Mountain Skies Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mountain Skies Observatory clear sky chart from 2005-11-06 to 2025-05-11.

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	4 6% 2 6% 4 4 5% 4 6% 7% 52%	117899
	Night	4 6% 2 6% 4 4 5% 4 6% 8% 51%	120004
Transparency	Day	29% 7% 12% 19% 20% 14%	117664
	Night	30% 8% 13% 21% 18% 10%	119828
Seeing	Day	7% 7% 19% 32% 26% 8%	15408
	Night	8% 4 16% 41% 27% 3	26046

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	3 8% 4 11% 8% 6% 6% 6% 11% 8% 30%	4098
	11th to 20th	Day	4 10% 5% 13% 7% 7% 6% 5% 6% 6% 31%	4100
	21st to end	Day	4 9% 5% 8% 5% 4 4 5% 6% 8% 43%	4510
February	1st to 10th	Day	6% 10% 3 9% 5% 7% 7% 6% 8% 6% 35%	1800
	11th to 20th	Day	4 11% 3 10% 7% 7% 6% 5% 6% 7% 35%	1844
	21st to end	Day	5% 11% 4 9% 6% 5% 5% 4 6% 7% 37%	1650
March	1st to 10th	Day	2 10% 3 8% 6% 7% 6% 4 5% 6% 42%	4818
	11th to 20th	Day	3 9% 3 8% 5% 5% 6% 5% 8% 8% 42%	5000
	21st to end	Day	4 10% 3 10% 7% 6% 7% 6% 7% 7% 32%	5486
April	1st to 10th	Day	5% 7% 3 7% 5% 6% 5% 4 6% 7% 44%	2207
	11th to 20th	Day	5% 7% 3 7% 5% 5% 5% 5% 6% 6% 46%	2533
	21st to end	Day	5% 6% 3 6% 5% 5% 5% 4 5% 8% 49%	2600
May	1st to 10th	Day	6% 9% 2 7% 5% 5% 5% 5% 6% 8% 42%	4240
	11th to 20th	Day	4 3 1 3 3 4 4 4 6% 9% 59%	4073
	21st to end	Day	4 4 2 5% 4 4 5% 5% 7% 8% 52%	4477
June	1st to 10th	Day	2 3 1 5% 3 3 4 4 7% 10% 57%	2470
	11th to 20th	Day	4 4 1 3 3 3 4 3 4 6% 65%	2470
	21st to end	Day	2 1 1 2 3 3 3 3 4 6% 73%	2470
July	1st to 10th	Day	1 1 2 2 2 3 2 4 5% 77%	4110
	11th to 20th	Day	2 2 1 3 3 4 5% 5% 6% 9% 59%	4110
	21st to end	Day	2 2 1 3 3 3 4 4 5% 7% 67%	4454
August	1st to 10th	Day	2 2 1 3 3 3 4 4 6% 8% 64%	4092
	11th to 20th	Day	2 1 1 3 3 3 4 4 6% 6% 67%	4028
	21st to end	Day	2 2 1 3 2 2 3 3 5% 6% 71%	4495
September	1st to 10th	Day	3 2 1 2 2 3 4 3 5% 6% 70%	2126
	11th to 20th	Day	4 2 1 3 3 3 4 3 6% 6% 65%	2090
	21st to end	Day	5% 2 1 3 2 3 2 2 4 6% 70%	2090
October	1st to 10th	Day	4 3 1 3 2 3 3 2 4 4 70%	3343
	11th to 20th	Day	5% 4 1 4 3 3 3 4 4 7% 62%	3150
	21st to end	Day	4 5% 1 5% 4 3 4 3 5% 6% 60%	3418
November	1st to 10th	Day	7% 5% 2 6% 4 4 5% 5% 6% 7% 49%	1764
	11th to 20th	Day	6% 7% 3 6% 4 4 4 4 7% 7% 48%	1800
	21st to end	Day	4 7% 4 9% 6% 5% 5% 6% 6% 8% 41%	1721
December	1st to 10th	Day	3 5% 2 5% 4 4 3 3 8% 12% 51%	2620
	11th to 20th	Day	4 7% 2 7% 5% 5% 6% 5% 8% 9% 40%	3132
	21st to end	Day	4 9% 3 10% 6% 7% 7% 6% 9% 9% 30%	4510
January	1st to 10th	Night	3 9% 3 9% 7% 6% 6% 6% 8% 9% 34%	5867
	11th to 20th	Night	4 11% 4 11% 6% 6% 6% 6% 9% 10% 27%	5880
	21st to end	Night	3 8% 3 8% 5% 6% 6% 5% 6% 8% 41%	6468
February	1st to 10th	Night	4 8% 4 7% 6% 6% 7% 6% 8% 10% 35%	2600
	11th to 20th	Night	4 9% 3 8% 5% 5% 6% 5% 5% 8% 42%	2441
	21st to end	Night	3 9% 3 8% 5% 4 6% 5% 6% 9% 41%	1980
