Battersea Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Battersea Observatory clear sky chart from 2005-11-06 to 2025-09-19.

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	17% 10% 3 8% 5% 4 4 4 5% 7% 33%	126725
	Night	17% 11% 4 9% 5% 5% 4 4 5% 6% 32%	122277
Transparency	Day	49% 6% 12% 21% 10% 1	130229
	Night	51% 6% 12% 20% 10% 1	126860
Seeing	Day	23% 16% 31% 19% 8% 3	15619
	Night	22% 14% 30% 24% 9% 1	24724

By Month

Percentage Cloud Cover NE (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% 14% 4 12% 6% 4 5% 3 4 6% 28%	3597
	11th to 20th	Day	11% 18% 8% 14% 6% 6% 5% 3 4 5% 19%	3570
	21st to end	Day	13% 19% 7% 14% 6% 5% 4 3 4 4 20%	4300
February	1st to 10th	Day	16% 16% 5% 11% 5% 6% 5% 4 6% 4 22%	1800
	11th to 20th	Day	13% 15% 4 12% 7% 5% 5% 4 4 5% 26%	1753
	21st to end	Day	20% 15% 5% 10% 6% 5% 5% 3 4 5% 23%	1644
March	1st to 10th	Day	20% 13% 4 10% 5% 4 4 2 4 5% 30%	4568
	11th to 20th	Day	10% 13% 4 9% 6% 4 5% 4 6% 6% 32%	4980
	21st to end	Day	19% 10% 3 8% 4 3 3 3 4 4 37%	5708
April	1st to 10th	Day	21% 13% 2 8% 4 4 4 3 4 5% 32%	2400
	11th to 20th	Day	22% 10% 3 5% 4 3 4 3 4 5% 37%	2374
	21st to end	Day	21% 10% 3 8% 5% 4 4 3 4 5% 33%	2492
May	1st to 10th	Day	25% 11% 4 8% 5% 4 4 3 4 5% 27%	6341
	11th to 20th	Day	18% 9% 3 8% 5% 4 4 3 5% 7% 34%	6318
	21st to end	Day	20% 8% 3 6% 4 4 4 4 6% 8% 33%	6949
June	1st to 10th	Day	20% 7% 4 7% 4 4 5% 4 6% 8% 32%	2800
	11th to 20th	Day	15% 6% 2 5% 5% 4 5% 4 6% 9% 38%	2800
	21st to end	Day	17% 8% 3 6% 4 4 5% 4 6% 7% 35%	2800
July	1st to 10th	Day	12% 6% 3 7% 5% 5% 5% 5% 7% 9% 37%	6360
	11th to 20th	Day	12% 5% 2 6% 4 5% 5% 5% 6% 8% 43%	6360
	21st to end	Day	10% 5% 2 6% 4 4 5% 4 7% 9% 44%	6926
August	1st to 10th	Day	16% 7% 2 7% 4 4 5% 4 6% 8% 37%	4351
	11th to 20th	Day	17% 6% 2 6% 4 4 5% 5% 7% 8% 36%	4127
	21st to end	Day	15% 6% 2 5% 4 4 4 4 7% 9% 40%	4312
September	1st to 10th	Day	12% 6% 2 6% 5% 4 4 4 7% 8% 43%	2400
	11th to 20th	Day	11% 5% 2 6% 4 4 4 3 5% 7% 49%	2376
	21st to end	Day	17% 7% 2 6% 4 4 5% 3 5% 7% 41%	2017
October	1st to 10th	Day	22% 7% 2 6% 5% 3 4 4 4 6% 37%	1900
	11th to 20th	Day	20% 9% 4 7% 5% 4 5% 3 5% 7% 32%	1900
	21st to end	Day	25% 10% 3 7% 4 5% 5% 3 4 5% 28%	1985
November	1st to 10th	Day	15% 9% 2 7% 6% 4 4 5% 6% 7% 34%	1761
	11th to 20th	Day	16% 12% 4 10% 5% 5% 4 3 4 6% 32%	1610
	21st to end	Day	21% 13% 5% 6% 6% 4 5% 4 6% 6% 24%	1600
December	1st to 10th	Day	21% 15% 5% 11% 6% 4 4 4 4 6% 20%	2600
	11th to 20th	Day	18% 19% 5% 11% 5% 4 4 3 4 4 23%	2986
	21st to end	Day	22% 17% 4 11% 6% 4 3 3 3 4 24%	3960
January	1st to 10th	Night	15% 16% 5% 11% 7% 6% 5% 3 4 5% 24%	6349
	11th to 20th	Night	11% 16% 7% 16% 8% 7% 5% 4 4 5% 18%	6315
	21st to end	Night	17% 15% 7% 13% 6% 5% 5% 3 4 4 20%	6651
February	1st to 10th	Night	16% 15% 4 11% 6% 6% 5% 3 5% 5% 23%	2600
	11th to 20th	Night	14% 14% 5% 10% 6% 5% 4 4 4 6% 29%	2533
