Birmingham
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Birmingham clear sky chart from 2005-11-06 to 2025-06-10.

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% 5% 4 5% 4 6% 8% 41%	119627
	Night	13% 7% 2 6% 4 4 4 3 5% 7% 45%	118614
Transparency	Day	40% 5% 10% 21% 19% 6%	123449
	Night	38% 4 9% 21% 21% 7%	122932
Seeing	Day	16% 9% 23% 25% 18% 9%	15269
	Night	15% 7% 23% 31% 21% 4	25289

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	11% 6% 5% 6% 3 3 3 3 4 5% 51%	4262
	21st to end	Day	21% 9% 2 6% 4 3 3 3 4 5% 39%	4972
February	1st to 10th	Day	17% 11% 2 7% 3 3 4 3 4 6% 38%	2000
	11th to 20th	Day	23% 9% 2 6% 3 3 4 3 4 5% 38%	1948
	21st to end	Day	20% 7% 4 7% 4 4 4 3 4 5% 38%	1650
March	1st to 10th	Day	19% 9% 3 7% 5% 4 4 3 5% 6% 35%	4480
	11th to 20th	Day	11% 7% 3 7% 4 3 4 3 5% 7% 46%	4768
	21st to end	Day	15% 6% 3 7% 4 4 5% 3 5% 7% 40%	5751
April	1st to 10th	Day	11% 6% 3 6% 4 3 4 4 5% 8% 46%	2400
	11th to 20th	Day	12% 6% 2 6% 4 4 4 4 5% 7% 46%	2374
	21st to end	Day	8% 5% 2 6% 5% 5% 4 5% 6% 8% 48%	2400
May	1st to 10th	Day	13% 6% 3 7% 5% 5% 6% 5% 7% 9% 35%	5766
	11th to 20th	Day	15% 8% 3 7% 5% 6% 6% 5% 7% 8% 32%	5861
	21st to end	Day	10% 7% 2 7% 5% 4 5% 4 7% 8% 41%	6433
June	1st to 10th	Day	9% 7% 2 7% 5% 6% 6% 5% 9% 10% 33%	2587
	11th to 20th	Day	6% 3 2 6% 5% 5% 6% 5% 8% 11% 43%	2470
	21st to end	Day	7% 5% 3 5% 6% 5% 6% 5% 8% 11% 40%	2470
July	1st to 10th	Day	6% 6% 3 8% 7% 8% 8% 6% 9% 12% 27%	4120
	11th to 20th	Day	8% 6% 2 7% 6% 7% 7% 7% 9% 11% 30%	4120
	21st to end	Day	10% 8% 4 10% 7% 7% 8% 6% 9% 9% 21%	4353
August	1st to 10th	Day	4 3 1 4 4 5% 5% 5% 8% 12% 50%	3785
	11th to 20th	Day	5% 4 2 6% 5% 5% 5% 5% 7% 10% 46%	3729
	21st to end	Day	6% 3 2 6% 5% 4 5% 6% 7% 8% 48%	4180
September	1st to 10th	Day	10% 6% 2 6% 5% 6% 6% 5% 6% 9% 40%	2234
	11th to 20th	Day	12% 7% 3 6% 4 4 5% 4 5% 7% 42%	2090
	21st to end	Day	10% 5% 2 6% 4 4 4 4 5% 7% 49%	2090
October	1st to 10th	Day	9% 4 2 5% 4 4 5% 3 4 6% 54%	2090
	11th to 20th	Day	9% 5% 2 4 3 3 3 3 5% 8% 56%	2090
	21st to end	Day	12% 5% 1 5% 4 4 3 2 4 6% 54%	1967
November	1st to 10th	Day	12% 6% 2 5% 4 4 4 2 4 5% 52%	1762
	11th to 20th	Day	14% 5% 2 4 3 3 4 2 4 6% 53%	1800
	21st to end	Day	13% 7% 1 5% 5% 4 4 4 5% 7% 45%	1800
December	1st to 10th	Day	19% 8% 2 5% 4 3 3 3 4 6% 43%	2930
	11th to 20th	Day	14% 9% 3 8% 5% 4 4 3 3 5% 42%	3373
	21st to end	Day	19% 10% 4 6% 3 4 3 2 4 6% 39%	4466
January	1st to 10th	Night	15% 10% 4 6% 3 3 3 3 4 4 44%	5889
	11th to 20th	Night	16% 9% 4 6% 3 3 3 2 4 5% 46%	5693
	21st to end	Night	19% 10% 3 6% 3 3 3 3 4 4 43%	5984
February	1st to 10th	Night	17% 8% 2 7% 4 4 3 3 5% 6% 42%	2400
	11th to 20th	Night	20% 8% 3 6% 5% 4 3 2 4 4 41%	2338
	21st to end	Night	18% 6% 2 6% 4 4 4 4 6% 6% 40%	1980
March	1st to 10th	Night	20% 9% 3 7% 5% 4 4 3 5% 6% 34%	5373
