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 2024-03-23.

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% 4 7% 7% 4 6% 5% 6% 8% 34%	79108
	Night	12% 7% 3 7% 6% 3 5% 5% 6% 8% 38%	80791
Transparency	Day	30% 39% 8% 11% 9% 2	78831
	Night	29% 39% 8% 12% 10% 2	80587
Seeing	Day	23% 18% 24% 19% 11% 5%	14240
	Night	20% 16% 22% 24% 15% 2	24427

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	13% 9% 5% 7% 6% 3 5% 4 6% 9% 32%	2836
	11th to 20th	Day	11% 9% 6% 11% 8% 5% 6% 5% 5% 5% 31%	2840
	21st to end	Day	15% 11% 5% 9% 6% 3 4 3 4 5% 35%	3124
February	1st to 10th	Day	11% 10% 5% 6% 6% 4 6% 5% 5% 5% 36%	1710
	11th to 20th	Day	14% 10% 4 8% 7% 2 4 4 5% 6% 35%	1823
	21st to end	Day	17% 8% 4 7% 5% 3 4 4 6% 7% 35%	1570
March	1st to 10th	Day	12% 7% 4 6% 5% 3 4 4 5% 6% 44%	2090
	11th to 20th	Day	15% 9% 4 7% 6% 3 5% 4 6% 7% 34%	2090
	21st to end	Day	16% 10% 3 7% 6% 3 4 4 6% 7% 34%	2200
April	1st to 10th	Day	11% 6% 3 5% 6% 3 5% 6% 6% 8% 41%	1980
	11th to 20th	Day	16% 8% 3 4 6% 3 5% 4 6% 6% 39%	2158
	21st to end	Day	15% 7% 3 6% 7% 3 5% 6% 4 7% 36%	2340
May	1st to 10th	Day	14% 6% 4 7% 7% 4 6% 6% 6% 8% 32%	2340
	11th to 20th	Day	17% 8% 4 7% 7% 4 6% 5% 5% 7% 28%	2301
	21st to end	Day	13% 7% 3 6% 7% 5% 7% 7% 8% 8% 29%	2531
June	1st to 10th	Day	12% 6% 3 7% 8% 5% 6% 6% 7% 9% 31%	2340
	11th to 20th	Day	9% 6% 3 7% 7% 5% 8% 7% 7% 10% 32%	2340
	21st to end	Day	10% 5% 3 7% 7% 5% 7% 7% 9% 8% 31%	2340
July	1st to 10th	Day	11% 6% 3 8% 7% 5% 9% 9% 7% 10% 27%	2340
	11th to 20th	Day	11% 6% 3 7% 8% 5% 8% 7% 8% 11% 26%	2340
	21st to end	Day	12% 7% 4 9% 7% 5% 9% 7% 8% 9% 23%	2547
August	1st to 10th	Day	11% 7% 3 8% 9% 6% 8% 7% 7% 10% 24%	2327
	11th to 20th	Day	12% 7% 3 8% 8% 7% 8% 7% 8% 9% 24%	2303
	21st to end	Day	11% 7% 3 7% 7% 5% 7% 6% 7% 10% 31%	2466
September	1st to 10th	Day	14% 6% 3 6% 8% 4 4 6% 6% 8% 36%	1985
	11th to 20th	Day	15% 8% 4 6% 7% 4 5% 5% 6% 8% 32%	1980
	21st to end	Day	19% 6% 3 6% 5% 4 5% 4 6% 7% 34%	1980
October	1st to 10th	Day	15% 7% 3 5% 6% 3 5% 4 4 6% 42%	1886
	11th to 20th	Day	12% 7% 3 6% 5% 3 5% 5% 5% 7% 45%	1800
	21st to end	Day	13% 5% 3 7% 6% 3 4 4 4 7% 45%	1980
November	1st to 10th	Day	10% 6% 2 5% 4 3 5% 4 5% 7% 50%	1814
	11th to 20th	Day	10% 6% 3 5% 6% 3 4 5% 6% 7% 45%	1710
	21st to end	Day	12% 7% 3 6% 5% 3 5% 4 6% 6% 44%	1710
December	1st to 10th	Day	15% 10% 4 8% 7% 4 4 4 6% 7% 31%	1710
	11th to 20th	Day	13% 6% 4 7% 5% 3 5% 5% 6% 8% 40%	2153
	21st to end	Day	16% 7% 5% 7% 5% 4 5% 6% 6% 7% 31%	3124
January	1st to 10th	Night	12% 11% 6% 10% 8% 4 5% 5% 6% 6% 27%	4109
	11th to 20th	Night	10% 9% 4 9% 8% 4 5% 5% 4 5% 38%	4120
	21st to end	Night	15% 10% 5% 7% 6% 3 4 4 5% 6% 34%	4532
February	1st to 10th	Night	11% 7% 4 8% 6% 3 4 5% 7% 7% 38%	2470
	11th to 20th	Night	14% 9% 4 7% 6% 3 5% 5% 6% 6% 34%	2241
	21st to end	Night	13% 6% 4 8% 8% 4 5% 6% 6% 7% 33%	1884
