Oak Ridge
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Oak Ridge clear sky chart from 2005-11-06 to 2025-04-26.

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% 7% 3 7% 5% 4 5% 4 6% 7% 40%	113923
	Night	15% 7% 2 6% 4 4 4 4 5% 7% 42%	114723
Transparency	Day	41% 6% 11% 23% 16% 3	117706
	Night	40% 5% 11% 23% 18% 4	119078
Seeing	Day	17% 9% 23% 25% 18% 8%	15375
	Night	16% 6% 23% 32% 19% 3	25497

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	16% 11% 3 7% 5% 4 4 4 5% 6% 36%	4056
	11th to 20th	Day	16% 8% 5% 8% 4 3 3 3 4 5% 40%	4060
	21st to end	Day	19% 9% 3 6% 3 3 3 3 5% 7% 39%	4482
February	1st to 10th	Day	19% 9% 3 7% 5% 5% 4 2 6% 5% 36%	2000
	11th to 20th	Day	22% 8% 3 8% 5% 4 4 2 3 6% 35%	1948
	21st to end	Day	20% 8% 3 7% 4 4 4 4 5% 4 38%	1650
March	1st to 10th	Day	19% 9% 4 7% 5% 4 4 3 4 5% 35%	4589
	11th to 20th	Day	13% 6% 2 6% 3 4 4 3 5% 7% 47%	4980
	21st to end	Day	15% 7% 3 6% 4 4 4 3 4 6% 44%	5465
April	1st to 10th	Day	13% 6% 2 5% 5% 5% 5% 4 6% 7% 42%	2294
	11th to 20th	Day	14% 6% 2 7% 3 4 4 4 5% 6% 45%	2379
	21st to end	Day	14% 6% 2 6% 5% 5% 4 4 6% 8% 41%	2535
May	1st to 10th	Day	18% 8% 3 7% 5% 4 5% 4 5% 7% 34%	4082
	11th to 20th	Day	15% 8% 3 8% 5% 5% 4 4 6% 7% 35%	4081
	21st to end	Day	11% 6% 2 7% 5% 5% 5% 5% 7% 8% 39%	4476
June	1st to 10th	Day	11% 6% 2 7% 5% 5% 5% 5% 8% 10% 36%	2470
	11th to 20th	Day	5% 3 2 5% 6% 6% 6% 6% 8% 11% 41%	2470
	21st to end	Day	7% 4 2 6% 6% 6% 7% 6% 7% 12% 38%	2470
July	1st to 10th	Day	6% 5% 2 7% 5% 6% 7% 6% 10% 12% 34%	4120
	11th to 20th	Day	10% 5% 2 7% 5% 6% 6% 5% 7% 11% 37%	4120
	21st to end	Day	17% 10% 5% 9% 7% 7% 6% 5% 7% 7% 18%	4466
August	1st to 10th	Day	6% 5% 3 7% 5% 5% 6% 6% 8% 11% 39%	4104
	11th to 20th	Day	7% 5% 2 6% 5% 5% 6% 4 8% 11% 41%	3921
	21st to end	Day	6% 4 2 5% 4 4 4 4 6% 7% 53%	4180
September	1st to 10th	Day	11% 6% 2 6% 4 5% 4 4 6% 9% 42%	2119
	11th to 20th	Day	13% 5% 2 7% 4 5% 4 4 6% 7% 43%	2090
	21st to end	Day	15% 6% 2 4 4 4 4 4 5% 7% 46%	2090
October	1st to 10th	Day	10% 6% 2 4 3 3 4 3 5% 6% 54%	2090
	11th to 20th	Day	13% 5% 2 5% 3 4 4 3 4 6% 53%	1958
	21st to end	Day	16% 5% 2 4 3 3 3 3 6% 6% 49%	2047
November	1st to 10th	Day	11% 6% 2 5% 3 3 3 3 4 5% 55%	1762
	11th to 20th	Day	14% 5% 2 6% 4 3 4 3 5% 7% 48%	1800
	21st to end	Day	15% 6% 3 6% 5% 3 4 3 4 6% 45%	1800
December	1st to 10th	Day	18% 7% 2 5% 4 4 4 3 4 5% 45%	2930
	11th to 20th	Day	24% 10% 3 8% 4 5% 4 3 5% 5% 29%	3373
	21st to end	Day	19% 9% 3 8% 5% 5% 5% 3 5% 7% 31%	4466
January	1st to 10th	Night	16% 9% 3 8% 5% 4 3 3 4 6% 41%	5889
	11th to 20th	Night	19% 8% 2 5% 4 3 4 3 4 5% 42%	5900
	21st to end	Night	18% 8% 2 6% 4 3 4 3 4 6% 41%	6472
February	1st to 10th	Night	18% 8% 2 6% 4 5% 5% 4 6% 7% 37%	2400
	11th to 20th	Night	23% 7% 3 7% 5% 4 4 3 4 5% 34%	2338
	21st to end	Night	22% 7% 2 6% 5% 4 4 3 5% 5% 35%	1980
