Cumberland Plateau Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cumberland Plateau Observatory clear sky chart from 2005-11-06 to 2024-01-13.

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% 8% 3 6% 6% 3 5% 5% 6% 7% 38%	74857
	Night	14% 8% 3 6% 5% 3 4 4 5% 7% 40%	74884
Transparency	Day	44% 7% 11% 20% 15% 3	74580
	Night	42% 7% 11% 21% 16% 3	74680
Seeing	Day	19% 12% 24% 22% 16% 7%	13538
	Night	18% 11% 23% 27% 17% 3	23637

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	18% 10% 4 7% 8% 4 5% 4 4 6% 31%	1706
	11th to 20th	Day	21% 15% 5% 9% 6% 3 4 3 2 5% 26%	1638
	21st to end	Day	17% 11% 4 5% 4 2 4 5% 4 6% 38%	1841
February	1st to 10th	Day	19% 12% 5% 8% 5% 3 4 4 5% 5% 31%	1800
	11th to 20th	Day	21% 12% 5% 8% 6% 2 4 3 4 5% 30%	1748
	21st to end	Day	21% 10% 5% 7% 5% 3 3 3 4 6% 33%	1480
March	1st to 10th	Day	16% 10% 3 6% 6% 3 5% 4 5% 6% 38%	1800
	11th to 20th	Day	21% 10% 4 8% 6% 3 5% 4 4 4 32%	1962
	21st to end	Day	18% 9% 4 8% 5% 4 5% 4 5% 5% 32%	2178
April	1st to 10th	Day	14% 6% 2 6% 5% 4 4 4 6% 7% 43%	2160
	11th to 20th	Day	15% 7% 3 6% 4 3 3 4 5% 7% 42%	2134
	21st to end	Day	15% 7% 3 6% 5% 4 5% 5% 5% 6% 37%	2290
May	1st to 10th	Day	17% 6% 3 5% 5% 3 6% 4 6% 8% 39%	2340
	11th to 20th	Day	17% 7% 3 7% 7% 4 5% 5% 5% 7% 34%	2301
	21st to end	Day	10% 5% 3 5% 7% 4 5% 6% 7% 9% 38%	2531
June	1st to 10th	Day	11% 6% 3 6% 5% 3 6% 6% 6% 8% 39%	2340
	11th to 20th	Day	6% 4 3 5% 6% 4 6% 7% 7% 9% 44%	2340
	21st to end	Day	7% 4 2 5% 6% 4 7% 6% 9% 11% 39%	2340
July	1st to 10th	Day	10% 5% 3 7% 7% 5% 8% 7% 8% 9% 31%	2340
	11th to 20th	Day	10% 5% 3 6% 7% 4 6% 6% 7% 10% 35%	2340
	21st to end	Day	10% 5% 3 6% 8% 4 6% 6% 8% 11% 33%	2547
August	1st to 10th	Day	9% 6% 2 6% 7% 4 7% 8% 8% 11% 32%	2327
	11th to 20th	Day	11% 5% 3 7% 6% 4 6% 7% 7% 11% 33%	2155
	21st to end	Day	11% 6% 2 5% 7% 4 7% 7% 7% 10% 35%	2376
September	1st to 10th	Day	15% 5% 3 5% 6% 4 6% 6% 5% 7% 39%	2066
	11th to 20th	Day	14% 6% 2 6% 5% 4 5% 5% 6% 7% 42%	1980
	21st to end	Day	13% 5% 2 5% 4 3 4 5% 5% 5% 50%	1980
October	1st to 10th	Day	10% 6% 2 4 4 3 4 3 5% 6% 54%	1980
	11th to 20th	Day	14% 5% 2 5% 4 2 3 4 4 6% 51%	1908
	21st to end	Day	18% 6% 2 4 4 2 4 3 5% 7% 44%	1850
November	1st to 10th	Day	12% 6% 2 4 4 2 4 3 4 6% 55%	1672
	11th to 20th	Day	15% 8% 2 5% 4 2 4 4 5% 6% 46%	1710
	21st to end	Day	16% 9% 3 5% 4 3 3 4 5% 5% 43%	1710
December	1st to 10th	Day	23% 11% 4 5% 5% 3 4 4 4 5% 31%	1710
	11th to 20th	Day	17% 13% 4 6% 6% 3 5% 3 4 6% 32%	2153
	21st to end	Day	18% 15% 5% 6% 5% 3 4 4 6% 7% 28%	3124
January	1st to 10th	Night	19% 9% 3 7% 6% 3 4 3 3 5% 36%	2459
	11th to 20th	Night	22% 13% 5% 8% 5% 3 4 4 4 5% 27%	2367
	21st to end	Night	17% 11% 4 7% 5% 3 4 4 4 5% 36%	2511
February	1st to 10th	Night	21% 10% 5% 7% 5% 3 4 4 5% 5% 30%	2160
	11th to 20th	Night	22% 9% 5% 8% 5% 3 4 3 3 5% 33%	2098
