Dunwoody
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dunwoody clear sky chart from 2005-11-06 to 2025-12-18.

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 4 4 6% 8% 42%	126921
	Night	14% 6% 2 6% 4 4 4 4 5% 7% 44%	124139
Transparency	Day	38% 8% 12% 20% 18% 5%	130707
	Night	36% 7% 11% 19% 20% 7%	128421
Seeing	Day	15% 9% 22% 23% 20% 11%	16341
	Night	15% 8% 23% 29% 21% 4	26365

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% 10% 3 5% 3 2 4 2 4 4 46%	4056
	11th to 20th	Day	12% 7% 3 5% 3 3 3 3 3 6% 52%	4060
	21st to end	Day	23% 9% 3 6% 4 3 2 3 4 5% 40%	4668
February	1st to 10th	Day	15% 8% 3 6% 5% 4 4 4 5% 7% 39%	2000
	11th to 20th	Day	21% 7% 2 5% 5% 3 4 3 5% 6% 40%	1948
	21st to end	Day	20% 6% 2 6% 4 4 4 3 5% 6% 40%	1650
March	1st to 10th	Day	22% 10% 2 7% 4 4 4 3 5% 5% 34%	4613
	11th to 20th	Day	10% 6% 2 6% 4 4 3 3 5% 7% 50%	4980
	21st to end	Day	17% 5% 2 6% 4 4 4 3 4 6% 44%	5465
April	1st to 10th	Day	12% 5% 2 6% 4 4 4 4 6% 7% 46%	2261
	11th to 20th	Day	11% 5% 3 6% 5% 4 3 3 5% 6% 49%	2374
	21st to end	Day	11% 4 2 5% 4 4 3 3 6% 9% 49%	2555
May	1st to 10th	Day	14% 7% 3 7% 4 5% 5% 4 7% 9% 36%	5862
	11th to 20th	Day	18% 7% 3 7% 5% 5% 5% 5% 7% 9% 30%	5861
	21st to end	Day	14% 6% 3 6% 4 4 4 3 6% 7% 41%	6433
June	1st to 10th	Day	10% 6% 2 7% 6% 5% 5% 4 7% 11% 36%	2600
	11th to 20th	Day	6% 4 1 4 4 5% 6% 5% 9% 11% 44%	2600
	21st to end	Day	7% 5% 2 7% 5% 5% 6% 6% 7% 10% 41%	2600
July	1st to 10th	Day	7% 5% 2 7% 5% 7% 6% 5% 9% 12% 35%	5900
	11th to 20th	Day	6% 4 2 6% 5% 5% 6% 6% 8% 12% 39%	5900
	21st to end	Day	10% 8% 3 8% 6% 6% 7% 6% 9% 11% 26%	6424
August	1st to 10th	Day	8% 5% 2 6% 5% 6% 6% 5% 8% 12% 38%	4234
	11th to 20th	Day	8% 5% 2 6% 5% 5% 5% 4 6% 9% 45%	3892
	21st to end	Day	8% 4 2 5% 4 4 5% 4 6% 9% 49%	4312
September	1st to 10th	Day	9% 7% 2 6% 5% 5% 5% 5% 6% 9% 41%	2210
	11th to 20th	Day	15% 7% 2 5% 3 4 3 5% 6% 8% 42%	2200
	21st to end	Day	13% 5% 2 5% 4 3 4 3 5% 7% 48%	2200
October	1st to 10th	Day	13% 5% 1 4 2 3 4 3 5% 5% 55%	2200
	11th to 20th	Day	11% 4 2 4 3 3 3 3 5% 6% 57%	2094
	21st to end	Day	13% 6% 2 5% 3 3 3 2 4 5% 55%	2200
November	1st to 10th	Day	13% 6% 2 4 3 3 3 3 4 5% 55%	1867
	11th to 20th	Day	12% 6% 1 4 3 4 3 3 4 6% 53%	1890
	21st to end	Day	14% 6% 2 5% 3 4 4 3 6% 6% 47%	1890
December	1st to 10th	Day	18% 7% 2 6% 3 2 4 3 4 7% 44%	3020
	11th to 20th	Day	15% 9% 3 7% 5% 4 4 3 4 5% 42%	3436
	21st to end	Day	22% 7% 3 5% 3 3 4 3 5% 6% 39%	4466
January	1st to 10th	Night	16% 6% 2 5% 3 3 4 3 4 6% 48%	5889
	11th to 20th	Night	14% 7% 3 6% 3 3 4 3 4 5% 48%	5900
	21st to end	Night	19% 9% 2 6% 4 3 3 3 4 5% 43%	6283
February	1st to 10th	Night	17% 7% 2 5% 4 4 4 3 5% 6% 44%	2400
	11th to 20th	Night	22% 8% 3 6% 3 4 3 3 4 5% 42%	2338
	21st to end	Night	20% 6% 2 6% 4 3 3 4 5% 6% 41%	1980
March	1st to 10th	Night	21% 10% 2 6% 4 4 4 3 5% 6% 36%	5240
