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 2025-02-21.

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%	109743
	Night	14% 9% 4 8% 5% 4 4 4 5% 7% 36%	108847
Transparency	Day	46% 6% 12% 21% 13% 2	113430
	Night	46% 5% 11% 22% 14% 2	113299
Seeing	Day	17% 10% 27% 23% 16% 8%	15593
	Night	17% 9% 27% 30% 15% 2	25265

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	14% 15% 8% 15% 7% 4 5% 4 4 5% 19%	4056
	11th to 20th	Day	13% 15% 6% 14% 6% 7% 6% 3 4 5% 22%	4060
	21st to end	Day	20% 13% 8% 11% 7% 6% 4 4 5% 4 17%	4466
February	1st to 10th	Day	20% 12% 5% 11% 7% 4 4 3 4 6% 23%	1818
	11th to 20th	Day	16% 10% 5% 12% 7% 5% 5% 4 4 6% 24%	1948
	21st to end	Day	18% 13% 4 9% 5% 5% 3 4 4 5% 30%	1570
March	1st to 10th	Day	21% 12% 6% 10% 5% 5% 5% 4 5% 6% 23%	3216
	11th to 20th	Day	13% 9% 3 7% 5% 5% 5% 4 6% 6% 39%	3480
	21st to end	Day	19% 9% 3 8% 5% 5% 4 3 5% 7% 33%	3815
April	1st to 10th	Day	12% 11% 4 9% 5% 5% 5% 3 6% 8% 33%	2174
	11th to 20th	Day	15% 9% 3 7% 5% 4 5% 4 4 7% 37%	2368
	21st to end	Day	17% 7% 3 8% 5% 5% 5% 4 6% 7% 34%	2470
May	1st to 10th	Day	20% 11% 3 8% 5% 4 4 4 5% 7% 31%	4082
	11th to 20th	Day	20% 8% 3 6% 5% 4 4 4 6% 7% 34%	4081
	21st to end	Day	10% 5% 2 7% 5% 5% 6% 5% 8% 10% 36%	4476
June	1st to 10th	Day	12% 7% 2 7% 5% 5% 5% 4 7% 9% 38%	2470
	11th to 20th	Day	9% 6% 2 6% 6% 5% 5% 4 7% 10% 40%	2571
	21st to end	Day	8% 5% 2 6% 5% 5% 6% 5% 8% 10% 40%	2660
July	1st to 10th	Day	9% 6% 3 8% 5% 5% 6% 5% 7% 9% 37%	4440
	11th to 20th	Day	7% 5% 2 5% 4 5% 5% 4 8% 10% 45%	4134
	21st to end	Day	9% 6% 3 7% 5% 5% 7% 6% 8% 9% 36%	4466
August	1st to 10th	Day	7% 6% 2 6% 5% 5% 5% 5% 7% 10% 42%	4104
	11th to 20th	Day	7% 5% 2 6% 5% 5% 5% 4 8% 10% 42%	4044
	21st to end	Day	6% 4 2 6% 4 5% 5% 5% 7% 9% 48%	4378
September	1st to 10th	Day	11% 6% 2 4 3 4 4 3 5% 7% 49%	2122
	11th to 20th	Day	9% 5% 2 6% 4 4 4 4 6% 9% 48%	2090
	21st to end	Day	16% 7% 2 5% 4 4 4 3 5% 7% 43%	2090
October	1st to 10th	Day	11% 6% 2 5% 5% 4 4 4 6% 7% 45%	2063
	11th to 20th	Day	15% 7% 2 6% 3 4 4 4 5% 6% 45%	1900
	21st to end	Day	18% 9% 3 7% 5% 5% 3 3 6% 7% 34%	2000
November	1st to 10th	Day	9% 6% 3 7% 5% 4 4 3 5% 7% 47%	1762
	11th to 20th	Day	14% 10% 4 8% 6% 5% 4 3 5% 7% 34%	1800
	21st to end	Day	17% 11% 5% 10% 5% 4 4 3 4 6% 31%	1800
December	1st to 10th	Day	16% 10% 5% 9% 5% 5% 4 4 5% 6% 32%	2930
	11th to 20th	Day	20% 15% 6% 10% 5% 4 4 3 4 5% 23%	3373
	21st to end	Day	22% 18% 6% 11% 6% 5% 5% 3 5% 5% 15%	4466
January	1st to 10th	Night	13% 14% 7% 13% 7% 5% 5% 4 6% 6% 21%	5889
	11th to 20th	Night	11% 15% 6% 14% 7% 6% 6% 5% 5% 5% 20%	5900
	21st to end	Night	20% 13% 6% 9% 4 5% 4 3 4 5% 28%	6490
February	1st to 10th	Night	18% 13% 4 9% 6% 4 5% 3 5% 5% 28%	2580
	11th to 20th	Night	17% 11% 3 9% 6% 5% 5% 4 6% 6% 29%	2338
	21st to end	Night	18% 14% 5% 10% 4 4 4 3 4 5% 28%	1885
