Sweetgum Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sweetgum Observatory clear sky chart from 2005-11-06 to 2026-02-01.

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	12% 6% 2 6% 4 4 5% 4 6% 8% 41%	127267
	Night	13% 7% 2 6% 4 4 4 4 5% 7% 44%	124802
Transparency	Day	39% 7% 12% 20% 18% 4	131043
	Night	37% 6% 11% 20% 19% 6%	129096
Seeing	Day	15% 9% 22% 23% 20% 11%	16265
	Night	15% 7% 22% 31% 21% 4	26319

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% 4 5% 3 3 4 3 3 4 45%	4146
	11th to 20th	Day	13% 7% 4 5% 4 3 3 2 4 5% 50%	4150
	21st to end	Day	22% 9% 2 6% 4 4 3 3 4 6% 37%	5006
February	1st to 10th	Day	18% 9% 3 6% 4 4 3 3 6% 6% 38%	2000
	11th to 20th	Day	22% 9% 3 6% 4 3 4 3 4 5% 38%	1948
	21st to end	Day	19% 8% 2 8% 4 3 4 2 6% 5% 39%	1650
March	1st to 10th	Day	19% 8% 3 7% 5% 5% 5% 4 5% 6% 34%	4480
	11th to 20th	Day	11% 6% 2 7% 4 3 4 4 5% 6% 48%	4925
	21st to end	Day	15% 6% 3 7% 5% 4 4 3 5% 5% 43%	5495
April	1st to 10th	Day	12% 6% 2 5% 5% 4 4 4 5% 7% 46%	2400
	11th to 20th	Day	12% 5% 3 5% 5% 4 3 3 5% 6% 48%	2374
	21st to end	Day	10% 4 2 6% 4 5% 5% 4 6% 7% 47%	2463
May	1st to 10th	Day	14% 7% 3 7% 5% 5% 5% 4 6% 9% 34%	5862
	11th to 20th	Day	15% 7% 3 8% 5% 5% 5% 4 7% 8% 32%	5861
	21st to end	Day	12% 7% 3 7% 5% 4 4 5% 6% 8% 40%	6433
June	1st to 10th	Day	9% 7% 2 7% 6% 5% 6% 4 8% 10% 36%	2600
	11th to 20th	Day	4 4 2 6% 5% 5% 5% 5% 9% 11% 45%	2600
	21st to end	Day	7% 5% 2 6% 6% 5% 7% 5% 8% 10% 39%	2600
July	1st to 10th	Day	6% 4 2 7% 6% 6% 6% 6% 9% 12% 35%	5900
	11th to 20th	Day	5% 5% 2 6% 5% 6% 7% 6% 9% 14% 34%	5900
	21st to end	Day	8% 8% 3 10% 6% 7% 7% 6% 9% 11% 24%	6424
August	1st to 10th	Day	5% 3 1 4 4 5% 6% 6% 9% 13% 45%	4051
	11th to 20th	Day	6% 4 2 6% 5% 5% 5% 4 7% 12% 44%	3849
	21st to end	Day	7% 4 2 5% 4 4 5% 4 7% 8% 51%	4312
September	1st to 10th	Day	9% 5% 3 6% 5% 5% 5% 4 7% 8% 43%	2290
	11th to 20th	Day	13% 6% 2 6% 4 4 5% 4 6% 8% 43%	2200
	21st to end	Day	11% 5% 2 6% 4 3 4 2 6% 7% 49%	2200
October	1st to 10th	Day	11% 4 2 5% 4 4 3 3 4 6% 54%	2200
	11th to 20th	Day	9% 5% 2 5% 3 3 3 3 4 7% 57%	2180
	21st to end	Day	13% 5% 2 4 3 3 4 2 4 5% 54%	2088
November	1st to 10th	Day	14% 5% 2 5% 4 3 3 2 4 6% 53%	1852
	11th to 20th	Day	12% 6% 2 6% 4 3 3 2 4 6% 52%	1890
	21st to end	Day	15% 5% 2 5% 3 4 4 3 6% 7% 45%	1890
December	1st to 10th	Day	18% 8% 2 6% 3 3 3 3 5% 6% 43%	3020
	11th to 20th	Day	20% 9% 3 6% 5% 4 4 3 3 4 39%	3463
	21st to end	Day	20% 8% 3 6% 4 4 4 3 4 7% 37%	4565
January	1st to 10th	Night	16% 9% 3 7% 3 3 3 3 4 5% 43%	6019
	11th to 20th	Night	16% 8% 3 7% 3 3 3 3 5% 5% 44%	6030
	21st to end	Night	19% 10% 2 5% 3 3 3 3 4 5% 43%	6186
February	1st to 10th	Night	17% 8% 3 6% 3 4 3 3 4 6% 43%	2401
	11th to 20th	Night	21% 7% 3 7% 4 2 3 3 4 4 41%	2338
	21st to end	Night	19% 7% 2 7% 4 4 4 3 4 6% 40%	1980
