Alabama Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Alabama Observatory clear sky chart from 2005-11-06 to 2026-09-14.

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% 7% 3 7% 5% 5% 5% 4 6% 8% 39%	148386
	Night	13% 7% 2 6% 4 4 4 4 5% 8% 43%	142722
Transparency	Day	40% 7% 12% 20% 17% 5%	152152
	Night	38% 6% 11% 20% 19% 6%	147031
Seeing	Day	17% 10% 25% 24% 17% 7%	16987
	Night	16% 9% 24% 29% 19% 4	26898

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	20% 10% 5% 5% 3 2 3 2 3 4 41%	5258
	11th to 20th	Day	10% 6% 4 6% 2 2 3 2 3 5% 56%	5478
	21st to end	Day	23% 9% 2 7% 4 4 4 3 4 4 36%	6436
February	1st to 10th	Day	18% 10% 3 6% 4 3 4 3 4 5% 40%	2100
	11th to 20th	Day	23% 9% 2 4 4 3 4 3 4 4 39%	2048
	21st to end	Day	19% 8% 3 7% 5% 3 4 3 3 6% 39%	1730
March	1st to 10th	Day	19% 9% 3 7% 5% 4 4 3 5% 5% 36%	4580
	11th to 20th	Day	12% 8% 2 5% 4 4 4 4 5% 7% 45%	4896
	21st to end	Day	15% 7% 3 7% 4 4 5% 3 5% 6% 41%	5850
April	1st to 10th	Day	11% 7% 3 5% 4 4 4 4 5% 7% 46%	2520
	11th to 20th	Day	11% 6% 2 6% 5% 4 4 3 5% 7% 48%	2494
	21st to end	Day	9% 5% 2 6% 4 4 5% 4 6% 8% 46%	2520
May	1st to 10th	Day	16% 8% 3 7% 5% 5% 5% 4 6% 7% 33%	7549
	11th to 20th	Day	13% 7% 3 8% 5% 5% 5% 5% 6% 8% 35%	7626
	21st to end	Day	16% 9% 3 9% 5% 5% 5% 4 6% 7% 31%	8391
June	1st to 10th	Day	10% 6% 2 8% 6% 6% 7% 5% 7% 10% 32%	2730
	11th to 20th	Day	6% 4 2 7% 5% 5% 6% 6% 8% 12% 39%	2730
	21st to end	Day	7% 5% 2 6% 6% 6% 7% 5% 7% 11% 39%	2730
July	1st to 10th	Day	5% 5% 2 8% 6% 7% 8% 7% 9% 11% 31%	7680
	11th to 20th	Day	7% 6% 2 7% 6% 7% 8% 6% 10% 12% 28%	7680
	21st to end	Day	8% 7% 3 9% 6% 7% 7% 7% 9% 12% 25%	8209
August	1st to 10th	Day	4 4 2 7% 6% 6% 7% 6% 9% 12% 37%	5545
	11th to 20th	Day	4 3 1 4 4 5% 5% 5% 7% 11% 50%	5486
	21st to end	Day	6% 4 2 5% 5% 5% 6% 5% 7% 10% 47%	6080
September	1st to 10th	Day	9% 4 2 7% 5% 6% 6% 5% 7% 8% 42%	2462
	11th to 20th	Day	12% 7% 2 6% 4 5% 4 4 6% 7% 42%	2233
	21st to end	Day	10% 5% 3 6% 4 4 4 3 5% 6% 49%	2200
October	1st to 10th	Day	9% 4 2 6% 4 4 4 3 5% 5% 54%	2200
	11th to 20th	Day	9% 4 2 5% 3 2 3 3 5% 7% 57%	2200
	21st to end	Day	13% 5% 2 5% 3 3 3 3 4 6% 53%	2065
November	1st to 10th	Day	12% 6% 2 5% 3 3 3 3 4 5% 53%	1852
	11th to 20th	Day	12% 6% 2 4 3 3 3 3 4 7% 52%	1890
	21st to end	Day	14% 7% 2 6% 4 3 4 4 5% 7% 45%	1890
December	1st to 10th	Day	20% 8% 2 5% 3 3 3 3 4 5% 43%	3020
	11th to 20th	Day	15% 9% 4 7% 4 4 4 3 4 5% 42%	3463
	21st to end	Day	19% 10% 4 6% 3 4 3 3 4 6% 38%	4565
January	1st to 10th	Night	17% 10% 3 6% 3 3 3 3 4 5% 41%	7635
	11th to 20th	Night	15% 9% 4 6% 3 2 3 2 3 5% 47%	7463
	21st to end	Night	20% 9% 2 8% 3 3 3 2 3 4 41%	7745
February	1st to 10th	Night	17% 8% 3 6% 4 4 4 3 4 5% 43%	2520
	11th to 20th	Night	20% 8% 3 7% 4 3 3 3 4 4 41%	2458
	21st to end	Night	19% 6% 2 6% 5% 4 4 3 5% 6% 40%	2076
