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 2026-08-30.

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% 5% 5% 4 6% 8% 38%	143422
	Night	15% 7% 2 6% 4 4 4 4 6% 7% 40%	138821
Transparency	Day	42% 5% 13% 22% 15% 3	147161
	Night	42% 5% 12% 22% 16% 4	143153
Seeing	Day	17% 9% 23% 25% 18% 8%	17208
	Night	16% 6% 22% 32% 19% 3	27184

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	19% 10% 3 7% 4 4 4 4 5% 6% 35%	5258
	11th to 20th	Day	13% 7% 4 8% 4 4 3 3 4 4 46%	5280
	21st to end	Day	20% 9% 3 8% 4 3 3 3 5% 7% 37%	5796
February	1st to 10th	Day	19% 9% 3 7% 5% 5% 4 2 5% 5% 36%	2100
	11th to 20th	Day	22% 8% 3 8% 5% 4 3 2 3 6% 35%	2048
	21st to end	Day	19% 8% 3 8% 4 4 4 4 5% 4 38%	1730
March	1st to 10th	Day	19% 9% 4 7% 5% 4 4 3 4 5% 34%	4691
	11th to 20th	Day	13% 6% 2 6% 4 4 4 3 5% 7% 47%	5090
	21st to end	Day	14% 7% 3 6% 4 4 4 3 4 6% 44%	5586
April	1st to 10th	Day	12% 6% 2 5% 5% 5% 5% 4 6% 7% 42%	2409
	11th to 20th	Day	13% 6% 2 7% 3 4 4 4 5% 6% 45%	2499
	21st to end	Day	13% 6% 2 7% 5% 5% 4 4 5% 8% 41%	2730
May	1st to 10th	Day	18% 8% 3 7% 5% 4 5% 4 6% 9% 32%	7616
	11th to 20th	Day	12% 7% 3 9% 6% 5% 6% 4 7% 8% 34%	7626
	21st to end	Day	19% 9% 3 9% 6% 5% 5% 4 6% 6% 29%	8391
June	1st to 10th	Day	11% 6% 2 7% 5% 5% 5% 5% 8% 10% 37%	2730
	11th to 20th	Day	6% 3 2 6% 6% 6% 6% 6% 8% 11% 40%	2730
	21st to end	Day	7% 4 2 6% 6% 5% 7% 5% 7% 12% 39%	2730
July	1st to 10th	Day	6% 5% 2 7% 5% 6% 6% 5% 9% 11% 38%	7680
	11th to 20th	Day	10% 6% 2 8% 6% 7% 6% 5% 8% 12% 28%	7680
	21st to end	Day	13% 7% 3 8% 6% 6% 6% 5% 8% 11% 27%	8340
August	1st to 10th	Day	6% 5% 3 7% 5% 5% 7% 6% 8% 11% 37%	4364
	11th to 20th	Day	7% 5% 2 6% 5% 5% 6% 5% 8% 11% 40%	4173
	21st to end	Day	6% 4 2 5% 4 4 4 4 6% 7% 53%	4420
September	1st to 10th	Day	11% 6% 3 6% 4 5% 4 4 6% 9% 42%	2230
	11th to 20th	Day	13% 5% 2 7% 4 5% 4 4 6% 7% 45%	2200
	21st to end	Day	14% 6% 2 5% 4 4 5% 4 5% 7% 45%	2200
October	1st to 10th	Day	10% 6% 2 5% 3 3 4 3 5% 6% 54%	2200
	11th to 20th	Day	12% 5% 2 5% 3 3 4 3 3 6% 54%	2061
	21st to end	Day	16% 5% 2 4 3 3 3 3 6% 6% 48%	2154
November	1st to 10th	Day	11% 6% 2 5% 3 3 3 3 4 6% 54%	1852
	11th to 20th	Day	13% 5% 2 6% 4 3 3 3 5% 7% 49%	1890
	21st to end	Day	15% 6% 3 6% 5% 3 4 3 4 6% 44%	1890
December	1st to 10th	Day	18% 7% 2 5% 4 4 4 3 4 5% 44%	3020
	11th to 20th	Day	23% 10% 3 8% 4 5% 4 3 5% 5% 30%	3463
	21st to end	Day	19% 9% 3 7% 5% 5% 5% 3 5% 7% 31%	4565
January	1st to 10th	Night	17% 9% 3 7% 4 4 3 3 4 6% 41%	7635
	11th to 20th	Night	16% 8% 2 5% 4 3 3 3 4 5% 46%	7680
	21st to end	Night	19% 10% 3 6% 4 4 4 3 4 6% 38%	8383
February	1st to 10th	Night	17% 8% 3 6% 4 5% 5% 4 6% 7% 37%	2520
	11th to 20th	Night	23% 7% 3 7% 5% 4 4 3 4 5% 35%	2458
	21st to end	Night	22% 8% 3 7% 5% 4 4 3 5% 5% 35%	2076
March	1st to 10th	Night	19% 8% 3 7% 5% 4 4 4 6% 6% 35%	5381
