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 2024-01-12.

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	15% 8% 3 6% 6% 3 5% 5% 6% 7% 35%	74896
	Night	15% 9% 3 6% 5% 3 4 5% 5% 7% 38%	74836
Transparency	Day	47% 9% 10% 19% 13% 2	74619
	Night	45% 8% 11% 19% 14% 3	74632
Seeing	Day	20% 14% 22% 22% 16% 6%	13906
	Night	19% 11% 20% 29% 17% 2	24038

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	17% 11% 6% 9% 7% 3 3 5% 4 6% 30%	1706
	11th to 20th	Day	17% 14% 6% 9% 6% 4 4 4 5% 5% 28%	1635
	21st to end	Day	17% 13% 4 7% 5% 3 4 4 5% 6% 34%	1782
February	1st to 10th	Day	18% 12% 5% 8% 6% 3 5% 4 5% 6% 29%	1665
	11th to 20th	Day	22% 14% 5% 7% 5% 3 3 4 4 6% 28%	1748
	21st to end	Day	23% 12% 4 6% 4 3 3 3 4 5% 34%	1480
March	1st to 10th	Day	15% 10% 4 5% 4 3 5% 4 4 7% 41%	1934
	11th to 20th	Day	21% 9% 4 5% 5% 3 5% 5% 5% 7% 32%	1980
	21st to end	Day	20% 10% 3 5% 5% 3 4 4 6% 6% 33%	2178
April	1st to 10th	Day	13% 7% 3 4 5% 3 4 4 7% 8% 39%	1980
	11th to 20th	Day	16% 6% 3 5% 5% 2 4 6% 5% 7% 42%	2172
	21st to end	Day	17% 6% 2 5% 6% 4 5% 6% 5% 7% 38%	2340
May	1st to 10th	Day	18% 5% 3 6% 6% 4 5% 5% 6% 8% 35%	2340
	11th to 20th	Day	18% 6% 3 6% 6% 4 6% 6% 5% 7% 31%	2301
	21st to end	Day	13% 5% 2 5% 7% 4 6% 5% 7% 10% 35%	2531
June	1st to 10th	Day	12% 5% 3 6% 6% 4 6% 7% 7% 9% 36%	2340
	11th to 20th	Day	9% 5% 2 7% 7% 5% 8% 6% 8% 9% 33%	2340
	21st to end	Day	12% 6% 2 6% 6% 4 7% 8% 8% 9% 33%	2340
July	1st to 10th	Day	12% 6% 3 7% 7% 5% 7% 7% 7% 9% 31%	2340
	11th to 20th	Day	13% 5% 3 6% 7% 4 7% 7% 7% 10% 30%	2340
	21st to end	Day	14% 6% 3 6% 7% 4 7% 7% 8% 9% 27%	2547
August	1st to 10th	Day	13% 5% 3 6% 7% 4 7% 7% 9% 11% 29%	2327
	11th to 20th	Day	15% 5% 2 6% 7% 5% 8% 7% 8% 8% 29%	2303
	21st to end	Day	12% 6% 2 5% 6% 4 5% 8% 7% 7% 37%	2427
September	1st to 10th	Day	11% 4 3 6% 5% 4 5% 6% 7% 8% 41%	1980
	11th to 20th	Day	15% 6% 2 5% 6% 4 5% 5% 6% 6% 40%	1980
	21st to end	Day	15% 6% 3 5% 6% 3 5% 5% 6% 8% 40%	1980
October	1st to 10th	Day	15% 5% 2 4 4 2 4 4 4 7% 48%	1934
	11th to 20th	Day	16% 5% 2 4 3 2 4 4 5% 7% 47%	1800
	21st to end	Day	17% 8% 2 4 3 2 3 3 4 6% 45%	1980
November	1st to 10th	Day	13% 5% 3 4 4 2 3 3 4 6% 52%	1759
	11th to 20th	Day	12% 8% 3 5% 4 4 4 4 5% 7% 44%	1710
	21st to end	Day	15% 7% 4 5% 5% 2 3 4 5% 6% 43%	1710
December	1st to 10th	Day	19% 14% 3 6% 5% 3 4 5% 4 5% 31%	1710
	11th to 20th	Day	16% 14% 3 6% 3 2 4 4 5% 8% 33%	2153
	21st to end	Day	15% 15% 5% 6% 6% 4 6% 5% 5% 6% 28%	3124
January	1st to 10th	Night	18% 12% 4 9% 5% 3 4 4 4 6% 31%	2459
	11th to 20th	Night	19% 14% 5% 8% 5% 3 4 4 5% 6% 28%	2365
	21st to end	Night	16% 13% 5% 7% 5% 3 4 3 4 5% 35%	2574
February	1st to 10th	Night	18% 11% 4 7% 4 2 5% 4 4 6% 35%	2292
	11th to 20th	Night	20% 11% 5% 8% 6% 2 4 4 4 5% 30%	2098
	21st to end	Night	21% 12% 4 8% 3 2 3 4 4 6% 32%	1776