	11th to 20th	Night	10% 5% 2 5% 3 3 4 4 6% 7% 52%	4980
	21st to end	Night	14% 6% 2 6% 5% 4 4 3 5% 6% 45%	5467
April	1st to 10th	Night	12% 5% 2 4 4 4 5% 4 6% 7% 47%	2154
	11th to 20th	Night	13% 5% 1 4 3 3 4 3 5% 7% 51%	1975
	21st to end	Night	9% 6% 2 6% 4 3 3 4 6% 8% 51%	1845
May	1st to 10th	Night	11% 7% 3 7% 6% 4 5% 4 6% 9% 37%	4033
	11th to 20th	Night	20% 7% 3 7% 5% 5% 5% 4 6% 8% 31%	4032
	21st to end	Night	12% 5% 2 6% 4 4 4 4 6% 9% 43%	4421
June	1st to 10th	Night	7% 5% 2 6% 5% 5% 7% 6% 8% 12% 36%	1800
	11th to 20th	Night	5% 4 1 5% 6% 4 5% 5% 9% 12% 43%	1800
	21st to end	Night	6% 4 2 6% 5% 4 5% 5% 8% 12% 43%	1800
July	1st to 10th	Night	8% 6% 3 8% 6% 5% 6% 5% 8% 11% 34%	4058
	11th to 20th	Night	5% 5% 2 6% 5% 6% 7% 6% 9% 12% 37%	4057
	21st to end	Night	11% 8% 3 8% 6% 6% 7% 5% 8% 10% 29%	4427
August	1st to 10th	Night	8% 5% 3 6% 5% 5% 5% 5% 9% 13% 37%	2915
	11th to 20th	Night	9% 5% 3 7% 5% 4 4 4 6% 8% 46%	3152
	21st to end	Night	8% 5% 2 5% 4 4 5% 4 6% 8% 50%	3586
September	1st to 10th	Night	9% 5% 3 5% 5% 5% 6% 4 7% 9% 41%	2170
	11th to 20th	Night	14% 5% 2 5% 4 5% 4 4 6% 7% 44%	2200
	21st to end	Night	13% 5% 2 6% 4 4 4 4 5% 7% 46%	2200
October	1st to 10th	Night	11% 4 2 5% 3 3 4 3 5% 6% 54%	2200
	11th to 20th	Night	10% 4 2 4 3 3 3 3 4 5% 60%	2305
	21st to end	Night	13% 5% 1 5% 2 3 3 2 4 5% 57%	2640
November	1st to 10th	Night	13% 5% 2 4 3 3 3 3 4 5% 56%	2648
	11th to 20th	Night	14% 5% 2 4 3 3 3 2 3 6% 55%	2730
	21st to end	Night	15% 6% 2 4 3 4 4 4 5% 6% 48%	2730
December	1st to 10th	Night	15% 7% 2 5% 3 3 3 2 4 5% 50%	4380
	11th to 20th	Night	18% 7% 2 5% 4 3 3 3 4 6% 46%	4914
	21st to end	Night	21% 6% 3 6% 4 3 4 3 4 6% 41%	6490

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	40% 5% 5% 21% 22% 6%	4057
	11th to 20th	Day	33% 5% 7% 21% 28% 7%	4060
	21st to end	Day	47% 3 6% 16% 21% 7%	4668
February	1st to 10th	Day	45% 4 8% 18% 20% 4	2000
	11th to 20th	Day	45% 4 7% 18% 19% 6%	1958
	21st to end	Day	45% 5% 7% 15% 20% 8%	1650
March	1st to 10th	Day	50% 6% 7% 12% 18% 7%	4613
	11th to 20th	Day	33% 5% 7% 14% 24% 16%	4980
	21st to end	Day	41% 5% 8% 22% 20% 4	5465
April	1st to 10th	Day	36% 6% 11% 22% 20% 5%	2261
	11th to 20th	Day	36% 6% 8% 19% 24% 7%	2374
	21st to end	Day	31% 7% 12% 28% 20% 2	2555
May	1st to 10th	Day	41% 9% 16% 20% 13% 1	5897
	11th to 20th	Day	47% 10% 16% 18% 8% 1	5865
	21st to end	Day	39% 8% 12% 23% 17% 1	6447
June	1st to 10th	Day	38% 11% 18% 22% 9% 2	2600
	11th to 20th	Day	28% 11% 21% 26% 12% 1	2600
	21st to end	Day	33% 12% 17% 24% 12% 3	2600
July	1st to 10th	Day	36% 14% 21% 23% 6% 1	5835
	11th to 20th	Day	33% 14% 29% 19% 4 1	5831
	21st to end	Day	47% 14% 18% 17% 3	6240
August	1st to 10th	Day	35% 11% 16% 31% 8%	4221
	11th to 20th	Day	33% 9% 17% 26% 14% 2	3929
	21st to end	Day	30% 9% 15% 24% 20% 3	4311
September	1st to 10th	Day	37% 10% 16% 16% 18% 4	2210