	21st to end	Night	23% 13% 5% 7% 5% 3 3 4 4 5% 29%	1776
March	1st to 10th	Night	16% 9% 3 6% 5% 3 4 4 5% 5% 39%	2160
	11th to 20th	Night	19% 8% 3 6% 5% 3 4 5% 5% 6% 35%	1996
	21st to end	Night	15% 11% 4 7% 5% 3 5% 5% 5% 7% 33%	2178
April	1st to 10th	Night	13% 5% 3 5% 5% 2 4 5% 6% 7% 45%	1814
	11th to 20th	Night	16% 7% 3 3 3 3 3 4 5% 7% 46%	1775
	21st to end	Night	14% 7% 3 5% 4 3 4 4 5% 7% 43%	1670
May	1st to 10th	Night	13% 7% 3 5% 5% 3 3 5% 6% 7% 42%	1620
	11th to 20th	Night	12% 6% 3 5% 6% 4 6% 5% 5% 9% 40%	1592
	21st to end	Night	6% 5% 3 5% 7% 4 5% 6% 8% 11% 40%	1744
June	1st to 10th	Night	6% 5% 2 6% 5% 4 6% 5% 7% 10% 43%	1620
	11th to 20th	Night	4 3 2 4 5% 3 5% 5% 7% 12% 50%	1620
	21st to end	Night	5% 4 2 4 6% 4 6% 7% 8% 12% 42%	1620
July	1st to 10th	Night	10% 5% 2 5% 6% 3 6% 6% 8% 10% 38%	1620
	11th to 20th	Night	8% 4 2 6% 5% 4 5% 6% 7% 9% 45%	1620
	21st to end	Night	7% 4 3 5% 6% 4 6% 7% 8% 11% 39%	1771
August	1st to 10th	Night	8% 4 2 5% 5% 3 5% 6% 7% 9% 45%	1611
	11th to 20th	Night	8% 5% 2 6% 5% 5% 5% 6% 7% 10% 43%	1742
	21st to end	Night	8% 5% 2 5% 4 2 5% 5% 7% 11% 47%	1980
September	1st to 10th	Night	11% 5% 3 5% 5% 3 4 4 6% 7% 47%	1876
	11th to 20th	Night	11% 6% 2 6% 5% 3 5% 4 5% 7% 46%	1980
	21st to end	Night	10% 4 2 5% 4 2 4 4 4 8% 52%	1980
October	1st to 10th	Night	11% 5% 2 3 3 1 3 3 4 6% 59%	1980
	11th to 20th	Night	18% 4 2 4 4 3 4 3 4 5% 49%	2052
	21st to end	Night	16% 7% 2 5% 4 2 3 3 4 6% 47%	2502
November	1st to 10th	Night	12% 5% 3 5% 4 2 3 4 3 6% 52%	2406
	11th to 20th	Night	15% 7% 2 4 4 2 3 4 5% 7% 48%	2470
	21st to end	Night	16% 9% 3 4 4 2 3 3 4 6% 44%	2470
December	1st to 10th	Night	21% 15% 4 6% 5% 3 4 3 4 5% 31%	2470
	11th to 20th	Night	18% 12% 5% 8% 4 2 3 3 3 5% 36%	3042
	21st to end	Night	14% 14% 5% 9% 7% 4 5% 5% 5% 6% 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	54% 5% 8% 17% 14% 3	1707
	11th to 20th	Day	62% 4 7% 14% 11% 1	1638
	21st to end	Day	47% 5% 10% 23% 14% 1	1841
February	1st to 10th	Day	56% 6% 7% 16% 14% 2	1800
	11th to 20th	Day	58% 4 7% 18% 11% 3	1758
	21st to end	Day	54% 4 10% 13% 14% 5%	1480
March	1st to 10th	Day	48% 4 7% 19% 17% 4	1800
	11th to 20th	Day	55% 5% 9% 16% 13% 3	1962
	21st to end	Day	53% 6% 10% 18% 12% 1	2178
April	1st to 10th	Day	40% 6% 11% 24% 16% 3	2160
	11th to 20th	Day	41% 6% 8% 19% 22% 3	2134
	21st to end	Day	45% 7% 10% 24% 13% 2	2290
May	1st to 10th	Day	42% 8% 12% 24% 12% 1	2340
	11th to 20th	Day	47% 8% 13% 20% 12%	2305
	21st to end	Day	41% 11% 14% 21% 12% 2	2531
June	1st to 10th	Day	40% 9% 13% 22% 14% 2	2340
	11th to 20th	Day	33% 11% 17% 22% 15% 2	2340
	21st to end	Day	35% 11% 17% 19% 13% 4	2340
July	1st to 10th	Day	44% 11% 15% 17% 10% 3	2275
	11th to 20th	Day	43% 10% 17% 16% 11% 3	2271
	21st to end	Day	40% 11% 18% 21% 9% 2	2405
August	1st to 10th	Day	42% 13% 17% 17% 10% 1	2314
