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 2025-09-04.

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	15% 11% 4 9% 5% 5% 5% 4 5% 7% 31%	126699
	Night	14% 12% 4 10% 5% 4 4 4 5% 6% 31%	121826
Transparency	Day	50% 6% 12% 19% 11% 1	130295
	Night	50% 6% 12% 19% 11% 1	126318
Seeing	Day	18% 12% 28% 24% 13% 5%	16098
	Night	19% 10% 29% 29% 12% 1	25049

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% 17% 7% 25% 6% 4 4 2 4 3 14%	3691
	11th to 20th	Day	11% 21% 9% 22% 8% 7% 4 3 3 4 8%	4026
	21st to end	Day	19% 19% 7% 14% 7% 5% 5% 3 4 4 14%	4466
February	1st to 10th	Day	16% 17% 5% 13% 7% 6% 5% 3 5% 4 18%	1800
	11th to 20th	Day	14% 15% 7% 15% 9% 5% 4 3 4 6% 18%	1753
	21st to end	Day	16% 15% 6% 12% 7% 5% 5% 4 4 5% 21%	1649
March	1st to 10th	Day	18% 12% 7% 13% 7% 4 3 3 3 5% 26%	4814
	11th to 20th	Day	12% 13% 5% 10% 5% 4 4 3 6% 8% 30%	4980
	21st to end	Day	19% 14% 6% 10% 5% 4 3 3 4 4 28%	5465
April	1st to 10th	Day	17% 14% 4 8% 5% 4 5% 3 5% 7% 28%	2273
	11th to 20th	Day	18% 11% 3 7% 4 4 4 4 5% 8% 32%	2554
	21st to end	Day	19% 11% 4 8% 4 4 3 3 5% 6% 34%	2600
May	1st to 10th	Day	19% 10% 3 8% 4 3 5% 4 5% 7% 32%	5888
	11th to 20th	Day	17% 9% 3 7% 4 4 5% 4 6% 9% 33%	5861
	21st to end	Day	17% 9% 2 7% 4 4 4 4 5% 6% 39%	6946
June	1st to 10th	Day	13% 6% 2 7% 5% 5% 5% 4 6% 8% 38%	2800
	11th to 20th	Day	11% 6% 3 6% 5% 4 5% 5% 7% 8% 39%	2974
	21st to end	Day	12% 6% 3 5% 4 4 5% 4 6% 9% 40%	3000
July	1st to 10th	Day	9% 5% 2 6% 5% 5% 5% 4 6% 9% 41%	6819
	11th to 20th	Day	9% 5% 2 6% 5% 5% 5% 4 7% 10% 42%	6009
	21st to end	Day	8% 5% 2 6% 4 5% 5% 4 6% 9% 45%	6424
August	1st to 10th	Day	14% 6% 2 6% 4 5% 5% 4 7% 9% 38%	4234
	11th to 20th	Day	10% 7% 3 7% 5% 5% 6% 5% 7% 10% 33%	4174
	21st to end	Day	9% 6% 2 6% 5% 6% 5% 4 6% 9% 41%	4614
September	1st to 10th	Day	10% 6% 3 7% 5% 5% 5% 4 6% 8% 40%	2263
	11th to 20th	Day	11% 5% 2 5% 4 4 4 3 6% 8% 47%	2090
	21st to end	Day	18% 6% 2 7% 4 5% 5% 4 6% 9% 34%	2090
October	1st to 10th	Day	17% 7% 3 7% 5% 4 4 4 6% 8% 36%	1939
	11th to 20th	Day	20% 9% 3 9% 5% 5% 5% 4 5% 7% 29%	1900
	21st to end	Day	24% 12% 4 8% 6% 5% 4 4 5% 7% 22%	2056
November	1st to 10th	Day	15% 12% 3 7% 6% 5% 4 4 6% 7% 32%	1762
	11th to 20th	Day	16% 13% 4 8% 5% 5% 5% 4 5% 7% 26%	1641
	21st to end	Day	19% 18% 6% 10% 6% 4 4 5% 5% 6% 16%	1600
December	1st to 10th	Day	18% 19% 6% 12% 6% 6% 4 4 5% 6% 13%	2600
	11th to 20th	Day	21% 19% 7% 10% 6% 4 4 3 4 5% 16%	2986
	21st to end	Day	26% 21% 8% 12% 6% 4 3 3 4 5% 8%	3958
January	1st to 10th	Night	14% 15% 8% 23% 6% 5% 4 3 4 4 15%	6052
	11th to 20th	Night	12% 17% 8% 19% 9% 7% 6% 4 4 4 9%	5859
	21st to end	Night	18% 16% 7% 14% 5% 4 4 3 5% 5% 19%	6490
February	1st to 10th	Night	16% 15% 5% 11% 8% 5% 6% 4 5% 5% 19%	2600
	11th to 20th	Night	15% 14% 5% 10% 7% 5% 5% 5% 6% 7% 22%	2532
	21st to end	Night	16% 13% 5% 11% 7% 6% 5% 4 4 5% 23%	1980
