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 2024-12-02.

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%	101482
	Night	18% 8% 3 6% 4 4 4 3 5% 6% 40%	98168
Transparency	Day	44% 5% 11% 23% 15% 2	105020
	Night	44% 5% 11% 22% 15% 2	102706
Seeing	Day	24% 14% 32% 20% 8% 2	15097
	Night	22% 13% 32% 25% 7% 1	24025

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	14% 7% 3 7% 4 4 3 3 5% 5% 44%	2517
	11th to 20th	Day	16% 7% 3 8% 5% 4 5% 4 4 5% 40%	2520
	21st to end	Day	21% 11% 3 6% 5% 4 5% 3 5% 4 35%	2772
February	1st to 10th	Day	19% 8% 3 6% 5% 4 5% 4 5% 7% 35%	1762
	11th to 20th	Day	13% 8% 3 7% 5% 4 5% 4 5% 7% 39%	1848
	21st to end	Day	19% 10% 3 7% 5% 4 3 3 4 5% 37%	1570
March	1st to 10th	Day	25% 13% 2 7% 4 4 4 3 4 4 31%	3116
	11th to 20th	Day	14% 7% 2 6% 4 4 4 4 6% 7% 42%	3315
	21st to end	Day	21% 9% 2 6% 4 3 4 3 5% 6% 34%	4057
April	1st to 10th	Day	18% 10% 2 7% 5% 4 4 3 4 6% 37%	2280
	11th to 20th	Day	18% 6% 2 5% 4 4 4 5% 6% 7% 38%	2259
	21st to end	Day	21% 7% 3 6% 5% 5% 4 3 4 6% 36%	2470
May	1st to 10th	Day	24% 11% 4 6% 4 4 4 4 5% 7% 27%	4106
	11th to 20th	Day	23% 8% 3 8% 5% 4 4 3 4 7% 33%	4194
	21st to end	Day	14% 5% 2 5% 4 4 5% 4 6% 8% 45%	4837
June	1st to 10th	Day	19% 6% 2 6% 5% 4 4 4 6% 8% 35%	2660
	11th to 20th	Day	16% 6% 2 5% 5% 5% 5% 5% 7% 9% 37%	2660
	21st to end	Day	18% 6% 2 7% 5% 4 5% 4 6% 9% 34%	2660
July	1st to 10th	Day	13% 6% 3 6% 5% 5% 5% 5% 7% 9% 37%	4440
	11th to 20th	Day	10% 5% 2 6% 5% 5% 5% 4 7% 11% 40%	4440
	21st to end	Day	16% 7% 3 7% 5% 4 4 4 5% 8% 36%	4814
August	1st to 10th	Day	14% 7% 3 6% 5% 5% 6% 4 6% 9% 35%	4179
	11th to 20th	Day	14% 8% 3 7% 4 5% 6% 4 6% 7% 38%	3963
	21st to end	Day	8% 3 2 6% 5% 4 6% 4 7% 9% 45%	4180
September	1st to 10th	Day	13% 6% 2 5% 4 3 5% 4 5% 7% 47%	2280
	11th to 20th	Day	16% 5% 2 5% 3 3 3 3 5% 6% 50%	2096
	21st to end	Day	16% 6% 2 6% 4 4 3 3 4 7% 44%	2090
October	1st to 10th	Day	23% 6% 2 5% 4 4 4 3 4 6% 40%	2090
	11th to 20th	Day	14% 6% 2 4 4 4 4 3 5% 7% 47%	1849
	21st to end	Day	21% 7% 2 6% 4 4 3 4 5% 5% 41%	1881
November	1st to 10th	Day	15% 6% 2 5% 5% 3 3 2 4 4 50%	1761
	11th to 20th	Day	18% 6% 2 4 3 3 4 3 4 5% 49%	1800
	21st to end	Day	18% 8% 2 5% 4 4 4 3 5% 5% 43%	1800
December	1st to 10th	Day	18% 8% 2 7% 4 4 4 4 5% 5% 38%	1538
	11th to 20th	Day	22% 7% 2 5% 3 2 3 2 4 5% 45%	1906
	21st to end	Day	22% 11% 3 5% 4 3 4 3 5% 6% 34%	2772
January	1st to 10th	Night	18% 8% 2 5% 3 3 3 3 4 5% 47%	4429
	11th to 20th	Night	14% 8% 3 9% 5% 4 4 3 4 6% 39%	4440
	21st to end	Night	22% 13% 4 7% 5% 4 3 3 4 4 31%	4884
February	1st to 10th	Night	16% 9% 3 6% 4 4 4 4 6% 6% 39%	2414
	11th to 20th	Night	14% 8% 3 6% 4 4 4 3 6% 5% 41%	2218
	21st to end	Night	17% 9% 2 7% 4 3 3 3 4 6% 41%	1884
March	1st to 10th	Night	27% 11% 2 5% 3 3 3 3 4 5% 33%	3724
