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 2025-02-06.

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% 4 4 4 4 5% 7% 42%	108377
	Night	14% 7% 3 6% 4 4 4 3 5% 6% 44%	110048
Transparency	Day	39% 5% 11% 25% 17% 3	112021
	Night	40% 4 10% 24% 19% 3	114545
Seeing	Day	16% 10% 24% 25% 17% 7%	15815
	Night	17% 9% 27% 30% 15% 2	25752

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% 11% 5% 11% 5% 5% 4 3 4 5% 32%	3617
	11th to 20th	Day	12% 7% 3 8% 5% 5% 5% 3 5% 6% 40%	3708
	21st to end	Day	20% 12% 3 8% 4 3 3 2 4 5% 35%	4488
February	1st to 10th	Day	16% 11% 4 8% 4 4 4 3 5% 5% 37%	1881
	11th to 20th	Day	16% 9% 3 9% 4 4 4 3 5% 5% 38%	1848
	21st to end	Day	15% 10% 4 9% 4 4 4 3 4 6% 37%	1570
March	1st to 10th	Day	21% 12% 4 6% 3 3 4 2 4 4 36%	3120
	11th to 20th	Day	14% 7% 2 5% 3 3 3 2 5% 6% 49%	3315
	21st to end	Day	12% 8% 3 8% 5% 4 4 4 5% 7% 38%	4155
April	1st to 10th	Day	11% 6% 3 6% 4 4 4 4 5% 8% 45%	2280
	11th to 20th	Day	14% 8% 3 6% 5% 4 4 4 5% 6% 41%	2254
	21st to end	Day	18% 7% 2 5% 5% 4 4 4 6% 6% 39%	2280
May	1st to 10th	Day	15% 7% 2 7% 5% 5% 6% 5% 7% 9% 32%	4013
	11th to 20th	Day	18% 7% 2 5% 4 4 4 4 5% 8% 38%	4081
	21st to end	Day	10% 7% 2 7% 5% 4 5% 4 6% 8% 43%	4789
June	1st to 10th	Day	9% 4 2 6% 5% 4 4 5% 6% 9% 45%	2660
	11th to 20th	Day	6% 4 2 5% 4 5% 6% 4 6% 10% 49%	2660
	21st to end	Day	5% 5% 2 5% 5% 5% 5% 5% 8% 10% 46%	2660
July	1st to 10th	Day	12% 4 2 5% 4 5% 5% 4 7% 9% 43%	4440
	11th to 20th	Day	5% 3 2 5% 5% 4 5% 4 7% 12% 49%	4440
	21st to end	Day	7% 7% 3 8% 6% 6% 6% 5% 8% 9% 33%	4764
August	1st to 10th	Day	5% 3 1 4 3 4 4 3 6% 9% 55%	3852
	11th to 20th	Day	8% 5% 2 6% 5% 5% 5% 4 7% 9% 42%	3729
	21st to end	Day	6% 4 1 5% 4 4 4 3 6% 9% 55%	4180
September	1st to 10th	Day	11% 5% 3 7% 4 3 4 3 4 8% 47%	2280
	11th to 20th	Day	7% 7% 2 6% 4 4 6% 4 5% 8% 49%	2173
	21st to end	Day	10% 5% 2 6% 4 3 3 3 5% 7% 53%	2090
October	1st to 10th	Day	9% 3 1 5% 4 3 3 3 5% 6% 57%	2041
	11th to 20th	Day	12% 6% 3 5% 3 3 3 4 4 7% 51%	1900
	21st to end	Day	18% 7% 2 5% 4 3 3 2 4 5% 47%	1982
November	1st to 10th	Day	9% 5% 2 5% 4 4 4 3 5% 5% 53%	1762
	11th to 20th	Day	14% 7% 3 6% 4 4 4 2 4 6% 46%	1800
	21st to end	Day	18% 10% 3 7% 4 2 3 3 4 6% 41%	1800
December	1st to 10th	Day	11% 10% 3 6% 4 3 3 3 4 6% 48%	2775
	11th to 20th	Day	13% 10% 4 9% 5% 4 5% 3 5% 6% 36%	3008
	21st to end	Day	26% 15% 5% 9% 5% 3 4 2 4 4 24%	3982
January	1st to 10th	Night	17% 11% 4 9% 4 4 5% 3 5% 6% 33%	6348
	11th to 20th	Night	14% 10% 4 9% 4 5% 5% 3 4 4 38%	6268
	21st to end	Night	19% 10% 4 6% 4 3 3 3 4 6% 37%	6490
February	1st to 10th	Night	18% 11% 5% 8% 5% 4 4 4 3 5% 33%	2407
	11th to 20th	Night	16% 9% 2 7% 4 4 3 3 4 6% 41%	2218
	21st to end	Night	19% 9% 4 7% 4 4 4 3 4 5% 38%	1884
March	1st to 10th	Night	22% 9% 2 6% 4 4 3 3 4 5% 38%	3733
