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 2026-03-03.

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	17% 7% 2 6% 4 4 4 4 5% 7% 39%	130579
	Night	17% 8% 2 6% 4 4 4 3 5% 6% 40%	130596
Transparency	Day	44% 4 11% 22% 15% 3	134099
	Night	44% 4 11% 22% 16% 3	135150
Seeing	Day	24% 15% 31% 19% 8% 2	16616
	Night	23% 13% 32% 25% 7% 1	25648

By Month

Percentage Cloud Cover NE (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	11% 7% 3 10% 6% 6% 5% 4 5% 5% 39%	4661
	11th to 20th	Day	13% 8% 3 7% 4 3 4 3 4 5% 45%	4650
	21st to end	Day	14% 9% 3 4 4 3 4 3 5% 6% 47%	5124
February	1st to 10th	Day	17% 8% 3 6% 4 4 5% 5% 5% 7% 36%	1949
	11th to 20th	Day	13% 8% 3 7% 5% 4 5% 4 5% 7% 39%	2048
	21st to end	Day	18% 11% 3 6% 5% 4 3 3 4 5% 38%	1730
March	1st to 10th	Day	21% 11% 3 6% 4 4 4 3 5% 4 36%	4496
	11th to 20th	Day	16% 8% 2 6% 4 4 4 4 6% 6% 40%	4745
	21st to end	Day	25% 9% 2 6% 4 3 4 3 5% 6% 32%	5819
April	1st to 10th	Day	19% 10% 2 6% 5% 4 4 3 4 6% 37%	2400
	11th to 20th	Day	18% 6% 2 5% 4 4 4 5% 6% 7% 39%	2379
	21st to end	Day	21% 7% 3 6% 5% 5% 4 3 4 6% 37%	2600
May	1st to 10th	Day	31% 10% 3 6% 4 4 4 3 5% 6% 25%	5886
	11th to 20th	Day	20% 9% 3 8% 5% 4 4 3 5% 7% 32%	6016
	21st to end	Day	19% 7% 2 7% 4 4 5% 4 6% 8% 35%	6949
June	1st to 10th	Day	19% 6% 2 6% 5% 4 4 4 6% 8% 35%	2800
	11th to 20th	Day	17% 6% 2 5% 5% 4 5% 5% 7% 8% 37%	2800
	21st to end	Day	18% 6% 2 7% 5% 4 4 4 6% 9% 35%	2800
July	1st to 10th	Day	16% 7% 3 6% 5% 5% 5% 5% 7% 9% 34%	6360
	11th to 20th	Day	8% 5% 2 7% 5% 5% 6% 5% 7% 11% 39%	6360
	21st to end	Day	14% 6% 2 6% 4 4 4 3 5% 8% 44%	6926
August	1st to 10th	Day	14% 7% 3 7% 5% 5% 6% 4 6% 9% 35%	4311
	11th to 20th	Day	14% 8% 3 7% 4 5% 5% 4 6% 8% 38%	4091
	21st to end	Day	8% 3 2 6% 5% 4 6% 4 7% 9% 46%	4312
September	1st to 10th	Day	13% 6% 2 5% 4 3 5% 4 5% 7% 47%	2400
	11th to 20th	Day	15% 5% 2 5% 3 3 3 3 5% 6% 51%	2206
	21st to end	Day	16% 6% 2 6% 4 4 4 3 5% 7% 43%	2200
October	1st to 10th	Day	22% 5% 2 5% 3 4 4 3 4 5% 42%	2200
	11th to 20th	Day	15% 6% 2 4 4 4 4 3 5% 7% 46%	1946
	21st to end	Day	20% 7% 2 6% 4 4 3 3 4 5% 42%	1980
November	1st to 10th	Day	15% 6% 2 5% 5% 3 3 3 4 4 49%	1851
	11th to 20th	Day	17% 6% 2 5% 3 3 4 3 4 5% 48%	1890
	21st to end	Day	18% 8% 2 5% 4 3 4 3 5% 5% 42%	1890
