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 2024-01-27.

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% 7% 3 5% 5% 3 5% 5% 5% 7% 42%	74914
	Night	14% 7% 3 5% 5% 3 4 4 5% 7% 44%	75145
Transparency	Day	41% 6% 11% 24% 15% 3	74617
	Night	41% 5% 10% 23% 18% 4	74963
Seeing	Day	16% 10% 24% 25% 18% 8%	14413
	Night	17% 9% 26% 30% 15% 2	24366

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% 9% 5% 8% 4 3 5% 4 4 5% 36%	1517
	11th to 20th	Day	19% 9% 5% 6% 6% 2 5% 4 5% 6% 33%	1554
	21st to end	Day	13% 12% 4 8% 6% 3 5% 3 4 5% 37%	1836
February	1st to 10th	Day	17% 10% 4 7% 5% 2 4 5% 4 5% 36%	1738
	11th to 20th	Day	17% 8% 4 9% 5% 3 4 4 5% 6% 36%	1748
	21st to end	Day	15% 10% 4 8% 6% 3 5% 4 4 6% 37%	1480
March	1st to 10th	Day	16% 7% 3 5% 5% 3 5% 4 5% 5% 42%	1800
	11th to 20th	Day	19% 9% 4 7% 6% 3 4 4 4 5% 35%	1885
	21st to end	Day	19% 9% 4 7% 7% 3 4 4 4 6% 32%	2366
April	1st to 10th	Day	11% 6% 3 5% 5% 3 4 5% 6% 8% 46%	2160
	11th to 20th	Day	15% 8% 3 5% 5% 3 4 4 5% 6% 40%	2134
	21st to end	Day	19% 6% 3 5% 5% 3 4 5% 5% 6% 39%	2160
May	1st to 10th	Day	17% 6% 3 5% 5% 3 5% 6% 6% 8% 36%	2298
	11th to 20th	Day	16% 7% 3 4 5% 3 4 4 5% 8% 40%	2301
	21st to end	Day	9% 6% 3 6% 7% 4 5% 6% 8% 10% 36%	2704
June	1st to 10th	Day	8% 4 2 5% 6% 3 4 6% 6% 10% 46%	2520
	11th to 20th	Day	6% 3 2 4 5% 3 6% 6% 6% 10% 49%	2520
	21st to end	Day	5% 4 2 5% 5% 4 6% 6% 7% 10% 46%	2520
July	1st to 10th	Day	11% 4 2 4 5% 3 7% 6% 7% 11% 40%	2520
	11th to 20th	Day	6% 3 2 3 6% 4 5% 6% 7% 11% 48%	2520
	21st to end	Day	7% 3 2 5% 6% 3 6% 6% 7% 10% 44%	2720
August	1st to 10th	Day	6% 4 2 5% 6% 4 6% 6% 6% 10% 46%	2188
	11th to 20th	Day	9% 5% 2 5% 6% 4 5% 6% 6% 9% 44%	2125
	21st to end	Day	7% 5% 2 4 5% 4 5% 6% 6% 9% 48%	2376
September	1st to 10th	Day	12% 5% 3 6% 5% 2 5% 4 4 8% 46%	2160
	11th to 20th	Day	7% 6% 2 5% 5% 3 5% 5% 5% 8% 49%	2059
	21st to end	Day	9% 5% 2 4 4 2 3 4 5% 7% 55%	1980
October	1st to 10th	Day	10% 3 2 4 4 3 3 3 5% 6% 55%	1934
	11th to 20th	Day	13% 6% 3 5% 4 2 4 4 4 7% 50%	1800
	21st to end	Day	18% 7% 2 5% 5% 2 3 3 4 6% 46%	1878
November	1st to 10th	Day	9% 5% 2 4 4 3 4 4 5% 5% 54%	1672
	11th to 20th	Day	14% 6% 3 5% 6% 2 4 3 4 6% 46%	1710
	21st to end	Day	18% 9% 3 7% 4 1 3 4 4 6% 42%	1710
December	1st to 10th	Day	16% 10% 4 5% 6% 4 3 4 5% 6% 38%	1620
	11th to 20th	Day	14% 10% 4 7% 6% 4 5% 5% 5% 5% 35%	1918
	21st to end	Day	21% 14% 5% 7% 6% 2 4 3 4 6% 27%	2783
January	1st to 10th	Night	16% 9% 4 6% 5% 3 4 4 5% 6% 38%	2648
	11th to 20th	Night	18% 9% 5% 7% 5% 3 5% 4 4 5% 35%	2622
	21st to end	Night	17% 10% 4 6% 6% 3 4 4 5% 6% 36%	2665
February	1st to 10th	Night	19% 10% 6% 8% 5% 3 4 4 3 5% 33%	2219
	11th to 20th	Night	17% 8% 3 6% 5% 3 4 4 4 6% 41%	2098
	21st to end	Night	19% 9% 4 7% 4 2 4 4 4 5% 37%	1776
March	1st to 10th	Night	16% 7% 3 5% 4 3 4 4 4 5% 44%	2160
