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 2025-03-12.

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% 3 6% 4 4 5% 4 6% 8% 42%	109235
	Night	13% 7% 2 6% 4 4 4 3 5% 7% 45%	109793
Transparency	Day	38% 6% 10% 20% 19% 6%	113055
	Night	37% 5% 9% 21% 21% 7%	114111
Seeing	Day	15% 10% 23% 24% 19% 9%	14913
	Night	14% 7% 23% 30% 21% 4	25008

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	15% 10% 5% 7% 3 3 3 3 4 4 44%	4056
	11th to 20th	Day	12% 6% 5% 6% 3 3 3 2 4 6% 51%	4267
	21st to end	Day	21% 9% 2 7% 4 4 3 2 4 5% 39%	4972
February	1st to 10th	Day	18% 10% 3 6% 4 4 4 3 4 6% 39%	2000
	11th to 20th	Day	23% 8% 2 5% 4 3 4 3 4 5% 39%	1948
	21st to end	Day	19% 8% 3 7% 5% 3 4 3 4 5% 38%	1650
March	1st to 10th	Day	21% 9% 3 7% 4 3 4 3 6% 6% 34%	3220
	11th to 20th	Day	14% 8% 3 6% 4 3 4 4 6% 7% 41%	3348
	21st to end	Day	16% 7% 3 7% 4 4 5% 4 6% 7% 38%	4005
April	1st to 10th	Day	12% 6% 3 6% 4 4 4 4 5% 7% 46%	2280
	11th to 20th	Day	12% 6% 2 6% 5% 4 4 3 6% 7% 45%	2254
	21st to end	Day	8% 4 2 6% 4 5% 4 4 5% 9% 49%	2280
May	1st to 10th	Day	9% 5% 2 5% 5% 4 5% 5% 7% 9% 43%	4011
	11th to 20th	Day	16% 6% 2 6% 4 4 4 4 6% 8% 38%	4081
	21st to end	Day	9% 5% 2 6% 5% 4 4 4 6% 9% 44%	4476
June	1st to 10th	Day	10% 7% 3 7% 5% 6% 6% 5% 8% 10% 33%	2470
	11th to 20th	Day	6% 4 2 5% 5% 4 7% 5% 8% 12% 44%	2470
	21st to end	Day	7% 5% 2 6% 5% 6% 5% 5% 8% 12% 39%	2470
July	1st to 10th	Day	6% 5% 2 9% 7% 8% 8% 6% 9% 12% 27%	4120
	11th to 20th	Day	8% 6% 2 7% 6% 6% 8% 7% 9% 12% 30%	4120
	21st to end	Day	9% 8% 4 9% 7% 7% 8% 6% 9% 10% 21%	4349
August	1st to 10th	Day	4 2 1 4 4 4 5% 5% 8% 12% 51%	3785
	11th to 20th	Day	6% 4 2 5% 5% 5% 6% 4 7% 10% 46%	3729
	21st to end	Day	7% 4 2 5% 5% 5% 5% 5% 7% 8% 47%	4180
September	1st to 10th	Day	10% 5% 2 7% 5% 5% 6% 5% 6% 8% 41%	2233
	11th to 20th	Day	13% 7% 2 6% 4 4 5% 4 4 8% 42%	2090
	21st to end	Day	10% 6% 2 5% 4 4 4 3 6% 7% 50%	2090
October	1st to 10th	Day	9% 4 2 6% 3 5% 5% 2 4 6% 55%	2090
	11th to 20th	Day	9% 4 2 5% 3 3 3 3 5% 6% 57%	2090
	21st to end	Day	12% 5% 2 5% 3 3 3 3 3 5% 55%	1970
November	1st to 10th	Day	12% 6% 2 5% 4 4 3 2 4 6% 53%	1762
	11th to 20th	Day	13% 6% 2 4 3 3 3 3 4 6% 53%	1800
	21st to end	Day	14% 7% 2 5% 5% 3 4 3 5% 7% 46%	1800
December	1st to 10th	Day	20% 7% 2 5% 3 3 3 2 4 6% 43%	2930
	11th to 20th	Day	14% 8% 3 7% 5% 4 5% 3 3 5% 42%	3373
	21st to end	Day	19% 10% 3 6% 4 3 3 2 4 6% 39%	4466
January	1st to 10th	Night	15% 10% 4 6% 3 3 3 3 4 5% 45%	5889
	11th to 20th	Night	15% 9% 4 6% 3 3 3 2 4 5% 46%	5688
	21st to end	Night	19% 10% 2 5% 3 3 3 3 3 4 43%	5984
February	1st to 10th	Night	17% 8% 2 7% 4 4 4 2 4 6% 42%	2400
	11th to 20th	Night	20% 8% 3 7% 4 3 3 2 4 5% 42%	2338
	21st to end	Night	19% 6% 2 6% 4 3 4 3 6% 7% 41%	1980
March	1st to 10th	Night	23% 11% 3 6% 4 3 3 3 5% 5% 35%	3853
