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 2026-06-16.

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% 9% 4 8% 5% 5% 5% 4 6% 7% 34%	138302
	Night	14% 9% 4 8% 5% 5% 5% 4 5% 7% 35%	133722
Transparency	Day	47% 6% 12% 21% 13% 1	141945
	Night	46% 5% 11% 22% 14% 2	138151
Seeing	Day	17% 10% 27% 23% 16% 7%	17393
	Night	17% 9% 27% 30% 15% 2	26965

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	16% 14% 7% 14% 6% 5% 5% 4 5% 6% 18%	5258
	11th to 20th	Day	12% 13% 6% 14% 8% 7% 6% 3 4 5% 22%	5280
	21st to end	Day	17% 12% 7% 12% 7% 7% 5% 5% 6% 6% 16%	5780
February	1st to 10th	Day	19% 12% 5% 11% 7% 4 4 3 4 6% 24%	1909
	11th to 20th	Day	16% 10% 5% 13% 7% 5% 5% 4 4 6% 24%	2048
	21st to end	Day	16% 13% 4 10% 5% 5% 4 4 4 5% 30%	1730
March	1st to 10th	Day	17% 10% 7% 12% 5% 5% 4 4 4 5% 27%	4721
	11th to 20th	Day	12% 8% 4 7% 5% 5% 4 4 5% 6% 40%	5090
	21st to end	Day	17% 9% 3 8% 5% 5% 4 3 5% 7% 33%	5586
April	1st to 10th	Day	12% 11% 4 9% 5% 5% 4 3 5% 8% 32%	2403
	11th to 20th	Day	14% 9% 3 7% 5% 5% 5% 4 4 7% 37%	2620
	21st to end	Day	16% 7% 3 8% 5% 5% 5% 4 6% 7% 35%	2730
May	1st to 10th	Day	17% 9% 3 8% 5% 5% 5% 4 6% 7% 31%	7616
	11th to 20th	Day	17% 8% 3 6% 5% 4 5% 4 6% 8% 35%	7626
	21st to end	Day	17% 7% 3 7% 5% 4 5% 4 7% 8% 33%	8391
June	1st to 10th	Day	12% 7% 2 7% 5% 5% 5% 4 6% 9% 38%	2730
	11th to 20th	Day	9% 6% 2 7% 6% 5% 5% 4 7% 10% 39%	2773
	21st to end	Day	8% 5% 2 6% 5% 5% 6% 5% 8% 10% 39%	2800
July	1st to 10th	Day	8% 6% 3 7% 5% 5% 6% 4 6% 9% 41%	6360
	11th to 20th	Day	8% 5% 2 7% 5% 6% 6% 5% 8% 10% 37%	5914
	21st to end	Day	8% 6% 3 7% 5% 5% 7% 5% 8% 9% 37%	6424
August	1st to 10th	Day	6% 6% 2 6% 5% 5% 5% 5% 7% 10% 43%	4234
	11th to 20th	Day	7% 5% 2 6% 5% 5% 5% 4 8% 10% 42%	4174
	21st to end	Day	6% 4 2 6% 4 5% 5% 5% 7% 9% 48%	4516
September	1st to 10th	Day	11% 6% 2 4 3 4 4 3 5% 7% 50%	2233
	11th to 20th	Day	8% 5% 2 6% 4 4 4 4 6% 9% 49%	2200
	21st to end	Day	16% 7% 3 6% 4 4 4 3 5% 6% 43%	2200
October	1st to 10th	Day	11% 6% 2 5% 4 4 4 3 6% 7% 47%	2171
	11th to 20th	Day	14% 6% 2 6% 3 4 4 3 5% 6% 46%	2000
	21st to end	Day	18% 9% 3 7% 5% 5% 3 3 6% 7% 35%	2105
November	1st to 10th	Day	9% 7% 3 7% 4 4 5% 3 5% 7% 46%	1852
	11th to 20th	Day	14% 11% 4 8% 6% 5% 4 3 5% 7% 34%	1890
	21st to end	Day	17% 11% 5% 9% 5% 4 4 3 4 6% 30%	1890
December	1st to 10th	Day	16% 10% 5% 10% 5% 5% 5% 4 5% 5% 31%	3020
	11th to 20th	Day	20% 15% 6% 10% 5% 4 4 3 4 5% 23%	3463
	21st to end	Day	22% 18% 6% 11% 6% 5% 5% 3 5% 5% 15%	4565
January	1st to 10th	Night	12% 14% 6% 13% 8% 6% 5% 4 6% 6% 20%	7635
	11th to 20th	Night	10% 14% 6% 15% 7% 6% 6% 5% 5% 6% 21%	7680
	21st to end	Night	17% 12% 5% 8% 4 5% 4 3 5% 6% 30%	8401
February	1st to 10th	Night	18% 13% 4 9% 6% 4 4 3 5% 5% 29%	2709
	11th to 20th	Night	17% 11% 3 9% 5% 4 5% 4 6% 6% 30%	2458
	21st to end	Night	17% 13% 5% 10% 4 4 4 3 4 5% 29%	2076
