Carlson R. Chambliss Astronomical Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Carlson R. Chambliss Astronomical 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	16% 7% 2 6% 4 4 5% 4 6% 8% 38%	131343
	Night	16% 7% 2 6% 4 4 4 3 5% 7% 41%	130172
Transparency	Day	42% 6% 12% 23% 14% 2	134887
	Night	42% 6% 11% 23% 16% 3	134714
Seeing	Day	21% 15% 28% 21% 11% 4	16585
	Night	20% 12% 29% 28% 10% 1	26004

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	13% 6% 2 7% 5% 5% 5% 4 5% 6% 41%	4661
	11th to 20th	Day	12% 10% 5% 9% 5% 4 4 3 4 6% 37%	4843
	21st to end	Day	15% 6% 2 7% 5% 5% 5% 4 6% 6% 37%	5780
February	1st to 10th	Day	15% 8% 3 7% 6% 5% 5% 4 6% 6% 37%	1890
	11th to 20th	Day	15% 8% 4 8% 4 4 4 3 6% 7% 36%	1956
	21st to end	Day	18% 10% 3 6% 5% 3 5% 4 5% 6% 35%	1730
March	1st to 10th	Day	21% 5% 2 5% 4 4 4 3 5% 6% 41%	4665
	11th to 20th	Day	13% 6% 2 7% 5% 5% 4 3 5% 7% 42%	5057
	21st to end	Day	18% 7% 2 6% 4 4 4 3 5% 7% 42%	5735
April	1st to 10th	Day	17% 9% 3 6% 5% 4 4 4 5% 8% 36%	2400
	11th to 20th	Day	16% 7% 2 7% 5% 4 3 4 5% 7% 40%	2374
	21st to end	Day	17% 6% 2 6% 4 4 4 4 6% 7% 40%	2400
May	1st to 10th	Day	26% 7% 2 5% 4 4 5% 4 6% 8% 29%	5917
	11th to 20th	Day	24% 8% 3 7% 4 4 4 3 5% 7% 30%	6318
	21st to end	Day	20% 6% 2 5% 4 4 4 4 6% 9% 36%	6949
June	1st to 10th	Day	12% 6% 2 6% 5% 5% 5% 4 7% 9% 37%	2800
	11th to 20th	Day	15% 6% 2 5% 5% 5% 5% 5% 7% 8% 37%	2800
	21st to end	Day	12% 5% 2 6% 4 5% 6% 5% 6% 9% 40%	2800
July	1st to 10th	Day	8% 5% 2 7% 5% 6% 5% 5% 7% 11% 38%	6360
	11th to 20th	Day	11% 7% 3 8% 5% 5% 6% 5% 8% 10% 32%	6360
	21st to end	Day	11% 7% 3 7% 5% 5% 5% 4 6% 9% 40%	6754
August	1st to 10th	Day	19% 6% 3 7% 4 5% 5% 4 6% 8% 32%	4234
	11th to 20th	Day	13% 6% 2 6% 5% 5% 5% 4 6% 8% 39%	3922
	21st to end	Day	13% 6% 2 5% 3 4 4 3 6% 8% 47%	4312
September	1st to 10th	Day	16% 6% 3 5% 4 4 5% 4 4 6% 44%	2400
	11th to 20th	Day	12% 5% 2 4 4 3 3 4 4 8% 50%	2395
	21st to end	Day	20% 7% 2 7% 4 4 4 3 5% 5% 38%	2155
October	1st to 10th	Day	19% 6% 1 5% 3 4 5% 4 5% 7% 43%	2000
	11th to 20th	Day	17% 5% 2 6% 4 4 4 3 5% 7% 44%	2000
	21st to end	Day	19% 5% 2 5% 3 3 4 4 5% 6% 43%	2151
November	1st to 10th	Day	13% 7% 2 4 3 3 3 3 4 7% 51%	1851
	11th to 20th	Day	15% 6% 2 5% 4 3 3 4 6% 7% 43%	1890
	21st to end	Day	18% 6% 3 5% 3 4 5% 4 5% 8% 39%	1690
December	1st to 10th	Day	15% 8% 3 6% 4 5% 4 4 6% 6% 39%	2680
	11th to 20th	Day	21% 9% 2 5% 3 4 4 3 5% 6% 38%	3066
	21st to end	Day	19% 7% 3 6% 4 5% 5% 4 6% 7% 35%	4048
January	1st to 10th	Night	12% 8% 3 8% 5% 4 4 3 5% 7% 40%	8234
	11th to 20th	Night	13% 8% 3 7% 4 4 4 3 6% 7% 40%	8038
	21st to end	Night	15% 7% 2 6% 4 3 4 3 5% 7% 44%	8401
February	1st to 10th	Night	15% 9% 3 6% 4 3 5% 4 5% 6% 40%	2730
	11th to 20th	Night	18% 8% 3 6% 5% 4 4 4 5% 6% 38%	2549
	21st to end	Night	17% 9% 2 6% 4 4 4 4 4 6% 40%	2076