	21st to end	Night	17% 13% 3 10% 5% 5% 5% 3 4 6% 28%	1985
March	1st to 10th	Night	20% 12% 4 9% 6% 5% 4 3 4 4 29%	5254
	11th to 20th	Night	15% 12% 4 7% 5% 4 4 4 6% 8% 32%	4979
	21st to end	Night	20% 11% 3 8% 4 4 3 3 4 5% 35%	5222
April	1st to 10th	Night	21% 10% 4 7% 4 4 4 4 4 5% 33%	2000
	11th to 20th	Night	20% 9% 3 7% 4 4 4 3 4 6% 37%	1975
	21st to end	Night	22% 9% 3 6% 5% 4 4 3 5% 5% 33%	1925
May	1st to 10th	Night	26% 11% 3 7% 5% 4 4 3 5% 6% 26%	3598
	11th to 20th	Night	16% 8% 3 6% 4 4 4 4 4 7% 39%	3575
	21st to end	Night	18% 8% 3 6% 4 5% 5% 4 6% 7% 35%	3926
June	1st to 10th	Night	15% 7% 3 7% 6% 5% 5% 4 6% 7% 34%	1600
	11th to 20th	Night	14% 6% 2 5% 4 5% 4 5% 6% 9% 42%	1600
	21st to end	Night	17% 6% 2 6% 6% 4 5% 4 6% 8% 36%	1600
July	1st to 10th	Night	12% 5% 3 7% 5% 5% 5% 5% 6% 9% 38%	3600
	11th to 20th	Night	12% 6% 2 6% 4 4 4 3 6% 8% 45%	3600
	21st to end	Night	10% 5% 2 5% 4 4 4 3 6% 8% 50%	3926
August	1st to 10th	Night	11% 7% 2 7% 4 4 5% 4 6% 8% 43%	2781
	11th to 20th	Night	13% 7% 3 6% 4 3 3 4 5% 7% 44%	2875
	21st to end	Night	9% 5% 2 6% 5% 6% 5% 5% 8% 9% 41%	3586
September	1st to 10th	Night	11% 6% 2 7% 4 4 4 3 5% 6% 48%	2000
	11th to 20th	Night	12% 5% 2 4 3 3 4 4 6% 7% 51%	1981
	21st to end	Night	17% 7% 2 6% 3 4 3 4 6% 7% 42%	2160
October	1st to 10th	Night	17% 9% 3 7% 4 5% 4 3 6% 7% 37%	2280
	11th to 20th	Night	20% 9% 3 8% 5% 4 6% 3 5% 5% 33%	2280
	21st to end	Night	23% 10% 3 8% 5% 5% 4 3 4 5% 30%	2612
November	1st to 10th	Night	18% 9% 3 6% 5% 3 4 3 4 6% 37%	2537
	11th to 20th	Night	17% 10% 3 7% 4 4 5% 4 5% 6% 34%	2788
	21st to end	Night	21% 12% 4 8% 5% 5% 5% 3 5% 6% 25%	2800
December	1st to 10th	Night	19% 16% 4 11% 6% 6% 5% 4 5% 6% 19%	4580
	11th to 20th	Night	23% 15% 4 9% 4 4 3 3 4 5% 27%	5208
	21st to end	Night	26% 15% 4 10% 4 4 3 3 5% 4 21%	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	57% 8% 11% 18% 6% 1	3598
	11th to 20th	Day	67% 5% 11% 10% 6% 1	3600
	21st to end	Day	67% 6% 7% 9% 9% 1	4300
February	1st to 10th	Day	63% 5% 10% 16% 5% 1	1800
	11th to 20th	Day	61% 5% 10% 15% 8% 1	1762
	21st to end	Day	63% 6% 9% 12% 9% 1	1644
March	1st to 10th	Day	56% 5% 9% 14% 13% 2	4571
	11th to 20th	Day	50% 6% 18% 14% 11% 1	4980
	21st to end	Day	49% 5% 6% 21% 16% 3	5708
April	1st to 10th	Day	54% 5% 8% 18% 13% 3	2400
	11th to 20th	Day	49% 5% 7% 22% 15% 2	2374
	21st to end	Day	54% 5% 8% 17% 13% 3	2492
May	1st to 10th	Day	58% 6% 9% 17% 8% 2	6352
	11th to 20th	Day	49% 6% 11% 19% 13% 1	6322
	21st to end	Day	47% 7% 14% 22% 10% 1	6949
June	1st to 10th	Day	49% 8% 11% 20% 12% 1	2800
	11th to 20th	Day	41% 6% 12% 26% 12% 2	2800
	21st to end	Day	44% 8% 14% 21% 12% 1	2800
July	1st to 10th	Day	39% 8% 14% 28% 11%	6290
	11th to 20th	Day	35% 8% 14% 28% 14%	6285
	21st to end	Day	34% 7% 14% 35% 10%	6728
August	1st to 10th	Day	43% 7% 15% 25% 10% 1	4337
