London
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the London clear sky chart from 2005-11-06 to 2024-01-31.

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	14% 8% 4 7% 6% 4 5% 5% 6% 7% 34%	75512
	Night	14% 8% 4 6% 6% 3 5% 5% 5% 7% 37%	74623
Transparency	Day	47% 7% 12% 20% 12% 2	75230
	Night	46% 6% 12% 21% 13% 2	74425
Seeing	Day	17% 10% 26% 23% 16% 8%	14173
	Night	17% 9% 27% 30% 15% 2	23868

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% 13% 6% 10% 8% 4 6% 5% 4 6% 21%	1706
	11th to 20th	Day	18% 13% 7% 12% 7% 5% 4 5% 5% 5% 19%	1710
	21st to end	Day	16% 14% 8% 10% 8% 4 5% 4 6% 6% 19%	1872
February	1st to 10th	Day	22% 12% 5% 10% 7% 3 5% 4 4 6% 22%	1636
	11th to 20th	Day	16% 10% 6% 11% 9% 3 6% 5% 4 6% 23%	1748
	21st to end	Day	17% 13% 4 9% 6% 4 4 5% 3 5% 29%	1480
March	1st to 10th	Day	18% 9% 5% 7% 6% 3 5% 5% 4 7% 30%	1854
	11th to 20th	Day	18% 10% 5% 8% 7% 3 4 5% 6% 6% 29%	1980
	21st to end	Day	21% 10% 4 8% 6% 4 4 4 5% 6% 28%	2178
April	1st to 10th	Day	12% 10% 4 8% 6% 4 4 4 5% 8% 34%	2059
	11th to 20th	Day	16% 8% 3 6% 6% 4 5% 5% 4 7% 37%	2241
	21st to end	Day	18% 7% 4 7% 5% 4 5% 4 6% 7% 33%	2340
May	1st to 10th	Day	17% 9% 3 7% 6% 4 5% 5% 5% 7% 33%	2340
	11th to 20th	Day	18% 8% 4 6% 6% 3 5% 5% 7% 7% 31%	2301
	21st to end	Day	9% 4 3 6% 7% 4 6% 6% 8% 9% 37%	2531
June	1st to 10th	Day	12% 6% 3 6% 5% 3 5% 5% 6% 9% 38%	2340
	11th to 20th	Day	10% 5% 2 6% 6% 4 5% 5% 7% 10% 40%	2435
	21st to end	Day	9% 4 2 5% 5% 4 7% 6% 8% 10% 40%	2520
July	1st to 10th	Day	11% 6% 3 6% 6% 4 6% 6% 6% 9% 39%	2520
	11th to 20th	Day	8% 6% 2 5% 5% 4 6% 6% 8% 8% 43%	2354
	21st to end	Day	8% 4 3 5% 6% 4 6% 7% 7% 9% 41%	2547
August	1st to 10th	Day	7% 5% 3 5% 5% 4 5% 7% 6% 10% 43%	2327
	11th to 20th	Day	8% 6% 2 6% 7% 4 6% 6% 8% 10% 38%	2303
	21st to end	Day	7% 4 2 5% 5% 4 4 7% 7% 10% 45%	2490
September	1st to 10th	Day	12% 6% 3 4 4 3 4 4 5% 7% 48%	2011
	11th to 20th	Day	9% 5% 2 5% 5% 3 4 4 6% 9% 47%	1980
	21st to end	Day	15% 6% 3 4 5% 3 4 4 5% 7% 45%	1980
October	1st to 10th	Day	12% 6% 2 5% 6% 4 4 5% 6% 7% 43%	1955
	11th to 20th	Day	15% 6% 2 6% 4 3 4 5% 5% 6% 43%	1800
	21st to end	Day	19% 8% 3 6% 7% 4 4 4 5% 7% 33%	1895
November	1st to 10th	Day	9% 6% 3 6% 5% 3 5% 4 5% 7% 47%	1672
	11th to 20th	Day	14% 10% 5% 7% 7% 3 5% 3 4 7% 35%	1710
	21st to end	Day	17% 9% 5% 8% 6% 3 5% 3 4 6% 32%	1710
December	1st to 10th	Day	19% 13% 6% 9% 6% 3 5% 5% 6% 6% 22%	1710
	11th to 20th	Day	16% 13% 6% 9% 7% 3 5% 5% 5% 6% 25%	2153
	21st to end	Day	22% 19% 7% 10% 7% 4 5% 4 4 5% 12%	3124
January	1st to 10th	Night	15% 13% 6% 10% 9% 4 5% 4 5% 6% 24%	2459
	11th to 20th	Night	17% 14% 5% 10% 7% 4 5% 5% 5% 6% 23%	2470
	21st to end	Night	16% 13% 6% 9% 7% 4 5% 5% 5% 5% 25%	2711
February	1st to 10th	Night	20% 12% 5% 8% 7% 3 4 4 5% 5% 27%	2322
	11th to 20th	Night	17% 10% 4 8% 6% 4 5% 6% 6% 6% 30%	2098
