Vestavia Hills Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Vestavia Hills Observatory clear sky chart from 2005-11-06 to 2026-07-16.

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 6% 4 4 5% 4 6% 8% 40%	137905
	Night	13% 7% 2 6% 4 4 4 3 5% 7% 44%	134807
Transparency	Day	40% 6% 11% 20% 18% 5%	141685
	Night	38% 5% 10% 20% 20% 7%	139102
Seeing	Day	15% 9% 24% 25% 19% 9%	16729
	Night	14% 7% 23% 31% 21% 4	26692

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	19% 10% 5% 6% 3 3 3 3 3 4 41%	5258
	11th to 20th	Day	11% 5% 4 5% 2 2 2 2 4 6% 56%	5557
	21st to end	Day	24% 9% 2 8% 4 4 3 3 4 5% 35%	6436
February	1st to 10th	Day	17% 10% 3 6% 4 4 4 3 4 6% 39%	2100
	11th to 20th	Day	23% 8% 2 5% 4 3 4 3 4 5% 39%	2048
	21st to end	Day	18% 8% 3 7% 5% 3 4 3 4 6% 39%	1730
March	1st to 10th	Day	18% 9% 3 7% 4 4 4 3 6% 5% 36%	4580
	11th to 20th	Day	11% 7% 3 6% 4 3 4 4 5% 7% 46%	4887
	21st to end	Day	14% 7% 3 7% 4 4 5% 4 5% 7% 41%	5864
April	1st to 10th	Day	11% 6% 3 6% 4 3 4 4 6% 8% 46%	2520
	11th to 20th	Day	11% 5% 2 6% 5% 4 4 3 6% 7% 48%	2494
	21st to end	Day	8% 5% 2 6% 5% 5% 5% 4 6% 9% 47%	2520
May	1st to 10th	Day	15% 7% 3 8% 5% 5% 5% 4 6% 8% 34%	7489
	11th to 20th	Day	12% 6% 3 7% 5% 5% 6% 5% 7% 9% 36%	7626
	21st to end	Day	15% 9% 3 9% 6% 5% 5% 4 6% 7% 32%	8391
June	1st to 10th	Day	10% 7% 2 7% 5% 6% 6% 5% 8% 11% 33%	2730
	11th to 20th	Day	6% 4 2 5% 5% 4 7% 5% 8% 12% 41%	2730
	21st to end	Day	7% 5% 2 6% 5% 6% 5% 5% 8% 12% 40%	2730
July	1st to 10th	Day	5% 5% 2 8% 6% 7% 7% 6% 9% 12% 32%	6030
	11th to 20th	Day	6% 5% 2 6% 5% 7% 8% 6% 11% 13% 31%	5965
	21st to end	Day	7% 6% 3 8% 6% 7% 9% 7% 10% 12% 24%	6251
August	1st to 10th	Day	5% 2 1 4 4 4 5% 5% 8% 11% 50%	3905
	11th to 20th	Day	6% 4 2 5% 5% 6% 6% 4 7% 10% 45%	3849
	21st to end	Day	7% 4 2 5% 5% 5% 5% 5% 7% 8% 47%	4312
September	1st to 10th	Day	10% 5% 2 7% 5% 4 6% 5% 6% 8% 42%	2350
	11th to 20th	Day	12% 7% 2 6% 4 4 5% 4 5% 8% 43%	2200
	21st to end	Day	10% 6% 2 5% 4 4 4 3 6% 7% 50%	2200
October	1st to 10th	Day	9% 4 2 6% 4 5% 5% 2 4 6% 54%	2200
	11th to 20th	Day	9% 4 2 5% 3 3 3 3 5% 6% 58%	2200
	21st to end	Day	13% 5% 2 5% 3 3 3 3 3 5% 54%	2073
November	1st to 10th	Day	12% 6% 2 5% 4 4 3 2 4 6% 53%	1852
	11th to 20th	Day	13% 6% 2 3 3 3 3 3 4 6% 54%	1890
	21st to end	Day	14% 7% 2 5% 5% 3 4 3 4 7% 45%	1890
December	1st to 10th	Day	20% 7% 2 6% 3 3 3 2 4 6% 42%	3020
	11th to 20th	Day	14% 8% 3 7% 5% 4 5% 3 3 5% 43%	3463
	21st to end	Day	19% 10% 3 6% 4 3 3 2 4 6% 39%	4565
January	1st to 10th	Night	17% 10% 3 6% 3 3 4 3 4 5% 42%	7635
	11th to 20th	Night	15% 8% 4 6% 3 3 3 2 4 5% 47%	7398
	21st to end	Night	21% 9% 2 7% 3 3 3 2 3 4 41%	7745
February	1st to 10th	Night	17% 8% 2 7% 4 4 4 2 4 6% 43%	2520
	11th to 20th	Night	20% 8% 3 7% 4 3 3 2 4 5% 42%	2458
	21st to end	Night	19% 6% 2 6% 4 3 4 3 6% 6% 41%	2076