March	1st to 10th	Night	21% 9% 3 8% 4 4 4 3 4 6% 34%	5493
	11th to 20th	Night	12% 6% 2 5% 3 3 3 3 5% 6% 49%	5283
	21st to end	Night	14% 7% 3 7% 4 4 4 3 4 5% 44%	5363
April	1st to 10th	Night	11% 5% 2 5% 4 4 4 4 6% 8% 45%	2100
	11th to 20th	Night	12% 7% 2 4 3 3 4 3 4 7% 51%	2075
	21st to end	Night	11% 6% 2 5% 4 3 4 4 5% 6% 50%	2100
May	1st to 10th	Night	15% 7% 3 8% 5% 5% 5% 4 6% 7% 36%	5302
	11th to 20th	Night	12% 8% 3 9% 5% 5% 5% 4 6% 8% 35%	5234
	21st to end	Night	14% 8% 3 7% 5% 5% 6% 4 7% 9% 32%	5761
June	1st to 10th	Night	6% 5% 2 7% 6% 6% 5% 5% 8% 11% 39%	1890
	11th to 20th	Night	4 4 1 4 4 4 5% 5% 7% 12% 49%	1890
	21st to end	Night	5% 5% 2 6% 6% 5% 4 5% 7% 11% 44%	1890
July	1st to 10th	Night	4 5% 2 6% 5% 5% 7% 6% 8% 13% 38%	5278
	11th to 20th	Night	7% 7% 2 7% 5% 4 6% 5% 10% 15% 32%	5277
	21st to end	Night	6% 7% 3 8% 6% 6% 6% 6% 10% 13% 30%	5849
August	1st to 10th	Night	4 4 2 6% 5% 5% 6% 5% 8% 12% 45%	4604
	11th to 20th	Night	4 3 1 5% 3 4 5% 4 8% 14% 48%	4560
	21st to end	Night	5% 4 1 4 4 4 4 4 6% 10% 54%	5046
September	1st to 10th	Night	7% 5% 2 6% 5% 5% 5% 5% 5% 8% 45%	2136
	11th to 20th	Night	11% 5% 2 7% 4 4 4 4 6% 7% 46%	2234
	21st to end	Night	7% 4 2 6% 4 4 4 3 7% 6% 54%	2200
October	1st to 10th	Night	8% 5% 1 5% 3 3 3 3 4 6% 60%	2200
	11th to 20th	Night	10% 5% 1 4 3 3 3 3 4 6% 57%	2200
	21st to end	Night	13% 7% 2 5% 3 3 2 2 3 4 55%	2769
November	1st to 10th	Night	11% 5% 2 4 3 4 3 3 4 5% 57%	2666
	11th to 20th	Night	12% 6% 2 4 3 2 3 3 4 6% 55%	2730
	21st to end	Night	14% 6% 2 5% 5% 4 4 3 5% 6% 48%	2730
December	1st to 10th	Night	20% 7% 2 5% 3 2 3 2 4 5% 46%	4380
	11th to 20th	Night	17% 8% 3 7% 4 3 3 2 3 4 45%	4952
	21st to end	Night	19% 8% 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	48% 3 6% 24% 17% 3	5259
	11th to 20th	Day	32% 3 4 21% 33% 7%	5478
	21st to end	Day	53% 2 5% 13% 19% 8%	6436
February	1st to 10th	Day	47% 3 7% 19% 20% 5%	2100
	11th to 20th	Day	48% 4 5% 18% 19% 6%	2058
	21st to end	Day	48% 4 6% 13% 21% 8%	1730
March	1st to 10th	Day	50% 4 7% 13% 19% 7%	4580
	11th to 20th	Day	39% 4 6% 16% 21% 15%	4896
	21st to end	Day	43% 5% 7% 21% 19% 5%	5852
April	1st to 10th	Day	37% 5% 11% 21% 18% 8%	2520
	11th to 20th	Day	37% 4 7% 20% 24% 7%	2494
	21st to end	Day	34% 6% 10% 25% 22% 3	2520
May	1st to 10th	Day	47% 7% 11% 21% 13% 2	7585
	11th to 20th	Day	45% 7% 14% 21% 12% 1	7630
	21st to end	Day	50% 8% 10% 18% 13% 1	8405
June	1st to 10th	Day	43% 10% 13% 18% 15% 1	2730
	11th to 20th	Day	34% 9% 17% 24% 14% 2	2730
	21st to end	Day	36% 9% 17% 23% 11% 3	2730
July	1st to 10th	Day	38% 14% 23% 18% 7%	7615
	11th to 20th	Day	41% 15% 27% 14% 3	7611
	21st to end	Day	46% 13% 19% 19% 4	8027
August	1st to 10th	Day	33% 12% 15% 27% 13% 1	5533
	11th to 20th	Day	26% 8% 22% 30% 13% 2	5522
	21st to end	Day	30% 8% 13% 26% 20% 3	6079