March	1st to 10th	Night	12% 6% 3 7% 6% 4 5% 5% 6% 6% 41%	2090
	11th to 20th	Night	13% 8% 3 8% 7% 4 6% 6% 5% 7% 33%	2090
	21st to end	Night	11% 8% 3 6% 7% 4 5% 6% 7% 6% 36%	2201
April	1st to 10th	Night	9% 7% 3 7% 5% 4 5% 5% 6% 9% 40%	1980
	11th to 20th	Night	12% 7% 3 5% 5% 2 5% 6% 6% 9% 41%	1764
	21st to end	Night	12% 6% 2 6% 5% 4 5% 5% 5% 8% 41%	1620
May	1st to 10th	Night	12% 7% 3 7% 6% 3 5% 5% 6% 9% 37%	1620
	11th to 20th	Night	13% 8% 3 7% 5% 3 5% 5% 7% 8% 35%	1592
	21st to end	Night	11% 8% 3 5% 5% 3 6% 6% 8% 10% 35%	1744
June	1st to 10th	Night	8% 5% 3 6% 6% 3 6% 6% 7% 11% 39%	1620
	11th to 20th	Night	7% 5% 2 4 5% 4 6% 7% 7% 12% 41%	1620
	21st to end	Night	6% 5% 3 5% 6% 4 5% 5% 9% 13% 39%	1620
July	1st to 10th	Night	8% 5% 3 5% 6% 3 5% 6% 8% 11% 39%	1620
	11th to 20th	Night	6% 4 2 4 5% 4 6% 7% 9% 12% 39%	1620
	21st to end	Night	5% 4 3 6% 6% 4 7% 7% 8% 13% 36%	1771
August	1st to 10th	Night	11% 6% 2 7% 7% 4 6% 6% 7% 9% 35%	1611
	11th to 20th	Night	12% 6% 3 6% 6% 4 6% 6% 7% 9% 36%	1598
	21st to end	Night	10% 6% 2 5% 6% 3 6% 5% 6% 10% 40%	1870
September	1st to 10th	Night	10% 6% 4 6% 7% 3 5% 5% 7% 7% 42%	1970
	11th to 20th	Night	15% 7% 3 5% 5% 3 4 4 5% 7% 41%	1980
	21st to end	Night	15% 8% 3 6% 5% 3 4 4 5% 7% 40%	1980
October	1st to 10th	Night	15% 7% 2 4 4 2 5% 4 5% 7% 44%	2072
	11th to 20th	Night	13% 7% 3 5% 4 3 4 4 5% 7% 45%	2160
	21st to end	Night	12% 6% 3 6% 5% 3 4 4 4 7% 48%	2376
November	1st to 10th	Night	11% 7% 3 5% 5% 3 4 4 4 6% 49%	2262
	11th to 20th	Night	11% 6% 3 6% 4 3 5% 5% 5% 8% 45%	2470
	21st to end	Night	11% 6% 3 6% 4 3 5% 5% 7% 6% 43%	2470
December	1st to 10th	Night	14% 9% 4 8% 7% 4 5% 5% 6% 7% 31%	2470
	11th to 20th	Night	13% 7% 4 11% 6% 3 5% 4 5% 7% 36%	3042
	21st to end	Night	16% 8% 5% 8% 7% 4 6% 6% 6% 8% 26%	4532

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	29% 36% 10% 11% 12% 2	2837
	11th to 20th	Day	35% 33% 9% 8% 14% 1	2840
	21st to end	Day	41% 27% 7% 11% 12% 2	3124
February	1st to 10th	Day	31% 38% 6% 10% 13% 2	1710
	11th to 20th	Day	32% 38% 7% 10% 10% 4	1833
	21st to end	Day	32% 37% 8% 9% 10% 4	1570
March	1st to 10th	Day	26% 37% 6% 14% 13% 3	2090
	11th to 20th	Day	35% 38% 7% 10% 8% 3	2090
	21st to end	Day	35% 39% 8% 12% 5% 1	2200
April	1st to 10th	Day	24% 39% 8% 15% 11% 3	1980
	11th to 20th	Day	28% 37% 7% 11% 13% 3	2158
	21st to end	Day	32% 39% 8% 13% 7% 1	2340
May	1st to 10th	Day	33% 42% 7% 11% 7%	2340
	11th to 20th	Day	37% 41% 8% 9% 4 1	2305
	21st to end	Day	30% 42% 11% 13% 4 1	2531
June	1st to 10th	Day	31% 42% 9% 12% 6%	2340
	11th to 20th	Day	30% 42% 9% 11% 7% 1	2340
	21st to end	Day	28% 42% 9% 11% 10% 1	2340
July	1st to 10th	Day	32% 43% 10% 9% 6% 1	2275
	11th to 20th	Day	32% 43% 10% 7% 7% 1	2271
	21st to end	Day	33% 44% 10% 9% 4	2405
August	1st to 10th	Day	33% 43% 11% 11% 3	2314
	11th to 20th	Day	35% 42% 8% 10% 5%	2303
