Birmingham Astronomical Society Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Birmingham Astronomical Society Observatory clear sky chart from 2005-11-06 to 2026-03-18.

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% 2 6% 5% 4 5% 4 6% 8% 41%	131367
	Night	13% 7% 2 6% 4 4 4 3 5% 7% 44%	130168
Transparency	Day	39% 5% 11% 21% 18% 5%	135128
	Night	38% 5% 10% 21% 20% 6%	134465
Seeing	Day	18% 8% 23% 24% 19% 9%	16318
	Night	16% 7% 20% 30% 22% 4	26401

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	21% 10% 4 5% 3 3 3 2 4 4 41%	5258
	11th to 20th	Day	11% 5% 4 6% 3 2 2 2 3 5% 57%	5358
	21st to end	Day	24% 9% 2 5% 4 4 3 3 4 5% 36%	6436
February	1st to 10th	Day	18% 9% 3 6% 4 3 3 4 5% 6% 39%	2100
	11th to 20th	Day	21% 8% 3 6% 4 3 3 3 4 5% 39%	2048
	21st to end	Day	19% 8% 3 7% 4 4 3 3 5% 5% 39%	1730
March	1st to 10th	Day	19% 9% 3 7% 5% 5% 5% 4 4 5% 36%	4580
	11th to 20th	Day	12% 8% 2 6% 4 4 4 3 6% 6% 46%	4911
	21st to end	Day	15% 7% 3 7% 4 4 4 4 5% 6% 42%	5635
April	1st to 10th	Day	12% 6% 2 5% 4 4 5% 4 5% 8% 46%	2400
	11th to 20th	Day	13% 6% 2 6% 5% 4 4 4 5% 5% 47%	2374
	21st to end	Day	9% 5% 2 6% 5% 4 5% 4 6% 8% 46%	2410
May	1st to 10th	Day	15% 6% 2 7% 5% 5% 6% 5% 7% 9% 33%	5862
	11th to 20th	Day	17% 7% 3 7% 5% 5% 5% 5% 7% 8% 30%	5861
	21st to end	Day	13% 6% 3 6% 5% 5% 5% 4 6% 7% 40%	6433
June	1st to 10th	Day	10% 6% 3 8% 7% 6% 6% 5% 7% 10% 33%	2600
	11th to 20th	Day	5% 4 2 6% 5% 6% 6% 5% 9% 10% 42%	2600
	21st to end	Day	6% 5% 3 6% 5% 6% 6% 5% 9% 10% 38%	2600
July	1st to 10th	Day	6% 5% 2 8% 7% 7% 7% 6% 8% 12% 34%	5900
	11th to 20th	Day	6% 4 2 6% 6% 6% 8% 6% 10% 13% 32%	5900
	21st to end	Day	9% 7% 3 9% 7% 8% 8% 6% 9% 12% 23%	6424
August	1st to 10th	Day	5% 3 1 5% 4 4 5% 5% 8% 11% 48%	3918
	11th to 20th	Day	6% 4 2 6% 5% 5% 5% 5% 7% 9% 45%	3849
	21st to end	Day	7% 4 2 5% 5% 4 4 4 5% 8% 51%	4312
September	1st to 10th	Day	10% 5% 2 6% 5% 5% 6% 5% 6% 8% 41%	2325
	11th to 20th	Day	14% 5% 1 7% 5% 5% 5% 4 6% 8% 42%	2200
	21st to end	Day	11% 6% 3 5% 5% 4 4 3 5% 5% 49%	2200
October	1st to 10th	Day	11% 5% 2 4 4 4 5% 3 5% 5% 53%	2200
	11th to 20th	Day	9% 5% 2 5% 3 2 3 3 5% 7% 56%	2200
	21st to end	Day	14% 5% 1 4 3 3 4 3 5% 5% 54%	2063
November	1st to 10th	Day	13% 6% 2 6% 3 4 2 2 4 5% 53%	1852
	11th to 20th	Day	11% 5% 2 5% 4 3 3 3 4 7% 52%	1890
	21st to end	Day	15% 6% 2 6% 4 4 4 4 5% 6% 46%	1890
December	1st to 10th	Day	18% 8% 2 6% 3 2 3 3 4 6% 44%	3020
	11th to 20th	Day	17% 9% 3 7% 5% 4 4 3 4 5% 41%	3463
	21st to end	Day	19% 7% 3 6% 4 4 4 3 5% 5% 39%	4565
January	1st to 10th	Night	18% 9% 3 6% 4 4 3 3 4 5% 41%	7635
	11th to 20th	Night	14% 8% 3 6% 3 3 3 2 4 5% 48%	7583
	21st to end	Night	20% 9% 2 6% 4 3 3 2 4 4 42%	7745
February	1st to 10th	Night	16% 7% 2 7% 4 4 3 3 4 6% 43%	2520
	11th to 20th	Night	21% 8% 2 7% 4 3 2 3 3 5% 41%	2458
