Shaggy Mountain Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Shaggy Mountain Observatory clear sky chart from 2005-11-06 to 2025-07-21.

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	3 6% 2 6% 4 4 5% 4 6% 7% 52%	123884
	Night	3 6% 2 6% 4 4 5% 4 6% 8% 52%	124207
Transparency	Day	28% 7% 10% 21% 21% 13%	123650
	Night	28% 7% 12% 21% 21% 11%	124028
Seeing	Day	8% 15% 32% 31% 12% 2	15693
	Night	8% 9% 23% 36% 21% 4	26264

By Month

Percentage Cloud Cover SW (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	2 7% 4 12% 7% 7% 6% 5% 8% 8% 34%	4098
	11th to 20th	Day	3 10% 4 14% 7% 6% 7% 4 6% 7% 33%	4100
	21st to end	Day	3 9% 5% 8% 5% 5% 5% 4 5% 7% 45%	4510
February	1st to 10th	Day	5% 10% 5% 9% 7% 6% 7% 4 5% 7% 35%	1800
	11th to 20th	Day	4 12% 4 10% 7% 6% 6% 5% 7% 7% 34%	1844
	21st to end	Day	4 11% 4 10% 7% 6% 5% 4 6% 8% 35%	1650
March	1st to 10th	Day	2 10% 4 8% 6% 6% 6% 5% 6% 7% 41%	4813
	11th to 20th	Day	2 10% 3 7% 5% 5% 5% 5% 7% 9% 43%	5000
	21st to end	Day	4 11% 4 11% 6% 6% 7% 6% 7% 8% 31%	5486
April	1st to 10th	Day	5% 8% 3 8% 6% 6% 5% 5% 6% 6% 43%	2205
	11th to 20th	Day	5% 7% 3 8% 6% 6% 5% 5% 7% 7% 44%	2541
	21st to end	Day	4 6% 3 7% 6% 4 5% 4 7% 8% 47%	2600
May	1st to 10th	Day	6% 6% 2 7% 4 4 5% 4 6% 7% 48%	5880
	11th to 20th	Day	5% 4 2 5% 3 4 5% 4 7% 9% 52%	5841
	21st to end	Day	3 4 1 4 4 4 5% 4 7% 8% 56%	6423
June	1st to 10th	Day	2 3 1 5% 4 4 4 5% 6% 10% 56%	2600
	11th to 20th	Day	4 3 1 4 3 3 4 3 5% 6% 64%	2600
	21st to end	Day	1 1 1 3 2 3 3 3 6% 5% 72%	2600
July	1st to 10th	Day	1 1 1 2 2 2 3 3 4 5% 76%	4240
	11th to 20th	Day	2 2 1 3 3 4 5% 5% 7% 9% 59%	4240
	21st to end	Day	2 2 1 3 3 3 4 4 5% 7% 67%	4455
August	1st to 10th	Day	2 2 1 3 3 4 4 4 6% 8% 64%	4092
	11th to 20th	Day	2 1 1 3 3 4 4 3 6% 8% 67%	4028
	21st to end	Day	2 2 1 3 2 3 3 3 5% 6% 70%	4500
September	1st to 10th	Day	2 2 1 2 2 3 3 3 5% 5% 70%	2123
	11th to 20th	Day	3 2 1 4 3 3 5% 3 5% 7% 64%	2090
	21st to end	Day	4 2 1 3 3 2 2 3 4 5% 69%	2090
October	1st to 10th	Day	3 3 1 3 2 3 3 2 4 5% 69%	3344
	11th to 20th	Day	6% 4 1 4 3 3 3 3 5% 7% 62%	3150
	21st to end	Day	4 5% 2 5% 4 4 5% 3 4 6% 59%	3408
November	1st to 10th	Day	6% 5% 2 5% 4 4 6% 5% 7% 7% 48%	1764
	11th to 20th	Day	5% 8% 3 6% 4 4 4 4 6% 8% 47%	1800
