Birmingham Astronomical Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Birmingham Astronomical Observatory clear sky chart from 2005-11-06 to 2026-06-01.

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 7% 4 4 4 4 6% 8% 42%	131985
	Night	14% 7% 3 6% 4 4 4 3 5% 7% 43%	131406
Transparency	Day	40% 5% 11% 25% 17% 3	135586
	Night	40% 4 11% 24% 18% 3	135882
Seeing	Day	16% 10% 25% 24% 17% 7%	17612
	Night	17% 9% 27% 30% 15% 2	27472

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	13% 10% 4 10% 5% 5% 4 3 4 5% 34%	4691
	11th to 20th	Day	10% 7% 3 7% 5% 4 5% 3 5% 6% 45%	4826
	21st to end	Day	18% 12% 4 8% 5% 4 4 2 4 6% 34%	5812
February	1st to 10th	Day	16% 10% 4 9% 4 4 4 3 5% 5% 37%	2028
	11th to 20th	Day	17% 9% 3 9% 4 3 4 3 5% 5% 38%	2048
	21st to end	Day	14% 9% 4 9% 5% 4 4 3 4 6% 38%	1730
March	1st to 10th	Day	18% 11% 3 5% 3 3 4 2 4 4 43%	4580
	11th to 20th	Day	11% 8% 2 5% 3 3 3 3 5% 7% 50%	4850
	21st to end	Day	12% 8% 3 8% 5% 4 4 4 5% 7% 39%	6089
April	1st to 10th	Day	13% 6% 2 6% 4 4 4 4 6% 8% 44%	2520
	11th to 20th	Day	14% 8% 3 6% 5% 4 4 4 5% 6% 41%	2494
	21st to end	Day	17% 7% 2 5% 5% 4 4 4 6% 7% 39%	2520
May	1st to 10th	Day	17% 8% 3 8% 5% 5% 6% 5% 7% 9% 28%	5893
	11th to 20th	Day	16% 8% 3 7% 5% 5% 4 4 6% 8% 35%	5991
	21st to end	Day	13% 7% 2 7% 5% 5% 5% 4 6% 7% 39%	7039
June	1st to 10th	Day	9% 4 2 5% 5% 4 4 5% 6% 9% 45%	2801
	11th to 20th	Day	6% 4 2 5% 4 5% 6% 4 6% 10% 48%	2800
	21st to end	Day	5% 5% 2 5% 5% 5% 5% 5% 8% 9% 46%	2800
July	1st to 10th	Day	10% 4 2 6% 4 4 5% 4 6% 9% 46%	6360
	11th to 20th	Day	6% 4 2 6% 5% 5% 6% 5% 8% 12% 41%	6360
	21st to end	Day	6% 6% 3 7% 6% 5% 6% 5% 8% 11% 37%	6847
August	1st to 10th	Day	5% 3 1 4 3 4 4 3 6% 9% 55%	3974
	11th to 20th	Day	8% 5% 2 6% 5% 5% 5% 4 7% 9% 43%	3849
	21st to end	Day	5% 4 2 5% 3 4 4 3 6% 9% 55%	4312
September	1st to 10th	Day	11% 5% 3 7% 4 3 4 3 5% 8% 48%	2400
	11th to 20th	Day	7% 7% 2 6% 4 4 5% 4 5% 8% 50%	2287
	21st to end	Day	10% 5% 2 5% 4 3 3 3 5% 7% 53%	2200
October	1st to 10th	Day	9% 3 1 4 4 3 3 3 5% 6% 57%	2148
	11th to 20th	Day	11% 5% 3 5% 3 3 3 4 4 7% 52%	2000
	21st to end	Day	18% 7% 2 5% 4 3 3 2 4 5% 47%	2086
November	1st to 10th	Day	9% 6% 2 6% 4 4 4 3 5% 5% 52%	1852
	11th to 20th	Day	14% 8% 3 5% 4 4 3 2 4 6% 46%	1890
	21st to end	Day	18% 10% 3 7% 4 2 3 3 4 5% 41%	1890
December	1st to 10th	Day	11% 10% 3 6% 4 3 3 2 4 6% 48%	2860
	11th to 20th	Day	13% 10% 4 9% 5% 4 5% 3 5% 6% 37%	3088
	21st to end	Day	25% 15% 5% 9% 5% 3 4 3 4 5% 24%	4070
January	1st to 10th	Night	16% 10% 4 8% 4 4 4 3 4 6% 37%	8232
	11th to 20th	Night	12% 9% 4 9% 4 5% 5% 3 4 5% 42%	8160
	21st to end	Night	17% 10% 4 7% 5% 4 3 3 4 6% 38%	8402
February	1st to 10th	Night	18% 11% 5% 9% 5% 4 4 4 3 5% 33%	2589
	11th to 20th	Night	17% 9% 2 7% 4 4 3 3 4 6% 40%	2458
	21st to end	Night	18% 9% 3 7% 4 3 4 3 4 5% 40%	2076