	21st to end	Night	19% 6% 2 6% 4 4 4 4 5% 6% 40%	2076
March	1st to 10th	Night	22% 9% 3 7% 4 5% 4 3 4 6% 33%	5493
	11th to 20th	Night	12% 6% 2 5% 3 3 3 4 5% 6% 50%	5202
	21st to end	Night	15% 8% 3 7% 5% 4 4 3 4 4 44%	5336
April	1st to 10th	Night	11% 6% 2 5% 4 4 5% 4 6% 7% 48%	2000
	11th to 20th	Night	13% 6% 2 5% 3 3 4 3 4 7% 50%	1975
	21st to end	Night	10% 5% 2 6% 5% 3 3 4 5% 7% 50%	1990
May	1st to 10th	Night	12% 7% 2 7% 5% 5% 5% 4 6% 8% 37%	4033
	11th to 20th	Night	19% 8% 3 8% 5% 5% 5% 3 5% 6% 31%	4032
	21st to end	Night	10% 6% 2 6% 5% 5% 5% 4 8% 9% 40%	4421
June	1st to 10th	Night	6% 5% 2 7% 5% 6% 6% 4 8% 10% 41%	1800
	11th to 20th	Night	4 4 1 5% 5% 5% 4 4 7% 12% 50%	1800
	21st to end	Night	6% 4 1 6% 4 4 5% 5% 8% 10% 47%	1800
July	1st to 10th	Night	6% 5% 2 7% 5% 6% 6% 5% 8% 12% 38%	4058
	11th to 20th	Night	6% 5% 2 6% 4 4 6% 5% 8% 15% 39%	4057
	21st to end	Night	8% 7% 3 8% 7% 7% 7% 5% 9% 12% 27%	4427
August	1st to 10th	Night	3 3 1 5% 3 3 5% 4 6% 12% 55%	3226
	11th to 20th	Night	6% 4 2 5% 4 4 4 4 5% 9% 53%	3200
	21st to end	Night	6% 4 2 5% 4 4 4 4 5% 8% 55%	3586
September	1st to 10th	Night	8% 5% 2 5% 5% 5% 6% 5% 7% 8% 44%	2054
	11th to 20th	Night	12% 5% 2 5% 5% 4 4 4 5% 8% 45%	2200
	21st to end	Night	8% 5% 2 6% 4 4 4 3 6% 8% 52%	2200
October	1st to 10th	Night	9% 4 3 5% 3 3 3 2 4 6% 59%	2200
	11th to 20th	Night	10% 5% 1 4 3 3 4 3 4 5% 57%	2200
	21st to end	Night	13% 6% 2 4 4 3 3 2 3 4 55%	2770
November	1st to 10th	Night	12% 4 2 5% 4 3 3 3 4 5% 55%	2666
	11th to 20th	Night	12% 6% 2 4 2 3 3 3 4 6% 55%	2730
	21st to end	Night	14% 6% 2 6% 3 4 4 3 4 6% 47%	2730
December	1st to 10th	Night	19% 7% 2 5% 4 3 3 3 4 5% 46%	4380
	11th to 20th	Night	18% 9% 3 6% 4 3 3 2 4 4 43%	4952
	21st to end	Night	21% 6% 3 6% 4 4 3 3 5% 6% 38%	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% 3 5% 25% 17% 2	5259
	11th to 20th	Day	31% 3 4 22% 34% 7%	5358
	21st to end	Day	51% 3 5% 15% 18% 8%	6436
February	1st to 10th	Day	45% 2 6% 22% 20% 4	2100
	11th to 20th	Day	48% 3 4 20% 19% 6%	2058
	21st to end	Day	47% 3 6% 14% 22% 7%	1730
March	1st to 10th	Day	49% 4 7% 13% 18% 7%	4580
	11th to 20th	Day	38% 4 6% 17% 21% 14%	4911
	21st to end	Day	43% 4 8% 21% 20% 4	5637
April	1st to 10th	Day	37% 3 11% 22% 19% 7%	2400
	11th to 20th	Day	37% 4 6% 21% 24% 7%	2374
	21st to end	Day	34% 5% 10% 26% 22% 3	2410
May	1st to 10th	Day	44% 8% 12% 21% 13% 2	5897
	11th to 20th	Day	48% 8% 15% 21% 8% 1	5865
	21st to end	Day	40% 7% 10% 23% 18% 2	6447
June	1st to 10th	Day	44% 8% 15% 18% 16% 1	2600
	11th to 20th	Day	33% 7% 18% 25% 16% 1	2600
	21st to end	Day	37% 8% 17% 23% 12% 3	2600
July	1st to 10th	Day	37% 10% 21% 22% 9%	5835
	11th to 20th	Day	37% 12% 29% 18% 4 1	5831
	21st to end	Day	49% 11% 20% 16% 4	6240
August	1st to 10th	Day	27% 6% 13% 37% 16% 1	3905
	11th to 20th	Day	32% 7% 17% 26% 16% 2	3885
	21st to end	Day	30% 6% 15% 21% 24% 4	4311
