Fernbank Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Fernbank Observatory clear sky chart from 2005-11-06 to 2026-03-18.

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	13% 6% 2 6% 4 4 5% 4 6% 8% 42%	131307
	Night	13% 6% 2 6% 4 4 4 4 5% 7% 44%	130216
Transparency	Day	39% 6% 12% 20% 18% 5%	135093
	Night	37% 5% 10% 20% 20% 7%	134499
Seeing	Day	16% 9% 21% 23% 20% 12%	16627
	Night	15% 7% 22% 30% 21% 4	26731

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	18% 9% 3 6% 4 3 3 3 3 4 42%	5258
	11th to 20th	Day	10% 7% 2 5% 3 2 2 3 3 5% 58%	5280
	21st to end	Day	24% 9% 3 6% 4 3 3 3 4 5% 36%	6056
February	1st to 10th	Day	14% 7% 3 7% 5% 5% 4 3 5% 6% 40%	2100
	11th to 20th	Day	22% 7% 3 5% 4 4 4 3 4 5% 40%	2048
	21st to end	Day	20% 7% 2 6% 5% 4 4 3 5% 6% 40%	1730
March	1st to 10th	Day	21% 10% 3 7% 4 4 4 3 5% 5% 34%	4717
	11th to 20th	Day	10% 6% 2 6% 4 4 4 3 5% 6% 50%	5057
	21st to end	Day	17% 6% 2 6% 4 4 4 3 4 5% 44%	5465
April	1st to 10th	Day	12% 6% 2 5% 4 5% 4 4 5% 7% 46%	2256
	11th to 20th	Day	10% 6% 2 6% 5% 4 4 3 4 7% 48%	2374
	21st to end	Day	9% 5% 1 5% 4 4 4 4 6% 8% 49%	2551
May	1st to 10th	Day	14% 6% 3 6% 5% 5% 4 4 6% 9% 38%	5862
	11th to 20th	Day	18% 7% 2 7% 5% 5% 6% 5% 7% 9% 30%	5861
	21st to end	Day	14% 6% 2 6% 4 4 5% 4 6% 8% 42%	6433
June	1st to 10th	Day	9% 6% 3 8% 5% 6% 5% 6% 7% 11% 35%	2600
	11th to 20th	Day	5% 4 2 5% 5% 4 6% 5% 9% 12% 44%	2600
	21st to end	Day	6% 4 2 6% 5% 5% 7% 6% 8% 10% 40%	2600
July	1st to 10th	Day	8% 5% 2 7% 5% 6% 7% 6% 9% 12% 33%	5900
	11th to 20th	Day	6% 4 2 6% 5% 6% 7% 5% 9% 11% 39%	5900
	21st to end	Day	10% 7% 3 8% 6% 7% 7% 6% 9% 11% 27%	6424
August	1st to 10th	Day	8% 6% 2 6% 5% 5% 6% 5% 8% 11% 38%	4234
	11th to 20th	Day	8% 4 2 6% 4 5% 5% 4 6% 10% 46%	3888
	21st to end	Day	8% 5% 1 5% 4 4 5% 4 6% 9% 50%	4312
September	1st to 10th	Day	10% 6% 2 6% 6% 5% 5% 5% 6% 9% 42%	2206
	11th to 20th	Day	15% 5% 3 6% 4 3 4 4 7% 8% 42%	2200
	21st to end	Day	13% 5% 2 6% 4 4 4 3 5% 6% 49%	2200
October	1st to 10th	Day	12% 5% 2 4 3 4 3 3 4 6% 55%	2200
	11th to 20th	Day	10% 4 1 4 3 2 3 3 5% 7% 56%	2095
	21st to end	Day	14% 5% 1 4 4 3 3 3 3 5% 55%	2200
November	1st to 10th	Day	13% 5% 2 5% 3 3 3 3 4 5% 54%	1872
	11th to 20th	Day	13% 5% 2 3 3 3 3 3 5% 5% 53%	1890
	21st to end	Day	12% 6% 2 5% 4 4 5% 3 5% 6% 48%	1890
December	1st to 10th	Day	18% 6% 2 5% 3 3 3 3 5% 7% 44%	3020
	11th to 20th	Day	15% 8% 4 7% 5% 4 4 3 4 5% 42%	3463
	21st to end	Day	22% 7% 2 5% 3 3 3 3 5% 6% 40%	4565
January	1st to 10th	Night	16% 6% 3 6% 4 4 4 3 5% 6% 45%	7635
	11th to 20th	Night	13% 7% 3 6% 3 3 3 2 5% 5% 50%	7680
	21st to end	Night	21% 10% 2 6% 3 3 3 3 3 4 41%	8120
February	1st to 10th	Night	15% 7% 2 6% 4 4 4 3 4 6% 44%	2520
	11th to 20th	Night	21% 8% 3 5% 4 4 3 3 5% 5% 41%	2458
	21st to end	Night	19% 7% 3 5% 5% 4 3 3 4 6% 42%	2076