March	1st to 10th	Night	18% 11% 3 6% 4 4 4 3 4 5% 37%	5493
	11th to 20th	Night	12% 7% 3 5% 4 3 4 3 5% 6% 48%	5373
	21st to end	Night	15% 8% 3 8% 5% 5% 5% 4 5% 6% 36%	5080
April	1st to 10th	Night	12% 6% 1 6% 4 3 5% 4 4 6% 47%	2100
	11th to 20th	Night	15% 6% 2 5% 3 4 5% 3 6% 7% 44%	2075
	21st to end	Night	18% 7% 3 6% 4 4 4 4 5% 6% 38%	2100
May	1st to 10th	Night	16% 8% 3 7% 5% 4 5% 4 6% 9% 33%	4221
	11th to 20th	Night	13% 6% 3 8% 6% 5% 5% 4 6% 7% 38%	4122
	21st to end	Night	13% 7% 2 7% 5% 5% 5% 4 5% 8% 39%	4074
June	1st to 10th	Night	10% 5% 2 5% 5% 3 5% 4 6% 9% 47%	1600
	11th to 20th	Night	5% 4 2 4 3 4 5% 5% 6% 10% 53%	1600
	21st to end	Night	4 3 1 5% 4 3 4 5% 7% 12% 52%	1600
July	1st to 10th	Night	9% 4 2 6% 3 3 4 3 6% 7% 50%	3618
	11th to 20th	Night	6% 4 2 5% 4 4 5% 4 6% 10% 49%	3617
	21st to end	Night	6% 5% 3 7% 5% 4 5% 4 8% 13% 40%	4006
August	1st to 10th	Night	5% 3 1 5% 2 3 4 3 6% 9% 58%	3176
	11th to 20th	Night	8% 4 2 4 4 4 4 4 6% 9% 51%	3200
	21st to end	Night	3 3 1 4 4 3 4 3 6% 10% 58%	3586
September	1st to 10th	Night	10% 6% 2 6% 4 4 4 3 4 6% 53%	2000
	11th to 20th	Night	7% 5% 1 6% 3 4 4 3 5% 8% 53%	2091
	21st to end	Night	12% 5% 2 5% 3 3 4 4 5% 6% 52%	2200
October	1st to 10th	Night	7% 4 2 5% 4 4 3 3 4 6% 60%	2250
	11th to 20th	Night	13% 4 2 5% 3 3 3 2 3 5% 56%	2400
	21st to end	Night	20% 7% 3 5% 3 2 3 2 4 5% 46%	2752
November	1st to 10th	Night	10% 6% 2 5% 3 3 2 3 5% 6% 56%	2666
	11th to 20th	Night	12% 6% 2 6% 3 3 3 3 4 7% 51%	2730
	21st to end	Night	18% 8% 3 6% 4 3 3 2 4 5% 45%	2730
December	1st to 10th	Night	16% 9% 3 7% 3 3 3 3 4 5% 44%	4540
	11th to 20th	Night	16% 8% 3 5% 4 4 4 3 4 6% 43%	5339
	21st to end	Night	25% 13% 4 8% 5% 4 4 3 4 4 26%	7150

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	52% 4 12% 24% 7% 1	4692
	11th to 20th	Day	39% 3 9% 29% 18% 2	4826
	21st to end	Day	53% 2 10% 17% 11% 6%	5812
February	1st to 10th	Day	49% 5% 11% 23% 11% 1	2028
	11th to 20th	Day	50% 3 9% 21% 14% 3	2058
	21st to end	Day	48% 3 9% 21% 14% 4	1730
March	1st to 10th	Day	46% 3 5% 20% 23% 4	4580
	11th to 20th	Day	35% 4 9% 27% 21% 4	4850
	21st to end	Day	43% 5% 12% 26% 13% 1	6091
April	1st to 10th	Day	39% 5% 12% 26% 17% 1	2520
	11th to 20th	Day	43% 4 12% 23% 15% 3	2494
	21st to end	Day	43% 6% 10% 27% 12% 2	2520
May	1st to 10th	Day	50% 9% 10% 20% 10% 1	5929
	11th to 20th	Day	46% 8% 11% 24% 12% 1	5995
	21st to end	Day	42% 6% 10% 21% 19% 2	7053
June	1st to 10th	Day	34% 7% 13% 26% 18% 2	2801
	11th to 20th	Day	32% 7% 10% 26% 21% 3	2800
	21st to end	Day	31% 7% 16% 22% 17% 5%	2800
July	1st to 10th	Day	34% 6% 11% 28% 18% 3	6291
	11th to 20th	Day	33% 9% 16% 29% 12% 1	6285
	21st to end	Day	37% 8% 18% 26% 9% 1	6652
August	1st to 10th	Day	25% 6% 14% 37% 17% 1	3960
	11th to 20th	Day	35% 7% 14% 26% 17% 1	3885
	21st to end	Day	25% 5% 12% 35% 19% 3	4311
