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 2024-11-30.

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	5% 5% 2 4 3 3 3 3 5% 7% 61%	105190
	Night	6% 6% 2 5% 4 3 4 3 5% 7% 55%	103178
Transparency	Day	23% 4 9% 19% 24% 21%	104949
	Night	26% 5% 11% 19% 20% 19%	103009
Seeing	Day	8% 10% 24% 31% 20% 6%	15545
	Night	9% 5% 15% 33% 30% 7%	24931

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	12% 9% 3 9% 4 4 4 3 6% 7% 38%	2736
	11th to 20th	Day	12% 10% 4 10% 6% 5% 6% 4 5% 6% 32%	2860
	21st to end	Day	14% 12% 4 10% 5% 6% 5% 4 6% 7% 29%	3146
February	1st to 10th	Day	13% 9% 3 6% 4 4 4 4 5% 5% 44%	1710
	11th to 20th	Day	12% 8% 3 6% 4 4 4 3 5% 7% 43%	1766
	21st to end	Day	10% 10% 4 6% 5% 4 3 2 4 6% 44%	1570
March	1st to 10th	Day	9% 13% 4 11% 7% 7% 6% 5% 5% 6% 26%	3381
	11th to 20th	Day	7% 5% 2 6% 4 3 4 3 5% 8% 52%	3490
	21st to end	Day	10% 9% 3 10% 6% 6% 7% 4 7% 8% 31%	3883
April	1st to 10th	Day	11% 8% 3 6% 5% 4 3 3 5% 7% 45%	2280
	11th to 20th	Day	9% 6% 2 4 3 3 3 4 6% 8% 52%	2249
	21st to end	Day	6% 5% 1 4 4 4 4 3 5% 8% 57%	2280
May	1st to 10th	Day	5% 3 1 3 2 2 3 2 3 5% 71%	3879
	11th to 20th	Day	2 2 1 2 1 2 2 2 4 7% 74%	4129
	21st to end	Day	2 3 1 3 2 3 3 2 4 6% 69%	4813
June	1st to 10th	Day	4 5% 1 3 3 3 3 3 5% 8% 62%	2660
	11th to 20th	Day	2 2 1 1 2 3 2 4 8% 74%	2660
	21st to end	Day	1 2 1 2 1 2 2 2 3 5% 79%	2660
July	1st to 10th	Day	1 1 1 1 1 1 2 5% 87%	4398
	11th to 20th	Day	1 1 1 1 1 2 2 4 6% 81%	4110
	21st to end	Day	1 1 1 1 2 2 4 7% 81%	4455
August	1st to 10th	Day	1 1 1 2 2 2 3 4 5% 9% 71%	4091
	11th to 20th	Day	1 1 1 1 1 2 3 6% 84%	3795
	21st to end	Day	1 2 2 2 2 2 4 7% 78%	4180
September	1st to 10th	Day	1 1 1 1 1 1 1 1 3 5% 82%	2280
	11th to 20th	Day	2 3 1 4 3 3 4 3 5% 8% 66%	2280
	21st to end	Day	2 2 1 2 2 2 2 2 4 5% 76%	2019
October	1st to 10th	Day	2 2 1 4 2 2 3 2 4 6% 72%	3180
	11th to 20th	Day	2 3 1 3 2 3 3 3 5% 8% 68%	3161
	21st to end	Day	3 3 1 4 3 4 3 3 6% 8% 61%	3448
November	1st to 10th	Day	6% 6% 2 5% 3 5% 4 3 6% 7% 54%	1934
	11th to 20th	Day	6% 6% 2 6% 4 4 5% 4 6% 8% 50%	1800
	21st to end	Day	10% 6% 2 4 4 3 3 3 5% 7% 52%	1665
December	1st to 10th	Day	7% 7% 2 8% 6% 5% 5% 3 6% 8% 42%	1520
	11th to 20th	Day	17% 15% 3 8% 5% 4 4 3 6% 5% 30%	1928
	21st to end	Day	11% 13% 4 8% 6% 5% 4 4 5% 7% 33%	2794
January	1st to 10th	Night	11% 10% 4 7% 5% 4 4 4 5% 7% 41%	4212
	11th to 20th	Night	11% 12% 4 12% 6% 5% 5% 4 5% 6% 30%	4110
	21st to end	Night	14% 14% 4 9% 5% 5% 5% 4 4 6% 30%	4521
February	1st to 10th	Night	10% 9% 2 6% 5% 4 4 3 5% 5% 46%	2470
	11th to 20th	Night	9% 8% 3 6% 4 4 4 4 6% 8% 44%	2300
	21st to end	Night	10% 8% 2 6% 4 3 3 3 5% 7% 49%	1884
March	1st to 10th	Night	10% 14% 4 9% 6% 5% 5% 4 5% 6% 31%	3490
	11th to 20th	Night	8% 7% 2 5% 4 3 3 2 4 7% 54%	3490
