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 2024-11-04.

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% 10% 4 8% 5% 5% 5% 4 6% 7% 31%	101858
	Night	15% 11% 4 8% 5% 4 4 4 5% 6% 32%	97013
Transparency	Day	49% 7% 12% 20% 11% 1	105464
	Night	49% 6% 12% 20% 11% 1	101496
Seeing	Day	18% 11% 28% 24% 13% 5%	15023
	Night	19% 9% 28% 30% 12% 1	24026

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% 20% 8% 12% 6% 4 4 2 4 4 18%	2565
	11th to 20th	Day	13% 21% 9% 19% 7% 6% 5% 4 4 4 9%	2840
	21st to end	Day	23% 22% 9% 11% 6% 4 4 2 3 4 12%	3124
February	1st to 10th	Day	16% 18% 5% 12% 8% 6% 5% 3 5% 4 17%	1710
	11th to 20th	Day	14% 15% 7% 15% 8% 5% 4 3 5% 6% 19%	1663
	21st to end	Day	17% 15% 6% 12% 6% 5% 5% 4 4 5% 22%	1569
March	1st to 10th	Day	20% 15% 6% 9% 5% 4 3 3 3 5% 26%	3356
	11th to 20th	Day	12% 15% 5% 10% 5% 5% 4 3 5% 7% 28%	3480
	21st to end	Day	18% 12% 3 9% 5% 4 4 3 5% 5% 31%	3815
April	1st to 10th	Day	16% 14% 4 8% 5% 4 5% 3 5% 8% 28%	2159
	11th to 20th	Day	18% 11% 3 7% 4 4 4 4 5% 8% 33%	2425
	21st to end	Day	19% 11% 4 7% 4 4 4 3 5% 6% 33%	2470
May	1st to 10th	Day	19% 8% 3 8% 4 4 5% 4 6% 7% 33%	4108
	11th to 20th	Day	16% 8% 3 7% 4 4 5% 4 6% 9% 32%	4081
	21st to end	Day	12% 6% 2 6% 4 4 4 4 6% 7% 46%	4834
June	1st to 10th	Day	13% 6% 2 7% 5% 5% 5% 4 6% 8% 38%	2660
	11th to 20th	Day	11% 6% 3 6% 5% 4 5% 5% 7% 8% 40%	2825
	21st to end	Day	12% 6% 3 6% 4 4 5% 4 6% 9% 40%	2850
July	1st to 10th	Day	11% 5% 2 6% 6% 5% 5% 4 7% 10% 40%	4760
	11th to 20th	Day	11% 5% 2 6% 4 4 5% 4 7% 11% 40%	4201
	21st to end	Day	8% 6% 3 7% 5% 6% 6% 5% 6% 9% 41%	4466
August	1st to 10th	Day	15% 6% 2 6% 4 5% 5% 4 7% 9% 37%	4104
	11th to 20th	Day	10% 7% 3 7% 5% 5% 6% 5% 7% 11% 33%	4044
	21st to end	Day	9% 6% 2 6% 5% 6% 5% 4 6% 9% 40%	4473
September	1st to 10th	Day	10% 6% 3 7% 5% 5% 5% 4 7% 8% 39%	2227
	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% 11% 3 7% 6% 5% 4 4 6% 7% 32%	1699
	11th to 20th	Day	16% 13% 4 8% 5% 5% 5% 4 5% 7% 26%	1559
	21st to end	Day	20% 18% 6% 9% 6% 5% 4 5% 5% 6% 16%	1520
December	1st to 10th	Day	16% 18% 6% 12% 6% 5% 4 5% 5% 6% 16%	1520
	11th to 20th	Day	18% 19% 8% 12% 7% 5% 5% 4 4 4 15%	1906
	21st to end	Day	28% 20% 8% 11% 5% 4 3 3 5% 5% 8%	2770
January	1st to 10th	Night	17% 17% 8% 14% 6% 5% 4 3 4 4 17%	4230
	11th to 20th	Night	15% 18% 8% 16% 7% 7% 7% 5% 4 4 10%	4120