March	1st to 10th	Night	21% 9% 2 6% 4 4 4 4 4 6% 36%	5355
	11th to 20th	Night	11% 5% 2 4 3 3 3 3 5% 7% 53%	5058
	21st to end	Night	14% 6% 2 6% 4 4 4 4 5% 6% 46%	5467
April	1st to 10th	Night	12% 5% 2 5% 4 4 5% 4 6% 7% 47%	2159
	11th to 20th	Night	13% 5% 2 4 3 3 4 3 5% 7% 50%	1975
	21st to end	Night	9% 5% 2 5% 4 3 4 3 5% 8% 52%	1846
May	1st to 10th	Night	10% 7% 3 8% 6% 5% 5% 4 7% 9% 37%	4033
	11th to 20th	Night	19% 7% 2 7% 4 5% 5% 3 6% 8% 32%	4032
	21st to end	Night	12% 5% 2 6% 3 4 5% 4 6% 10% 43%	4421
June	1st to 10th	Night	7% 6% 3 6% 5% 5% 5% 5% 9% 12% 37%	1800
	11th to 20th	Night	4 3 1 5% 5% 6% 6% 4 9% 13% 44%	1800
	21st to end	Night	5% 4 2 5% 4 5% 7% 5% 9% 12% 43%	1800
July	1st to 10th	Night	8% 6% 2 8% 6% 6% 7% 5% 8% 12% 34%	4058
	11th to 20th	Night	5% 4 2 7% 5% 6% 6% 6% 10% 14% 36%	4057
	21st to end	Night	10% 7% 3 9% 5% 6% 6% 6% 8% 11% 29%	4427
August	1st to 10th	Night	8% 4 2 7% 5% 5% 6% 5% 9% 13% 35%	2915
	11th to 20th	Night	8% 5% 2 6% 4 4 5% 5% 6% 9% 47%	3157
	21st to end	Night	8% 5% 2 5% 4 4 4 4 5% 8% 51%	3586
September	1st to 10th	Night	8% 5% 2 6% 4 5% 6% 6% 7% 10% 42%	2171
	11th to 20th	Night	13% 6% 2 6% 4 4 5% 3 6% 7% 44%	2200
	21st to end	Night	12% 5% 2 6% 5% 4 4 3 5% 8% 47%	2200
October	1st to 10th	Night	11% 5% 2 6% 2 3 4 4 4 6% 53%	2200
	11th to 20th	Night	9% 5% 2 4 3 2 3 3 4 5% 59%	2304
	21st to end	Night	13% 4 1 4 3 3 3 3 4 5% 58%	2640
November	1st to 10th	Night	13% 5% 2 5% 3 4 2 2 4 5% 56%	2641
	11th to 20th	Night	13% 5% 2 5% 3 3 2 2 4 6% 54%	2730
	21st to end	Night	14% 6% 2 5% 4 4 4 3 5% 6% 48%	2730
December	1st to 10th	Night	15% 6% 2 5% 3 3 3 2 4 5% 51%	4380
	11th to 20th	Night	17% 6% 2 5% 3 3 3 3 5% 6% 47%	4952
	21st to end	Night	21% 7% 2 5% 4 4 3 3 4 5% 41%	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	45% 3 7% 22% 18% 5%	5259
	11th to 20th	Day	30% 2 5% 21% 31% 11%	5280
	21st to end	Day	52% 2 4 15% 19% 7%	6056
February	1st to 10th	Day	44% 4 8% 18% 22% 5%	2100
	11th to 20th	Day	47% 3 6% 19% 20% 6%	2058
	21st to end	Day	46% 3 7% 15% 21% 8%	1730
March	1st to 10th	Day	52% 3 9% 12% 19% 6%	4717
	11th to 20th	Day	34% 4 8% 15% 23% 16%	5057
	21st to end	Day	43% 3 7% 22% 22% 4	5465
April	1st to 10th	Day	37% 4 10% 24% 20% 5%	2256
	11th to 20th	Day	36% 4 7% 21% 25% 7%	2374
	21st to end	Day	32% 5% 11% 27% 22% 2	2551
May	1st to 10th	Day	42% 6% 16% 21% 14% 1	5897
	11th to 20th	Day	48% 8% 16% 20% 8% 1	5865
	21st to end	Day	39% 6% 12% 24% 18% 1	6447
June	1st to 10th	Day	41% 7% 17% 23% 10% 2	2600
	11th to 20th	Day	28% 8% 22% 26% 15% 1	2600
	21st to end	Day	34% 9% 18% 23% 13% 3	2600
July	1st to 10th	Day	38% 13% 20% 22% 6% 1	5835
	11th to 20th	Day	33% 14% 28% 20% 4 1	5831
	21st to end	Day	46% 13% 19% 18% 3	6240
August	1st to 10th	Day	36% 8% 17% 31% 7%	4221
	11th to 20th	Day	31% 8% 18% 27% 14% 2	3925
	21st to end	Day	29% 7% 17% 23% 21% 3	4311