	11th to 20th	Night	18% 8% 4 6% 5% 2 4 4 5% 7% 38%	2089
	21st to end	Night	20% 8% 4 7% 5% 3 5% 4 5% 7% 34%	1988
April	1st to 10th	Night	12% 6% 2 5% 6% 2 4 5% 4 6% 48%	1800
	11th to 20th	Night	16% 6% 2 4 4 3 5% 4 5% 7% 44%	1775
	21st to end	Night	19% 6% 3 5% 5% 3 4 5% 5% 6% 39%	1800
May	1st to 10th	Night	15% 6% 2 6% 5% 2 5% 4 4 6% 44%	1661
	11th to 20th	Night	12% 6% 3 7% 6% 4 5% 4 5% 7% 41%	1592
	21st to end	Night	5% 5% 3 6% 7% 4 6% 5% 6% 10% 43%	1568
June	1st to 10th	Night	10% 4 2 5% 5% 2 4 5% 6% 8% 48%	1440
	11th to 20th	Night	4 3 2 3 4 3 4 5% 6% 11% 54%	1440
	21st to end	Night	4 3 1 4 4 3 4 6% 7% 13% 52%	1440
July	1st to 10th	Night	8% 4 1 4 4 2 4 5% 8% 10% 50%	1440
	11th to 20th	Night	5% 3 2 4 4 3 5% 5% 4 10% 55%	1440
	21st to end	Night	7% 4 2 5% 4 3 5% 5% 6% 9% 49%	1594
August	1st to 10th	Night	7% 3 2 4 4 2 4 5% 6% 10% 53%	1749
	11th to 20th	Night	7% 4 2 4 5% 4 4 5% 5% 10% 51%	1776
	21st to end	Night	6% 4 2 4 5% 2 4 4 5% 10% 53%	1980
September	1st to 10th	Night	10% 5% 3 5% 5% 3 3 4 4 6% 53%	1800
	11th to 20th	Night	7% 5% 2 5% 4 2 5% 4 5% 7% 53%	1881
	21st to end	Night	11% 4 2 4 3 2 4 5% 5% 6% 55%	1980
October	1st to 10th	Night	7% 4 2 5% 4 3 3 4 4 6% 58%	2024
	11th to 20th	Night	14% 4 2 4 4 2 3 3 3 6% 54%	2160
	21st to end	Night	20% 7% 3 4 4 2 3 3 4 5% 46%	2476
November	1st to 10th	Night	11% 6% 2 3 4 2 3 3 4 6% 57%	2406
	11th to 20th	Night	13% 6% 2 5% 4 2 4 4 4 7% 51%	2470
	21st to end	Night	17% 7% 3 6% 4 2 3 3 4 5% 46%	2470
December	1st to 10th	Night	18% 9% 3 6% 5% 3 4 4 5% 6% 38%	2555
	11th to 20th	Night	17% 8% 3 5% 5% 3 4 4 4 5% 41%	3279
	21st to end	Night	19% 14% 5% 7% 7% 4 4 4 4 5% 27%	4884

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	51% 4 9% 22% 11% 3	1518
	11th to 20th	Day	51% 4 9% 24% 10%	1554
	21st to end	Day	52% 4 10% 24% 9% 2	1836
February	1st to 10th	Day	51% 5% 10% 21% 11% 1	1738
	11th to 20th	Day	51% 3 9% 21% 13% 3	1758
	21st to end	Day	50% 3 9% 20% 13% 4	1480
March	1st to 10th	Day	45% 5% 8% 23% 16% 3	1800
	11th to 20th	Day	53% 5% 10% 19% 11% 2	1885
	21st to end	Day	54% 6% 10% 20% 9% 1	2366
April	1st to 10th	Day	37% 5% 12% 28% 17% 1	2160
	11th to 20th	Day	44% 4 11% 22% 16% 3	2134
	21st to end	Day	45% 6% 10% 25% 12% 2	2160
May	1st to 10th	Day	46% 7% 12% 25% 9% 1	2298
	11th to 20th	Day	43% 7% 13% 21% 15% 1	2305
	21st to end	Day	41% 9% 15% 22% 11% 2	2704
June	1st to 10th	Day	33% 7% 13% 26% 18% 2	2520
	11th to 20th	Day	30% 8% 11% 27% 21% 3	2520
	21st to end	Day	31% 7% 16% 22% 18% 5%	2520
July	1st to 10th	Day	36% 8% 14% 23% 14% 5%	2451
	11th to 20th	Day	29% 8% 15% 26% 19% 3	2445
	21st to end	Day	33% 8% 13% 30% 14% 2	2569
August	1st to 10th	Day	33% 9% 13% 29% 15% 1	2174
	11th to 20th	Day	35% 8% 14% 27% 14% 2	2125
	21st to end	Day	31% 6% 14% 25% 18% 6%	2376
