ETSU Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the ETSU Observatory clear sky chart from 2005-11-06 to 2025-11-18.

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	14% 9% 3 7% 5% 5% 5% 4 6% 7% 35%	126838
	Night	14% 9% 3 7% 4 4 4 4 5% 7% 37%	123543
Transparency	Day	45% 8% 11% 20% 13% 3	130536
	Night	45% 7% 11% 20% 14% 3	127914
Seeing	Day	20% 14% 23% 22% 15% 6%	16427
	Night	19% 11% 21% 30% 16% 2	26468

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% 18% 8% 11% 4 3 3 3 4 4 28%	4056
	11th to 20th	Day	11% 18% 9% 10% 6% 4 3 2 3 4 31%	4060
	21st to end	Day	15% 15% 4 9% 4 3 4 2 4 5% 35%	4466
February	1st to 10th	Day	17% 12% 4 9% 5% 4 4 3 5% 6% 31%	1850
	11th to 20th	Day	21% 15% 5% 8% 4 4 2 3 4 5% 28%	1948
	21st to end	Day	21% 12% 3 7% 4 4 2 3 4 5% 35%	1650
March	1st to 10th	Day	16% 14% 4 7% 4 4 4 3 4 5% 35%	4837
	11th to 20th	Day	12% 9% 3 5% 4 3 4 3 5% 6% 47%	4980
	21st to end	Day	16% 8% 2 6% 4 4 4 3 5% 7% 39%	5465
April	1st to 10th	Day	14% 9% 2 5% 5% 4 4 4 7% 8% 38%	2200
	11th to 20th	Day	15% 8% 2 6% 4 3 5% 4 5% 6% 41%	2417
	21st to end	Day	16% 7% 2 6% 5% 5% 5% 4 5% 6% 38%	2600
May	1st to 10th	Day	17% 9% 3 9% 6% 6% 6% 5% 7% 8% 25%	5862
	11th to 20th	Day	17% 8% 3 9% 6% 6% 6% 5% 7% 8% 25%	5861
	21st to end	Day	15% 6% 2 7% 6% 5% 5% 5% 7% 10% 31%	6433
June	1st to 10th	Day	12% 6% 2 6% 5% 6% 6% 6% 7% 8% 35%	2600
	11th to 20th	Day	9% 5% 2 8% 5% 7% 7% 5% 9% 10% 32%	2600
	21st to end	Day	11% 6% 2 6% 5% 5% 7% 6% 8% 9% 35%	2600
July	1st to 10th	Day	7% 5% 2 6% 5% 6% 6% 6% 8% 12% 37%	5900
	11th to 20th	Day	11% 6% 3 7% 6% 6% 7% 5% 8% 10% 30%	5900
	21st to end	Day	15% 8% 3 9% 7% 7% 7% 5% 8% 9% 22%	6424
August	1st to 10th	Day	13% 6% 3 7% 6% 6% 6% 6% 9% 10% 28%	4234
	11th to 20th	Day	13% 5% 2 7% 5% 6% 6% 5% 8% 10% 33%	4174
	21st to end	Day	7% 4 1 4 4 4 4 5% 7% 7% 52%	4393
September	1st to 10th	Day	11% 5% 2 7% 5% 5% 5% 5% 6% 7% 42%	2200
	11th to 20th	Day	14% 6% 2 6% 5% 5% 5% 3 6% 6% 41%	2200
	21st to end	Day	15% 6% 2 6% 5% 4 5% 5% 6% 8% 38%	2200
October	1st to 10th	Day	13% 5% 2 5% 3 3 4 3 4 7% 49%	2148
	11th to 20th	Day	15% 5% 2 4 3 3 3 3 5% 7% 50%	2000
	21st to end	Day	17% 9% 2 4 3 3 3 2 4 6% 46%	2200
November	1st to 10th	Day	12% 6% 3 5% 4 3 3 3 5% 6% 51%	1948
	11th to 20th	Day	11% 9% 3 5% 4 5% 4 4 5% 7% 44%	1863
	21st to end	Day	15% 9% 4 6% 4 3 3 4 5% 6% 42%	1800
December	1st to 10th	Day	13% 12% 3 11% 5% 3 4 3 4 5% 37%	2930
	11th to 20th	Day	18% 14% 4 9% 4 4 4 4 6% 7% 27%	3373
	21st to end	Day	12% 12% 6% 7% 5% 5% 5% 4 6% 7% 30%	4466
January	1st to 10th	Night	14% 16% 6% 12% 5% 3 4 3 3 4 28%	5889
	11th to 20th	Night	14% 14% 5% 11% 6% 4 4 2 4 4 32%	5900
	21st to end	Night	16% 10% 4 8% 5% 4 4 3 4 5% 38%	6490
February	1st to 10th	Night	17% 11% 3 7% 3 4 5% 3 5% 6% 36%	2547
	11th to 20th	Night	20% 12% 4 9% 5% 3 4 3 4 5% 30%	2338
	21st to end	Night	20% 12% 3 9% 3 3 3 3 5% 6% 33%	1980