September	1st to 10th	Day	38% 7% 16% 18% 17% 4	2206
	11th to 20th	Day	39% 7% 11% 20% 22% 1	2200
	21st to end	Day	36% 5% 11% 20% 22% 6%	2200
October	1st to 10th	Day	25% 4 7% 14% 37% 13%	3590
	11th to 20th	Day	19% 3 5% 17% 30% 27%	3387
	21st to end	Day	27% 3 6% 20% 32% 12%	3538
November	1st to 10th	Day	33% 4 7% 15% 31% 10%	1872
	11th to 20th	Day	33% 3 7% 20% 27% 10%	1890
	21st to end	Day	37% 4 8% 19% 26% 7%	1890
December	1st to 10th	Day	41% 3 5% 18% 26% 7%	3020
	11th to 20th	Day	45% 3 8% 18% 20% 6%	3460
	21st to end	Day	44% 3 8% 26% 18% 2	4565
January	1st to 10th	Night	39% 4 8% 22% 22% 5%	7636
	11th to 20th	Night	36% 3 5% 19% 28% 9%	7680
	21st to end	Night	48% 2 4 15% 24% 8%	8120
February	1st to 10th	Night	41% 3 8% 19% 20% 8%	2520
	11th to 20th	Night	46% 3 6% 18% 20% 8%	2459
	21st to end	Night	45% 3 7% 17% 18% 11%	2076
March	1st to 10th	Night	48% 4 8% 15% 18% 8%	5355
	11th to 20th	Night	31% 4 8% 15% 25% 18%	5058
	21st to end	Night	39% 4 6% 21% 21% 9%	5467
April	1st to 10th	Night	35% 4 10% 20% 23% 8%	2159
	11th to 20th	Night	34% 2 7% 21% 25% 10%	1975
	21st to end	Night	31% 5% 11% 27% 21% 6%	1846
May	1st to 10th	Night	41% 6% 17% 20% 13% 2	4060
	11th to 20th	Night	49% 6% 14% 21% 9% 1	4033
	21st to end	Night	35% 5% 15% 24% 18% 2	4429
June	1st to 10th	Night	36% 10% 18% 21% 13% 2	1800
	11th to 20th	Night	29% 8% 21% 28% 13% 1	1800
	21st to end	Night	30% 11% 21% 19% 15% 4	1800
July	1st to 10th	Night	40% 12% 19% 22% 5% 1	4015
	11th to 20th	Night	33% 15% 29% 17% 5% 1	4006
	21st to end	Night	47% 12% 21% 16% 4 1	4297
August	1st to 10th	Night	36% 9% 18% 29% 7% 1	2904
	11th to 20th	Night	32% 9% 16% 27% 15% 1	3186
	21st to end	Night	30% 7% 15% 21% 24% 3	3586
September	1st to 10th	Night	35% 8% 17% 18% 17% 4	2171
	11th to 20th	Night	37% 7% 12% 22% 19% 3	2200
	21st to end	Night	35% 6% 13% 18% 20% 8%	2200
October	1st to 10th	Night	26% 5% 8% 13% 35% 13%	3590
	11th to 20th	Night	17% 2 4 18% 32% 27%	3751
	21st to end	Night	23% 3 4 22% 31% 17%	4252
November	1st to 10th	Night	32% 3 8% 17% 28% 12%	2641
	11th to 20th	Night	32% 3 5% 19% 29% 12%	2730
	21st to end	Night	36% 3 7% 19% 26% 8%	2730
December	1st to 10th	Night	38% 3 6% 19% 28% 8%	4380
	11th to 20th	Night	39% 4 11% 15% 25% 6%	4954
	21st to end	Night	45% 3 5% 27% 18% 2	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	25% 20% 36% 17% 2	330
	11th to 20th	Day	26% 21% 29% 17% 5% 2	330
	21st to end	Day	25% 21% 35% 15% 4 1	363
February	1st to 10th	Day	19% 19% 36% 20% 5%	330
	11th to 20th	Day	37% 16% 32% 13% 2	330
	21st to end	Day	30% 13% 38% 15% 4	273
March	1st to 10th	Day	23% 17% 29% 23% 7% 1	389
	11th to 20th	Day	22% 16% 31% 24% 7% 1	528
	21st to end	Day	20% 14% 34% 24% 8% 1	550
April	1st to 10th	Day	18% 15% 30% 21% 14% 2	500
	11th to 20th	Day	12% 13% 32% 25% 16% 2	492
	21st to end	Day	15% 8% 23% 28% 21% 5%	500