March	1st to 10th	Night	14% 11% 4 10% 6% 4 5% 4 5% 6% 32%	5025
	11th to 20th	Night	10% 9% 4 8% 5% 5% 4 4 7% 9% 36%	4979
	21st to end	Night	18% 11% 3 9% 5% 4 4 3 4 4 35%	5467
April	1st to 10th	Night	18% 11% 4 8% 4 4 4 3 6% 7% 31%	2126
	11th to 20th	Night	17% 10% 3 5% 5% 5% 4 3 5% 7% 35%	1812
	21st to end	Night	15% 9% 2 7% 6% 4 5% 4 6% 8% 34%	1800
May	1st to 10th	Night	20% 9% 2 7% 4 4 4 3 5% 6% 35%	4053
	11th to 20th	Night	13% 9% 3 8% 5% 4 5% 4 5% 6% 38%	4032
	21st to end	Night	18% 7% 2 4 3 3 4 3 5% 6% 45%	3926
June	1st to 10th	Night	12% 6% 3 5% 4 3 4 4 6% 8% 45%	1600
	11th to 20th	Night	9% 6% 2 5% 4 4 5% 5% 8% 10% 41%	1420
	21st to end	Night	9% 5% 3 5% 4 5% 5% 4 5% 8% 47%	1400
July	1st to 10th	Night	7% 5% 2 6% 4 4 4 5% 6% 10% 48%	3160
	11th to 20th	Night	9% 6% 2 5% 4 4 3 4 5% 9% 49%	3948
	21st to end	Night	7% 6% 2 5% 4 4 4 4 6% 8% 51%	4428
August	1st to 10th	Night	11% 6% 2 7% 3 4 4 4 6% 8% 46%	2915
	11th to 20th	Night	11% 8% 3 5% 3 4 4 4 5% 7% 47%	2875
	21st to end	Night	8% 5% 2 6% 4 4 4 4 6% 7% 48%	3250
September	1st to 10th	Night	9% 6% 3 7% 3 4 4 3 6% 7% 48%	1981
	11th to 20th	Night	10% 4 2 4 4 3 4 4 5% 7% 53%	2090
	21st to end	Night	17% 7% 2 7% 4 4 4 3 6% 6% 40%	2090
October	1st to 10th	Night	14% 8% 3 7% 5% 4 4 3 5% 7% 42%	2239
	11th to 20th	Night	18% 11% 3 7% 5% 3 4 3 5% 7% 34%	2280
	21st to end	Night	21% 11% 3 8% 5% 5% 4 3 5% 7% 27%	2541
November	1st to 10th	Night	15% 9% 3 7% 5% 3 4 4 6% 7% 38%	2536
	11th to 20th	Night	16% 14% 4 8% 5% 4 5% 4 6% 6% 26%	2756
	21st to end	Night	21% 14% 5% 8% 6% 5% 5% 4 6% 6% 20%	2800
December	1st to 10th	Night	13% 18% 7% 15% 7% 5% 5% 5% 5% 6% 14%	4580
	11th to 20th	Night	15% 22% 6% 12% 5% 4 4 3 4 5% 20%	5208
	21st to end	Night	23% 21% 6% 11% 6% 4 4 3 4 5% 13%	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	73% 6% 8% 9% 4	3692
	11th to 20th	Day	79% 5% 8% 5% 3	4060
	21st to end	Day	73% 5% 9% 7% 6%	4466
February	1st to 10th	Day	69% 4 10% 13% 4 1	1800
	11th to 20th	Day	67% 4 8% 14% 6%	1762
	21st to end	Day	64% 5% 10% 14% 6% 1	1649
March	1st to 10th	Day	61% 5% 12% 17% 5%	4817
	11th to 20th	Day	51% 5% 17% 19% 7% 2	4980
	21st to end	Day	58% 5% 10% 17% 9% 1	5465
April	1st to 10th	Day	55% 5% 10% 20% 10% 1	2273
	11th to 20th	Day	50% 6% 11% 20% 13% 1	2554
	21st to end	Day	51% 5% 11% 18% 12% 2	2600
May	1st to 10th	Day	50% 7% 9% 19% 10% 5%	5897
	11th to 20th	Day	47% 9% 12% 20% 11% 1	5865
	21st to end	Day	45% 6% 9% 19% 17% 4	6946
June	1st to 10th	Day	41% 7% 13% 25% 13% 2	2800
	11th to 20th	Day	40% 8% 12% 23% 16% 2	2974
	21st to end	Day	38% 9% 12% 24% 16% 1	3000
July	1st to 10th	Day	36% 8% 12% 26% 16% 2	6746
	11th to 20th	Day	35% 7% 13% 29% 15% 1	5936
	21st to end	Day	34% 8% 12% 25% 21% 1	6240
August	1st to 10th	Day	40% 9% 13% 23% 14% 1	4221
	11th to 20th	Day	41% 8% 15% 26% 9% 1	4213
