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 2024-11-08.

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% 9% 3 7% 5% 5% 5% 4 6% 7% 35%	101186
	Night	15% 9% 3 7% 4 4 4 4 5% 7% 37%	97870
Transparency	Day	45% 8% 11% 20% 13% 3	104928
	Night	44% 8% 11% 20% 14% 4	102267
Seeing	Day	20% 14% 23% 23% 15% 6%	15054
	Night	19% 11% 21% 30% 17% 2	25105

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	15% 15% 10% 11% 3 3 3 3 5% 5% 27%	2836
	11th to 20th	Day	14% 18% 8% 11% 7% 5% 3 2 4 4 25%	2840
	21st to end	Day	18% 18% 4 7% 4 4 4 3 5% 4 29%	3124
February	1st to 10th	Day	18% 12% 4 9% 5% 4 4 3 5% 5% 31%	1757
	11th to 20th	Day	21% 14% 5% 8% 4 4 2 3 4 6% 29%	1848
	21st to end	Day	22% 12% 3 7% 4 4 2 3 4 6% 33%	1570
March	1st to 10th	Day	19% 13% 3 6% 4 4 3 3 4 6% 34%	3365
	11th to 20th	Day	15% 8% 3 5% 4 3 4 3 4 5% 46%	3480
	21st to end	Day	18% 10% 2 6% 4 4 4 3 6% 6% 37%	3815
April	1st to 10th	Day	13% 9% 2 5% 5% 4 4 4 7% 8% 38%	2090
	11th to 20th	Day	16% 8% 2 6% 4 3 5% 4 5% 6% 41%	2295
	21st to end	Day	17% 7% 2 6% 5% 5% 5% 4 5% 6% 39%	2470
May	1st to 10th	Day	21% 9% 3 7% 5% 5% 5% 4 6% 7% 29%	4082
	11th to 20th	Day	19% 8% 3 8% 5% 5% 5% 4 5% 6% 30%	4081
	21st to end	Day	11% 6% 2 6% 5% 5% 5% 4 8% 11% 37%	4476
June	1st to 10th	Day	12% 6% 2 6% 5% 6% 6% 5% 7% 8% 35%	2470
	11th to 20th	Day	9% 5% 2 8% 5% 7% 7% 5% 9% 10% 33%	2470
	21st to end	Day	11% 7% 2 6% 5% 5% 7% 6% 8% 9% 34%	2470
July	1st to 10th	Day	8% 4 2 6% 6% 6% 6% 6% 9% 12% 34%	4120
	11th to 20th	Day	12% 6% 3 6% 6% 5% 6% 5% 7% 9% 37%	4120
	21st to end	Day	18% 9% 4 10% 8% 7% 7% 5% 7% 7% 18%	4466
August	1st to 10th	Day	13% 6% 3 7% 6% 6% 7% 6% 8% 11% 28%	4104
	11th to 20th	Day	13% 5% 2 7% 5% 6% 6% 5% 8% 10% 33%	4044
	21st to end	Day	7% 4 1 4 4 4 4 5% 6% 7% 52%	4259
September	1st to 10th	Day	11% 5% 2 7% 5% 5% 5% 5% 6% 7% 42%	2090
	11th to 20th	Day	15% 6% 2 6% 5% 5% 5% 3 6% 6% 40%	2090
	21st to end	Day	15% 6% 2 5% 5% 4 5% 4 6% 8% 39%	2090
October	1st to 10th	Day	14% 5% 2 5% 4 3 4 3 4 7% 49%	2041
	11th to 20th	Day	15% 6% 2 4 3 3 3 3 5% 7% 49%	1900
	21st to end	Day	17% 9% 2 4 3 3 3 2 4 6% 47%	2090
November	1st to 10th	Day	12% 6% 3 5% 4 3 3 2 5% 6% 52%	1826
	11th to 20th	Day	11% 9% 3 5% 4 5% 4 3 5% 7% 44%	1710
	21st to end	Day	15% 8% 4 6% 4 3 3 4 5% 6% 43%	1710
December	1st to 10th	Day	19% 14% 3 7% 5% 4 4 3 4 5% 31%	1710
	11th to 20th	Day	16% 15% 3 7% 3 3 4 4 5% 8% 33%	2153
	21st to end	Day	14% 15% 5% 7% 5% 5% 5% 4 5% 5% 28%	3124
January	1st to 10th	Night	19% 17% 6% 9% 4 3 3 3 4 5% 27%	4109
	11th to 20th	Night	18% 13% 4 9% 6% 4 4 3 4 4 32%	4120
	21st to end	Night	21% 11% 4 7% 5% 4 4 3 4 5% 32%	4532
