Grayson Highlands
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Grayson Highlands clear sky chart from 2005-11-06 to 2025-08-05.

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% 7% 3 8% 6% 6% 6% 5% 7% 8% 34%	125792
	Night	11% 7% 3 7% 5% 5% 5% 4 6% 8% 38%	122286
Transparency	Day	26% 41% 9% 12% 8% 3	129451
	Night	24% 42% 9% 13% 9% 4	126696
Seeing	Day	23% 18% 24% 19% 11% 5%	16127
	Night	20% 16% 22% 24% 15% 2	26188

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% 9% 5% 9% 5% 4 4 3 5% 7% 39%	4056
	11th to 20th	Day	8% 8% 5% 10% 5% 5% 5% 4 5% 6% 40%	4060
	21st to end	Day	12% 9% 4 10% 5% 4 4 3 4 5% 41%	4466
February	1st to 10th	Day	10% 11% 4 8% 6% 5% 5% 4 5% 5% 36%	1800
	11th to 20th	Day	14% 11% 3 9% 7% 4 4 3 5% 6% 34%	1922
	21st to end	Day	16% 9% 3 8% 4 4 4 3 6% 6% 36%	1650
March	1st to 10th	Day	14% 7% 3 7% 5% 4 5% 3 6% 7% 37%	4935
	11th to 20th	Day	9% 7% 3 7% 4 4 4 3 6% 7% 47%	4980
	21st to end	Day	13% 8% 2 7% 5% 4 4 3 6% 7% 41%	5465
April	1st to 10th	Day	10% 8% 3 7% 5% 4 5% 5% 6% 7% 39%	2200
	11th to 20th	Day	15% 8% 2 5% 5% 4 5% 4 6% 7% 39%	2403
	21st to end	Day	14% 8% 3 7% 7% 5% 4 5% 5% 7% 36%	2600
May	1st to 10th	Day	11% 8% 4 10% 7% 7% 7% 6% 8% 8% 25%	5862
	11th to 20th	Day	17% 8% 4 9% 6% 7% 6% 5% 7% 8% 23%	5861
	21st to end	Day	13% 6% 2 7% 6% 6% 7% 6% 8% 10% 30%	6433
June	1st to 10th	Day	11% 6% 2 8% 7% 6% 6% 5% 7% 10% 32%	2600
	11th to 20th	Day	8% 6% 3 8% 6% 7% 7% 5% 8% 10% 32%	2600
	21st to end	Day	9% 6% 2 8% 7% 6% 6% 6% 9% 9% 32%	2600
July	1st to 10th	Day	6% 5% 2 7% 6% 7% 8% 6% 8% 10% 36%	5900
	11th to 20th	Day	7% 6% 2 8% 7% 8% 8% 6% 10% 12% 27%	5900
	21st to end	Day	12% 8% 3 10% 7% 8% 9% 6% 9% 9% 19%	6424
August	1st to 10th	Day	12% 7% 3 9% 7% 7% 7% 6% 8% 11% 24%	4156
	11th to 20th	Day	9% 5% 2 8% 7% 8% 7% 6% 9% 10% 28%	4044
	21st to end	Day	6% 5% 2 6% 5% 6% 6% 4 7% 11% 41%	4340
September	1st to 10th	Day	13% 7% 2 7% 6% 5% 4 5% 6% 8% 37%	2095
	11th to 20th	Day	16% 8% 4 7% 6% 5% 5% 4 6% 8% 32%	2090
	21st to end	Day	20% 7% 3 7% 5% 5% 5% 3 6% 7% 33%	2090
October	1st to 10th	Day	15% 7% 2 6% 4 4 4 3 5% 6% 43%	1990
	11th to 20th	Day	11% 7% 2 6% 4 4 4 4 5% 7% 46%	1900
	21st to end	Day	12% 6% 3 8% 5% 3 3 3 4 7% 47%	2090
November	1st to 10th	Day	10% 6% 2 5% 4 4 4 4 5% 7% 49%	1911
	11th to 20th	Day	10% 6% 2 6% 5% 4 5% 4 6% 7% 45%	1800
	21st to end	Day	11% 7% 2 8% 5% 4 4 3 6% 6% 44%	1800
December	1st to 10th	Day	9% 7% 3 8% 5% 4 4 4 6% 7% 44%	2930
	11th to 20th	Day	14% 6% 4 9% 6% 5% 6% 4 6% 7% 35%	3373
	21st to end	Day	16% 8% 4 8% 4 5% 5% 4 7% 8% 31%	4466
January	1st to 10th	Night	9% 9% 4 9% 6% 5% 5% 4 6% 7% 36%	5889
	11th to 20th	Night	8% 9% 3 9% 7% 5% 4 3 4 5% 42%	5900
	21st to end	Night	12% 8% 4 7% 4 4 4 3 5% 6% 44%	6490
February	1st to 10th	Night	11% 8% 3 8% 6% 4 4 4 7% 7% 38%	2600
	11th to 20th	Night	14% 11% 3 8% 5% 4 5% 4 6% 6% 33%	2362
