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 2024-01-15.

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% 3 5% 5% 3 5% 5% 6% 8% 42%	74887
	Night	13% 6% 2 5% 5% 3 4 5% 5% 8% 45%	74907
Transparency	Day	39% 5% 12% 20% 18% 4	74610
	Night	38% 5% 11% 20% 20% 6%	74703
Seeing	Day	15% 8% 20% 22% 21% 13%	13762
	Night	15% 7% 22% 30% 22% 4	23855

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	17% 6% 3 5% 4 3 4 4 5% 6% 43%	1706
	11th to 20th	Day	16% 8% 3 4 4 2 3 5% 4 6% 45%	1662
	21st to end	Day	19% 7% 3 6% 5% 2 3 4 5% 5% 42%	1863
February	1st to 10th	Day	16% 7% 3 7% 5% 3 5% 4 5% 7% 38%	1800
	11th to 20th	Day	21% 6% 3 5% 4 3 4 4 4 5% 40%	1748
	21st to end	Day	21% 7% 2 5% 5% 3 4 4 5% 6% 38%	1480
March	1st to 10th	Day	14% 5% 2 6% 5% 3 4 4 4 8% 45%	1850
	11th to 20th	Day	17% 6% 3 6% 5% 3 4 5% 6% 6% 38%	1980
	21st to end	Day	15% 6% 2 5% 6% 3 5% 5% 6% 7% 40%	2178
April	1st to 10th	Day	12% 6% 2 4 5% 3 4 5% 5% 7% 47%	2030
	11th to 20th	Day	12% 6% 3 6% 5% 3 4 4 5% 7% 46%	2134
	21st to end	Day	9% 4 2 4 5% 3 4 5% 5% 9% 50%	2295
May	1st to 10th	Day	9% 5% 2 4 5% 2 3 4 5% 9% 51%	2340
	11th to 20th	Day	11% 6% 2 5% 5% 3 6% 6% 5% 9% 41%	2301
	21st to end	Day	10% 4 3 5% 5% 3 4 5% 6% 9% 45%	2531
June	1st to 10th	Day	9% 6% 3 7% 7% 4 6% 6% 6% 11% 35%	2340
	11th to 20th	Day	5% 4 2 4 6% 3 6% 6% 8% 12% 45%	2340
	21st to end	Day	6% 4 2 6% 6% 3 7% 7% 8% 11% 39%	2340
July	1st to 10th	Day	9% 5% 3 6% 7% 5% 8% 8% 8% 11% 30%	2340
	11th to 20th	Day	9% 5% 3 6% 6% 4 7% 7% 9% 12% 31%	2340
	21st to end	Day	5% 5% 3 6% 7% 4 8% 8% 9% 12% 31%	2547
August	1st to 10th	Day	7% 6% 2 6% 7% 4 7% 9% 10% 12% 30%	2327
	11th to 20th	Day	11% 6% 3 6% 6% 5% 6% 6% 7% 10% 33%	2150
	21st to end	Day	13% 7% 3 6% 6% 4 6% 7% 8% 9% 32%	2376
September	1st to 10th	Day	10% 6% 2 5% 6% 4 5% 6% 6% 9% 41%	1986
	11th to 20th	Day	15% 5% 3 5% 5% 3 4 5% 7% 8% 42%	1980
	21st to end	Day	13% 5% 3 5% 4 3 3 4 4 6% 50%	1980
October	1st to 10th	Day	12% 5% 2 3 4 3 3 3 4 6% 55%	1980
	11th to 20th	Day	11% 4 1 4 4 2 3 4 5% 8% 53%	1886
	21st to end	Day	14% 4 1 4 4 2 4 4 3 5% 54%	1980
November	1st to 10th	Day	12% 4 3 4 3 2 3 3 4 5% 56%	1690
	11th to 20th	Day	13% 5% 2 3 4 2 4 4 5% 5% 52%	1710
	21st to end	Day	13% 6% 3 4 4 3 5% 4 5% 7% 46%	1710
December	1st to 10th	Day	19% 7% 3 6% 4 3 5% 5% 5% 7% 37%	1710
	11th to 20th	Day	16% 7% 4 6% 5% 3 3 4 5% 5% 43%	2153
	21st to end	Day	22% 7% 3 5% 5% 2 4 4 5% 7% 35%	3124
January	1st to 10th	Night	18% 7% 3 5% 4 2 4 3 4 6% 44%	2459
	11th to 20th	Night	16% 8% 3 4 5% 3 4 4 5% 6% 42%	2404
	21st to end	Night	17% 7% 3 5% 4 2 3 3 4 5% 47%	2488
February	1st to 10th	Night	17% 6% 3 5% 5% 2 4 4 4 7% 43%	2160
	11th to 20th	Night	20% 7% 3 4 4 3 3 4 4 5% 43%	2098
	21st to end	Night	21% 6% 3 5% 5% 3 3 4 5% 6% 40%	1776
March	1st to 10th	Night	13% 7% 2 5% 4 3 4 4 4 6% 49%	2109