	11th to 20th	Day	41% 8% 15% 26% 10% 1	4166
	21st to end	Day	38% 7% 15% 29% 10%	4312
September	1st to 10th	Day	37% 7% 16% 28% 12% 1	2400
	11th to 20th	Day	34% 5% 13% 31% 16% 1	2376
	21st to end	Day	43% 6% 13% 27% 10%	2017
October	1st to 10th	Day	41% 6% 13% 29% 9%	3160
	11th to 20th	Day	44% 6% 16% 27% 7%	3140
	21st to end	Day	48% 6% 15% 23% 8%	3249
November	1st to 10th	Day	47% 7% 15% 22% 8% 1	1761
	11th to 20th	Day	53% 6% 10% 19% 10% 1	1610
	21st to end	Day	59% 8% 13% 13% 7%	1600
December	1st to 10th	Day	66% 6% 15% 10% 3	2600
	11th to 20th	Day	65% 6% 9% 14% 6%	2986
	21st to end	Day	64% 6% 8% 17% 5%	3960
January	1st to 10th	Night	62% 6% 12% 14% 5%	6349
	11th to 20th	Night	67% 7% 10% 11% 5%	6360
	21st to end	Night	66% 6% 8% 10% 9% 1	6651
February	1st to 10th	Night	62% 5% 11% 15% 6% 1	2600
	11th to 20th	Night	56% 5% 10% 19% 9% 1	2535
	21st to end	Night	57% 5% 11% 18% 8% 1	1985
March	1st to 10th	Night	57% 5% 7% 17% 10% 3	5264
	11th to 20th	Night	48% 6% 18% 17% 9% 3	4980
	21st to end	Night	49% 6% 7% 22% 15% 2	5222
April	1st to 10th	Night	52% 4 8% 20% 14% 2	2000
	11th to 20th	Night	49% 5% 9% 19% 17% 1	1975
	21st to end	Night	52% 5% 9% 16% 15% 3	1925
May	1st to 10th	Night	58% 5% 9% 16% 9% 3	3604
	11th to 20th	Night	44% 6% 15% 21% 13% 2	3576
	21st to end	Night	47% 7% 13% 19% 13% 1	3927
June	1st to 10th	Night	46% 7% 13% 21% 12% 1	1600
	11th to 20th	Night	38% 7% 14% 21% 18% 2	1600
	21st to end	Night	44% 8% 13% 21% 13% 1	1600
July	1st to 10th	Night	40% 7% 15% 21% 16% 1	3560
	11th to 20th	Night	36% 7% 12% 32% 13% 1	3552
	21st to end	Night	32% 7% 13% 34% 13% 1	3809
August	1st to 10th	Night	38% 7% 14% 27% 13% 1	2771
	11th to 20th	Night	37% 7% 13% 31% 10% 1	2902
	21st to end	Night	36% 9% 15% 29% 10% 1	3586
September	1st to 10th	Night	37% 5% 13% 31% 13% 1	2000
	11th to 20th	Night	30% 7% 15% 34% 13% 1	1981
	21st to end	Night	41% 6% 12% 29% 12% 1	2160
October	1st to 10th	Night	38% 6% 12% 28% 16% 1	3800
	11th to 20th	Night	42% 6% 15% 27% 10%	3779
	21st to end	Night	48% 6% 14% 23% 8%	4298
November	1st to 10th	Night	46% 7% 13% 23% 10% 1	2537
	11th to 20th	Night	49% 7% 13% 19% 10% 2	2788
	21st to end	Night	59% 7% 12% 16% 6%	2800
December	1st to 10th	Night	65% 7% 13% 13% 3	4580
	11th to 20th	Night	60% 6% 9% 15% 9% 1	5208
	21st to end	Night	66% 5% 8% 11% 10%	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% 20% 37% 13% 4 1	200
	11th to 20th	Day	31% 19% 34% 14% 3 1	200
	21st to end	Day	27% 23% 31% 16% 4	293
February	1st to 10th	Day	33% 27% 29% 9% 2	300
	11th to 20th	Day	33% 28% 33% 7%	300
	21st to end	Day	33% 20% 33% 12% 1	408
March	1st to 10th	Day	27% 21% 32% 15% 5%	500
	11th to 20th	Day	29% 21% 32% 15% 4	500
	21st to end	Day	32% 21% 28% 13% 6% 1	550
April	1st to 10th	Day	30% 22% 31% 12% 4	500
	11th to 20th	Day	32% 22% 23% 14% 8% 2	492
	21st to end	Day	26% 23% 31% 13% 5% 2	500