	11th to 20th	Night	12% 7% 2 5% 3 3 4 4 5% 7% 50%	5177
	21st to end	Night	14% 7% 3 7% 5% 4 4 3 4 5% 44%	5206
April	1st to 10th	Night	11% 5% 1 6% 4 4 4 4 6% 8% 46%	2000
	11th to 20th	Night	12% 6% 2 5% 3 3 3 3 5% 7% 50%	1975
	21st to end	Night	10% 6% 2 5% 4 3 4 3 4 7% 52%	2000
May	1st to 10th	Night	11% 8% 3 7% 5% 5% 5% 4 6% 9% 37%	4127
	11th to 20th	Night	15% 10% 3 8% 5% 5% 5% 5% 6% 8% 32%	4032
	21st to end	Night	9% 6% 2 6% 4 4 5% 5% 8% 10% 41%	4421
June	1st to 10th	Night	5% 5% 2 7% 6% 6% 6% 5% 8% 10% 40%	1797
	11th to 20th	Night	4 2 1 4 4 4 5% 4 9% 12% 52%	1710
	21st to end	Night	5% 5% 2 6% 5% 4 6% 5% 7% 12% 43%	1710
July	1st to 10th	Night	5% 6% 3 8% 6% 6% 7% 6% 8% 11% 33%	2840
	11th to 20th	Night	6% 5% 2 7% 5% 5% 5% 6% 9% 13% 34%	2840
	21st to end	Night	8% 9% 4 10% 7% 7% 6% 6% 7% 10% 26%	3196
August	1st to 10th	Night	2 2 1 4 3 4 5% 4 7% 11% 57%	3144
	11th to 20th	Night	5% 4 2 5% 4 4 5% 4 5% 9% 53%	3100
	21st to end	Night	5% 4 2 4 4 4 4 4 7% 8% 54%	3476
September	1st to 10th	Night	8% 6% 2 7% 6% 5% 5% 4 6% 8% 44%	1924
	11th to 20th	Night	10% 6% 2 6% 5% 4 5% 4 5% 7% 46%	2090
	21st to end	Night	7% 5% 2 6% 4 4 3 3 6% 7% 53%	2090
October	1st to 10th	Night	7% 5% 2 4 3 3 2 3 5% 5% 61%	2090
	11th to 20th	Night	10% 4 2 5% 3 3 3 2 4 6% 57%	2090
	21st to end	Night	13% 6% 2 5% 3 2 3 2 3 4 56%	2624
November	1st to 10th	Night	11% 5% 1 5% 3 3 4 3 4 5% 56%	2536
	11th to 20th	Night	12% 6% 2 4 4 3 3 3 5% 5% 54%	2600
	21st to end	Night	13% 6% 2 5% 4 4 4 3 4 6% 48%	2600
December	1st to 10th	Night	19% 7% 1 5% 3 3 3 2 4 6% 48%	4250
	11th to 20th	Night	15% 8% 3 6% 4 4 3 3 4 5% 45%	4822
	21st to end	Night	19% 8% 3 6% 4 3 3 3 5% 6% 39%	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% 3 4 25% 21% 3	4057
	11th to 20th	Day	37% 3 4 24% 28% 4	4262
	21st to end	Day	50% 2 5% 15% 20% 8%	4972
February	1st to 10th	Day	48% 3 6% 20% 18% 5%	2000
	11th to 20th	Day	49% 3 4 18% 19% 7%	1958
	21st to end	Day	49% 4 5% 14% 22% 7%	1650
March	1st to 10th	Day	50% 3 7% 13% 20% 7%	4480
	11th to 20th	Day	38% 3 6% 17% 21% 15%	4768
	21st to end	Day	44% 4 8% 20% 20% 4	5753
April	1st to 10th	Day	37% 5% 11% 21% 19% 8%	2400
	11th to 20th	Day	38% 4 7% 20% 24% 8%	2374
	21st to end	Day	33% 5% 11% 26% 23% 3	2400
May	1st to 10th	Day	43% 7% 12% 24% 12% 2	5802
	11th to 20th	Day	48% 5% 16% 22% 9% 1	5865
	21st to end	Day	39% 6% 12% 25% 17% 1	6447
June	1st to 10th	Day	40% 8% 16% 20% 15% 1	2587
	11th to 20th	Day	31% 7% 18% 26% 16% 2	2470
	21st to end	Day	36% 8% 16% 24% 11% 4	2470
July	1st to 10th	Day	44% 8% 22% 20% 5% 1	4055
	11th to 20th	Day	43% 8% 22% 21% 6% 1	4051
	21st to end	Day	54% 10% 14% 16% 5%	4214
August	1st to 10th	Day	25% 7% 12% 37% 19% 1	3773
	11th to 20th	Day	32% 5% 18% 26% 16% 3	3765
	21st to end	Day	32% 6% 16% 21% 22% 4	4179
September	1st to 10th	Day	39% 9% 12% 18% 17% 6%	2234