February	1st to 10th	Night	17% 12% 3 7% 3 4 5% 3 5% 6% 36%	2420
	11th to 20th	Night	20% 12% 4 9% 5% 3 4 3 4 5% 30%	2218
	21st to end	Night	21% 12% 4 9% 3 3 3 3 4 6% 31%	1884
March	1st to 10th	Night	24% 14% 2 6% 4 4 4 2 4 5% 32%	3486
	11th to 20th	Night	16% 7% 4 5% 3 3 4 3 5% 6% 45%	3480
	21st to end	Night	15% 7% 2 5% 5% 4 5% 4 6% 7% 42%	3817
April	1st to 10th	Night	14% 9% 3 5% 4 4 4 3 4 8% 41%	2090
	11th to 20th	Night	15% 8% 2 6% 4 4 4 4 6% 6% 42%	1846
	21st to end	Night	15% 7% 3 6% 5% 5% 4 3 5% 6% 42%	1710
May	1st to 10th	Night	18% 8% 3 7% 6% 5% 4 4 6% 7% 31%	2813
	11th to 20th	Night	20% 7% 3 6% 5% 4 4 4 6% 8% 33%	2812
	21st to end	Night	13% 7% 2 6% 4 5% 5% 4 7% 9% 38%	3079
June	1st to 10th	Night	11% 6% 2 7% 6% 5% 5% 4 7% 10% 38%	1710
	11th to 20th	Night	7% 5% 2 7% 5% 5% 6% 5% 9% 12% 39%	1710
	21st to end	Night	10% 6% 2 6% 5% 6% 5% 5% 7% 11% 38%	1710
July	1st to 10th	Night	11% 6% 3 6% 5% 5% 6% 6% 8% 12% 33%	2840
	11th to 20th	Night	9% 6% 3 6% 4 4 5% 5% 7% 10% 40%	2840
	21st to end	Night	14% 10% 5% 11% 7% 5% 5% 5% 7% 8% 22%	3087
August	1st to 10th	Night	12% 6% 3 7% 5% 5% 6% 5% 8% 11% 34%	2825
	11th to 20th	Night	14% 7% 3 6% 5% 5% 5% 4 6% 7% 38%	2785
	21st to end	Night	7% 3 1 4 3 4 4 4 8% 10% 52%	3359
September	1st to 10th	Night	12% 6% 2 6% 5% 5% 4 3 5% 7% 44%	2090
	11th to 20th	Night	16% 5% 2 6% 4 4 4 3 6% 8% 42%	2090
	21st to end	Night	14% 6% 2 5% 4 4 3 2 5% 6% 47%	2090
October	1st to 10th	Night	16% 6% 2 5% 3 3 3 3 4 6% 50%	2137
	11th to 20th	Night	16% 7% 2 4 2 3 3 3 4 6% 49%	2280
	21st to end	Night	15% 7% 2 5% 3 2 4 3 5% 7% 48%	2508
November	1st to 10th	Night	15% 7% 2 4 3 3 3 3 5% 6% 49%	2409
	11th to 20th	Night	14% 9% 2 5% 4 4 3 3 6% 6% 45%	2470
	21st to end	Night	13% 10% 2 5% 3 3 4 3 5% 6% 46%	2470
December	1st to 10th	Night	19% 12% 5% 9% 4 4 4 3 5% 6% 30%	2470
	11th to 20th	Night	18% 12% 4 10% 4 3 4 3 4 5% 34%	3042
	21st to end	Night	15% 16% 6% 8% 5% 3 5% 4 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	57% 6% 5% 15% 14% 3	2837
	11th to 20th	Day	62% 5% 6% 12% 14% 1	2840
	21st to end	Day	58% 5% 6% 14% 16% 1	3124
February	1st to 10th	Day	55% 5% 7% 16% 15% 2	1757
	11th to 20th	Day	58% 5% 6% 15% 14% 2	1858
	21st to end	Day	54% 5% 7% 14% 15% 5%	1570
March	1st to 10th	Day	51% 5% 7% 18% 14% 4	3365
	11th to 20th	Day	41% 5% 7% 23% 18% 5%	3480
	21st to end	Day	46% 6% 10% 25% 11% 3	3815
April	1st to 10th	Day	42% 7% 11% 22% 14% 3	2090
	11th to 20th	Day	42% 7% 8% 19% 19% 4	2295
	21st to end	Day	45% 8% 10% 23% 11% 2	2470
May	1st to 10th	Day	52% 10% 11% 18% 8% 1	4120
	11th to 20th	Day	52% 10% 10% 22% 5% 1	4085
