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 2024-12-06.

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	13% 6% 2 6% 5% 5% 5% 4 6% 8% 41%	101144
	Night	13% 7% 2 6% 4 4 4 4 5% 7% 45%	98554
Transparency	Day	39% 5% 11% 21% 19% 5%	104947
	Night	37% 5% 10% 21% 21% 7%	102876
Seeing	Day	18% 7% 22% 24% 20% 9%	14689
	Night	16% 7% 20% 30% 23% 4	24698

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	17% 11% 3 7% 5% 4 4 3 6% 5% 36%	2836
	11th to 20th	Day	17% 6% 3 6% 4 3 3 2 3 4 48%	2876
	21st to end	Day	25% 10% 2 4 2 3 3 2 3 6% 41%	3476
February	1st to 10th	Day	19% 9% 3 6% 4 3 3 4 5% 6% 38%	1900
	11th to 20th	Day	22% 9% 3 5% 4 3 3 3 4 5% 40%	1848
	21st to end	Day	20% 8% 3 7% 4 4 3 3 5% 5% 37%	1570
March	1st to 10th	Day	23% 10% 3 6% 4 4 4 3 4 5% 33%	3120
	11th to 20th	Day	14% 8% 2 6% 4 4 3 3 5% 7% 42%	3379
	21st to end	Day	16% 7% 3 7% 4 4 4 4 6% 7% 38%	3931
April	1st to 10th	Day	12% 6% 2 5% 4 5% 5% 4 5% 8% 45%	2280
	11th to 20th	Day	13% 6% 2 6% 5% 4 4 4 5% 5% 46%	2254
	21st to end	Day	9% 5% 2 6% 4 4 4 4 6% 8% 48%	2289
May	1st to 10th	Day	11% 5% 2 6% 5% 5% 5% 4 6% 8% 41%	4082
	11th to 20th	Day	16% 6% 2 6% 5% 4 4 4 7% 8% 37%	4081
	21st to end	Day	10% 5% 2 6% 5% 5% 5% 4 7% 9% 43%	4476
June	1st to 10th	Day	10% 6% 3 8% 7% 6% 6% 5% 7% 10% 32%	2470
	11th to 20th	Day	5% 4 2 6% 4 6% 6% 5% 9% 10% 43%	2470
	21st to end	Day	7% 5% 3 6% 5% 6% 6% 5% 8% 10% 38%	2470
July	1st to 10th	Day	8% 6% 2 8% 7% 7% 7% 6% 8% 12% 28%	4120
	11th to 20th	Day	8% 5% 3 7% 6% 7% 8% 6% 9% 11% 31%	4120
	21st to end	Day	11% 9% 4 10% 7% 8% 7% 6% 8% 9% 20%	4466
August	1st to 10th	Day	4 3 1 5% 4 4 5% 5% 8% 11% 49%	3798
	11th to 20th	Day	6% 4 2 6% 5% 5% 5% 5% 7% 9% 46%	3729
	21st to end	Day	7% 4 2 5% 5% 4 4 4 5% 8% 51%	4180
September	1st to 10th	Day	10% 5% 2 7% 5% 5% 6% 5% 7% 8% 40%	2209
	11th to 20th	Day	14% 5% 1 7% 5% 5% 5% 4 5% 7% 41%	2090
	21st to end	Day	11% 6% 2 5% 5% 4 4 3 5% 5% 49%	2090
October	1st to 10th	Day	10% 5% 2 4 4 3 4 3 5% 5% 54%	2090
	11th to 20th	Day	10% 5% 2 5% 3 2 3 3 5% 7% 55%	2090
	21st to end	Day	13% 5% 1 3 4 4 4 3 5% 5% 54%	1960
November	1st to 10th	Day	13% 6% 2 6% 3 4 3 2 4 5% 52%	1762
	11th to 20th	Day	12% 6% 2 5% 4 4 3 4 4 7% 51%	1800
	21st to end	Day	15% 6% 2 6% 4 3 4 4 5% 6% 46%	1800
December	1st to 10th	Day	19% 9% 2 6% 4 3 4 4 5% 6% 39%	1755
	11th to 20th	Day	17% 8% 2 6% 5% 4 4 3 4 5% 42%	2153
	21st to end	Day	18% 9% 4 7% 5% 5% 4 3 5% 5% 35%	3124
January	1st to 10th	Night	18% 12% 2 5% 4 4 3 3 5% 5% 40%	4109
	11th to 20th	Night	16% 9% 4 6% 3 3 3 3 5% 5% 43%	4079
	21st to end	Night	21% 13% 2 5% 4 3 3 3 3 4 39%	4180
February	1st to 10th	Night	16% 8% 2 7% 4 4 3 3 4 6% 42%	2280
	11th to 20th	Night	21% 8% 2 7% 4 3 2 2 3 5% 43%	2218
	21st to end	Night	19% 7% 3 6% 4 4 4 4 5% 6% 38%	1884
March	1st to 10th	Night	25% 10% 3 7% 4 4 4 3 3 5% 33%	3733