September	1st to 10th	Day	39% 6% 15% 19% 16% 6%	2325
	11th to 20th	Day	40% 5% 12% 21% 19% 3	2200
	21st to end	Day	36% 4 11% 21% 21% 7%	2200
October	1st to 10th	Day	24% 3 5% 14% 36% 17%	3590
	11th to 20th	Day	19% 2 4 17% 33% 25%	3568
	21st to end	Day	25% 3 7% 21% 33% 11%	3299
November	1st to 10th	Day	35% 3 6% 19% 27% 10%	1852
	11th to 20th	Day	33% 3 7% 21% 27% 10%	1890
	21st to end	Day	38% 4 6% 19% 26% 7%	1890
December	1st to 10th	Day	43% 2 6% 18% 26% 5%	3020
	11th to 20th	Day	47% 3 7% 19% 18% 6%	3460
	21st to end	Day	46% 4 6% 25% 17% 2	4565
January	1st to 10th	Night	46% 3 7% 24% 17% 3	7636
	11th to 20th	Night	39% 3 5% 18% 26% 8%	7583
	21st to end	Night	47% 2 3 18% 22% 7%	7745
February	1st to 10th	Night	43% 2 5% 21% 22% 7%	2520
	11th to 20th	Night	47% 2 6% 19% 18% 8%	2459
	21st to end	Night	44% 5% 6% 14% 21% 9%	2076
March	1st to 10th	Night	52% 4 6% 14% 19% 5%	5493
	11th to 20th	Night	33% 5% 7% 16% 23% 16%	5202
	21st to end	Night	43% 3 5% 23% 18% 6%	5336
April	1st to 10th	Night	35% 4 11% 23% 20% 8%	2000
	11th to 20th	Night	35% 4 5% 22% 23% 12%	1975
	21st to end	Night	33% 5% 11% 25% 21% 5%	1990
May	1st to 10th	Night	42% 9% 13% 19% 15% 2	4060
	11th to 20th	Night	51% 7% 13% 19% 8% 1	4033
	21st to end	Night	36% 8% 13% 25% 16% 2	4429
June	1st to 10th	Night	36% 7% 16% 22% 18% 2	1800
	11th to 20th	Night	26% 7% 21% 24% 19% 3	1800
	21st to end	Night	30% 8% 18% 23% 17% 4	1800
July	1st to 10th	Night	37% 10% 21% 21% 10% 1	4015
	11th to 20th	Night	31% 12% 31% 19% 5% 1	4006
	21st to end	Night	46% 13% 21% 15% 4 1	4297
August	1st to 10th	Night	21% 6% 16% 41% 14% 2	3215
	11th to 20th	Night	28% 7% 18% 30% 15% 3	3230
	21st to end	Night	27% 6% 14% 20% 28% 4	3586
September	1st to 10th	Night	34% 9% 17% 18% 16% 6%	2054
	11th to 20th	Night	38% 7% 10% 21% 21% 4	2200
	21st to end	Night	29% 5% 10% 24% 24% 8%	2200
October	1st to 10th	Night	21% 3 6% 18% 33% 19%	3590
	11th to 20th	Night	18% 2 4 18% 32% 26%	3571
	21st to end	Night	24% 3 3 24% 32% 14%	4471
November	1st to 10th	Night	32% 3 8% 19% 25% 13%	2666
	11th to 20th	Night	32% 2 5% 20% 28% 12%	2730
	21st to end	Night	37% 3 6% 22% 24% 8%	2730
December	1st to 10th	Night	42% 3 5% 21% 26% 4	4380
	11th to 20th	Night	45% 3 7% 17% 24% 4	4954
	21st to end	Night	45% 4 5% 27% 17% 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	31% 17% 34% 15% 3	330
	11th to 20th	Day	29% 18% 32% 14% 5% 1	330
	21st to end	Day	28% 18% 37% 12% 3 1	363
February	1st to 10th	Day	26% 18% 35% 15% 5%	330
	11th to 20th	Day	38% 16% 34% 11% 1	330
	21st to end	Day	36% 15% 37% 8% 3	273
March	1st to 10th	Day	29% 18% 33% 15% 4 1	330
	11th to 20th	Day	25% 15% 34% 18% 6% 1	460
	21st to end	Day	23% 15% 34% 22% 6%	550
April	1st to 10th	Day	23% 13% 30% 22% 10% 2	500
	11th to 20th	Day	22% 12% 30% 23% 12% 1	492
	21st to end	Day	14% 11% 31% 27% 15% 2	500