March	1st to 10th	Night	23% 9% 3 7% 4 4 4 3 5% 5% 32%	5373
	11th to 20th	Night	11% 6% 2 5% 4 3 3 4 5% 7% 51%	5033
	21st to end	Night	15% 7% 2 7% 5% 4 4 3 4 5% 43%	5467
April	1st to 10th	Night	11% 5% 2 4 4 4 4 3 7% 9% 48%	2010
	11th to 20th	Night	13% 6% 2 5% 3 2 3 3 4 7% 51%	1975
	21st to end	Night	9% 5% 2 6% 4 4 4 3 4 7% 52%	1935
May	1st to 10th	Night	12% 7% 3 8% 6% 4 6% 4 6% 9% 35%	4033
	11th to 20th	Night	17% 8% 3 8% 5% 5% 5% 4 6% 7% 31%	4032
	21st to end	Night	10% 6% 2 7% 5% 4 5% 5% 6% 9% 41%	4421
June	1st to 10th	Night	7% 5% 2 7% 5% 5% 6% 5% 9% 10% 40%	1800
	11th to 20th	Night	4 3 1 5% 5% 5% 5% 4 7% 11% 51%	1800
	21st to end	Night	5% 4 2 5% 5% 5% 5% 6% 6% 11% 47%	1800
July	1st to 10th	Night	6% 5% 3 8% 5% 6% 6% 5% 8% 12% 37%	4058
	11th to 20th	Night	5% 4 2 5% 5% 5% 6% 6% 9% 14% 40%	4057
	21st to end	Night	7% 7% 3 9% 6% 6% 7% 6% 8% 11% 28%	4427
August	1st to 10th	Night	4 3 2 5% 4 4 5% 4 8% 11% 50%	3093
	11th to 20th	Night	7% 5% 2 6% 4 4 4 4 6% 10% 49%	3200
	21st to end	Night	8% 5% 2 5% 3 3 4 3 5% 7% 54%	3586
September	1st to 10th	Night	7% 5% 3 7% 6% 5% 6% 5% 7% 8% 43%	2088
	11th to 20th	Night	12% 5% 3 6% 5% 4 4 4 6% 8% 44%	2200
	21st to end	Night	9% 6% 2 5% 3 4 4 4 6% 8% 50%	2200
October	1st to 10th	Night	10% 4 2 5% 4 3 3 3 4 6% 58%	2200
	11th to 20th	Night	11% 5% 2 5% 2 3 3 3 4 5% 57%	2220
	21st to end	Night	13% 5% 2 5% 3 3 3 3 4 4 54%	2749
November	1st to 10th	Night	12% 5% 2 5% 3 3 3 3 5% 6% 54%	2666
	11th to 20th	Night	12% 6% 2 4 3 3 3 2 4 6% 55%	2730
	21st to end	Night	14% 7% 2 5% 4 3 4 4 4 6% 46%	2730
December	1st to 10th	Night	17% 8% 3 6% 4 2 3 3 5% 6% 44%	4380
	11th to 20th	Night	19% 9% 3 7% 3 3 3 2 4 5% 42%	4952
	21st to end	Night	20% 7% 3 6% 4 4 4 3 5% 7% 38%	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	44% 4 6% 23% 22% 3	4147
	11th to 20th	Day	37% 3 6% 24% 27% 3	4150
	21st to end	Day	49% 2 7% 16% 20% 5%	5006
February	1st to 10th	Day	47% 5% 8% 19% 19% 3	2000
	11th to 20th	Day	49% 4 5% 19% 18% 5%	1958
	21st to end	Day	47% 5% 7% 14% 21% 6%	1650
March	1st to 10th	Day	50% 6% 8% 13% 17% 7%	4480
	11th to 20th	Day	36% 5% 6% 16% 23% 14%	4925
	21st to end	Day	43% 5% 9% 20% 20% 3	5497
April	1st to 10th	Day	37% 5% 12% 22% 18% 6%	2400
	11th to 20th	Day	36% 5% 8% 19% 25% 7%	2374
	21st to end	Day	32% 8% 13% 24% 20% 3	2463
May	1st to 10th	Day	43% 8% 14% 21% 12% 2	5897
	11th to 20th	Day	47% 8% 16% 20% 8%	5865
	21st to end	Day	40% 8% 11% 23% 17% 2	6447
June	1st to 10th	Day	40% 11% 17% 19% 12% 2	2600
	11th to 20th	Day	30% 11% 19% 24% 15% 1	2600
	21st to end	Day	35% 12% 17% 22% 12% 2	2600
July	1st to 10th	Day	36% 13% 20% 22% 9% 1	5835
	11th to 20th	Day	35% 13% 30% 17% 5% 1	5831
	21st to end	Day	48% 13% 20% 15% 4	6240
August	1st to 10th	Day	25% 10% 16% 36% 12%	4038
	11th to 20th	Day	31% 9% 18% 26% 14% 3	3885
	21st to end	Day	30% 9% 14% 19% 24% 4	4311