	21st to end	Night	19% 13% 6% 9% 5% 3 4 4 4 5% 28%	1776
March	1st to 10th	Night	17% 8% 4 6% 7% 4 4 4 5% 5% 36%	2106
	11th to 20th	Night	16% 8% 3 6% 6% 3 4 4 6% 7% 36%	1980
	21st to end	Night	21% 9% 3 7% 5% 3 3 4 5% 6% 35%	2178
April	1st to 10th	Night	13% 8% 3 5% 6% 3 5% 5% 6% 7% 39%	1917
	11th to 20th	Night	14% 6% 3 6% 6% 3 5% 5% 6% 7% 40%	1666
	21st to end	Night	16% 6% 3 6% 7% 3 5% 4 5% 7% 38%	1620
May	1st to 10th	Night	18% 7% 3 6% 6% 3 5% 7% 5% 7% 33%	1620
	11th to 20th	Night	17% 7% 4 5% 4 3 4 4 6% 9% 37%	1592
	21st to end	Night	7% 4 2 5% 6% 4 4 7% 8% 11% 43%	1744
June	1st to 10th	Night	7% 5% 3 4 5% 4 5% 4 5% 8% 48%	1620
	11th to 20th	Night	8% 5% 1 3 5% 3 5% 5% 8% 10% 47%	1518
	21st to end	Night	7% 3 2 4 5% 3 7% 5% 8% 10% 46%	1440
July	1st to 10th	Night	7% 6% 2 5% 5% 4 4 5% 6% 7% 48%	1440
	11th to 20th	Night	6% 3 2 5% 6% 3 5% 5% 6% 9% 49%	1602
	21st to end	Night	8% 4 2 5% 7% 3 6% 5% 5% 7% 48%	1771
August	1st to 10th	Night	6% 4 2 5% 5% 4 5% 7% 7% 9% 47%	1611
	11th to 20th	Night	7% 5% 3 4 5% 3 5% 5% 6% 10% 47%	1598
	21st to end	Night	6% 3 2 4 4 4 5% 6% 6% 8% 52%	1862
September	1st to 10th	Night	13% 5% 2 4 4 2 4 4 5% 7% 50%	1930
	11th to 20th	Night	7% 4 2 4 4 3 4 4 4 8% 55%	1980
	21st to end	Night	13% 6% 2 4 4 2 3 4 5% 6% 50%	1980
October	1st to 10th	Night	8% 5% 2 5% 5% 2 4 4 6% 7% 51%	2003
	11th to 20th	Night	15% 6% 3 5% 5% 3 5% 4 5% 7% 43%	2160
	21st to end	Night	19% 8% 3 6% 5% 2 4 5% 5% 5% 39%	2459
November	1st to 10th	Night	12% 6% 3 5% 5% 2 3 4 5% 7% 48%	2406
	11th to 20th	Night	16% 8% 3 6% 5% 3 4 4 5% 6% 40%	2470
	21st to end	Night	18% 10% 4 7% 7% 3 4 5% 5% 7% 30%	2470
December	1st to 10th	Night	20% 12% 5% 7% 6% 4 5% 5% 5% 5% 26%	2470
	11th to 20th	Night	17% 9% 5% 8% 5% 3 6% 5% 6% 8% 28%	3042
	21st to end	Night	22% 16% 7% 9% 7% 3 4 5% 4 5% 17%	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	64% 6% 9% 14% 7%	1707
	11th to 20th	Day	67% 5% 12% 12% 3	1710
	21st to end	Day	65% 5% 12% 12% 5% 1	1872
February	1st to 10th	Day	64% 4 12% 12% 7% 1	1636
	11th to 20th	Day	62% 6% 10% 15% 6% 1	1758
	21st to end	Day	59% 5% 11% 16% 7% 2	1480
March	1st to 10th	Day	53% 5% 14% 21% 6% 1	1854
	11th to 20th	Day	55% 6% 11% 18% 10% 1	1980
	21st to end	Day	57% 5% 11% 20% 6% 1	2178
April	1st to 10th	Day	49% 6% 13% 20% 12% 1	2059
	11th to 20th	Day	47% 6% 13% 19% 13% 3	2241
	21st to end	Day	50% 5% 12% 20% 10% 2	2340
May	1st to 10th	Day	51% 6% 13% 20% 8% 2	2340
	11th to 20th	Day	50% 9% 13% 18% 10% 1	2305
	21st to end	Day	39% 10% 16% 23% 11% 1	2531
June	1st to 10th	Day	40% 8% 12% 24% 14% 2	2340
	11th to 20th	Day	38% 8% 15% 22% 15% 1	2435
	21st to end	Day	36% 9% 16% 21% 14% 3	2520
July	1st to 10th	Day	41% 8% 13% 19% 15% 3	2451
	11th to 20th	Day	35% 9% 13% 25% 16% 1	2284
	21st to end	Day	35% 10% 16% 24% 12% 3	2405
August	1st to 10th	Day	34% 9% 16% 25% 14% 2	2314