	11th to 20th	Night	12% 5% 2 5% 4 3 4 4 6% 7% 48%	3579
	21st to end	Night	16% 9% 3 7% 5% 3 4 3 5% 5% 40%	3728
April	1st to 10th	Night	11% 6% 2 5% 4 4 5% 4 6% 7% 47%	1900
	11th to 20th	Night	14% 6% 2 5% 3 3 4 3 4 7% 49%	1875
	21st to end	Night	10% 5% 2 6% 4 3 3 4 5% 7% 51%	1891
May	1st to 10th	Night	10% 6% 2 6% 5% 5% 4 4 6% 8% 43%	2813
	11th to 20th	Night	16% 7% 2 7% 4 4 5% 3 5% 7% 40%	2812
	21st to end	Night	6% 4 1 5% 4 4 5% 5% 9% 11% 46%	3079
June	1st to 10th	Night	6% 5% 2 7% 5% 7% 6% 5% 9% 10% 39%	1710
	11th to 20th	Night	3 4 1 4 5% 5% 4 4 7% 12% 51%	1710
	21st to end	Night	5% 4 1 6% 4 4 5% 5% 8% 10% 47%	1710
July	1st to 10th	Night	8% 5% 2 8% 5% 6% 6% 6% 7% 12% 35%	2840
	11th to 20th	Night	7% 5% 2 6% 4 5% 6% 5% 8% 12% 39%	2840
	21st to end	Night	11% 9% 4 10% 7% 7% 7% 5% 8% 9% 24%	3087
August	1st to 10th	Night	2 2 1 4 3 3 5% 4 6% 12% 56%	3127
	11th to 20th	Night	6% 4 2 5% 4 4 4 4 5% 9% 53%	3100
	21st to end	Night	6% 4 1 5% 4 4 4 4 5% 8% 56%	3476
September	1st to 10th	Night	8% 5% 2 5% 5% 5% 6% 5% 7% 8% 44%	1951
	11th to 20th	Night	13% 5% 2 6% 5% 4 4 3 5% 8% 44%	2090
	21st to end	Night	8% 5% 2 5% 4 4 3 3 5% 8% 53%	2090
October	1st to 10th	Night	8% 4 2 5% 3 3 3 2 4 6% 60%	2090
	11th to 20th	Night	10% 5% 2 4 3 3 4 3 5% 6% 55%	2090
	21st to end	Night	13% 6% 2 4 3 3 3 2 3 4 55%	2631
November	1st to 10th	Night	13% 4 2 5% 4 3 3 3 4 5% 55%	2536
	11th to 20th	Night	13% 6% 2 4 2 3 3 3 4 6% 54%	2600
	21st to end	Night	14% 6% 2 6% 3 4 4 3 4 6% 48%	2600
December	1st to 10th	Night	19% 7% 3 6% 4 4 4 3 4 5% 40%	2542
	11th to 20th	Night	17% 7% 3 5% 4 3 3 2 5% 5% 47%	3042
	21st to end	Night	18% 8% 3 8% 4 4 4 4 6% 7% 33%	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	50% 3 5% 17% 22% 4	2837
	11th to 20th	Day	40% 4 5% 24% 24% 3	2876
	21st to end	Day	47% 2 6% 19% 21% 5%	3476
February	1st to 10th	Day	47% 2 6% 22% 19% 4	1900
	11th to 20th	Day	48% 2 4 19% 20% 6%	1858
	21st to end	Day	49% 3 6% 13% 22% 6%	1570
March	1st to 10th	Day	52% 5% 8% 12% 15% 9%	3120
	11th to 20th	Day	42% 4 7% 19% 22% 7%	3379
	21st to end	Day	44% 5% 10% 17% 20% 4	3931
April	1st to 10th	Day	38% 3 11% 21% 19% 7%	2280
	11th to 20th	Day	39% 4 6% 20% 23% 7%	2254
	21st to end	Day	33% 5% 10% 27% 24% 3	2289
May	1st to 10th	Day	37% 8% 11% 25% 17% 2	4120
	11th to 20th	Day	42% 7% 12% 27% 12% 1	4085
	21st to end	Day	35% 7% 12% 26% 18% 1	4489
June	1st to 10th	Day	44% 7% 15% 18% 16% 1	2470
	11th to 20th	Day	32% 6% 18% 26% 17% 2	2470
	21st to end	Day	37% 8% 17% 23% 12% 3	2470
July	1st to 10th	Day	41% 10% 23% 19% 6% 1	4055
	11th to 20th	Day	41% 10% 21% 22% 6% 1	4051
	21st to end	Day	56% 9% 14% 15% 5%	4324
August	1st to 10th	Day	26% 6% 13% 38% 17% 1	3785
	11th to 20th	Day	31% 7% 17% 27% 16% 2	3765
	21st to end	Day	30% 6% 15% 21% 24% 4	4179
September	1st to 10th	Day	39% 6% 15% 18% 16% 6%	2209
