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 2024-03-23.

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	13% 6% 3 5% 5% 3 5% 5% 6% 8% 41%	79211
	Night	12% 6% 3 5% 5% 3 4 4 5% 7% 45%	80680
Transparency	Day	40% 6% 11% 20% 19% 5%	78938
	Night	38% 5% 10% 20% 20% 6%	80472
Seeing	Day	16% 8% 22% 25% 19% 9%	13638
	Night	15% 7% 23% 30% 21% 4	23755

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% 12% 4 7% 5% 2 4 4 5% 4 37%	2836
	11th to 20th	Day	14% 7% 3 6% 4 2 4 4 3 5% 49%	2972
	21st to end	Day	23% 8% 3 4 4 2 3 3 3 6% 41%	3476
February	1st to 10th	Day	18% 11% 3 6% 4 2 4 4 4 6% 38%	1900
	11th to 20th	Day	23% 7% 3 5% 4 2 4 4 4 5% 39%	1848
	21st to end	Day	21% 7% 4 6% 5% 3 4 3 4 5% 37%	1570
March	1st to 10th	Day	15% 8% 2 5% 6% 2 4 3 4 6% 44%	1900
	11th to 20th	Day	15% 8% 4 7% 5% 3 4 5% 5% 7% 37%	2000
	21st to end	Day	15% 6% 4 6% 4 3 5% 4 6% 8% 39%	2308
April	1st to 10th	Day	11% 6% 3 5% 4 2 5% 5% 5% 7% 47%	2160
	11th to 20th	Day	13% 5% 2 5% 6% 3 4 5% 4 8% 45%	2134
	21st to end	Day	9% 4 2 5% 5% 3 5% 5% 5% 8% 50%	2160
May	1st to 10th	Day	11% 4 2 4 4 3 4 5% 6% 8% 48%	2299
	11th to 20th	Day	9% 5% 3 5% 5% 3 5% 5% 6% 9% 44%	2301
	21st to end	Day	8% 5% 2 6% 6% 4 5% 5% 6% 9% 43%	2531
June	1st to 10th	Day	9% 7% 3 6% 5% 4 6% 7% 8% 10% 33%	2340
	11th to 20th	Day	6% 3 2 5% 5% 4 6% 6% 8% 11% 44%	2340
	21st to end	Day	7% 4 3 5% 6% 4 7% 6% 8% 11% 40%	2340
July	1st to 10th	Day	8% 5% 4 6% 7% 5% 8% 8% 9% 11% 30%	2340
	11th to 20th	Day	8% 5% 2 6% 7% 5% 8% 9% 9% 12% 30%	2340
	21st to end	Day	7% 4 3 6% 7% 5% 9% 8% 10% 12% 30%	2487
August	1st to 10th	Day	6% 4 2 6% 7% 5% 7% 9% 10% 13% 31%	2148
	11th to 20th	Day	9% 6% 3 6% 8% 4 7% 7% 8% 9% 31%	2125
	21st to end	Day	11% 6% 3 7% 8% 4 6% 7% 7% 9% 32%	2376
September	1st to 10th	Day	10% 5% 2 5% 5% 4 6% 6% 6% 9% 40%	2117
	11th to 20th	Day	12% 7% 2 5% 5% 3 5% 5% 5% 7% 43%	1980
	21st to end	Day	10% 5% 2 4 5% 3 4 5% 5% 7% 50%	1980
October	1st to 10th	Day	9% 4 2 4 5% 3 5% 4 4 6% 53%	1980
	11th to 20th	Day	9% 5% 2 4 3 3 3 4 5% 8% 54%	1980
	21st to end	Day	13% 5% 2 4 5% 3 4 3 4 6% 53%	1864
November	1st to 10th	Day	12% 6% 2 4 4 3 3 3 4 5% 54%	1672
	11th to 20th	Day	14% 5% 2 4 3 2 4 4 4 6% 53%	1710
	21st to end	Day	14% 6% 2 5% 5% 3 4 5% 5% 7% 45%	1710
December	1st to 10th	Day	21% 8% 3 6% 5% 3 5% 4 4 6% 38%	1710
	11th to 20th	Day	17% 8% 3 6% 6% 3 4 4 3 4 43%	2153
	21st to end	Day	18% 11% 4 6% 5% 3 4 4 5% 7% 34%	3124
January	1st to 10th	Night	17% 13% 3 4 4 2 4 4 4 5% 40%	4109
	11th to 20th	Night	16% 9% 4 5% 5% 2 3 3 4 5% 45%	3983
	21st to end	Night	21% 12% 3 5% 4 2 3 3 3 3 39%	4180
February	1st to 10th	Night	17% 8% 2 6% 5% 3 4 3 4 6% 42%	2280
	11th to 20th	Night	20% 8% 3 5% 5% 3 3 3 4 5% 43%	2218
	21st to end	Night	19% 6% 3 5% 5% 3 4 5% 6% 6% 39%	1884
March	1st to 10th	Night	16% 7% 2 4 4 2 4 4 4 6% 45%	2280