	21st to end	Day	31% 41% 10% 11% 5% 2	2466
September	1st to 10th	Day	29% 39% 8% 11% 9% 3	1985
	11th to 20th	Day	31% 39% 6% 14% 9% 1	1980
	21st to end	Day	30% 38% 8% 10% 11% 3	1980
October	1st to 10th	Day	25% 37% 6% 15% 14% 3	1886
	11th to 20th	Day	25% 39% 7% 15% 11% 3	1800
	21st to end	Day	23% 40% 6% 14% 12% 4	1980
November	1st to 10th	Day	17% 42% 6% 12% 19% 5%	1814
	11th to 20th	Day	22% 42% 6% 13% 15% 3	1710
	21st to end	Day	24% 40% 6% 14% 12% 4	1710
December	1st to 10th	Day	31% 43% 7% 10% 9% 1	1710
	11th to 20th	Day	25% 37% 9% 10% 16% 2	2150
	21st to end	Day	32% 31% 11% 16% 9% 1	3124
January	1st to 10th	Night	33% 39% 10% 8% 7% 4	4109
	11th to 20th	Night	33% 31% 6% 11% 14% 5%	4120
	21st to end	Night	41% 28% 6% 12% 11% 3	4532
February	1st to 10th	Night	28% 38% 8% 12% 12% 2	2470
	11th to 20th	Night	31% 40% 8% 9% 9% 4	2242
	21st to end	Night	34% 39% 6% 11% 7% 2	1884
March	1st to 10th	Night	28% 39% 7% 11% 13% 2	2090
	11th to 20th	Night	31% 40% 8% 12% 6% 2	2090
	21st to end	Night	30% 41% 8% 13% 6% 1	2201
April	1st to 10th	Night	25% 41% 9% 14% 9% 3	1980
	11th to 20th	Night	24% 39% 9% 12% 13% 3	1764
	21st to end	Night	28% 39% 9% 15% 7% 1	1620
May	1st to 10th	Night	31% 42% 9% 10% 7%	1620
	11th to 20th	Night	31% 43% 10% 12% 4 1	1593
	21st to end	Night	26% 43% 13% 12% 4 1	1745
June	1st to 10th	Night	25% 42% 11% 13% 8%	1620
	11th to 20th	Night	24% 43% 13% 14% 7%	1620
	21st to end	Night	21% 44% 12% 13% 9% 2	1620
July	1st to 10th	Night	25% 42% 11% 12% 8% 1	1575
	11th to 20th	Night	25% 45% 11% 10% 7% 2	1566
	21st to end	Night	21% 47% 15% 14% 3	1672
August	1st to 10th	Night	27% 42% 10% 13% 7%	1600
	11th to 20th	Night	29% 42% 10% 12% 6% 1	1598
	21st to end	Night	26% 41% 12% 14% 5% 1	1870
September	1st to 10th	Night	27% 41% 8% 11% 10% 3	1970
	11th to 20th	Night	27% 39% 9% 14% 11%	1980
	21st to end	Night	28% 39% 6% 12% 12% 2	1980
October	1st to 10th	Night	23% 38% 7% 14% 15% 3	2072
	11th to 20th	Night	25% 39% 7% 16% 10% 2	2160
	21st to end	Night	23% 40% 6% 14% 13% 5%	2376
November	1st to 10th	Night	18% 43% 6% 11% 16% 5%	2262
	11th to 20th	Night	24% 42% 6% 12% 13% 3	2470
	21st to end	Night	24% 42% 5% 13% 13% 4	2470
December	1st to 10th	Night	29% 45% 6% 10% 9% 2	2470
	11th to 20th	Night	32% 39% 5% 7% 14% 3	3044
	21st to end	Night	35% 34% 10% 12% 9% 1	4532

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	43% 27% 22% 7% 1 1	270
	11th to 20th	Day	41% 26% 25% 7%	270
	21st to end	Day	36% 37% 18% 8% 1	297
February	1st to 10th	Day	35% 33% 24% 7% 1	270
	11th to 20th	Day	49% 27% 20% 3	270
	21st to end	Day	44% 32% 22% 2	225
March	1st to 10th	Day	30% 28% 25% 13% 4	460
	11th to 20th	Day	36% 29% 20% 11% 3 1	460
	21st to end	Day	29% 27% 26% 14% 3 1	470
April	1st to 10th	Day	27% 26% 26% 13% 7% 1	420
	11th to 20th	Day	29% 22% 23% 16% 8% 2	412
	21st to end	Day	26% 23% 24% 15% 9% 3	420