March	1st to 10th	Night	15% 14% 3 5% 4 3 4 3 4 5% 40%	2024
	11th to 20th	Night	20% 10% 3 6% 4 3 4 5% 5% 7% 33%	1980
	21st to end	Night	17% 9% 3 4 5% 4 5% 5% 5% 7% 37%	2178
April	1st to 10th	Night	15% 7% 3 4 5% 3 4 5% 4 8% 42%	1980
	11th to 20th	Night	14% 7% 3 5% 4 2 5% 4 6% 7% 43%	1749
	21st to end	Night	16% 6% 3 6% 5% 3 5% 4 5% 6% 42%	1620
May	1st to 10th	Night	16% 7% 3 5% 7% 3 4 4 6% 8% 37%	1620
	11th to 20th	Night	16% 6% 3 5% 5% 4 6% 5% 7% 10% 34%	1592
	21st to end	Night	11% 6% 2 5% 5% 4 5% 6% 7% 10% 39%	1744
June	1st to 10th	Night	11% 5% 3 6% 6% 2 6% 5% 7% 10% 39%	1620
	11th to 20th	Night	7% 4 2 6% 6% 3 6% 5% 8% 12% 41%	1620
	21st to end	Night	10% 5% 2 6% 5% 4 6% 5% 7% 12% 38%	1620
July	1st to 10th	Night	12% 6% 3 5% 6% 4 5% 6% 7% 10% 36%	1620
	11th to 20th	Night	9% 5% 3 5% 4 4 6% 8% 8% 11% 36%	1620
	21st to end	Night	10% 5% 3 7% 6% 4 6% 7% 8% 10% 33%	1771
August	1st to 10th	Night	10% 4 3 6% 6% 3 6% 5% 8% 10% 39%	1611
	11th to 20th	Night	12% 6% 3 6% 7% 4 6% 5% 7% 8% 36%	1598
	21st to end	Night	11% 4 2 5% 5% 4 6% 5% 8% 9% 41%	1905
September	1st to 10th	Night	12% 5% 3 5% 5% 4 5% 5% 5% 8% 44%	1980
	11th to 20th	Night	16% 5% 3 6% 4 3 4 4 6% 7% 42%	1980
	21st to end	Night	14% 5% 2 4 5% 3 3 3 5% 6% 49%	1980
October	1st to 10th	Night	16% 6% 2 4 3 3 3 4 4 6% 49%	2024
	11th to 20th	Night	17% 6% 2 4 3 3 3 4 4 6% 48%	2160
	21st to end	Night	16% 7% 2 5% 3 2 4 4 4 7% 46%	2376
November	1st to 10th	Night	15% 7% 1 4 4 2 3 4 4 6% 50%	2316
	11th to 20th	Night	14% 8% 3 4 4 3 4 4 5% 7% 45%	2470
	21st to end	Night	13% 10% 2 5% 4 2 4 4 4 6% 46%	2470
December	1st to 10th	Night	19% 11% 6% 8% 4 3 4 4 4 7% 30%	2470
	11th to 20th	Night	19% 11% 4 9% 4 3 4 4 4 5% 34%	3042
	21st to end	Night	15% 16% 6% 7% 6% 3 4 6% 6% 7% 24%	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	56% 5% 7% 17% 10% 4	1707
	11th to 20th	Day	59% 5% 7% 16% 10% 2	1635
	21st to end	Day	51% 4 8% 21% 14% 1	1782
February	1st to 10th	Day	57% 5% 7% 16% 13% 1	1665
	11th to 20th	Day	59% 4 7% 16% 12% 2	1758
	21st to end	Day	54% 5% 7% 15% 14% 5%	1480
March	1st to 10th	Day	45% 5% 5% 22% 18% 4	1934
	11th to 20th	Day	53% 6% 9% 17% 11% 3	1980
	21st to end	Day	52% 6% 11% 21% 9% 1	2178
April	1st to 10th	Day	41% 7% 12% 23% 14% 3	1980
	11th to 20th	Day	42% 7% 8% 19% 20% 5%	2172
	21st to end	Day	46% 8% 10% 23% 11% 2	2340
May	1st to 10th	Day	46% 9% 12% 21% 11% 2	2340
	11th to 20th	Day	51% 10% 13% 18% 7% 1	2305
	21st to end	Day	43% 11% 15% 22% 9% 1	2531
June	1st to 10th	Day	42% 12% 15% 20% 10% 1	2340
	11th to 20th	Day	44% 13% 14% 15% 12% 1	2340
	21st to end	Day	43% 12% 13% 19% 12% 2	2340
July	1st to 10th	Day	46% 14% 14% 15% 9% 1	2275
	11th to 20th	Day	46% 15% 14% 12% 11% 2	2271
	21st to end	Day	48% 15% 13% 17% 7% 1	2405
August	1st to 10th	Day	46% 15% 15% 18% 7%	2314
	11th to 20th	Day	51% 13% 12% 14% 9%	2303