September	1st to 10th	Day	38% 5% 9% 28% 17% 4	2160
	11th to 20th	Day	34% 7% 8% 25% 24% 2	2059
	21st to end	Day	29% 5% 9% 26% 27% 4	1980
October	1st to 10th	Day	30% 4 9% 27% 26% 5%	1934
	11th to 20th	Day	34% 4 9% 25% 21% 7%	1800
	21st to end	Day	42% 4 7% 26% 18% 3	1878
November	1st to 10th	Day	33% 4 10% 28% 19% 6%	1672
	11th to 20th	Day	40% 4 8% 23% 20% 5%	1710
	21st to end	Day	46% 3 8% 25% 16% 2	1710
December	1st to 10th	Day	48% 4 10% 24% 13% 1	1620
	11th to 20th	Day	50% 3 9% 24% 12% 2	1915
	21st to end	Day	59% 4 12% 15% 10% 1	2783
January	1st to 10th	Night	48% 4 9% 22% 15% 1	2648
	11th to 20th	Night	52% 4 9% 22% 12% 1	2622
	21st to end	Night	49% 5% 11% 21% 13% 1	2665
February	1st to 10th	Night	56% 4 8% 20% 11% 1	2219
	11th to 20th	Night	47% 4 8% 23% 16% 2	2099
	21st to end	Night	51% 4 9% 19% 13% 4	1776
March	1st to 10th	Night	44% 4 8% 20% 20% 4	2160
	11th to 20th	Night	47% 5% 11% 19% 14% 3	2089
	21st to end	Night	52% 5% 11% 20% 10% 2	1988
April	1st to 10th	Night	37% 5% 10% 26% 19% 3	1800
	11th to 20th	Night	39% 5% 9% 22% 19% 5%	1775
	21st to end	Night	46% 5% 8% 23% 15% 3	1800
May	1st to 10th	Night	41% 6% 10% 27% 14% 2	1661
	11th to 20th	Night	43% 6% 12% 21% 16% 2	1593
	21st to end	Night	36% 7% 15% 23% 15% 4	1569
June	1st to 10th	Night	33% 6% 13% 24% 22% 3	1440
	11th to 20th	Night	24% 7% 12% 28% 24% 6%	1440
	21st to end	Night	23% 8% 17% 26% 18% 7%	1440
July	1st to 10th	Night	30% 7% 16% 25% 17% 6%	1400
	11th to 20th	Night	27% 6% 13% 30% 18% 6%	1392
	21st to end	Night	29% 7% 14% 30% 19% 2	1505
August	1st to 10th	Night	26% 7% 18% 28% 20% 1	1739
	11th to 20th	Night	29% 7% 13% 27% 22% 3	1776
	21st to end	Night	27% 6% 14% 25% 21% 6%	1980
September	1st to 10th	Night	34% 4 7% 28% 20% 7%	1800
	11th to 20th	Night	30% 4 7% 30% 24% 5%	1881
	21st to end	Night	29% 5% 9% 22% 28% 7%	1980
October	1st to 10th	Night	29% 4 8% 23% 31% 5%	2024
	11th to 20th	Night	34% 3 7% 24% 25% 7%	2160
	21st to end	Night	42% 3 7% 25% 17% 6%	2476
November	1st to 10th	Night	30% 4 10% 26% 25% 6%	2406
	11th to 20th	Night	35% 4 7% 25% 24% 6%	2470
	21st to end	Night	42% 3 8% 24% 21% 2	2470
December	1st to 10th	Night	48% 4 11% 23% 13% 1	2555
	11th to 20th	Night	45% 4 8% 23% 17% 2	3281
	21st to end	Night	59% 4 11% 15% 9% 2	4884

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% 21% 4 1	270
	11th to 20th	Day	29% 22% 31% 16% 3	270
	21st to end	Day	24% 21% 38% 14% 3	282
February	1st to 10th	Day	29% 19% 39% 10% 3	240
	11th to 20th	Day	33% 20% 33% 11% 3	240
	21st to end	Day	30% 18% 32% 17% 3 1	198
March	1st to 10th	Day	22% 20% 38% 16% 3	240
	11th to 20th	Day	28% 15% 38% 15% 3	240
	21st to end	Day	28% 15% 32% 17% 8%	452
April	1st to 10th	Day	14% 20% 31% 27% 7% 1	420
	11th to 20th	Day	22% 17% 25% 22% 12% 2	412
	21st to end	Day	23% 13% 32% 20% 10% 2	420
May	1st to 10th	Day	19% 13% 32% 23% 10% 3	423