May	1st to 10th	Day	12% 10% 27% 26% 19% 7%	503
	11th to 20th	Day	13% 5% 20% 25% 22% 14%	527
	21st to end	Day	14% 5% 15% 24% 24% 17%	573
June	1st to 10th	Day	11% 4 13% 25% 30% 17%	530
	11th to 20th	Day	7% 2 13% 26% 22% 30%	530
	21st to end	Day	10% 2 9% 26% 30% 23%	530
July	1st to 10th	Day	7% 2 5% 20% 35% 31%	525
	11th to 20th	Day	7% 2 4 17% 32% 37%	524
	21st to end	Day	6% 2 7% 19% 40% 28%	576
August	1st to 10th	Day	13% 4 11% 21% 26% 25%	526
	11th to 20th	Day	10% 3 8% 21% 33% 25%	519
	21st to end	Day	16% 2 7% 24% 28% 23%	583
September	1st to 10th	Day	9% 4 14% 25% 30% 19%	530
	11th to 20th	Day	15% 5% 10% 21% 35% 15%	530
	21st to end	Day	15% 3 16% 24% 31% 12%	530
October	1st to 10th	Day	16% 3 19% 33% 20% 9%	530
	11th to 20th	Day	11% 9% 22% 26% 25% 7%	530
	21st to end	Day	18% 8% 21% 28% 20% 4	583
November	1st to 10th	Day	21% 9% 24% 24% 18% 4	350
	11th to 20th	Day	16% 14% 36% 23% 10% 2	330
	21st to end	Day	15% 15% 31% 27% 9% 2	330
December	1st to 10th	Day	29% 16% 33% 14% 7%	330
	11th to 20th	Day	23% 15% 28% 25% 7% 1	330
	21st to end	Day	29% 14% 31% 20% 5% 1	363
January	1st to 10th	Night	23% 15% 32% 24% 5%	840
	11th to 20th	Night	20% 15% 32% 26% 8%	840
	21st to end	Night	21% 14% 36% 25% 3	924
February	1st to 10th	Night	19% 12% 31% 33% 6%	840
	11th to 20th	Night	25% 12% 36% 25% 2	823
	21st to end	Night	22% 14% 35% 25% 3	692
March	1st to 10th	Night	18% 13% 31% 33% 6%	840
	11th to 20th	Night	18% 11% 34% 27% 10%	829
	21st to end	Night	17% 12% 29% 31% 10%	880
April	1st to 10th	Night	15% 11% 30% 32% 12% 1	759
	11th to 20th	Night	16% 8% 29% 31% 15% 1	593
	21st to end	Night	11% 6% 26% 35% 21% 2	600
May	1st to 10th	Night	10% 7% 23% 36% 23% 1	600
	11th to 20th	Night	13% 4 21% 33% 25% 4	591
	21st to end	Night	12% 2 12% 34% 32% 8%	650
June	1st to 10th	Night	10% 1 9% 38% 35% 7%	600
	11th to 20th	Night	5% 2 12% 29% 38% 14%	600
	21st to end	Night	6% 2 7% 32% 44% 10%	600
July	1st to 10th	Night	10% 2 7% 21% 46% 14%	585
	11th to 20th	Night	7% 2 4 19% 52% 16%	582
	21st to end	Night	6% 2 5% 28% 46% 13%	657
August	1st to 10th	Night	9% 2 8% 26% 43% 12%	597
	11th to 20th	Night	11% 1 7% 26% 44% 9%	594
	21st to end	Night	14% 3 7% 27% 41% 10%	660
September	1st to 10th	Night	9% 2 10% 28% 39% 12%	771
	11th to 20th	Night	15% 2 10% 27% 43% 4	800
	21st to end	Night	16% 3 12% 32% 35% 4	800
October	1st to 10th	Night	13% 3 17% 35% 26% 7%	800
	11th to 20th	Night	12% 6% 23% 34% 22% 3	800
	21st to end	Night	15% 6% 22% 35% 20% 3	880
November	1st to 10th	Night	16% 6% 22% 34% 21% 1	820
	11th to 20th	Night	16% 9% 31% 30% 14% 1	840
	21st to end	Night	17% 10% 28% 32% 13% 1	840
December	1st to 10th	Night	19% 11% 30% 31% 9% 1	840
	11th to 20th	Night	22% 8% 29% 30% 10%	840
	21st to end	Night	26% 10% 27% 30% 7%	924

Page generated at 2026-03-15 02:23:55. Version ac07

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