May	1st to 10th	Day	26% 14% 32% 20% 6% 1	503
	11th to 20th	Day	20% 13% 37% 19% 9% 2	527
	21st to end	Day	19% 14% 31% 24% 9% 2	573
June	1st to 10th	Day	23% 12% 35% 21% 7% 2	530
	11th to 20th	Day	15% 16% 35% 20% 11% 3	530
	21st to end	Day	21% 14% 32% 22% 9% 2	530
July	1st to 10th	Day	12% 11% 32% 28% 14% 3	525
	11th to 20th	Day	14% 11% 31% 26% 14% 4	524
	21st to end	Day	12% 9% 37% 29% 9% 4	576
August	1st to 10th	Day	14% 7% 31% 29% 15% 4	526
	11th to 20th	Day	13% 7% 30% 29% 16% 6%	519
	21st to end	Day	15% 10% 32% 25% 12% 6%	583
September	1st to 10th	Day	14% 10% 32% 25% 12% 6%	530
	11th to 20th	Day	11% 9% 23% 27% 19% 12%	522
	21st to end	Day	19% 11% 29% 26% 13% 3	490
October	1st to 10th	Day	27% 15% 26% 17% 11% 4	490
	11th to 20th	Day	26% 21% 30% 15% 7% 1	490
	21st to end	Day	30% 19% 27% 20% 4	483
November	1st to 10th	Day	28% 19% 28% 17% 7% 1	395
	11th to 20th	Day	25% 16% 42% 10% 6% 2	210
	21st to end	Day	31% 22% 30% 13% 5%	200
December	1st to 10th	Day	33% 21% 29% 15% 3 1	200
	11th to 20th	Day	28% 26% 34% 11% 3	200
	21st to end	Day	34% 18% 31% 14% 3	220
January	1st to 10th	Night	30% 18% 30% 18% 4	800
	11th to 20th	Night	29% 18% 29% 18% 5%	800
	21st to end	Night	21% 16% 36% 22% 4	880
February	1st to 10th	Night	26% 20% 34% 19% 2	800
	11th to 20th	Night	23% 20% 37% 18% 3	783
	21st to end	Night	25% 20% 33% 18% 3	660
March	1st to 10th	Night	25% 16% 36% 20% 3	800
	11th to 20th	Night	28% 17% 30% 21% 4 1	800
	21st to end	Night	24% 16% 34% 20% 6%	880
April	1st to 10th	Night	26% 16% 34% 21% 3	800
	11th to 20th	Night	23% 19% 32% 18% 7% 1	791
	21st to end	Night	27% 12% 29% 23% 9%	725
May	1st to 10th	Night	25% 9% 27% 28% 10% 1	404
	11th to 20th	Night	14% 9% 37% 33% 7%	394
	21st to end	Night	12% 9% 36% 30% 12% 1	433
June	1st to 10th	Night	17% 7% 34% 33% 9% 1	400
	11th to 20th	Night	16% 10% 28% 33% 13% 1	400
	21st to end	Night	18% 5% 28% 37% 13%	400
July	1st to 10th	Night	9% 5% 30% 39% 16%	390
	11th to 20th	Night	13% 4 28% 37% 18% 1	388
	21st to end	Night	14% 4 26% 41% 14% 1	438
August	1st to 10th	Night	12% 3 28% 32% 24% 1	515
	11th to 20th	Night	13% 4 24% 33% 23% 3	594
	21st to end	Night	11% 7% 30% 35% 16% 2	880
September	1st to 10th	Night	15% 7% 29% 31% 16% 3	800
	11th to 20th	Night	14% 7% 20% 27% 25% 6%	793
	21st to end	Night	18% 9% 24% 28% 18% 3	760
October	1st to 10th	Night	21% 12% 28% 20% 17% 2	760
	11th to 20th	Night	25% 15% 28% 23% 8% 1	760
	21st to end	Night	27% 15% 29% 24% 5%	836
November	1st to 10th	Night	26% 16% 29% 22% 8%	780
	11th to 20th	Night	24% 17% 32% 19% 8% 1	800
	21st to end	Night	30% 19% 28% 16% 7%	800
December	1st to 10th	Night	30% 19% 29% 18% 4	800
	11th to 20th	Night	29% 19% 33% 16% 3 1	800
	21st to end	Night	33% 18% 30% 17% 3	880

Page generated at 2025-09-16 01:49:12. Version ac07

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