March	1st to 10th	Night	20% 10% 3 7% 4 4 4 3 5% 5% 34%	5493
	11th to 20th	Night	12% 6% 2 5% 3 3 4 3 5% 6% 50%	5291
	21st to end	Night	14% 6% 3 8% 4 4 4 3 4 5% 45%	5336
April	1st to 10th	Night	11% 4 2 6% 4 3 4 4 6% 8% 46%	2100
	11th to 20th	Night	13% 5% 2 5% 3 3 3 3 5% 6% 52%	2075
	21st to end	Night	11% 5% 2 6% 4 4 4 3 4 6% 51%	2100
May	1st to 10th	Night	15% 7% 3 8% 5% 5% 5% 4 6% 7% 35%	5361
	11th to 20th	Night	12% 8% 2 8% 5% 5% 6% 4 6% 8% 36%	5234
	21st to end	Night	12% 8% 3 8% 5% 5% 5% 5% 7% 9% 33%	5761
June	1st to 10th	Night	5% 6% 3 6% 5% 6% 6% 5% 8% 11% 38%	1890
	11th to 20th	Night	5% 3 1 4 4 4 5% 5% 9% 11% 48%	1890
	21st to end	Night	5% 4 1 5% 6% 5% 6% 4 8% 11% 44%	1890
July	1st to 10th	Night	6% 6% 2 8% 6% 6% 6% 6% 8% 12% 36%	4148
	11th to 20th	Night	6% 5% 2 7% 6% 5% 6% 6% 9% 15% 34%	4102
	21st to end	Night	6% 7% 3 8% 7% 7% 7% 7% 10% 10% 28%	4595
August	1st to 10th	Night	3 2 1 4 3 4 4 4 7% 11% 55%	3244
	11th to 20th	Night	5% 4 2 5% 4 4 5% 4 6% 9% 52%	3200
	21st to end	Night	5% 3 2 5% 4 5% 5% 4 7% 8% 53%	3586
September	1st to 10th	Night	8% 5% 2 8% 5% 5% 5% 4 6% 9% 43%	2028
	11th to 20th	Night	11% 6% 2 6% 5% 4 5% 4 5% 8% 46%	2200
	21st to end	Night	6% 5% 2 6% 4 4 4 3 6% 7% 53%	2200
October	1st to 10th	Night	7% 5% 2 5% 3 3 2 3 4 5% 60%	2200
	11th to 20th	Night	10% 4 2 4 3 3 3 3 4 6% 59%	2200
	21st to end	Night	13% 6% 1 5% 3 3 3 2 4 4 56%	2760
November	1st to 10th	Night	10% 5% 2 5% 3 3 3 2 5% 5% 57%	2666
	11th to 20th	Night	11% 7% 2 3 3 3 3 2 5% 5% 55%	2730
	21st to end	Night	12% 6% 1 5% 4 4 4 4 5% 6% 48%	2730
December	1st to 10th	Night	19% 7% 2 4 3 3 3 3 3 6% 48%	4380
	11th to 20th	Night	15% 8% 2 7% 4 3 3 3 4 5% 46%	4952
	21st to end	Night	19% 8% 3 6% 4 3 3 3 5% 6% 40%	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% 2 7% 24% 16% 3	5259
	11th to 20th	Day	31% 3 5% 23% 32% 6%	5557
	21st to end	Day	54% 2 5% 13% 18% 8%	6436
February	1st to 10th	Day	47% 3 7% 20% 19% 5%	2100
	11th to 20th	Day	48% 4 4 18% 19% 7%	2058
	21st to end	Day	47% 4 6% 13% 22% 8%	1730
March	1st to 10th	Day	49% 4 7% 13% 20% 6%	4580
	11th to 20th	Day	38% 3 6% 17% 20% 16%	4887
	21st to end	Day	43% 4 8% 19% 20% 5%	5866
April	1st to 10th	Day	36% 6% 10% 22% 19% 7%	2520
	11th to 20th	Day	36% 5% 7% 20% 25% 7%	2494
	21st to end	Day	33% 6% 10% 25% 23% 3	2520
May	1st to 10th	Day	46% 6% 11% 22% 13% 2	7525
	11th to 20th	Day	43% 6% 16% 22% 12% 1	7630
	21st to end	Day	49% 8% 11% 19% 12% 1	8405
June	1st to 10th	Day	41% 9% 15% 19% 15% 1	2730
	11th to 20th	Day	33% 9% 18% 23% 15% 1	2730
	21st to end	Day	34% 9% 18% 24% 11% 4	2730
July	1st to 10th	Day	39% 10% 21% 22% 8% 1	5965
	11th to 20th	Day	38% 12% 29% 17% 4 1	5896
	21st to end	Day	46% 12% 22% 17% 3	6070
August	1st to 10th	Day	24% 9% 13% 34% 19% 1	3893
	11th to 20th	Day	32% 7% 18% 26% 15% 3	3885
	21st to end	Day	31% 8% 16% 20% 21% 4	4311