	21st to end	Day	37% 8% 14% 29% 12% 1	4614
September	1st to 10th	Day	39% 8% 14% 25% 13% 1	2263
	11th to 20th	Day	35% 6% 14% 30% 16%	2090
	21st to end	Day	46% 7% 12% 24% 10% 1	2090
October	1st to 10th	Day	38% 7% 14% 22% 17% 1	3225
	11th to 20th	Day	41% 5% 14% 22% 18%	3140
	21st to end	Day	50% 6% 17% 21% 6%	3370
November	1st to 10th	Day	50% 6% 14% 21% 7% 2	1762
	11th to 20th	Day	54% 6% 16% 14% 9% 1	1641
	21st to end	Day	67% 6% 13% 12% 3	1600
December	1st to 10th	Day	69% 7% 14% 8% 2	2600
	11th to 20th	Day	69% 6% 10% 9% 6%	2986
	21st to end	Day	78% 5% 9% 7% 1	3958
January	1st to 10th	Night	69% 6% 10% 11% 3	6052
	11th to 20th	Night	75% 6% 8% 8% 3	5900
	21st to end	Night	66% 5% 9% 11% 8% 1	6490
February	1st to 10th	Night	64% 6% 11% 14% 5%	2600
	11th to 20th	Night	60% 5% 12% 16% 7%	2534
	21st to end	Night	62% 5% 9% 17% 6%	1980
March	1st to 10th	Night	51% 5% 14% 19% 9% 1	5035
	11th to 20th	Night	43% 6% 17% 22% 11% 1	4980
	21st to end	Night	52% 6% 9% 18% 15% 1	5467
April	1st to 10th	Night	51% 6% 10% 19% 13% 2	2126
	11th to 20th	Night	48% 5% 9% 20% 17% 2	1812
	21st to end	Night	46% 6% 11% 19% 15% 3	1800
May	1st to 10th	Night	49% 7% 9% 15% 12% 8%	4060
	11th to 20th	Night	46% 7% 10% 23% 11% 3	4033
	21st to end	Night	39% 7% 12% 22% 18% 3	3927
June	1st to 10th	Night	36% 6% 11% 28% 16% 3	1600
	11th to 20th	Night	34% 7% 13% 26% 17% 2	1420
	21st to end	Night	35% 7% 13% 24% 21% 1	1400
July	1st to 10th	Night	31% 7% 13% 27% 18% 5%	3125
	11th to 20th	Night	32% 6% 12% 28% 20% 2	3898
	21st to end	Night	30% 7% 12% 22% 24% 4	4297
August	1st to 10th	Night	36% 7% 13% 26% 18% 1	2904
	11th to 20th	Night	35% 6% 14% 31% 13% 1	2902
	21st to end	Night	31% 7% 12% 32% 17% 1	3250
September	1st to 10th	Night	35% 6% 13% 26% 17% 2	1981
	11th to 20th	Night	30% 5% 12% 32% 19% 1	2090
	21st to end	Night	43% 6% 13% 24% 13% 1	2090
October	1st to 10th	Night	31% 5% 10% 28% 24% 2	3733
	11th to 20th	Night	42% 4 14% 23% 16% 1	3779
	21st to end	Night	42% 6% 15% 25% 11%	4177
November	1st to 10th	Night	43% 6% 13% 25% 12% 1	2536
	11th to 20th	Night	53% 7% 13% 18% 8% 1	2756
	21st to end	Night	61% 6% 16% 12% 3 1	2800
December	1st to 10th	Night	68% 7% 14% 9% 3	4580
	11th to 20th	Night	65% 5% 10% 12% 7% 1	5208
	21st to end	Night	72% 6% 10% 9% 3	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	20% 23% 32% 17% 5% 1	265
	11th to 20th	Day	19% 24% 40% 13% 3 2	300
	21st to end	Day	26% 17% 32% 20% 3 1	330
February	1st to 10th	Day	24% 23% 36% 14% 3 1	300
	11th to 20th	Day	26% 21% 38% 13% 2	300
	21st to end	Day	26% 25% 36% 12% 1	249
March	1st to 10th	Day	24% 13% 39% 19% 4 1	443
	11th to 20th	Day	22% 17% 33% 22% 4 1	500
	21st to end	Day	30% 14% 29% 21% 6% 1	550
April	1st to 10th	Day	21% 17% 32% 23% 7% 1	500
	11th to 20th	Day	26% 17% 27% 16% 10% 4	492
	21st to end	Day	25% 11% 33% 21% 8% 2	500
