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 2024-01-22.

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% 9% 4 7% 6% 3 5% 5% 5% 7% 33%	74797
	Night	18% 10% 4 7% 6% 3 5% 4 5% 6% 33%	75133
Transparency	Day	50% 7% 12% 20% 10% 1	74502
	Night	51% 6% 12% 20% 10% 1	74962
Seeing	Day	23% 16% 31% 20% 8% 3	13421
	Night	22% 13% 30% 24% 9% 1	22727

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	17% 16% 5% 9% 8% 3 5% 4 3 5% 24%	1517
	11th to 20th	Day	19% 17% 6% 9% 7% 3 4 4 3 4 23%	1520
	21st to end	Day	13% 15% 5% 11% 9% 4 6% 5% 5% 5% 23%	1722
February	1st to 10th	Day	17% 15% 6% 11% 6% 4 6% 5% 5% 5% 21%	1620
	11th to 20th	Day	14% 15% 5% 11% 7% 3 6% 5% 4 5% 27%	1573
	21st to end	Day	21% 14% 5% 8% 7% 4 6% 4 3 5% 23%	1475
March	1st to 10th	Day	16% 12% 5% 11% 8% 4 5% 3 4 4 29%	1833
	11th to 20th	Day	17% 13% 4 8% 6% 3 5% 4 4 5% 29%	1980
	21st to end	Day	20% 12% 4 6% 4 2 3 3 4 5% 37%	2268
April	1st to 10th	Day	21% 11% 4 7% 5% 3 4 4 4 5% 33%	2160
	11th to 20th	Day	22% 8% 3 4 4 3 4 3 4 5% 39%	2134
	21st to end	Day	22% 10% 3 7% 5% 3 4 4 4 5% 32%	2242
May	1st to 10th	Day	19% 8% 3 6% 5% 3 5% 6% 5% 6% 33%	2515
	11th to 20th	Day	16% 7% 3 6% 5% 3 4 5% 5% 8% 38%	2478
	21st to end	Day	14% 5% 3 6% 5% 4 5% 5% 7% 8% 39%	2725
June	1st to 10th	Day	21% 6% 4 6% 5% 3 5% 5% 5% 8% 33%	2520
	11th to 20th	Day	15% 6% 2 4 5% 3 5% 5% 5% 9% 39%	2520
	21st to end	Day	16% 8% 3 6% 5% 3 5% 5% 6% 7% 36%	2520
July	1st to 10th	Day	12% 5% 3 5% 3 3 5% 5% 7% 9% 43%	2520
	11th to 20th	Day	13% 5% 2 5% 4 4 5% 6% 7% 9% 39%	2520
	21st to end	Day	12% 5% 2 5% 6% 4 5% 7% 7% 9% 38%	2744
August	1st to 10th	Day	12% 5% 2 6% 5% 3 5% 5% 7% 8% 40%	2398
	11th to 20th	Day	11% 5% 2 6% 6% 3 6% 6% 7% 8% 39%	2285
	21st to end	Day	14% 6% 2 5% 4 3 5% 4 7% 9% 41%	2376
September	1st to 10th	Day	13% 6% 2 6% 6% 2 4 5% 6% 8% 43%	2160
	11th to 20th	Day	13% 5% 2 5% 4 3 4 3 5% 7% 48%	2160
	21st to end	Day	17% 6% 2 6% 4 3 4 4 5% 7% 41%	1912
October	1st to 10th	Day	23% 7% 2 6% 6% 3 4 5% 4 6% 36%	1800
	11th to 20th	Day	21% 9% 4 7% 6% 3 5% 4 5% 7% 31%	1800
	21st to end	Day	27% 10% 4 6% 5% 4 4 4 4 6% 27%	1881
November	1st to 10th	Day	16% 8% 3 7% 7% 3 5% 6% 6% 7% 34%	1671
	11th to 20th	Day	17% 11% 4 8% 6% 4 5% 4 4 6% 31%	1530
	21st to end	Day	22% 13% 5% 6% 7% 3 5% 5% 5% 5% 24%	1520
December	1st to 10th	Day	23% 15% 6% 8% 5% 3 5% 5% 4 6% 20%	1520
	11th to 20th	Day	20% 15% 5% 10% 6% 4 4 4 4 5% 23%	1906
	21st to end	Day	27% 17% 5% 9% 7% 3 3 4 3 4 20%	2772
January	1st to 10th	Night	19% 16% 5% 9% 7% 4 6% 5% 4 5% 21%	2649
	11th to 20th	Night	17% 15% 5% 10% 8% 5% 5% 5% 4 5% 22%	2660
	21st to end	Night	13% 13% 6% 10% 7% 3 6% 5% 5% 6% 28%	2663
February	1st to 10th	Night	18% 13% 5% 11% 7% 4 6% 5% 5% 5% 22%	2340
	11th to 20th	Night	15% 12% 6% 8% 7% 3 5% 4 4 6% 30%	2273
	21st to end	Night	19% 11% 4 8% 7% 4 6% 4 4 6% 29%	1780