March	1st to 10th	Night	18% 11% 4 8% 4 4 3 3 4 5% 35%	5338
	11th to 20th	Night	10% 6% 2 6% 5% 4 5% 4 6% 8% 42%	5090
	21st to end	Night	18% 8% 3 8% 4 4 4 3 5% 6% 36%	5588
April	1st to 10th	Night	15% 9% 3 6% 5% 5% 5% 4 6% 7% 37%	2235
	11th to 20th	Night	13% 7% 3 7% 5% 5% 5% 4 6% 7% 39%	1947
	21st to end	Night	15% 7% 3 8% 6% 4 4 4 5% 7% 39%	1890
May	1st to 10th	Night	21% 8% 3 7% 5% 4 5% 5% 5% 7% 30%	5237
	11th to 20th	Night	15% 7% 2 6% 4 4 4 3 6% 10% 39%	5234
	21st to end	Night	14% 7% 2 7% 5% 4 4 4 6% 8% 37%	5761
June	1st to 10th	Night	8% 5% 2 5% 5% 5% 4 4 5% 8% 47%	1890
	11th to 20th	Night	8% 6% 2 4 4 4 4 4 8% 10% 46%	1730
	21st to end	Night	7% 4 2 4 4 5% 6% 4 8% 10% 46%	1600
July	1st to 10th	Night	8% 6% 2 7% 4 4 4 5% 6% 7% 46%	3618
	11th to 20th	Night	6% 5% 3 8% 5% 5% 5% 5% 7% 9% 42%	4013
	21st to end	Night	8% 5% 3 7% 5% 5% 5% 5% 7% 9% 42%	4427
August	1st to 10th	Night	8% 6% 3 7% 4 5% 5% 5% 7% 8% 42%	2915
	11th to 20th	Night	7% 6% 3 7% 5% 5% 5% 4 6% 8% 45%	2875
	21st to end	Night	4 3 2 5% 4 5% 5% 5% 8% 9% 52%	3378
September	1st to 10th	Night	11% 5% 2 4 3 3 4 3 4 6% 53%	2146
	11th to 20th	Night	7% 4 2 4 3 4 4 3 5% 8% 56%	2200
	21st to end	Night	15% 6% 2 6% 4 3 3 3 5% 6% 48%	2200
October	1st to 10th	Night	8% 5% 2 6% 4 3 4 4 5% 7% 53%	2227
	11th to 20th	Night	14% 7% 2 6% 3 4 4 3 5% 7% 46%	2400
	21st to end	Night	17% 8% 3 7% 4 4 4 3 5% 5% 40%	2733
November	1st to 10th	Night	12% 7% 2 6% 4 3 3 3 5% 7% 48%	2666
	11th to 20th	Night	16% 9% 2 7% 4 3 5% 4 5% 6% 39%	2730
	21st to end	Night	18% 11% 4 9% 6% 4 4 4 5% 7% 29%	2730
December	1st to 10th	Night	15% 10% 4 8% 6% 5% 5% 4 5% 6% 32%	4380
	11th to 20th	Night	17% 13% 6% 11% 4 4 5% 3 5% 7% 24%	4952
	21st to end	Night	22% 17% 6% 11% 6% 5% 4 3 4 5% 17%	6633

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	66% 4 11% 14% 5%	5259
	11th to 20th	Day	65% 3 9% 10% 12% 1	5280
	21st to end	Day	68% 4 11% 10% 6% 1	5780
February	1st to 10th	Day	61% 4 12% 14% 8%	1909
	11th to 20th	Day	61% 5% 10% 16% 8% 1	2058
	21st to end	Day	57% 5% 12% 17% 8% 1	1730
March	1st to 10th	Day	58% 4 10% 20% 7% 1	4721
	11th to 20th	Day	43% 5% 12% 27% 13% 1	5090
	21st to end	Day	51% 4 12% 21% 10% 2	5586
April	1st to 10th	Day	50% 5% 12% 20% 12% 1	2403
	11th to 20th	Day	46% 6% 13% 20% 12% 2	2620
	21st to end	Day	48% 5% 13% 22% 11% 2	2730
May	1st to 10th	Day	52% 6% 11% 20% 10% 1	7651
	11th to 20th	Day	46% 6% 12% 24% 12%	7630
	21st to end	Day	46% 7% 11% 20% 12% 4	8405
June	1st to 10th	Day	40% 8% 11% 23% 15% 2	2730
	11th to 20th	Day	38% 8% 15% 22% 16% 1	2773
	21st to end	Day	36% 9% 16% 21% 15% 3	2800
July	1st to 10th	Day	39% 6% 11% 26% 16% 2	6291
	11th to 20th	Day	39% 8% 13% 25% 14%	5844
	21st to end	Day	40% 8% 13% 21% 15% 3	6240
August	1st to 10th	Day	34% 9% 17% 27% 13% 1	4221
	11th to 20th	Day	33% 7% 13% 31% 15% 1	4213