	11th to 20th	Night	15% 6% 2 5% 5% 2 4 4 5% 7% 44%	1980
	21st to end	Night	12% 6% 2 5% 4 3 5% 5% 5% 7% 46%	2178
April	1st to 10th	Night	11% 5% 2 4 4 3 5% 5% 5% 7% 48%	1944
	11th to 20th	Night	14% 5% 2 4 4 2 4 4 5% 8% 50%	1775
	21st to end	Night	8% 4 2 4 4 2 4 4 5% 8% 54%	1662
May	1st to 10th	Night	8% 4 2 5% 4 3 5% 4 6% 7% 51%	1620
	11th to 20th	Night	9% 5% 2 5% 6% 3 6% 5% 6% 8% 43%	1592
	21st to end	Night	9% 5% 2 6% 5% 3 5% 6% 5% 8% 46%	1744
June	1st to 10th	Night	7% 5% 3 5% 5% 4 6% 6% 10% 13% 37%	1620
	11th to 20th	Night	4 3 2 5% 5% 4 6% 5% 8% 13% 46%	1620
	21st to end	Night	5% 4 2 4 5% 4 7% 7% 8% 12% 42%	1620
July	1st to 10th	Night	7% 5% 3 7% 5% 4 7% 6% 8% 13% 35%	1620
	11th to 20th	Night	6% 5% 3 6% 7% 4 6% 7% 8% 14% 34%	1620
	21st to end	Night	5% 5% 3 6% 6% 5% 7% 9% 8% 12% 34%	1771
August	1st to 10th	Night	7% 4 3 7% 7% 4 7% 7% 9% 12% 34%	1611
	11th to 20th	Night	10% 5% 2 6% 5% 3 5% 8% 8% 10% 39%	1747
	21st to end	Night	12% 7% 4 5% 7% 4 5% 6% 6% 10% 35%	1980
September	1st to 10th	Night	8% 5% 2 6% 4 4 6% 7% 6% 10% 43%	1953
	11th to 20th	Night	12% 6% 2 5% 5% 3 5% 4 6% 7% 44%	1980
	21st to end	Night	12% 5% 2 5% 5% 3 4 4 5% 7% 49%	1980
October	1st to 10th	Night	11% 5% 2 4 3 3 4 4 4 6% 54%	1980
	11th to 20th	Night	10% 5% 2 4 4 2 2 4 4 6% 56%	2073
	21st to end	Night	12% 4 1 4 4 2 3 4 4 5% 57%	2376
November	1st to 10th	Night	13% 4 1 4 3 3 2 3 4 5% 57%	2383
	11th to 20th	Night	13% 5% 2 4 4 2 3 3 4 6% 54%	2470
	21st to end	Night	15% 6% 2 4 4 3 4 4 5% 6% 47%	2470
December	1st to 10th	Night	17% 6% 2 6% 4 3 4 4 5% 6% 41%	2470
	11th to 20th	Night	17% 8% 3 4 3 2 3 3 4 5% 48%	3042
	21st to end	Night	19% 9% 3 6% 6% 3 4 4 5% 7% 34%	4532

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% 3 10% 18% 22% 4	1707
	11th to 20th	Day	41% 3 8% 21% 23% 4	1662
	21st to end	Day	44% 3 6% 20% 21% 6%	1863
February	1st to 10th	Day	46% 4 8% 17% 20% 4	1800
	11th to 20th	Day	46% 2 7% 19% 20% 6%	1758
	21st to end	Day	48% 3 8% 15% 19% 8%	1480
March	1st to 10th	Day	39% 3 9% 18% 22% 9%	1850
	11th to 20th	Day	45% 5% 12% 15% 17% 6%	1980
	21st to end	Day	42% 5% 10% 23% 17% 4	2178
April	1st to 10th	Day	37% 4 10% 24% 20% 6%	2030
	11th to 20th	Day	38% 4 7% 21% 24% 6%	2134
	21st to end	Day	31% 5% 11% 28% 24% 2	2295
May	1st to 10th	Day	31% 4 13% 24% 24% 3	2340
	11th to 20th	Day	39% 7% 16% 23% 13% 2	2305
	21st to end	Day	34% 6% 13% 28% 18% 1	2531
June	1st to 10th	Day	41% 7% 17% 23% 11% 2	2340
	11th to 20th	Day	29% 7% 22% 27% 14% 1	2340
	21st to end	Day	36% 9% 18% 22% 12% 2	2340
July	1st to 10th	Day	42% 10% 21% 18% 8% 2	2275
	11th to 20th	Day	41% 10% 21% 17% 8% 3	2271
	21st to end	Day	39% 10% 23% 21% 7%	2405
August	1st to 10th	Day	41% 11% 21% 22% 5%	2314
	11th to 20th	Day	44% 9% 18% 18% 9% 2	2150
	21st to end	Day	45% 8% 16% 17% 11% 2	2376