March	1st to 10th	Night	20% 12% 3 7% 4 5% 3 3 4 6% 33%	3623
	11th to 20th	Night	11% 7% 2 6% 5% 4 5% 4 6% 8% 43%	3480
	21st to end	Night	19% 8% 2 6% 4 4 4 3 4 5% 41%	3817
April	1st to 10th	Night	13% 9% 2 6% 5% 4 5% 4 6% 7% 38%	2023
	11th to 20th	Night	14% 7% 3 6% 5% 4 5% 4 6% 7% 40%	1759
	21st to end	Night	15% 7% 3 7% 6% 5% 4 4 5% 7% 38%	1710
May	1st to 10th	Night	23% 10% 3 7% 5% 4 4 4 5% 6% 28%	2813
	11th to 20th	Night	19% 7% 2 5% 3 4 3 3 6% 9% 40%	2812
	21st to end	Night	8% 5% 2 5% 4 5% 5% 5% 7% 9% 45%	3079
June	1st to 10th	Night	8% 5% 2 5% 5% 5% 5% 4 5% 9% 47%	1710
	11th to 20th	Night	7% 5% 1 4 4 4 5% 4 8% 10% 46%	1602
	21st to end	Night	7% 4 2 4 4 5% 6% 4 8% 10% 46%	1520
July	1st to 10th	Night	10% 7% 2 6% 4 4 4 4 5% 6% 48%	2530
	11th to 20th	Night	5% 3 2 6% 5% 4 4 5% 6% 9% 50%	2809
	21st to end	Night	8% 5% 3 7% 6% 5% 6% 5% 6% 8% 42%	3087
August	1st to 10th	Night	9% 7% 3 7% 4 5% 5% 5% 7% 8% 41%	2825
	11th to 20th	Night	7% 6% 3 7% 5% 5% 5% 4 6% 8% 44%	2785
	21st to end	Night	4 3 2 5% 4 5% 5% 5% 8% 9% 52%	3274
September	1st to 10th	Night	12% 5% 2 4 3 3 4 3 4 7% 51%	2038
	11th to 20th	Night	7% 4 2 4 3 4 4 3 5% 8% 56%	2090
	21st to end	Night	15% 6% 2 5% 4 3 3 3 5% 6% 48%	2090
October	1st to 10th	Night	7% 5% 2 6% 4 3 4 4 6% 7% 52%	2115
	11th to 20th	Night	15% 7% 2 6% 4 4 4 3 5% 7% 45%	2280
	21st to end	Night	17% 9% 3 7% 4 4 4 3 5% 5% 40%	2596
November	1st to 10th	Night	12% 7% 2 6% 4 3 3 3 5% 7% 48%	2536
	11th to 20th	Night	16% 9% 2 7% 4 3 5% 4 5% 6% 39%	2600
	21st to end	Night	17% 11% 4 9% 6% 4 4 4 6% 7% 29%	2600
December	1st to 10th	Night	15% 10% 4 8% 6% 5% 5% 4 5% 6% 32%	4250
	11th to 20th	Night	18% 13% 6% 11% 4 4 5% 3 5% 7% 24%	4822
	21st to end	Night	22% 17% 6% 11% 6% 5% 4 3 4 5% 17%	6490

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 8% 15% 6%	4057
	11th to 20th	Day	66% 3 10% 10% 10% 1	4060
	21st to end	Day	69% 4 11% 9% 6% 1	4466
February	1st to 10th	Day	62% 5% 12% 13% 8%	1818
	11th to 20th	Day	61% 5% 10% 16% 8% 1	1958
	21st to end	Day	58% 5% 11% 17% 7% 1	1570
March	1st to 10th	Day	61% 5% 11% 18% 4 1	3216
	11th to 20th	Day	43% 6% 12% 25% 13% 1	3480
	21st to end	Day	50% 5% 13% 23% 8%	3815
April	1st to 10th	Day	50% 6% 12% 20% 12% 1	2174
	11th to 20th	Day	46% 6% 13% 20% 13% 2	2368
	21st to end	Day	49% 5% 13% 20% 11% 2	2470
May	1st to 10th	Day	52% 6% 11% 22% 7% 1	4120
	11th to 20th	Day	47% 8% 13% 22% 10% 1	4085
	21st to end	Day	38% 11% 15% 21% 12% 3	4489
June	1st to 10th	Day	40% 8% 11% 24% 14% 2	2470
	11th to 20th	Day	38% 8% 15% 23% 16% 1	2571
	21st to end	Day	36% 9% 16% 21% 15% 3	2660
July	1st to 10th	Day	42% 7% 12% 20% 16% 3	4371
	11th to 20th	Day	32% 7% 12% 29% 19% 1	4064
	21st to end	Day	41% 9% 13% 18% 15% 4	4324
August	1st to 10th	Day	34% 9% 17% 27% 13% 1	4091
	11th to 20th	Day	34% 7% 13% 30% 16% 1	4083
	21st to end	Day	30% 8% 14% 29% 16% 3	4377
