Glens of Grandfather Mountain
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Glens of Grandfather Mountain clear sky chart from 2005-11-06 to 2025-05-26.

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	13% 7% 2 7% 5% 5% 5% 4 6% 8% 38%	114606
	Night	13% 6% 2 6% 4 4 4 4 6% 8% 43%	114681
Transparency	Day	39% 10% 12% 19% 14% 5%	118322
	Night	35% 10% 14% 20% 15% 6%	119091
Seeing	Day	20% 18% 25% 20% 12% 6%	15821
	Night	18% 16% 22% 25% 16% 3	25952

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	11% 5% 2 5% 3 4 3 3 5% 8% 51%	4056
	11th to 20th	Day	10% 6% 2 5% 4 4 3 3 4 6% 52%	4060
	21st to end	Day	15% 6% 2 6% 4 4 4 3 5% 6% 45%	4466
February	1st to 10th	Day	11% 9% 3 5% 5% 5% 6% 5% 6% 7% 38%	1812
	11th to 20th	Day	16% 10% 3 6% 5% 4 4 3 5% 7% 38%	1948
	21st to end	Day	16% 7% 3 6% 5% 4 4 4 6% 7% 38%	1650
March	1st to 10th	Day	18% 6% 2 5% 4 4 5% 3 5% 6% 42%	4921
	11th to 20th	Day	11% 5% 2 5% 4 4 3 3 5% 7% 50%	4980
	21st to end	Day	15% 7% 2 6% 5% 4 4 3 5% 7% 42%	5465
April	1st to 10th	Day	11% 6% 2 6% 4 5% 4 4 6% 8% 43%	2200
	11th to 20th	Day	15% 7% 2 6% 4 4 4 4 7% 8% 40%	2390
	21st to end	Day	15% 6% 3 7% 5% 4 5% 4 5% 6% 40%	2600
May	1st to 10th	Day	13% 8% 3 8% 6% 6% 6% 5% 7% 8% 31%	4212
	11th to 20th	Day	17% 8% 3 8% 6% 6% 6% 5% 7% 7% 27%	4211
	21st to end	Day	10% 5% 2 7% 5% 5% 6% 5% 8% 11% 37%	4541
June	1st to 10th	Day	13% 6% 2 7% 7% 7% 6% 5% 6% 8% 31%	2470
	11th to 20th	Day	9% 7% 3 8% 6% 7% 6% 6% 8% 9% 32%	2470
	21st to end	Day	9% 7% 2 8% 6% 7% 7% 6% 9% 10% 31%	2470
July	1st to 10th	Day	8% 6% 2 8% 7% 7% 7% 6% 9% 10% 31%	4120
	11th to 20th	Day	10% 6% 3 9% 7% 8% 7% 5% 7% 9% 30%	4120
	21st to end	Day	14% 11% 4 11% 9% 8% 7% 5% 8% 8% 16%	4466
August	1st to 10th	Day	15% 8% 3 8% 6% 7% 7% 5% 9% 10% 23%	4104
	11th to 20th	Day	10% 7% 3 8% 6% 7% 8% 6% 7% 11% 27%	4044
	21st to end	Day	9% 5% 2 6% 5% 6% 7% 5% 7% 9% 39%	4286
September	1st to 10th	Day	13% 8% 3 8% 5% 5% 6% 4 6% 8% 34%	2090
	11th to 20th	Day	18% 9% 3 7% 5% 4 6% 4 7% 7% 30%	2090
	21st to end	Day	20% 7% 2 6% 5% 5% 4 5% 6% 7% 33%	2090
October	1st to 10th	Day	18% 5% 2 6% 3 3 4 3 5% 6% 45%	2012
	11th to 20th	Day	13% 6% 1 5% 3 4 4 3 4 6% 51%	1900
	21st to end	Day	15% 6% 1 5% 3 3 4 3 4 6% 51%	2090
November	1st to 10th	Day	11% 7% 2 4 3 3 3 4 4 7% 52%	1903
	11th to 20th	Day	12% 5% 2 5% 4 4 4 3 5% 7% 50%	1800
	21st to end	Day	13% 5% 2 6% 4 4 4 4 4 7% 48%	1800
December	1st to 10th	Day	10% 6% 3 6% 5% 4 4 3 4 7% 48%	2930
	11th to 20th	Day	13% 6% 2 6% 4 5% 5% 4 7% 7% 41%	3373
	21st to end	Day	17% 9% 2 5% 3 4 5% 3 5% 8% 40%	4466
January	1st to 10th	Night	10% 6% 2 5% 3 4 4 4 5% 8% 51%	5889
	11th to 20th	Night	10% 6% 2 7% 4 4 4 4 4 5% 50%	5900
	21st to end	Night	14% 6% 2 5% 4 4 3 3 5% 6% 48%	6490
February	1st to 10th	Night	12% 7% 2 6% 4 4 5% 5% 7% 8% 41%	2585
	11th to 20th	Night	17% 9% 2 5% 4 4 5% 4 6% 6% 39%	2338
	21st to end	Night	12% 7% 2 7% 6% 4 5% 4 7% 7% 38%	1980
