Spruce Knob Mountain Center
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Spruce Knob Mountain Center clear sky chart from 2005-11-06 to 2026-03-18.

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	11% 8% 3 9% 6% 6% 6% 4 6% 7% 33%	130873
	Night	10% 8% 4 9% 6% 6% 5% 4 6% 7% 34%	130593
Transparency	Day	45% 9% 10% 20% 13% 3	134479
	Night	44% 10% 11% 19% 13% 3	135057
Seeing	Day	24% 20% 26% 17% 9% 3	16637
	Night	22% 21% 24% 22% 10% 1	26616

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	10% 8% 6% 12% 6% 5% 4 3 5% 7% 33%	5258
	11th to 20th	Day	6% 10% 5% 14% 6% 7% 5% 3 5% 6% 33%	5280
	21st to end	Day	10% 8% 4 9% 5% 7% 5% 4 5% 6% 37%	5780
February	1st to 10th	Day	8% 11% 3 9% 6% 5% 5% 4 6% 8% 33%	1890
	11th to 20th	Day	12% 10% 3 11% 5% 5% 5% 4 7% 8% 31%	1843
	21st to end	Day	10% 13% 3 9% 6% 5% 4 4 5% 6% 34%	1795
March	1st to 10th	Day	12% 7% 4 10% 5% 5% 5% 4 6% 7% 35%	5045
	11th to 20th	Day	9% 8% 3 8% 5% 4 4 4 5% 7% 43%	5057
	21st to end	Day	10% 10% 3 10% 7% 5% 6% 4 5% 5% 34%	5465
April	1st to 10th	Day	10% 10% 5% 10% 7% 5% 4 4 5% 7% 34%	2312
	11th to 20th	Day	12% 9% 4 7% 4 4 4 3 6% 7% 39%	2374
	21st to end	Day	16% 8% 3 8% 5% 5% 4 4 6% 7% 34%	2540
May	1st to 10th	Day	14% 10% 3 9% 7% 6% 7% 5% 7% 7% 26%	5862
	11th to 20th	Day	20% 9% 3 7% 5% 5% 5% 4 7% 8% 26%	5861
	21st to end	Day	15% 8% 3 10% 6% 7% 6% 5% 7% 7% 27%	6542
June	1st to 10th	Day	12% 7% 3 7% 5% 5% 6% 5% 6% 8% 36%	2800
	11th to 20th	Day	11% 8% 3 8% 7% 6% 7% 5% 7% 8% 32%	2800
	21st to end	Day	9% 6% 3 7% 6% 6% 7% 5% 8% 8% 35%	2772
July	1st to 10th	Day	6% 6% 3 9% 6% 6% 6% 5% 7% 10% 35%	5900
	11th to 20th	Day	8% 6% 3 10% 8% 8% 8% 6% 8% 10% 27%	5900
	21st to end	Day	10% 9% 3 10% 7% 6% 7% 6% 7% 8% 25%	6424
August	1st to 10th	Day	13% 7% 3 9% 7% 7% 7% 5% 8% 9% 25%	4234
	11th to 20th	Day	8% 6% 3 10% 7% 8% 8% 6% 8% 9% 25%	4174
	21st to end	Day	6% 5% 2 7% 5% 7% 6% 5% 7% 9% 40%	4352
September	1st to 10th	Day	13% 6% 3 7% 6% 6% 5% 3 5% 7% 39%	2400
	11th to 20th	Day	11% 8% 2 7% 6% 5% 4 4 6% 7% 39%	2226
	21st to end	Day	20% 9% 3 7% 4 4 4 4 5% 6% 33%	2182
October	1st to 10th	Day	13% 7% 2 7% 5% 5% 4 3 5% 7% 43%	2000
	11th to 20th	Day	13% 11% 3 8% 4 4 4 3 4 5% 43%	2000
	21st to end	Day	12% 9% 3 6% 4 4 5% 4 6% 7% 41%	2200
November	1st to 10th	Day	8% 7% 2 6% 5% 4 4 5% 5% 7% 47%	2057
