Spruce Mountain Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Spruce Mountain Observatory clear sky chart from 2005-11-06 to 2025-11-03.

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 5% 2 5% 4 4 5% 4 6% 8% 53%	135395
	Night	3 5% 2 5% 4 4 4 3 6% 8% 57%	132083
Transparency	Day	25% 8% 12% 22% 23% 11%	130368
	Night	23% 7% 12% 24% 24% 9%	128114
Seeing	Day	8% 15% 33% 27% 14% 3	16911
	Night	5% 9% 37% 35% 12% 1	26885

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	5% 3 6% 4 2 3 3 5% 8% 62%	3638
	11th to 20th	Day	1 3 2 7% 6% 4 5% 4 6% 7% 56%	3796
	21st to end	Day	1 5% 2 7% 4 3 3 3 4 6% 62%	4510
February	1st to 10th	Day	1 7% 3 6% 4 4 3 2 5% 7% 57%	1805
	11th to 20th	Day	1 4 2 5% 4 3 5% 4 6% 8% 58%	1949
	21st to end	Day	1 6% 2 7% 4 3 3 4 5% 5% 59%	1650
March	1st to 10th	Day	1 7% 2 6% 3 3 3 3 5% 6% 62%	4500
	11th to 20th	Day	3 10% 4 8% 5% 5% 4 3 5% 7% 47%	4908
	21st to end	Day	2 7% 2 5% 4 4 5% 4 7% 8% 52%	5832
April	1st to 10th	Day	4 9% 2 5% 4 3 4 4 7% 7% 49%	2400
	11th to 20th	Day	6% 9% 2 8% 5% 4 4 4 6% 7% 46%	2371
	21st to end	Day	6% 9% 2 6% 4 5% 5% 4 6% 8% 45%	2400
May	1st to 10th	Day	10% 9% 2 6% 4 4 4 4 6% 7% 45%	5711
	11th to 20th	Day	6% 6% 2 6% 4 4 6% 4 6% 9% 47%	6198
	21st to end	Day	5% 5% 2 5% 4 4 5% 5% 8% 9% 48%	6925
June	1st to 10th	Day	5% 4 2 4 4 5% 6% 6% 8% 9% 48%	2800
	11th to 20th	Day	4 4 2 4 4 5% 5% 5% 7% 9% 51%	2800
	21st to end	Day	3 4 2 6% 5% 6% 6% 7% 7% 9% 47%	2800
July	1st to 10th	Day	3 3 1 3 4 4 5% 5% 8% 10% 54%	6339
	11th to 20th	Day	3 3 2 5% 4 6% 6% 6% 9% 11% 43%	6340
	21st to end	Day	5% 5% 2 6% 6% 6% 6% 5% 7% 9% 43%	6733
August	1st to 10th	Day	5% 4 2 6% 5% 6% 7% 6% 9% 10% 39%	5704
	11th to 20th	Day	3 3 1 5% 4 5% 6% 5% 8% 10% 49%	5366
	21st to end	Day	5% 4 2 6% 5% 5% 6% 5% 8% 10% 43%	5984
September	1st to 10th	Day	5% 5% 1 4 4 4 5% 4 6% 7% 56%	2400
	11th to 20th	Day	6% 2 2 4 4 4 5% 4 6% 8% 57%	2368
	21st to end	Day	5% 4 1 4 3 4 4 3 4 6% 63%	2200
October	1st to 10th	Day	3 3 1 4 2 3 3 2 4 5% 71%	3317
	11th to 20th	Day	4 4 2 6% 3 3 4 3 5% 6% 60%	3250
	21st to end	Day	3 5% 2 3 2 1 2 3 4 6% 69%	3430
November	1st to 10th	Day	1 4 2 3 3 2 2 4 6% 8% 64%	1781
	11th to 20th	Day	1 4 2 4 3 3 3 2 5% 7% 64%	1800
	21st to end	Day	2 6% 2 4 3 3 3 4 4 7% 63%	1740
December	1st to 10th	Day	1 5% 3 5% 2 2 2 2 4 5% 69%	2620
	11th to 20th	Day	1 3 2 4 3 3 3 2 4 7% 68%	3026
	21st to end	Day	1 4 2 7% 3 4 3 3 6% 9% 58%	4004
January	1st to 10th	Night	5% 4 9% 4 3 3 3 6% 7% 56%	6327
	11th to 20th	Night	1 3 1 5% 4 4 4 4 5% 7% 63%	6227
	21st to end	Night	1 5% 2 6% 4 3 4 3 4 6% 62%	6468
February	1st to 10th	Night	1 6% 3 6% 3 3 4 3 5% 7% 59%	2595
	11th to 20th	Night	1 5% 3 6% 4 4 3 4 6% 8% 55%	2338
	21st to end	Night	1 7% 2 5% 3 4 3 3 3 6% 63%	1980
March	1st to 10th	Night	1 6% 3 6% 3 4 4 3 5% 6% 59%	5374