	11th to 20th	Night	13% 6% 3 5% 4 3 3 3 5% 7% 49%	3673
	21st to end	Night	16% 7% 3 8% 5% 4 4 3 5% 7% 38%	3474
April	1st to 10th	Night	12% 7% 2 7% 4 3 5% 4 4 6% 47%	1900
	11th to 20th	Night	16% 7% 2 4 3 4 5% 3 6% 6% 44%	1875
	21st to end	Night	19% 6% 3 6% 4 4 4 4 5% 6% 38%	1900
May	1st to 10th	Night	16% 7% 2 6% 5% 4 5% 3 5% 8% 39%	2881
	11th to 20th	Night	15% 6% 2 7% 5% 4 4 3 5% 6% 43%	2812
	21st to end	Night	9% 6% 2 7% 5% 5% 5% 4 6% 9% 43%	2773
June	1st to 10th	Night	10% 5% 2 5% 5% 3 4 4 6% 9% 48%	1520
	11th to 20th	Night	5% 4 1 4 3 4 5% 4 6% 10% 54%	1520
	21st to end	Night	4 3 1 5% 4 3 4 5% 7% 12% 52%	1520
July	1st to 10th	Night	12% 5% 2 5% 3 3 4 3 6% 7% 49%	2530
	11th to 20th	Night	5% 2 1 4 4 4 4 4 5% 8% 58%	2530
	21st to end	Night	7% 7% 3 8% 5% 5% 6% 5% 7% 9% 36%	2795
August	1st to 10th	Night	5% 3 1 5% 3 3 3 3 6% 9% 59%	3078
	11th to 20th	Night	8% 4 2 5% 4 4 4 4 5% 9% 51%	3100
	21st to end	Night	4 3 1 4 4 3 4 3 6% 10% 58%	3476
September	1st to 10th	Night	10% 6% 2 6% 4 3 3 3 4 6% 53%	1900
	11th to 20th	Night	7% 6% 1 6% 3 4 4 3 5% 8% 52%	1986
	21st to end	Night	12% 5% 2 5% 3 3 4 4 5% 6% 53%	2090
October	1st to 10th	Night	7% 4 2 5% 4 4 3 3 4 6% 59%	2137
	11th to 20th	Night	13% 4 2 5% 3 3 3 2 4 5% 55%	2280
	21st to end	Night	19% 7% 2 5% 3 2 3 2 4 5% 46%	2614
November	1st to 10th	Night	11% 6% 2 5% 3 2 2 3 4 6% 56%	2536
	11th to 20th	Night	13% 6% 2 6% 4 3 3 3 4 7% 50%	2600
	21st to end	Night	17% 8% 3 6% 3 3 3 2 4 5% 46%	2600
December	1st to 10th	Night	16% 9% 3 7% 3 3 3 3 4 5% 45%	4405
	11th to 20th	Night	17% 8% 3 5% 4 4 4 3 4 6% 43%	5199
	21st to end	Night	25% 14% 4 8% 5% 4 4 3 4 4 26%	6996

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	55% 4 11% 21% 8% 2	3618
	11th to 20th	Day	44% 3 9% 26% 16% 2	3708
	21st to end	Day	53% 2 9% 16% 14% 6%	4488
February	1st to 10th	Day	49% 5% 11% 22% 11% 1	1881
	11th to 20th	Day	50% 3 9% 21% 14% 3	1858
	21st to end	Day	49% 4 9% 19% 14% 5%	1570
March	1st to 10th	Day	53% 3 6% 19% 14% 5%	3120
	11th to 20th	Day	37% 4 10% 28% 16% 5%	3315
	21st to end	Day	44% 6% 14% 22% 13% 1	4155
April	1st to 10th	Day	38% 5% 12% 27% 17% 1	2280
	11th to 20th	Day	43% 4 11% 23% 16% 3	2254
	21st to end	Day	44% 6% 10% 26% 12% 2	2280
May	1st to 10th	Day	45% 10% 12% 23% 9% 1	4050
	11th to 20th	Day	44% 8% 10% 25% 12% 1	4085
	21st to end	Day	38% 7% 11% 20% 23% 1	4802
June	1st to 10th	Day	34% 7% 13% 26% 18% 2	2660
	11th to 20th	Day	30% 7% 10% 27% 22% 3	2660
	21st to end	Day	31% 7% 16% 22% 17% 5%	2660
July	1st to 10th	Day	37% 6% 12% 26% 16% 3	4371
	11th to 20th	Day	27% 7% 13% 34% 17% 2	4365
	21st to end	Day	43% 9% 14% 24% 9% 1	4614
August	1st to 10th	Day	26% 6% 14% 36% 17% 1	3838
	11th to 20th	Day	36% 7% 14% 25% 18% 1	3765
	21st to end	Day	26% 5% 12% 35% 19% 3	4179
September	1st to 10th	Day	37% 5% 9% 27% 18% 5%	2280