September	1st to 10th	Day	37% 9% 13% 20% 16% 6%	2462
	11th to 20th	Day	38% 7% 11% 20% 19% 4	2233
	21st to end	Day	35% 6% 10% 21% 22% 6%	2200
October	1st to 10th	Day	23% 3 6% 14% 33% 21%	3590
	11th to 20th	Day	19% 3 5% 15% 31% 28%	3568
	21st to end	Day	25% 4 5% 21% 34% 12%	3302
November	1st to 10th	Day	34% 3 7% 18% 27% 11%	1852
	11th to 20th	Day	33% 3 7% 20% 27% 11%	1890
	21st to end	Day	39% 5% 6% 18% 25% 7%	1890
December	1st to 10th	Day	44% 3 5% 17% 26% 5%	3020
	11th to 20th	Day	46% 3 9% 18% 17% 6%	3460
	21st to end	Day	47% 3 6% 24% 18% 1	4565
January	1st to 10th	Night	46% 3 7% 24% 18% 2	7636
	11th to 20th	Night	41% 3 4 17% 26% 9%	7463
	21st to end	Night	48% 2 4 16% 22% 7%	7745
February	1st to 10th	Night	44% 3 5% 19% 21% 7%	2520
	11th to 20th	Night	48% 4 5% 18% 17% 8%	2459
	21st to end	Night	45% 4 7% 13% 21% 9%	2076
March	1st to 10th	Night	51% 4 6% 15% 18% 7%	5493
	11th to 20th	Night	35% 4 7% 17% 21% 16%	5283
	21st to end	Night	43% 4 6% 23% 18% 6%	5363
April	1st to 10th	Night	35% 5% 12% 21% 20% 7%	2100
	11th to 20th	Night	33% 4 8% 21% 23% 11%	2075
	21st to end	Night	34% 5% 11% 24% 20% 5%	2100
May	1st to 10th	Night	46% 8% 12% 17% 16% 2	5329
	11th to 20th	Night	46% 6% 14% 22% 11% 1	5235
	21st to end	Night	47% 9% 12% 19% 12% 1	5771
June	1st to 10th	Night	36% 9% 15% 20% 18% 2	1890
	11th to 20th	Night	26% 10% 20% 25% 17% 3	1890
	21st to end	Night	31% 9% 16% 24% 16% 4	1890
July	1st to 10th	Night	33% 11% 27% 21% 7% 1	5235
	11th to 20th	Night	36% 14% 31% 15% 3 1	5226
	21st to end	Night	41% 13% 24% 16% 6%	5718
August	1st to 10th	Night	29% 9% 19% 30% 12% 1	4592
	11th to 20th	Night	24% 8% 24% 30% 12% 2	4590
	21st to end	Night	25% 7% 14% 28% 23% 4	5046
September	1st to 10th	Night	34% 9% 17% 19% 16% 6%	2136
	11th to 20th	Night	37% 8% 10% 21% 20% 5%	2234
	21st to end	Night	29% 5% 11% 23% 24% 8%	2200
October	1st to 10th	Night	22% 4 5% 18% 30% 21%	3590
	11th to 20th	Night	18% 3 4 17% 30% 27%	3571
	21st to end	Night	23% 3 4 23% 33% 15%	4482
November	1st to 10th	Night	31% 4 7% 19% 26% 13%	2666
	11th to 20th	Night	31% 3 6% 19% 28% 12%	2730
	21st to end	Night	38% 5% 6% 19% 24% 9%	2730
December	1st to 10th	Night	41% 3 6% 19% 26% 5%	4380
	11th to 20th	Night	45% 3 7% 16% 24% 5%	4954
	21st to end	Night	45% 4 8% 24% 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	33% 20% 33% 9% 4	330
	11th to 20th	Day	30% 24% 29% 14% 3 1	330
	21st to end	Day	27% 27% 34% 9% 3	363
February	1st to 10th	Day	25% 24% 36% 12% 2 1	330
	11th to 20th	Day	42% 19% 29% 8% 2	330
	21st to end	Day	35% 22% 34% 8% 2	273
March	1st to 10th	Day	29% 19% 35% 12% 4 1	330
	11th to 20th	Day	25% 20% 34% 16% 4 1	450
	21st to end	Day	22% 17% 36% 20% 5%	594
April	1st to 10th	Day	23% 14% 32% 20% 10% 1	540
	11th to 20th	Day	18% 14% 35% 21% 10% 1	532
	21st to end	Day	14% 15% 30% 29% 11% 1	540