March	1st to 10th	Night	18% 8% 2 5% 4 4 4 3 5% 6% 40%	4930
	11th to 20th	Night	11% 5% 2 4 3 4 4 4 6% 8% 51%	4980
	21st to end	Night	15% 6% 2 5% 4 5% 5% 3 5% 8% 43%	5467
April	1st to 10th	Night	11% 6% 2 6% 4 4 5% 4 6% 9% 42%	2200
	11th to 20th	Night	15% 5% 2 5% 4 3 4 4 6% 8% 46%	1972
	21st to end	Night	12% 7% 2 6% 4 4 4 5% 6% 8% 43%	1800
May	1st to 10th	Night	14% 7% 2 7% 5% 6% 4 4 7% 9% 34%	2903
	11th to 20th	Night	20% 7% 2 7% 5% 4 4 3 7% 9% 31%	2902
	21st to end	Night	10% 5% 2 6% 4 4 4 5% 7% 11% 42%	3130
June	1st to 10th	Night	10% 6% 3 7% 4 5% 6% 5% 8% 10% 35%	1710
	11th to 20th	Night	9% 6% 2 5% 5% 5% 5% 4 9% 10% 40%	1710
	21st to end	Night	7% 5% 2 5% 6% 4 6% 5% 9% 11% 39%	1710
July	1st to 10th	Night	9% 6% 2 6% 4 5% 7% 5% 9% 12% 34%	2840
	11th to 20th	Night	9% 5% 1 5% 4 4 5% 5% 8% 12% 43%	2840
	21st to end	Night	11% 9% 4 9% 7% 6% 7% 6% 8% 11% 22%	3087
August	1st to 10th	Night	14% 8% 2 7% 5% 4 5% 5% 7% 12% 31%	2825
	11th to 20th	Night	11% 7% 2 6% 4 5% 6% 4 7% 12% 35%	2785
	21st to end	Night	8% 5% 2 4 4 4 4 4 7% 10% 50%	3331
September	1st to 10th	Night	13% 8% 2 8% 4 4 5% 4 5% 7% 41%	2090
	11th to 20th	Night	20% 7% 2 6% 4 4 4 3 5% 7% 37%	2090
	21st to end	Night	21% 8% 2 6% 3 4 4 3 4 5% 41%	2090
October	1st to 10th	Night	17% 6% 2 5% 3 4 3 3 5% 6% 46%	2166
	11th to 20th	Night	14% 6% 1 5% 4 3 3 3 5% 6% 51%	2280
	21st to end	Night	14% 5% 1 5% 3 3 3 3 4 5% 54%	2508
November	1st to 10th	Night	13% 8% 1 5% 3 3 3 3 5% 7% 49%	2391
	11th to 20th	Night	14% 5% 2 5% 3 3 4 3 6% 7% 49%	2600
	21st to end	Night	12% 5% 2 5% 3 4 3 4 6% 7% 48%	2600
December	1st to 10th	Night	10% 5% 2 5% 4 4 4 3 5% 7% 49%	4250
	11th to 20th	Night	14% 10% 4 7% 5% 5% 5% 3 4 6% 38%	4822
	21st to end	Night	19% 7% 2 5% 4 4 4 3 5% 8% 40%	6490

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	30% 7% 10% 22% 26% 6%	4057
	11th to 20th	Day	30% 8% 9% 16% 29% 8%	4060
	21st to end	Day	37% 6% 10% 18% 22% 6%	4466
February	1st to 10th	Day	41% 10% 12% 18% 15% 4	1812
	11th to 20th	Day	45% 8% 10% 17% 16% 4	1958
	21st to end	Day	42% 9% 11% 16% 15% 6%	1650
March	1st to 10th	Day	40% 7% 8% 17% 17% 11%	4921
	11th to 20th	Day	31% 7% 8% 21% 18% 15%	4980
	21st to end	Day	40% 7% 11% 25% 14% 4	5465
April	1st to 10th	Day	36% 9% 13% 22% 16% 3	2200
	11th to 20th	Day	39% 9% 11% 20% 17% 5%	2390
	21st to end	Day	41% 11% 13% 21% 12% 2	2600
May	1st to 10th	Day	46% 13% 13% 19% 8% 1	4250
	11th to 20th	Day	50% 13% 12% 19% 6% 1	4215
	21st to end	Day	35% 13% 16% 19% 14% 2	4554
June	1st to 10th	Day	46% 13% 15% 17% 9% 1	2470
	11th to 20th	Day	43% 13% 15% 16% 12% 1	2470
	21st to end	Day	42% 13% 15% 17% 12% 1	2470
July	1st to 10th	Day	42% 17% 16% 16% 7% 1	4055
	11th to 20th	Day	45% 15% 19% 16% 5% 1	4051
	21st to end	Day	60% 14% 11% 11% 3 1	4324
August	1st to 10th	Day	48% 16% 17% 15% 3	4091
	11th to 20th	Day	41% 16% 20% 13% 6% 3	4083