September	1st to 10th	Day	36% 5% 9% 27% 18% 5%	2400
	11th to 20th	Day	33% 6% 9% 24% 25% 2	2287
	21st to end	Day	32% 4 9% 24% 27% 4	2200
October	1st to 10th	Day	17% 2 5% 24% 45% 7%	3499
	11th to 20th	Day	23% 3 9% 23% 35% 8%	3240
	21st to end	Day	33% 3 6% 26% 29% 4	3350
November	1st to 10th	Day	35% 4 10% 28% 18% 5%	1852
	11th to 20th	Day	40% 3 7% 23% 20% 5%	1890
	21st to end	Day	47% 3 8% 24% 16% 2	1890
December	1st to 10th	Day	39% 3 12% 30% 16% 1	2860
	11th to 20th	Day	49% 3 10% 24% 13% 1	3085
	21st to end	Day	64% 3 12% 13% 7% 1	4070
January	1st to 10th	Night	49% 3 14% 25% 9%	8233
	11th to 20th	Night	45% 3 6% 24% 19% 3	8160
	21st to end	Night	50% 3 11% 20% 16% 1	8402
February	1st to 10th	Night	54% 4 8% 21% 11% 1	2589
	11th to 20th	Night	47% 3 8% 23% 16% 2	2459
	21st to end	Night	47% 4 9% 21% 15% 4	2076
March	1st to 10th	Night	50% 3 7% 18% 19% 3	5493
	11th to 20th	Night	36% 5% 10% 22% 20% 7%	5373
	21st to end	Night	47% 5% 12% 24% 11% 1	5080
April	1st to 10th	Night	37% 5% 11% 26% 18% 3	2100
	11th to 20th	Night	39% 6% 10% 22% 19% 5%	2075
	21st to end	Night	46% 5% 9% 23% 15% 2	2100
May	1st to 10th	Night	48% 7% 14% 21% 9% 1	4249
	11th to 20th	Night	44% 7% 13% 21% 14% 1	4123
	21st to end	Night	42% 6% 12% 24% 13% 3	4082
June	1st to 10th	Night	34% 6% 13% 24% 21% 3	1600
	11th to 20th	Night	25% 7% 12% 26% 25% 6%	1600
	21st to end	Night	23% 7% 17% 26% 19% 7%	1600
July	1st to 10th	Night	33% 6% 10% 25% 23% 3	3580
	11th to 20th	Night	30% 8% 17% 30% 13% 2	3572
	21st to end	Night	33% 7% 22% 26% 12% 1	3890
August	1st to 10th	Night	22% 6% 17% 36% 19% 1	3166
	11th to 20th	Night	30% 6% 14% 28% 21% 2	3230
	21st to end	Night	22% 5% 13% 35% 23% 3	3586
September	1st to 10th	Night	34% 4 7% 27% 21% 7%	2000
	11th to 20th	Night	30% 4 8% 29% 25% 4	2091
	21st to end	Night	32% 5% 9% 20% 27% 6%	2200
October	1st to 10th	Night	17% 3 5% 16% 49% 10%	3679
	11th to 20th	Night	22% 2 6% 23% 40% 7%	3899
	21st to end	Night	32% 3 6% 24% 25% 9%	4438
November	1st to 10th	Night	31% 4 9% 26% 25% 5%	2666
	11th to 20th	Night	35% 4 7% 25% 23% 6%	2730
	21st to end	Night	44% 3 8% 24% 19% 2	2730
December	1st to 10th	Night	44% 3 9% 27% 16% 1	4540
	11th to 20th	Night	44% 3 12% 22% 18% 1	5341
	21st to end	Night	61% 4 11% 16% 7% 1	7150

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% 18% 31% 20% 5% 1	330
	11th to 20th	Day	25% 26% 32% 14% 3	330
	21st to end	Day	23% 21% 40% 13% 3	363
February	1st to 10th	Day	23% 19% 41% 14% 2	330
	11th to 20th	Day	29% 24% 34% 10% 2 1	330
	21st to end	Day	30% 18% 33% 16% 2	273
March	1st to 10th	Day	23% 20% 37% 15% 4 1	330
	11th to 20th	Day	27% 19% 38% 12% 2 1	330
	21st to end	Day	25% 17% 34% 16% 6%	584
April	1st to 10th	Day	18% 19% 31% 25% 6% 1	540
	11th to 20th	Day	23% 18% 28% 19% 10% 1	532
	21st to end	Day	21% 14% 33% 20% 9% 2	540