September	1st to 10th	Day	39% 7% 15% 18% 17% 4	1986
	11th to 20th	Day	39% 7% 11% 19% 23% 1	1980
	21st to end	Day	36% 5% 8% 21% 23% 7%	1980
October	1st to 10th	Day	32% 4 9% 19% 29% 7%	1980
	11th to 20th	Day	30% 4 8% 25% 25% 8%	1886
	21st to end	Day	34% 2 6% 17% 30% 10%	1980
November	1st to 10th	Day	32% 4 7% 15% 32% 11%	1690
	11th to 20th	Day	34% 3 7% 20% 26% 10%	1710
	21st to end	Day	38% 4 7% 19% 25% 7%	1710
December	1st to 10th	Day	47% 4 7% 19% 17% 6%	1710
	11th to 20th	Day	45% 2 5% 17% 24% 7%	2150
	21st to end	Day	47% 3 8% 20% 19% 3	3124
January	1st to 10th	Night	43% 2 7% 19% 24% 5%	2459
	11th to 20th	Night	44% 3 7% 20% 21% 5%	2404
	21st to end	Night	40% 3 4 20% 26% 7%	2488
February	1st to 10th	Night	43% 3 8% 19% 20% 8%	2160
	11th to 20th	Night	45% 3 5% 18% 21% 8%	2099
	21st to end	Night	46% 3 8% 16% 18% 9%	1776
March	1st to 10th	Night	37% 3 8% 20% 20% 12%	2109
	11th to 20th	Night	40% 4 12% 16% 21% 7%	1980
	21st to end	Night	37% 4 10% 22% 21% 5%	2178
April	1st to 10th	Night	34% 4 10% 21% 23% 8%	1944
	11th to 20th	Night	35% 3 7% 21% 24% 11%	1775
	21st to end	Night	29% 5% 10% 28% 22% 6%	1662
May	1st to 10th	Night	32% 5% 14% 23% 23% 4	1620
	11th to 20th	Night	37% 6% 17% 23% 16% 2	1593
	21st to end	Night	35% 7% 11% 25% 19% 3	1745
June	1st to 10th	Night	36% 10% 18% 21% 13% 2	1620
	11th to 20th	Night	28% 8% 21% 29% 12% 2	1620
	21st to end	Night	30% 11% 22% 20% 14% 3	1620
July	1st to 10th	Night	39% 10% 19% 21% 9% 2	1575
	11th to 20th	Night	35% 12% 25% 16% 10% 3	1566
	21st to end	Night	37% 10% 25% 19% 8% 2	1672
August	1st to 10th	Night	39% 10% 19% 25% 6% 1	1600
	11th to 20th	Night	38% 10% 19% 18% 13% 2	1747
	21st to end	Night	44% 7% 18% 18% 11% 2	1980
September	1st to 10th	Night	35% 8% 17% 18% 18% 4	1953
	11th to 20th	Night	37% 7% 12% 21% 20% 4	1980
	21st to end	Night	34% 6% 12% 18% 21% 9%	1980
October	1st to 10th	Night	31% 5% 10% 19% 27% 8%	1980
	11th to 20th	Night	30% 4 6% 23% 27% 10%	2073
	21st to end	Night	30% 3 5% 18% 31% 13%	2376
November	1st to 10th	Night	30% 3 8% 17% 28% 13%	2383
	11th to 20th	Night	33% 3 5% 18% 28% 13%	2470
	21st to end	Night	37% 3 7% 19% 26% 8%	2470
December	1st to 10th	Night	43% 4 9% 19% 20% 6%	2470
	11th to 20th	Night	39% 2 8% 16% 28% 7%	3044
	21st to end	Night	49% 3 6% 20% 18% 3	4532

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	22% 21% 37% 18% 2	270
	11th to 20th	Day	24% 18% 29% 20% 7% 2	254
	21st to end	Day	22% 22% 35% 15% 5% 1	264
February	1st to 10th	Day	21% 20% 33% 20% 5%	240
	11th to 20th	Day	35% 15% 34% 13% 2	240
	21st to end	Day	30% 14% 38% 15% 3	198
March	1st to 10th	Day	18% 16% 31% 24% 8% 2	290
	11th to 20th	Day	22% 16% 28% 25% 8%	420
	21st to end	Day	19% 14% 33% 24% 9% 1	462
April	1st to 10th	Day	18% 14% 28% 22% 16% 2	420
	11th to 20th	Day	12% 14% 30% 24% 18% 3	412
	21st to end	Day	15% 8% 23% 26% 22% 5%	420