September	1st to 10th	Day	38% 9% 13% 18% 17% 5%	2350
	11th to 20th	Day	39% 7% 10% 21% 19% 4	2200
	21st to end	Day	34% 6% 9% 22% 22% 7%	2200
October	1st to 10th	Day	24% 3 6% 13% 32% 22%	3590
	11th to 20th	Day	18% 3 4 15% 32% 28%	3568
	21st to end	Day	25% 3 6% 19% 36% 12%	3323
November	1st to 10th	Day	34% 3 7% 18% 27% 12%	1852
	11th to 20th	Day	33% 3 7% 19% 27% 11%	1890
	21st to end	Day	39% 4 7% 17% 24% 8%	1890
December	1st to 10th	Day	44% 3 5% 17% 27% 4	3020
	11th to 20th	Day	46% 3 9% 18% 18% 6%	3460
	21st to end	Day	46% 3 6% 24% 20% 1	4565
January	1st to 10th	Night	45% 3 7% 24% 19% 2	7636
	11th to 20th	Night	41% 2 5% 17% 26% 9%	7398
	21st to end	Night	49% 2 3 16% 23% 7%	7745
February	1st to 10th	Night	44% 3 5% 18% 21% 8%	2520
	11th to 20th	Night	47% 3 5% 18% 17% 9%	2459
	21st to end	Night	43% 5% 7% 14% 20% 10%	2076
March	1st to 10th	Night	50% 4 5% 15% 18% 7%	5493
	11th to 20th	Night	34% 4 8% 16% 22% 17%	5291
	21st to end	Night	42% 4 6% 23% 18% 7%	5336
April	1st to 10th	Night	34% 6% 11% 21% 20% 8%	2100
	11th to 20th	Night	33% 4 7% 22% 23% 11%	2075
	21st to end	Night	34% 5% 10% 24% 22% 5%	2100
May	1st to 10th	Night	46% 7% 12% 16% 16% 3	5389
	11th to 20th	Night	44% 6% 14% 22% 12% 1	5235
	21st to end	Night	45% 9% 12% 19% 13% 1	5771
June	1st to 10th	Night	36% 10% 14% 21% 18% 2	1890
	11th to 20th	Night	26% 8% 21% 27% 16% 2	1890
	21st to end	Night	31% 9% 17% 23% 16% 4	1890
July	1st to 10th	Night	38% 9% 22% 21% 8% 1	4105
	11th to 20th	Night	35% 11% 32% 17% 4 1	4051
	21st to end	Night	45% 12% 23% 16% 4	4462
August	1st to 10th	Night	20% 8% 15% 39% 17% 2	3232
	11th to 20th	Night	27% 8% 18% 29% 15% 3	3230
	21st to end	Night	28% 6% 16% 21% 25% 4	3586
September	1st to 10th	Night	36% 7% 16% 18% 17% 6%	2028
	11th to 20th	Night	36% 8% 10% 20% 21% 5%	2200
	21st to end	Night	30% 5% 11% 24% 24% 7%	2200
October	1st to 10th	Night	23% 4 6% 15% 29% 23%	3590
	11th to 20th	Night	18% 3 4 17% 29% 30%	3571
	21st to end	Night	23% 2 4 22% 32% 16%	4460
November	1st to 10th	Night	30% 4 8% 18% 26% 14%	2666
	11th to 20th	Night	31% 3 6% 19% 28% 13%	2730
	21st to end	Night	36% 5% 7% 19% 24% 9%	2730
December	1st to 10th	Night	40% 3 6% 21% 26% 5%	4380
	11th to 20th	Night	42% 3 9% 16% 24% 5%	4954
	21st to end	Night	45% 3 7% 24% 19% 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	27% 22% 28% 19% 4	330
	11th to 20th	Day	24% 23% 32% 15% 6%	330
	21st to end	Day	25% 24% 36% 12% 2 1	363
February	1st to 10th	Day	22% 22% 40% 13% 3	330
	11th to 20th	Day	38% 16% 33% 12% 2	330
	21st to end	Day	27% 23% 37% 10% 3	273
March	1st to 10th	Day	25% 19% 34% 18% 4 1	330
	11th to 20th	Day	21% 17% 39% 17% 5% 1	441
	21st to end	Day	20% 16% 34% 22% 7%	594
April	1st to 10th	Day	20% 13% 32% 24% 10% 1	540
	11th to 20th	Day	16% 14% 34% 23% 12% 2	532
	21st to end	Day	12% 10% 31% 31% 14% 2	540