	11th to 20th	Day	42% 11% 15% 18% 13% 1	2155
	21st to end	Day	41% 12% 13% 20% 12% 2	2376
September	1st to 10th	Day	43% 9% 13% 17% 16% 3	2066
	11th to 20th	Day	42% 7% 12% 21% 17% 2	1980
	21st to end	Day	34% 7% 11% 24% 21% 4	1980
October	1st to 10th	Day	32% 6% 9% 23% 26% 4	1980
	11th to 20th	Day	35% 6% 9% 19% 26% 6%	1908
	21st to end	Day	39% 6% 8% 21% 22% 4	1850
November	1st to 10th	Day	32% 5% 9% 23% 27% 4	1672
	11th to 20th	Day	38% 5% 7% 22% 22% 5%	1710
	21st to end	Day	42% 5% 7% 21% 20% 4	1710
December	1st to 10th	Day	55% 5% 7% 19% 12% 2	1710
	11th to 20th	Day	53% 5% 8% 17% 15% 2	2150
	21st to end	Day	53% 6% 10% 20% 9% 2	3124
January	1st to 10th	Night	51% 5% 9% 18% 15% 2	2459
	11th to 20th	Night	60% 4 8% 14% 12% 2	2367
	21st to end	Night	50% 5% 9% 20% 13% 2	2511
February	1st to 10th	Night	56% 5% 9% 17% 13% 1	2160
	11th to 20th	Night	56% 4 7% 18% 12% 3	2099
	21st to end	Night	58% 5% 6% 13% 15% 3	1776
March	1st to 10th	Night	47% 6% 8% 15% 18% 6%	2160
	11th to 20th	Night	49% 6% 9% 21% 13% 2	1996
	21st to end	Night	50% 7% 10% 20% 11% 1	2178
April	1st to 10th	Night	37% 7% 10% 24% 19% 3	1814
	11th to 20th	Night	38% 5% 9% 20% 22% 6%	1775
	21st to end	Night	40% 7% 10% 28% 14% 1	1670
May	1st to 10th	Night	40% 7% 12% 25% 14% 2	1620
	11th to 20th	Night	39% 10% 15% 22% 13% 1	1593
	21st to end	Night	34% 12% 16% 23% 13% 3	1745
June	1st to 10th	Night	34% 9% 13% 28% 14% 2	1620
	11th to 20th	Night	26% 9% 21% 26% 16% 2	1620
	21st to end	Night	32% 11% 17% 24% 12% 4	1620
July	1st to 10th	Night	36% 13% 17% 19% 12% 2	1575
	11th to 20th	Night	32% 11% 18% 19% 16% 4	1566
	21st to end	Night	35% 12% 18% 22% 12% 1	1672
August	1st to 10th	Night	33% 10% 18% 26% 13%	1600
	11th to 20th	Night	33% 12% 19% 21% 14% 2	1742
	21st to end	Night	30% 9% 15% 27% 15% 3	1980
September	1st to 10th	Night	35% 9% 13% 22% 17% 4	1876
	11th to 20th	Night	38% 7% 11% 22% 22% 2	1980
	21st to end	Night	31% 6% 11% 23% 24% 5%	1980
October	1st to 10th	Night	27% 5% 9% 23% 28% 7%	1980
	11th to 20th	Night	37% 5% 9% 21% 22% 6%	2052
	21st to end	Night	38% 4 7% 21% 25% 6%	2502
November	1st to 10th	Night	35% 4 7% 23% 25% 7%	2406
	11th to 20th	Night	36% 5% 8% 21% 24% 7%	2470
	21st to end	Night	43% 3 7% 24% 19% 4	2470
December	1st to 10th	Night	56% 5% 7% 17% 11% 2	2470
	11th to 20th	Night	51% 5% 7% 18% 17% 3	3044
	21st to end	Night	56% 8% 10% 15% 10% 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% 23% 34% 11% 2 1	270
	11th to 20th	Day	38% 28% 20% 10% 3	246
	21st to end	Day	28% 28% 34% 7% 3	264
February	1st to 10th	Day	31% 28% 28% 11% 2	240
	11th to 20th	Day	43% 26% 25% 5% 1	240
	21st to end	Day	41% 23% 31% 4 2	198
March	1st to 10th	Day	30% 23% 37% 10% 1	240
	11th to 20th	Day	31% 23% 26% 15% 4 1	402
	21st to end	Day	29% 20% 27% 16% 7%	462
April	1st to 10th	Day	26% 17% 27% 20% 9%	420
	11th to 20th	Day	21% 17% 31% 21% 9% 1	412