	11th to 20th	Day	12% 8% 3 7% 5% 5% 4 4 6% 7% 39%	1838
	21st to end	Day	10% 10% 4 8% 6% 5% 5% 4 4 7% 38%	1680
December	1st to 10th	Day	7% 8% 4 10% 6% 5% 5% 4 7% 7% 36%	2680
	11th to 20th	Day	14% 9% 4 9% 5% 5% 6% 4 6% 6% 33%	3070
	21st to end	Day	14% 9% 4 10% 6% 6% 5% 4 7% 7% 27%	4280
January	1st to 10th	Night	8% 10% 5% 14% 10% 7% 6% 4 5% 5% 26%	7635
	11th to 20th	Night	6% 10% 3 10% 10% 7% 5% 4 5% 7% 35%	7680
	21st to end	Night	9% 9% 3 7% 6% 6% 5% 4 6% 7% 39%	8401
February	1st to 10th	Night	8% 8% 4 9% 6% 6% 5% 5% 7% 9% 33%	2730
	11th to 20th	Night	12% 9% 3 10% 8% 7% 7% 4 6% 7% 27%	2663
	21st to end	Night	10% 10% 3 11% 6% 5% 5% 4 6% 7% 32%	2008
March	1st to 10th	Night	13% 9% 3 8% 5% 6% 4 4 5% 6% 36%	5028
	11th to 20th	Night	9% 7% 3 9% 4 4 3 3 5% 8% 43%	5058
	21st to end	Night	9% 9% 4 8% 6% 5% 5% 4 6% 7% 38%	5467
April	1st to 10th	Night	11% 9% 5% 8% 6% 6% 5% 4 6% 6% 35%	2087
	11th to 20th	Night	10% 9% 3 6% 6% 5% 5% 4 6% 8% 38%	1975
	21st to end	Night	12% 8% 3 7% 5% 5% 5% 4 6% 6% 38%	1875
May	1st to 10th	Night	16% 10% 4 10% 5% 5% 5% 4 6% 8% 26%	4033
	11th to 20th	Night	21% 10% 3 8% 5% 5% 5% 5% 6% 6% 26%	4032
	21st to end	Night	13% 9% 4 11% 6% 6% 6% 4 6% 8% 27%	4294
June	1st to 10th	Night	10% 6% 2 7% 4 4 6% 4 7% 10% 40%	1600
	11th to 20th	Night	9% 6% 3 8% 6% 4 6% 5% 7% 9% 39%	1600
	21st to end	Night	7% 5% 2 5% 4 5% 6% 5% 7% 11% 42%	1621
July	1st to 10th	Night	5% 6% 3 8% 6% 6% 6% 5% 7% 10% 39%	4058
	11th to 20th	Night	7% 8% 4 11% 7% 6% 6% 5% 7% 10% 30%	4057
	21st to end	Night	8% 8% 4 11% 7% 6% 6% 5% 8% 10% 28%	4427
August	1st to 10th	Night	13% 6% 3 8% 6% 6% 5% 4 7% 10% 32%	2915
	11th to 20th	Night	9% 9% 4 9% 6% 6% 5% 4 7% 7% 35%	2875
	21st to end	Night	6% 6% 2 6% 5% 4 5% 4 7% 10% 45%	3511
September	1st to 10th	Night	11% 6% 2 6% 4 4 6% 4 6% 6% 45%	2000
	11th to 20th	Night	10% 7% 2 7% 5% 5% 4 3 5% 7% 44%	2170
	21st to end	Night	17% 10% 2 6% 5% 5% 5% 4 5% 6% 33%	2216
October	1st to 10th	Night	13% 7% 3 6% 4 4 5% 3 5% 7% 43%	2400
	11th to 20th	Night	10% 9% 3 8% 4 4 4 3 6% 8% 42%	2400
	21st to end	Night	11% 9% 3 7% 5% 4 5% 4 5% 6% 41%	2640
November	1st to 10th	Night	7% 9% 3 7% 4 5% 5% 4 5% 6% 43%	2461
	11th to 20th	Night	10% 9% 4 9% 4 4 6% 4 6% 6% 37%	2772
	21st to end	Night	10% 9% 4 9% 6% 6% 6% 3 6% 7% 34%	2940
December	1st to 10th	Night	7% 7% 3 11% 6% 6% 5% 5% 9% 7% 32%	4720