	11th to 20th	Night	12% 6% 2 5% 3 4 4 4 6% 7% 48%	3639
	21st to end	Night	15% 7% 3 8% 5% 4 4 3 4 6% 40%	3640
April	1st to 10th	Night	11% 4 2 6% 5% 3 4 4 6% 8% 46%	1900
	11th to 20th	Night	13% 6% 2 5% 3 3 3 3 6% 6% 50%	1875
	21st to end	Night	10% 4 2 5% 4 4 4 3 4 6% 53%	1900
May	1st to 10th	Night	8% 6% 2 7% 5% 5% 5% 4 6% 9% 42%	2881
	11th to 20th	Night	16% 8% 2 6% 4 4 5% 4 5% 8% 38%	2812
	21st to end	Night	5% 3 1 5% 4 4 5% 6% 8% 11% 47%	3079
June	1st to 10th	Night	5% 6% 3 6% 5% 6% 6% 5% 9% 11% 38%	1710
	11th to 20th	Night	4 2 1 4 3 4 5% 5% 9% 11% 51%	1710
	21st to end	Night	6% 4 2 5% 6% 5% 5% 5% 8% 10% 44%	1710
July	1st to 10th	Night	6% 6% 2 9% 6% 6% 7% 6% 8% 11% 32%	2840
	11th to 20th	Night	7% 6% 2 7% 5% 5% 6% 6% 9% 13% 34%	2840
	21st to end	Night	8% 9% 3 10% 7% 6% 7% 6% 8% 9% 27%	3199
August	1st to 10th	Night	3 2 1 4 3 3 4 4 7% 11% 56%	3144
	11th to 20th	Night	5% 4 2 5% 4 4 5% 4 6% 9% 53%	3100
	21st to end	Night	5% 3 1 5% 4 4 4 4 7% 8% 54%	3476
September	1st to 10th	Night	8% 5% 2 8% 5% 5% 5% 4 6% 9% 43%	1926
	11th to 20th	Night	11% 6% 2 6% 5% 4 5% 3 5% 7% 46%	2090
	21st to end	Night	7% 5% 2 6% 4 4 4 3 5% 7% 53%	2090
October	1st to 10th	Night	7% 5% 2 4 3 3 2 3 4 5% 61%	2090
	11th to 20th	Night	10% 5% 2 4 3 3 4 3 4 6% 57%	2090
	21st to end	Night	13% 6% 1 5% 3 3 3 2 4 4 56%	2622
November	1st to 10th	Night	11% 5% 2 5% 3 3 4 2 5% 5% 56%	2536
	11th to 20th	Night	12% 7% 2 4 3 3 3 2 5% 6% 54%	2600
	21st to end	Night	12% 7% 1 5% 4 4 4 4 5% 6% 48%	2600
December	1st to 10th	Night	19% 7% 2 4 3 3 2 3 3 6% 48%	4250
	11th to 20th	Night	15% 8% 2 7% 4 3 4 3 4 5% 46%	4822
	21st to end	Night	19% 8% 3 6% 4 3 3 3 5% 6% 40%	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	44% 2 6% 24% 21% 4	4057
	11th to 20th	Day	36% 3 5% 23% 28% 4	4267
	21st to end	Day	49% 2 6% 15% 18% 9%	4972
February	1st to 10th	Day	48% 3 7% 19% 18% 5%	2000
	11th to 20th	Day	49% 4 4 17% 19% 7%	1958
	21st to end	Day	48% 4 6% 13% 21% 7%	1650
March	1st to 10th	Day	50% 5% 8% 11% 18% 7%	3220
	11th to 20th	Day	42% 5% 8% 17% 19% 9%	3348
	21st to end	Day	44% 6% 9% 17% 20% 5%	4005
April	1st to 10th	Day	36% 6% 10% 21% 19% 8%	2280
	11th to 20th	Day	39% 5% 7% 19% 22% 8%	2254
	21st to end	Day	31% 6% 9% 26% 24% 3	2280
May	1st to 10th	Day	33% 9% 12% 27% 16% 2	4048
	11th to 20th	Day	41% 7% 12% 27% 11% 1	4085
	21st to end	Day	34% 9% 13% 27% 15% 2	4489
June	1st to 10th	Day	41% 10% 15% 19% 14% 1	2470
	11th to 20th	Day	30% 10% 17% 25% 17% 2	2470
	21st to end	Day	35% 10% 17% 23% 11% 4	2470
July	1st to 10th	Day	42% 11% 22% 18% 5% 1	4055
	11th to 20th	Day	41% 12% 21% 20% 5% 1	4051
	21st to end	Day	53% 12% 14% 15% 5%	4210
August	1st to 10th	Day	23% 9% 13% 35% 19% 1	3773
	11th to 20th	Day	31% 7% 18% 26% 15% 3	3765
	21st to end	Day	31% 8% 16% 20% 21% 4	4179