March	1st to 10th	Night	19% 8% 3 7% 5% 4 4 4 6% 6% 35%	5263
	11th to 20th	Night	11% 5% 2 5% 4 4 4 3 6% 8% 49%	4980
	21st to end	Night	14% 7% 3 7% 4 4 5% 4 4 7% 41%	5467
April	1st to 10th	Night	13% 4 2 5% 4 5% 4 4 6% 7% 47%	2121
	11th to 20th	Night	13% 7% 2 5% 3 4 4 3 5% 6% 47%	1970
	21st to end	Night	12% 6% 2 6% 5% 4 5% 4 6% 8% 42%	1761
May	1st to 10th	Night	15% 7% 3 8% 5% 5% 5% 4 7% 7% 34%	2813
	11th to 20th	Night	19% 6% 2 6% 4 3 4 4 6% 7% 39%	2812
	21st to end	Night	7% 6% 2 7% 5% 4 5% 4 7% 9% 42%	3079
June	1st to 10th	Night	9% 5% 3 6% 6% 5% 4 4 9% 10% 39%	1710
	11th to 20th	Night	4 3 1 5% 4 4 4 5% 9% 11% 49%	1710
	21st to end	Night	6% 5% 3 6% 5% 6% 5% 4 7% 11% 42%	1710
July	1st to 10th	Night	9% 5% 2 6% 6% 6% 6% 6% 9% 10% 36%	2840
	11th to 20th	Night	8% 5% 2 5% 4 4 5% 4 7% 11% 46%	2840
	21st to end	Night	15% 9% 4 10% 7% 6% 6% 5% 7% 9% 23%	3087
August	1st to 10th	Night	5% 5% 2 6% 4 4 5% 5% 7% 10% 45%	2825
	11th to 20th	Night	8% 5% 2 7% 4 4 4 4 5% 9% 48%	2903
	21st to end	Night	5% 4 1 4 3 4 4 3 6% 9% 56%	3476
September	1st to 10th	Night	10% 5% 2 6% 4 4 6% 4 6% 6% 46%	2040
	11th to 20th	Night	10% 6% 2 7% 6% 4 5% 4 6% 8% 42%	2090
	21st to end	Night	14% 4 2 6% 3 4 3 3 6% 6% 49%	2090
October	1st to 10th	Night	11% 4 2 5% 3 3 2 2 4 6% 56%	2090
	11th to 20th	Night	15% 5% 2 4 3 3 3 2 5% 5% 53%	2220
	21st to end	Night	13% 5% 2 6% 3 3 4 2 4 5% 53%	2549
November	1st to 10th	Night	12% 5% 2 5% 4 4 3 3 4 5% 52%	2536
	11th to 20th	Night	13% 7% 3 5% 3 3 4 3 5% 6% 48%	2600
	21st to end	Night	16% 6% 3 6% 4 3 4 4 5% 7% 43%	2600
December	1st to 10th	Night	20% 7% 2 6% 4 4 4 3 4 5% 41%	4250
	11th to 20th	Night	25% 8% 3 6% 4 3 3 3 4 5% 36%	4822
	21st to end	Night	20% 8% 3 6% 5% 5% 6% 4 6% 7% 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	47% 5% 7% 25% 15% 1	4057
	11th to 20th	Day	45% 5% 6% 20% 22% 1	4060
	21st to end	Day	46% 3 10% 23% 16% 1	4482
February	1st to 10th	Day	51% 3 9% 18% 16% 2	2000
	11th to 20th	Day	52% 4 9% 20% 14% 2	1958
	21st to end	Day	49% 4 8% 17% 16% 6%	1650
March	1st to 10th	Day	50% 5% 6% 16% 15% 8%	4589
	11th to 20th	Day	37% 4 6% 21% 24% 7%	4980
	21st to end	Day	42% 3 8% 31% 15% 1	5465
April	1st to 10th	Day	39% 6% 11% 24% 16% 3	2294
	11th to 20th	Day	39% 5% 9% 19% 24% 4	2379
	21st to end	Day	41% 6% 11% 26% 15% 2	2535
May	1st to 10th	Day	48% 6% 11% 22% 12% 1	4120
	11th to 20th	Day	46% 8% 11% 23% 10% 1	4085
	21st to end	Day	38% 9% 14% 20% 17% 2	4489
June	1st to 10th	Day	39% 10% 14% 22% 14% 2	2470
	11th to 20th	Day	32% 11% 17% 23% 14% 2	2470
	21st to end	Day	36% 10% 17% 19% 14% 4	2470
July	1st to 10th	Day	37% 10% 22% 20% 11% 1	4055
	11th to 20th	Day	39% 7% 21% 24% 7% 2	4051
	21st to end	Day	60% 8% 12% 13% 5% 1	4324
August	1st to 10th	Day	36% 7% 18% 29% 10%	4091
	11th to 20th	Day	35% 7% 18% 24% 15% 1	3960
	21st to end	Day	28% 7% 14% 35% 15% 2	4179