	21st to end	Day	30% 7% 14% 29% 17% 3	4515
September	1st to 10th	Day	33% 6% 12% 24% 21% 4	2233
	11th to 20th	Day	31% 6% 12% 26% 23% 3	2200
	21st to end	Day	40% 5% 9% 21% 23% 1	2200
October	1st to 10th	Day	24% 5% 10% 31% 28% 2	3535
	11th to 20th	Day	31% 4 11% 28% 23% 2	3240
	21st to end	Day	38% 3 12% 25% 20% 2	3381
November	1st to 10th	Day	38% 5% 10% 27% 17% 3	1852
	11th to 20th	Day	49% 5% 11% 20% 12% 2	1890
	21st to end	Day	56% 4 10% 19% 9% 1	1890
December	1st to 10th	Day	54% 5% 14% 20% 7%	3020
	11th to 20th	Day	63% 5% 10% 16% 6%	3460
	21st to end	Day	71% 5% 9% 10% 5% 1	4565
January	1st to 10th	Night	63% 4 11% 15% 6%	7636
	11th to 20th	Night	63% 3 10% 12% 9% 2	7680
	21st to end	Night	55% 3 13% 19% 8% 1	8401
February	1st to 10th	Night	57% 4 11% 18% 9% 1	2709
	11th to 20th	Night	54% 5% 10% 19% 11% 1	2459
	21st to end	Night	57% 5% 11% 16% 9% 2	2076
March	1st to 10th	Night	51% 4 8% 25% 11% 2	5338
	11th to 20th	Night	38% 6% 12% 30% 12% 1	5090
	21st to end	Night	48% 4 9% 24% 13% 2	5588
April	1st to 10th	Night	45% 6% 13% 22% 12% 2	2235
	11th to 20th	Night	42% 5% 14% 18% 18% 3	1947
	21st to end	Night	45% 5% 11% 23% 14% 2	1890
May	1st to 10th	Night	52% 6% 10% 20% 8% 3	5264
	11th to 20th	Night	41% 7% 11% 26% 14% 1	5235
	21st to end	Night	42% 7% 12% 24% 11% 5%	5771
June	1st to 10th	Night	34% 7% 11% 23% 22% 2	1890
	11th to 20th	Night	30% 10% 15% 25% 17% 3	1730
	21st to end	Night	29% 10% 17% 23% 18% 3	1600
July	1st to 10th	Night	36% 6% 12% 25% 18% 4	3580
	11th to 20th	Night	35% 8% 14% 27% 14% 1	3963
	21st to end	Night	37% 6% 14% 22% 18% 3	4297
August	1st to 10th	Night	38% 7% 15% 26% 13% 1	2904
	11th to 20th	Night	36% 8% 10% 27% 20%	2902
	21st to end	Night	26% 7% 15% 29% 20% 3	3378
September	1st to 10th	Night	32% 5% 12% 25% 20% 6%	2146
	11th to 20th	Night	26% 6% 9% 27% 28% 4	2200
	21st to end	Night	37% 6% 10% 21% 26% 2	2200
October	1st to 10th	Night	24% 4 8% 29% 34% 2	3643
	11th to 20th	Night	29% 3 9% 27% 28% 3	3899
	21st to end	Night	33% 4 7% 25% 25% 5%	4407
November	1st to 10th	Night	37% 4 11% 25% 21% 3	2666
	11th to 20th	Night	45% 4 10% 23% 15% 3	2730
	21st to end	Night	56% 4 11% 19% 9% 1	2730
December	1st to 10th	Night	52% 4 12% 23% 8%	4380
	11th to 20th	Night	60% 4 12% 17% 6%	4954
	21st to end	Night	70% 3 10% 13% 3	6633

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	24% 17% 33% 20% 6% 1	330
	11th to 20th	Day	22% 24% 38% 12% 4 1	330
	21st to end	Day	25% 20% 33% 17% 6%	363
February	1st to 10th	Day	27% 14% 36% 18% 5%	330
	11th to 20th	Day	25% 19% 37% 15% 4	330
	21st to end	Day	22% 18% 40% 18% 2	273
March	1st to 10th	Day	22% 17% 36% 20% 5%	393
	11th to 20th	Day	23% 16% 36% 17% 7% 1	540
	21st to end	Day	26% 15% 34% 20% 5%	594
April	1st to 10th	Day	17% 17% 36% 20% 8% 2	540
	11th to 20th	Day	18% 18% 29% 23% 9% 4	532
	21st to end	Day	19% 12% 34% 22% 11% 3	540