	11th to 20th	Night	11% 9% 3 7% 5% 5% 4 4 6% 6% 39%	3644
	21st to end	Night	24% 9% 3 6% 3 4 4 4 6% 6% 34%	3572
April	1st to 10th	Night	18% 8% 3 6% 6% 4 4 3 6% 6% 37%	1900
	11th to 20th	Night	19% 7% 2 5% 4 4 4 3 5% 7% 39%	1870
	21st to end	Night	20% 7% 3 6% 4 4 4 3 4 6% 40%	1710
May	1st to 10th	Night	25% 10% 3 7% 4 4 4 3 5% 6% 27%	2834
	11th to 20th	Night	21% 9% 3 7% 5% 4 4 5% 6% 7% 28%	2727
	21st to end	Night	16% 6% 2 6% 4 4 3 4 6% 7% 42%	2738
June	1st to 10th	Night	18% 9% 2 6% 4 3 3 3 5% 7% 38%	1520
	11th to 20th	Night	20% 7% 2 4 4 4 4 4 5% 7% 39%	1520
	21st to end	Night	15% 6% 3 5% 4 4 5% 4 6% 9% 39%	1520
July	1st to 10th	Night	11% 6% 3 6% 6% 5% 5% 5% 7% 10% 36%	2520
	11th to 20th	Night	11% 7% 3 6% 5% 5% 4 3 7% 8% 41%	2520
	21st to end	Night	14% 8% 3 7% 5% 4 4 4 5% 7% 38%	2739
August	1st to 10th	Night	19% 7% 3 7% 5% 5% 5% 4 5% 8% 33%	2718
	11th to 20th	Night	11% 6% 3 7% 6% 5% 6% 5% 7% 8% 36%	2860
	21st to end	Night	9% 3 2 4 3 4 3 3 5% 8% 55%	3476
September	1st to 10th	Night	13% 6% 2 4 4 3 4 3 5% 6% 50%	1900
	11th to 20th	Night	14% 5% 2 4 3 3 3 3 4 6% 53%	2083
	21st to end	Night	17% 6% 3 6% 3 4 3 3 5% 6% 44%	2090
October	1st to 10th	Night	21% 7% 2 5% 4 3 3 3 5% 4 43%	2090
	11th to 20th	Night	16% 5% 1 5% 4 3 5% 3 6% 6% 47%	2327
	21st to end	Night	22% 8% 2 5% 4 3 4 3 4 6% 39%	2717
November	1st to 10th	Night	15% 6% 2 5% 4 4 4 3 4 5% 49%	2537
	11th to 20th	Night	18% 7% 2 4 3 3 3 3 5% 4 48%	2600
	21st to end	Night	17% 8% 2 5% 4 3 4 3 5% 6% 44%	2600
December	1st to 10th	Night	18% 9% 3 6% 4 4 4 3 5% 6% 38%	2671
	11th to 20th	Night	19% 7% 2 5% 3 3 4 3 5% 5% 42%	3288
	21st to end	Night	20% 10% 3 6% 5% 4 4 3 5% 6% 34%	4884

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	42% 4 8% 33% 11% 2	2518
	11th to 20th	Day	47% 5% 9% 25% 13% 2	2520
	21st to end	Day	51% 5% 9% 23% 10% 2	2772
February	1st to 10th	Day	48% 4 12% 21% 12% 3	1762
	11th to 20th	Day	44% 6% 9% 22% 15% 3	1857
	21st to end	Day	50% 4 8% 21% 14% 3	1570
March	1st to 10th	Day	57% 4 10% 15% 11% 3	3119
	11th to 20th	Day	36% 8% 13% 25% 15% 3	3315
	21st to end	Day	50% 4 5% 17% 17% 7%	4057
April	1st to 10th	Day	48% 4 9% 19% 16% 4	2280
	11th to 20th	Day	43% 6% 11% 20% 18% 3	2259
	21st to end	Day	49% 5% 8% 18% 17% 4	2470
May	1st to 10th	Day	56% 6% 11% 15% 10% 1	4120
	11th to 20th	Day	52% 5% 14% 17% 10% 2	4198
	21st to end	Day	36% 6% 11% 25% 19% 2	4837
June	1st to 10th	Day	45% 7% 12% 24% 12% 1	2660
	11th to 20th	Day	42% 7% 13% 21% 16% 2	2660
	21st to end	Day	46% 7% 12% 21% 14% 1	2660
July	1st to 10th	Day	43% 6% 12% 22% 16% 1	4370
	11th to 20th	Day	39% 5% 16% 26% 13% 1	4365
	21st to end	Day	47% 4 13% 21% 14% 2	4661
August	1st to 10th	Day	46% 6% 16% 22% 9% 1	4165
	11th to 20th	Day	44% 4 15% 27% 9% 1	4002
	21st to end	Day	33% 5% 12% 31% 17% 1	4180
September	1st to 10th	Day	36% 6% 11% 26% 20% 2	2280