	21st to end	Day	37% 12% 16% 20% 13% 2	4489
June	1st to 10th	Day	42% 12% 15% 20% 10% 1	2470
	11th to 20th	Day	43% 12% 14% 16% 13% 1	2470
	21st to end	Day	42% 12% 13% 19% 12% 2	2470
July	1st to 10th	Day	39% 14% 19% 18% 10% 1	4055
	11th to 20th	Day	42% 11% 17% 23% 6% 1	4051
	21st to end	Day	62% 10% 9% 13% 5%	4324
August	1st to 10th	Day	46% 12% 18% 18% 5%	4091
	11th to 20th	Day	43% 10% 17% 20% 9%	4083
	21st to end	Day	30% 9% 16% 36% 9% 1	4259
September	1st to 10th	Day	40% 11% 12% 19% 15% 4	2090
	11th to 20th	Day	43% 10% 9% 21% 15% 2	2090
	21st to end	Day	43% 9% 12% 18% 14% 4	2090
October	1st to 10th	Day	29% 6% 7% 20% 29% 9%	3392
	11th to 20th	Day	27% 5% 7% 26% 18% 18%	3140
	21st to end	Day	29% 4 6% 30% 18% 13%	3428
November	1st to 10th	Day	34% 5% 8% 20% 27% 6%	1826
	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	60% 6% 6% 13% 12% 4	4109
	11th to 20th	Night	54% 6% 8% 12% 17% 3	4120
	21st to end	Night	55% 7% 7% 13% 16% 2	4532
February	1st to 10th	Night	50% 5% 9% 19% 15% 2	2420
	11th to 20th	Night	57% 5% 7% 16% 12% 4	2219
	21st to end	Night	54% 6% 8% 17% 12% 4	1884
March	1st to 10th	Night	55% 6% 8% 16% 13% 2	3486
	11th to 20th	Night	39% 6% 11% 22% 14% 8%	3480
	21st to end	Night	42% 7% 9% 25% 14% 3	3817
April	1st to 10th	Night	42% 7% 12% 21% 13% 3	2090
	11th to 20th	Night	40% 7% 10% 21% 17% 5%	1846
	21st to end	Night	43% 6% 12% 26% 12% 1	1710
May	1st to 10th	Night	48% 12% 13% 19% 7% 1	2840
	11th to 20th	Night	46% 12% 13% 23% 4 1	2813
	21st to end	Night	37% 14% 15% 24% 10% 1	3087
June	1st to 10th	Night	39% 12% 17% 20% 11% 1	1710
	11th to 20th	Night	34% 12% 21% 23% 9% 1	1710
	21st to end	Night	38% 14% 14% 20% 12% 3	1710
July	1st to 10th	Night	42% 11% 20% 18% 8% 1	2795
	11th to 20th	Night	35% 11% 23% 23% 5% 2	2786
	21st to end	Night	56% 11% 13% 15% 4	2988
August	1st to 10th	Night	42% 10% 22% 20% 5%	2814
	11th to 20th	Night	42% 9% 18% 23% 8%	2812
	21st to end	Night	25% 9% 17% 38% 11% 1	3359
September	1st to 10th	Night	38% 9% 14% 20% 15% 3	2090
	11th to 20th	Night	41% 9% 12% 21% 17% 1	2090
	21st to end	Night	37% 8% 11% 23% 17% 3	2090
October	1st to 10th	Night	34% 5% 8% 20% 26% 7%	3566
	11th to 20th	Night	29% 5% 6% 23% 20% 17%	3779
	21st to end	Night	26% 4 6% 30% 22% 13%	4120
November	1st to 10th	Night	37% 6% 9% 20% 23% 6%	2409
	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	36% 26% 25% 13% 1	270
	21st to end	Day	27% 30% 29% 11% 2	297
February	1st to 10th	Day	27% 26% 33% 11% 3	270
	11th to 20th	Day	41% 31% 21% 6% 1	270
	21st to end	Day	40% 23% 29% 7% 1	225
March	1st to 10th	Day	25% 23% 27% 18% 7%	412
