Oak Ridge Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Oak Ridge Observatory clear sky chart from 2005-11-06 to 2026-02-16.

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	4 4 2 4 3 3 3 3 5% 7% 62%	139522
	Night	5% 6% 2 5% 3 3 3 3 5% 8% 56%	138832
Transparency	Day	21% 5% 9% 19% 26% 21%	134455
	Night	25% 6% 10% 18% 22% 18%	134886
Seeing	Day	8% 11% 24% 31% 20% 6%	17066
	Night	9% 6% 16% 33% 30% 6%	26549

By Month

Percentage Cloud Cover SW (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	10% 8% 4 8% 3 4 4 3 6% 8% 42%	5115
	11th to 20th	Day	6% 5% 2 6% 4 3 4 3 4 6% 57%	5340
	21st to end	Day	8% 9% 3 8% 5% 5% 4 4 6% 8% 41%	5843
February	1st to 10th	Day	12% 10% 3 6% 4 4 4 4 5% 5% 44%	1890
	11th to 20th	Day	11% 8% 3 6% 4 4 4 4 6% 7% 42%	1909
	21st to end	Day	10% 10% 4 7% 5% 4 3 3 5% 6% 44%	1650
March	1st to 10th	Day	9% 11% 4 10% 6% 7% 6% 5% 6% 6% 31%	4865
	11th to 20th	Day	7% 6% 3 7% 5% 5% 5% 4 6% 7% 45%	5000
	21st to end	Day	7% 8% 3 9% 6% 6% 7% 5% 8% 9% 32%	5570
April	1st to 10th	Day	11% 8% 3 6% 5% 4 3 3 5% 7% 46%	2400
	11th to 20th	Day	8% 6% 2 5% 3 3 3 4 6% 8% 53%	2369
	21st to end	Day	6% 5% 2 5% 4 3 4 3 5% 8% 56%	2400
May	1st to 10th	Day	4 3 1 2 2 1 2 2 3 6% 74%	5545
	11th to 20th	Day	2 2 1 2 1 1 2 2 4 7% 76%	5925
	21st to end	Day	2 2 1 3 2 2 2 2 4 6% 75%	6902
June	1st to 10th	Day	3 4 1 3 3 3 2 3 5% 9% 63%	2800
	11th to 20th	Day	2 2 1 1 2 3 2 4 8% 75%	2800
	21st to end	Day	1 1 1 2 1 2 2 2 3 5% 80%	2800
July	1st to 10th	Day	1 1 1 1 1 2 5% 88%	6284
	11th to 20th	Day	1 1 1 1 1 1 2 3 6% 84%	5880
	21st to end	Day	1 1 2 2 2 2 2 5% 9% 75%	6402
August	1st to 10th	Day	1 1 1 1 1 1 2 3 4 7% 79%	5777
	11th to 20th	Day	1 1 1 1 1 2 3 6% 83%	5474
	21st to end	Day	1 2 2 2 2 2 4 8% 78%	5984
September	1st to 10th	Day	1 1 1 1 1 1 1 2 3 6% 80%	2400
	11th to 20th	Day	2 3 1 4 2 3 4 3 5% 8% 65%	2400
	21st to end	Day	2 2 1 2 2 2 2 2 4 5% 75%	2124
October	1st to 10th	Day	2 2 1 4 2 3 3 2 4 6% 71%	3280
	11th to 20th	Day	2 3 1 3 2 3 3 3 5% 8% 68%	3261
	21st to end	Day	3 3 1 4 3 4 3 3 6% 8% 61%	3558
November	1st to 10th	Day	6% 6% 2 5% 3 4 3 3 6% 7% 55%	2032
	11th to 20th	Day	6% 6% 2 7% 4 4 5% 4 6% 8% 47%	1890