May	1st to 10th	Day	23% 14% 30% 21% 9% 3	543
	11th to 20th	Day	19% 10% 27% 24% 14% 6%	567
	21st to end	Day	15% 6% 25% 25% 18% 11%	617
June	1st to 10th	Day	11% 7% 24% 30% 20% 8%	570
	11th to 20th	Day	11% 6% 24% 29% 22% 7%	658
	21st to end	Day	9% 6% 25% 29% 21% 10%	730
July	1st to 10th	Day	13% 2 15% 26% 24% 19%	721
	11th to 20th	Day	10% 3 19% 23% 29% 15%	537
	21st to end	Day	10% 2 15% 30% 25% 18%	576
August	1st to 10th	Day	10% 2 14% 24% 30% 19%	526
	11th to 20th	Day	10% 3 19% 26% 29% 13%	519
	21st to end	Day	7% 3 14% 28% 30% 19%	583
September	1st to 10th	Day	12% 4 15% 25% 30% 14%	530
	11th to 20th	Day	8% 3 11% 23% 35% 21%	530
	21st to end	Day	19% 9% 17% 25% 19% 11%	530
October	1st to 10th	Day	10% 5% 23% 29% 23% 9%	530
	11th to 20th	Day	14% 12% 32% 23% 16% 4	530
	21st to end	Day	20% 13% 33% 25% 9% 1	488
November	1st to 10th	Day	16% 11% 35% 24% 10% 4	330
	11th to 20th	Day	18% 18% 33% 18% 12% 2	330
	21st to end	Day	24% 12% 39% 20% 5%	330
December	1st to 10th	Day	20% 20% 37% 16% 6% 1	330
	11th to 20th	Day	24% 13% 40% 18% 4	330
	21st to end	Day	30% 18% 32% 16% 5%	363
January	1st to 10th	Night	20% 18% 35% 23% 5%	840
	11th to 20th	Night	23% 17% 36% 20% 4	840
	21st to end	Night	21% 14% 37% 25% 3	924
February	1st to 10th	Night	25% 13% 33% 27% 2	840
	11th to 20th	Night	24% 14% 35% 25% 2	823
	21st to end	Night	24% 12% 38% 22% 3	692
March	1st to 10th	Night	22% 13% 34% 24% 6%	840
	11th to 20th	Night	18% 16% 35% 25% 6%	840
	21st to end	Night	23% 11% 34% 25% 7%	924
April	1st to 10th	Night	18% 13% 34% 29% 5% 1	765
	11th to 20th	Night	14% 11% 35% 27% 11% 1	623
	21st to end	Night	17% 6% 38% 29% 9%	630
May	1st to 10th	Night	21% 9% 31% 28% 11%	630
	11th to 20th	Night	19% 5% 28% 31% 15% 2	621
	21st to end	Night	10% 2 20% 40% 24% 3	683
June	1st to 10th	Night	10% 3 26% 40% 21% 1	630
	11th to 20th	Night	10% 5% 19% 42% 24% 1	615
	21st to end	Night	7% 3 21% 39% 26% 3	600
July	1st to 10th	Night	9% 3 13% 36% 33% 6%	585
	11th to 20th	Night	6% 2 15% 37% 35% 5%	582
	21st to end	Night	8% 3 13% 37% 37% 2	657
August	1st to 10th	Night	7% 2 12% 41% 34% 4	597
	11th to 20th	Night	9% 2 12% 41% 34% 4	594
	21st to end	Night	7% 2 13% 34% 38% 7%	660
September	1st to 10th	Night	13% 3 16% 33% 30% 5%	746
	11th to 20th	Night	9% 3 11% 34% 38% 6%	800
	21st to end	Night	17% 6% 18% 33% 24% 3	800
October	1st to 10th	Night	10% 5% 25% 33% 23% 5%	800
	11th to 20th	Night	17% 9% 28% 32% 13% 2	800
	21st to end	Night	20% 11% 30% 28% 10%	880
November	1st to 10th	Night	15% 9% 30% 29% 15% 1	820
	11th to 20th	Night	19% 15% 31% 27% 8%	840
	21st to end	Night	22% 14% 28% 29% 7%	840
December	1st to 10th	Night	26% 12% 33% 24% 5% 1	840
	11th to 20th	Night	22% 12% 34% 26% 5%	840
	21st to end	Night	28% 15% 31% 23% 4	924

Page generated at 2026-06-13 02:31:40. Version ac07

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