	21st to end	Day	33% 14% 17% 29% 6% 2	4286
September	1st to 10th	Day	44% 11% 11% 18% 13% 3	2090
	11th to 20th	Day	47% 10% 11% 18% 13% 2	2090
	21st to end	Day	47% 8% 12% 16% 12% 4	2090
October	1st to 10th	Day	27% 7% 12% 16% 25% 13%	3337
	11th to 20th	Day	23% 4 7% 21% 23% 22%	3140
	21st to end	Day	24% 7% 11% 25% 16% 16%	3428
November	1st to 10th	Day	32% 7% 10% 19% 25% 6%	1903
	11th to 20th	Day	32% 7% 9% 20% 24% 8%	1800
	21st to end	Day	35% 6% 10% 22% 22% 5%	1800
December	1st to 10th	Day	35% 7% 9% 18% 25% 6%	2930
	11th to 20th	Day	38% 8% 11% 18% 18% 5%	3370
	21st to end	Day	40% 8% 14% 23% 12% 3	4466
January	1st to 10th	Night	28% 8% 11% 25% 18% 11%	5889
	11th to 20th	Night	32% 9% 9% 18% 22% 11%	5900
	21st to end	Night	37% 6% 9% 15% 24% 9%	6490
February	1st to 10th	Night	37% 10% 14% 20% 16% 3	2585
	11th to 20th	Night	43% 8% 12% 17% 14% 5%	2339
	21st to end	Night	40% 10% 13% 17% 14% 6%	1980
March	1st to 10th	Night	40% 9% 9% 18% 17% 8%	4930
	11th to 20th	Night	28% 8% 10% 22% 17% 15%	4980
	21st to end	Night	37% 9% 13% 25% 11% 5%	5467
April	1st to 10th	Night	35% 10% 16% 20% 15% 4	2200
	11th to 20th	Night	34% 9% 12% 23% 16% 6%	1972
	21st to end	Night	36% 10% 16% 23% 14% 1	1800
May	1st to 10th	Night	43% 13% 16% 21% 6% 1	2930
	11th to 20th	Night	45% 13% 17% 19% 6%	2903
	21st to end	Night	32% 14% 21% 22% 10% 1	3138
June	1st to 10th	Night	40% 14% 17% 20% 8% 1	1710
	11th to 20th	Night	34% 13% 22% 22% 8% 1	1710
	21st to end	Night	33% 15% 18% 19% 11% 3	1710
July	1st to 10th	Night	38% 16% 21% 18% 6% 1	2795
	11th to 20th	Night	31% 14% 28% 22% 4 1	2786
	21st to end	Night	50% 18% 16% 12% 4	2988
August	1st to 10th	Night	43% 14% 25% 14% 5%	2814
	11th to 20th	Night	37% 16% 21% 17% 7% 1	2812
	21st to end	Night	27% 12% 20% 31% 7% 3	3331
September	1st to 10th	Night	41% 11% 14% 18% 14% 3	2090
	11th to 20th	Night	45% 10% 14% 17% 15% 1	2090
	21st to end	Night	45% 7% 12% 18% 15% 2	2090
October	1st to 10th	Night	27% 8% 12% 19% 18% 15%	3608
	11th to 20th	Night	23% 5% 9% 21% 22% 21%	3779
	21st to end	Night	27% 7% 8% 22% 20% 15%	4120
November	1st to 10th	Night	35% 7% 10% 20% 21% 7%	2391
	11th to 20th	Night	32% 7% 10% 20% 22% 8%	2600
	21st to end	Night	33% 7% 11% 23% 22% 5%	2600
December	1st to 10th	Night	31% 9% 12% 21% 23% 4	4250
	11th to 20th	Night	44% 8% 11% 16% 16% 4	4824
	21st to end	Night	38% 10% 17% 20% 12% 3	6490

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	37% 33% 22% 7% 1	300
	11th to 20th	Day	33% 28% 29% 9% 1	300
	21st to end	Day	30% 35% 26% 7% 1	330
February	1st to 10th	Day	22% 34% 33% 9% 1	300
	11th to 20th	Day	43% 33% 20% 4	300
	21st to end	Day	35% 34% 27% 4	249
March	1st to 10th	Day	26% 28% 27% 14% 4	486
	11th to 20th	Day	26% 34% 24% 12% 3 1	500
	21st to end	Day	29% 29% 26% 11% 4	550
April	1st to 10th	Day	24% 28% 26% 15% 6% 1	500
	11th to 20th	Day	21% 28% 23% 18% 8% 2	492
	21st to end	Day	21% 20% 29% 15% 11% 4	500