	11th to 20th	Day	41% 6% 10% 19% 19% 4	2090
	21st to end	Day	35% 5% 8% 23% 22% 7%	2090
October	1st to 10th	Day	23% 3 5% 13% 33% 23%	3480
	11th to 20th	Day	19% 3 4 15% 32% 28%	3458
	21st to end	Day	25% 2 5% 19% 36% 13%	3217
November	1st to 10th	Day	35% 3 6% 18% 27% 11%	1762
	11th to 20th	Day	34% 3 6% 19% 27% 12%	1800
	21st to end	Day	39% 4 7% 17% 25% 8%	1800
December	1st to 10th	Day	44% 2 5% 18% 27% 4	2930
	11th to 20th	Day	48% 2 8% 19% 18% 5%	3370
	21st to end	Day	47% 3 5% 24% 20% 1	4466
January	1st to 10th	Night	44% 3 3 23% 24% 3	5889
	11th to 20th	Night	43% 3 3 20% 24% 8%	5693
	21st to end	Night	47% 2 3 15% 27% 6%	5984
February	1st to 10th	Night	44% 3 6% 19% 21% 8%	2400
	11th to 20th	Night	48% 2 5% 18% 18% 10%	2339
	21st to end	Night	43% 6% 6% 15% 20% 10%	1980
March	1st to 10th	Night	51% 3 5% 15% 18% 8%	5373
	11th to 20th	Night	35% 3 8% 16% 22% 17%	5177
	21st to end	Night	44% 4 5% 23% 18% 7%	5206
April	1st to 10th	Night	35% 4 12% 21% 20% 8%	2000
	11th to 20th	Night	34% 4 8% 21% 23% 11%	1975
	21st to end	Night	33% 4 11% 25% 22% 5%	2000
May	1st to 10th	Night	42% 7% 13% 20% 15% 2	4155
	11th to 20th	Night	50% 7% 14% 20% 8% 1	4033
	21st to end	Night	35% 8% 13% 26% 17% 2	4429
June	1st to 10th	Night	35% 9% 16% 21% 18% 2	1797
	11th to 20th	Night	23% 6% 22% 29% 17% 3	1710
	21st to end	Night	31% 8% 16% 22% 17% 4	1710
July	1st to 10th	Night	42% 8% 24% 18% 7% 1	2795
	11th to 20th	Night	38% 7% 26% 23% 5% 1	2786
	21st to end	Night	51% 11% 16% 16% 6% 1	3094
August	1st to 10th	Night	21% 6% 15% 40% 17% 2	3132
	11th to 20th	Night	28% 6% 17% 30% 16% 3	3130
	21st to end	Night	27% 5% 15% 22% 26% 5%	3476
September	1st to 10th	Night	37% 6% 16% 18% 16% 6%	1924
	11th to 20th	Night	36% 9% 10% 19% 21% 5%	2090
	21st to end	Night	31% 4 10% 24% 24% 8%	2090
October	1st to 10th	Night	22% 2 5% 17% 30% 24%	3480
	11th to 20th	Night	19% 2 3 18% 29% 29%	3461
	21st to end	Night	23% 2 3 23% 32% 16%	4324
November	1st to 10th	Night	30% 3 9% 19% 25% 13%	2536
	11th to 20th	Night	33% 2 5% 18% 28% 13%	2600
	21st to end	Night	37% 4 6% 19% 24% 9%	2600
December	1st to 10th	Night	40% 3 5% 21% 26% 5%	4250
	11th to 20th	Night	44% 2 9% 16% 24% 4	4824
	21st to end	Night	45% 4 6% 24% 19% 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	27% 21% 31% 17% 3	300
	11th to 20th	Day	26% 22% 31% 15% 6%	300
	21st to end	Day	27% 22% 36% 12% 2 1	330
February	1st to 10th	Day	24% 20% 39% 12% 4	300
	11th to 20th	Day	38% 16% 32% 12% 2	300
	21st to end	Day	27% 21% 40% 9% 2	249
March	1st to 10th	Day	28% 19% 34% 15% 4 1	300
	11th to 20th	Day	22% 19% 35% 17% 6% 1	405
	21st to end	Day	23% 14% 35% 20% 7% 1	550
April	1st to 10th	Day	22% 12% 33% 23% 9% 1	500
	11th to 20th	Day	18% 11% 36% 22% 12% 1	492
	21st to end	Day	14% 10% 31% 31% 13% 2	500
May	1st to 10th	Day	14% 8% 29% 30% 15% 4	503