	11th to 20th	Night	14% 7% 3 5% 5% 3 5% 5% 6% 7% 41%	2179
	21st to end	Night	14% 5% 3 5% 5% 2 5% 4 5% 7% 44%	2107
April	1st to 10th	Night	11% 4 2 6% 5% 3 4 5% 6% 9% 46%	1800
	11th to 20th	Night	13% 6% 2 4 3 2 3 4 4 7% 50%	1775
	21st to end	Night	10% 4 2 4 5% 2 5% 3 5% 7% 53%	1800
May	1st to 10th	Night	9% 4 2 4 4 3 4 4 6% 9% 52%	1659
	11th to 20th	Night	9% 4 2 5% 5% 3 5% 6% 6% 10% 45%	1592
	21st to end	Night	6% 4 1 5% 4 3 5% 6% 7% 11% 47%	1744
June	1st to 10th	Night	5% 4 3 6% 6% 4 6% 6% 9% 11% 39%	1620
	11th to 20th	Night	4 2 1 3 3 3 6% 5% 8% 12% 53%	1620
	21st to end	Night	5% 4 2 6% 5% 3 6% 6% 7% 12% 44%	1620
July	1st to 10th	Night	5% 4 4 5% 5% 4 8% 9% 8% 10% 38%	1620
	11th to 20th	Night	5% 4 3 5% 6% 4 6% 7% 8% 12% 41%	1620
	21st to end	Night	5% 4 3 6% 7% 4 6% 7% 7% 11% 41%	1827
August	1st to 10th	Night	4 3 2 5% 5% 4 7% 7% 9% 14% 41%	1790
	11th to 20th	Night	7% 5% 3 5% 6% 4 6% 6% 7% 10% 41%	1776
	21st to end	Night	9% 6% 3 5% 6% 4 6% 5% 7% 8% 41%	1980
September	1st to 10th	Night	8% 5% 2 6% 7% 4 5% 5% 6% 8% 44%	1822
	11th to 20th	Night	9% 6% 2 5% 6% 3 5% 5% 6% 7% 46%	1980
	21st to end	Night	6% 5% 2 5% 5% 3 3 4 5% 8% 54%	1980
October	1st to 10th	Night	7% 5% 2 4 4 2 3 3 4 6% 61%	1980
	11th to 20th	Night	11% 4 2 4 4 2 4 3 4 6% 55%	1980
	21st to end	Night	14% 6% 3 4 4 2 3 3 2 4 55%	2485
November	1st to 10th	Night	11% 5% 1 3 4 2 4 4 4 5% 58%	2406
	11th to 20th	Night	12% 6% 2 4 4 2 3 3 4 6% 54%	2470
	21st to end	Night	13% 5% 2 5% 5% 4 4 4 4 6% 47%	2470
December	1st to 10th	Night	18% 7% 2 5% 5% 2 4 4 4 7% 41%	2470
	11th to 20th	Night	16% 8% 3 5% 5% 2 3 3 3 5% 47%	3042
	21st to end	Night	18% 8% 3 6% 5% 3 4 5% 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% 4 5% 15% 22% 5%	2837
	11th to 20th	Day	39% 5% 6% 26% 21% 4	2972
	21st to end	Day	46% 3 6% 18% 21% 6%	3476
February	1st to 10th	Day	48% 3 6% 20% 18% 5%	1900
	11th to 20th	Day	49% 3 4 17% 19% 7%	1858
	21st to end	Day	50% 4 5% 13% 21% 7%	1570
March	1st to 10th	Day	43% 3 8% 15% 21% 10%	1900
	11th to 20th	Day	47% 4 10% 16% 17% 6%	2000
	21st to end	Day	43% 5% 9% 21% 19% 3	2308
April	1st to 10th	Day	36% 5% 11% 21% 19% 8%	2160
	11th to 20th	Day	39% 4 7% 20% 23% 7%	2134
	21st to end	Day	32% 5% 9% 27% 24% 3	2160
May	1st to 10th	Day	33% 7% 10% 23% 22% 4	2299
	11th to 20th	Day	36% 6% 17% 24% 16% 2	2305
	21st to end	Day	36% 6% 13% 25% 18% 3	2531
June	1st to 10th	Day	41% 9% 15% 19% 15% 1	2340
	11th to 20th	Day	31% 8% 19% 26% 15% 2	2340
	21st to end	Day	36% 9% 17% 24% 11% 4	2340
July	1st to 10th	Day	40% 12% 19% 19% 9% 1	2275
	11th to 20th	Day	40% 12% 21% 16% 9% 2	2271
	21st to end	Day	39% 13% 19% 20% 9% 1	2348
August	1st to 10th	Day	38% 11% 20% 24% 7%	2136
	11th to 20th	Day	44% 9% 17% 17% 10% 3	2125
	21st to end	Day	44% 10% 15% 18% 9% 4	2376