September	1st to 10th	Day	39% 7% 13% 19% 17% 6%	2119
	11th to 20th	Day	39% 7% 11% 22% 18% 3	2090
	21st to end	Day	37% 5% 13% 21% 19% 6%	2090
October	1st to 10th	Day	24% 4 6% 22% 32% 11%	3480
	11th to 20th	Day	25% 2 8% 24% 32% 10%	3237
	21st to end	Day	29% 4 10% 26% 22% 9%	3349
November	1st to 10th	Day	33% 3 8% 24% 27% 5%	1762
	11th to 20th	Day	36% 3 6% 24% 25% 5%	1800
	21st to end	Day	41% 4 8% 24% 20% 2	1800
December	1st to 10th	Day	42% 4 5% 23% 23% 2	2930
	11th to 20th	Day	57% 4 9% 18% 11% 2	3370
	21st to end	Day	51% 4 15% 20% 9% 1	4466
January	1st to 10th	Night	46% 3 9% 24% 17% 2	5889
	11th to 20th	Night	44% 4 6% 20% 20% 5%	5900
	21st to end	Night	45% 3 8% 21% 19% 4	6472
February	1st to 10th	Night	46% 4 10% 21% 16% 2	2400
	11th to 20th	Night	52% 4 8% 19% 14% 4	2339
	21st to end	Night	51% 4 7% 17% 16% 4	1980
March	1st to 10th	Night	49% 4 8% 17% 17% 4	5263
	11th to 20th	Night	33% 4 7% 23% 23% 10%	4980
	21st to end	Night	44% 3 9% 27% 15% 2	5467
April	1st to 10th	Night	36% 6% 11% 24% 21% 3	2121
	11th to 20th	Night	38% 4 8% 21% 23% 6%	1970
	21st to end	Night	38% 7% 12% 27% 14% 2	1761
May	1st to 10th	Night	45% 8% 12% 22% 10% 1	2840
	11th to 20th	Night	42% 8% 15% 20% 14% 1	2813
	21st to end	Night	34% 9% 14% 24% 17% 1	3087
June	1st to 10th	Night	35% 8% 17% 23% 15% 2	1710
	11th to 20th	Night	25% 9% 22% 28% 15% 2	1710
	21st to end	Night	36% 8% 14% 24% 13% 5%	1710
July	1st to 10th	Night	39% 8% 22% 20% 10% 1	2795
	11th to 20th	Night	32% 6% 22% 28% 8% 3	2786
	21st to end	Night	55% 9% 15% 14% 6% 1	2988
August	1st to 10th	Night	32% 6% 19% 30% 12% 1	2814
	11th to 20th	Night	33% 6% 18% 28% 14% 1	2930
	21st to end	Night	24% 5% 15% 39% 15% 2	3476
September	1st to 10th	Night	36% 8% 14% 19% 17% 6%	2040
	11th to 20th	Night	39% 7% 14% 17% 21% 2	2090
	21st to end	Night	35% 5% 13% 20% 22% 5%	2090
October	1st to 10th	Night	27% 3 8% 22% 27% 14%	3480
	11th to 20th	Night	22% 3 5% 25% 31% 14%	3680
	21st to end	Night	25% 3 5% 28% 28% 10%	4197
November	1st to 10th	Night	35% 3 8% 22% 24% 8%	2536
	11th to 20th	Night	38% 4 6% 21% 25% 6%	2600
	21st to end	Night	41% 4 7% 25% 20% 3	2600
December	1st to 10th	Night	46% 4 6% 22% 21% 2	4250
	11th to 20th	Night	50% 3 8% 21% 15% 2	4824
	21st to end	Night	51% 4 15% 19% 11% 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	28% 16% 33% 19% 3 1	300
	11th to 20th	Day	28% 18% 32% 16% 6%	300
	21st to end	Day	21% 21% 39% 15% 4	330
February	1st to 10th	Day	26% 17% 35% 19% 3	300
	11th to 20th	Day	35% 21% 33% 9% 2	300
	21st to end	Day	34% 18% 35% 10% 2	249
March	1st to 10th	Day	25% 18% 34% 18% 4 1	344
	11th to 20th	Day	23% 18% 31% 21% 6% 1	500
	21st to end	Day	25% 14% 32% 21% 7% 1	550
April	1st to 10th	Day	21% 17% 26% 23% 11% 2	500
	11th to 20th	Day	19% 12% 31% 26% 10% 2	492
	21st to end	Day	20% 10% 31% 24% 12% 3	480