May	1st to 10th	Day	23% 13% 28% 22% 11% 3	503
	11th to 20th	Day	17% 10% 29% 22% 16% 6%	527
	21st to end	Day	15% 8% 26% 28% 17% 6%	573
June	1st to 10th	Day	13% 8% 33% 28% 14% 4	530
	11th to 20th	Day	12% 9% 29% 32% 14% 5%	704
	21st to end	Day	12% 8% 30% 33% 14% 4	730
July	1st to 10th	Day	10% 6% 23% 31% 22% 8%	721
	11th to 20th	Day	14% 6% 24% 28% 18% 9%	570
	21st to end	Day	9% 5% 22% 32% 23% 10%	576
August	1st to 10th	Day	10% 5% 18% 31% 24% 13%	526
	11th to 20th	Day	12% 5% 18% 27% 23% 14%	519
	21st to end	Day	11% 7% 20% 27% 24% 11%	583
September	1st to 10th	Day	11% 6% 23% 28% 21% 12%	502
	11th to 20th	Day	8% 3 20% 26% 29% 14%	490
	21st to end	Day	18% 9% 25% 25% 16% 8%	490
October	1st to 10th	Day	19% 10% 26% 23% 16% 6%	490
	11th to 20th	Day	20% 15% 32% 24% 7% 3	490
	21st to end	Day	28% 13% 32% 21% 5% 1	505
November	1st to 10th	Day	19% 24% 31% 17% 7% 2	300
	11th to 20th	Day	27% 15% 34% 16% 6% 2	220
	21st to end	Day	26% 19% 35% 18% 3	200
December	1st to 10th	Day	25% 24% 29% 16% 7% 1	200
	11th to 20th	Day	28% 23% 29% 18% 3	200
	21st to end	Day	30% 19% 32% 15% 2 1	220
January	1st to 10th	Night	23% 16% 38% 17% 5% 1	802
	11th to 20th	Night	29% 16% 32% 20% 4	800
	21st to end	Night	22% 13% 34% 25% 6%	880
February	1st to 10th	Night	24% 15% 35% 23% 2	800
	11th to 20th	Night	24% 15% 33% 24% 5%	783
	21st to end	Night	23% 15% 38% 21% 3	660
March	1st to 10th	Night	20% 12% 36% 26% 6%	800
	11th to 20th	Night	19% 14% 36% 24% 6% 1	800
	21st to end	Night	24% 11% 30% 26% 9% 1	880
April	1st to 10th	Night	23% 14% 32% 25% 5%	800
	11th to 20th	Night	21% 12% 32% 25% 9% 1	628
	21st to end	Night	18% 9% 35% 31% 8% 1	600
May	1st to 10th	Night	20% 7% 30% 31% 12% 1	600
	11th to 20th	Night	15% 5% 28% 37% 14% 2	591
	21st to end	Night	13% 5% 27% 37% 17% 1	433
June	1st to 10th	Night	15% 3 28% 42% 12%	400
	11th to 20th	Night	12% 3 30% 43% 12%	400
	21st to end	Night	11% 5% 24% 45% 16% 1	400
July	1st to 10th	Night	6% 3 18% 48% 23% 2	390
	11th to 20th	Night	10% 4 17% 40% 27% 2	571
	21st to end	Night	10% 2 16% 46% 26% 1	657
August	1st to 10th	Night	8% 3 14% 42% 30% 3	597
	11th to 20th	Night	9% 3 13% 40% 31% 4	594
	21st to end	Night	9% 3 18% 40% 26% 3	674
September	1st to 10th	Night	11% 4 21% 35% 25% 5%	773
	11th to 20th	Night	11% 3 16% 37% 29% 4	760
	21st to end	Night	18% 6% 20% 34% 18% 4	760
October	1st to 10th	Night	17% 7% 26% 31% 17% 3	760
	11th to 20th	Night	23% 10% 27% 28% 11% 1	760
	21st to end	Night	27% 10% 31% 27% 5%	836
November	1st to 10th	Night	19% 13% 27% 31% 10% 1	780
	11th to 20th	Night	23% 13% 36% 22% 6% 2	800
	21st to end	Night	28% 12% 29% 25% 6%	800
December	1st to 10th	Night	28% 16% 33% 19% 5%	800
	11th to 20th	Night	24% 18% 37% 18% 4	800
	21st to end	Night	28% 13% 34% 22% 3	880

Page generated at 2025-09-01 01:51:09. Version ac07

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