May	1st to 10th	Day	16% 7% 31% 24% 17% 5%	503
	11th to 20th	Day	13% 6% 26% 29% 16% 9%	527
	21st to end	Day	13% 5% 21% 27% 22% 12%	573
June	1st to 10th	Day	15% 2 17% 28% 25% 14%	530
	11th to 20th	Day	8% 4 15% 27% 26% 20%	530
	21st to end	Day	10% 2 12% 27% 31% 19%	530
July	1st to 10th	Day	10% 1 7% 26% 33% 24%	525
	11th to 20th	Day	8% 1 6% 22% 34% 29%	524
	21st to end	Day	8% 3 7% 23% 36% 23%	576
August	1st to 10th	Day	13% 2 8% 25% 30% 22%	526
	11th to 20th	Day	11% 2 7% 26% 36% 18%	519
	21st to end	Day	15% 1 8% 25% 33% 19%	583
September	1st to 10th	Day	10% 2 12% 29% 34% 12%	530
	11th to 20th	Day	16% 3 11% 25% 32% 13%	530
	21st to end	Day	13% 4 16% 31% 28% 8%	530
October	1st to 10th	Day	14% 4 23% 31% 21% 8%	530
	11th to 20th	Day	12% 8% 24% 31% 22% 2	530
	21st to end	Day	19% 9% 24% 32% 14% 2	421
November	1st to 10th	Day	19% 8% 32% 24% 15% 2	330
	11th to 20th	Day	17% 11% 42% 24% 6%	330
	21st to end	Day	17% 17% 34% 22% 8% 2	330
December	1st to 10th	Day	34% 15% 30% 16% 5%	330
	11th to 20th	Day	26% 16% 30% 19% 7% 2	330
	21st to end	Day	29% 15% 30% 21% 5% 1	363
January	1st to 10th	Night	25% 15% 28% 27% 5%	840
	11th to 20th	Night	22% 16% 27% 28% 7%	840
	21st to end	Night	21% 14% 34% 28% 3	924
February	1st to 10th	Night	23% 12% 29% 31% 5%	840
	11th to 20th	Night	28% 14% 29% 27% 3	823
	21st to end	Night	25% 14% 32% 26% 3	692
March	1st to 10th	Night	21% 13% 28% 32% 6%	840
	11th to 20th	Night	19% 11% 31% 29% 9% 1	829
	21st to end	Night	19% 12% 27% 31% 11%	826
April	1st to 10th	Night	15% 12% 28% 31% 14% 1	600
	11th to 20th	Night	17% 8% 30% 28% 15% 1	593
	21st to end	Night	13% 7% 23% 39% 17% 2	600
May	1st to 10th	Night	12% 5% 23% 34% 24% 3	600
	11th to 20th	Night	14% 3 17% 34% 26% 7%	591
	21st to end	Night	10% 2 9% 34% 37% 7%	650
June	1st to 10th	Night	10% 1 9% 38% 38% 6%	600
	11th to 20th	Night	6% 1 9% 30% 38% 16%	600
	21st to end	Night	9% 6% 30% 45% 11%	600
July	1st to 10th	Night	9% 1 4 21% 48% 17%	585
	11th to 20th	Night	6% 1 3 19% 51% 20%	582
	21st to end	Night	7% 1 5% 24% 47% 16%	657
August	1st to 10th	Night	6% 1 3 28% 48% 14%	597
	11th to 20th	Night	10% 1 5% 25% 46% 12%	594
	21st to end	Night	14% 1 4 29% 41% 11%	660
September	1st to 10th	Night	10% 1 8% 29% 42% 10%	654
	11th to 20th	Night	14% 1 10% 27% 42% 6%	800
	21st to end	Night	11% 2 13% 30% 41% 3	800
October	1st to 10th	Night	11% 3 18% 35% 27% 7%	800
	11th to 20th	Night	12% 6% 20% 36% 25% 2	800
	21st to end	Night	17% 7% 18% 34% 22% 2	880
November	1st to 10th	Night	15% 6% 20% 35% 22% 2	820
	11th to 20th	Night	15% 7% 28% 36% 13% 1	840
	21st to end	Night	18% 11% 24% 35% 12% 1	840
December	1st to 10th	Night	24% 10% 25% 30% 9% 1	840
	11th to 20th	Night	23% 10% 26% 30% 10% 1	840
	21st to end	Night	27% 11% 23% 32% 7%	924

Page generated at 2026-03-15 01:23:56. Version ac07

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