	11th to 20th	Day	29% 23% 28% 15% 3 1	460
	21st to end	Day	28% 24% 24% 19% 6%	506
April	1st to 10th	Day	25% 20% 27% 18% 9% 1	460
	11th to 20th	Day	22% 20% 24% 22% 10% 2	452
	21st to end	Day	24% 14% 28% 19% 11% 3	460
May	1st to 10th	Day	24% 17% 25% 18% 11% 4	463
	11th to 20th	Day	18% 12% 26% 24% 14% 6%	487
	21st to end	Day	14% 9% 22% 23% 22% 10%	529
June	1st to 10th	Day	14% 6% 22% 30% 19% 8%	490
	11th to 20th	Day	11% 6% 21% 27% 26% 10%	490
	21st to end	Day	14% 7% 25% 25% 20% 9%	490
July	1st to 10th	Day	13% 6% 15% 26% 26% 14%	485
	11th to 20th	Day	11% 4 14% 30% 27% 13%	484
	21st to end	Day	14% 5% 14% 28% 26% 12%	532
August	1st to 10th	Day	15% 5% 20% 24% 23% 13%	486
	11th to 20th	Day	13% 5% 20% 25% 25% 12%	479
	21st to end	Day	13% 2 11% 29% 26% 19%	539
September	1st to 10th	Day	11% 4 16% 28% 27% 13%	490
	11th to 20th	Day	16% 5% 15% 24% 29% 12%	490
	21st to end	Day	18% 8% 20% 28% 21% 6%	490
October	1st to 10th	Day	18% 7% 21% 28% 18% 7%	490
	11th to 20th	Day	17% 17% 21% 26% 17% 3	490
	21st to end	Day	22% 12% 23% 28% 12% 3	539
November	1st to 10th	Day	16% 18% 26% 26% 12% 3	382
	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	28% 18% 26% 24% 4	760
	21st to end	Night	23% 20% 31% 23% 3	836
February	1st to 10th	Night	23% 16% 29% 28% 4	760
	11th to 20th	Night	29% 23% 29% 17% 1	743
	21st to end	Night	28% 24% 27% 19% 2	628
March	1st to 10th	Night	22% 19% 29% 26% 4	760
	11th to 20th	Night	26% 18% 27% 21% 8%	760
	21st to end	Night	22% 14% 31% 26% 8%	836
April	1st to 10th	Night	21% 18% 26% 27% 8%	760
	11th to 20th	Night	19% 14% 29% 27% 11% 1	653
	21st to end	Night	19% 9% 27% 34% 9% 1	570
May	1st to 10th	Night	18% 10% 27% 29% 15% 1	570
	11th to 20th	Night	18% 6% 22% 34% 19% 2	561
	21st to end	Night	12% 3 17% 33% 31% 5%	617
June	1st to 10th	Night	10% 4 13% 41% 29% 4	570
	11th to 20th	Night	8% 4 14% 38% 30% 6%	570
	21st to end	Night	11% 2 12% 41% 29% 4	570
July	1st to 10th	Night	14% 4 8% 30% 40% 4	555
	11th to 20th	Night	12% 3 7% 31% 40% 7%	552
	21st to end	Night	11% 4 8% 33% 36% 8%	624
August	1st to 10th	Night	12% 3 8% 32% 39% 6%	567
	11th to 20th	Night	12% 4 8% 35% 36% 5%	564
	21st to end	Night	12% 2 7% 32% 39% 7%	676
September	1st to 10th	Night	13% 3 10% 33% 35% 6%	760
	11th to 20th	Night	18% 4 12% 30% 32% 5%	760
	21st to end	Night	17% 5% 15% 35% 26% 2	760
October	1st to 10th	Night	18% 6% 16% 28% 26% 6%	760
	11th to 20th	Night	19% 8% 23% 33% 15% 2	760
	21st to end	Night	19% 11% 22% 34% 12% 2	836
November	1st to 10th	Night	19% 8% 21% 34% 16% 3	771
	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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