	21st to end	Night	12% 7% 3 9% 6% 5% 5% 5% 7% 7% 34%	1980
March	1st to 10th	Night	16% 8% 3 8% 5% 4 4 3 5% 5% 38%	4918
	11th to 20th	Night	10% 5% 2 7% 4 4 4 3 5% 6% 49%	4980
	21st to end	Night	11% 7% 2 7% 5% 5% 5% 4 6% 6% 41%	5467
April	1st to 10th	Night	9% 8% 3 8% 5% 5% 4 4 6% 9% 39%	2200
	11th to 20th	Night	11% 7% 2 6% 5% 3 5% 5% 6% 9% 41%	1961
	21st to end	Night	12% 7% 2 7% 5% 5% 4 4 5% 8% 41%	1800
May	1st to 10th	Night	12% 9% 3 8% 5% 5% 6% 5% 7% 10% 30%	4033
	11th to 20th	Night	18% 7% 3 8% 6% 5% 5% 4 7% 9% 28%	4032
	21st to end	Night	14% 6% 2 7% 5% 5% 5% 5% 8% 10% 32%	4421
June	1st to 10th	Night	7% 6% 2 7% 5% 4 6% 5% 7% 11% 38%	1800
	11th to 20th	Night	7% 6% 1 5% 4 5% 6% 5% 9% 12% 40%	1800
	21st to end	Night	6% 6% 2 7% 6% 6% 4 3 9% 12% 40%	1800
July	1st to 10th	Night	6% 6% 2 6% 4 5% 6% 6% 9% 12% 39%	4058
	11th to 20th	Night	6% 5% 2 7% 5% 6% 7% 5% 10% 14% 34%	4057
	21st to end	Night	7% 9% 3 9% 7% 6% 7% 6% 9% 11% 25%	4427
August	1st to 10th	Night	12% 8% 3 8% 5% 5% 5% 5% 8% 11% 31%	2861
	11th to 20th	Night	8% 5% 2 7% 4 6% 6% 5% 8% 11% 36%	2785
	21st to end	Night	6% 4 1 4 4 3 5% 4 6% 9% 54%	3282
September	1st to 10th	Night	10% 7% 3 7% 6% 4 5% 4 7% 7% 42%	2080
	11th to 20th	Night	16% 8% 3 6% 4 4 4 3 6% 7% 40%	2090
	21st to end	Night	16% 9% 3 7% 4 4 4 3 5% 6% 38%	2090
October	1st to 10th	Night	15% 7% 2 5% 3 3 4 3 5% 7% 45%	2188
	11th to 20th	Night	12% 8% 3 5% 4 4 4 3 5% 6% 46%	2280
	21st to end	Night	11% 6% 3 6% 4 3 4 3 4 7% 49%	2508
November	1st to 10th	Night	12% 8% 2 6% 4 4 4 3 4 6% 47%	2385
	11th to 20th	Night	10% 7% 3 7% 4 4 5% 4 5% 8% 44%	2600
	21st to end	Night	10% 7% 3 6% 4 4 5% 4 7% 6% 43%	2600
December	1st to 10th	Night	9% 6% 2 6% 5% 4 4 4 7% 8% 45%	4250
	11th to 20th	Night	14% 10% 4 11% 6% 5% 5% 3 5% 6% 31%	4822
	21st to end	Night	15% 7% 3 8% 5% 5% 6% 5% 7% 9% 31%	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	25% 33% 7% 11% 20% 4	4057
	11th to 20th	Day	26% 33% 6% 9% 21% 5%	4060
	21st to end	Day	30% 31% 8% 13% 15% 4	4466
February	1st to 10th	Day	19% 63% 4 6% 6% 2	1800
	11th to 20th	Day	21% 63% 3 5% 7% 1	1932
	21st to end	Day	18% 63% 3 5% 7% 3	1650
March	1st to 10th	Day	31% 33% 7% 10% 10% 8%	4935
	11th to 20th	Day	20% 32% 5% 16% 15% 12%	4980
	21st to end	Day	28% 30% 7% 19% 13% 4	5465
April	1st to 10th	Day	18% 64% 5% 7% 5% 1	2200
	11th to 20th	Day	17% 64% 4 9% 4 1	2403
	21st to end	Day	17% 64% 4 9% 5% 1	2600
May	1st to 10th	Day	37% 35% 10% 13% 4	5897
	11th to 20th	Day	39% 33% 10% 14% 3	5865
	21st to end	Day	30% 34% 12% 13% 9% 3	6447
June	1st to 10th	Day	16% 65% 6% 8% 5%	2600
	11th to 20th	Day	16% 66% 7% 6% 4	2600
	21st to end	Day	18% 63% 7% 9% 3	2600
July	1st to 10th	Day	29% 32% 14% 17% 7% 2	5835
	11th to 20th	Day	34% 33% 18% 14% 1	5831
	21st to end	Day	44% 31% 11% 9% 4	6240
August	1st to 10th	Day	34% 40% 13% 11% 2	4143
	11th to 20th	Day	23% 42% 14% 14% 4 2	4083