March	1st to 10th	Night	20% 13% 3 5% 4 3 4 2 4 5% 36%	5014
	11th to 20th	Night	15% 6% 3 4 2 3 3 3 5% 7% 50%	4980
	21st to end	Night	12% 7% 2 5% 5% 5% 5% 4 6% 7% 41%	5467
April	1st to 10th	Night	15% 9% 3 5% 4 4 4 3 4 8% 41%	2200
	11th to 20th	Night	14% 8% 2 6% 4 4 4 4 6% 6% 43%	1944
	21st to end	Night	15% 7% 3 7% 5% 5% 4 3 5% 6% 41%	1800
May	1st to 10th	Night	18% 8% 3 8% 6% 5% 5% 4 7% 9% 28%	4033
	11th to 20th	Night	18% 8% 3 9% 6% 5% 5% 4 6% 9% 28%	4032
	21st to end	Night	18% 9% 3 7% 4 5% 4 4 6% 7% 34%	4421
June	1st to 10th	Night	11% 6% 2 7% 6% 5% 5% 4 7% 10% 38%	1800
	11th to 20th	Night	7% 5% 2 7% 5% 5% 6% 5% 8% 12% 39%	1800
	21st to end	Night	10% 6% 2 6% 5% 6% 5% 5% 7% 11% 39%	1800
July	1st to 10th	Night	12% 6% 3 6% 5% 5% 6% 5% 8% 11% 33%	4058
	11th to 20th	Night	12% 7% 3 7% 4 5% 6% 5% 8% 10% 33%	4057
	21st to end	Night	13% 9% 4 11% 7% 6% 6% 6% 7% 9% 23%	4427
August	1st to 10th	Night	12% 6% 3 7% 5% 5% 6% 5% 8% 11% 34%	2915
	11th to 20th	Night	13% 7% 3 7% 5% 5% 5% 4 6% 8% 38%	2875
	21st to end	Night	7% 3 1 4 3 4 4 3 8% 10% 52%	3466
September	1st to 10th	Night	12% 6% 2 6% 5% 5% 4 3 5% 8% 45%	2200
	11th to 20th	Night	15% 5% 2 6% 4 4 4 4 6% 8% 43%	2200
	21st to end	Night	15% 6% 2 6% 4 4 4 2 5% 7% 46%	2200
October	1st to 10th	Night	15% 6% 2 5% 3 3 3 3 4 6% 50%	2250
	11th to 20th	Night	16% 7% 2 4 2 3 3 3 4 6% 50%	2400
	21st to end	Night	15% 8% 2 5% 3 2 4 3 4 6% 48%	2640
November	1st to 10th	Night	14% 8% 2 5% 3 3 4 3 5% 5% 49%	2566
	11th to 20th	Night	13% 9% 2 5% 4 4 3 3 5% 6% 45%	2692
	21st to end	Night	13% 11% 2 5% 3 3 4 3 5% 6% 45%	2600
December	1st to 10th	Night	14% 12% 5% 8% 4 4 3 3 4 6% 36%	4250
	11th to 20th	Night	19% 12% 5% 10% 4 4 4 3 4 5% 29%	4822
	21st to end	Night	13% 15% 6% 7% 5% 3 4 4 5% 8% 30%	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	58% 5% 5% 18% 12% 2	4057
	11th to 20th	Day	57% 5% 5% 12% 21% 2	4060
	21st to end	Day	53% 4 6% 19% 16% 1	4466
February	1st to 10th	Day	55% 5% 8% 16% 15% 2	1850
	11th to 20th	Day	59% 4 6% 14% 14% 2	1958
	21st to end	Day	52% 5% 7% 16% 15% 5%	1650
March	1st to 10th	Day	51% 5% 6% 15% 16% 8%	4837
	11th to 20th	Day	39% 4 6% 20% 22% 9%	4980
	21st to end	Day	44% 5% 8% 28% 13% 2	5465
April	1st to 10th	Day	42% 7% 12% 22% 14% 3	2200
	11th to 20th	Day	42% 7% 8% 20% 18% 4	2417
	21st to end	Day	46% 8% 10% 23% 11% 2	2600
May	1st to 10th	Day	53% 9% 12% 18% 7% 1	5897
	11th to 20th	Day	55% 9% 13% 18% 4 1	5865
	21st to end	Day	45% 11% 13% 18% 12% 1	6447
June	1st to 10th	Day	42% 12% 15% 19% 10% 1	2600
	11th to 20th	Day	43% 12% 14% 16% 12% 1	2600
	21st to end	Day	41% 12% 13% 20% 12% 2	2600
July	1st to 10th	Day	37% 13% 19% 20% 11% 1	5835
	11th to 20th	Day	46% 11% 21% 17% 4 1	5831
	21st to end	Day	56% 11% 13% 14% 7%	6240
August	1st to 10th	Day	46% 12% 18% 19% 5%	4221
	11th to 20th	Day	43% 10% 18% 20% 9%	4213