	11th to 20th	Day	38% 9% 14% 27% 11% 1	2303
	21st to end	Day	32% 9% 15% 22% 17% 5%	2490
September	1st to 10th	Day	35% 7% 12% 23% 19% 4	2011
	11th to 20th	Day	33% 6% 13% 25% 21% 2	1980
	21st to end	Day	38% 6% 10% 23% 23% 1	1980
October	1st to 10th	Day	38% 7% 11% 24% 19% 2	1955
	11th to 20th	Day	40% 5% 13% 24% 17% 3	1800
	21st to end	Day	49% 4 14% 20% 11% 1	1895
November	1st to 10th	Day	37% 5% 10% 27% 18% 3	1672
	11th to 20th	Day	49% 5% 11% 20% 13% 2	1710
	21st to end	Day	54% 5% 11% 20% 10% 1	1710
December	1st to 10th	Day	62% 6% 11% 16% 4 1	1710
	11th to 20th	Day	58% 6% 10% 17% 8%	2150
	21st to end	Day	74% 5% 8% 8% 4 1	3124
January	1st to 10th	Night	61% 4 10% 16% 8% 1	2459
	11th to 20th	Night	62% 5% 13% 14% 5% 1	2470
	21st to end	Night	60% 5% 11% 16% 7% 1	2711
February	1st to 10th	Night	60% 4 11% 17% 7% 1	2322
	11th to 20th	Night	54% 5% 11% 19% 10% 1	2099
	21st to end	Night	59% 5% 9% 16% 8% 2	1776
March	1st to 10th	Night	49% 6% 10% 22% 12% 2	2106
	11th to 20th	Night	47% 6% 12% 22% 10% 3	1980
	21st to end	Night	51% 5% 10% 22% 11% 2	2178
April	1st to 10th	Night	42% 6% 14% 23% 12% 2	1917
	11th to 20th	Night	41% 5% 13% 18% 19% 4	1666
	21st to end	Night	46% 5% 12% 21% 13% 2	1620
May	1st to 10th	Night	49% 7% 12% 20% 9% 3	1620
	11th to 20th	Night	44% 8% 12% 20% 14% 2	1593
	21st to end	Night	31% 9% 16% 28% 12% 3	1745
June	1st to 10th	Night	33% 7% 12% 25% 20% 2	1620
	11th to 20th	Night	29% 10% 15% 25% 17% 3	1518
	21st to end	Night	29% 10% 17% 23% 18% 3	1440
July	1st to 10th	Night	35% 7% 13% 23% 17% 4	1400
	11th to 20th	Night	30% 7% 15% 28% 18% 2	1549
	21st to end	Night	33% 7% 14% 28% 15% 3	1672
August	1st to 10th	Night	31% 9% 16% 27% 16% 1	1600
	11th to 20th	Night	32% 10% 13% 26% 19% 1	1598
	21st to end	Night	28% 8% 14% 25% 20% 5%	1862
September	1st to 10th	Night	34% 6% 13% 24% 18% 6%	1930
	11th to 20th	Night	28% 6% 10% 28% 26% 3	1980
	21st to end	Night	34% 6% 10% 22% 26% 2	1980
October	1st to 10th	Night	32% 6% 12% 26% 22% 2	2003
	11th to 20th	Night	41% 5% 9% 24% 19% 2	2160
	21st to end	Night	47% 5% 8% 23% 15% 2	2459
November	1st to 10th	Night	37% 4 11% 24% 21% 3	2406
	11th to 20th	Night	45% 4 9% 23% 15% 3	2470
	21st to end	Night	54% 4 12% 20% 9% 1	2470
December	1st to 10th	Night	60% 5% 10% 17% 7%	2470
	11th to 20th	Night	54% 5% 13% 21% 7% 1	3044
	21st to end	Night	70% 4 10% 12% 4	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	23% 16% 34% 20% 7% 1	270
	11th to 20th	Day	22% 21% 39% 13% 5% 1	270
	21st to end	Day	27% 18% 33% 16% 6%	294
February	1st to 10th	Day	33% 15% 35% 15% 3	240
	11th to 20th	Day	25% 20% 37% 16% 2	240
	21st to end	Day	25% 18% 40% 15% 2	198
March	1st to 10th	Day	22% 16% 36% 21% 5%	294
	11th to 20th	Day	21% 15% 36% 18% 8% 1	420
	21st to end	Day	28% 13% 33% 20% 6%	462
April	1st to 10th	Day	17% 14% 35% 22% 9% 3	420
	11th to 20th	Day	20% 16% 26% 24% 9% 5%	412