May	1st to 10th	Day	17% 22% 34% 14% 8% 4	503
	11th to 20th	Day	22% 13% 27% 21% 10% 6%	527
	21st to end	Day	18% 10% 24% 22% 15% 10%	549
June	1st to 10th	Day	15% 9% 28% 26% 15% 7%	490
	11th to 20th	Day	11% 7% 25% 30% 17% 10%	490
	21st to end	Day	10% 10% 31% 27% 14% 9%	490
July	1st to 10th	Day	9% 4 21% 29% 25% 13%	485
	11th to 20th	Day	9% 5% 17% 33% 24% 12%	484
	21st to end	Day	10% 6% 19% 31% 21% 13%	532
August	1st to 10th	Day	17% 8% 23% 23% 15% 14%	486
	11th to 20th	Day	11% 5% 24% 25% 23% 11%	479
	21st to end	Day	14% 4 16% 27% 21% 17%	539
September	1st to 10th	Day	14% 5% 18% 27% 25% 12%	490
	11th to 20th	Day	18% 9% 16% 23% 26% 10%	490
	21st to end	Day	23% 10% 23% 25% 15% 4	490
October	1st to 10th	Day	22% 11% 21% 22% 18% 6%	490
	11th to 20th	Day	17% 19% 24% 21% 16% 3	490
	21st to end	Day	18% 17% 27% 25% 10% 3	539
November	1st to 10th	Day	18% 19% 28% 22% 10% 3	441
	11th to 20th	Day	20% 28% 31% 17% 3 1	300
	21st to end	Day	21% 24% 33% 17% 4 1	300
December	1st to 10th	Day	28% 28% 31% 10% 2	300
	11th to 20th	Day	24% 22% 39% 13% 3	300
	21st to end	Day	27% 24% 34% 10% 4 1	330
January	1st to 10th	Night	22% 29% 26% 19% 4	800
	11th to 20th	Night	22% 27% 27% 20% 4	800
	21st to end	Night	17% 32% 29% 19% 3	880
February	1st to 10th	Night	16% 26% 31% 25% 3	800
	11th to 20th	Night	26% 32% 26% 15% 1	783
	21st to end	Night	21% 30% 31% 17% 2	660
March	1st to 10th	Night	18% 27% 30% 22% 4	800
	11th to 20th	Night	23% 22% 30% 18% 7%	800
	21st to end	Night	21% 22% 30% 20% 7%	880
April	1st to 10th	Night	20% 23% 25% 24% 7%	800
	11th to 20th	Night	20% 20% 25% 22% 12% 1	746
	21st to end	Night	15% 16% 24% 32% 12% 1	600
May	1st to 10th	Night	15% 15% 31% 25% 13% 1	600
	11th to 20th	Night	16% 12% 23% 29% 18% 2	591
	21st to end	Night	18% 7% 16% 26% 26% 7%	634
June	1st to 10th	Night	11% 3 19% 34% 28% 5%	570
	11th to 20th	Night	11% 3 20% 27% 33% 7%	570
	21st to end	Night	7% 5% 19% 31% 34% 4	570
July	1st to 10th	Night	12% 2 12% 27% 42% 6%	555
	11th to 20th	Night	9% 3 11% 28% 41% 9%	552
	21st to end	Night	7% 3 12% 29% 40% 9%	624
August	1st to 10th	Night	16% 3 11% 26% 37% 7%	567
	11th to 20th	Night	14% 4 13% 30% 33% 7%	564
	21st to end	Night	14% 3 10% 27% 38% 9%	710
September	1st to 10th	Night	15% 5% 13% 27% 33% 7%	760
	11th to 20th	Night	23% 4 12% 24% 33% 5%	760
	21st to end	Night	23% 7% 14% 26% 26% 3	760
October	1st to 10th	Night	18% 11% 19% 23% 22% 7%	760
	11th to 20th	Night	19% 14% 23% 28% 14% 3	760
	21st to end	Night	17% 16% 21% 31% 11% 3	836
November	1st to 10th	Night	17% 14% 20% 32% 14% 3	780
	11th to 20th	Night	18% 19% 22% 30% 9% 2	800
	21st to end	Night	18% 19% 24% 27% 10% 1	800
December	1st to 10th	Night	21% 24% 25% 24% 7%	800
	11th to 20th	Night	19% 22% 26% 27% 6% 1	800
	21st to end	Night	25% 20% 26% 24% 5%	880

Page generated at 2025-05-23 09:21:00. Version ac07

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