September	1st to 10th	Day	39% 10% 13% 17% 17% 5%	2233
	11th to 20th	Day	40% 7% 10% 19% 19% 4	2090
	21st to end	Day	34% 6% 9% 22% 22% 7%	2090
October	1st to 10th	Day	23% 3 6% 13% 32% 23%	3480
	11th to 20th	Day	19% 3 4 15% 31% 29%	3458
	21st to end	Day	24% 3 6% 19% 36% 13%	3220
November	1st to 10th	Day	35% 3 6% 17% 27% 11%	1762
	11th to 20th	Day	34% 3 6% 18% 27% 12%	1800
	21st to end	Day	38% 5% 8% 17% 25% 8%	1800
December	1st to 10th	Day	43% 3 5% 17% 27% 4	2930
	11th to 20th	Day	46% 3 9% 18% 18% 5%	3370
	21st to end	Day	46% 3 6% 23% 20% 1	4466
January	1st to 10th	Night	44% 3 4 23% 24% 3	5889
	11th to 20th	Night	43% 2 4 19% 24% 8%	5688
	21st to end	Night	47% 2 3 15% 26% 6%	5984
February	1st to 10th	Night	44% 3 6% 18% 21% 8%	2400
	11th to 20th	Night	47% 3 4 18% 18% 10%	2339
	21st to end	Night	43% 6% 7% 15% 20% 9%	1980
March	1st to 10th	Night	51% 5% 7% 15% 16% 7%	3853
	11th to 20th	Night	34% 5% 10% 18% 22% 11%	3639
	21st to end	Night	45% 5% 7% 20% 19% 5%	3640
April	1st to 10th	Night	35% 6% 11% 21% 20% 8%	1900
	11th to 20th	Night	34% 5% 7% 20% 22% 12%	1875
	21st to end	Night	32% 6% 9% 25% 23% 6%	1900
May	1st to 10th	Night	35% 9% 12% 20% 20% 3	2909
	11th to 20th	Night	40% 8% 15% 23% 12% 2	2813
	21st to end	Night	26% 10% 14% 30% 18% 3	3087
June	1st to 10th	Night	36% 11% 14% 21% 17% 2	1710
	11th to 20th	Night	23% 8% 21% 29% 16% 3	1710
	21st to end	Night	31% 10% 16% 22% 17% 4	1710
July	1st to 10th	Night	41% 11% 23% 18% 6% 2	2795
	11th to 20th	Night	36% 11% 26% 20% 6% 1	2786
	21st to end	Night	49% 12% 17% 15% 6% 1	3097
August	1st to 10th	Night	19% 8% 14% 39% 17% 2	3132
	11th to 20th	Night	27% 8% 17% 30% 16% 3	3130
	21st to end	Night	27% 6% 15% 22% 26% 4	3476
September	1st to 10th	Night	36% 7% 17% 18% 17% 6%	1926
	11th to 20th	Night	36% 8% 11% 19% 21% 5%	2090
	21st to end	Night	30% 5% 10% 24% 23% 8%	2090
October	1st to 10th	Night	22% 4 6% 15% 29% 24%	3480
	11th to 20th	Night	18% 3 4 17% 29% 30%	3461
	21st to end	Night	22% 2 4 22% 32% 17%	4322
November	1st to 10th	Night	30% 4 8% 18% 25% 14%	2536
	11th to 20th	Night	33% 3 5% 18% 28% 13%	2600
	21st to end	Night	36% 5% 7% 19% 24% 9%	2600
December	1st to 10th	Night	39% 3 6% 21% 26% 5%	4250
	11th to 20th	Night	43% 3 9% 16% 24% 5%	4824
	21st to end	Night	45% 3 7% 24% 20% 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% 24% 28% 19% 4	300
	11th to 20th	Day	25% 22% 32% 15% 6%	300
	21st to end	Day	25% 24% 37% 12% 2 1	330
February	1st to 10th	Day	23% 21% 38% 14% 3	300
	11th to 20th	Day	39% 16% 32% 12% 2	300
	21st to end	Day	26% 24% 39% 10% 2	249
March	1st to 10th	Day	26% 21% 33% 16% 3 1	300
	11th to 20th	Day	21% 17% 38% 17% 6% 1	373
	21st to end	Day	22% 18% 32% 20% 8%	506
April	1st to 10th	Day	21% 13% 33% 23% 10% 1	460
	11th to 20th	Day	18% 14% 32% 22% 13% 1	452
	21st to end	Day	12% 11% 32% 30% 13% 2	460