	21st to end	Day	9% 6% 2 4 4 3 3 3 5% 7% 53%	1753
December	1st to 10th	Day	4 5% 2 6% 4 4 4 4 6% 9% 52%	2700
	11th to 20th	Day	11% 11% 3 8% 6% 5% 5% 4 6% 7% 34%	3108
	21st to end	Day	10% 12% 5% 10% 7% 6% 5% 4 5% 6% 29%	4092
January	1st to 10th	Night	10% 10% 4 8% 4 4 3 3 5% 7% 42%	7796
	11th to 20th	Night	6% 6% 2 7% 4 4 4 3 5% 7% 53%	7650
	21st to end	Night	8% 9% 3 8% 5% 4 5% 3 5% 8% 41%	8371
February	1st to 10th	Night	10% 9% 3 6% 5% 4 4 3 4 5% 47%	2730
	11th to 20th	Night	9% 9% 3 7% 4 4 4 4 6% 8% 43%	2488
	21st to end	Night	10% 8% 2 6% 4 3 4 3 5% 7% 49%	1980
March	1st to 10th	Night	10% 13% 4 10% 6% 5% 4 3 5% 6% 33%	5026
	11th to 20th	Night	9% 8% 4 7% 5% 4 4 3 4 7% 47%	5000
	21st to end	Night	8% 14% 4 8% 4 5% 4 3 5% 7% 37%	5404
April	1st to 10th	Night	8% 7% 3 7% 5% 4 4 4 6% 7% 47%	2000
	11th to 20th	Night	7% 7% 2 5% 4 4 3 3 6% 7% 51%	1979
	21st to end	Night	4 4 1 3 3 3 4 3 7% 9% 57%	2000
May	1st to 10th	Night	1 4 1 3 2 3 2 2 4 8% 69%	4432
	11th to 20th	Night	1 2 1 3 3 3 4 4 6% 9% 64%	3982
	21st to end	Night	1 2 1 2 2 2 3 3 5% 6% 73%	3972
June	1st to 10th	Night	2 3 2 4 2 2 2 4 5% 11% 63%	1600
	11th to 20th	Night	1 1 2 2 2 2 3 5% 8% 75%	1600
	21st to end	Night	1 1 1 3 2 2 1 2 3 6% 78%	1600
July	1st to 10th	Night	1 1 1 1 1 2 7% 86%	3672
	11th to 20th	Night	1 1 1 2 2 3 5% 11% 74%	4100
	21st to end	Night	2 1 4 4 5% 4 3 6% 10% 62%	4472
August	1st to 10th	Night	1 1 2 1 2 2 3 5% 12% 70%	4035
	11th to 20th	Night	2 2 1 2 2 5% 8% 77%	4390
	21st to end	Night	1 1 2 2 2 3 3 4 9% 74%	5015
September	1st to 10th	Night	1 2 1 2 3 2 4 8% 76%	2000
	11th to 20th	Night	2 3 1 3 2 4 4 3 5% 9% 65%	2000
	21st to end	Night	2 2 1 3 2 2 3 2 4 6% 73%	2270
October	1st to 10th	Night	2 2 1 2 2 2 3 2 4 6% 73%	3920
	11th to 20th	Night	2 4 2 5% 3 3 3 2 5% 6% 65%	3897
	21st to end	Night	3 4 2 4 3 3 3 3 5% 7% 63%	4253
November	1st to 10th	Night	4 6% 2 5% 4 3 4 3 5% 8% 57%	2485
	11th to 20th	Night	8% 5% 2 6% 4 4 4 4 7% 8% 48%	2730
	21st to end	Night	6% 6% 2 5% 3 4 4 3 5% 8% 54%	2861
December	1st to 10th	Night	5% 5% 2 5% 4 4 4 4 5% 7% 56%	4700
	11th to 20th	Night	12% 12% 3 7% 5% 5% 5% 4 7% 9% 30%	5316
	21st to end	Night	11% 12% 4 11% 7% 6% 5% 4 5% 6% 28%	7106