	21st to end	Day	17% 40% 15% 21% 7% 1	4340
September	1st to 10th	Day	16% 61% 6% 8% 5% 3	2095
	11th to 20th	Day	21% 62% 4 6% 6%	2090
	21st to end	Day	21% 62% 6% 7% 3	2090
October	1st to 10th	Day	15% 39% 5% 11% 21% 8%	3302
	11th to 20th	Day	12% 38% 4 18% 9% 20%	3140
	21st to end	Day	13% 38% 6% 21% 9% 14%	3428
November	1st to 10th	Day	19% 57% 5% 9% 8% 2	1911
	11th to 20th	Day	16% 58% 4 6% 12% 4	1800
	21st to end	Day	17% 60% 4 9% 10% 1	1800
December	1st to 10th	Day	21% 38% 6% 15% 17% 3	2930
	11th to 20th	Day	31% 34% 8% 10% 14% 2	3370
	21st to end	Day	33% 31% 13% 15% 6% 2	4466
January	1st to 10th	Night	25% 35% 6% 14% 12% 9%	5889
	11th to 20th	Night	28% 30% 4 13% 17% 8%	5900
	21st to end	Night	27% 32% 6% 13% 19% 3	6490
February	1st to 10th	Night	16% 64% 6% 7% 6% 1	2600
	11th to 20th	Night	20% 63% 5% 5% 5% 1	2363
	21st to end	Night	16% 65% 3 8% 5% 3	1980
March	1st to 10th	Night	30% 34% 5% 14% 9% 7%	4918
	11th to 20th	Night	19% 31% 6% 16% 17% 11%	4980
	21st to end	Night	29% 31% 9% 17% 11% 3	5467
April	1st to 10th	Night	17% 64% 5% 7% 5% 1	2200
	11th to 20th	Night	15% 64% 6% 9% 5% 1	1961
	21st to end	Night	15% 64% 6% 11% 4	1800
May	1st to 10th	Night	32% 39% 10% 14% 5%	4060
	11th to 20th	Night	36% 34% 14% 14% 2	4033
	21st to end	Night	30% 36% 14% 15% 5% 1	4429
June	1st to 10th	Night	14% 66% 7% 8% 4	1800
	11th to 20th	Night	11% 67% 8% 10% 3 1	1800
	21st to end	Night	14% 64% 8% 10% 4	1800
July	1st to 10th	Night	26% 33% 16% 21% 4	4015
	11th to 20th	Night	27% 33% 24% 14% 1	4006
	21st to end	Night	39% 32% 14% 10% 5%	4297
August	1st to 10th	Night	31% 40% 18% 10% 2	2850
	11th to 20th	Night	19% 43% 18% 17% 3	2812
	21st to end	Night	13% 39% 13% 28% 7%	3282
September	1st to 10th	Night	15% 62% 8% 8% 6% 1	2080
	11th to 20th	Night	18% 61% 6% 9% 6%	2090
	21st to end	Night	20% 62% 5% 10% 2	2090
October	1st to 10th	Night	16% 39% 6% 15% 14% 9%	3656
	11th to 20th	Night	12% 38% 5% 14% 13% 18%	3779
	21st to end	Night	14% 37% 6% 16% 12% 14%	4120
November	1st to 10th	Night	21% 57% 5% 9% 7% 1	2385
	11th to 20th	Night	15% 59% 3 8% 10% 5%	2600
	21st to end	Night	15% 59% 6% 9% 9% 1	2600
December	1st to 10th	Night	19% 39% 7% 20% 14% 1	4250
	11th to 20th	Night	36% 36% 6% 11% 10% 2	4824
	21st to end	Night	30% 33% 12% 16% 7% 2	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	47% 25% 21% 6% 1 1	300
	11th to 20th	Day	43% 25% 24% 7% 1	300
	21st to end	Day	36% 37% 18% 8% 1	330
February	1st to 10th	Day	36% 33% 24% 7% 1	300
	11th to 20th	Day	52% 27% 19% 3	300
	21st to end	Day	43% 31% 23% 2	249
March	1st to 10th	Day	31% 28% 24% 12% 4	500
	11th to 20th	Day	36% 29% 21% 10% 3 1	500
	21st to end	Day	31% 29% 23% 13% 3 1	550
April	1st to 10th	Day	30% 26% 25% 11% 6% 1	500
	11th to 20th	Day	29% 24% 23% 15% 7% 2	492
	21st to end	Day	25% 23% 25% 16% 9% 2	500
May	1st to 10th	Day	23% 26% 29% 14% 4 5%	503