	11th to 20th	Night	11% 5% 2 5% 4 4 4 3 5% 8% 49%	5090
	21st to end	Night	14% 7% 3 7% 4 4 5% 4 4 7% 42%	5588
April	1st to 10th	Night	13% 4 2 5% 4 5% 4 4 6% 7% 47%	2227
	11th to 20th	Night	13% 6% 2 5% 3 4 4 3 5% 7% 48%	2070
	21st to end	Night	12% 6% 2 7% 5% 4 5% 4 6% 8% 43%	1890
May	1st to 10th	Night	16% 7% 3 8% 5% 5% 5% 4 7% 8% 32%	5237
	11th to 20th	Night	16% 8% 3 7% 5% 4 5% 4 6% 7% 36%	5234
	21st to end	Night	18% 9% 3 8% 5% 5% 5% 4 5% 7% 31%	5761
June	1st to 10th	Night	9% 5% 3 7% 5% 5% 4 4 8% 10% 40%	1890
	11th to 20th	Night	6% 3 1 5% 4 4 4 5% 8% 11% 47%	1890
	21st to end	Night	6% 5% 3 6% 5% 5% 5% 4 7% 11% 42%	1890
July	1st to 10th	Night	8% 6% 2 7% 6% 5% 6% 5% 8% 11% 36%	5278
	11th to 20th	Night	10% 6% 3 8% 5% 6% 6% 5% 8% 11% 33%	5277
	21st to end	Night	13% 7% 3 9% 6% 6% 6% 5% 8% 12% 25%	5736
August	1st to 10th	Night	6% 6% 3 7% 5% 4 5% 5% 7% 10% 43%	3005
	11th to 20th	Night	8% 5% 2 7% 4 4 4 4 5% 9% 48%	3090
	21st to end	Night	5% 4 1 4 3 4 4 3 6% 9% 56%	3676
September	1st to 10th	Night	10% 5% 2 7% 4 4 6% 4 6% 7% 46%	2148
	11th to 20th	Night	10% 6% 2 7% 5% 4 5% 4 6% 8% 43%	2200
	21st to end	Night	14% 5% 2 6% 3 4 4 3 6% 6% 49%	2200
October	1st to 10th	Night	11% 4 2 6% 3 3 3 2 5% 6% 55%	2200
	11th to 20th	Night	14% 5% 2 4 3 2 3 2 5% 5% 54%	2337
	21st to end	Night	14% 5% 2 5% 3 3 4 2 3 5% 53%	2683
November	1st to 10th	Night	12% 6% 2 5% 4 3 3 3 4 5% 52%	2666
	11th to 20th	Night	13% 7% 3 5% 3 3 4 3 5% 6% 48%	2730
	21st to end	Night	16% 6% 3 6% 4 3 4 4 5% 7% 43%	2730
December	1st to 10th	Night	20% 7% 2 6% 4 4 4 3 4 5% 41%	4380
	11th to 20th	Night	25% 8% 3 6% 4 3 4 3 4 5% 36%	4952
	21st to end	Night	20% 8% 3 6% 5% 5% 6% 4 6% 7% 30%	6633

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	49% 4 10% 24% 12% 1	5259
	11th to 20th	Day	40% 4 5% 22% 24% 4	5280
	21st to end	Day	49% 3 9% 23% 14% 2	5796
February	1st to 10th	Day	51% 3 9% 18% 16% 2	2100
	11th to 20th	Day	52% 4 9% 20% 13% 2	2058
	21st to end	Day	49% 4 8% 18% 16% 5%	1730
March	1st to 10th	Day	50% 5% 6% 15% 15% 8%	4691
	11th to 20th	Day	37% 4 7% 21% 24% 7%	5090
	21st to end	Day	41% 3 9% 31% 15% 1	5586
April	1st to 10th	Day	39% 6% 12% 25% 16% 3	2409
	11th to 20th	Day	38% 5% 10% 20% 24% 4	2499
	21st to end	Day	40% 5% 11% 27% 15% 2	2730
May	1st to 10th	Day	49% 5% 12% 21% 12% 1	7651
	11th to 20th	Day	47% 5% 16% 20% 10% 2	7630
	21st to end	Day	54% 6% 11% 16% 11% 2	8405
June	1st to 10th	Day	38% 10% 14% 22% 14% 2	2730
	11th to 20th	Day	34% 10% 17% 23% 14% 2	2730
	21st to end	Day	35% 9% 18% 20% 14% 3	2730
July	1st to 10th	Day	38% 7% 22% 21% 11% 1	7615
	11th to 20th	Day	46% 6% 24% 18% 4 1	7611
	21st to end	Day	49% 6% 19% 18% 7% 1	8156
August	1st to 10th	Day	38% 7% 18% 28% 9%	4351
	11th to 20th	Day	35% 7% 19% 24% 14% 1	4212
	21st to end	Day	28% 6% 14% 35% 15% 2	4419