May	1st to 10th	Day	19% 14% 33% 22% 9% 3	543
	11th to 20th	Day	18% 12% 27% 24% 13% 6%	567
	21st to end	Day	15% 7% 25% 29% 17% 7%	820
June	1st to 10th	Day	11% 4 21% 24% 28% 11%	731
	11th to 20th	Day	8% 4 19% 33% 27% 9%	730
	21st to end	Day	6% 5% 24% 34% 22% 8%	730
July	1st to 10th	Day	11% 3 13% 28% 24% 21%	721
	11th to 20th	Day	7% 3 14% 33% 29% 13%	718
	21st to end	Day	8% 4 14% 30% 31% 13%	794
August	1st to 10th	Day	9% 2 13% 30% 29% 17%	570
	11th to 20th	Day	12% 3 15% 27% 31% 12%	519
	21st to end	Day	8% 2 13% 28% 32% 17%	583
September	1st to 10th	Day	9% 4 14% 31% 31% 12%	530
	11th to 20th	Day	8% 3 9% 29% 34% 16%	530
	21st to end	Day	16% 8% 20% 23% 22% 11%	530
October	1st to 10th	Day	8% 5% 21% 35% 22% 9%	478
	11th to 20th	Day	16% 12% 29% 27% 12% 4	330
	21st to end	Day	27% 15% 28% 23% 7%	363
November	1st to 10th	Day	13% 12% 35% 24% 12% 4	330
	11th to 20th	Day	19% 16% 32% 24% 7% 2	330
	21st to end	Day	31% 13% 27% 21% 7% 1	330
December	1st to 10th	Day	17% 19% 37% 17% 7% 3	330
	11th to 20th	Day	24% 15% 38% 18% 5%	330
	21st to end	Day	28% 19% 34% 15% 4	363
January	1st to 10th	Night	22% 16% 34% 24% 4	1050
	11th to 20th	Night	25% 18% 36% 18% 4	1050
	21st to end	Night	21% 15% 40% 23% 2	1155
February	1st to 10th	Night	23% 14% 38% 22% 2	909
	11th to 20th	Night	23% 16% 37% 22% 1	823
	21st to end	Night	24% 16% 36% 23% 1	692
March	1st to 10th	Night	22% 15% 36% 23% 4	840
	11th to 20th	Night	21% 13% 35% 25% 6%	757
	21st to end	Night	24% 12% 32% 26% 7%	693
April	1st to 10th	Night	17% 13% 36% 28% 6%	630
	11th to 20th	Night	20% 11% 31% 25% 12% 1	623
	21st to end	Night	21% 9% 30% 30% 9% 1	630
May	1st to 10th	Night	18% 7% 29% 34% 11% 1	630
	11th to 20th	Night	15% 5% 25% 35% 17% 2	621
	21st to end	Night	11% 3 20% 41% 21% 3	683
June	1st to 10th	Night	12% 3 18% 39% 25% 4	600
	11th to 20th	Night	7% 2 15% 48% 27% 2	600
	21st to end	Night	4 3 16% 44% 29% 3	600
July	1st to 10th	Night	11% 2 8% 33% 37% 9%	585
	11th to 20th	Night	6% 2 10% 40% 36% 6%	582
	21st to end	Night	8% 2 11% 36% 39% 5%	657
August	1st to 10th	Night	9% 2 8% 36% 40% 6%	597
	11th to 20th	Night	9% 1 9% 39% 37% 6%	594
	21st to end	Night	6% 1 10% 37% 39% 7%	660
September	1st to 10th	Night	13% 2 11% 33% 35% 6%	600
	11th to 20th	Night	9% 1 12% 32% 39% 7%	691
	21st to end	Night	14% 4 21% 30% 25% 5%	800
October	1st to 10th	Night	9% 5% 18% 38% 26% 5%	800
	11th to 20th	Night	15% 7% 30% 29% 17% 2	800
	21st to end	Night	23% 9% 31% 26% 10% 1	880
November	1st to 10th	Night	14% 8% 34% 32% 11% 1	820
	11th to 20th	Night	16% 12% 30% 32% 10%	840
	21st to end	Night	24% 11% 29% 26% 9%	840
December	1st to 10th	Night	23% 12% 32% 27% 5%	935
	11th to 20th	Night	22% 11% 33% 28% 5% 1	1050
	21st to end	Night	27% 14% 36% 21% 3	1155

Page generated at 2026-05-29 00:53:26. Version ac07

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