	11th to 20th	Day	38% 9% 11% 19% 21% 1	2200
	21st to end	Day	36% 6% 13% 18% 22% 6%	2200
October	1st to 10th	Day	25% 4 6% 13% 38% 13%	3590
	11th to 20th	Day	19% 3 5% 16% 30% 27%	3386
	21st to end	Day	27% 4 6% 22% 29% 12%	3538
November	1st to 10th	Day	33% 4 8% 15% 30% 10%	1867
	11th to 20th	Day	33% 4 7% 19% 28% 9%	1890
	21st to end	Day	36% 6% 7% 19% 25% 6%	1890
December	1st to 10th	Day	41% 5% 5% 18% 26% 6%	3020
	11th to 20th	Day	46% 4 8% 17% 19% 6%	3433
	21st to end	Day	42% 6% 8% 25% 16% 2	4466
January	1st to 10th	Night	35% 6% 6% 20% 28% 6%	5889
	11th to 20th	Night	37% 5% 6% 19% 26% 8%	5900
	21st to end	Night	43% 4 5% 15% 27% 6%	6283
February	1st to 10th	Night	41% 5% 9% 18% 20% 8%	2400
	11th to 20th	Night	46% 4 6% 17% 20% 8%	2339
	21st to end	Night	43% 5% 7% 16% 18% 10%	1980
March	1st to 10th	Night	47% 6% 8% 15% 18% 7%	5240
	11th to 20th	Night	29% 6% 8% 15% 26% 17%	4980
	21st to end	Night	38% 5% 7% 21% 19% 9%	5467
April	1st to 10th	Night	34% 6% 10% 19% 23% 7%	2154
	11th to 20th	Night	32% 5% 9% 20% 25% 10%	1975
	21st to end	Night	31% 7% 11% 26% 21% 5%	1845
May	1st to 10th	Night	40% 11% 17% 19% 12% 2	4060
	11th to 20th	Night	48% 8% 16% 19% 9% 1	4033
	21st to end	Night	34% 9% 14% 23% 17% 2	4429
June	1st to 10th	Night	35% 14% 18% 19% 13% 2	1800
	11th to 20th	Night	29% 10% 22% 25% 13% 1	1800
	21st to end	Night	30% 12% 19% 21% 14% 4	1800
July	1st to 10th	Night	41% 11% 19% 23% 5% 1	4015
	11th to 20th	Night	34% 16% 28% 17% 5% 1	4006
	21st to end	Night	48% 12% 19% 16% 4 1	4297
August	1st to 10th	Night	35% 11% 17% 29% 7% 1	2904
	11th to 20th	Night	34% 9% 15% 26% 15% 1	3181
	21st to end	Night	30% 9% 13% 21% 25% 3	3586
September	1st to 10th	Night	36% 10% 16% 17% 17% 4	2170
	11th to 20th	Night	37% 9% 12% 21% 19% 3	2200
	21st to end	Night	35% 8% 13% 17% 20% 8%	2200
October	1st to 10th	Night	25% 5% 9% 13% 36% 11%	3590
	11th to 20th	Night	17% 3 4 17% 33% 25%	3752
	21st to end	Night	23% 3 5% 22% 30% 17%	4252
November	1st to 10th	Night	31% 5% 8% 17% 27% 12%	2648
	11th to 20th	Night	32% 4 5% 18% 29% 12%	2730
	21st to end	Night	35% 5% 7% 19% 25% 8%	2730
December	1st to 10th	Night	38% 4 6% 18% 27% 7%	4380
	11th to 20th	Night	39% 5% 11% 15% 24% 6%	4916
	21st to end	Night	43% 5% 6% 27% 16% 2	6490

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% 21% 38% 15% 1	300
	11th to 20th	Day	22% 22% 31% 18% 6% 2	300
	21st to end	Day	22% 22% 35% 15% 4 1	330
February	1st to 10th	Day	19% 18% 37% 22% 4	300
	11th to 20th	Day	36% 18% 29% 15% 2	300
	21st to end	Day	27% 17% 38% 15% 3	249
March	1st to 10th	Day	24% 18% 30% 21% 7% 1	355
	11th to 20th	Day	19% 16% 33% 23% 7%	500
	21st to end	Day	18% 15% 35% 24% 7% 1	550
April	1st to 10th	Day	17% 16% 31% 24% 12% 1	500
	11th to 20th	Day	12% 13% 32% 26% 16% 1	492
	21st to end	Day	15% 9% 22% 31% 19% 4	500
May	1st to 10th	Day	12% 9% 29% 26% 17% 5%	503