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	32% 12% 15% 21% 15% 4	5116
	11th to 20th	Day	28% 5% 6% 10% 22% 29%	5340
	21st to end	Day	37% 7% 6% 15% 22% 13%	5843
February	1st to 10th	Day	38% 4 7% 20% 18% 12%	1890
	11th to 20th	Day	38% 6% 9% 19% 21% 7%	1918
	21st to end	Day	39% 5% 8% 22% 18% 7%	1650
March	1st to 10th	Day	44% 11% 8% 15% 19% 2	4865
	11th to 20th	Day	29% 11% 13% 28% 12% 7%	5000
	21st to end	Day	37% 12% 18% 20% 11% 2	5570
April	1st to 10th	Day	36% 5% 12% 22% 21% 4	2400
	11th to 20th	Day	28% 6% 13% 25% 21% 6%	2372
	21st to end	Day	25% 4 14% 29% 23% 5%	2400
May	1st to 10th	Day	12% 3 6% 30% 35% 14%	5545
	11th to 20th	Day	10% 2 17% 28% 34% 10%	5928
	21st to end	Day	12% 3 11% 28% 41% 6%	6904
June	1st to 10th	Day	19% 5% 13% 26% 29% 8%	2800
	11th to 20th	Day	9% 4 11% 22% 36% 18%	2800
	21st to end	Day	10% 1 6% 15% 32% 36%	2800
July	1st to 10th	Day	4 1 2 8% 22% 64%	6216
	11th to 20th	Day	5% 1 4 8% 17% 65%	5811
	21st to end	Day	7% 3 5% 19% 30% 36%	6217
August	1st to 10th	Day	10% 3 4 15% 30% 38%	4206
	11th to 20th	Day	5% 2 4 11% 38% 41%	3955
	21st to end	Day	7% 2 6% 15% 34% 35%	4311
September	1st to 10th	Day	8% 2 5% 12% 31% 42%	2400
	11th to 20th	Day	16% 4 9% 23% 31% 17%	2400
	21st to end	Day	11% 3 7% 20% 35% 24%	2124
October	1st to 10th	Day	13% 3 5% 11% 28% 39%	3280
	11th to 20th	Day	15% 4 8% 24% 32% 17%	3261
	21st to end	Day	17% 5% 7% 22% 29% 18%	3558
November	1st to 10th	Day	27% 6% 9% 21% 25% 12%	2032
	11th to 20th	Day	31% 7% 10% 21% 22% 8%	1890
	21st to end	Day	30% 3 9% 18% 29% 10%	1753
December	1st to 10th	Day	26% 5% 9% 19% 32% 9%	2700
	11th to 20th	Day	44% 8% 10% 16% 15% 8%	3108
	21st to end	Day	48% 11% 11% 17% 10% 3	4092
January	1st to 10th	Night	33% 12% 16% 19% 16% 3	7796
	11th to 20th	Night	28% 7% 8% 9% 25% 23%	7650
	21st to end	Night	37% 8% 7% 14% 19% 14%	8371
February	1st to 10th	Night	37% 4 8% 18% 20% 13%	2730
	11th to 20th	Night	36% 6% 11% 19% 21% 7%	2490
	21st to end	Night	32% 5% 11% 29% 17% 6%	1980
March	1st to 10th	Night	44% 10% 11% 14% 17% 4	5026
	11th to 20th	Night	32% 9% 13% 21% 17% 7%	5000
	21st to end	Night	39% 12% 13% 22% 12% 3	5404
April	1st to 10th	Night	33% 6% 15% 22% 19% 5%	2000
	11th to 20th	Night	29% 6% 14% 26% 18% 8%	1979
	21st to end	Night	20% 6% 17% 29% 22% 7%	2000
May	1st to 10th	Night	13% 5% 11% 26% 28% 17%	4429
	11th to 20th	Night	12% 4 27% 28% 22% 8%	3983
	21st to end	Night	11% 4 16% 22% 39% 8%	3972
June	1st to 10th	Night	16% 5% 16% 27% 27% 10%	1600
	11th to 20th	Night	9% 4 10% 21% 31% 25%	1600
	21st to end	Night	12% 2 8% 18% 26% 35%	1600
July	1st to 10th	Night	3 1 3 9% 22% 62%	3632
	11th to 20th	Night	5% 2 5% 13% 17% 58%	4046
	21st to end	Night	12% 4 10% 15% 24% 35%	4345
August	1st to 10th	Night	13% 2 5% 20% 24% 36%	2929
	11th to 20th	Night	7% 3 7% 10% 29% 45%	3178
	21st to end	Night	9% 3 7% 17% 30% 34%	3608
September	1st to 10th	Night	8% 3 5% 15% 27% 43%	2000
	11th to 20th	Night	16% 4 10% 23% 29% 18%	2000
	21st to end	Night	12% 3 7% 18% 33% 26%	2270
October	1st to 10th	Night	12% 3 7% 10% 21% 46%	3920
	11th to 20th	Night	18% 6% 7% 23% 28% 19%	3897
	21st to end	Night	19% 5% 7% 21% 26% 22%	4253
November	1st to 10th	Night	24% 6% 9% 25% 22% 14%	2485
	11th to 20th	Night	30% 6% 10% 23% 23% 8%	2730
	21st to end	Night	28% 3 10% 20% 24% 14%	2861
December	1st to 10th	Night	25% 5% 7% 22% 27% 14%	4700
	11th to 20th	Night	44% 8% 10% 14% 18% 7%	5316
	21st to end	Night	49% 12% 13% 14% 9% 4	7106