March	1st to 10th	Night	18% 9% 2 6% 4 3 4 3 4 5% 41%	5390
	11th to 20th	Night	11% 6% 2 5% 4 4 4 3 5% 6% 50%	5057
	21st to end	Night	16% 8% 2 7% 4 3 3 3 5% 6% 43%	5197
April	1st to 10th	Night	18% 7% 3 6% 4 4 5% 4 6% 6% 37%	2000
	11th to 20th	Night	15% 7% 3 7% 4 4 4 4 6% 7% 41%	1975
	21st to end	Night	19% 4 2 5% 4 4 4 3 6% 7% 44%	2000
May	1st to 10th	Night	26% 9% 2 6% 4 4 4 4 5% 7% 28%	4064
	11th to 20th	Night	23% 9% 3 7% 4 4 4 3 5% 6% 32%	3575
	21st to end	Night	22% 6% 3 6% 4 4 4 3 5% 8% 37%	3926
June	1st to 10th	Night	16% 6% 2 7% 4 3 3 4 6% 8% 41%	1600
	11th to 20th	Night	13% 6% 2 6% 5% 4 4 5% 7% 8% 41%	1600
	21st to end	Night	12% 4 2 5% 4 4 4 3 8% 10% 43%	1600
July	1st to 10th	Night	9% 7% 3 7% 6% 5% 6% 4 6% 9% 38%	3600
	11th to 20th	Night	12% 9% 3 8% 5% 5% 6% 5% 7% 9% 31%	3600
	21st to end	Night	12% 6% 2 5% 4 4 4 4 6% 9% 45%	4094
August	1st to 10th	Night	20% 7% 3 7% 5% 4 5% 4 6% 8% 34%	2915
	11th to 20th	Night	14% 7% 3 7% 5% 4 4 4 5% 6% 41%	3094
	21st to end	Night	13% 5% 2 5% 3 3 5% 4 5% 8% 48%	3586
September	1st to 10th	Night	14% 7% 2 5% 4 3 3 3 4 7% 47%	2000
	11th to 20th	Night	12% 4 2 5% 4 4 3 3 5% 6% 51%	2005
	21st to end	Night	20% 6% 2 6% 4 4 4 3 5% 6% 41%	2240
October	1st to 10th	Night	20% 6% 2 5% 3 3 3 4 4 7% 44%	2400
	11th to 20th	Night	17% 5% 2 4 3 3 3 3 5% 6% 48%	2400
	21st to end	Night	19% 6% 2 4 2 3 4 3 5% 6% 47%	2687
November	1st to 10th	Night	12% 5% 2 4 4 4 4 3 5% 6% 51%	2667
	11th to 20th	Night	16% 7% 3 5% 5% 3 3 3 3 7% 45%	2730
	21st to end	Night	15% 7% 2 6% 4 4 4 3 5% 8% 41%	2924
December	1st to 10th	Night	13% 8% 3 7% 4 4 4 3 5% 6% 42%	4720
	11th to 20th	Night	22% 7% 2 5% 4 3 3 3 5% 5% 39%	5348
	21st to end	Night	19% 9% 3 7% 4 4 4 3 4 6% 38%	7150

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 12% 25% 16% 1	4662
	11th to 20th	Day	47% 4 9% 20% 15% 5%	4874
	21st to end	Day	45% 6% 13% 22% 13% 1	5780
February	1st to 10th	Day	44% 7% 13% 22% 12% 2	1890
	11th to 20th	Day	46% 6% 11% 23% 12% 2	1965
	21st to end	Day	49% 5% 11% 22% 10% 2	1730
March	1st to 10th	Day	43% 5% 9% 24% 12% 6%	4668
	11th to 20th	Day	39% 7% 12% 27% 11% 4	5057
	21st to end	Day	43% 5% 11% 16% 21% 4	5735
April	1st to 10th	Day	46% 6% 10% 20% 14% 5%	2400
	11th to 20th	Day	43% 6% 11% 22% 15% 4	2374
	21st to end	Day	41% 7% 11% 22% 15% 5%	2400
May	1st to 10th	Day	51% 8% 13% 18% 9% 2	5926
	11th to 20th	Day	53% 7% 8% 21% 10% 1	6322
	21st to end	Day	43% 7% 11% 24% 13% 2	6949
June	1st to 10th	Day	40% 9% 13% 23% 14% 2	2800
	11th to 20th	Day	42% 9% 14% 23% 10% 2	2800
	21st to end	Day	37% 9% 14% 24% 14% 1	2800
July	1st to 10th	Day	39% 10% 13% 22% 14% 1	6290
	11th to 20th	Day	43% 10% 16% 21% 9% 1	6285
	21st to end	Day	41% 6% 10% 20% 22% 1	6559
August	1st to 10th	Day	48% 8% 12% 21% 10%	4221
	11th to 20th	Day	39% 7% 12% 29% 11% 1	3960
	21st to end	Day	34% 6% 11% 30% 17% 2	4312