	11th to 20th	Night	12% 9% 4 12% 6% 6% 5% 4 5% 6% 31%	5345
	21st to end	Night	11% 8% 6% 11% 7% 6% 6% 5% 7% 8% 25%	6899

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	46% 9% 8% 18% 16% 4	5259
	11th to 20th	Day	48% 7% 8% 13% 18% 7%	5280
	21st to end	Day	45% 7% 8% 22% 14% 4	5780
February	1st to 10th	Day	49% 9% 7% 18% 14% 3	1890
	11th to 20th	Day	51% 11% 7% 16% 14% 2	1852
	21st to end	Day	50% 10% 7% 17% 13% 4	1795
March	1st to 10th	Day	44% 7% 9% 18% 15% 6%	5045
	11th to 20th	Day	38% 8% 7% 25% 16% 5%	5057
	21st to end	Day	50% 6% 9% 15% 17% 4	5465
April	1st to 10th	Day	49% 8% 10% 16% 14% 3	2312
	11th to 20th	Day	44% 8% 9% 20% 16% 3	2374
	21st to end	Day	49% 8% 9% 21% 11% 2	2540
May	1st to 10th	Day	53% 9% 9% 18% 7% 2	5897
	11th to 20th	Day	53% 9% 10% 22% 5% 1	5865
	21st to end	Day	49% 11% 12% 17% 8% 2	6557
June	1st to 10th	Day	44% 10% 12% 21% 13% 1	2800
	11th to 20th	Day	45% 12% 14% 18% 11% 1	2800
	21st to end	Day	42% 11% 13% 22% 10% 1	2772
July	1st to 10th	Day	41% 10% 13% 23% 10% 3	5835
	11th to 20th	Day	46% 13% 14% 22% 6%	5831
	21st to end	Day	50% 9% 12% 20% 9%	6240
August	1st to 10th	Day	49% 11% 15% 19% 6%	4221
	11th to 20th	Day	46% 10% 12% 22% 10%	4213
	21st to end	Day	36% 10% 14% 24% 15% 1	4352
September	1st to 10th	Day	44% 7% 10% 16% 19% 4	2400
	11th to 20th	Day	43% 7% 8% 23% 17% 4	2226
	21st to end	Day	51% 5% 8% 18% 15% 2	2182
October	1st to 10th	Day	30% 5% 8% 23% 27% 8%	3260
	11th to 20th	Day	36% 5% 8% 23% 15% 13%	3240
	21st to end	Day	30% 6% 8% 26% 20% 9%	3538
November	1st to 10th	Day	37% 8% 8% 22% 19% 6%	2057
	11th to 20th	Day	42% 9% 8% 16% 20% 5%	1838
	21st to end	Day	46% 9% 9% 19% 16% 2	1680
December	1st to 10th	Day	43% 9% 11% 18% 16% 2	2680
	11th to 20th	Day	46% 11% 10% 16% 15% 2	3066
	21st to end	Day	51% 10% 11% 15% 12% 1	4280
January	1st to 10th	Night	50% 13% 9% 13% 13% 3	7636
	11th to 20th	Night	46% 9% 9% 13% 16% 6%	7680
	21st to end	Night	43% 8% 10% 20% 16% 2	8401
February	1st to 10th	Night	47% 11% 10% 17% 13% 2	2730
	11th to 20th	Night	54% 10% 8% 15% 10% 3	2665
	21st to end	Night	51% 10% 8% 17% 11% 4	2008
March	1st to 10th	Night	44% 7% 10% 19% 14% 5%	5028
	11th to 20th	Night	37% 9% 8% 27% 15% 4	5058
	21st to end	Night	43% 8% 13% 17% 17% 3	5467
April	1st to 10th	Night	48% 9% 10% 18% 13% 2	2087
