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 2005-11-05 to 2024-10-14.

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% 6% 2 6% 5% 4 5% 4 6% 8% 41%	100607
	Night	12% 7% 2 6% 4 4 4 4 6% 7% 45%	97907
Transparency	Day	39% 7% 11% 21% 18% 4	100420
	Night	38% 6% 10% 20% 20% 6%	97768
Seeing	Day	15% 9% 23% 25% 19% 9%	14458
	Night	14% 7% 23% 30% 21% 4	24412

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	16% 13% 4 8% 4 3 4 3 5% 4 37%	2836
	11th to 20th	Day	15% 6% 3 6% 3 3 3 3 4 5% 49%	2977
	21st to end	Day	23% 9% 1 5% 2 3 3 2 4 6% 41%	3476
February	1st to 10th	Day	18% 11% 3 6% 3 4 4 3 4 6% 38%	1900
	11th to 20th	Day	23% 8% 2 5% 4 3 4 3 4 5% 39%	1848
	21st to end	Day	20% 9% 3 7% 5% 3 4 3 4 5% 37%	1570
March	1st to 10th	Day	21% 10% 3 7% 4 3 4 3 6% 6% 34%	3120
	11th to 20th	Day	14% 9% 3 6% 4 3 4 4 6% 7% 41%	3338
	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% 7% 55%	2013
	21st to end	Day	13% 5% 2 5% 3 3 3 3 3 6% 54%	1867
November	1st to 10th	Day	12% 6% 2 5% 4 3 3 2 4 5% 54%	1672
	11th to 20th	Day	14% 6% 2 4 3 3 3 3 4 6% 52%	1710
	21st to end	Day	14% 6% 2 5% 5% 3 4 3 5% 7% 45%	1710
December	1st to 10th	Day	21% 7% 2 6% 4 4 3 2 5% 6% 38%	1710
	11th to 20th	Day	18% 7% 3 7% 5% 4 4 2 3 4 43%	2153
	21st to end	Day	18% 10% 4 7% 5% 4 4 3 5% 7% 34%	3124
January	1st to 10th	Night	17% 13% 2 5% 3 3 3 3 4 5% 40%	4109
	11th to 20th	Night	15% 9% 4 5% 3 4 3 3 4 5% 45%	3978
	21st to end	Night	21% 12% 2 6% 3 3 3 3 3 3 40%	4180
February	1st to 10th	Night	17% 8% 2 7% 4 4 4 2 4 6% 42%	2280
	11th to 20th	Night	20% 8% 3 6% 4 3 3 2 4 5% 43%	2218
	21st to end	Night	19% 7% 2 6% 5% 3 4 3 6% 7% 39%	1884
March	1st to 10th	Night	23% 11% 3 6% 4 3 3 2 5% 5% 35%	3733
	11th to 20th	Night	12% 6% 2 5% 3 4 4 4 6% 7% 48%	3620
	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	11% 5% 2 4 3 3 4 3 4 6% 56%	2020
	21st to end	Night	14% 6% 1 5% 3 3 3 2 4 4 54%	2483
November	1st to 10th	Night	11% 5% 1 4 3 3 4 2 5% 4 58%	2406
	11th to 20th	Night	11% 7% 2 4 3 3 3 3 5% 5% 54%	2470
	21st to end	Night	13% 7% 1 5% 4 4 4 4 5% 6% 47%	2470
December	1st to 10th	Night	18% 7% 3 5% 4 4 4 3 4 6% 41%	2470
	11th to 20th	Night	17% 8% 2 5% 4 3 3 3 4 5% 47%	3042
	21st to end	Night	18% 9% 3 7% 4 4 4 4 6% 7% 34%	4532