September	1st to 10th	Day	40% 6% 11% 21% 20% 2	2400
	11th to 20th	Day	33% 5% 11% 24% 23% 3	2395
	21st to end	Day	48% 5% 10% 17% 19% 2	2155
October	1st to 10th	Day	38% 4 9% 21% 23% 5%	3260
	11th to 20th	Day	30% 4 11% 28% 22% 5%	3240
	21st to end	Day	27% 4 15% 24% 27% 3	3453
November	1st to 10th	Day	34% 4 14% 25% 20% 3	1851
	11th to 20th	Day	39% 5% 11% 25% 18% 2	1890
	21st to end	Day	41% 7% 12% 29% 9% 2	1690
December	1st to 10th	Day	45% 5% 16% 24% 8% 2	2680
	11th to 20th	Day	47% 5% 14% 25% 9% 1	3066
	21st to end	Day	47% 6% 14% 21% 12% 1	4048
January	1st to 10th	Night	43% 5% 14% 23% 14% 2	8234
	11th to 20th	Night	42% 5% 12% 20% 16% 5%	8082
	21st to end	Night	39% 5% 10% 25% 19% 2	8401
February	1st to 10th	Night	44% 6% 12% 25% 10% 2	2730
	11th to 20th	Night	46% 5% 11% 20% 16% 3	2551
	21st to end	Night	44% 5% 9% 22% 16% 3	2076
March	1st to 10th	Night	45% 4 10% 22% 12% 7%	5400
	11th to 20th	Night	34% 6% 15% 27% 13% 5%	5058
	21st to end	Night	43% 5% 7% 17% 23% 5%	5197
April	1st to 10th	Night	45% 6% 11% 22% 14% 3	2000
	11th to 20th	Night	42% 7% 11% 20% 20% 2	1975
	21st to end	Night	38% 6% 12% 22% 17% 5%	2000
May	1st to 10th	Night	55% 6% 10% 17% 10% 2	4071
	11th to 20th	Night	52% 8% 10% 17% 12% 1	3576
	21st to end	Night	46% 8% 9% 23% 12% 2	3927
June	1st to 10th	Night	40% 7% 10% 22% 19% 1	1600
	11th to 20th	Night	37% 10% 14% 24% 13% 1	1600
	21st to end	Night	35% 9% 14% 27% 13% 1	1600
July	1st to 10th	Night	41% 10% 13% 19% 15% 2	3560
	11th to 20th	Night	47% 8% 14% 20% 10% 1	3552
	21st to end	Night	35% 7% 12% 23% 21% 1	3974
August	1st to 10th	Night	47% 8% 16% 17% 10% 1	2904
	11th to 20th	Night	43% 6% 12% 23% 15% 1	3122
	21st to end	Night	34% 7% 10% 29% 19% 1	3586
September	1st to 10th	Night	37% 6% 10% 23% 21% 3	2000
	11th to 20th	Night	33% 6% 9% 24% 25% 3	2005
	21st to end	Night	43% 6% 9% 20% 20% 2	2240
October	1st to 10th	Night	42% 5% 9% 21% 21% 2	3920
	11th to 20th	Night	28% 4 9% 29% 25% 4	3899
	21st to end	Night	27% 3 12% 32% 23% 3	4335
November	1st to 10th	Night	33% 5% 14% 24% 19% 4	2667
	11th to 20th	Night	41% 4 10% 24% 17% 4	2730
	21st to end	Night	40% 5% 13% 28% 13% 1	2924
December	1st to 10th	Night	43% 4 13% 29% 10% 1	4720
	11th to 20th	Night	46% 5% 11% 25% 12% 2	5348
	21st to end	Night	47% 5% 11% 20% 15% 1	7150

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% 29% 29% 13% 3 1	220
	11th to 20th	Day	33% 27% 29% 9% 2	254
	21st to end	Day	23% 31% 31% 11% 4 1	363
February	1st to 10th	Day	25% 26% 33% 13% 2 1	330
	11th to 20th	Day	30% 23% 35% 11% 1	444
	21st to end	Day	28% 20% 31% 18% 2	446
March	1st to 10th	Day	28% 21% 29% 18% 4	540
	11th to 20th	Day	26% 25% 31% 13% 4 1	528
	21st to end	Day	29% 22% 29% 13% 6% 1	550
April	1st to 10th	Day	28% 23% 28% 14% 5% 1	500
	11th to 20th	Day	23% 25% 28% 15% 6% 3	492
	21st to end	Day	27% 15% 33% 17% 5% 3	500