	11th to 20th	Day	36% 4 8% 27% 23% 3	2096
	21st to end	Day	42% 3 8% 22% 22% 2	2090
October	1st to 10th	Day	43% 5% 9% 22% 20% 3	3480
	11th to 20th	Day	31% 3 8% 28% 26% 4	3039
	21st to end	Day	33% 3 11% 31% 19% 3	3081
November	1st to 10th	Day	39% 3 12% 26% 17% 2	1761
	11th to 20th	Day	38% 4 9% 22% 23% 3	1800
	21st to end	Day	43% 4 14% 25% 13% 2	1800
December	1st to 10th	Day	47% 4 14% 16% 17% 1	1538
	11th to 20th	Day	43% 4 13% 19% 18% 4	1906
	21st to end	Day	51% 5% 11% 20% 11% 1	2772
January	1st to 10th	Night	40% 5% 12% 29% 12% 3	4429
	11th to 20th	Night	46% 6% 11% 22% 13% 3	4440
	21st to end	Night	57% 4 8% 18% 11% 2	4884
February	1st to 10th	Night	46% 5% 13% 21% 13% 3	2414
	11th to 20th	Night	44% 5% 10% 21% 17% 4	2220
	21st to end	Night	45% 4 8% 23% 16% 4	1884
March	1st to 10th	Night	54% 5% 8% 21% 10% 3	3734
	11th to 20th	Night	43% 7% 13% 19% 16% 3	3645
	21st to end	Night	50% 4 5% 13% 20% 7%	3572
April	1st to 10th	Night	48% 5% 9% 20% 16% 3	1900
	11th to 20th	Night	45% 4 9% 21% 19% 2	1870
	21st to end	Night	45% 5% 10% 19% 16% 4	1710
May	1st to 10th	Night	56% 5% 12% 17% 8% 2	2840
	11th to 20th	Night	54% 7% 14% 14% 9% 2	2728
	21st to end	Night	40% 6% 15% 22% 16% 2	2739
June	1st to 10th	Night	46% 6% 11% 23% 13% 1	1520
	11th to 20th	Night	45% 6% 13% 21% 13% 2	1520
	21st to end	Night	39% 8% 14% 23% 15% 1	1520
July	1st to 10th	Night	42% 7% 14% 22% 15% 1	2480
	11th to 20th	Night	41% 5% 16% 22% 15% 1	2472
	21st to end	Night	46% 5% 11% 21% 16% 1	2651
August	1st to 10th	Night	50% 7% 12% 20% 11% 1	2708
	11th to 20th	Night	43% 7% 15% 25% 10%	2887
	21st to end	Night	28% 4 15% 31% 22% 1	3476
September	1st to 10th	Night	36% 4 13% 26% 19% 1	1900
	11th to 20th	Night	33% 4 10% 23% 27% 2	2083
	21st to end	Night	41% 4 8% 25% 20% 3	2090
October	1st to 10th	Night	39% 4 15% 24% 15% 2	3480
	11th to 20th	Night	27% 3 11% 27% 28% 4	3863
	21st to end	Night	35% 3 11% 32% 17% 2	4467
November	1st to 10th	Night	38% 5% 10% 23% 21% 4	2537
	11th to 20th	Night	39% 4 10% 22% 21% 4	2600
	21st to end	Night	40% 5% 13% 26% 14% 2	2600
December	1st to 10th	Night	47% 4 11% 23% 13% 1	2671
	11th to 20th	Night	43% 4 15% 18% 18% 2	3288
	21st to end	Night	50% 5% 9% 25% 10% 1	4884

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% 22% 31% 15% 4 1	180
	11th to 20th	Day	36% 18% 32% 10% 4	180
	21st to end	Day	25% 24% 33% 14% 3 1	198
February	1st to 10th	Day	33% 20% 32% 13% 2	356
	11th to 20th	Day	28% 26% 32% 11% 2	456
	21st to end	Day	31% 20% 32% 13% 4 1	382
March	1st to 10th	Day	32% 18% 34% 13% 3	460
	11th to 20th	Day	28% 23% 31% 14% 3 1	460
	21st to end	Day	32% 17% 33% 12% 5% 2	506
April	1st to 10th	Day	32% 19% 28% 15% 5% 1	460
	11th to 20th	Day	31% 21% 31% 10% 7% 1	452
	21st to end	Day	29% 16% 33% 14% 5% 2	460
May	1st to 10th	Day	34% 15% 30% 15% 5% 2	463