	21st to end	Night	8% 16% 4 9% 5% 6% 5% 4 5% 6% 33%	3769
April	1st to 10th	Night	9% 7% 3 6% 5% 4 4 4 6% 7% 47%	1900
	11th to 20th	Night	7% 7% 2 5% 4 3 3 4 6% 6% 52%	1879
	21st to end	Night	4 4 1 3 3 3 4 3 7% 9% 58%	1900
May	1st to 10th	Night	1 4 1 4 3 3 3 3 5% 6% 68%	3094
	11th to 20th	Night	1 2 1 2 3 3 4 4 7% 11% 62%	2770
	21st to end	Night	2 2 1 3 2 3 3 3 6% 8% 68%	2762
June	1st to 10th	Night	2 3 2 4 2 2 2 4 5% 10% 63%	1520
	11th to 20th	Night	1 1 3 2 2 2 3 5% 8% 74%	1520
	21st to end	Night	1 1 1 3 2 2 2 2 3 6% 78%	1520
July	1st to 10th	Night	1 1 1 1 1 3 6% 85%	2558
	11th to 20th	Night	1 2 1 2 2 3 5% 8% 75%	2860
	21st to end	Night	1 2 2 3 4 3 5% 11% 69%	3109
August	1st to 10th	Night	1 2 1 3 2 2 3 3 6% 14% 62%	2848
	11th to 20th	Night	2 2 2 2 2 4 7% 79%	3052
	21st to end	Night	1 1 2 2 2 2 3 4 8% 75%	3498
September	1st to 10th	Night	1 2 1 2 2 2 4 8% 77%	1900
	11th to 20th	Night	2 3 1 3 3 4 3 3 5% 9% 65%	1900
	21st to end	Night	2 2 1 2 2 2 2 2 4 6% 75%	2157
October	1st to 10th	Night	2 2 1 2 2 2 3 2 4 6% 74%	3800
	11th to 20th	Night	2 3 2 5% 3 3 3 2 5% 7% 65%	3777
	21st to end	Night	3 4 2 4 3 3 3 3 5% 7% 63%	4121
November	1st to 10th	Night	5% 6% 2 5% 4 3 3 3 5% 8% 57%	2364
	11th to 20th	Night	7% 5% 2 6% 4 4 4 4 7% 8% 50%	2600
	21st to end	Night	6% 6% 2 5% 3 4 4 3 4 8% 54%	2725
December	1st to 10th	Night	9% 8% 3 7% 4 5% 3 3 4 7% 46%	2660
	11th to 20th	Night	16% 16% 4 7% 4 4 4 3 5% 7% 30%	3276
	21st to end	Night	11% 11% 4 9% 6% 6% 5% 4 5% 6% 34%	4862

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	41% 10% 11% 27% 8% 3	2736
	11th to 20th	Day	49% 8% 11% 15% 10% 8%	2860
	21st to end	Day	50% 8% 8% 16% 14% 5%	3146
February	1st to 10th	Day	39% 4 8% 21% 18% 10%	1710
	11th to 20th	Day	38% 5% 8% 19% 22% 8%	1775
	21st to end	Day	39% 5% 8% 22% 18% 7%	1570
March	1st to 10th	Day	50% 10% 9% 14% 13% 3	3381
	11th to 20th	Day	29% 5% 11% 27% 16% 10%	3490
	21st to end	Day	44% 8% 15% 19% 11% 2	3883
April	1st to 10th	Day	37% 4 12% 21% 21% 4	2280
	11th to 20th	Day	28% 5% 14% 26% 21% 6%	2252
	21st to end	Day	24% 4 14% 29% 24% 5%	2280
May	1st to 10th	Day	16% 4 9% 31% 28% 13%	3879
	11th to 20th	Day	11% 2 20% 23% 31% 13%	4132
	21st to end	Day	17% 3 14% 28% 34% 5%	4814
June	1st to 10th	Day	20% 5% 14% 26% 27% 9%	2660
	11th to 20th	Day	9% 4 11% 23% 37% 17%	2660
	21st to end	Day	11% 2 7% 15% 31% 35%	2660
July	1st to 10th	Day	5% 1 3 8% 14% 69%	4329
	11th to 20th	Day	7% 1 6% 10% 15% 61%	4041
	21st to end	Day	4 2 4 10% 30% 50%	4315
August	1st to 10th	Day	10% 3 4 15% 29% 39%	4076
	11th to 20th	Day	5% 2 4 11% 39% 41%	3832
	21st to end	Day	7% 2 6% 15% 34% 35%	4179
September	1st to 10th	Day	8% 2 5% 11% 31% 44%	2280
	11th to 20th	Day	16% 4 9% 23% 30% 18%	2280
	21st to end	Day	11% 3 7% 20% 35% 24%	2019
October	1st to 10th	Day	13% 3 5% 11% 28% 40%	3180