March	1st to 10th	Night	4 9% 4 8% 6% 7% 6% 5% 6% 7% 40%	5075
	11th to 20th	Night	3 7% 3 7% 5% 5% 5% 5% 8% 9% 44%	5000
	21st to end	Night	4 7% 3 6% 5% 5% 5% 4 7% 8% 46%	5489
April	1st to 10th	Night	6% 8% 2 6% 5% 4 4 3 5% 7% 51%	2192
	11th to 20th	Night	4 5% 2 5% 5% 4 6% 5% 6% 8% 50%	1811
	21st to end	Night	7% 5% 2 5% 4 4 5% 4 7% 6% 51%	1800
May	1st to 10th	Night	5% 7% 2 6% 5% 5% 5% 4 6% 8% 46%	2950
	11th to 20th	Night	3 4 1 4 3 3 4 3 5% 7% 62%	2838
	21st to end	Night	5% 5% 2 5% 4 3 4 3 5% 9% 56%	3110
June	1st to 10th	Night	4 4 2 3 4 5% 4 3 6% 7% 58%	1710
	11th to 20th	Night	3 3 1 3 2 3 4 3 4 7% 67%	1710
	21st to end	Night	1 2 1 3 3 3 4 3 5% 8% 67%	1710
July	1st to 10th	Night	1 1 1 3 3 3 3 2 4 5% 73%	2860
	11th to 20th	Night	2 3 1 4 3 5% 6% 4 7% 9% 56%	2860
	21st to end	Night	2 3 1 4 4 4 4 4 6% 6% 61%	3109
August	1st to 10th	Night	2 1 1 4 3 3 4 4 7% 9% 62%	2847
	11th to 20th	Night	2 2 1 5% 3 3 5% 4 6% 7% 64%	2811
	21st to end	Night	3 2 1 3 2 3 3 3 4 6% 70%	3172
September	1st to 10th	Night	3 3 1 4 2 3 2 3 4 6% 68%	2050
	11th to 20th	Night	3 3 1 4 3 3 3 3 5% 7% 66%	2090
	21st to end	Night	5% 2 1 3 2 2 3 2 3 5% 72%	2090
October	1st to 10th	Night	3 2 1 3 2 2 3 2 3 4 74%	3631
	11th to 20th	Night	5% 3 1 4 3 3 4 3 4 7% 63%	3778
	21st to end	Night	5% 5% 2 4 2 3 3 4 5% 6% 62%	4138
November	1st to 10th	Night	6% 6% 2 6% 4 3 4 4 5% 8% 52%	2535
	11th to 20th	Night	5% 5% 2 5% 4 4 5% 4 6% 8% 53%	2600
	21st to end	Night	5% 7% 2 5% 5% 4 5% 5% 6% 8% 47%	2652
December	1st to 10th	Night	3 4 2 5% 4 3 3 3 9% 10% 53%	4570
	11th to 20th	Night	5% 7% 2 6% 4 4 5% 4 6% 8% 47%	5112
	21st to end	Night	5% 9% 3 9% 6% 7% 8% 5% 8% 9% 32%	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	42% 13% 23% 15% 7%	4099
	11th to 20th	Day	49% 10% 17% 16% 6% 3	4100
	21st to end	Day	39% 12% 15% 20% 14% 1	4510
February	1st to 10th	Day	49% 9% 14% 15% 11% 1	1800
	11th to 20th	Day	45% 9% 14% 21% 11% 1	1853
	21st to end	Day	42% 8% 16% 20% 12% 1	1650
March	1st to 10th	Day	36% 8% 13% 25% 14% 4	4818
	11th to 20th	Day	36% 5% 15% 29% 13% 1	5000
	21st to end	Day	44% 10% 18% 19% 9% 1	5486
April	1st to 10th	Day	35% 8% 14% 20% 17% 6%	2207
	11th to 20th	Day	34% 6% 13% 21% 21% 5%	2536
	21st to end	Day	30% 7% 12% 24% 23% 3	2600
May	1st to 10th	Day	32% 6% 14% 21% 20% 7%	4240
	11th to 20th	Day	19% 5% 13% 27% 31% 5%	4076
	21st to end	Day	25% 7% 13% 22% 28% 6%	4477
June	1st to 10th	Day	21% 5% 9% 19% 32% 13%	2470
	11th to 20th	Day	19% 4 8% 15% 29% 25%	2470
	21st to end	Day	14% 2 6% 10% 27% 42%	2470
July	1st to 10th	Day	9% 2 5% 7% 14% 62%	4046
	11th to 20th	Day	18% 3 9% 10% 24% 36%	4041
	21st to end	Day	14% 4 8% 10% 23% 41%	4314
August	1st to 10th	Day	17% 4 8% 17% 27% 28%	4078
	11th to 20th	Day	16% 4 7% 13% 32% 28%	4067
	21st to end	Day	14% 3 6% 14% 28% 34%	4494
September	1st to 10th	Day	14% 3 8% 14% 28% 33%	2126