	21st to end	Day	25% 15% 31% 17% 10% 2	420
May	1st to 10th	Day	20% 17% 33% 19% 7% 4	423
	11th to 20th	Day	16% 12% 29% 25% 11% 8%	447
	21st to end	Day	15% 7% 23% 26% 18% 11%	485
June	1st to 10th	Day	12% 5% 23% 29% 20% 11%	450
	11th to 20th	Day	7% 4 20% 30% 28% 11%	450
	21st to end	Day	9% 5% 24% 29% 20% 12%	450
July	1st to 10th	Day	12% 2 17% 25% 26% 18%	445
	11th to 20th	Day	9% 6% 16% 26% 27% 16%	444
	21st to end	Day	10% 3 13% 30% 30% 14%	488
August	1st to 10th	Day	13% 2 15% 26% 31% 12%	446
	11th to 20th	Day	12% 4 13% 34% 29% 7%	439
	21st to end	Day	13% 4 10% 28% 31% 15%	495
September	1st to 10th	Day	13% 4 13% 27% 30% 13%	450
	11th to 20th	Day	15% 4 13% 27% 27% 14%	450
	21st to end	Day	14% 6% 19% 29% 23% 9%	450
October	1st to 10th	Day	12% 7% 22% 31% 22% 7%	450
	11th to 20th	Day	15% 14% 28% 25% 14% 3	450
	21st to end	Day	25% 12% 27% 25% 8% 2	365
November	1st to 10th	Day	17% 16% 31% 27% 9% 2	270
	11th to 20th	Day	21% 17% 38% 20% 5%	270
	21st to end	Day	22% 18% 33% 18% 7% 2	270
December	1st to 10th	Day	34% 20% 27% 14% 4	270
	11th to 20th	Day	30% 21% 26% 15% 7%	270
	21st to end	Day	31% 24% 28% 13% 3	297
January	1st to 10th	Night	26% 21% 29% 21% 4	760
	11th to 20th	Night	30% 23% 25% 18% 3	729
	21st to end	Night	24% 19% 36% 19% 2	792
February	1st to 10th	Night	27% 16% 34% 20% 3	720
	11th to 20th	Night	30% 21% 33% 15% 1	703
	21st to end	Night	34% 21% 28% 16% 1	592
March	1st to 10th	Night	23% 17% 33% 23% 3	720
	11th to 20th	Night	25% 18% 30% 19% 6% 1	720
	21st to end	Night	24% 17% 29% 23% 7%	792
April	1st to 10th	Night	16% 16% 31% 29% 8%	554
	11th to 20th	Night	20% 11% 33% 24% 12%	533
	21st to end	Night	18% 10% 31% 30% 10% 1	540
May	1st to 10th	Night	17% 9% 30% 30% 13% 1	540
	11th to 20th	Night	13% 6% 26% 34% 18% 3	531
	21st to end	Night	8% 3 20% 35% 30% 5%	584
June	1st to 10th	Night	8% 4 16% 36% 30% 5%	540
	11th to 20th	Night	5% 2 16% 37% 33% 8%	540
	21st to end	Night	5% 2 16% 39% 32% 6%	540
July	1st to 10th	Night	11% 3 11% 26% 42% 8%	525
	11th to 20th	Night	8% 3 11% 25% 44% 8%	522
	21st to end	Night	8% 2 7% 33% 44% 7%	591
August	1st to 10th	Night	11% 2 7% 29% 44% 7%	537
	11th to 20th	Night	9% 1 9% 40% 36% 5%	534
	21st to end	Night	9% 2 7% 33% 40% 8%	594
September	1st to 10th	Night	11% 4 11% 30% 38% 6%	616
	11th to 20th	Night	13% 3 13% 29% 35% 7%	720
	21st to end	Night	12% 4 18% 32% 29% 5%	720
October	1st to 10th	Night	12% 6% 18% 30% 29% 5%	720
	11th to 20th	Night	20% 8% 21% 33% 17% 2	720
	21st to end	Night	20% 10% 27% 27% 14% 2	792
November	1st to 10th	Night	14% 9% 24% 35% 16% 2	740
	11th to 20th	Night	17% 16% 29% 27% 10% 1	760
	21st to end	Night	22% 15% 27% 28% 9%	760
December	1st to 10th	Night	29% 16% 27% 25% 4	760
	11th to 20th	Night	24% 16% 29% 24% 6% 1	760
	21st to end	Night	27% 20% 26% 24% 4	836

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