	11th to 20th	Day	15% 4 8% 24% 32% 18%	3161
	21st to end	Day	17% 5% 8% 23% 29% 19%	3448
November	1st to 10th	Day	28% 6% 9% 22% 24% 12%	1934
	11th to 20th	Day	30% 7% 10% 22% 23% 9%	1800
	21st to end	Day	31% 3 9% 16% 30% 10%	1665
December	1st to 10th	Day	37% 5% 13% 17% 17% 11%	1520
	11th to 20th	Day	53% 5% 10% 13% 14% 5%	1928
	21st to end	Day	47% 7% 12% 20% 12% 3	2794
January	1st to 10th	Night	36% 10% 17% 23% 11% 3	4212
	11th to 20th	Night	48% 9% 13% 12% 9% 8%	4110
	21st to end	Night	50% 10% 9% 12% 13% 7%	4521
February	1st to 10th	Night	37% 4 8% 19% 19% 12%	2470
	11th to 20th	Night	35% 5% 11% 18% 23% 7%	2302
	21st to end	Night	33% 5% 11% 29% 17% 5%	1884
March	1st to 10th	Night	47% 10% 13% 13% 13% 3	3490
	11th to 20th	Night	30% 6% 13% 20% 21% 10%	3490
	21st to end	Night	44% 11% 12% 19% 10% 3	3769
April	1st to 10th	Night	34% 6% 15% 21% 19% 5%	1900
	11th to 20th	Night	29% 5% 14% 27% 17% 7%	1879
	21st to end	Night	20% 5% 17% 29% 22% 7%	1900
May	1st to 10th	Night	15% 6% 13% 27% 26% 13%	3094
	11th to 20th	Night	12% 3 36% 22% 16% 11%	2771
	21st to end	Night	13% 4 23% 26% 29% 5%	2762
June	1st to 10th	Night	17% 5% 16% 26% 26% 10%	1520
	11th to 20th	Night	10% 4 11% 22% 31% 22%	1520
	21st to end	Night	12% 2 8% 19% 25% 34%	1520
July	1st to 10th	Night	5% 1 3 11% 15% 66%	2518
	11th to 20th	Night	7% 3 5% 15% 14% 57%	2806
	21st to end	Night	9% 2 7% 15% 21% 45%	3011
August	1st to 10th	Night	13% 2 5% 20% 23% 36%	2839
	11th to 20th	Night	7% 3 6% 9% 29% 45%	3081
	21st to end	Night	9% 2 7% 18% 29% 35%	3498
September	1st to 10th	Night	7% 3 5% 14% 27% 45%	1900
	11th to 20th	Night	16% 4 11% 22% 29% 18%	1900
	21st to end	Night	12% 3 7% 19% 33% 27%	2157
October	1st to 10th	Night	12% 3 7% 10% 21% 48%	3800
	11th to 20th	Night	17% 6% 7% 23% 28% 19%	3777
	21st to end	Night	19% 5% 7% 21% 26% 22%	4121
November	1st to 10th	Night	25% 6% 8% 25% 22% 15%	2364
	11th to 20th	Night	29% 6% 10% 23% 23% 9%	2600
	21st to end	Night	29% 3 10% 20% 24% 14%	2725
December	1st to 10th	Night	37% 5% 10% 18% 17% 13%	2660
	11th to 20th	Night	52% 4 9% 14% 16% 5%	3276
	21st to end	Night	46% 9% 13% 16% 12% 4	4862

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% 12% 25% 18% 15% 2	299
	11th to 20th	Day	15% 14% 25% 29% 14% 3	370
	21st to end	Day	16% 11% 27% 27% 15% 4	407
February	1st to 10th	Day	18% 16% 24% 23% 13% 5%	370
	11th to 20th	Day	18% 20% 27% 23% 10% 2	414
	21st to end	Day	13% 16% 33% 25% 12% 2	382
March	1st to 10th	Day	23% 15% 31% 22% 9% 1	460
	11th to 20th	Day	18% 18% 26% 22% 14% 4	460
	21st to end	Day	14% 19% 32% 26% 8% 2	506
April	1st to 10th	Day	16% 17% 28% 25% 11% 3	460
	11th to 20th	Day	12% 19% 29% 25% 12% 3	452
	21st to end	Day	6% 18% 30% 29% 11% 6%	460
May	1st to 10th	Day	8% 13% 36% 26% 14% 3	463