	11th to 20th	Day	11% 5% 30% 28% 18% 8%	527
	21st to end	Day	14% 4 21% 28% 18% 14%	573
June	1st to 10th	Day	13% 4 16% 32% 23% 13%	526
	11th to 20th	Day	7% 3 13% 27% 30% 20%	490
	21st to end	Day	11% 4 9% 28% 30% 18%	490
July	1st to 10th	Day	7% 2 6% 27% 34% 24%	485
	11th to 20th	Day	8% 1 8% 21% 32% 29%	484
	21st to end	Day	8% 3 7% 24% 34% 24%	532
August	1st to 10th	Day	8% 1 9% 26% 32% 24%	486
	11th to 20th	Day	9% 3 7% 29% 35% 17%	479
	21st to end	Day	13% 1 9% 25% 32% 19%	539
September	1st to 10th	Day	9% 2 11% 34% 30% 14%	490
	11th to 20th	Day	14% 3 12% 29% 31% 10%	490
	21st to end	Day	12% 3 16% 30% 30% 8%	490
October	1st to 10th	Day	11% 6% 23% 33% 21% 6%	490
	11th to 20th	Day	11% 7% 27% 35% 18% 1	490
	21st to end	Day	17% 10% 26% 32% 13% 1	349
November	1st to 10th	Day	21% 8% 34% 27% 8% 2	300
	11th to 20th	Day	18% 14% 41% 21% 6%	300
	21st to end	Day	16% 18% 36% 22% 8% 1	300
December	1st to 10th	Day	28% 14% 34% 21% 3	300
	11th to 20th	Day	25% 15% 38% 16% 5% 1	300
	21st to end	Day	30% 15% 31% 18% 5%	330
January	1st to 10th	Night	21% 14% 35% 26% 4 1	800
	11th to 20th	Night	24% 14% 30% 26% 7%	800
	21st to end	Night	21% 13% 35% 27% 4	880
February	1st to 10th	Night	22% 11% 33% 30% 5%	800
	11th to 20th	Night	25% 13% 33% 25% 3	783
	21st to end	Night	22% 14% 36% 26% 2	660
March	1st to 10th	Night	20% 12% 32% 30% 6%	800
	11th to 20th	Night	17% 10% 37% 26% 9% 1	800
	21st to end	Night	18% 11% 33% 29% 8%	774
April	1st to 10th	Night	14% 11% 34% 29% 12% 1	600
	11th to 20th	Night	16% 7% 33% 29% 13% 2	593
	21st to end	Night	13% 8% 27% 33% 18% 2	600
May	1st to 10th	Night	11% 5% 26% 35% 19% 3	600
	11th to 20th	Night	11% 3 24% 33% 24% 4	591
	21st to end	Night	8% 2 14% 36% 34% 6%	650
June	1st to 10th	Night	9% 2 10% 40% 33% 6%	599
	11th to 20th	Night	5% 1 10% 29% 42% 13%	570
	21st to end	Night	7% 2 8% 29% 42% 12%	570
July	1st to 10th	Night	6% 1 6% 29% 43% 15%	555
	11th to 20th	Night	7% 1 4 23% 45% 20%	552
	21st to end	Night	7% 2 6% 24% 46% 14%	624
August	1st to 10th	Night	5% 1 3 27% 49% 16%	567
	11th to 20th	Night	10% 1 6% 29% 44% 10%	564
	21st to end	Night	11% 1 7% 28% 43% 11%	627
September	1st to 10th	Night	10% 3 9% 31% 38% 9%	594
	11th to 20th	Night	13% 3 9% 32% 38% 5%	760
	21st to end	Night	9% 1 14% 33% 39% 4	760
October	1st to 10th	Night	10% 3 20% 35% 26% 6%	760
	11th to 20th	Night	12% 5% 23% 38% 20% 3	760
	21st to end	Night	15% 7% 20% 37% 19% 2	836
November	1st to 10th	Night	13% 6% 21% 38% 19% 3	780
	11th to 20th	Night	16% 6% 29% 35% 12% 1	800
	21st to end	Night	17% 7% 29% 33% 12% 2	800
December	1st to 10th	Night	22% 11% 27% 32% 8% 1	800
	11th to 20th	Night	24% 9% 29% 30% 8%	800
	21st to end	Night	26% 10% 30% 27% 7%	880

Page generated at 2025-06-07 09:28:12. Version ac07

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