	11th to 20th	Day	24% 12% 41% 16% 5% 2	487
	21st to end	Day	19% 12% 36% 23% 9% 2	529
June	1st to 10th	Day	19% 11% 37% 20% 10% 2	490
	11th to 20th	Day	19% 12% 36% 23% 9% 2	490
	21st to end	Day	20% 12% 36% 20% 8% 3	490
July	1st to 10th	Day	18% 8% 29% 28% 13% 4	485
	11th to 20th	Day	15% 5% 33% 31% 13% 3	484
	21st to end	Day	14% 8% 34% 32% 10% 2	532
August	1st to 10th	Day	12% 9% 30% 34% 13% 3	486
	11th to 20th	Day	18% 7% 29% 28% 14% 4	479
	21st to end	Day	13% 8% 31% 27% 15% 6%	539
September	1st to 10th	Day	18% 9% 28% 23% 16% 6%	490
	11th to 20th	Day	21% 7% 27% 21% 13% 11%	490
	21st to end	Day	20% 9% 31% 24% 11% 5%	490
October	1st to 10th	Day	28% 10% 25% 20% 12% 4	490
	11th to 20th	Day	18% 16% 29% 23% 11% 2	439
	21st to end	Day	28% 12% 31% 19% 9% 1	429
November	1st to 10th	Day	22% 14% 34% 18% 9% 3	395
	11th to 20th	Day	31% 19% 25% 20% 5% 1	400
	21st to end	Day	30% 17% 29% 18% 6% 1	400
December	1st to 10th	Day	20% 29% 30% 17% 4 1	192
	11th to 20th	Day	33% 22% 33% 9% 2 1	180
	21st to end	Day	27% 21% 31% 16% 4 1	198
January	1st to 10th	Night	23% 17% 38% 19% 3	760
	11th to 20th	Night	28% 17% 35% 18% 2	760
	21st to end	Night	21% 18% 36% 21% 3	836
February	1st to 10th	Night	23% 17% 38% 20% 2	760
	11th to 20th	Night	23% 20% 39% 17% 1	743
	21st to end	Night	26% 19% 33% 19% 3	628
March	1st to 10th	Night	26% 17% 35% 18% 3	760
	11th to 20th	Night	23% 20% 36% 17% 3	760
	21st to end	Night	27% 17% 32% 19% 5%	836
April	1st to 10th	Night	25% 18% 33% 21% 3	760
	11th to 20th	Night	25% 17% 32% 20% 6%	746
	21st to end	Night	23% 19% 34% 21% 3	570
May	1st to 10th	Night	29% 12% 32% 23% 4	570
	11th to 20th	Night	21% 15% 37% 23% 5%	515
	21st to end	Night	20% 7% 37% 29% 7%	411
June	1st to 10th	Night	23% 4 36% 28% 9%	380
	11th to 20th	Night	22% 7% 33% 31% 6% 1	380
	21st to end	Night	17% 4 35% 35% 8% 1	380
July	1st to 10th	Night	15% 4 28% 42% 11% 1	370
	11th to 20th	Night	12% 4 26% 45% 13%	368
	21st to end	Night	13% 4 28% 44% 10%	416
August	1st to 10th	Night	14% 3 27% 42% 13% 1	499
	11th to 20th	Night	14% 5% 27% 39% 14% 1	601
	21st to end	Night	16% 3 24% 38% 17% 2	836
September	1st to 10th	Night	15% 5% 30% 34% 14% 1	760
	11th to 20th	Night	18% 6% 23% 30% 19% 3	760
	21st to end	Night	21% 7% 26% 31% 14% 2	760
October	1st to 10th	Night	24% 9% 27% 26% 12% 2	760
	11th to 20th	Night	19% 11% 32% 24% 12% 1	760
	21st to end	Night	29% 11% 31% 22% 6%	836
November	1st to 10th	Night	20% 12% 31% 27% 9% 1	780
	11th to 20th	Night	24% 16% 33% 20% 7% 1	800
	21st to end	Night	25% 17% 33% 20% 5% 1	800
December	1st to 10th	Night	27% 17% 34% 18% 4	768
	11th to 20th	Night	25% 17% 32% 22% 3	760
	21st to end	Night	27% 15% 36% 19% 3	836

Page generated at 2024-11-29 10:43:03. Version ac07

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