May	1st to 10th	Day	23% 26% 28% 12% 4 5%	423
	11th to 20th	Day	21% 18% 28% 19% 9% 5%	447
	21st to end	Day	19% 11% 24% 23% 14% 10%	485
June	1st to 10th	Day	14% 11% 29% 23% 16% 7%	450
	11th to 20th	Day	11% 11% 27% 28% 18% 6%	450
	21st to end	Day	13% 17% 28% 23% 13% 6%	450
July	1st to 10th	Day	12% 9% 19% 27% 21% 12%	445
	11th to 20th	Day	9% 9% 20% 33% 20% 9%	444
	21st to end	Day	10% 6% 26% 26% 22% 10%	488
August	1st to 10th	Day	14% 8% 23% 27% 16% 12%	446
	11th to 20th	Day	11% 7% 23% 24% 23% 12%	439
	21st to end	Day	13% 5% 21% 26% 18% 17%	495
September	1st to 10th	Day	12% 6% 20% 24% 24% 13%	450
	11th to 20th	Day	16% 7% 17% 27% 23% 10%	450
	21st to end	Day	24% 11% 20% 23% 16% 5%	450
October	1st to 10th	Day	22% 12% 21% 21% 18% 6%	450
	11th to 20th	Day	19% 21% 22% 21% 14% 2	450
	21st to end	Day	21% 20% 26% 21% 8% 3	495
November	1st to 10th	Day	19% 19% 29% 20% 9% 3	412
	11th to 20th	Day	32% 21% 24% 17% 6%	270
	21st to end	Day	26% 26% 29% 16% 3 1	270
December	1st to 10th	Day	33% 30% 28% 8% 1	270
	11th to 20th	Day	30% 24% 31% 10% 3	270
	21st to end	Day	31% 28% 27% 9% 4	297
January	1st to 10th	Night	32% 26% 23% 15% 3 1	760
	11th to 20th	Night	29% 26% 26% 17% 2	760
	21st to end	Night	27% 29% 25% 17% 2	836
February	1st to 10th	Night	25% 23% 31% 19% 2	760
	11th to 20th	Night	35% 32% 21% 11% 1	743
	21st to end	Night	26% 32% 28% 12% 2	628
March	1st to 10th	Night	24% 24% 30% 19% 3	760
	11th to 20th	Night	29% 24% 24% 16% 6%	760
	21st to end	Night	21% 22% 32% 20% 6%	801
April	1st to 10th	Night	23% 22% 27% 21% 6%	720
	11th to 20th	Night	21% 21% 25% 23% 10% 1	685
	21st to end	Night	17% 20% 23% 32% 7% 1	540
May	1st to 10th	Night	18% 16% 29% 23% 12% 1	540
	11th to 20th	Night	16% 11% 24% 31% 16% 2	531
	21st to end	Night	16% 6% 18% 30% 25% 5%	584
June	1st to 10th	Night	10% 6% 19% 36% 25% 4	540
	11th to 20th	Night	9% 6% 18% 31% 31% 4	540
	21st to end	Night	7% 7% 19% 38% 25% 4	540
July	1st to 10th	Night	10% 4 14% 31% 34% 7%	525
	11th to 20th	Night	7% 4 12% 34% 36% 6%	522
	21st to end	Night	4 5% 13% 34% 37% 7%	591
August	1st to 10th	Night	13% 4 12% 30% 37% 4	537
	11th to 20th	Night	14% 4 10% 33% 34% 6%	534
	21st to end	Night	11% 2 13% 30% 35% 7%	682
September	1st to 10th	Night	12% 5% 15% 28% 32% 7%	720
	11th to 20th	Night	17% 5% 16% 28% 29% 6%	720
	21st to end	Night	20% 8% 17% 28% 25% 2	720
October	1st to 10th	Night	19% 11% 17% 24% 23% 5%	720
	11th to 20th	Night	20% 17% 24% 23% 14% 3	720
	21st to end	Night	21% 15% 24% 27% 10% 2	792
November	1st to 10th	Night	18% 16% 25% 24% 15% 3	740
	11th to 20th	Night	25% 16% 23% 27% 9% 1	760
	21st to end	Night	24% 18% 25% 22% 11% 1	760
December	1st to 10th	Night	27% 23% 25% 20% 5%	760
	11th to 20th	Night	25% 24% 25% 21% 5%	760
	21st to end	Night	27% 22% 27% 20% 4	836

Page generated at 2024-03-19 23:31:03. Version ac07

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