September	1st to 10th	Day	39% 9% 12% 18% 17% 5%	2117
	11th to 20th	Day	40% 6% 11% 19% 20% 5%	1980
	21st to end	Day	34% 5% 8% 22% 24% 7%	1980
October	1st to 10th	Day	32% 5% 8% 20% 25% 9%	1980
	11th to 20th	Day	30% 4 7% 23% 28% 8%	1980
	21st to end	Day	34% 3 5% 17% 31% 10%	1864
November	1st to 10th	Day	34% 3 6% 18% 28% 12%	1672
	11th to 20th	Day	34% 3 6% 20% 26% 11%	1710
	21st to end	Day	39% 4 7% 18% 24% 8%	1710
December	1st to 10th	Day	48% 4 8% 16% 19% 6%	1710
	11th to 20th	Day	47% 3 5% 17% 22% 6%	2150
	21st to end	Day	49% 5% 7% 19% 19% 2	3124
January	1st to 10th	Night	47% 3 5% 16% 26% 3	4109
	11th to 20th	Night	43% 4 4 23% 19% 8%	3983
	21st to end	Night	51% 3 3 15% 23% 5%	4180
February	1st to 10th	Night	45% 3 6% 18% 21% 7%	2280
	11th to 20th	Night	46% 3 5% 18% 19% 10%	2219
	21st to end	Night	45% 6% 7% 14% 19% 10%	1884
March	1st to 10th	Night	40% 3 7% 18% 21% 11%	2280
	11th to 20th	Night	41% 6% 11% 17% 19% 7%	2179
	21st to end	Night	39% 5% 8% 24% 19% 5%	2107
April	1st to 10th	Night	35% 5% 12% 21% 20% 8%	1800
	11th to 20th	Night	34% 4 8% 21% 22% 11%	1775
	21st to end	Night	32% 5% 10% 25% 23% 6%	1800
May	1st to 10th	Night	29% 6% 11% 24% 25% 5%	1659
	11th to 20th	Night	34% 7% 18% 21% 17% 3	1593
	21st to end	Night	29% 9% 12% 28% 17% 4	1745
June	1st to 10th	Night	35% 9% 16% 21% 17% 1	1620
	11th to 20th	Night	23% 7% 22% 30% 16% 3	1620
	21st to end	Night	31% 9% 17% 23% 17% 4	1620
July	1st to 10th	Night	36% 11% 20% 20% 10% 3	1575
	11th to 20th	Night	32% 11% 24% 23% 9% 2	1566
	21st to end	Night	32% 13% 22% 22% 10% 1	1725
August	1st to 10th	Night	28% 10% 24% 29% 8% 1	1778
	11th to 20th	Night	36% 11% 21% 17% 12% 4	1776
	21st to end	Night	39% 9% 16% 20% 12% 5%	1980
September	1st to 10th	Night	37% 6% 16% 18% 17% 5%	1822
	11th to 20th	Night	35% 9% 11% 19% 21% 6%	1980
	21st to end	Night	29% 4 11% 23% 25% 8%	1980
October	1st to 10th	Night	26% 4 8% 24% 25% 13%	1980
	11th to 20th	Night	31% 4 6% 22% 25% 12%	1980
	21st to end	Night	35% 3 5% 15% 30% 12%	2485
November	1st to 10th	Night	29% 4 8% 18% 27% 14%	2406
	11th to 20th	Night	33% 2 6% 19% 27% 13%	2470
	21st to end	Night	37% 4 6% 19% 24% 9%	2470
December	1st to 10th	Night	43% 6% 8% 18% 20% 5%	2470
	11th to 20th	Night	41% 3 7% 16% 28% 5%	3044
	21st to end	Night	47% 5% 7% 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	26% 21% 32% 18% 3	270
	11th to 20th	Day	27% 20% 31% 15% 7%	270
	21st to end	Day	26% 23% 36% 12% 2 1	297
February	1st to 10th	Day	26% 21% 38% 12% 3	270
	11th to 20th	Day	38% 15% 32% 13% 2	270
	21st to end	Day	30% 21% 39% 10% 1	225
March	1st to 10th	Day	27% 18% 33% 17% 4 1	270
	11th to 20th	Day	22% 17% 35% 18% 7% 1	370
	21st to end	Day	24% 14% 32% 21% 8% 1	470
April	1st to 10th	Day	19% 10% 34% 26% 10% 1	420
	11th to 20th	Day	19% 12% 32% 23% 13% 1	412
	21st to end	Day	14% 11% 30% 29% 14% 2	420