December	1st to 10th	Day	15% 8% 2 5% 3 4 3 3 4 6% 47%	2690
	11th to 20th	Day	20% 7% 2 5% 3 2 3 2 4 5% 47%	3066
	21st to end	Day	20% 11% 3 5% 4 3 3 3 4 5% 38%	4048
January	1st to 10th	Night	14% 7% 2 6% 4 4 4 3 5% 6% 45%	8234
	11th to 20th	Night	14% 10% 3 9% 5% 4 4 3 4 6% 38%	8235
	21st to end	Night	14% 10% 3 7% 5% 4 4 3 5% 5% 41%	9057
February	1st to 10th	Night	16% 9% 3 6% 4 4 4 4 5% 6% 39%	2667
	11th to 20th	Night	14% 8% 3 7% 4 5% 4 3 6% 5% 40%	2458
	21st to end	Night	17% 10% 2 7% 4 3 3 3 4 6% 40%	2076
March	1st to 10th	Night	23% 9% 2 5% 3 3 3 3 4 6% 38%	5389
	11th to 20th	Night	15% 10% 3 7% 4 4 4 3 5% 6% 38%	5214
	21st to end	Night	25% 9% 3 6% 3 4 4 4 5% 5% 34%	5110
April	1st to 10th	Night	19% 8% 3 6% 6% 4 5% 3 6% 6% 36%	2000
	11th to 20th	Night	20% 7% 2 6% 4 4 4 3 5% 7% 39%	1970
	21st to end	Night	20% 7% 3 6% 4 4 4 3 4 6% 40%	1800
May	1st to 10th	Night	33% 9% 3 5% 3 3 4 3 5% 6% 25%	4054
	11th to 20th	Night	17% 10% 3 8% 5% 5% 5% 5% 6% 7% 29%	3905
	21st to end	Night	20% 8% 3 8% 4 5% 4 4 5% 6% 33%	3926
June	1st to 10th	Night	18% 9% 2 7% 4 3 3 3 5% 7% 39%	1600
	11th to 20th	Night	20% 7% 2 4 4 4 4 4 5% 7% 38%	1600
	21st to end	Night	15% 6% 2 5% 4 4 4 4 6% 9% 39%	1600
July	1st to 10th	Night	12% 7% 2 7% 5% 4 4 5% 6% 9% 38%	3600
	11th to 20th	Night	11% 7% 2 7% 5% 4 5% 4 7% 8% 40%	3600
	21st to end	Night	11% 7% 2 6% 4 4 4 4 5% 6% 47%	3926
August	1st to 10th	Night	19% 7% 2 7% 5% 5% 5% 4 5% 8% 34%	2805
	11th to 20th	Night	11% 6% 3 7% 6% 5% 6% 5% 7% 8% 37%	2952
	21st to end	Night	9% 3 2 4 3 4 3 3 5% 8% 55%	3586
September	1st to 10th	Night	13% 6% 2 4 4 3 4 3 5% 6% 50%	2000
	11th to 20th	Night	14% 5% 2 4 3 3 3 3 4 6% 53%	2193
	21st to end	Night	17% 6% 2 6% 3 4 3 3 5% 6% 44%	2200
October	1st to 10th	Night	20% 7% 2 5% 4 3 3 3 5% 4 45%	2200
	11th to 20th	Night	16% 5% 1 4 3 3 4 3 6% 6% 47%	2450
	21st to end	Night	22% 8% 2 5% 4 3 4 3 4 6% 39%	2860
November	1st to 10th	Night	15% 6% 2 5% 4 3 4 3 4 5% 49%	2667
	11th to 20th	Night	17% 6% 2 5% 3 3 3 4 5% 5% 48%	2730
	21st to end	Night	16% 8% 2 5% 4 3 3 3 5% 7% 44%	2730
December	1st to 10th	Night	17% 9% 2 6% 4 4 4 3 5% 5% 42%	4704
	11th to 20th	Night	22% 8% 2 5% 3 3 3 3 4 4 42%	5348
	21st to end	Night	18% 10% 3 6% 4 4 3 3 5% 6% 39%	7150