	11th to 20th	Day	22% 20% 26% 19% 9% 5%	527
	21st to end	Day	21% 13% 25% 21% 13% 8%	573
June	1st to 10th	Day	13% 12% 32% 22% 14% 7%	530
	11th to 20th	Day	10% 10% 28% 28% 17% 6%	530
	21st to end	Day	12% 15% 27% 25% 15% 6%	530
July	1st to 10th	Day	11% 8% 21% 29% 20% 11%	525
	11th to 20th	Day	9% 8% 19% 35% 21% 9%	524
	21st to end	Day	11% 7% 26% 25% 21% 10%	576
August	1st to 10th	Day	17% 8% 25% 25% 15% 10%	502
	11th to 20th	Day	10% 8% 26% 23% 22% 11%	479
	21st to end	Day	12% 5% 20% 27% 19% 17%	539
September	1st to 10th	Day	12% 6% 21% 25% 23% 12%	490
	11th to 20th	Day	17% 8% 17% 26% 22% 10%	490
	21st to end	Day	26% 11% 21% 22% 15% 4	490
October	1st to 10th	Day	20% 12% 22% 22% 17% 6%	490
	11th to 20th	Day	19% 21% 22% 22% 14% 2	490
	21st to end	Day	21% 19% 27% 22% 9% 3	539
November	1st to 10th	Day	19% 19% 30% 20% 9% 3	449
	11th to 20th	Day	32% 22% 25% 16% 5%	300
	21st to end	Day	28% 26% 28% 14% 3 1	300
December	1st to 10th	Day	33% 31% 27% 7% 1	300
	11th to 20th	Day	31% 24% 32% 10% 3	300
	21st to end	Day	32% 27% 27% 10% 4	330
January	1st to 10th	Night	34% 26% 23% 15% 3 1	800
	11th to 20th	Night	30% 26% 26% 16% 2	800
	21st to end	Night	27% 29% 26% 16% 2	880
February	1st to 10th	Night	26% 23% 31% 19% 2	800
	11th to 20th	Night	36% 31% 21% 11% 1	783
	21st to end	Night	26% 30% 29% 13% 2	660
March	1st to 10th	Night	25% 24% 29% 19% 3	800
	11th to 20th	Night	29% 24% 25% 16% 6%	800
	21st to end	Night	22% 22% 32% 19% 5%	880
April	1st to 10th	Night	25% 22% 27% 20% 6%	800
	11th to 20th	Night	21% 21% 26% 22% 9% 1	761
	21st to end	Night	16% 20% 24% 32% 8% 1	600
May	1st to 10th	Night	18% 15% 30% 23% 12% 1	600
	11th to 20th	Night	18% 11% 24% 30% 15% 2	591
	21st to end	Night	18% 7% 19% 29% 23% 5%	650
June	1st to 10th	Night	10% 6% 23% 34% 24% 4	600
	11th to 20th	Night	9% 6% 17% 33% 31% 4	600
	21st to end	Night	7% 6% 17% 36% 29% 5%	600
July	1st to 10th	Night	10% 4 14% 31% 35% 6%	585
	11th to 20th	Night	6% 4 12% 34% 37% 6%	582
	21st to end	Night	6% 5% 12% 33% 37% 7%	657
August	1st to 10th	Night	14% 4 12% 30% 36% 4	579
	11th to 20th	Night	13% 4 11% 34% 33% 5%	564
	21st to end	Night	11% 3 13% 30% 36% 8%	720
September	1st to 10th	Night	12% 6% 15% 29% 32% 7%	760
	11th to 20th	Night	18% 5% 16% 28% 28% 5%	760
	21st to end	Night	21% 9% 17% 28% 24% 2	760
October	1st to 10th	Night	18% 11% 17% 26% 23% 5%	760
	11th to 20th	Night	19% 17% 23% 23% 14% 3	760
	21st to end	Night	21% 15% 23% 29% 11% 2	836
November	1st to 10th	Night	18% 15% 26% 24% 14% 3	780
	11th to 20th	Night	24% 16% 23% 27% 9% 1	800
	21st to end	Night	25% 18% 24% 22% 10% 1	800
December	1st to 10th	Night	27% 24% 26% 19% 5%	800
	11th to 20th	Night	25% 24% 25% 21% 5%	800
	21st to end	Night	27% 22% 27% 21% 4	880

Page generated at 2025-08-02 01:32:23. Version ac07

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