September	1st to 10th	Day	39% 7% 13% 19% 17% 6%	2230
	11th to 20th	Day	38% 6% 12% 24% 17% 3	2200
	21st to end	Day	37% 5% 13% 21% 18% 6%	2200
October	1st to 10th	Day	25% 4 6% 23% 32% 11%	3590
	11th to 20th	Day	24% 2 7% 24% 32% 10%	3340
	21st to end	Day	29% 4 10% 27% 22% 9%	3456
November	1st to 10th	Day	33% 3 8% 24% 27% 5%	1852
	11th to 20th	Day	36% 3 7% 25% 24% 5%	1890
	21st to end	Day	41% 4 8% 25% 20% 2	1890
December	1st to 10th	Day	43% 4 6% 23% 23% 1	3020
	11th to 20th	Day	56% 4 9% 18% 11% 2	3460
	21st to end	Day	51% 4 15% 21% 9% 1	4565
January	1st to 10th	Night	45% 3 14% 23% 13% 1	7636
	11th to 20th	Night	41% 3 6% 22% 24% 5%	7680
	21st to end	Night	49% 2 9% 19% 16% 5%	8383
February	1st to 10th	Night	46% 5% 10% 22% 16% 2	2520
	11th to 20th	Night	52% 4 7% 19% 14% 4	2459
	21st to end	Night	52% 4 7% 17% 16% 4	2076
March	1st to 10th	Night	49% 4 9% 17% 17% 4	5381
	11th to 20th	Night	33% 4 7% 24% 22% 10%	5090
	21st to end	Night	44% 3 9% 27% 14% 2	5588
April	1st to 10th	Night	35% 5% 12% 24% 21% 3	2227
	11th to 20th	Night	37% 4 8% 23% 23% 6%	2070
	21st to end	Night	38% 7% 12% 27% 15% 2	1890
May	1st to 10th	Night	48% 5% 14% 21% 11% 1	5264
	11th to 20th	Night	47% 5% 16% 17% 14% 1	5235
	21st to end	Night	52% 6% 11% 19% 11% 1	5771
June	1st to 10th	Night	37% 7% 16% 23% 14% 2	1890
	11th to 20th	Night	27% 9% 21% 27% 14% 2	1890
	21st to end	Night	35% 9% 14% 24% 13% 5%	1890
July	1st to 10th	Night	39% 8% 22% 21% 9% 1	5235
	11th to 20th	Night	44% 7% 24% 19% 5% 1	5226
	21st to end	Night	50% 7% 21% 14% 6% 1	5606
August	1st to 10th	Night	35% 6% 18% 29% 11% 1	2994
	11th to 20th	Night	33% 5% 19% 29% 13% 1	3117
	21st to end	Night	24% 5% 15% 38% 16% 2	3676
September	1st to 10th	Night	36% 8% 14% 19% 17% 6%	2148
	11th to 20th	Night	38% 6% 14% 19% 21% 2	2200
	21st to end	Night	36% 5% 13% 20% 21% 5%	2200
October	1st to 10th	Night	28% 3 7% 22% 26% 13%	3590
	11th to 20th	Night	21% 3 5% 25% 32% 14%	3797
	21st to end	Night	25% 3 5% 28% 29% 10%	4331
November	1st to 10th	Night	36% 3 8% 22% 24% 8%	2666
	11th to 20th	Night	37% 4 6% 22% 25% 6%	2730
	21st to end	Night	41% 4 7% 25% 20% 3	2730
December	1st to 10th	Night	46% 4 6% 22% 20% 2	4380
	11th to 20th	Night	50% 3 9% 21% 16% 2	4954
	21st to end	Night	51% 4 15% 19% 11% 1	6633

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% 15% 34% 18% 3 1	330
	11th to 20th	Day	27% 19% 32% 16% 5%	330
	21st to end	Day	21% 22% 38% 15% 3	363
February	1st to 10th	Day	25% 16% 36% 19% 3	330
	11th to 20th	Day	34% 21% 33% 9% 2	330
	21st to end	Day	33% 17% 35% 11% 3	273
March	1st to 10th	Day	25% 18% 35% 17% 4 1	376
	11th to 20th	Day	24% 18% 31% 21% 6% 1	540
	21st to end	Day	24% 14% 34% 21% 7% 1	594
April	1st to 10th	Day	20% 16% 26% 24% 12% 2	540
	11th to 20th	Day	17% 13% 32% 26% 10% 2	532
	21st to end	Day	19% 10% 32% 24% 12% 3	540
May	1st to 10th	Day	17% 11% 33% 23% 10% 5%	543