	11th to 20th	Day	8% 13% 29% 30% 14% 6%	488
	21st to end	Day	5% 13% 27% 31% 16% 7%	529
June	1st to 10th	Day	5% 10% 28% 31% 19% 7%	490
	11th to 20th	Day	2 10% 30% 35% 19% 4	490
	21st to end	Day	1 4 23% 40% 26% 6%	490
July	1st to 10th	Day	1 2 17% 40% 33% 7%	486
	11th to 20th	Day	1 14% 43% 31% 10%	484
	21st to end	Day	1 12% 43% 32% 12%	531
August	1st to 10th	Day	2 2 19% 41% 27% 9%	487
	11th to 20th	Day	1 1 20% 39% 26% 13%	477
	21st to end	Day	2 15% 41% 31% 10%	539
September	1st to 10th	Day	1 4 12% 43% 30% 10%	490
	11th to 20th	Day	3 6% 24% 38% 20% 8%	490
	21st to end	Day	3 7% 18% 36% 28% 9%	490
October	1st to 10th	Day	3 7% 23% 32% 27% 9%	490
	11th to 20th	Day	6% 10% 20% 27% 29% 9%	490
	21st to end	Day	6% 8% 25% 30% 25% 7%	539
November	1st to 10th	Day	7% 14% 20% 25% 26% 8%	470
	11th to 20th	Day	8% 11% 21% 27% 24% 9%	300
	21st to end	Day	11% 12% 21% 26% 24% 6%	234
December	1st to 10th	Day	13% 12% 19% 28% 19% 8%	180
	11th to 20th	Day	12% 18% 23% 19% 17% 10%	180
	21st to end	Day	16% 22% 23% 21% 12% 5%	198
January	1st to 10th	Night	18% 7% 17% 29% 24% 5%	760
	11th to 20th	Night	14% 7% 16% 31% 24% 8%	760
	21st to end	Night	17% 6% 14% 33% 25% 6%	836
February	1st to 10th	Night	16% 8% 16% 32% 23% 6%	760
	11th to 20th	Night	15% 10% 19% 33% 19% 6%	744
	21st to end	Night	13% 10% 21% 33% 19% 3	628
March	1st to 10th	Night	16% 10% 21% 29% 21% 3	760
	11th to 20th	Night	16% 8% 18% 30% 24% 5%	760
	21st to end	Night	13% 10% 22% 35% 18% 3	836
April	1st to 10th	Night	13% 6% 24% 34% 20% 3	760
	11th to 20th	Night	11% 12% 20% 32% 20% 5%	751
	21st to end	Night	6% 8% 28% 31% 21% 6%	760
May	1st to 10th	Night	5% 4 27% 34% 25% 4	713
	11th to 20th	Night	5% 2 15% 37% 35% 6%	524
	21st to end	Night	4 5% 21% 40% 27% 3	411
June	1st to 10th	Night	4 4 16% 44% 29% 3	380
	11th to 20th	Night	1 5% 14% 49% 30% 2	380
	21st to end	Night	1 1 12% 43% 40% 4	380
July	1st to 10th	Night	1 8% 35% 53% 3	375
	11th to 20th	Night	1 2 34% 51% 12%	552
	21st to end	Night	1 3 32% 53% 10%	624
August	1st to 10th	Night	2 1 7% 35% 48% 7%	567
	11th to 20th	Night	1 11% 36% 44% 7%	702
	21st to end	Night	1 8% 32% 48% 10%	836
September	1st to 10th	Night	1 8% 35% 44% 12%	760
	11th to 20th	Night	3 3 11% 38% 38% 8%	760
	21st to end	Night	3 3 12% 31% 41% 11%	760
October	1st to 10th	Night	4 3 12% 35% 35% 11%	760
	11th to 20th	Night	6% 4 12% 28% 40% 10%	760
	21st to end	Night	6% 6% 10% 33% 31% 15%	836
November	1st to 10th	Night	7% 5% 16% 28% 34% 9%	780
	11th to 20th	Night	9% 5% 16% 32% 31% 7%	800
	21st to end	Night	11% 4 15% 37% 26% 7%	800
December	1st to 10th	Night	14% 6% 17% 32% 25% 6%	760
	11th to 20th	Night	19% 6% 16% 32% 22% 4	760
	21st to end	Night	16% 13% 20% 30% 18% 3	836

Page generated at 2024-11-27 14:30:48. Version ac07

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