	11th to 20th	Day	13% 5% 23% 27% 20% 12%	527
	21st to end	Day	15% 5% 15% 25% 26% 13%	573
June	1st to 10th	Day	12% 3 15% 24% 31% 15%	530
	11th to 20th	Day	6% 3 14% 25% 24% 28%	530
	21st to end	Day	9% 2 11% 27% 29% 22%	530
July	1st to 10th	Day	9% 2 4 23% 33% 30%	525
	11th to 20th	Day	9% 1 8% 16% 32% 34%	524
	21st to end	Day	8% 2 7% 19% 37% 27%	576
August	1st to 10th	Day	13% 3 11% 22% 28% 23%	526
	11th to 20th	Day	9% 3 10% 21% 36% 22%	519
	21st to end	Day	13% 2 10% 25% 27% 24%	583
September	1st to 10th	Day	9% 5% 12% 26% 30% 18%	530
	11th to 20th	Day	17% 4 11% 21% 34% 13%	530
	21st to end	Day	15% 3 17% 23% 32% 10%	530
October	1st to 10th	Day	15% 4 16% 35% 22% 8%	530
	11th to 20th	Day	10% 9% 24% 24% 27% 5%	530
	21st to end	Day	18% 8% 21% 30% 20% 4	583
November	1st to 10th	Day	21% 8% 27% 22% 18% 4	345
	11th to 20th	Day	15% 14% 37% 22% 9% 2	330
	21st to end	Day	11% 17% 37% 25% 9% 1	330
December	1st to 10th	Day	28% 17% 34% 15% 5% 1	330
	11th to 20th	Day	21% 16% 31% 21% 9% 1	321
	21st to end	Day	30% 13% 31% 18% 6% 1	330
January	1st to 10th	Night	24% 17% 32% 21% 6% 1	800
	11th to 20th	Night	20% 15% 34% 24% 8%	800
	21st to end	Night	20% 15% 38% 25% 3	880
February	1st to 10th	Night	19% 13% 31% 32% 6%	800
	11th to 20th	Night	25% 14% 34% 25% 2	783
	21st to end	Night	22% 14% 35% 25% 3	660
March	1st to 10th	Night	17% 12% 35% 31% 5%	800
	11th to 20th	Night	17% 11% 36% 26% 10% 1	800
	21st to end	Night	17% 12% 29% 32% 9%	880
April	1st to 10th	Night	14% 13% 30% 31% 12% 1	754
	11th to 20th	Night	15% 9% 28% 34% 14% 1	593
	21st to end	Night	11% 7% 26% 36% 20% 2	600
May	1st to 10th	Night	11% 8% 23% 35% 23% 1	600
	11th to 20th	Night	14% 3 22% 33% 24% 4	591
	21st to end	Night	11% 2 13% 34% 33% 7%	650
June	1st to 10th	Night	10% 1 10% 39% 34% 6%	600
	11th to 20th	Night	6% 2 12% 30% 39% 13%	600
	21st to end	Night	6% 3 8% 32% 41% 10%	600
July	1st to 10th	Night	9% 3 6% 23% 46% 14%	585
	11th to 20th	Night	8% 1 7% 16% 52% 16%	582
	21st to end	Night	8% 1 7% 24% 49% 11%	657
August	1st to 10th	Night	11% 2 7% 25% 44% 10%	597
	11th to 20th	Night	10% 2 9% 27% 43% 9%	594
	21st to end	Night	14% 2 8% 26% 40% 10%	660
September	1st to 10th	Night	10% 1 11% 28% 40% 10%	770
	11th to 20th	Night	16% 2 11% 28% 40% 4	800
	21st to end	Night	15% 3 12% 33% 34% 3	800
October	1st to 10th	Night	13% 3 17% 35% 24% 7%	800
	11th to 20th	Night	13% 5% 23% 35% 22% 3	800
	21st to end	Night	14% 8% 23% 35% 19% 3	880
November	1st to 10th	Night	15% 7% 22% 35% 20% 1	820
	11th to 20th	Night	17% 8% 30% 29% 14% 1	840
	21st to end	Night	18% 10% 28% 30% 13%	840
December	1st to 10th	Night	19% 13% 32% 28% 8% 1	840
	11th to 20th	Night	22% 10% 30% 29% 9%	829
	21st to end	Night	27% 11% 26% 29% 6%	880

Page generated at 2025-12-15 01:13:52. Version ac07

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