May	1st to 10th	Day	16% 11% 33% 24% 11% 6%	463
	11th to 20th	Day	14% 9% 27% 23% 18% 10%	487
	21st to end	Day	14% 7% 18% 26% 21% 15%	529
June	1st to 10th	Day	14% 2 20% 28% 23% 12%	490
	11th to 20th	Day	6% 3 14% 31% 30% 16%	490
	21st to end	Day	8% 2 20% 30% 25% 16%	490
July	1st to 10th	Day	10% 3 10% 25% 28% 23%	485
	11th to 20th	Day	11% 4 11% 24% 33% 17%	484
	21st to end	Day	14% 2 8% 26% 32% 18%	532
August	1st to 10th	Day	11% 3 9% 28% 34% 15%	486
	11th to 20th	Day	11% 2 13% 31% 33% 10%	479
	21st to end	Day	12% 2 7% 24% 36% 19%	539
September	1st to 10th	Day	10% 3 12% 25% 31% 18%	490
	11th to 20th	Day	13% 3 11% 26% 31% 16%	490
	21st to end	Day	19% 4 13% 28% 28% 7%	490
October	1st to 10th	Day	13% 3 19% 28% 29% 8%	490
	11th to 20th	Day	14% 9% 23% 30% 21% 3	490
	21st to end	Day	19% 8% 19% 34% 16% 3	496
November	1st to 10th	Day	18% 7% 30% 30% 11% 3	300
	11th to 20th	Day	17% 11% 33% 30% 9%	300
	21st to end	Day	15% 12% 33% 28% 11% 1	300
December	1st to 10th	Day	28% 15% 32% 20% 6% 1	300
	11th to 20th	Day	24% 13% 33% 23% 7% 1	300
	21st to end	Day	25% 16% 30% 21% 8% 1	330
January	1st to 10th	Night	21% 13% 31% 30% 5%	800
	11th to 20th	Night	25% 10% 32% 30% 4	800
	21st to end	Night	20% 9% 38% 30% 2	880
February	1st to 10th	Night	22% 8% 35% 31% 4	800
	11th to 20th	Night	27% 13% 34% 24% 2	783
	21st to end	Night	26% 14% 32% 25% 3	660
March	1st to 10th	Night	21% 11% 30% 33% 4	800
	11th to 20th	Night	18% 10% 34% 30% 7% 1	800
	21st to end	Night	20% 9% 33% 31% 7%	880
April	1st to 10th	Night	17% 10% 30% 34% 9%	721
	11th to 20th	Night	16% 7% 31% 32% 13%	593
	21st to end	Night	14% 7% 30% 37% 12% 1	587
May	1st to 10th	Night	12% 4 25% 37% 18% 3	570
	11th to 20th	Night	15% 4 19% 36% 22% 4	561
	21st to end	Night	9% 2 16% 30% 36% 6%	617
June	1st to 10th	Night	10% 2 14% 36% 33% 5%	570
	11th to 20th	Night	4 2 13% 36% 36% 9%	570
	21st to end	Night	8% 2 9% 36% 38% 7%	570
July	1st to 10th	Night	11% 2 8% 26% 45% 8%	555
	11th to 20th	Night	9% 2 5% 29% 44% 11%	552
	21st to end	Night	10% 2 8% 30% 41% 10%	624
August	1st to 10th	Night	8% 3 7% 29% 44% 10%	567
	11th to 20th	Night	12% 2 8% 33% 40% 5%	564
	21st to end	Night	10% 1 6% 29% 44% 9%	627
September	1st to 10th	Night	11% 2 9% 29% 41% 7%	710
	11th to 20th	Night	14% 2 11% 31% 36% 6%	760
	21st to end	Night	15% 3 16% 34% 29% 3	760
October	1st to 10th	Night	12% 6% 14% 30% 31% 7%	760
	11th to 20th	Night	16% 4 21% 37% 20% 2	760
	21st to end	Night	15% 6% 20% 39% 18% 2	836
November	1st to 10th	Night	15% 5% 21% 38% 18% 3	780
	11th to 20th	Night	16% 8% 30% 35% 11% 1	800
	21st to end	Night	18% 7% 29% 34% 12% 1	800
December	1st to 10th	Night	24% 10% 26% 33% 7%	800
	11th to 20th	Night	23% 6% 31% 34% 7%	800
	21st to end	Night	25% 11% 24% 35% 6%	880

Page generated at 2025-04-23 09:25:08. Version ac07

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