September	1st to 10th	Day	34% 7% 12% 23% 21% 5%	2122
	11th to 20th	Day	32% 6% 13% 25% 22% 3	2090
	21st to end	Day	40% 5% 9% 22% 22% 1	2090
October	1st to 10th	Day	24% 5% 10% 31% 28% 2	3427
	11th to 20th	Day	32% 4 11% 28% 23% 2	3140
	21st to end	Day	38% 3 12% 25% 19% 2	3276
November	1st to 10th	Day	37% 5% 10% 28% 18% 3	1762
	11th to 20th	Day	50% 5% 11% 20% 13% 2	1800
	21st to end	Day	55% 4 11% 19% 9% 1	1800
December	1st to 10th	Day	53% 5% 14% 21% 7%	2930
	11th to 20th	Day	63% 5% 10% 15% 6%	3370
	21st to end	Day	71% 5% 9% 10% 5% 1	4466
January	1st to 10th	Night	63% 4 11% 15% 7%	5889
	11th to 20th	Night	65% 4 11% 11% 8% 1	5900
	21st to end	Night	59% 3 12% 17% 8% 2	6490
February	1st to 10th	Night	58% 4 11% 18% 8% 1	2580
	11th to 20th	Night	54% 5% 10% 18% 11% 1	2339
	21st to end	Night	59% 5% 10% 16% 9% 2	1885
March	1st to 10th	Night	51% 6% 9% 23% 10% 1	3623
	11th to 20th	Night	38% 7% 12% 28% 13% 2	3480
	21st to end	Night	45% 5% 10% 28% 11% 1	3817
April	1st to 10th	Night	43% 6% 14% 23% 13% 2	2023
	11th to 20th	Night	42% 5% 13% 18% 18% 4	1759
	21st to end	Night	45% 6% 12% 21% 13% 2	1710
May	1st to 10th	Night	55% 7% 10% 20% 8% 2	2840
	11th to 20th	Night	41% 8% 13% 27% 10% 1	2813
	21st to end	Night	30% 9% 15% 29% 13% 4	3087
June	1st to 10th	Night	34% 7% 12% 24% 21% 2	1710
	11th to 20th	Night	29% 10% 15% 25% 17% 3	1602
	21st to end	Night	29% 10% 17% 23% 18% 3	1520
July	1st to 10th	Night	37% 6% 11% 25% 16% 6%	2490
	11th to 20th	Night	29% 6% 13% 32% 19% 1	2756
	21st to end	Night	38% 6% 14% 22% 16% 4	2988
August	1st to 10th	Night	38% 8% 15% 26% 13% 1	2814
	11th to 20th	Night	36% 8% 10% 26% 20%	2812
	21st to end	Night	26% 8% 15% 29% 20% 3	3274
September	1st to 10th	Night	33% 5% 13% 24% 19% 6%	2038
	11th to 20th	Night	27% 6% 9% 27% 27% 4	2090
	21st to end	Night	37% 6% 10% 21% 25% 2	2090
October	1st to 10th	Night	24% 4 8% 29% 34% 2	3531
	11th to 20th	Night	29% 3 9% 27% 28% 3	3779
	21st to end	Night	33% 4 7% 25% 25% 5%	4270
November	1st to 10th	Night	37% 4 11% 24% 21% 3	2536
	11th to 20th	Night	45% 4 9% 22% 15% 4	2600
	21st to end	Night	55% 4 12% 20% 9% 1	2600
December	1st to 10th	Night	52% 4 12% 23% 8%	4250
	11th to 20th	Night	60% 4 13% 17% 6%	4824
	21st to end	Night	70% 3 10% 13% 3	6490

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% 17% 33% 20% 6% 1	300
	11th to 20th	Day	21% 22% 39% 13% 4 1	300
	21st to end	Day	25% 18% 35% 16% 5%	330
February	1st to 10th	Day	28% 15% 36% 16% 4	300
	11th to 20th	Day	25% 20% 37% 14% 4	300
	21st to end	Day	25% 18% 38% 17% 2	225
March	1st to 10th	Day	23% 17% 33% 21% 5%	327
	11th to 20th	Day	21% 16% 36% 18% 7% 1	460
	21st to end	Day	27% 14% 33% 21% 5%	506
April	1st to 10th	Day	17% 16% 35% 21% 8% 3	460
	11th to 20th	Day	19% 17% 26% 24% 9% 4	452
	21st to end	Day	20% 12% 34% 22% 9% 2	460
May	1st to 10th	Day	24% 13% 28% 22% 9% 4	463