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% 13% 25% 17% 13% 2	353
	11th to 20th	Day	14% 14% 23% 30% 15% 4	430
	21st to end	Day	15% 12% 28% 26% 15% 4	473
February	1st to 10th	Day	20% 17% 23% 24% 12% 4	430
	11th to 20th	Day	19% 20% 28% 22% 9% 2	467
	21st to end	Day	12% 16% 33% 26% 12% 1	414
March	1st to 10th	Day	22% 16% 30% 22% 9% 1	500
	11th to 20th	Day	19% 18% 27% 20% 13% 3	500
	21st to end	Day	15% 19% 32% 25% 8% 2	550
April	1st to 10th	Day	16% 17% 29% 25% 11% 3	500
	11th to 20th	Day	11% 19% 30% 24% 13% 3	492
	21st to end	Day	6% 18% 29% 30% 11% 6%	500
May	1st to 10th	Day	8% 13% 35% 27% 14% 4	503
	11th to 20th	Day	9% 13% 30% 29% 14% 6%	528
	21st to end	Day	5% 13% 29% 30% 17% 6%	573
June	1st to 10th	Day	4 10% 27% 32% 20% 7%	530
	11th to 20th	Day	2 10% 31% 34% 19% 4	530
	21st to end	Day	1 4 23% 39% 26% 7%	530
July	1st to 10th	Day	1 2 17% 39% 33% 7%	526
	11th to 20th	Day	1 14% 42% 31% 11%	524
	21st to end	Day	1 12% 42% 34% 12%	575
August	1st to 10th	Day	2 2 19% 40% 26% 11%	527
	11th to 20th	Day	1 1 21% 39% 26% 13%	517
	21st to end	Day	2 14% 41% 32% 11%	583
September	1st to 10th	Day	2 4 13% 43% 29% 10%	530
	11th to 20th	Day	4 6% 23% 38% 21% 9%	530
	21st to end	Day	4 7% 19% 34% 28% 8%	530
October	1st to 10th	Day	3 8% 23% 30% 26% 9%	530
	11th to 20th	Day	6% 10% 20% 27% 28% 9%	530
	21st to end	Day	6% 8% 24% 30% 25% 7%	583
November	1st to 10th	Day	7% 13% 20% 26% 26% 8%	508
	11th to 20th	Day	10% 13% 22% 25% 22% 8%	330
	21st to end	Day	10% 11% 22% 28% 24% 5%	258
December	1st to 10th	Day	11% 11% 20% 31% 19% 8%	220
	11th to 20th	Day	13% 17% 21% 20% 18% 10%	220
	21st to end	Day	19% 24% 24% 19% 10% 4	242
January	1st to 10th	Night	20% 7% 17% 29% 22% 5%	840
	11th to 20th	Night	13% 7% 16% 32% 25% 7%	840
	21st to end	Night	16% 5% 14% 35% 25% 5%	924
February	1st to 10th	Night	16% 9% 16% 33% 21% 5%	840
	11th to 20th	Night	16% 9% 20% 33% 17% 5%	804
	21st to end	Night	12% 10% 22% 35% 18% 3	660
March	1st to 10th	Night	17% 10% 21% 29% 21% 3	800
	11th to 20th	Night	18% 8% 18% 29% 23% 5%	800
	21st to end	Night	13% 10% 22% 34% 18% 3	880
April	1st to 10th	Night	13% 7% 24% 34% 20% 3	800
	11th to 20th	Night	10% 12% 20% 33% 19% 5%	791
	21st to end	Night	6% 8% 27% 31% 22% 6%	800
May	1st to 10th	Night	5% 4 27% 34% 25% 5%	750
	11th to 20th	Night	5% 3 16% 37% 33% 5%	552
	21st to end	Night	3 5% 22% 40% 27% 3	433
June	1st to 10th	Night	4 4 15% 45% 29% 3	400
	11th to 20th	Night	1 5% 15% 49% 30% 2	400
	21st to end	Night	2 1 12% 42% 40% 4	400
July	1st to 10th	Night	1 8% 36% 52% 3	396
	11th to 20th	Night	1 2 34% 50% 13%	582
	21st to end	Night	1 3 33% 53% 10%	657
August	1st to 10th	Night	2 1 7% 36% 47% 8%	597
	11th to 20th	Night	1 12% 37% 43% 7%	739
	21st to end	Night	1 8% 32% 48% 10%	880
September	1st to 10th	Night	1 1 8% 36% 43% 11%	800
	11th to 20th	Night	3 3 12% 37% 38% 8%	800
	21st to end	Night	3 3 12% 31% 40% 10%	800
October	1st to 10th	Night	5% 3 13% 34% 34% 11%	800
	11th to 20th	Night	6% 5% 12% 28% 39% 10%	800
	21st to end	Night	6% 6% 11% 33% 31% 14%	880
November	1st to 10th	Night	7% 5% 15% 27% 35% 10%	820
	11th to 20th	Night	10% 6% 16% 31% 30% 6%	840
	21st to end	Night	11% 4 15% 37% 26% 7%	840
December	1st to 10th	Night	13% 5% 17% 34% 25% 6%	840
	11th to 20th	Night	18% 7% 16% 33% 23% 4	840
	21st to end	Night	18% 14% 21% 29% 17% 3	924

Page generated at 2026-02-12 22:42:14. Version ac07

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