March	1st to 10th	Night	15% 12% 6% 7% 7% 3 5% 4 4 5% 31%	2110
	11th to 20th	Night	18% 12% 5% 7% 5% 3 3 4 4 5% 34%	1979
	21st to end	Night	19% 9% 4 7% 5% 2 3 4 3 5% 39%	2086
April	1st to 10th	Night	21% 9% 4 7% 4 2 4 4 5% 5% 35%	1800
	11th to 20th	Night	19% 8% 3 6% 5% 3 4 4 4 6% 38%	1775
	21st to end	Night	22% 9% 3 6% 5% 3 4 4 5% 6% 32%	1733
May	1st to 10th	Night	22% 8% 2 4 6% 3 4 4 6% 6% 36%	1444
	11th to 20th	Night	13% 6% 3 5% 5% 3 5% 5% 4 8% 43%	1415
	21st to end	Night	12% 4 3 5% 5% 3 5% 5% 5% 9% 45%	1550
June	1st to 10th	Night	16% 6% 4 6% 6% 4 5% 5% 6% 7% 35%	1440
	11th to 20th	Night	14% 5% 3 4 4 4 4 5% 6% 8% 43%	1440
	21st to end	Night	16% 6% 2 5% 7% 3 5% 5% 6% 9% 37%	1440
July	1st to 10th	Night	9% 4 3 5% 5% 2 6% 5% 5% 7% 48%	1440
	11th to 20th	Night	11% 5% 2 6% 5% 3 4 5% 6% 11% 43%	1440
	21st to end	Night	10% 5% 2 4 5% 3 4 4 6% 9% 48%	1574
August	1st to 10th	Night	9% 5% 2 6% 5% 2 5% 5% 6% 8% 46%	1537
	11th to 20th	Night	10% 7% 3 6% 5% 3 4 5% 6% 8% 43%	1598
	21st to end	Night	10% 6% 2 5% 6% 3 5% 6% 6% 8% 45%	1980
September	1st to 10th	Night	12% 6% 2 6% 5% 3 4 3 5% 7% 46%	1800
	11th to 20th	Night	13% 5% 2 4 3 2 3 4 6% 7% 50%	1800
	21st to end	Night	16% 6% 2 5% 4 3 3 4 5% 7% 43%	2046
October	1st to 10th	Night	18% 8% 3 6% 5% 4 4 4 6% 7% 36%	2160
	11th to 20th	Night	21% 8% 4 7% 6% 3 6% 5% 5% 5% 32%	2160
	21st to end	Night	24% 9% 3 7% 6% 3 4 4 4 5% 29%	2474
November	1st to 10th	Night	19% 8% 3 6% 5% 3 4 4 4 7% 37%	2407
	11th to 20th	Night	18% 10% 4 6% 5% 2 5% 6% 5% 6% 34%	2648
	21st to end	Night	21% 11% 4 7% 6% 3 6% 4 5% 6% 25%	2660
December	1st to 10th	Night	21% 15% 5% 8% 7% 4 5% 5% 4 5% 21%	2660
	11th to 20th	Night	22% 13% 5% 8% 6% 3 4 4 4 4 28%	3288
	21st to end	Night	31% 14% 4 9% 5% 3 4 4 4 5% 17%	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	62% 7% 10% 15% 5% 1	1518
	11th to 20th	Day	65% 5% 10% 11% 9% 1	1520
	21st to end	Day	62% 7% 9% 13% 8% 1	1722
February	1st to 10th	Day	65% 5% 10% 14% 5% 1	1620
	11th to 20th	Day	61% 5% 10% 15% 8% 1	1582
	21st to end	Day	63% 6% 8% 12% 9% 1	1475
March	1st to 10th	Day	60% 4 10% 16% 9% 2	1836
	11th to 20th	Day	57% 5% 10% 14% 11% 2	1980
	21st to end	Day	51% 4 7% 19% 13% 5%	2268
April	1st to 10th	Day	54% 4 8% 18% 13% 3	2160
	11th to 20th	Day	47% 5% 7% 23% 15% 3	2134
	21st to end	Day	54% 5% 9% 16% 13% 4	2242
May	1st to 10th	Day	50% 6% 9% 18% 15% 3	2515
	11th to 20th	Day	44% 6% 10% 23% 15% 2	2482
	21st to end	Day	41% 7% 16% 21% 11% 3	2725
June	1st to 10th	Day	49% 8% 11% 19% 12% 1	2520
	11th to 20th	Day	41% 6% 12% 26% 13% 2	2520
	21st to end	Day	44% 9% 15% 20% 12% 1	2520
July	1st to 10th	Day	35% 8% 13% 27% 15% 1	2450
	11th to 20th	Day	38% 9% 16% 28% 9%	2445
	21st to end	Day	39% 9% 17% 24% 11%	2591
August	1st to 10th	Day	38% 9% 16% 26% 10% 1	2384
	11th to 20th	Day	39% 9% 16% 24% 11% 1	2285