May	1st to 10th	Day	13% 9% 28% 30% 15% 5%	463
	11th to 20th	Day	10% 5% 27% 28% 22% 8%	487
	21st to end	Day	12% 4 21% 30% 19% 15%	529
June	1st to 10th	Day	13% 3 16% 32% 23% 13%	490
	11th to 20th	Day	6% 4 13% 27% 29% 21%	490
	21st to end	Day	10% 4 9% 28% 30% 19%	490
July	1st to 10th	Day	7% 2 8% 27% 33% 23%	485
	11th to 20th	Day	7% 2 8% 21% 31% 30%	484
	21st to end	Day	7% 2 10% 22% 35% 24%	532
August	1st to 10th	Day	7% 2 9% 25% 34% 23%	486
	11th to 20th	Day	9% 2 8% 29% 35% 17%	479
	21st to end	Day	13% 2 9% 24% 31% 19%	539
September	1st to 10th	Day	9% 4 9% 34% 30% 13%	490
	11th to 20th	Day	12% 4 14% 28% 33% 9%	490
	21st to end	Day	12% 3 17% 29% 30% 8%	490
October	1st to 10th	Day	11% 6% 25% 31% 23% 5%	490
	11th to 20th	Day	12% 6% 27% 33% 20% 1	490
	21st to end	Day	17% 12% 24% 30% 15% 2	349
November	1st to 10th	Day	18% 10% 33% 27% 9% 3	300
	11th to 20th	Day	18% 14% 40% 21% 6% 1	300
	21st to end	Day	15% 18% 37% 20% 8% 1	300
December	1st to 10th	Day	28% 16% 32% 21% 3	300
	11th to 20th	Day	22% 16% 40% 17% 4 1	300
	21st to end	Day	28% 19% 30% 18% 6%	330
January	1st to 10th	Night	20% 15% 36% 24% 5% 1	800
	11th to 20th	Night	23% 16% 28% 25% 7%	800
	21st to end	Night	19% 14% 36% 27% 4	880
February	1st to 10th	Night	22% 11% 34% 30% 5%	800
	11th to 20th	Night	25% 13% 33% 25% 3	783
	21st to end	Night	22% 14% 36% 25% 2	660
March	1st to 10th	Night	20% 13% 31% 31% 6%	800
	11th to 20th	Night	17% 11% 37% 26% 9% 1	767
	21st to end	Night	17% 13% 33% 28% 9%	736
April	1st to 10th	Night	13% 11% 34% 28% 13%	570
	11th to 20th	Night	15% 9% 32% 29% 13% 2	563
	21st to end	Night	12% 10% 27% 33% 17% 2	570
May	1st to 10th	Night	9% 6% 28% 33% 21% 3	570
	11th to 20th	Night	11% 4 24% 32% 25% 4	561
	21st to end	Night	7% 2 13% 36% 35% 6%	617
June	1st to 10th	Night	8% 3 10% 39% 33% 6%	570
	11th to 20th	Night	5% 1 10% 28% 43% 13%	570
	21st to end	Night	6% 1 8% 29% 44% 12%	570
July	1st to 10th	Night	6% 1 6% 28% 45% 14%	555
	11th to 20th	Night	6% 1 5% 22% 46% 21%	552
	21st to end	Night	7% 1 7% 25% 47% 13%	624
August	1st to 10th	Night	6% 1 4 26% 50% 15%	567
	11th to 20th	Night	9% 1 7% 31% 43% 9%	564
	21st to end	Night	10% 2 7% 28% 42% 11%	627
September	1st to 10th	Night	10% 2 9% 33% 36% 9%	596
	11th to 20th	Night	13% 3 12% 29% 38% 6%	760
	21st to end	Night	9% 1 14% 33% 39% 4	760
October	1st to 10th	Night	9% 3 20% 36% 26% 6%	760
	11th to 20th	Night	11% 6% 24% 36% 21% 3	760
	21st to end	Night	15% 7% 20% 35% 20% 2	836
November	1st to 10th	Night	14% 6% 20% 39% 18% 3	780
	11th to 20th	Night	14% 8% 29% 34% 13% 1	800
	21st to end	Night	16% 9% 30% 32% 12% 1	800
December	1st to 10th	Night	22% 11% 28% 31% 8% 1	800
	11th to 20th	Night	24% 9% 29% 30% 8%	800
	21st to end	Night	24% 12% 30% 27% 7%	880

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