May	1st to 10th	Day	15% 10% 29% 30% 13% 5%	543
	11th to 20th	Day	10% 6% 28% 29% 19% 7%	567
	21st to end	Day	14% 5% 21% 29% 18% 13%	617
June	1st to 10th	Day	13% 4 16% 31% 24% 12%	570
	11th to 20th	Day	8% 4 15% 27% 28% 19%	570
	21st to end	Day	9% 4 9% 27% 31% 21%	570
July	1st to 10th	Day	6% 2 8% 28% 33% 24%	565
	11th to 20th	Day	7% 2 7% 21% 34% 29%	544
	21st to end	Day	7% 2 9% 22% 36% 24%	576
August	1st to 10th	Day	8% 3 8% 25% 33% 22%	526
	11th to 20th	Day	8% 2 8% 29% 34% 19%	519
	21st to end	Day	13% 2 10% 26% 30% 19%	583
September	1st to 10th	Day	8% 4 11% 35% 29% 12%	530
	11th to 20th	Day	11% 4 13% 28% 34% 9%	530
	21st to end	Day	12% 3 17% 31% 30% 8%	530
October	1st to 10th	Day	12% 5% 26% 31% 22% 5%	530
	11th to 20th	Day	12% 6% 26% 34% 21% 2	530
	21st to end	Day	18% 13% 25% 29% 14% 2	383
November	1st to 10th	Day	18% 10% 33% 27% 9% 2	330
	11th to 20th	Day	17% 13% 41% 23% 7% 1	330
	21st to end	Day	15% 18% 39% 19% 8% 1	330
December	1st to 10th	Day	29% 17% 31% 20% 3	330
	11th to 20th	Day	21% 16% 40% 17% 5% 1	330
	21st to end	Day	27% 18% 31% 18% 6%	363
January	1st to 10th	Night	20% 14% 35% 25% 4	840
	11th to 20th	Night	23% 16% 29% 25% 6%	840
	21st to end	Night	20% 14% 36% 27% 3	924
February	1st to 10th	Night	22% 11% 33% 30% 4	840
	11th to 20th	Night	25% 13% 33% 25% 3	823
	21st to end	Night	22% 15% 36% 24% 3	692
March	1st to 10th	Night	19% 12% 30% 32% 6%	840
	11th to 20th	Night	17% 11% 37% 26% 9%	840
	21st to end	Night	16% 12% 34% 30% 8%	814
April	1st to 10th	Night	13% 10% 34% 30% 12%	630
	11th to 20th	Night	14% 8% 31% 31% 13% 1	623
	21st to end	Night	13% 9% 25% 34% 17% 2	630
May	1st to 10th	Night	10% 6% 28% 33% 20% 3	630
	11th to 20th	Night	12% 4 24% 33% 24% 4	621
	21st to end	Night	9% 3 13% 36% 34% 6%	683
June	1st to 10th	Night	9% 3 10% 39% 33% 6%	630
	11th to 20th	Night	7% 1 10% 29% 41% 12%	630
	21st to end	Night	6% 1 8% 28% 44% 13%	630
July	1st to 10th	Night	6% 1 6% 27% 46% 14%	615
	11th to 20th	Night	6% 2 5% 21% 46% 21%	597
	21st to end	Night	7% 1 7% 24% 48% 14%	657
August	1st to 10th	Night	6% 1 3 26% 49% 14%	597
	11th to 20th	Night	9% 2 6% 30% 43% 10%	594
	21st to end	Night	10% 2 7% 29% 41% 11%	660
September	1st to 10th	Night	10% 2 9% 34% 36% 9%	628
	11th to 20th	Night	12% 3 11% 29% 39% 6%	800
	21st to end	Night	9% 1 14% 34% 38% 4	800
October	1st to 10th	Night	10% 4 20% 36% 25% 5%	800
	11th to 20th	Night	10% 6% 23% 37% 21% 3	800
	21st to end	Night	16% 7% 20% 36% 19% 2	880
November	1st to 10th	Night	13% 6% 20% 39% 18% 3	820
	11th to 20th	Night	14% 8% 30% 34% 13% 1	840
	21st to end	Night	15% 9% 30% 33% 12% 1	840
December	1st to 10th	Night	22% 11% 28% 31% 7% 1	840
	11th to 20th	Night	23% 10% 28% 30% 8%	840
	21st to end	Night	24% 12% 29% 28% 7%	924

Page generated at 2026-07-13 02:14:50. Version ac07

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