May	1st to 10th	Day	17% 10% 31% 25% 12% 4	543
	11th to 20th	Day	11% 8% 31% 29% 15% 6%	567
	21st to end	Day	16% 6% 24% 28% 17% 10%	617
June	1st to 10th	Day	12% 5% 21% 30% 23% 9%	570
	11th to 20th	Day	9% 4 20% 28% 24% 15%	570
	21st to end	Day	9% 5% 15% 30% 26% 15%	570
July	1st to 10th	Day	8% 2 11% 28% 32% 19%	565
	11th to 20th	Day	8% 3 8% 25% 31% 24%	564
	21st to end	Day	9% 2 10% 25% 35% 19%	620
August	1st to 10th	Day	10% 4 11% 27% 29% 19%	566
	11th to 20th	Day	9% 3 9% 32% 33% 14%	559
	21st to end	Day	12% 4 10% 30% 30% 14%	627
September	1st to 10th	Day	9% 3 13% 35% 29% 11%	570
	11th to 20th	Day	13% 4 15% 28% 30% 10%	542
	21st to end	Day	12% 6% 19% 31% 24% 8%	530
October	1st to 10th	Day	11% 5% 29% 31% 17% 6%	530
	11th to 20th	Day	13% 8% 27% 35% 15% 2	530
	21st to end	Day	20% 11% 27% 29% 11% 1	392
November	1st to 10th	Day	22% 10% 34% 25% 8% 2	330
	11th to 20th	Day	17% 18% 42% 18% 6%	330
	21st to end	Day	20% 20% 35% 18% 7% 1	330
December	1st to 10th	Day	33% 18% 33% 13% 3	330
	11th to 20th	Day	26% 22% 33% 15% 5% 1	330
	21st to end	Day	31% 15% 34% 17% 4	363
January	1st to 10th	Night	24% 16% 35% 21% 4	840
	11th to 20th	Night	25% 16% 30% 23% 6%	840
	21st to end	Night	21% 18% 36% 23% 2	924
February	1st to 10th	Night	22% 15% 34% 26% 3	840
	11th to 20th	Night	28% 15% 34% 21% 3	823
	21st to end	Night	24% 18% 34% 22% 2	692
March	1st to 10th	Night	22% 14% 32% 28% 5%	840
	11th to 20th	Night	17% 14% 37% 25% 7% 1	840
	21st to end	Night	18% 14% 35% 26% 8%	828
April	1st to 10th	Night	14% 13% 34% 27% 11%	630
	11th to 20th	Night	15% 9% 36% 26% 13% 1	623
	21st to end	Night	14% 9% 27% 32% 17% 1	630
May	1st to 10th	Night	14% 7% 28% 28% 19% 3	630
	11th to 20th	Night	10% 6% 23% 36% 20% 4	621
	21st to end	Night	8% 4 15% 36% 30% 6%	683
June	1st to 10th	Night	8% 3 13% 38% 32% 6%	630
	11th to 20th	Night	6% 2 11% 30% 38% 13%	630
	21st to end	Night	8% 3 9% 28% 41% 11%	630
July	1st to 10th	Night	7% 2 6% 28% 41% 16%	615
	11th to 20th	Night	7% 1 6% 23% 45% 18%	612
	21st to end	Night	7% 2 8% 28% 42% 13%	690
August	1st to 10th	Night	5% 2 6% 29% 43% 15%	627
	11th to 20th	Night	9% 3 7% 31% 40% 12%	624
	21st to end	Night	12% 2 7% 33% 36% 9%	693
September	1st to 10th	Night	9% 3 12% 31% 36% 9%	666
	11th to 20th	Night	13% 2 12% 31% 37% 5%	813
	21st to end	Night	8% 2 16% 34% 35% 4	800
October	1st to 10th	Night	11% 5% 23% 33% 24% 5%	800
	11th to 20th	Night	12% 6% 24% 36% 19% 2	800
	21st to end	Night	17% 8% 21% 34% 18% 2	880
November	1st to 10th	Night	15% 8% 22% 35% 17% 3	820
	11th to 20th	Night	16% 10% 30% 32% 11% 1	840
	21st to end	Night	18% 11% 31% 28% 11% 1	840
December	1st to 10th	Night	25% 14% 28% 26% 7% 1	840
	11th to 20th	Night	25% 12% 30% 25% 8%	840
	21st to end	Night	26% 13% 31% 24% 6%	924

Page generated at 2026-09-11 03:11:51. Version ac07

Copyright © 2026 Attilla Danko.