	21st to end	Day	3 9% 3 8% 6% 5% 6% 5% 6% 8% 42%	1721
December	1st to 10th	Day	3 6% 2 6% 4 3 4 3 4 7% 59%	2620
	11th to 20th	Day	3 8% 3 7% 6% 6% 5% 4 8% 9% 40%	3118
	21st to end	Day	3 9% 4 9% 7% 7% 7% 5% 8% 9% 32%	4510
January	1st to 10th	Night	3 9% 3 10% 6% 6% 5% 5% 6% 9% 38%	5867
	11th to 20th	Night	3 11% 4 10% 5% 5% 5% 7% 8% 8% 33%	5880
	21st to end	Night	3 8% 3 7% 5% 5% 6% 4 6% 8% 46%	6468
February	1st to 10th	Night	4 8% 3 8% 7% 6% 7% 5% 7% 9% 37%	2600
	11th to 20th	Night	3 10% 3 8% 5% 6% 5% 4 6% 8% 42%	2439
	21st to end	Night	3 10% 3 7% 5% 6% 6% 4 7% 8% 42%	1980
March	1st to 10th	Night	2 7% 4 9% 6% 7% 6% 4 7% 8% 40%	5079
	11th to 20th	Night	3 7% 3 7% 5% 5% 5% 5% 8% 9% 44%	5000
	21st to end	Night	3 7% 2 7% 5% 5% 6% 5% 7% 8% 45%	5489
April	1st to 10th	Night	6% 8% 2 7% 4 4 4 3 5% 7% 50%	2195
	11th to 20th	Night	4 6% 2 5% 5% 5% 6% 4 7% 7% 50%	1804
	21st to end	Night	6% 6% 2 6% 4 4 5% 4 7% 7% 50%	1800
May	1st to 10th	Night	4 5% 2 6% 4 5% 5% 4 6% 7% 53%	4100
	11th to 20th	Night	3 3 1 3 3 2 4 3 5% 9% 63%	4074
	21st to end	Night	3 4 1 4 3 3 3 4 5% 8% 60%	4474
June	1st to 10th	Night	3 5% 1 4 4 4 4 4 5% 7% 59%	1800
	11th to 20th	Night	3 3 2 3 2 3 4 3 5% 6% 66%	1800
	21st to end	Night	1 3 1 3 3 3 4 3 6% 8% 66%	1800
July	1st to 10th	Night	1 1 1 3 3 3 3 3 4 5% 73%	2950
	11th to 20th	Night	2 3 1 5% 4 5% 5% 5% 7% 9% 56%	2950
	21st to end	Night	2 3 1 5% 3 4 4 4 6% 7% 60%	3109
August	1st to 10th	Night	2 1 1 4 3 4 3 4 7% 9% 62%	2847
	11th to 20th	Night	2 2 1 4 3 4 4 4 6% 6% 64%	2811
	21st to end	Night	3 2 1 2 2 2 3 3 4 6% 71%	3167
September	1st to 10th	Night	3 3 1 4 2 3 3 3 4 6% 68%	2054
	11th to 20th	Night	3 3 1 4 3 3 3 3 4 7% 66%	2090
	21st to end	Night	5% 2 1 3 3 3 2 3 3 5% 71%	2090
October	1st to 10th	Night	3 2 1 3 2 2 3 3 3 5% 73%	3630
	11th to 20th	Night	5% 3 1 3 3 3 4 3 5% 7% 62%	3778
	21st to end	Night	6% 4 1 4 3 3 3 3 5% 7% 60%	4145
November	1st to 10th	Night	5% 6% 2 6% 4 4 4 4 6% 8% 50%	2535
	11th to 20th	Night	5% 5% 2 5% 3 4 6% 4 7% 9% 51%	2600
	21st to end	Night	5% 7% 2 6% 5% 4 4 4 5% 9% 48%	2652
December	1st to 10th	Night	2 4 2 5% 4 4 3 3 4 7% 61%	4570
	11th to 20th	Night	4 7% 2 6% 5% 4 4 4 5% 9% 48%	5112
	21st to end	Night	4 9% 3 9% 6% 7% 6% 5% 8% 8% 35%	6468