	11th to 20th	Day	16% 5% 9% 18% 27% 24%	2090
	21st to end	Day	16% 4 7% 19% 31% 22%	2090
October	1st to 10th	Day	18% 4 6% 12% 39% 21%	3343
	11th to 20th	Day	25% 3 9% 26% 28% 10%	3150
	21st to end	Day	25% 5% 11% 28% 26% 5%	3418
November	1st to 10th	Day	33% 7% 13% 23% 20% 4	1764
	11th to 20th	Day	34% 8% 13% 28% 16% 1	1800
	21st to end	Day	38% 9% 16% 22% 14% 1	1721
December	1st to 10th	Day	28% 12% 16% 35% 8% 1	2620
	11th to 20th	Day	37% 11% 19% 26% 8%	3130
	21st to end	Day	45% 12% 16% 18% 9%	4510
January	1st to 10th	Night	39% 15% 19% 19% 7%	5867
	11th to 20th	Night	48% 14% 14% 18% 6% 1	5880
	21st to end	Night	38% 13% 16% 24% 9%	6468
February	1st to 10th	Night	42% 12% 18% 19% 9% 1	2600
	11th to 20th	Night	40% 7% 15% 23% 14% 1	2442
	21st to end	Night	39% 7% 14% 25% 13% 2	1980
March	1st to 10th	Night	41% 6% 12% 28% 12% 1	5075
	11th to 20th	Night	33% 7% 13% 29% 17% 2	5000
	21st to end	Night	33% 10% 18% 24% 13% 2	5489
April	1st to 10th	Night	34% 5% 12% 25% 18% 7%	2192
	11th to 20th	Night	29% 7% 10% 23% 26% 5%	1811
	21st to end	Night	31% 7% 14% 23% 23% 3	1800
May	1st to 10th	Night	30% 6% 13% 20% 26% 5%	2950
	11th to 20th	Night	20% 5% 8% 28% 29% 9%	2839
	21st to end	Night	25% 6% 11% 25% 29% 4	3110
June	1st to 10th	Night	23% 5% 10% 20% 29% 14%	1710
	11th to 20th	Night	17% 4 8% 16% 31% 24%	1710
	21st to end	Night	15% 3 6% 9% 26% 41%	1710
July	1st to 10th	Night	15% 2 5% 7% 10% 61%	2815
	11th to 20th	Night	23% 3 9% 9% 24% 31%	2806
	21st to end	Night	23% 4 8% 11% 22% 33%	3011
August	1st to 10th	Night	20% 5% 9% 14% 25% 27%	2838
	11th to 20th	Night	19% 6% 7% 15% 26% 27%	2838
	21st to end	Night	16% 3 7% 9% 29% 35%	3172
September	1st to 10th	Night	17% 3 8% 13% 25% 34%	2050
	11th to 20th	Night	18% 5% 8% 17% 27% 26%	2090
	21st to end	Night	18% 3 4 16% 33% 26%	2090
October	1st to 10th	Night	15% 3 6% 16% 37% 24%	3631
	11th to 20th	Night	24% 4 7% 27% 30% 9%	3778
	21st to end	Night	22% 4 11% 34% 24% 4	4138
November	1st to 10th	Night	29% 6% 11% 27% 23% 5%	2535
	11th to 20th	Night	28% 7% 15% 31% 16% 3	2600
	21st to end	Night	33% 9% 13% 27% 15% 2	2652
December	1st to 10th	Night	24% 14% 24% 27% 9% 1	4570
	11th to 20th	Night	35% 8% 27% 23% 7%	5113
	21st to end	Night	47% 9% 20% 17% 7%	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	14% 9% 17% 34% 17% 8%	300
	11th to 20th	Day	9% 7% 21% 35% 20% 8%	300
	21st to end	Day	11% 4 18% 38% 22% 7%	330
February	1st to 10th	Day	13% 11% 26% 34% 14% 4	300
	11th to 20th	Day	10% 8% 25% 36% 19% 3	389
	21st to end	Day	7% 9% 26% 38% 16% 3	414
March	1st to 10th	Day	12% 9% 23% 36% 16% 5%	500
	11th to 20th	Day	8% 10% 29% 29% 20% 4	500
	21st to end	Day	14% 13% 27% 30% 15% 2	550
April	1st to 10th	Day	10% 10% 27% 29% 19% 6%	500
	11th to 20th	Day	10% 12% 28% 30% 17% 3	492
	21st to end	Day	10% 12% 24% 29% 20% 4	500