	11th to 20th	Night	42% 11% 12% 21% 13% 3	1975
	21st to end	Night	44% 9% 11% 23% 11% 2	1875
May	1st to 10th	Night	52% 12% 12% 15% 8% 1	4060
	11th to 20th	Night	53% 11% 11% 20% 5%	4033
	21st to end	Night	49% 14% 15% 16% 4 2	4301
June	1st to 10th	Night	36% 13% 13% 26% 12% 1	1600
	11th to 20th	Night	39% 12% 15% 22% 10% 1	1600
	21st to end	Night	33% 12% 17% 26% 12% 1	1621
July	1st to 10th	Night	37% 12% 18% 23% 9% 2	4015
	11th to 20th	Night	44% 14% 17% 20% 5% 1	4006
	21st to end	Night	44% 11% 18% 19% 9%	4297
August	1st to 10th	Night	44% 11% 18% 20% 6%	2904
	11th to 20th	Night	43% 9% 14% 23% 10% 1	2902
	21st to end	Night	32% 9% 17% 30% 11% 1	3511
September	1st to 10th	Night	37% 9% 14% 19% 18% 3	2000
	11th to 20th	Night	38% 7% 10% 26% 17% 2	2170
	21st to end	Night	49% 9% 10% 17% 14% 1	2216
October	1st to 10th	Night	34% 7% 9% 26% 20% 4	3920
	11th to 20th	Night	35% 7% 9% 18% 16% 14%	3899
	21st to end	Night	29% 8% 8% 25% 19% 12%	4252
November	1st to 10th	Night	40% 8% 10% 17% 18% 7%	2461
	11th to 20th	Night	44% 12% 5% 16% 19% 4	2772
	21st to end	Night	47% 11% 9% 17% 14% 3	2940
December	1st to 10th	Night	43% 11% 12% 21% 11% 1	4720
	11th to 20th	Night	51% 10% 11% 12% 15% 2	5348
	21st to end	Night	51% 13% 11% 14% 11% 1	6899

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	40% 32% 17% 8% 3	330
	11th to 20th	Day	45% 30% 20% 5% 1	330
	21st to end	Day	43% 27% 24% 5% 1	363
February	1st to 10th	Day	39% 35% 18% 7% 1	330
	11th to 20th	Day	53% 26% 18% 2 1	330
	21st to end	Day	39% 33% 21% 7%	362
March	1st to 10th	Day	34% 27% 27% 10% 3	540
	11th to 20th	Day	35% 29% 21% 11% 4	528
	21st to end	Day	31% 28% 27% 10% 4	550
April	1st to 10th	Day	29% 30% 27% 10% 4 1	500
	11th to 20th	Day	28% 23% 28% 12% 7% 2	492
	21st to end	Day	28% 25% 28% 12% 5% 1	500
May	1st to 10th	Day	29% 25% 25% 12% 7% 2	503
	11th to 20th	Day	25% 20% 28% 19% 5% 3	527
	21st to end	Day	21% 15% 27% 20% 13% 5%	573
June	1st to 10th	Day	17% 18% 30% 19% 12% 4	530
	11th to 20th	Day	17% 15% 31% 21% 13% 4	530
	21st to end	Day	11% 18% 32% 24% 12% 4	530
July	1st to 10th	Day	10% 13% 29% 24% 15% 8%	525
	11th to 20th	Day	7% 12% 33% 29% 12% 8%	524
	21st to end	Day	14% 10% 29% 24% 17% 6%	576
August	1st to 10th	Day	16% 10% 26% 25% 14% 9%	526
	11th to 20th	Day	12% 11% 28% 24% 16% 9%	519
	21st to end	Day	9% 11% 29% 26% 18% 7%	583