	21st to end	Day	38% 7% 13% 28% 12% 1	2376
September	1st to 10th	Day	38% 7% 16% 27% 12% 1	2160
	11th to 20th	Day	36% 6% 12% 30% 16% 1	2160
	21st to end	Day	43% 7% 13% 28% 9%	1912
October	1st to 10th	Day	49% 5% 12% 22% 12% 1	1800
	11th to 20th	Day	52% 7% 15% 19% 6% 1	1800
	21st to end	Day	59% 6% 10% 16% 8% 1	1881
November	1st to 10th	Day	47% 7% 15% 21% 8% 1	1671
	11th to 20th	Day	55% 6% 10% 18% 9% 1	1530
	21st to end	Day	59% 8% 13% 12% 7%	1520
December	1st to 10th	Day	65% 6% 12% 13% 4	1520
	11th to 20th	Day	63% 7% 13% 13% 3	1906
	21st to end	Day	69% 6% 6% 15% 3	2772
January	1st to 10th	Night	65% 6% 10% 14% 5%	2649
	11th to 20th	Night	66% 5% 9% 12% 7% 1	2660
	21st to end	Night	57% 6% 10% 17% 9% 1	2663
February	1st to 10th	Night	64% 5% 11% 14% 6% 1	2340
	11th to 20th	Night	56% 5% 10% 19% 9%	2275
	21st to end	Night	57% 5% 11% 17% 9% 1	1780
March	1st to 10th	Night	55% 4 11% 19% 10% 1	2120
	11th to 20th	Night	53% 4 11% 19% 12% 1	1980
	21st to end	Night	50% 3 7% 20% 16% 3	2086
April	1st to 10th	Night	51% 4 8% 21% 14% 2	1800
	11th to 20th	Night	48% 5% 10% 19% 17% 1	1775
	21st to end	Night	52% 5% 9% 15% 15% 3	1733
May	1st to 10th	Night	48% 5% 9% 17% 16% 5%	1444
	11th to 20th	Night	39% 6% 11% 25% 16% 3	1416
	21st to end	Night	36% 7% 16% 20% 17% 3	1551
June	1st to 10th	Night	46% 8% 13% 22% 11% 1	1440
	11th to 20th	Night	38% 6% 14% 21% 19% 2	1440
	21st to end	Night	42% 9% 13% 22% 13% 1	1440
July	1st to 10th	Night	33% 8% 12% 25% 20% 2	1400
	11th to 20th	Night	35% 7% 13% 31% 14%	1392
	21st to end	Night	33% 8% 15% 31% 12% 1	1486
August	1st to 10th	Night	35% 8% 16% 28% 12% 1	1527
	11th to 20th	Night	37% 7% 15% 27% 13% 1	1598
	21st to end	Night	35% 9% 14% 29% 13%	1980
September	1st to 10th	Night	38% 6% 14% 29% 13% 1	1800
	11th to 20th	Night	32% 7% 15% 33% 13% 1	1800
	21st to end	Night	40% 6% 12% 30% 11% 1	2046
October	1st to 10th	Night	48% 6% 11% 21% 13% 1	2160
	11th to 20th	Night	51% 7% 13% 20% 8% 1	2160
	21st to end	Night	57% 6% 14% 14% 9% 1	2474
November	1st to 10th	Night	46% 7% 14% 22% 10% 1	2407
	11th to 20th	Night	49% 7% 13% 19% 10% 2	2648
	21st to end	Night	59% 7% 12% 16% 6%	2660
December	1st to 10th	Night	64% 7% 12% 14% 4	2660
	11th to 20th	Night	60% 6% 11% 16% 6% 1	3288
	21st to end	Night	69% 6% 10% 12% 3	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	28% 19% 33% 14% 4 1	180
	11th to 20th	Day	33% 18% 33% 13% 2 1	180
	21st to end	Day	28% 19% 31% 18% 4	236
February	1st to 10th	Day	36% 28% 29% 6% 1	240
	11th to 20th	Day	33% 28% 33% 6%	240
	21st to end	Day	35% 21% 31% 12% 2	341
March	1st to 10th	Day	24% 21% 33% 16% 5%	420
	11th to 20th	Day	29% 21% 31% 15% 4	420
	21st to end	Day	31% 22% 27% 13% 6% 1	462
April	1st to 10th	Day	28% 22% 31% 14% 5%	420
	11th to 20th	Day	29% 22% 23% 15% 8% 3	412
	21st to end	Day	26% 22% 31% 14% 5% 2	420