	11th to 20th	Day	19% 11% 26% 25% 14% 6%	447
	21st to end	Day	13% 5% 23% 30% 20% 9%	658
June	1st to 10th	Day	10% 3 20% 23% 30% 13%	630
	11th to 20th	Day	7% 4 20% 34% 27% 9%	630
	21st to end	Day	7% 6% 24% 33% 22% 8%	630
July	1st to 10th	Day	12% 3 12% 27% 25% 23%	621
	11th to 20th	Day	7% 3 15% 32% 29% 14%	618
	21st to end	Day	9% 2 14% 32% 30% 13%	684
August	1st to 10th	Day	10% 1 13% 29% 30% 17%	486
	11th to 20th	Day	14% 4 14% 26% 32% 10%	439
	21st to end	Day	9% 2 13% 27% 31% 19%	495
September	1st to 10th	Day	11% 4 12% 28% 32% 13%	450
	11th to 20th	Day	8% 3 10% 30% 34% 15%	450
	21st to end	Day	14% 8% 20% 23% 23% 12%	450
October	1st to 10th	Day	9% 5% 24% 35% 19% 7%	404
	11th to 20th	Day	17% 14% 31% 26% 10% 2	270
	21st to end	Day	27% 14% 28% 23% 7%	297
November	1st to 10th	Day	13% 10% 35% 24% 14% 5%	270
	11th to 20th	Day	19% 17% 30% 25% 7% 2	270
	21st to end	Day	30% 11% 29% 21% 7% 1	270
December	1st to 10th	Day	17% 19% 36% 17% 8% 4	270
	11th to 20th	Day	24% 14% 35% 21% 6%	270
	21st to end	Day	27% 20% 35% 14% 4	297
January	1st to 10th	Night	22% 17% 33% 24% 4	950
	11th to 20th	Night	25% 17% 35% 18% 5%	950
	21st to end	Night	22% 14% 38% 24% 2	1025
February	1st to 10th	Night	25% 14% 36% 22% 2	779
	11th to 20th	Night	23% 16% 38% 22% 1	703
	21st to end	Night	25% 16% 35% 22% 2	592
March	1st to 10th	Night	21% 14% 37% 24% 4	720
	11th to 20th	Night	21% 12% 34% 26% 7%	649
	21st to end	Night	25% 10% 30% 28% 7% 1	594
April	1st to 10th	Night	16% 12% 35% 29% 7%	540
	11th to 20th	Night	20% 11% 28% 27% 14% 1	533
	21st to end	Night	22% 8% 29% 31% 8% 1	540
May	1st to 10th	Night	18% 6% 28% 34% 13% 1	540
	11th to 20th	Night	16% 3 25% 35% 19% 2	531
	21st to end	Night	9% 2 20% 42% 24% 4	584
June	1st to 10th	Night	12% 2 17% 38% 27% 5%	540
	11th to 20th	Night	6% 2 15% 48% 27% 2	540
	21st to end	Night	4 3 17% 44% 29% 3	540
July	1st to 10th	Night	10% 1 8% 34% 38% 10%	525
	11th to 20th	Night	6% 2 10% 38% 37% 6%	522
	21st to end	Night	8% 2 11% 37% 37% 5%	591
August	1st to 10th	Night	9% 2 7% 35% 41% 6%	537
	11th to 20th	Night	9% 1 8% 40% 36% 6%	534
	21st to end	Night	6% 1 10% 35% 40% 8%	594
September	1st to 10th	Night	13% 2 11% 33% 35% 7%	540
	11th to 20th	Night	10% 1 13% 30% 38% 8%	621
	21st to end	Night	13% 4 21% 30% 26% 6%	720
October	1st to 10th	Night	9% 6% 19% 37% 24% 5%	720
	11th to 20th	Night	16% 7% 31% 29% 15% 2	720
	21st to end	Night	24% 8% 30% 27% 10% 1	792
November	1st to 10th	Night	13% 7% 34% 33% 11% 1	740
	11th to 20th	Night	16% 12% 30% 31% 11%	760
	21st to end	Night	24% 10% 29% 26% 10%	760
December	1st to 10th	Night	23% 12% 32% 27% 5%	845
	11th to 20th	Night	23% 11% 32% 29% 5% 1	950
	21st to end	Night	26% 14% 36% 21% 3	1045

Page generated at 2024-01-24 08:37:29. Version ac07

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