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	49% 3 7% 14% 23% 5%	2837
	11th to 20th	Day	39% 4 7% 25% 21% 4	2977
	21st to end	Day	46% 3 7% 18% 19% 8%	3476
February	1st to 10th	Day	48% 3 7% 19% 18% 4	1900
	11th to 20th	Day	48% 4 4 17% 19% 7%	1858
	21st to end	Day	49% 5% 6% 12% 21% 7%	1570
March	1st to 10th	Day	50% 6% 8% 11% 18% 7%	3120
	11th to 20th	Day	42% 5% 8% 17% 19% 9%	3338
	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	31% 5% 9% 18% 25% 11%	2090
	11th to 20th	Day	29% 4 7% 23% 28% 9%	2014
	21st to end	Day	33% 3 6% 16% 31% 10%	1867
November	1st to 10th	Day	34% 3 6% 17% 28% 12%	1672
	11th to 20th	Day	34% 3 6% 19% 26% 11%	1710
	21st to end	Day	39% 5% 8% 17% 24% 8%	1710
December	1st to 10th	Day	47% 5% 7% 16% 19% 6%	1710
	11th to 20th	Day	47% 3 6% 16% 23% 5%	2150
	21st to end	Day	49% 3 8% 19% 19% 2	3124
January	1st to 10th	Night	46% 4 5% 15% 25% 4	4109
	11th to 20th	Night	43% 3 5% 22% 19% 8%	3978
	21st to end	Night	51% 3 3 15% 23% 5%	4180
February	1st to 10th	Night	45% 3 6% 18% 21% 8%	2280
	11th to 20th	Night	46% 3 4 18% 19% 10%	2219
	21st to end	Night	44% 6% 8% 14% 19% 9%	1884
March	1st to 10th	Night	51% 5% 7% 15% 16% 7%	3733
	11th to 20th	Night	34% 5% 10% 18% 22% 11%	3620
	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	26% 5% 8% 21% 25% 15%	2090
	11th to 20th	Night	30% 4 6% 21% 25% 13%	2024
	21st to end	Night	35% 3 6% 15% 30% 13%	2483
November	1st to 10th	Night	29% 4 7% 18% 26% 14%	2406
	11th to 20th	Night	33% 3 6% 18% 27% 13%	2470
	21st to end	Night	37% 5% 7% 18% 24% 9%	2470
December	1st to 10th	Night	43% 5% 9% 17% 20% 5%	2470
	11th to 20th	Night	41% 3 7% 14% 29% 6%	3044
	21st to end	Night	47% 4 8% 19% 19% 3	4532

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	24% 24% 29% 19% 4	270
	11th to 20th	Day	26% 20% 31% 15% 7%	270
	21st to end	Day	25% 25% 37% 11% 2 1	297
February	1st to 10th	Day	26% 23% 37% 13% 2	270
	11th to 20th	Day	39% 15% 31% 13% 2	270
	21st to end	Day	29% 24% 38% 9%	225
March	1st to 10th	Day	26% 21% 32% 17% 4 1	270
	11th to 20th	Day	21% 17% 38% 17% 6% 1	370
	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	13% 6% 26% 33% 20% 2	462
	21st to end	Day	19% 13% 25% 28% 13% 2	315
November	1st to 10th	Day	17% 10% 33% 26% 10% 3	270
	11th to 20th	Day	18% 13% 41% 21% 6%	270
	21st to end	Day	15% 17% 38% 20% 9% 1	270
December	1st to 10th	Day	29% 16% 30% 22% 3	270
	11th to 20th	Day	24% 16% 38% 17% 4 1	270
	21st to end	Day	27% 20% 31% 18% 4	297
January	1st to 10th	Night	20% 15% 35% 24% 5% 1	760
	11th to 20th	Night	22% 17% 29% 25% 7%	760
	21st to end	Night	20% 14% 36% 26% 4	836
February	1st to 10th	Night	22% 11% 33% 30% 4	760
	11th to 20th	Night	24% 13% 34% 26% 3	743
	21st to end	Night	23% 15% 36% 24% 2	628
March	1st to 10th	Night	20% 12% 31% 31% 6%	760
	11th to 20th	Night	17% 11% 37% 25% 9% 1	760
	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% 35% 21% 3	735
	21st to end	Night	16% 7% 21% 34% 19% 2	792
November	1st to 10th	Night	13% 6% 19% 39% 19% 3	740
	11th to 20th	Night	14% 8% 29% 34% 13% 1	760
	21st to end	Night	16% 9% 30% 32% 12% 1	760
December	1st to 10th	Night	21% 11% 27% 31% 8% 1	760
	11th to 20th	Night	24% 9% 29% 29% 9%	760
	21st to end	Night	25% 11% 30% 27% 7%	836

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