	21st to end	Day	21% 13% 33% 22% 9% 2	420
May	1st to 10th	Day	25% 12% 28% 22% 9% 4	423
	11th to 20th	Day	20% 8% 26% 26% 14% 6%	447
	21st to end	Day	13% 6% 24% 25% 20% 13%	485
June	1st to 10th	Day	12% 6% 22% 31% 21% 8%	450
	11th to 20th	Day	11% 6% 25% 27% 23% 8%	545
	21st to end	Day	10% 6% 26% 29% 20% 10%	630
July	1st to 10th	Day	14% 2 14% 26% 23% 21%	621
	11th to 20th	Day	10% 4 17% 23% 30% 16%	457
	21st to end	Day	9% 3 15% 30% 25% 19%	488
August	1st to 10th	Day	11% 2 13% 24% 30% 20%	446
	11th to 20th	Day	11% 3 18% 25% 29% 13%	439
	21st to end	Day	8% 3 13% 29% 28% 19%	495
September	1st to 10th	Day	13% 3 16% 24% 30% 14%	450
	11th to 20th	Day	9% 3 13% 24% 33% 18%	450
	21st to end	Day	17% 10% 18% 26% 18% 11%	450
October	1st to 10th	Day	10% 6% 25% 29% 21% 8%	450
	11th to 20th	Day	15% 13% 32% 22% 16% 2	450
	21st to end	Day	22% 12% 34% 24% 8%	410
November	1st to 10th	Day	15% 10% 36% 24% 11% 4	270
	11th to 20th	Day	17% 18% 30% 19% 14% 2	270
	21st to end	Day	24% 10% 39% 21% 6%	270
December	1st to 10th	Day	21% 20% 34% 16% 7% 1	270
	11th to 20th	Day	22% 12% 41% 19% 5%	270
	21st to end	Day	29% 19% 29% 17% 5%	297
January	1st to 10th	Night	20% 17% 35% 22% 5%	760
	11th to 20th	Night	24% 16% 34% 21% 4 1	760
	21st to end	Night	22% 14% 35% 26% 3	835
February	1st to 10th	Night	28% 11% 34% 25% 2	720
	11th to 20th	Night	24% 14% 36% 25% 2	703
	21st to end	Night	26% 11% 38% 21% 3	592
March	1st to 10th	Night	22% 13% 35% 24% 6% 1	720
	11th to 20th	Night	20% 13% 34% 26% 7% 1	720
	21st to end	Night	24% 9% 33% 25% 8%	792
April	1st to 10th	Night	16% 12% 35% 30% 6% 1	657
	11th to 20th	Night	14% 10% 34% 28% 13% 1	533
	21st to end	Night	18% 6% 38% 29% 8% 1	540
May	1st to 10th	Night	20% 8% 32% 29% 12% 1	540
	11th to 20th	Night	19% 5% 27% 31% 16% 2	531
	21st to end	Night	9% 3 20% 38% 27% 4	584
June	1st to 10th	Night	8% 3 25% 41% 21% 1	540
	11th to 20th	Night	9% 5% 19% 41% 25% 1	540
	21st to end	Night	7% 3 21% 41% 26% 2	540
July	1st to 10th	Night	10% 3 13% 34% 34% 7%	525
	11th to 20th	Night	6% 2 15% 37% 35% 5%	522
	21st to end	Night	7% 3 14% 37% 37% 2	591
August	1st to 10th	Night	7% 1 11% 40% 36% 4	537
	11th to 20th	Night	10% 2 12% 40% 34% 4	534
	21st to end	Night	7% 2 14% 33% 38% 7%	594
September	1st to 10th	Night	14% 3 16% 31% 31% 6%	670
	11th to 20th	Night	9% 3 12% 34% 36% 6%	720
	21st to end	Night	15% 6% 19% 33% 25% 3	720
October	1st to 10th	Night	10% 5% 26% 33% 22% 5%	720
	11th to 20th	Night	18% 9% 28% 31% 13% 1	720
	21st to end	Night	22% 11% 29% 28% 9% 1	792
November	1st to 10th	Night	16% 9% 28% 29% 17% 1	740
	11th to 20th	Night	19% 15% 30% 26% 9% 1	760
	21st to end	Night	22% 13% 29% 29% 7% 1	760
December	1st to 10th	Night	27% 12% 33% 23% 5% 1	760
	11th to 20th	Night	22% 11% 35% 26% 6% 1	760
	21st to end	Night	28% 14% 31% 23% 4	836

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