May	1st to 10th	Day	8% 9% 26% 33% 17% 7%	503
	11th to 20th	Day	8% 6% 23% 32% 24% 8%	489
	21st to end	Day	7% 7% 21% 25% 29% 11%	529
June	1st to 10th	Day	3 7% 18% 29% 34% 9%	490
	11th to 20th	Day	6% 6% 21% 34% 25% 7%	490
	21st to end	Day	2 3 22% 34% 31% 7%	490
July	1st to 10th	Day	1 2 15% 36% 34% 11%	486
	11th to 20th	Day	4 2 12% 34% 37% 11%	484
	21st to end	Day	3 9% 31% 44% 13%	531
August	1st to 10th	Day	5% 2 12% 35% 37% 10%	487
	11th to 20th	Day	3 3 14% 31% 36% 14%	477
	21st to end	Day	4 3 13% 29% 40% 10%	539
September	1st to 10th	Day	3 3 12% 31% 34% 17%	490
	11th to 20th	Day	4 6% 18% 28% 31% 13%	490
	21st to end	Day	6% 5% 16% 24% 36% 14%	490
October	1st to 10th	Day	5% 5% 20% 25% 27% 17%	408
	11th to 20th	Day	7% 5% 15% 31% 28% 14%	300
	21st to end	Day	6% 6% 20% 35% 26% 6%	330
November	1st to 10th	Day	12% 8% 15% 32% 23% 10%	300
	11th to 20th	Day	7% 8% 15% 39% 21% 10%	300
	21st to end	Day	11% 5% 16% 33% 26% 9%	300
December	1st to 10th	Day	8% 7% 13% 34% 26% 11%	300
	11th to 20th	Day	10% 6% 15% 44% 21% 4	300
	21st to end	Day	14% 8% 20% 34% 21% 2	330
January	1st to 10th	Night	13% 5% 15% 42% 22% 5%	800
	11th to 20th	Night	11% 5% 18% 36% 26% 5%	800
	21st to end	Night	11% 5% 14% 42% 23% 4	880
February	1st to 10th	Night	10% 6% 20% 46% 14% 4	800
	11th to 20th	Night	11% 4 19% 46% 18% 2	784
	21st to end	Night	9% 5% 21% 45% 19% 1	660
March	1st to 10th	Night	10% 7% 20% 44% 17% 3	800
	11th to 20th	Night	9% 5% 18% 44% 23% 2	800
	21st to end	Night	11% 7% 22% 44% 15% 2	880
April	1st to 10th	Night	13% 7% 19% 40% 18% 2	792
	11th to 20th	Night	6% 8% 23% 48% 14% 1	593
	21st to end	Night	9% 5% 22% 41% 23% 2	600
May	1st to 10th	Night	8% 6% 22% 41% 21% 2	600
	11th to 20th	Night	6% 4 15% 41% 31% 3	563
	21st to end	Night	8% 6% 17% 36% 29% 4	617
June	1st to 10th	Night	5% 6% 16% 40% 31% 2	570
	11th to 20th	Night	4 6% 21% 42% 26% 1	570
	21st to end	Night	2 5% 14% 43% 34% 2	570
July	1st to 10th	Night	3 1 13% 37% 41% 5%	555
	11th to 20th	Night	4 2 9% 43% 39% 4	552
	21st to end	Night	5% 1 6% 32% 53% 3	624
August	1st to 10th	Night	4 1 11% 41% 41% 2	567
	11th to 20th	Night	3 1 9% 44% 39% 4	564
	21st to end	Night	3 2 10% 39% 44% 2	627
September	1st to 10th	Night	5% 2 9% 38% 40% 6%	720
	11th to 20th	Night	4 2 14% 37% 36% 7%	760
	21st to end	Night	7% 3 11% 32% 38% 8%	760
October	1st to 10th	Night	7% 3 15% 39% 29% 6%	760
	11th to 20th	Night	6% 3 14% 40% 31% 5%	760
	21st to end	Night	6% 4 17% 42% 28% 4	836
November	1st to 10th	Night	11% 4 17% 36% 28% 4	780
	11th to 20th	Night	7% 4 17% 48% 20% 5%	800
	21st to end	Night	10% 4 17% 41% 24% 5%	852
December	1st to 10th	Night	11% 4 15% 45% 22% 3	1000
	11th to 20th	Night	16% 3 17% 44% 18% 2	970
	21st to end	Night	15% 6% 18% 44% 16% 2	880

Page generated at 2025-05-08 07:33:56. Version ac07

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