	11th to 20th	Night	3 11% 2 5% 3 3 3 3 4 7% 55%	5071
	21st to end	Night	3 8% 2 5% 4 4 4 3 6% 8% 53%	5126
April	1st to 10th	Night	3 7% 3 5% 4 3 4 3 4 6% 58%	2000
	11th to 20th	Night	7% 9% 3 6% 3 3 4 3 4 6% 50%	1977
	21st to end	Night	7% 10% 2 6% 3 3 5% 3 5% 7% 48%	2000
May	1st to 10th	Night	9% 9% 3 7% 4 3 4 3 5% 7% 45%	4267
	11th to 20th	Night	6% 5% 2 5% 4 4 4 4 7% 9% 50%	3714
	21st to end	Night	6% 5% 2 5% 4 4 5% 3 5% 7% 54%	3972
June	1st to 10th	Night	5% 4 2 3 3 3 4 4 7% 10% 55%	1600
	11th to 20th	Night	3 3 1 3 2 4 4 4 6% 9% 61%	1600
	21st to end	Night	4 3 2 5% 4 3 5% 6% 8% 8% 53%	1600
July	1st to 10th	Night	3 4 2 5% 4 5% 4 5% 7% 8% 54%	3640
	11th to 20th	Night	3 3 1 4 4 5% 6% 6% 8% 12% 48%	3640
	21st to end	Night	3 4 2 5% 4 5% 5% 4 8% 11% 48%	4122
August	1st to 10th	Night	3 5% 2 6% 4 6% 5% 5% 8% 11% 45%	4106
	11th to 20th	Night	3 3 1 4 4 5% 6% 5% 8% 11% 49%	4483
	21st to end	Night	5% 5% 2 7% 5% 5% 5% 5% 7% 11% 43%	4993
September	1st to 10th	Night	5% 4 2 4 3 5% 6% 4 6% 7% 53%	2000
	11th to 20th	Night	5% 2 1 3 3 3 5% 4 6% 8% 60%	2031
	21st to end	Night	5% 4 2 4 3 3 3 3 5% 6% 62%	2200
October	1st to 10th	Night	4 4 1 4 2 2 3 2 4 5% 69%	3854
	11th to 20th	Night	6% 4 2 6% 4 3 3 3 5% 7% 58%	3898
	21st to end	Night	2 4 2 5% 3 2 3 3 4 5% 68%	4338
November	1st to 10th	Night	2 4 2 4 3 3 2 3 5% 8% 66%	2562
	11th to 20th	Night	1 3 3 5% 3 2 3 3 4 7% 67%	2600
	21st to end	Night	2 6% 2 5% 3 3 3 3 5% 7% 61%	2655
December	1st to 10th	Night	1 3 2 4 3 2 2 2 4 6% 73%	4570
	11th to 20th	Night	1 4 2 4 3 3 3 3 5% 7% 67%	5181
	21st to end	Night	2 6% 3 5% 4 3 5% 4 5% 8% 56%	6974

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	18% 7% 10% 30% 25% 10%	3639
	11th to 20th	Day	20% 8% 11% 23% 27% 10%	3796
	21st to end	Day	21% 6% 10% 25% 26% 12%	4510
February	1st to 10th	Day	26% 5% 13% 25% 20% 11%	1805
	11th to 20th	Day	20% 7% 12% 27% 27% 7%	1959
	21st to end	Day	24% 5% 9% 21% 24% 17%	1650
March	1st to 10th	Day	20% 6% 8% 15% 30% 21%	4500
	11th to 20th	Day	29% 8% 12% 20% 24% 7%	4908
	21st to end	Day	23% 8% 13% 36% 17% 3	5832
April	1st to 10th	Day	29% 8% 12% 23% 19% 9%	2400
	11th to 20th	Day	32% 9% 13% 19% 18% 9%	2374
	21st to end	Day	32% 10% 14% 23% 17% 3	2400
May	1st to 10th	Day	32% 9% 12% 23% 17% 6%	5711
	11th to 20th	Day	27% 11% 12% 17% 22% 11%	6202
	21st to end	Day	25% 11% 16% 22% 20% 7%	6926
June	1st to 10th	Day	26% 12% 15% 21% 16% 9%	2800
	11th to 20th	Day	25% 9% 12% 16% 21% 16%	2800
	21st to end	Day	27% 10% 13% 18% 20% 12%	2800
July	1st to 10th	Day	20% 8% 8% 19% 26% 18%	6271
	11th to 20th	Day	28% 8% 15% 24% 15% 9%	6265
	21st to end	Day	32% 9% 14% 14% 15% 15%	6538
August	1st to 10th	Day	37% 11% 15% 20% 12% 4	4155
	11th to 20th	Day	27% 10% 15% 17% 18% 13%	3882
	21st to end	Day	27% 8% 14% 19% 24% 7%	4311
September	1st to 10th	Day	25% 6% 11% 19% 25% 14%	2400
