night	7% 6% 2 6% 5% 6% 7% 5% 7% 10% 40%	2520
	11th to 20th	Night	7% 5% 2 7% 6% 6% 6% 5% 7% 11% 38%	2520
	21st to end	Night	14% 7% 2 7% 4 5% 4 5% 5% 8% 39%	3018
August	1st to 10th	Night	19% 5% 3 6% 5% 5% 4 4 6% 8% 35%	2825
	11th to 20th	Night	13% 7% 2 7% 4 4 5% 4 5% 7% 40%	2928
	21st to end	Night	14% 5% 2 5% 4 4 4 4 6% 7% 46%	3476
September	1st to 10th	Night	14% 5% 2 5% 4 4 4 4 5% 6% 47%	1900
	11th to 20th	Night	12% 5% 2 4 3 4 3 3 4 7% 52%	1900
	21st to end	Night	21% 6% 3 5% 3 4 4 3 5% 6% 39%	2162
October	1st to 10th	Night	20% 6% 2 5% 4 3 3 3 4 5% 45%	2280
	11th to 20th	Night	16% 5% 1 6% 3 3 3 3 5% 6% 49%	2280
	21st to end	Night	17% 5% 2 5% 3 3 3 3 5% 7% 47%	2508
November	1st to 10th	Night	11% 5% 2 5% 3 3 4 3 5% 6% 53%	2513
	11th to 20th	Night	15% 6% 3 5% 4 3 3 3 5% 6% 46%	2635
	21st to end	Night	14% 5% 2 6% 4 5% 4 4 5% 7% 44%	2798
December	1st to 10th	Night	18% 8% 2 6% 4 4 5% 4 5% 6% 37%	2660
	11th to 20th	Night	18% 6% 2 6% 4 5% 5% 4 5% 7% 39%	3285
	21st to end	Night	17% 7% 2 7% 6% 5% 5% 5% 6% 7% 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	44% 4 14% 25% 10% 3	2518
	11th to 20th	Day	54% 5% 9% 17% 13% 3	2750
	21st to end	Day	53% 6% 10% 22% 8% 1	3124
February	1st to 10th	Day	46% 5% 12% 24% 11% 2	1710
	11th to 20th	Day	44% 5% 9% 26% 14% 2	1729
	21st to end	Day	51% 4 9% 22% 11% 4	1570
March	1st to 10th	Day	48% 4 11% 26% 9% 2	3301
	11th to 20th	Day	37% 6% 10% 27% 16% 4	3480
	21st to end	Day	44% 5% 8% 22% 18% 3	3929
April	1st to 10th	Day	45% 5% 11% 20% 16% 4	2280
	11th to 20th	Day	41% 6% 10% 23% 17% 4	2254
	21st to end	Day	43% 5% 9% 23% 16% 4	2280
May	1st to 10th	Day	52% 6% 8% 20% 12% 2	4103
	11th to 20th	Day	54% 7% 12% 18% 8% 1	4402
	21st to end	Day	35% 7% 13% 26% 17% 2	4837
June	1st to 10th	Day	39% 7% 11% 24% 17% 2	2660
	11th to 20th	Day	39% 8% 13% 23% 13% 3	2660
	21st to end	Day	34% 8% 13% 28% 16% 1	2660
July	1st to 10th	Day	37% 9% 14% 23% 14% 3	4370
	11th to 20th	Day	35% 8% 14% 27% 14% 2	4365
	21st to end	Day	44% 7% 10% 20% 18% 2	4386
August	1st to 10th	Day	46% 8% 12% 25% 10%	4091
	11th to 20th	Day	38% 6% 11% 28% 16% 1	3906
	21st to end	Day	34% 5% 10% 29% 19% 2	4180
September	1st to 10th	Day	39% 4 11% 21% 21% 4	2280
	11th to 20th	Day	33% 4 10% 26% 23% 4	2280
	21st to end	Day	47% 4 8% 18% 20% 2	2014
October	1st to 10th	Day	37% 4 8% 23% 24% 4	3160
	11th to 20th	Day	28% 4 12% 28% 24% 5%	3140
	21st to end	Day	27% 3 11% 30% 27% 2	3428
November	1st to 10th	Day	34% 4 14% 24% 21% 4	1785
	11th to 20th	Day	39% 5% 11% 24% 19% 3	1760
	21st to end	Day	42% 5% 12% 30% 10% 2	1598
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	45% 5% 12% 26% 10% 2	4429
	11th to 20th	Night	46% 5% 13% 24% 11% 2	4207
	21st to end	Night	52% 4 9% 25% 9% 1	4532
February	1st to 10th	Night	45% 5% 13% 25% 11% 1	2470
	11th to 20th	Night	45% 4 9% 22% 18% 2	2348
	21st to end	Night	47% 4 10% 20% 15% 4	1884
March	1st to 10th	Night	48% 5% 9% 25% 12% 1	3549
	11th to 20th	Night	36% 6% 10% 31% 15% 2	3480
	21st to end	Night	42% 5% 6% 21% 22% 4	3702
April	1st to 10th	Night	47% 4 9% 20% 16% 4	1900
	11th to 20th	Night	39% 6% 10% 21% 19% 4	1875
	21st to end	Night	40% 5% 10% 22% 17% 5%	1900
May	1st to 10th	Night	54% 6% 7% 18% 12% 2	2886
	11th to 20th	Night	54% 8% 11% 15% 10% 1	2496
	21st to end	Night	38% 8% 13% 25% 13% 4	2739
June	1st to 10th	Night	36% 8% 10% 24% 20% 2	1520
	11th to 20th	Night	36% 9% 13% 22% 18% 2	1520
	21st to end	Night	35% 8% 12% 27% 16% 2	1520
July	1st to 10th	Night	37% 9% 16% 18% 14% 5%	2480
	11th to 20th	Night	39% 8% 13% 23% 15% 1	2472
	21st to end	Night	42% 7% 13% 17% 20% 1	2922
August	1st to 10th	Night	46% 7% 15% 23% 9% 1	2814