September	1st to 10th	Day	37% 10% 16% 16% 16% 6%	2290
	11th to 20th	Day	39% 8% 12% 21% 17% 3	2200
	21st to end	Day	35% 6% 13% 18% 22% 7%	2200
October	1st to 10th	Day	25% 5% 6% 12% 37% 15%	3590
	11th to 20th	Day	21% 3 5% 17% 31% 24%	3536
	21st to end	Day	24% 4 7% 23% 32% 10%	3351
November	1st to 10th	Day	34% 4 7% 18% 29% 8%	1852
	11th to 20th	Day	34% 4 8% 20% 26% 8%	1890
	21st to end	Day	37% 5% 8% 19% 25% 6%	1890
December	1st to 10th	Day	42% 4 6% 18% 25% 5%	3020
	11th to 20th	Day	49% 3 8% 19% 16% 5%	3460
	21st to end	Day	47% 4 8% 25% 14% 1	4565
January	1st to 10th	Night	43% 4 6% 23% 21% 3	6019
	11th to 20th	Night	41% 4 6% 21% 23% 6%	6030
	21st to end	Night	45% 3 5% 18% 24% 5%	6186
February	1st to 10th	Night	44% 3 7% 20% 20% 6%	2401
	11th to 20th	Night	49% 3 6% 17% 18% 7%	2339
	21st to end	Night	45% 5% 7% 15% 19% 8%	1980
March	1st to 10th	Night	52% 5% 6% 14% 18% 4	5373
	11th to 20th	Night	33% 4 8% 17% 23% 15%	5033
	21st to end	Night	42% 4 7% 22% 19% 6%	5467
April	1st to 10th	Night	32% 7% 12% 21% 20% 7%	2010
	11th to 20th	Night	34% 5% 6% 22% 22% 11%	1975
	21st to end	Night	32% 6% 12% 26% 20% 5%	1935
May	1st to 10th	Night	42% 9% 15% 18% 13% 1	4060
	11th to 20th	Night	50% 8% 14% 19% 8%	4033
	21st to end	Night	36% 9% 14% 24% 16% 2	4429
June	1st to 10th	Night	35% 11% 15% 21% 16% 2	1800
	11th to 20th	Night	25% 10% 23% 23% 18% 2	1800
	21st to end	Night	29% 10% 20% 21% 16% 4	1800
July	1st to 10th	Night	37% 12% 19% 23% 9% 1	4015
	11th to 20th	Night	29% 14% 30% 20% 5% 1	4006
	21st to end	Night	44% 15% 20% 16% 4 1	4297
August	1st to 10th	Night	25% 9% 15% 37% 12% 1	3082
	11th to 20th	Night	30% 8% 17% 27% 16% 2	3230
	21st to end	Night	28% 7% 14% 17% 28% 5%	3586
September	1st to 10th	Night	35% 10% 16% 17% 15% 6%	2088
	11th to 20th	Night	37% 9% 11% 20% 20% 3	2200
	21st to end	Night	31% 6% 13% 21% 22% 7%	2200
October	1st to 10th	Night	23% 4 6% 16% 33% 17%	3590
	11th to 20th	Night	19% 3 4 18% 30% 27%	3603
	21st to end	Night	23% 4 5% 24% 31% 13%	4436
November	1st to 10th	Night	32% 5% 9% 19% 24% 11%	2666
	11th to 20th	Night	32% 4 6% 21% 28% 10%	2730
	21st to end	Night	38% 4 8% 20% 23% 7%	2730
December	1st to 10th	Night	41% 4 6% 21% 25% 3	4380
	11th to 20th	Night	45% 4 7% 16% 23% 4	4954
	21st to end	Night	45% 4 9% 26% 15% 1	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	23% 22% 34% 16% 4	330
	11th to 20th	Day	22% 21% 34% 16% 5% 1	330
	21st to end	Day	24% 21% 38% 11% 5% 1	363
February	1st to 10th	Day	25% 16% 32% 20% 5%	300
	11th to 20th	Day	37% 21% 27% 13% 2	300
	21st to end	Day	27% 21% 38% 12% 2	249
March	1st to 10th	Day	24% 20% 35% 15% 4 2	300
	11th to 20th	Day	21% 17% 34% 21% 6% 1	471
	21st to end	Day	19% 15% 36% 23% 7%	550
April	1st to 10th	Day	21% 13% 30% 21% 14% 1	500
	11th to 20th	Day	16% 14% 30% 25% 14% 1	492
	21st to end	Day	15% 10% 27% 28% 17% 3	500