	11th to 20th	Day	23% 7% 11% 17% 27% 15%	2368
	21st to end	Day	21% 5% 9% 21% 32% 11%	2200
October	1st to 10th	Day	15% 4 8% 16% 37% 20%	3317
	11th to 20th	Day	23% 4 8% 24% 31% 8%	3250
	21st to end	Day	17% 3 7% 21% 39% 13%	3430
November	1st to 10th	Day	15% 6% 13% 22% 27% 17%	1781
	11th to 20th	Day	17% 6% 10% 25% 31% 12%	1800
	21st to end	Day	20% 5% 10% 22% 29% 14%	1740
December	1st to 10th	Day	18% 4 8% 30% 30% 10%	2620
	11th to 20th	Day	16% 5% 9% 29% 31% 11%	3024
	21st to end	Day	21% 7% 11% 26% 25% 10%	4004
January	1st to 10th	Night	23% 7% 9% 28% 26% 7%	6327
	11th to 20th	Night	18% 7% 11% 24% 24% 15%	6227
	21st to end	Night	20% 8% 10% 24% 25% 13%	6468
February	1st to 10th	Night	23% 5% 12% 27% 22% 10%	2595
	11th to 20th	Night	26% 6% 12% 28% 21% 8%	2338
	21st to end	Night	21% 5% 7% 25% 27% 15%	1980
March	1st to 10th	Night	20% 8% 9% 14% 35% 13%	5374
	11th to 20th	Night	26% 7% 11% 16% 29% 11%	5071
	21st to end	Night	25% 9% 12% 33% 19% 2	5126
April	1st to 10th	Night	25% 7% 12% 25% 23% 8%	2000
	11th to 20th	Night	31% 7% 13% 24% 19% 6%	1977
	21st to end	Night	33% 9% 13% 25% 17% 4	2000
May	1st to 10th	Night	32% 10% 14% 29% 10% 6%	4264
	11th to 20th	Night	25% 10% 15% 23% 18% 9%	3715
	21st to end	Night	25% 11% 17% 26% 17% 5%	3972
June	1st to 10th	Night	22% 10% 21% 25% 13% 9%	1600
	11th to 20th	Night	17% 9% 15% 22% 21% 16%	1600
	21st to end	Night	23% 10% 13% 21% 23% 10%	1600
July	1st to 10th	Night	25% 8% 12% 19% 30% 7%	3600
	11th to 20th	Night	25% 9% 20% 28% 15% 4	3592
	21st to end	Night	27% 9% 19% 20% 15% 10%	4005
August	1st to 10th	Night	36% 11% 18% 20% 11% 3	2979
	11th to 20th	Night	26% 10% 19% 21% 18% 5%	3243
	21st to end	Night	25% 10% 18% 25% 18% 4	3597
September	1st to 10th	Night	27% 7% 11% 23% 22% 11%	2000
	11th to 20th	Night	20% 7% 13% 22% 25% 14%	2031
	21st to end	Night	21% 6% 10% 23% 29% 10%	2200
October	1st to 10th	Night	18% 5% 8% 20% 34% 16%	3854
	11th to 20th	Night	26% 4 8% 27% 26% 9%	3898
	21st to end	Night	18% 4 7% 26% 32% 13%	4338
November	1st to 10th	Night	17% 4 11% 24% 29% 15%	2562
	11th to 20th	Night	18% 4 10% 26% 31% 12%	2600
	21st to end	Night	22% 5% 10% 22% 30% 11%	2655
December	1st to 10th	Night	14% 4 9% 33% 32% 9%	4570
	11th to 20th	Night	16% 5% 9% 28% 34% 8%	5182
	21st to end	Night	24% 7% 10% 22% 25% 11%	6974

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	7% 12% 44% 23% 13% 1	300
	11th to 20th	Day	8% 15% 42% 28% 8%	300
	21st to end	Day	3 19% 43% 25% 10% 1	330
February	1st to 10th	Day	5% 29% 47% 17% 2 1	300
	11th to 20th	Day	4 25% 46% 19% 5%	300
	21st to end	Day	8% 24% 46% 18% 3	249
March	1st to 10th	Day	8% 23% 39% 21% 9%	300
	11th to 20th	Day	13% 23% 36% 19% 7% 1	460
	21st to end	Day	13% 30% 34% 18% 5%	550
April	1st to 10th	Day	13% 25% 36% 21% 4	500
	11th to 20th	Day	17% 33% 31% 15% 4 1	492
	21st to end	Day	16% 26% 34% 19% 5% 1	500