	21st to end	Day	44% 12% 13% 19% 11% 2	2427
September	1st to 10th	Day	41% 11% 13% 19% 13% 4	1980
	11th to 20th	Day	44% 10% 9% 21% 15% 2	1980
	21st to end	Day	42% 9% 11% 18% 15% 4	1980
October	1st to 10th	Day	37% 7% 9% 22% 22% 4	1934
	11th to 20th	Day	37% 6% 9% 24% 20% 5%	1800
	21st to end	Day	40% 5% 7% 23% 19% 5%	1980
November	1st to 10th	Day	34% 5% 7% 19% 28% 6%	1759
	11th to 20th	Day	40% 5% 7% 20% 22% 5%	1710
	21st to end	Day	42% 5% 8% 21% 19% 4	1710
December	1st to 10th	Day	55% 6% 8% 17% 12% 2	1710
	11th to 20th	Day	49% 5% 11% 15% 17% 3	2150
	21st to end	Day	56% 6% 8% 20% 9% 1	3124
January	1st to 10th	Night	55% 5% 8% 15% 12% 4	2459
	11th to 20th	Night	58% 6% 8% 13% 13% 2	2365
	21st to end	Night	54% 6% 7% 18% 15% 2	2574
February	1st to 10th	Night	51% 5% 10% 19% 13% 1	2292
	11th to 20th	Night	57% 5% 7% 16% 11% 4	2099
	21st to end	Night	54% 6% 8% 17% 11% 4	1776
March	1st to 10th	Night	48% 5% 8% 16% 19% 4	2024
	11th to 20th	Night	51% 7% 11% 19% 11% 2	1980
	21st to end	Night	48% 8% 10% 21% 12% 2	2178
April	1st to 10th	Night	42% 7% 12% 22% 14% 3	1980
	11th to 20th	Night	39% 7% 10% 21% 17% 5%	1749
	21st to end	Night	43% 5% 12% 26% 12% 1	1620
May	1st to 10th	Night	44% 10% 14% 22% 9% 1	1620
	11th to 20th	Night	44% 13% 15% 21% 6% 1	1593
	21st to end	Night	39% 12% 16% 23% 9% 1	1745
June	1st to 10th	Night	39% 12% 17% 20% 11% 1	1620
	11th to 20th	Night	35% 12% 21% 22% 10% 1	1620
	21st to end	Night	38% 14% 14% 20% 12% 3	1620
July	1st to 10th	Night	41% 11% 17% 17% 12% 1	1575
	11th to 20th	Night	37% 16% 21% 15% 9% 3	1566
	21st to end	Night	40% 14% 17% 20% 8%	1672
August	1st to 10th	Night	38% 13% 20% 20% 8% 1	1600
	11th to 20th	Night	43% 12% 16% 18% 10% 1	1598
	21st to end	Night	36% 12% 16% 24% 11% 1	1905
September	1st to 10th	Night	38% 9% 14% 21% 15% 3	1980
	11th to 20th	Night	41% 9% 12% 20% 17% 1	1980
	21st to end	Night	36% 8% 11% 24% 18% 3	1980
October	1st to 10th	Night	37% 6% 10% 21% 23% 4	2024
	11th to 20th	Night	36% 6% 9% 26% 18% 4	2160
	21st to end	Night	38% 5% 6% 22% 22% 6%	2376
November	1st to 10th	Night	36% 6% 8% 20% 24% 6%	2316
	11th to 20th	Night	39% 5% 7% 21% 22% 6%	2470
	21st to end	Night	40% 5% 6% 26% 19% 3	2470
December	1st to 10th	Night	56% 7% 8% 16% 11% 3	2470
	11th to 20th	Night	53% 5% 7% 15% 16% 3	3044
	21st to end	Night	57% 9% 8% 15% 11% 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	29% 30% 26% 13% 1	270
	11th to 20th	Day	31% 27% 26% 14% 1	245
	21st to end	Day	28% 31% 28% 10% 3	264
February	1st to 10th	Day	28% 29% 32% 10% 2	240
	11th to 20th	Day	42% 32% 21% 5% 1	240
	21st to end	Day	40% 23% 29% 7% 1	198
March	1st to 10th	Day	22% 24% 29% 19% 6%	374
	11th to 20th	Day	29% 24% 26% 15% 4 1	420
	21st to end	Day	28% 23% 23% 19% 7%	462
April	1st to 10th	Day	23% 19% 26% 20% 10% 1	420
	11th to 20th	Day	21% 20% 23% 23% 10% 2	412
	21st to end	Day	25% 14% 27% 20% 10% 4	420