	11th to 20th	Day	20% 9% 25% 25% 14% 7%	487
	21st to end	Day	12% 6% 25% 26% 19% 12%	529
June	1st to 10th	Day	11% 7% 23% 30% 21% 8%	490
	11th to 20th	Day	11% 6% 24% 28% 24% 8%	591
	21st to end	Day	9% 6% 27% 29% 19% 9%	680
July	1st to 10th	Day	14% 2 15% 26% 23% 20%	671
	11th to 20th	Day	9% 4 19% 23% 30% 15%	497
	21st to end	Day	10% 2 15% 31% 25% 17%	532
August	1st to 10th	Day	11% 2 14% 24% 30% 19%	486
	11th to 20th	Day	11% 4 20% 26% 28% 12%	479
	21st to end	Day	8% 3 12% 29% 29% 19%	539
September	1st to 10th	Day	12% 4 15% 25% 30% 14%	490
	11th to 20th	Day	8% 3 12% 25% 33% 19%	490
	21st to end	Day	20% 9% 17% 25% 18% 11%	490
October	1st to 10th	Day	10% 6% 24% 30% 21% 9%	490
	11th to 20th	Day	14% 12% 32% 22% 16% 4	490
	21st to end	Day	20% 12% 33% 24% 9% 1	449
November	1st to 10th	Day	15% 10% 36% 24% 11% 4	300
	11th to 20th	Day	17% 17% 32% 19% 13% 2	300
	21st to end	Day	23% 12% 38% 21% 5%	300
December	1st to 10th	Day	21% 20% 36% 15% 7% 1	300
	11th to 20th	Day	24% 12% 40% 19% 4	300
	21st to end	Day	29% 18% 30% 17% 5%	330
January	1st to 10th	Night	20% 18% 35% 22% 5%	800
	11th to 20th	Night	24% 16% 35% 21% 4 1	800
	21st to end	Night	21% 14% 36% 25% 3	880
February	1st to 10th	Night	26% 13% 34% 26% 2	800
	11th to 20th	Night	24% 14% 35% 25% 2	783
	21st to end	Night	25% 12% 38% 22% 3	629
March	1st to 10th	Night	22% 13% 34% 25% 6% 1	760
	11th to 20th	Night	19% 14% 34% 26% 7% 1	760
	21st to end	Night	23% 10% 34% 25% 8%	836
April	1st to 10th	Night	17% 13% 35% 29% 5% 1	693
	11th to 20th	Night	14% 10% 35% 28% 12% 1	563
	21st to end	Night	17% 6% 39% 29% 8% 1	570
May	1st to 10th	Night	20% 8% 31% 29% 11% 1	570
	11th to 20th	Night	19% 5% 27% 31% 16% 2	561
	21st to end	Night	9% 3 20% 39% 26% 4	617
June	1st to 10th	Night	8% 3 26% 41% 21% 1	570
	11th to 20th	Night	9% 5% 18% 41% 25% 1	570
	21st to end	Night	7% 3 22% 40% 26% 2	570
July	1st to 10th	Night	9% 3 14% 35% 32% 7%	555
	11th to 20th	Night	6% 2 15% 38% 34% 5%	552
	21st to end	Night	8% 3 13% 38% 36% 2	624
August	1st to 10th	Night	8% 2 12% 40% 35% 4	567
	11th to 20th	Night	9% 2 12% 41% 32% 3	564
	21st to end	Night	7% 2 13% 33% 39% 7%	627
September	1st to 10th	Night	14% 3 16% 32% 30% 5%	708
	11th to 20th	Night	9% 3 12% 34% 36% 6%	760
	21st to end	Night	17% 6% 19% 33% 23% 3	760
October	1st to 10th	Night	9% 5% 26% 33% 22% 5%	760
	11th to 20th	Night	18% 9% 28% 31% 13% 2	760
	21st to end	Night	21% 11% 30% 28% 11%	836
November	1st to 10th	Night	16% 9% 28% 29% 16% 1	780
	11th to 20th	Night	19% 14% 31% 27% 9% 1	800
	21st to end	Night	22% 14% 29% 29% 7% 1	800
December	1st to 10th	Night	26% 12% 33% 23% 5% 1	800
	11th to 20th	Night	22% 11% 34% 26% 6% 1	800
	21st to end	Night	28% 14% 30% 24% 4	880

Page generated at 2025-02-18 03:26:29. Version ac07

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