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	46% 3 9% 24% 15% 2	4662
	11th to 20th	Day	42% 3 9% 21% 20% 5%	4680
	21st to end	Day	39% 3 9% 23% 23% 2	5124
February	1st to 10th	Day	47% 4 12% 21% 14% 3	1949
	11th to 20th	Day	44% 6% 9% 23% 15% 3	2057
	21st to end	Day	49% 4 8% 21% 15% 3	1730
March	1st to 10th	Day	51% 4 9% 15% 11% 10%	4499
	11th to 20th	Day	42% 5% 12% 26% 12% 2	4745
	21st to end	Day	53% 4 5% 15% 16% 7%	5819
April	1st to 10th	Day	49% 4 9% 18% 17% 4	2400
	11th to 20th	Day	43% 5% 10% 20% 19% 3	2379
	21st to end	Day	48% 5% 7% 18% 17% 4	2600
May	1st to 10th	Day	60% 5% 12% 13% 8% 1	5897
	11th to 20th	Day	53% 4 14% 17% 10% 1	6020
	21st to end	Day	47% 5% 11% 22% 14% 1	6949
June	1st to 10th	Day	45% 7% 12% 24% 12% 1	2800
	11th to 20th	Day	42% 6% 13% 20% 16% 2	2800
	21st to end	Day	45% 6% 12% 21% 15% 1	2800
July	1st to 10th	Day	47% 5% 12% 21% 14% 1	6290
	11th to 20th	Day	40% 5% 16% 26% 13% 1	6285
	21st to end	Day	40% 3 12% 28% 16% 1	6728
August	1st to 10th	Day	45% 6% 16% 23% 9% 1	4297
	11th to 20th	Day	44% 4 15% 27% 10% 1	4130
	21st to end	Day	33% 5% 12% 31% 18% 1	4312
September	1st to 10th	Day	37% 6% 11% 26% 20% 2	2400
	11th to 20th	Day	35% 4 8% 27% 23% 3	2206
	21st to end	Day	42% 4 8% 22% 22% 2	2200
October	1st to 10th	Day	42% 4 9% 22% 20% 3	3590
	11th to 20th	Day	32% 3 8% 29% 25% 3	3136
	21st to end	Day	33% 3 10% 31% 19% 3	3180
November	1st to 10th	Day	39% 3 12% 26% 18% 2	1851
	11th to 20th	Day	39% 4 9% 23% 23% 3	1890
	21st to end	Day	44% 4 13% 25% 12% 2	1890
December	1st to 10th	Day	40% 3 15% 27% 13% 1	2690
	11th to 20th	Day	42% 3 13% 19% 20% 4	3066
	21st to end	Day	49% 4 8% 19% 15% 5%	4048
January	1st to 10th	Night	41% 4 11% 28% 14% 2	8234
	11th to 20th	Night	48% 4 10% 18% 15% 5%	8280
	21st to end	Night	46% 3 8% 19% 21% 4	9057
February	1st to 10th	Night	45% 4 12% 21% 15% 2	2667
	11th to 20th	Night	44% 4 10% 22% 17% 4	2460
	21st to end	Night	46% 3 8% 24% 15% 4	2076
March	1st to 10th	Night	48% 4 8% 17% 16% 7%	5399
	11th to 20th	Night	46% 5% 16% 18% 13% 2	5215
	21st to end	Night	52% 4 4 12% 22% 6%	5110
April	1st to 10th	Night	49% 4 9% 20% 15% 3	2000
	11th to 20th	Night	46% 4 8% 21% 19% 2	1970
	21st to end	Night	45% 5% 10% 19% 17% 4	1800
May	1st to 10th	Night	60% 4 12% 13% 8% 2	4060
	11th to 20th	Night	53% 5% 14% 16% 10% 2	3906
	21st to end	Night	51% 5% 13% 18% 11% 1	3927
June	1st to 10th	Night	46% 6% 11% 23% 13% 1	1600
	11th to 20th	Night	46% 6% 13% 21% 13% 2	1600
	21st to end	Night	39% 8% 14% 24% 15% 1	1600
July	1st to 10th	Night	42% 6% 13% 23% 16%	3560
	11th to 20th	Night	41% 4 19% 20% 15% 1	3552
	21st to end	Night	38% 4 11% 30% 16% 1	3809
August	1st to 10th	Night	49% 7% 12% 20% 11% 1	2795
	11th to 20th	Night	42% 7% 15% 26% 10%	2979
	21st to end	Night	28% 4 15% 31% 22% 1	3586
September	1st to 10th	Night	36% 4 13% 27% 20% 1	2000
	11th to 20th	Night	33% 4 10% 24% 27% 2	2193
	21st to end	Night	42% 4 8% 25% 19% 3	2200
October	1st to 10th	Night	38% 4 14% 25% 16% 2	3590
	11th to 20th	Night	28% 3 11% 27% 28% 3	3986
	21st to end	Night	35% 2 11% 32% 17% 2	4610
November	1st to 10th	Night	38% 5% 10% 23% 20% 4	2667
	11th to 20th	Night	39% 4 10% 22% 21% 4	2730
	21st to end	Night	40% 5% 13% 27% 14% 2	2730
December	1st to 10th	Night	45% 3 11% 29% 12% 1	4704
	11th to 20th	Night	46% 3 14% 15% 20% 2	5348
	21st to end	Night	46% 4 8% 23% 16% 3	7150