	21st to end	Day	30% 9% 16% 36% 9% 1	4393
September	1st to 10th	Day	40% 11% 12% 18% 15% 4	2200
	11th to 20th	Day	43% 10% 9% 23% 14% 2	2200
	21st to end	Day	43% 9% 13% 18% 13% 4	2200
October	1st to 10th	Day	29% 6% 7% 20% 29% 9%	3499
	11th to 20th	Day	27% 5% 7% 27% 18% 17%	3240
	21st to end	Day	30% 4 5% 30% 18% 13%	3538
November	1st to 10th	Day	34% 5% 9% 20% 27% 5%	1948
	11th to 20th	Day	39% 5% 7% 20% 23% 5%	1863
	21st to end	Day	43% 5% 7% 22% 19% 4	1800
December	1st to 10th	Day	50% 6% 7% 14% 22% 1	2930
	11th to 20th	Day	56% 4 10% 15% 12% 2	3370
	21st to end	Day	51% 5% 13% 20% 10% 1	4466
January	1st to 10th	Night	58% 7% 5% 16% 11% 3	5889
	11th to 20th	Night	54% 6% 6% 12% 17% 5%	5900
	21st to end	Night	48% 6% 6% 18% 18% 2	6490
February	1st to 10th	Night	50% 5% 10% 19% 15% 2	2547
	11th to 20th	Night	58% 5% 7% 15% 11% 4	2339
	21st to end	Night	52% 6% 7% 17% 13% 4	1980
March	1st to 10th	Night	50% 6% 6% 19% 14% 5%	5014
	11th to 20th	Night	34% 5% 9% 19% 23% 10%	4980
	21st to end	Night	41% 7% 11% 26% 13% 2	5467
April	1st to 10th	Night	43% 7% 12% 21% 14% 3	2200
	11th to 20th	Night	40% 7% 9% 22% 17% 5%	1944
	21st to end	Night	44% 6% 13% 25% 12% 1	1800
May	1st to 10th	Night	49% 11% 15% 17% 7% 1	4060
	11th to 20th	Night	52% 11% 14% 20% 3	4033
	21st to end	Night	45% 11% 13% 20% 9% 1	4429
June	1st to 10th	Night	40% 12% 17% 20% 11% 1	1800
	11th to 20th	Night	35% 12% 20% 22% 10% 1	1800
	21st to end	Night	37% 13% 14% 21% 12% 3	1800
July	1st to 10th	Night	42% 10% 19% 20% 9% 1	4015
	11th to 20th	Night	43% 11% 24% 16% 4 1	4006
	21st to end	Night	55% 10% 14% 13% 7%	4297
August	1st to 10th	Night	42% 10% 22% 20% 5%	2904
	11th to 20th	Night	42% 9% 19% 22% 7%	2902
	21st to end	Night	25% 9% 17% 37% 11% 1	3466
September	1st to 10th	Night	38% 8% 15% 20% 16% 3	2200
	11th to 20th	Night	40% 8% 13% 22% 16% 1	2200
	21st to end	Night	38% 8% 11% 23% 17% 2	2200
October	1st to 10th	Night	34% 5% 8% 20% 26% 7%	3679
	11th to 20th	Night	28% 5% 6% 23% 21% 16%	3899
	21st to end	Night	27% 4 6% 30% 22% 13%	4252
November	1st to 10th	Night	37% 5% 9% 21% 22% 6%	2566
	11th to 20th	Night	40% 4 7% 21% 23% 6%	2692
	21st to end	Night	40% 5% 6% 26% 19% 3	2600
December	1st to 10th	Night	50% 5% 7% 18% 18% 1	4250
	11th to 20th	Night	57% 5% 9% 15% 12% 2	4824
	21st to end	Night	51% 8% 11% 17% 12% 1	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	32% 30% 24% 12% 1	300
	11th to 20th	Day	36% 27% 23% 12% 2	300
	21st to end	Day	28% 32% 28% 11% 2	330
February	1st to 10th	Day	26% 27% 34% 10% 3	300
	11th to 20th	Day	43% 30% 20% 5% 1	300
	21st to end	Day	39% 23% 31% 7% 1	249
March	1st to 10th	Day	26% 23% 26% 18% 6%	450
	11th to 20th	Day	29% 22% 29% 15% 3 1	500
	21st to end	Day	28% 24% 25% 17% 6%	550
April	1st to 10th	Day	24% 21% 28% 18% 8% 1	500
	11th to 20th	Day	22% 21% 26% 21% 9% 2	492