May	1st to 10th	Day	17% 7% 28% 26% 16% 6%	503
	11th to 20th	Day	12% 6% 22% 29% 21% 10%	527
	21st to end	Day	12% 6% 18% 27% 22% 16%	573
June	1st to 10th	Day	13% 3 18% 27% 25% 15%	530
	11th to 20th	Day	5% 3 16% 23% 29% 24%	530
	21st to end	Day	9% 3 12% 23% 29% 24%	530
July	1st to 10th	Day	8% 3 8% 25% 29% 27%	525
	11th to 20th	Day	7% 2 6% 20% 33% 32%	524
	21st to end	Day	8% 2 9% 18% 35% 28%	576
August	1st to 10th	Day	10% 1 10% 22% 34% 23%	526
	11th to 20th	Day	10% 3 7% 27% 32% 21%	519
	21st to end	Day	13% 4 8% 22% 31% 23%	583
September	1st to 10th	Day	8% 4 9% 31% 31% 17%	530
	11th to 20th	Day	15% 4 10% 22% 33% 16%	530
	21st to end	Day	13% 4 14% 26% 34% 9%	530
October	1st to 10th	Day	13% 3 21% 30% 24% 9%	530
	11th to 20th	Day	10% 8% 20% 33% 26% 3	530
	21st to end	Day	16% 12% 20% 29% 21% 3	471
November	1st to 10th	Day	18% 10% 26% 28% 16% 2	330
	11th to 20th	Day	15% 11% 37% 26% 9% 2	330
	21st to end	Day	15% 12% 37% 25% 10% 1	330
December	1st to 10th	Day	28% 17% 32% 17% 5% 1	330
	11th to 20th	Day	23% 16% 28% 22% 9% 1	330
	21st to end	Day	26% 17% 32% 18% 6% 1	363
January	1st to 10th	Night	22% 15% 32% 25% 5%	840
	11th to 20th	Night	21% 15% 30% 28% 6%	840
	21st to end	Night	21% 14% 36% 26% 3	924
February	1st to 10th	Night	20% 12% 30% 33% 5%	801
	11th to 20th	Night	25% 14% 32% 26% 3	783
	21st to end	Night	23% 14% 35% 26% 2	660
March	1st to 10th	Night	21% 12% 32% 31% 5%	800
	11th to 20th	Night	18% 12% 33% 29% 9% 1	800
	21st to end	Night	20% 12% 28% 31% 9%	880
April	1st to 10th	Night	14% 10% 32% 30% 14% 1	610
	11th to 20th	Night	15% 9% 29% 33% 12% 1	593
	21st to end	Night	11% 8% 25% 39% 18% 1	600
May	1st to 10th	Night	11% 6% 24% 33% 24% 2	600
	11th to 20th	Night	12% 5% 17% 35% 26% 5%	591
	21st to end	Night	10% 3 11% 34% 36% 6%	650
June	1st to 10th	Night	8% 2 12% 37% 36% 5%	600
	11th to 20th	Night	3 2 10% 33% 39% 14%	600
	21st to end	Night	6% 1 8% 28% 45% 11%	600
July	1st to 10th	Night	8% 2 6% 23% 47% 14%	585
	11th to 20th	Night	5% 1 4 22% 48% 19%	582
	21st to end	Night	6% 2 6% 26% 46% 14%	657
August	1st to 10th	Night	6% 1 5% 28% 48% 12%	597
	11th to 20th	Night	11% 2 6% 27% 42% 11%	594
	21st to end	Night	14% 2 7% 26% 40% 10%	660
September	1st to 10th	Night	8% 3 8% 31% 40% 10%	688
	11th to 20th	Night	14% 2 10% 30% 39% 6%	800
	21st to end	Night	11% 3 15% 32% 37% 2	800
October	1st to 10th	Night	11% 3 18% 36% 26% 6%	800
	11th to 20th	Night	13% 5% 23% 35% 23% 2	800
	21st to end	Night	16% 7% 21% 36% 18% 2	880
November	1st to 10th	Night	13% 8% 20% 38% 21% 1	820
	11th to 20th	Night	15% 7% 30% 35% 13% 1	840
	21st to end	Night	17% 9% 28% 35% 11%	840
December	1st to 10th	Night	24% 11% 28% 28% 8%	840
	11th to 20th	Night	23% 10% 28% 30% 9%	840
	21st to end	Night	27% 10% 27% 30% 7%	924

Page generated at 2026-01-29 00:21:43. Version ac07

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