	11th to 20th	Day	34% 6% 8% 25% 24% 2	2173
	21st to end	Day	31% 4 9% 25% 26% 4	2090
October	1st to 10th	Day	17% 2 5% 24% 45% 7%	3392
	11th to 20th	Day	23% 3 8% 23% 35% 8%	3140
	21st to end	Day	32% 3 6% 26% 30% 4	3246
November	1st to 10th	Day	34% 4 10% 28% 18% 6%	1762
	11th to 20th	Day	40% 4 7% 23% 21% 5%	1800
	21st to end	Day	46% 3 8% 25% 16% 2	1800
December	1st to 10th	Day	39% 3 12% 30% 16% 1	2775
	11th to 20th	Day	49% 3 10% 24% 13% 1	3005
	21st to end	Day	64% 3 12% 13% 7% 1	3982
January	1st to 10th	Night	52% 3 11% 24% 9% 1	6348
	11th to 20th	Night	49% 3 7% 23% 15% 3	6268
	21st to end	Night	50% 3 9% 18% 18% 1	6490
February	1st to 10th	Night	54% 4 8% 20% 12% 1	2407
	11th to 20th	Night	46% 3 8% 23% 17% 2	2219
	21st to end	Night	50% 4 9% 18% 15% 4	1884
March	1st to 10th	Night	49% 4 8% 19% 17% 3	3733
	11th to 20th	Night	35% 6% 12% 22% 19% 6%	3673
	21st to end	Night	45% 5% 12% 26% 10% 1	3474
April	1st to 10th	Night	38% 5% 10% 26% 18% 3	1900
	11th to 20th	Night	39% 5% 9% 22% 19% 5%	1875
	21st to end	Night	46% 5% 8% 23% 15% 3	1900
May	1st to 10th	Night	43% 7% 15% 24% 9% 1	2909
	11th to 20th	Night	42% 7% 13% 22% 15% 1	2813
	21st to end	Night	35% 8% 13% 25% 16% 2	2781
June	1st to 10th	Night	33% 6% 13% 23% 22% 3	1520
	11th to 20th	Night	24% 7% 12% 27% 25% 6%	1520
	21st to end	Night	23% 8% 17% 25% 19% 7%	1520
July	1st to 10th	Night	34% 5% 11% 27% 18% 5%	2490
	11th to 20th	Night	24% 6% 12% 36% 19% 4	2482
	21st to end	Night	41% 8% 17% 22% 12% 1	2706
August	1st to 10th	Night	21% 6% 17% 35% 19% 1	3068
	11th to 20th	Night	30% 6% 14% 27% 21% 2	3130
	21st to end	Night	22% 5% 13% 35% 22% 3	3476
September	1st to 10th	Night	33% 4 6% 27% 21% 7%	1900
	11th to 20th	Night	31% 4 7% 29% 24% 5%	1986
	21st to end	Night	32% 5% 9% 21% 26% 6%	2090
October	1st to 10th	Night	17% 3 5% 16% 49% 10%	3566
	11th to 20th	Night	22% 2 5% 23% 41% 7%	3779
	21st to end	Night	32% 3 6% 25% 25% 9%	4300
November	1st to 10th	Night	31% 4 9% 25% 25% 6%	2536
	11th to 20th	Night	35% 4 7% 25% 24% 6%	2600
	21st to end	Night	43% 3 8% 24% 20% 2	2600
December	1st to 10th	Night	44% 3 9% 27% 17% 1	4405
	11th to 20th	Night	44% 3 12% 22% 19% 1	5201
	21st to end	Night	62% 4 11% 16% 7% 2	6996

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% 19% 29% 20% 4 1	300
	11th to 20th	Day	26% 24% 32% 15% 3	300
	21st to end	Day	23% 20% 40% 14% 3	330
February	1st to 10th	Day	26% 19% 40% 13% 2	285
	11th to 20th	Day	30% 21% 34% 11% 3 1	270
	21st to end	Day	32% 18% 32% 16% 2	225
March	1st to 10th	Day	23% 20% 37% 16% 4 1	270
	11th to 20th	Day	27% 19% 37% 14% 3	270
	21st to end	Day	27% 16% 33% 17% 7%	496
April	1st to 10th	Day	17% 20% 30% 26% 6% 1	460
	11th to 20th	Day	23% 18% 26% 21% 11% 2	452
	21st to end	Day	23% 15% 32% 19% 9% 2	460
May	1st to 10th	Day	19% 13% 31% 23% 10% 3	463