	11th to 20th	Day	41% 5% 12% 19% 19% 3	2090
	21st to end	Day	36% 4 10% 21% 21% 7%	2090
October	1st to 10th	Day	23% 3 5% 14% 37% 18%	3480
	11th to 20th	Day	20% 2 4 17% 33% 25%	3458
	21st to end	Day	24% 3 7% 21% 34% 11%	3196
November	1st to 10th	Day	36% 2 6% 19% 27% 10%	1762
	11th to 20th	Day	34% 3 7% 19% 28% 10%	1800
	21st to end	Day	39% 4 6% 19% 26% 7%	1800
December	1st to 10th	Day	46% 3 8% 17% 21% 5%	1755
	11th to 20th	Day	46% 2 5% 19% 21% 6%	2150
	21st to end	Day	49% 4 7% 19% 19% 2	3124
January	1st to 10th	Night	47% 3 6% 18% 24% 3	4109
	11th to 20th	Night	43% 3 5% 22% 19% 7%	4079
	21st to end	Night	51% 2 3 17% 23% 5%	4180
February	1st to 10th	Night	44% 2 5% 20% 21% 7%	2280
	11th to 20th	Night	47% 2 5% 18% 19% 9%	2219
	21st to end	Night	45% 5% 7% 14% 21% 9%	1884
March	1st to 10th	Night	54% 4 6% 14% 15% 7%	3733
	11th to 20th	Night	33% 5% 9% 19% 23% 10%	3579
	21st to end	Night	46% 4 6% 21% 20% 4	3728
April	1st to 10th	Night	35% 4 11% 23% 21% 7%	1900
	11th to 20th	Night	36% 4 5% 21% 22% 12%	1875
	21st to end	Night	33% 4 10% 26% 22% 5%	1891
May	1st to 10th	Night	36% 9% 11% 22% 20% 2	2840
	11th to 20th	Night	42% 7% 13% 24% 12% 1	2813
	21st to end	Night	27% 9% 15% 29% 18% 2	3087
June	1st to 10th	Night	37% 7% 16% 22% 17% 2	1710
	11th to 20th	Night	25% 7% 22% 25% 19% 3	1710
	21st to end	Night	29% 8% 19% 23% 17% 4	1710
July	1st to 10th	Night	40% 10% 23% 18% 7% 2	2795
	11th to 20th	Night	32% 11% 25% 24% 8% 2	2786
	21st to end	Night	53% 12% 14% 15% 6% 1	2988
August	1st to 10th	Night	20% 6% 16% 42% 15% 2	3116
	11th to 20th	Night	27% 6% 18% 30% 16% 3	3130
	21st to end	Night	27% 7% 14% 20% 28% 5%	3476
September	1st to 10th	Night	34% 9% 17% 18% 16% 6%	1951
	11th to 20th	Night	39% 7% 10% 20% 21% 5%	2090
	21st to end	Night	29% 5% 9% 24% 24% 8%	2090
October	1st to 10th	Night	21% 3 6% 18% 33% 20%	3480
	11th to 20th	Night	19% 2 4 17% 32% 26%	3461
	21st to end	Night	23% 3 3 24% 32% 14%	4332
November	1st to 10th	Night	32% 3 8% 19% 25% 13%	2536
	11th to 20th	Night	33% 2 5% 20% 29% 12%	2600
	21st to end	Night	37% 3 6% 22% 24% 8%	2600
December	1st to 10th	Night	45% 4 7% 19% 21% 4	2542
	11th to 20th	Night	41% 2 7% 17% 28% 5%	3044
	21st to end	Night	47% 4 6% 21% 19% 2	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	30% 17% 34% 16% 3	270
	11th to 20th	Day	31% 16% 31% 15% 6% 1	270
	21st to end	Day	28% 18% 39% 11% 4 1	297
February	1st to 10th	Day	30% 17% 32% 17% 5%	270
	11th to 20th	Day	37% 16% 33% 11% 1	270
	21st to end	Day	40% 16% 38% 6% 1	225
March	1st to 10th	Day	31% 16% 35% 13% 4 1	270
	11th to 20th	Day	24% 14% 34% 19% 7% 1	398
	21st to end	Day	23% 15% 33% 22% 7%	506
April	1st to 10th	Day	24% 12% 30% 22% 11% 2	460
	11th to 20th	Day	23% 13% 28% 24% 12% 1	452
	21st to end	Day	15% 11% 31% 26% 15% 2	460
May	1st to 10th	Day	15% 8% 30% 24% 18% 5%	463