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	30% 24% 29% 14% 3	220
	11th to 20th	Day	36% 21% 31% 9% 3	220
	21st to end	Day	29% 26% 30% 12% 2	242
February	1st to 10th	Day	32% 21% 32% 13% 2	423
	11th to 20th	Day	30% 25% 33% 11% 2	536
	21st to end	Day	30% 22% 32% 12% 3 1	446
March	1st to 10th	Day	33% 20% 32% 12% 3	508
	11th to 20th	Day	29% 23% 31% 14% 3 1	500
	21st to end	Day	33% 17% 32% 11% 4 2	550
April	1st to 10th	Day	35% 19% 27% 14% 5% 1	500
	11th to 20th	Day	33% 22% 29% 10% 6% 1	492
	21st to end	Day	29% 16% 33% 15% 5% 1	500
May	1st to 10th	Day	35% 15% 29% 15% 5% 1	503
	11th to 20th	Day	25% 13% 40% 16% 5% 2	527
	21st to end	Day	20% 12% 36% 23% 8% 2	573
June	1st to 10th	Day	19% 11% 37% 20% 10% 2	530
	11th to 20th	Day	21% 12% 36% 22% 8% 2	530
	21st to end	Day	20% 12% 35% 21% 8% 4	530
July	1st to 10th	Day	19% 9% 29% 28% 13% 3	525
	11th to 20th	Day	15% 5% 33% 31% 13% 3	524
	21st to end	Day	14% 8% 34% 31% 10% 2	576
August	1st to 10th	Day	11% 8% 30% 34% 14% 3	526
	11th to 20th	Day	18% 7% 27% 29% 15% 4	519
	21st to end	Day	13% 8% 33% 27% 14% 6%	583
September	1st to 10th	Day	18% 9% 29% 23% 15% 6%	530
	11th to 20th	Day	19% 6% 27% 22% 15% 10%	530
	21st to end	Day	20% 9% 31% 24% 11% 5%	530
October	1st to 10th	Day	27% 10% 25% 20% 13% 5%	530
	11th to 20th	Day	20% 16% 29% 22% 11% 2	474
	21st to end	Day	28% 13% 31% 18% 9% 1	462
November	1st to 10th	Day	24% 14% 33% 18% 8% 2	425
	11th to 20th	Day	30% 20% 26% 19% 4 1	430
	21st to end	Day	31% 17% 29% 17% 6%	430
December	1st to 10th	Day	23% 26% 29% 18% 4	230
	11th to 20th	Day	35% 23% 30% 10% 2	220
	21st to end	Day	29% 20% 30% 16% 3 1	242
January	1st to 10th	Night	24% 18% 38% 18% 3	840
	11th to 20th	Night	29% 17% 34% 18% 2	840
	21st to end	Night	21% 19% 37% 20% 3	924
February	1st to 10th	Night	23% 17% 38% 20% 2	840
	11th to 20th	Night	24% 19% 39% 18% 1	823
	21st to end	Night	26% 19% 34% 18% 3	692
March	1st to 10th	Night	26% 18% 35% 18% 3	809
	11th to 20th	Night	23% 20% 35% 18% 3	800
	21st to end	Night	27% 17% 32% 19% 4	880
April	1st to 10th	Night	26% 18% 32% 20% 3	800
	11th to 20th	Night	26% 18% 32% 19% 5%	786
	21st to end	Night	23% 19% 34% 21% 3	600
May	1st to 10th	Night	31% 11% 32% 22% 4	600
	11th to 20th	Night	20% 14% 37% 23% 5%	542
	21st to end	Night	21% 7% 36% 29% 6%	433
June	1st to 10th	Night	23% 4 36% 29% 9%	400
	11th to 20th	Night	22% 7% 34% 32% 6% 1	400
	21st to end	Night	17% 4 35% 35% 8% 1	400
July	1st to 10th	Night	14% 5% 28% 42% 10% 1	390
	11th to 20th	Night	13% 4 27% 45% 12%	388
	21st to end	Night	12% 4 30% 44% 10%	438
August	1st to 10th	Night	14% 3 27% 43% 13% 1	526
	11th to 20th	Night	13% 5% 27% 39% 14% 1	633
	21st to end	Night	16% 4 25% 37% 17% 1	880
September	1st to 10th	Night	15% 5% 30% 35% 14% 1	800
	11th to 20th	Night	18% 6% 23% 30% 20% 3	800
	21st to end	Night	21% 7% 26% 31% 14% 2	800
October	1st to 10th	Night	24% 9% 27% 26% 13% 2	800
	11th to 20th	Night	20% 11% 32% 25% 12% 1	800
	21st to end	Night	28% 12% 32% 22% 6%	880
November	1st to 10th	Night	21% 13% 31% 26% 8% 1	820
	11th to 20th	Night	24% 16% 33% 19% 6% 1	840
	21st to end	Night	25% 17% 32% 21% 5% 1	840
December	1st to 10th	Night	27% 17% 33% 18% 4	840
	11th to 20th	Night	26% 17% 32% 21% 3	840
	21st to end	Night	27% 15% 35% 19% 3	924

Page generated at 2026-02-28 01:40:18. Version ac07

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