May	1st to 10th	Day	11% 9% 26% 26% 21% 7%	423
	11th to 20th	Day	11% 4 17% 26% 25% 17%	447
	21st to end	Day	14% 4 12% 24% 27% 20%	485
June	1st to 10th	Day	12% 3 11% 24% 31% 19%	450
	11th to 20th	Day	7% 2 10% 25% 22% 34%	450
	21st to end	Day	11% 2 9% 26% 27% 24%	450
July	1st to 10th	Day	8% 2 4 18% 35% 33%	445
	11th to 20th	Day	8% 2 5% 16% 29% 40%	444
	21st to end	Day	5% 1 7% 15% 40% 32%	488
August	1st to 10th	Day	11% 3 11% 20% 27% 28%	446
	11th to 20th	Day	12% 3 6% 20% 33% 26%	439
	21st to end	Day	17% 2 5% 23% 30% 23%	495
September	1st to 10th	Day	9% 4 10% 22% 32% 22%	450
	11th to 20th	Day	14% 4 11% 20% 34% 16%	450
	21st to end	Day	15% 2 14% 22% 33% 12%	450
October	1st to 10th	Day	16% 4 16% 31% 22% 11%	450
	11th to 20th	Day	12% 9% 22% 24% 25% 8%	450
	21st to end	Day	18% 7% 19% 29% 21% 5%	495
November	1st to 10th	Day	20% 7% 25% 25% 19% 5%	288
	11th to 20th	Day	17% 14% 33% 24% 10% 2	270
	21st to end	Day	12% 13% 32% 29% 11% 2	270
December	1st to 10th	Day	28% 14% 33% 16% 9%	270
	11th to 20th	Day	23% 16% 29% 24% 8% 1	270
	21st to end	Day	31% 13% 31% 19% 6% 1	297
January	1st to 10th	Night	23% 14% 34% 24% 5% 1	760
	11th to 20th	Night	19% 14% 31% 27% 9%	740
	21st to end	Night	20% 13% 37% 27% 4	792
February	1st to 10th	Night	20% 10% 32% 32% 7%	720
	11th to 20th	Night	24% 10% 37% 26% 2	703
	21st to end	Night	23% 13% 35% 26% 4	592
March	1st to 10th	Night	16% 13% 33% 33% 5%	720
	11th to 20th	Night	18% 10% 34% 26% 11%	720
	21st to end	Night	17% 11% 28% 32% 11%	792
April	1st to 10th	Night	14% 10% 29% 34% 13% 1	684
	11th to 20th	Night	17% 8% 28% 31% 16% 1	533
	21st to end	Night	11% 5% 26% 34% 22% 2	540
May	1st to 10th	Night	10% 7% 23% 35% 24% 1	540
	11th to 20th	Night	11% 4 19% 34% 27% 5%	531
	21st to end	Night	12% 2 10% 32% 35% 9%	584
June	1st to 10th	Night	9% 1 7% 39% 36% 7%	540
	11th to 20th	Night	4 2 11% 28% 40% 15%	540
	21st to end	Night	6% 1 7% 31% 43% 11%	540
July	1st to 10th	Night	10% 1 6% 22% 47% 15%	525
	11th to 20th	Night	7% 2 4 17% 52% 17%	522
	21st to end	Night	6% 2 4 27% 48% 14%	591
August	1st to 10th	Night	8% 1 7% 24% 47% 14%	537
	11th to 20th	Night	11% 1 6% 26% 46% 9%	534
	21st to end	Night	14% 3 5% 25% 43% 10%	594
September	1st to 10th	Night	9% 2 7% 26% 43% 13%	693
	11th to 20th	Night	15% 1 11% 26% 43% 4	720
	21st to end	Night	16% 2 11% 31% 36% 4	720
October	1st to 10th	Night	13% 3 16% 34% 27% 7%	720
	11th to 20th	Night	13% 5% 24% 33% 22% 3	720
	21st to end	Night	15% 5% 21% 35% 20% 3	792
November	1st to 10th	Night	15% 6% 22% 34% 22% 2	740
	11th to 20th	Night	16% 9% 30% 30% 14% 1	760
	21st to end	Night	17% 8% 27% 33% 14% 1	760
December	1st to 10th	Night	19% 10% 29% 32% 9% 1	760
	11th to 20th	Night	22% 8% 30% 30% 10%	760
	21st to end	Night	27% 10% 27% 29% 7%	836

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