	11th to 20th	Day	19% 10% 25% 25% 14% 7%	487
	21st to end	Day	13% 6% 25% 29% 19% 8%	712
June	1st to 10th	Day	11% 4 21% 23% 29% 12%	680
	11th to 20th	Day	7% 4 20% 33% 28% 9%	680
	21st to end	Day	7% 6% 25% 33% 21% 8%	680
July	1st to 10th	Day	11% 3 13% 28% 24% 21%	671
	11th to 20th	Day	7% 3 15% 33% 30% 13%	668
	21st to end	Day	9% 4 14% 31% 29% 13%	739
August	1st to 10th	Day	10% 2 13% 29% 29% 17%	528
	11th to 20th	Day	13% 4 16% 28% 30% 9%	479
	21st to end	Day	9% 1 12% 27% 32% 18%	539
September	1st to 10th	Day	10% 4 13% 30% 31% 12%	490
	11th to 20th	Day	9% 3 10% 29% 34% 15%	490
	21st to end	Day	16% 8% 20% 23% 22% 11%	490
October	1st to 10th	Day	8% 5% 23% 35% 20% 9%	441
	11th to 20th	Day	16% 13% 30% 25% 12% 4	300
	21st to end	Day	25% 15% 28% 24% 7%	330
November	1st to 10th	Day	13% 11% 35% 24% 13% 4	300
	11th to 20th	Day	19% 16% 31% 24% 8% 2	300
	21st to end	Day	31% 12% 28% 21% 7% 1	300
December	1st to 10th	Day	17% 20% 36% 17% 7% 4	300
	11th to 20th	Day	25% 14% 36% 19% 6%	300
	21st to end	Day	29% 19% 33% 14% 4	330
January	1st to 10th	Night	22% 16% 33% 24% 4	1000
	11th to 20th	Night	25% 17% 36% 18% 4	1000
	21st to end	Night	21% 14% 39% 23% 2	1100
February	1st to 10th	Night	24% 14% 37% 23% 2	847
	11th to 20th	Night	22% 16% 38% 23% 1	743
	21st to end	Night	25% 16% 36% 22% 1	628
March	1st to 10th	Night	21% 14% 37% 23% 4	760
	11th to 20th	Night	20% 12% 35% 26% 7%	685
	21st to end	Night	25% 11% 31% 26% 7%	627
April	1st to 10th	Night	17% 13% 35% 28% 6%	570
	11th to 20th	Night	21% 11% 29% 26% 13% 1	563
	21st to end	Night	21% 9% 31% 30% 8% 1	570
May	1st to 10th	Night	18% 6% 28% 34% 12% 1	570
	11th to 20th	Night	16% 4 24% 36% 19% 2	561
	21st to end	Night	9% 3 20% 42% 23% 4	617
June	1st to 10th	Night	12% 2 17% 38% 26% 4	570
	11th to 20th	Night	6% 2 15% 48% 27% 2	570
	21st to end	Night	4 3 17% 44% 28% 3	570
July	1st to 10th	Night	11% 1 8% 34% 37% 9%	555
	11th to 20th	Night	6% 2 10% 39% 37% 6%	552
	21st to end	Night	8% 2 11% 37% 37% 5%	624
August	1st to 10th	Night	9% 2 7% 36% 40% 6%	567
	11th to 20th	Night	9% 1 9% 40% 35% 5%	564
	21st to end	Night	6% 1 10% 36% 40% 7%	627
September	1st to 10th	Night	12% 2 11% 32% 35% 6%	570
	11th to 20th	Night	10% 1 12% 31% 39% 7%	656
	21st to end	Night	14% 4 21% 30% 25% 6%	760
October	1st to 10th	Night	9% 5% 18% 37% 25% 5%	760
	11th to 20th	Night	15% 7% 30% 29% 16% 2	760
	21st to end	Night	23% 8% 31% 27% 11% 1	836
November	1st to 10th	Night	14% 8% 33% 33% 11% 1	780
	11th to 20th	Night	16% 12% 30% 32% 11%	800
	21st to end	Night	24% 11% 29% 26% 9%	800
December	1st to 10th	Night	23% 12% 32% 27% 5%	890
	11th to 20th	Night	23% 11% 32% 29% 5% 1	1000
	21st to end	Night	27% 14% 35% 21% 3	1100

Page generated at 2025-02-03 02:05:53. Version ac07

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