May	1st to 10th	Day	17% 17% 35% 21% 8% 2	618
	11th to 20th	Day	13% 14% 32% 28% 10% 2	725
	21st to end	Day	10% 17% 31% 26% 13% 2	789
June	1st to 10th	Day	6% 12% 28% 31% 19% 4	730
	11th to 20th	Day	7% 13% 33% 29% 16% 3	730
	21st to end	Day	5% 10% 34% 29% 19% 4	730
July	1st to 10th	Day	6% 6% 23% 34% 25% 6%	721
	11th to 20th	Day	6% 2 22% 37% 27% 6%	718
	21st to end	Day	7% 4 19% 35% 28% 6%	725
August	1st to 10th	Day	9% 7% 26% 37% 17% 4	527
	11th to 20th	Day	6% 4 30% 35% 20% 5%	517
	21st to end	Day	7% 6% 32% 35% 16% 4	583
September	1st to 10th	Day	8% 8% 30% 33% 17% 4	530
	11th to 20th	Day	6% 11% 36% 29% 15% 3	498
	21st to end	Day	6% 9% 28% 32% 19% 5%	330
October	1st to 10th	Day	7% 14% 30% 24% 18% 6%	330
	11th to 20th	Day	4 18% 38% 27% 11% 3	330
	21st to end	Day	8% 17% 38% 24% 11% 2	363
November	1st to 10th	Day	8% 20% 35% 22% 13% 3	306
	11th to 20th	Day	4 16% 42% 26% 10% 2	300
	21st to end	Day	8% 18% 39% 26% 8% 1	300
December	1st to 10th	Day	6% 16% 42% 25% 10% 1	300
	11th to 20th	Day	7% 16% 47% 23% 7%	300
	21st to end	Day	7% 18% 48% 20% 6% 1	330
January	1st to 10th	Night	2 9% 45% 33% 10% 1	1000
	11th to 20th	Night	3 13% 47% 30% 6% 1	952
	21st to end	Night	2 8% 46% 38% 6%	880
February	1st to 10th	Night	2 12% 56% 25% 4 1	800
	11th to 20th	Night	3 12% 54% 27% 5%	784
	21st to end	Night	4 12% 54% 27% 3	660
March	1st to 10th	Night	3 12% 51% 28% 6%	800
	11th to 20th	Night	7% 10% 39% 36% 7% 1	800
	21st to end	Night	6% 11% 48% 29% 5%	724
April	1st to 10th	Night	7% 13% 47% 29% 5%	600
	11th to 20th	Night	13% 15% 41% 26% 5% 1	593
	21st to end	Night	13% 14% 34% 31% 7%	600
May	1st to 10th	Night	12% 10% 32% 35% 10% 1	600
	11th to 20th	Night	14% 5% 29% 39% 11% 1	591
	21st to end	Night	9% 10% 32% 33% 13% 2	650
June	1st to 10th	Night	6% 7% 23% 44% 18% 2	600
	11th to 20th	Night	5% 7% 31% 44% 13% 1	600
	21st to end	Night	4 3 26% 45% 20% 1	600
July	1st to 10th	Night	6% 3 23% 40% 26% 2	585
	11th to 20th	Night	5% 2 16% 42% 31% 3	582
	21st to end	Night	7% 2 12% 43% 33% 3	657
August	1st to 10th	Night	7% 4 16% 41% 28% 3	597
	11th to 20th	Night	6% 3 19% 47% 23% 2	594
	21st to end	Night	7% 2 18% 43% 26% 4	660
September	1st to 10th	Night	7% 3 19% 44% 25% 3	600
	11th to 20th	Night	6% 4 23% 45% 21% 2	600
	21st to end	Night	6% 6% 26% 40% 20% 2	600
October	1st to 10th	Night	8% 7% 33% 37% 13% 2	760
	11th to 20th	Night	4 6% 39% 38% 12% 2	800
	21st to end	Night	6% 8% 37% 34% 13% 2	930
November	1st to 10th	Night	3 11% 37% 37% 11% 1	986
	11th to 20th	Night	2 11% 45% 34% 8% 2	1000
	21st to end	Night	5% 10% 39% 37% 9% 1	1000
December	1st to 10th	Night	2 9% 47% 34% 8% 1	1000
	11th to 20th	Night	4 11% 48% 29% 8% 1	1000
	21st to end	Night	4 13% 48% 29% 5% 1	1100

Page generated at 2025-10-31 00:02:40. Version ac07

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