	11th to 20th	Night	41% 6% 10% 25% 17% 1	2955
	21st to end	Night	36% 6% 10% 29% 17% 2	3476
September	1st to 10th	Night	38% 5% 11% 21% 21% 4	1900
	11th to 20th	Night	32% 4 8% 27% 24% 4	1900
	21st to end	Night	45% 5% 8% 19% 20% 3	2162
October	1st to 10th	Night	43% 4 9% 22% 21% 2	3800
	11th to 20th	Night	28% 4 9% 28% 26% 4	3779
	21st to end	Night	26% 3 10% 34% 24% 2	4120
November	1st to 10th	Night	32% 4 11% 26% 22% 4	2513
	11th to 20th	Night	40% 4 8% 25% 18% 5%	2635
	21st to end	Night	40% 5% 12% 27% 15% 2	2798
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	32% 23% 28% 15% 1	247
	21st to end	Day	24% 26% 33% 14% 3 1	297
February	1st to 10th	Day	27% 25% 30% 15% 3	270
	11th to 20th	Day	29% 26% 35% 8% 2	327
	21st to end	Day	33% 21% 25% 19% 2	382
March	1st to 10th	Day	25% 22% 30% 18% 5%	460
	11th to 20th	Day	24% 25% 32% 14% 4 1	460
	21st to end	Day	26% 21% 33% 14% 4 1	506
April	1st to 10th	Day	24% 23% 30% 15% 7% 1	460
	11th to 20th	Day	22% 22% 27% 16% 10% 4	452
	21st to end	Day	27% 13% 31% 18% 8% 2	460
May	1st to 10th	Day	29% 15% 28% 17% 9% 3	463
	11th to 20th	Day	23% 14% 28% 21% 10% 5%	487
	21st to end	Day	19% 8% 22% 26% 17% 8%	529
June	1st to 10th	Day	16% 12% 27% 23% 16% 6%	490
	11th to 20th	Day	16% 9% 26% 26% 19% 5%	490
	21st to end	Day	11% 8% 30% 27% 18% 6%	490
July	1st to 10th	Day	11% 4 22% 30% 22% 11%	485
	11th to 20th	Day	6% 6% 20% 34% 24% 10%	484
	21st to end	Day	12% 4 26% 33% 20% 5%	532
August	1st to 10th	Day	13% 3 20% 29% 25% 10%	486
	11th to 20th	Day	14% 6% 18% 31% 21% 10%	479
	21st to end	Day	13% 5% 19% 31% 20% 12%	539
September	1st to 10th	Day	18% 8% 20% 21% 22% 11%	490
	11th to 20th	Day	15% 4 18% 29% 23% 10%	490
	21st to end	Day	29% 10% 21% 23% 13% 5%	490
October	1st to 10th	Day	22% 8% 22% 24% 18% 7%	490
	11th to 20th	Day	19% 16% 23% 27% 12% 3	490
	21st to end	Day	18% 14% 33% 23% 10% 3	539
November	1st to 10th	Day	17% 13% 32% 24% 13% 2	408
	11th to 20th	Day	21% 18% 30% 21% 9% 1	360
	21st to end	Day	26% 22% 31% 16% 5% 1	200
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% 30% 25% 4	760
	21st to end	Night	19% 17% 39% 23% 3	836
February	1st to 10th	Night	22% 17% 31% 28% 2	760
	11th to 20th	Night	24% 21% 37% 17% 1	743
	21st to end	Night	24% 19% 35% 18% 4	628
March	1st to 10th	Night	21% 17% 35% 23% 4	760
	11th to 20th	Night	22% 19% 33% 21% 5% 1	760
	21st to end	Night	22% 13% 38% 21% 6%	836
April	1st to 10th	Night	22% 17% 36% 20% 5%	760
	11th to 20th	Night	19% 12% 35% 25% 8%	751
	21st to end	Night	22% 11% 32% 26% 9%	760
May	1st to 10th	Night	22% 9% 30% 28% 11%	603
	11th to 20th	Night	22% 8% 30% 29% 11%	374
	21st to end	Night	15% 5% 20% 39% 20% 1	411
June	1st to 10th	Night	14% 3 24% 37% 20% 1	380
	11th to 20th	Night	13% 5% 27% 37% 18% 1	380
	21st to end	Night	9% 2 26% 47% 16%	380
July	1st to 10th	Night	14% 3 19% 39% 23% 3	370
	11th to 20th	Night	7% 2 15% 44% 30% 2	368
	21st to end	Night	12% 3 15% 42% 26% 2	581
August	1st to 10th	Night	13% 2 12% 43% 29% 2	567
	11th to 20th	Night	13% 4 12% 40% 28% 4	642
	21st to end	Night	14% 3 16% 36% 29% 3	836
September	1st to 10th	Night	16% 4 23% 31% 24% 3	760
	11th to 20th	Night	13% 4 17% 36% 27% 4	760
	21st to end	Night	23% 5% 20% 30% 20% 2	760
October	1st to 10th	Night	21% 6% 22% 28% 19% 3	760
	11th to 20th	Night	18% 10% 27% 31% 12% 1	760
	21st to end	Night	19% 11% 30% 30% 10%	836
November	1st to 10th	Night	14% 9% 34% 30% 12% 1	780
	11th to 20th	Night	19% 16% 31% 25% 8% 1	800
	21st to end	Night	18% 16% 34% 25% 7%	800
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

Page generated at 2024-11-27 17:30:49. Version ac07

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