May	1st to 10th	Day	24% 17% 26% 18% 10% 4	423
	11th to 20th	Day	19% 11% 26% 24% 14% 6%	447
	21st to end	Day	14% 8% 20% 23% 23% 11%	485
June	1st to 10th	Day	14% 6% 21% 30% 20% 9%	450
	11th to 20th	Day	12% 6% 20% 27% 25% 10%	450
	21st to end	Day	15% 6% 24% 25% 20% 9%	450
July	1st to 10th	Day	13% 5% 15% 24% 27% 15%	445
	11th to 20th	Day	11% 4 14% 29% 29% 13%	444
	21st to end	Day	13% 5% 13% 29% 27% 14%	488
August	1st to 10th	Day	15% 5% 17% 25% 24% 14%	446
	11th to 20th	Day	13% 3 18% 26% 27% 13%	439
	21st to end	Day	15% 2 12% 28% 24% 20%	495
September	1st to 10th	Day	11% 4 15% 27% 28% 14%	450
	11th to 20th	Day	16% 5% 13% 24% 30% 12%	450
	21st to end	Day	17% 7% 18% 29% 22% 6%	450
October	1st to 10th	Day	19% 7% 21% 26% 20% 8%	450
	11th to 20th	Day	17% 16% 21% 25% 17% 3	450
	21st to end	Day	24% 13% 22% 27% 11% 3	495
November	1st to 10th	Day	16% 18% 24% 26% 12% 3	357
	11th to 20th	Day	19% 24% 26% 25% 6%	270
	21st to end	Day	21% 18% 30% 23% 7% 1	270
December	1st to 10th	Day	30% 22% 30% 15% 3	270
	11th to 20th	Day	26% 20% 27% 21% 5%	270
	21st to end	Day	23% 24% 33% 13% 7%	297
January	1st to 10th	Night	24% 23% 25% 25% 4	760
	11th to 20th	Night	27% 18% 26% 25% 4	728
	21st to end	Night	22% 20% 30% 24% 3	792
February	1st to 10th	Night	24% 17% 29% 27% 4	720
	11th to 20th	Night	29% 23% 29% 17% 1	703
	21st to end	Night	28% 25% 25% 20% 2	592
March	1st to 10th	Night	22% 19% 29% 27% 4	720
	11th to 20th	Night	26% 18% 27% 21% 8%	720
	21st to end	Night	22% 14% 30% 26% 8%	792
April	1st to 10th	Night	21% 18% 24% 28% 9%	720
	11th to 20th	Night	18% 14% 28% 27% 12% 1	619
	21st to end	Night	20% 9% 26% 34% 10% 1	540
May	1st to 10th	Night	18% 9% 28% 29% 15% 1	540
	11th to 20th	Night	18% 5% 22% 33% 20% 2	531
	21st to end	Night	12% 3 15% 33% 32% 5%	584
June	1st to 10th	Night	10% 3 13% 39% 30% 4	540
	11th to 20th	Night	9% 4 13% 37% 31% 7%	540
	21st to end	Night	11% 2 12% 40% 31% 4	540
July	1st to 10th	Night	14% 4 7% 29% 42% 5%	525
	11th to 20th	Night	12% 2 7% 30% 41% 8%	522
	21st to end	Night	10% 3 7% 33% 38% 8%	591
August	1st to 10th	Night	11% 3 8% 31% 41% 6%	537
	11th to 20th	Night	11% 4 7% 35% 38% 5%	534
	21st to end	Night	13% 2 7% 32% 39% 8%	640
September	1st to 10th	Night	13% 3 10% 32% 36% 7%	720
	11th to 20th	Night	17% 4 11% 29% 34% 5%	720
	21st to end	Night	16% 5% 14% 35% 27% 3	720
October	1st to 10th	Night	18% 6% 17% 26% 27% 6%	720
	11th to 20th	Night	19% 8% 23% 31% 16% 2	720
	21st to end	Night	19% 11% 22% 33% 13% 2	792
November	1st to 10th	Night	18% 7% 21% 34% 16% 3	740
	11th to 20th	Night	20% 13% 21% 33% 12% 1	760
	21st to end	Night	18% 15% 26% 30% 10% 1	760
December	1st to 10th	Night	25% 15% 27% 27% 6%	760
	11th to 20th	Night	25% 13% 24% 30% 6% 1	760
	21st to end	Night	26% 17% 24% 28% 4	836

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