September	1st to 10th	Day	17% 11% 23% 24% 17% 8%	530
	11th to 20th	Day	14% 7% 25% 27% 19% 8%	530
	21st to end	Day	26% 16% 26% 18% 11% 3	530
October	1st to 10th	Day	19% 13% 25% 24% 15% 3	530
	11th to 20th	Day	24% 20% 26% 18% 9% 2	530
	21st to end	Day	24% 24% 29% 15% 7% 1	583
November	1st to 10th	Day	21% 26% 25% 18% 8% 2	535
	11th to 20th	Day	34% 24% 26% 12% 4 1	312
	21st to end	Day	35% 20% 33% 9% 3	220
December	1st to 10th	Day	34% 33% 24% 7% 2 1	220
	11th to 20th	Day	39% 26% 22% 11% 2	220
	21st to end	Day	34% 29% 23% 10% 3	296
January	1st to 10th	Night	31% 35% 21% 10% 2	840
	11th to 20th	Night	32% 30% 25% 12% 1	840
	21st to end	Night	29% 35% 23% 11% 1	924
February	1st to 10th	Night	26% 34% 25% 14% 1	840
	11th to 20th	Night	34% 34% 23% 8%	823
	21st to end	Night	32% 33% 26% 8% 1	692
March	1st to 10th	Night	27% 30% 25% 16% 2	840
	11th to 20th	Night	31% 27% 23% 14% 4 1	829
	21st to end	Night	26% 29% 26% 15% 4	880
April	1st to 10th	Night	23% 30% 29% 15% 3	800
	11th to 20th	Night	22% 23% 26% 20% 8%	791
	21st to end	Night	21% 20% 29% 25% 5%	675
May	1st to 10th	Night	21% 19% 23% 26% 10% 1	600
	11th to 20th	Night	19% 16% 25% 28% 10% 2	591
	21st to end	Night	17% 9% 27% 28% 17% 3	600
June	1st to 10th	Night	12% 10% 30% 29% 19% 1	400
	11th to 20th	Night	13% 8% 23% 40% 16% 1	400
	21st to end	Night	9% 7% 29% 37% 17% 1	421
July	1st to 10th	Night	10% 6% 22% 35% 22% 5%	585
	11th to 20th	Night	7% 5% 24% 34% 26% 5%	582
	21st to end	Night	11% 5% 21% 32% 30% 2	657
August	1st to 10th	Night	12% 4 15% 41% 25% 2	597
	11th to 20th	Night	10% 8% 17% 37% 24% 4	594
	21st to end	Night	11% 6% 22% 36% 21% 3	831
September	1st to 10th	Night	15% 7% 20% 32% 21% 5%	800
	11th to 20th	Night	13% 8% 18% 32% 25% 4	800
	21st to end	Night	23% 12% 24% 23% 17% 1	800
October	1st to 10th	Night	18% 13% 20% 28% 19% 3	800
	11th to 20th	Night	20% 20% 24% 24% 11% 1	800
	21st to end	Night	22% 21% 27% 21% 8% 1	880
November	1st to 10th	Night	17% 21% 26% 25% 9% 2	820
	11th to 20th	Night	27% 23% 24% 18% 7% 1	840
	21st to end	Night	26% 24% 28% 17% 5% 1	840
December	1st to 10th	Night	27% 32% 23% 14% 3	840
	11th to 20th	Night	25% 31% 24% 16% 3	840
	21st to end	Night	25% 28% 27% 18% 2	924

Page generated at 2026-03-15 02:23:52. Version ac07

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