	11th to 20th	Day	14% 9% 28% 24% 17% 8%	567
	21st to end	Day	18% 8% 18% 25% 19% 13%	617
June	1st to 10th	Day	14% 3 20% 29% 24% 11%	570
	11th to 20th	Day	7% 4 15% 31% 30% 14%	570
	21st to end	Day	8% 2 19% 28% 26% 17%	570
July	1st to 10th	Day	9% 3 10% 26% 28% 24%	565
	11th to 20th	Day	12% 4 10% 23% 34% 18%	564
	21st to end	Day	13% 3 10% 27% 30% 18%	620
August	1st to 10th	Day	12% 4 10% 27% 33% 15%	566
	11th to 20th	Day	11% 2 12% 30% 33% 12%	559
	21st to end	Day	11% 2 8% 25% 36% 18%	619
September	1st to 10th	Day	11% 3 12% 25% 31% 17%	530
	11th to 20th	Day	12% 2 11% 26% 31% 17%	530
	21st to end	Day	19% 4 15% 28% 27% 7%	530
October	1st to 10th	Day	13% 3 18% 28% 29% 8%	530
	11th to 20th	Day	14% 8% 22% 32% 21% 3	530
	21st to end	Day	20% 9% 20% 33% 15% 3	537
November	1st to 10th	Day	18% 8% 30% 30% 11% 3	330
	11th to 20th	Day	15% 11% 35% 29% 9%	330
	21st to end	Day	16% 12% 34% 27% 11% 1	330
December	1st to 10th	Day	27% 15% 32% 20% 5% 1	330
	11th to 20th	Day	23% 13% 34% 23% 6% 1	330
	21st to end	Day	24% 18% 30% 21% 7% 1	363
January	1st to 10th	Night	22% 12% 32% 30% 5%	840
	11th to 20th	Night	24% 10% 32% 29% 4	840
	21st to end	Night	20% 9% 38% 30% 2	924
February	1st to 10th	Night	22% 8% 36% 31% 4	840
	11th to 20th	Night	27% 13% 35% 23% 2	823
	21st to end	Night	27% 14% 32% 25% 3	692
March	1st to 10th	Night	21% 11% 31% 33% 4	840
	11th to 20th	Night	18% 10% 33% 31% 7% 1	840
	21st to end	Night	20% 9% 34% 31% 7%	924
April	1st to 10th	Night	16% 9% 30% 34% 9%	757
	11th to 20th	Night	16% 7% 30% 34% 13%	623
	21st to end	Night	14% 7% 29% 39% 11% 1	630
May	1st to 10th	Night	13% 5% 26% 37% 17% 3	630
	11th to 20th	Night	15% 4 19% 36% 22% 4	621
	21st to end	Night	12% 2 17% 30% 33% 6%	683
June	1st to 10th	Night	10% 2 14% 36% 33% 5%	630
	11th to 20th	Night	6% 2 12% 36% 36% 8%	630
	21st to end	Night	8% 2 8% 35% 40% 7%	630
July	1st to 10th	Night	11% 2 7% 25% 46% 9%	615
	11th to 20th	Night	10% 2 5% 28% 44% 11%	612
	21st to end	Night	10% 2 7% 31% 40% 10%	690
August	1st to 10th	Night	10% 3 6% 29% 43% 10%	627
	11th to 20th	Night	12% 2 7% 30% 42% 6%	624
	21st to end	Night	10% 1 6% 31% 43% 8%	687
September	1st to 10th	Night	11% 2 9% 30% 41% 7%	748
	11th to 20th	Night	13% 2 10% 30% 38% 6%	800
	21st to end	Night	16% 3 15% 35% 29% 3	800
October	1st to 10th	Night	12% 6% 14% 30% 31% 7%	800
	11th to 20th	Night	16% 4 20% 38% 20% 2	800
	21st to end	Night	16% 5% 21% 38% 18% 2	880
November	1st to 10th	Night	15% 5% 20% 40% 17% 2	820
	11th to 20th	Night	15% 8% 30% 35% 11% 1	840
	21st to end	Night	18% 7% 29% 34% 11%	840
December	1st to 10th	Night	24% 10% 26% 33% 7%	840
	11th to 20th	Night	23% 6% 31% 34% 6%	840
	21st to end	Night	25% 11% 24% 35% 6%	924

Page generated at 2026-08-27 04:07:09. Version ac07

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