	11th to 20th	Day	13% 4 24% 31% 18% 10%	487
	21st to end	Day	13% 4 20% 27% 23% 13%	529
June	1st to 10th	Day	14% 1 17% 29% 24% 14%	490
	11th to 20th	Day	7% 3 15% 26% 27% 22%	490
	21st to end	Day	11% 2 12% 27% 31% 18%	490
July	1st to 10th	Day	11% 1 7% 26% 33% 24%	485
	11th to 20th	Day	9% 1 6% 22% 33% 29%	484
	21st to end	Day	9% 2 7% 22% 36% 23%	532
August	1st to 10th	Day	11% 1 8% 24% 31% 23%	486
	11th to 20th	Day	11% 1 7% 28% 36% 16%	479
	21st to end	Day	16% 6% 23% 35% 19%	539
September	1st to 10th	Day	10% 2 10% 28% 36% 13%	490
	11th to 20th	Day	17% 3 11% 25% 30% 13%	490
	21st to end	Day	14% 4 15% 30% 29% 9%	490
October	1st to 10th	Day	13% 3 22% 30% 22% 9%	490
	11th to 20th	Day	12% 9% 25% 30% 22% 2	490
	21st to end	Day	19% 8% 22% 33% 15% 2	385
November	1st to 10th	Day	20% 7% 32% 24% 14% 3	300
	11th to 20th	Day	18% 12% 40% 24% 6%	300
	21st to end	Day	17% 16% 34% 23% 9% 2	300
December	1st to 10th	Day	32% 13% 32% 17% 6%	285
	11th to 20th	Day	25% 16% 32% 19% 6% 2	270
	21st to end	Day	31% 14% 29% 21% 5%	297
January	1st to 10th	Night	25% 15% 28% 26% 6%	760
	11th to 20th	Night	22% 15% 27% 28% 8%	760
	21st to end	Night	22% 14% 34% 28% 3	836
February	1st to 10th	Night	23% 11% 29% 31% 6%	760
	11th to 20th	Night	27% 13% 28% 28% 3	743
	21st to end	Night	25% 14% 34% 25% 3	628
March	1st to 10th	Night	21% 13% 29% 31% 6%	760
	11th to 20th	Night	19% 11% 32% 28% 9% 1	760
	21st to end	Night	19% 12% 26% 31% 11%	785
April	1st to 10th	Night	16% 11% 28% 31% 14% 1	570
	11th to 20th	Night	18% 8% 29% 28% 16% 1	563
	21st to end	Night	13% 7% 23% 39% 17% 2	570
May	1st to 10th	Night	11% 5% 22% 34% 25% 3	570
	11th to 20th	Night	13% 3 16% 35% 27% 7%	561
	21st to end	Night	10% 1 9% 34% 39% 7%	617
June	1st to 10th	Night	10% 9% 37% 38% 6%	570
	11th to 20th	Night	5% 1 9% 31% 38% 17%	570
	21st to end	Night	9% 6% 31% 44% 10%	570
July	1st to 10th	Night	9% 1 4 21% 48% 17%	555
	11th to 20th	Night	6% 1 4 19% 50% 20%	552
	21st to end	Night	7% 1 6% 25% 45% 16%	624
August	1st to 10th	Night	5% 1 2 29% 48% 14%	567
	11th to 20th	Night	11% 1 5% 26% 46% 11%	564
	21st to end	Night	14% 1 4 27% 42% 11%	627
September	1st to 10th	Night	10% 1 8% 28% 43% 10%	621
	11th to 20th	Night	15% 1 11% 26% 41% 6%	760
	21st to end	Night	11% 2 13% 29% 41% 3	760
October	1st to 10th	Night	10% 3 18% 34% 28% 7%	760
	11th to 20th	Night	13% 6% 20% 35% 25% 2	760
	21st to end	Night	16% 7% 17% 35% 23% 2	836
November	1st to 10th	Night	15% 6% 20% 35% 22% 3	780
	11th to 20th	Night	16% 7% 28% 35% 13% 1	800
	21st to end	Night	18% 11% 23% 34% 12% 1	800
December	1st to 10th	Night	24% 10% 26% 30% 10% 1	783
	11th to 20th	Night	23% 10% 27% 29% 10% 1	760
	21st to end	Night	27% 10% 23% 32% 7%	836

Page generated at 2024-12-03 05:45:09. Version ac07

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