	21st to end	Day	24% 14% 29% 19% 11% 3	500
May	1st to 10th	Day	24% 18% 27% 17% 11% 4	503
	11th to 20th	Day	19% 13% 26% 24% 13% 6%	527
	21st to end	Day	16% 11% 22% 21% 21% 9%	573
June	1st to 10th	Day	14% 7% 23% 30% 19% 7%	530
	11th to 20th	Day	10% 6% 22% 28% 25% 9%	530
	21st to end	Day	13% 7% 24% 27% 20% 9%	530
July	1st to 10th	Day	12% 6% 15% 28% 26% 14%	525
	11th to 20th	Day	11% 4 14% 30% 27% 13%	524
	21st to end	Day	14% 5% 13% 29% 26% 13%	576
August	1st to 10th	Day	16% 5% 19% 24% 23% 13%	526
	11th to 20th	Day	14% 4 19% 26% 25% 12%	519
	21st to end	Day	13% 3 13% 29% 26% 17%	583
September	1st to 10th	Day	11% 5% 17% 28% 27% 13%	530
	11th to 20th	Day	15% 5% 15% 25% 29% 11%	530
	21st to end	Day	17% 9% 20% 28% 20% 5%	530
October	1st to 10th	Day	17% 7% 22% 27% 20% 7%	530
	11th to 20th	Day	16% 16% 23% 26% 16% 3	530
	21st to end	Day	24% 13% 23% 26% 11% 2	583
November	1st to 10th	Day	17% 18% 27% 25% 11% 3	426
	11th to 20th	Day	20% 24% 28% 22% 6%	321
	21st to end	Day	23% 19% 30% 22% 6% 1	300
December	1st to 10th	Day	30% 24% 30% 14% 2	300
	11th to 20th	Day	28% 20% 27% 20% 5%	300
	21st to end	Day	23% 24% 32% 15% 7%	330
January	1st to 10th	Night	25% 22% 25% 24% 3	800
	11th to 20th	Night	28% 19% 27% 23% 4	800
	21st to end	Night	23% 20% 32% 23% 3	880
February	1st to 10th	Night	24% 17% 29% 27% 4	800
	11th to 20th	Night	30% 23% 29% 17% 1	783
	21st to end	Night	27% 23% 28% 19% 2	660
March	1st to 10th	Night	23% 19% 29% 26% 4	800
	11th to 20th	Night	25% 18% 28% 22% 7%	800
	21st to end	Night	22% 14% 32% 25% 7%	880
April	1st to 10th	Night	21% 19% 25% 27% 8%	800
	11th to 20th	Night	19% 14% 29% 27% 11% 1	688
	21st to end	Night	19% 10% 28% 33% 9% 1	600
May	1st to 10th	Night	18% 10% 28% 30% 14% 1	600
	11th to 20th	Night	18% 6% 23% 33% 18% 2	591
	21st to end	Night	14% 4 17% 32% 29% 4	650
June	1st to 10th	Night	11% 4 13% 41% 28% 4	600
	11th to 20th	Night	9% 4 14% 39% 28% 6%	600
	21st to end	Night	11% 3 12% 42% 29% 4	600
July	1st to 10th	Night	14% 4 8% 31% 39% 4	585
	11th to 20th	Night	12% 4 7% 29% 41% 7%	582
	21st to end	Night	12% 4 7% 33% 36% 8%	657
August	1st to 10th	Night	13% 3 8% 33% 38% 6%	597
	11th to 20th	Night	12% 4 8% 37% 36% 5%	594
	21st to end	Night	12% 2 8% 33% 38% 7%	712
September	1st to 10th	Night	13% 3 11% 34% 34% 6%	800
	11th to 20th	Night	17% 3 12% 31% 32% 5%	800
	21st to end	Night	18% 5% 15% 35% 25% 2	800
October	1st to 10th	Night	18% 6% 16% 29% 26% 6%	800
	11th to 20th	Night	18% 8% 23% 34% 15% 2	800
	21st to end	Night	20% 11% 23% 32% 12% 2	880
November	1st to 10th	Night	18% 8% 23% 33% 15% 3	820
	11th to 20th	Night	21% 14% 22% 32% 11% 1	829
	21st to end	Night	19% 15% 26% 30% 10% 1	800
December	1st to 10th	Night	24% 15% 29% 26% 6%	800
	11th to 20th	Night	26% 13% 25% 29% 6% 1	800
	21st to end	Night	26% 17% 25% 28% 4	880

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