May	1st to 10th	Day	29% 18% 27% 17% 6% 3	503
	11th to 20th	Day	23% 15% 30% 20% 8% 2	527
	21st to end	Day	20% 9% 28% 24% 14% 5%	573
June	1st to 10th	Day	15% 14% 31% 23% 14% 3	530
	11th to 20th	Day	20% 11% 28% 25% 13% 3	530
	21st to end	Day	14% 11% 31% 26% 13% 4	530
July	1st to 10th	Day	11% 4 24% 31% 22% 7%	525
	11th to 20th	Day	8% 6% 26% 31% 23% 6%	524
	21st to end	Day	11% 7% 28% 34% 15% 4	576
August	1st to 10th	Day	13% 6% 23% 32% 19% 9%	526
	11th to 20th	Day	13% 6% 20% 34% 20% 7%	519
	21st to end	Day	13% 7% 25% 28% 18% 10%	583
September	1st to 10th	Day	18% 9% 22% 25% 20% 7%	530
	11th to 20th	Day	13% 5% 23% 29% 20% 10%	530
	21st to end	Day	24% 13% 23% 22% 13% 4	530
October	1st to 10th	Day	23% 8% 25% 22% 15% 7%	530
	11th to 20th	Day	20% 15% 27% 22% 12% 3	530
	21st to end	Day	21% 16% 32% 18% 11% 1	555
November	1st to 10th	Day	17% 16% 30% 24% 10% 3	425
	11th to 20th	Day	21% 20% 31% 22% 5%	430
	21st to end	Day	27% 23% 32% 13% 4	230
December	1st to 10th	Day	30% 23% 32% 10% 5%	220
	11th to 20th	Day	32% 25% 26% 12% 4	220
	21st to end	Day	27% 22% 30% 14% 6% 1	242
January	1st to 10th	Night	19% 20% 39% 20% 2	840
	11th to 20th	Night	25% 18% 32% 23% 3	840
	21st to end	Night	18% 19% 37% 24% 2	924
February	1st to 10th	Night	22% 16% 35% 25% 2	840
	11th to 20th	Night	26% 20% 36% 16% 1	823
	21st to end	Night	23% 18% 37% 19% 2	692
March	1st to 10th	Night	23% 19% 32% 23% 4	840
	11th to 20th	Night	24% 18% 33% 20% 5% 1	829
	21st to end	Night	22% 16% 38% 20% 5%	880
April	1st to 10th	Night	23% 19% 34% 20% 4	800
	11th to 20th	Night	19% 15% 35% 23% 7%	791
	21st to end	Night	22% 12% 31% 28% 7%	800
May	1st to 10th	Night	25% 10% 30% 28% 7%	611
	11th to 20th	Night	20% 7% 32% 32% 8% 1	394
	21st to end	Night	19% 7% 21% 39% 13% 1	433
June	1st to 10th	Night	16% 6% 27% 35% 17%	400
	11th to 20th	Night	14% 6% 25% 42% 13% 1	400
	21st to end	Night	11% 4 25% 46% 15%	400
July	1st to 10th	Night	11% 4 19% 41% 23% 2	390
	11th to 20th	Night	10% 3 20% 39% 26% 2	388
	21st to end	Night	11% 3 18% 42% 23% 2	505
August	1st to 10th	Night	14% 3 14% 42% 27% 2	597
	11th to 20th	Night	14% 5% 15% 37% 28% 2	723
	21st to end	Night	15% 3 18% 37% 25% 2	880
September	1st to 10th	Night	16% 4 25% 33% 20% 2	800
	11th to 20th	Night	14% 4 17% 36% 24% 5%	800
	21st to end	Night	22% 5% 20% 32% 19% 2	800
October	1st to 10th	Night	22% 7% 24% 26% 18% 4	800
	11th to 20th	Night	19% 12% 25% 33% 11% 1	800
	21st to end	Night	22% 11% 29% 28% 9%	880
November	1st to 10th	Night	15% 11% 34% 28% 11% 1	820
	11th to 20th	Night	20% 18% 28% 27% 7% 1	840
	21st to end	Night	18% 17% 32% 27% 6%	840
December	1st to 10th	Night	24% 14% 35% 21% 4 1	840
	11th to 20th	Night	22% 16% 33% 25% 5%	840
	21st to end	Night	23% 13% 34% 27% 3	924

Page generated at 2026-03-15 02:23:57. Version ac07

Copyright © 2026 Attilla Danko.