May	1st to 10th	Day	13% 8% 26% 31% 16% 4	423
	11th to 20th	Day	9% 4 28% 30% 20% 9%	447
	21st to end	Day	13% 3 17% 29% 21% 16%	485
June	1st to 10th	Day	13% 4 13% 31% 24% 14%	450
	11th to 20th	Day	8% 3 11% 28% 29% 22%	450
	21st to end	Day	11% 4 9% 27% 30% 18%	450
July	1st to 10th	Day	7% 2 6% 26% 34% 25%	445
	11th to 20th	Day	8% 1 8% 20% 32% 30%	444
	21st to end	Day	8% 3 6% 23% 34% 26%	488
August	1st to 10th	Day	9% 2 9% 24% 31% 26%	446
	11th to 20th	Day	10% 3 5% 29% 36% 18%	439
	21st to end	Day	15% 2 8% 24% 32% 19%	495
September	1st to 10th	Day	10% 2 8% 33% 32% 15%	450
	11th to 20th	Day	14% 3 11% 29% 33% 10%	450
	21st to end	Day	11% 3 15% 31% 31% 8%	450
October	1st to 10th	Day	11% 5% 24% 31% 22% 7%	450
	11th to 20th	Day	12% 6% 27% 35% 18% 2	450
	21st to end	Day	19% 11% 26% 30% 12% 1	315
November	1st to 10th	Day	20% 9% 34% 27% 9% 3	270
	11th to 20th	Day	19% 13% 42% 21% 6%	270
	21st to end	Day	16% 17% 37% 21% 8% 1	270
December	1st to 10th	Day	28% 14% 32% 22% 3	270
	11th to 20th	Day	25% 14% 38% 16% 5% 1	270
	21st to end	Day	30% 15% 33% 18% 4	297
January	1st to 10th	Night	21% 13% 34% 26% 4 1	760
	11th to 20th	Night	24% 14% 29% 26% 7%	760
	21st to end	Night	21% 14% 35% 27% 4	836
February	1st to 10th	Night	22% 10% 33% 30% 5%	760
	11th to 20th	Night	24% 12% 34% 26% 3	743
	21st to end	Night	23% 14% 36% 25% 2	628
March	1st to 10th	Night	21% 11% 32% 31% 6%	760
	11th to 20th	Night	17% 9% 37% 26% 10% 1	760
	21st to end	Night	18% 11% 33% 29% 9%	707
April	1st to 10th	Night	13% 10% 34% 30% 14% 1	540
	11th to 20th	Night	17% 7% 32% 29% 14% 2	533
	21st to end	Night	12% 8% 26% 34% 18% 2	540
May	1st to 10th	Night	11% 5% 27% 33% 21% 4	540
	11th to 20th	Night	10% 2 24% 34% 26% 5%	531
	21st to end	Night	7% 2 12% 35% 37% 7%	584
June	1st to 10th	Night	9% 1 9% 40% 35% 6%	540
	11th to 20th	Night	5% 1 9% 29% 43% 14%	540
	21st to end	Night	5% 2 9% 29% 43% 12%	540
July	1st to 10th	Night	6% 1 5% 28% 44% 15%	525
	11th to 20th	Night	6% 1 4 23% 45% 20%	522
	21st to end	Night	6% 2 6% 24% 47% 15%	591
August	1st to 10th	Night	5% 1 3 26% 49% 16%	537
	11th to 20th	Night	10% 1 5% 29% 45% 10%	534
	21st to end	Night	11% 1 7% 28% 41% 11%	594
September	1st to 10th	Night	10% 3 7% 31% 40% 9%	562
	11th to 20th	Night	12% 3 9% 32% 38% 6%	720
	21st to end	Night	8% 1 14% 33% 40% 4	720
October	1st to 10th	Night	10% 3 20% 34% 26% 6%	720
	11th to 20th	Night	12% 5% 23% 37% 20% 3	720
	21st to end	Night	16% 7% 21% 35% 18% 2	792
November	1st to 10th	Night	13% 6% 20% 38% 19% 3	740
	11th to 20th	Night	16% 7% 29% 35% 12% 1	760
	21st to end	Night	17% 7% 29% 34% 12% 2	760
December	1st to 10th	Night	22% 11% 27% 32% 8% 1	760
	11th to 20th	Night	24% 9% 29% 30% 8%	760
	21st to end	Night	25% 10% 31% 27% 7%	836

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