May	1st to 10th	Day	25% 14% 31% 20% 7% 2	423
	11th to 20th	Day	20% 13% 37% 19% 9% 2	447
	21st to end	Day	18% 14% 32% 24% 9% 2	485
June	1st to 10th	Day	24% 11% 34% 21% 8% 2	450
	11th to 20th	Day	17% 15% 33% 21% 11% 3	450
	21st to end	Day	20% 12% 30% 24% 11% 2	450
July	1st to 10th	Day	13% 10% 28% 30% 16% 4	445
	11th to 20th	Day	15% 10% 29% 27% 15% 5%	444
	21st to end	Day	13% 8% 36% 30% 9% 3	488
August	1st to 10th	Day	13% 8% 30% 29% 15% 4	446
	11th to 20th	Day	11% 7% 30% 29% 16% 6%	439
	21st to end	Day	15% 9% 32% 26% 12% 6%	495
September	1st to 10th	Day	14% 9% 33% 25% 12% 7%	450
	11th to 20th	Day	12% 10% 26% 25% 16% 10%	450
	21st to end	Day	18% 11% 30% 24% 13% 3	450
October	1st to 10th	Day	28% 14% 25% 17% 12% 4	450
	11th to 20th	Day	26% 21% 30% 15% 6% 1	450
	21st to end	Day	32% 18% 27% 20% 4	445
November	1st to 10th	Day	27% 19% 28% 18% 7% 1	365
	11th to 20th	Day	27% 16% 39% 11% 5% 3	190
	21st to end	Day	27% 22% 32% 13% 5%	180
December	1st to 10th	Day	33% 22% 28% 14% 2 1	180
	11th to 20th	Day	26% 24% 37% 11% 2	180
	21st to end	Day	34% 19% 32% 13% 2	198
January	1st to 10th	Night	31% 17% 29% 19% 4	760
	11th to 20th	Night	30% 18% 29% 18% 5%	760
	21st to end	Night	21% 16% 35% 24% 5%	800
February	1st to 10th	Night	26% 20% 33% 18% 2	720
	11th to 20th	Night	22% 19% 39% 18% 3	703
	21st to end	Night	26% 20% 34% 17% 3	592
March	1st to 10th	Night	23% 16% 38% 20% 3	720
	11th to 20th	Night	28% 17% 30% 21% 4 1	720
	21st to end	Night	23% 14% 34% 21% 7%	792
April	1st to 10th	Night	26% 16% 34% 22% 3	720
	11th to 20th	Night	21% 18% 33% 19% 7% 1	711
	21st to end	Night	27% 11% 30% 23% 9%	653
May	1st to 10th	Night	24% 9% 27% 28% 11% 1	364
	11th to 20th	Night	14% 9% 36% 33% 7%	354
	21st to end	Night	12% 9% 35% 30% 13% 1	389
June	1st to 10th	Night	18% 7% 32% 33% 9% 1	360
	11th to 20th	Night	16% 9% 28% 33% 13% 1	360
	21st to end	Night	18% 4 26% 38% 14%	360
July	1st to 10th	Night	9% 5% 29% 38% 18%	350
	11th to 20th	Night	12% 3 28% 36% 19% 1	348
	21st to end	Night	13% 4 27% 41% 13% 1	394
August	1st to 10th	Night	12% 3 28% 32% 24% 1	463
	11th to 20th	Night	13% 4 23% 34% 23% 3	534
	21st to end	Night	11% 7% 30% 34% 16% 2	792
September	1st to 10th	Night	15% 6% 29% 31% 16% 3	720
	11th to 20th	Night	15% 8% 22% 28% 22% 5%	720
	21st to end	Night	18% 9% 25% 26% 18% 3	720
October	1st to 10th	Night	22% 13% 28% 19% 17% 2	720
	11th to 20th	Night	25% 15% 29% 23% 8% 1	720
	21st to end	Night	28% 14% 29% 24% 5%	792
November	1st to 10th	Night	25% 15% 30% 22% 8%	740
	11th to 20th	Night	24% 18% 31% 18% 8% 1	760
	21st to end	Night	30% 18% 28% 17% 7%	760
December	1st to 10th	Night	30% 20% 28% 18% 4	760
	11th to 20th	Night	28% 19% 33% 16% 3 1	760
	21st to end	Night	33% 18% 30% 17% 2	836

Page generated at 2024-01-19 09:28:32. Version ac07

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