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% 10% 18% 19% 9% 1	4099
	11th to 20th	Day	44% 13% 13% 20% 7% 3	4100
	21st to end	Day	37% 9% 13% 20% 19% 1	4510
February	1st to 10th	Day	47% 10% 14% 16% 11% 1	1800
	11th to 20th	Day	47% 10% 13% 19% 10% 1	1853
	21st to end	Day	45% 9% 15% 19% 11% 2	1650
March	1st to 10th	Day	35% 12% 11% 23% 13% 5%	4813
	11th to 20th	Day	32% 8% 13% 32% 15% 1	5000
	21st to end	Day	46% 9% 18% 17% 9% 1	5486
April	1st to 10th	Day	39% 9% 12% 20% 16% 5%	2205
	11th to 20th	Day	36% 8% 12% 22% 18% 3	2544
	21st to end	Day	33% 8% 12% 25% 20% 2	2600
May	1st to 10th	Day	29% 7% 12% 23% 23% 6%	5880
	11th to 20th	Day	25% 6% 9% 25% 30% 5%	5844
	21st to end	Day	21% 7% 9% 27% 32% 5%	6424
June	1st to 10th	Day	21% 7% 9% 23% 30% 10%	2600
	11th to 20th	Day	20% 5% 6% 18% 29% 22%	2600
	21st to end	Day	13% 4 5% 12% 32% 35%	2600
July	1st to 10th	Day	10% 4 4 7% 15% 60%	4176
	11th to 20th	Day	17% 5% 5% 12% 26% 35%	4171
	21st to end	Day	15% 5% 7% 9% 26% 38%	4315
August	1st to 10th	Day	16% 5% 6% 17% 29% 28%	4078
	11th to 20th	Day	15% 4 8% 13% 33% 27%	4067
	21st to end	Day	15% 3 7% 14% 29% 32%	4499
September	1st to 10th	Day	15% 4 6% 16% 28% 30%	2123
	11th to 20th	Day	18% 7% 8% 18% 27% 22%	2090
	21st to end	Day	18% 5% 6% 19% 33% 19%	2090
October	1st to 10th	Day	19% 4 5% 12% 37% 22%	3344
	11th to 20th	Day	23% 4 7% 28% 29% 9%	3150
	21st to end	Day	25% 7% 11% 30% 23% 4	3408
November	1st to 10th	Day	34% 8% 13% 23% 19% 3	1764
	11th to 20th	Day	33% 9% 12% 30% 16% 1	1800
	21st to end	Day	38% 10% 13% 24% 14% 1	1721
December	1st to 10th	Day	26% 6% 14% 43% 10% 1	2620
	11th to 20th	Day	38% 9% 15% 28% 10%	3116
	21st to end	Day	44% 9% 16% 20% 12%	4510
January	1st to 10th	Night	40% 9% 18% 21% 11% 1	5867
	11th to 20th	Night	42% 11% 15% 22% 9% 1	5880
	21st to end	Night	32% 11% 14% 23% 19% 1	6468
February	1st to 10th	Night	41% 10% 18% 21% 9% 2	2600
	11th to 20th	Night	39% 7% 15% 21% 16% 1	2440
	21st to end	Night	38% 9% 12% 24% 14% 3	1980
March	1st to 10th	Night	38% 9% 11% 23% 14% 5%	5079
	11th to 20th	Night	32% 7% 14% 29% 17% 2	5000
	21st to end	Night	33% 10% 18% 22% 16% 2	5489
April	1st to 10th	Night	33% 6% 13% 23% 18% 7%	2195
	11th to 20th	Night	30% 9% 11% 23% 22% 5%	1804
	21st to end	Night	31% 8% 14% 23% 22% 2	1800
May	1st to 10th	Night	26% 8% 12% 21% 26% 6%	4097
	11th to 20th	Night	18% 5% 8% 28% 35% 6%	4075
	21st to end	Night	20% 6% 9% 21% 40% 4	4474
June	1st to 10th	Night	23% 7% 9% 21% 29% 11%	1800
	11th to 20th	Night	18% 5% 7% 17% 31% 22%	1800
	21st to end	Night	16% 4 5% 9% 28% 37%	1800
July	1st to 10th	Night	14% 3 3 8% 10% 61%	2905
	11th to 20th	Night	22% 5% 4 10% 24% 36%	2896
	21st to end	Night	22% 3 7% 10% 22% 36%	3011
August	1st to 10th	Night	17% 4 7% 13% 28% 31%	2838
	11th to 20th	Night	18% 3 9% 12% 30% 27%	2838
	21st to end	Night	15% 4 6% 10% 25% 40%	3167
September	1st to 10th	Night	18% 3 6% 15% 25% 33%	2054
	11th to 20th	Night	18% 5% 7% 17% 27% 25%	2090
	21st to end	Night	18% 3 5% 15% 33% 25%	2090
October	1st to 10th	Night	16% 4 6% 13% 33% 29%	3630
	11th to 20th	Night	21% 6% 6% 26% 28% 12%	3778
	21st to end	Night	23% 6% 6% 28% 30% 6%	4145
November	1st to 10th	Night	31% 8% 10% 24% 22% 5%	2535
	11th to 20th	Night	28% 9% 12% 29% 18% 3	2600
	21st to end	Night	33% 8% 10% 29% 16% 3	2652
December	1st to 10th	Night	24% 6% 16% 38% 14% 2	4570
	11th to 20th	Night	34% 6% 21% 26% 12%	5113
	21st to end	Night	43% 8% 18% 20% 10%	6468

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	13% 20% 34% 21% 12% 1	300
	11th to 20th	Day	8% 19% 35% 25% 12% 1	300
	21st to end	Day	8% 15% 35% 29% 11% 1	330
February	1st to 10th	Day	12% 28% 30% 23% 7%	300
	11th to 20th	Day	11% 21% 38% 22% 8% 1	389
	21st to end	Day	10% 23% 39% 22% 5% 1	414
March	1st to 10th	Day	11% 23% 36% 23% 6%	500
	11th to 20th	Day	11% 18% 41% 22% 7%	500
	21st to end	Day	17% 25% 34% 21% 4	550
April	1st to 10th	Day	13% 22% 29% 24% 10% 1	500
	11th to 20th	Day	14% 26% 32% 22% 6%	492
	21st to end	Day	12% 24% 30% 25% 8% 1	500
May	1st to 10th	Day	10% 21% 37% 23% 9% 1	503
	11th to 20th	Day	12% 17% 30% 28% 10% 3	528
	21st to end	Day	8% 15% 30% 30% 13% 3	573
June	1st to 10th	Day	3 13% 33% 36% 11% 4	530
	11th to 20th	Day	7% 15% 38% 28% 11% 1	530
	21st to end	Day	2 13% 34% 40% 9% 2	530
July	1st to 10th	Day	2 6% 33% 45% 12% 2	526
	11th to 20th	Day	3 6% 33% 41% 14% 3	524
	21st to end	Day	2 2 28% 47% 18% 3	532
August	1st to 10th	Day	4 7% 29% 44% 14% 2	487
	11th to 20th	Day	3 8% 28% 40% 17% 4	477
	21st to end	Day	4 7% 29% 38% 18% 3	539
September	1st to 10th	Day	3 9% 26% 39% 18% 6%	490
	11th to 20th	Day	6% 11% 30% 34% 16% 3	490
	21st to end	Day	6% 12% 25% 35% 19% 3	490
October	1st to 10th	Day	6% 15% 29% 28% 19% 4	409
	11th to 20th	Day	6% 13% 31% 25% 21% 3	300
	21st to end	Day	8% 15% 32% 30% 14% 1	330
November	1st to 10th	Day	14% 16% 26% 27% 14% 3	300
	11th to 20th	Day	8% 18% 31% 27% 11% 4	300
	21st to end	Day	12% 15% 26% 26% 19% 3	300
December	1st to 10th	Day	9% 13% 29% 30% 15% 3	300
	11th to 20th	Day	8% 14% 34% 34% 8% 1	300
	21st to end	Day	16% 15% 33% 28% 7%	330
January	1st to 10th	Night	12% 11% 24% 33% 17% 4	800
	11th to 20th	Night	9% 11% 24% 33% 17% 5%	800
	21st to end	Night	10% 10% 23% 34% 19% 4	880
February	1st to 10th	Night	9% 11% 30% 33% 14% 3	800
	11th to 20th	Night	9% 9% 30% 35% 14% 3	784
	21st to end	Night	9% 11% 28% 36% 14% 2	660
March	1st to 10th	Night	9% 13% 29% 35% 11% 3	800
	11th to 20th	Night	7% 11% 27% 35% 18% 3	800
	21st to end	Night	8% 15% 30% 35% 10% 2	880
April	1st to 10th	Night	12% 14% 27% 33% 13% 2	795
	11th to 20th	Night	7% 14% 30% 37% 11% 2	593
	21st to end	Night	9% 10% 26% 35% 16% 3	600
May	1st to 10th	Night	8% 10% 28% 36% 17% 2	600
	11th to 20th	Night	7% 6% 21% 37% 25% 4	591
	21st to end	Night	8% 7% 25% 32% 24% 5%	650
June	1st to 10th	Night	6% 7% 19% 41% 25% 3	600
	11th to 20th	Night	5% 11% 23% 39% 20% 2	600
	21st to end	Night	3 4 23% 39% 27% 4	600
July	1st to 10th	Night	3 4 20% 38% 32% 3	585
	11th to 20th	Night	4 2 17% 45% 30% 2	582
	21st to end	Night	5% 2 12% 34% 45% 2	624
August	1st to 10th	Night	4 2 18% 41% 33% 2	567
	11th to 20th	Night	3 2 19% 43% 29% 4	564
	21st to end	Night	3 4 18% 40% 32% 4	627
September	1st to 10th	Night	5% 4 15% 38% 32% 5%	724
	11th to 20th	Night	4 5% 18% 38% 27% 7%	760
	21st to end	Night	6% 6% 15% 34% 29% 10%	760
October	1st to 10th	Night	6% 7% 21% 36% 26% 4	760
	11th to 20th	Night	6% 5% 21% 36% 25% 7%	760
	21st to end	Night	7% 9% 24% 33% 22% 5%	836
November	1st to 10th	Night	11% 10% 20% 33% 21% 5%	780
	11th to 20th	Night	8% 8% 27% 37% 15% 5%	800
	21st to end	Night	11% 7% 24% 34% 19% 5%	852
December	1st to 10th	Night	9% 10% 24% 38% 19% 2	1000
	11th to 20th	Night	14% 9% 28% 35% 13% 2	970
	21st to end	Night	15% 14% 27% 31% 12% 1	880

Page generated at 2025-07-17 23:10:00. Version ac07

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