	21st to end	Night	23% 18% 8% 14% 5% 4 4 3 4 4 13%	4532
February	1st to 10th	Night	16% 15% 5% 11% 8% 5% 5% 4 5% 5% 20%	2470
	11th to 20th	Night	15% 15% 5% 10% 7% 5% 5% 5% 6% 7% 22%	2403
	21st to end	Night	16% 14% 5% 11% 7% 6% 5% 4 5% 5% 23%	1884
March	1st to 10th	Night	16% 12% 4 9% 5% 4 4 3 5% 6% 32%	3483
	11th to 20th	Night	11% 10% 4 8% 6% 4 4 4 6% 8% 34%	3479
	21st to end	Night	17% 11% 3 9% 5% 4 3 3 4 4 37%	3817
April	1st to 10th	Night	18% 11% 3 7% 4 4 4 3 6% 7% 33%	2020
	11th to 20th	Night	17% 10% 3 5% 5% 4 4 3 5% 7% 35%	1720
	21st to end	Night	15% 9% 2 7% 5% 4 5% 4 6% 7% 34%	1710
May	1st to 10th	Night	19% 8% 2 6% 4 3 4 3 6% 7% 38%	2833
	11th to 20th	Night	11% 9% 3 7% 5% 5% 6% 4 6% 7% 38%	2812
	21st to end	Night	11% 5% 2 4 3 3 4 4 6% 7% 52%	2738
June	1st to 10th	Night	12% 6% 3 5% 4 3 4 4 6% 8% 44%	1520
	11th to 20th	Night	9% 6% 2 5% 4 5% 5% 5% 8% 10% 41%	1349
	21st to end	Night	9% 5% 3 5% 4 5% 5% 4 5% 8% 46%	1330
July	1st to 10th	Night	7% 4 2 5% 4 4 3 4 6% 10% 51%	2210
	11th to 20th	Night	9% 5% 2 5% 3 4 3 4 6% 9% 52%	2754
	21st to end	Night	5% 5% 2 5% 4 5% 5% 5% 7% 9% 48%	3087
August	1st to 10th	Night	11% 6% 2 7% 3 4 4 4 6% 7% 45%	2825
	11th to 20th	Night	11% 8% 3 5% 3 4 4 4 5% 7% 47%	2785
	21st to end	Night	8% 5% 2 6% 5% 4 4 4 6% 7% 48%	3150
September	1st to 10th	Night	9% 6% 3 7% 3 4 4 3 6% 7% 47%	1950
	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	14% 9% 3 6% 5% 4 4 4 6% 7% 38%	2452
	11th to 20th	Night	16% 14% 4 9% 5% 4 5% 4 7% 6% 27%	2618
	21st to end	Night	20% 15% 5% 8% 6% 5% 5% 4 6% 6% 21%	2660
December	1st to 10th	Night	20% 17% 5% 10% 7% 5% 4 4 5% 6% 17%	2660
	11th to 20th	Night	15% 20% 6% 12% 5% 4 5% 3 5% 6% 20%	3288
	21st to end	Night	26% 22% 7% 11% 5% 3 4 3 4 4 11%	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	70% 5% 10% 11% 4	2566
	11th to 20th	Day	75% 5% 10% 7% 2	2840
	21st to end	Day	77% 6% 7% 6% 3	3124
February	1st to 10th	Day	69% 4 10% 12% 4 1	1710
	11th to 20th	Day	67% 4 8% 14% 7%	1672
	21st to end	Day	64% 5% 10% 14% 6% 1	1569
March	1st to 10th	Day	61% 5% 8% 21% 5% 1	3359
	11th to 20th	Day	55% 5% 15% 17% 7% 1	3480
	21st to end	Day	54% 6% 10% 18% 11% 1	3815
April	1st to 10th	Day	55% 5% 10% 20% 10% 1	2159
	11th to 20th	Day	49% 6% 11% 20% 13% 1	2425
	21st to end	Day	52% 5% 11% 18% 12% 2	2470
May	1st to 10th	Day	47% 8% 11% 21% 11% 3	4120
	11th to 20th	Day	46% 11% 14% 20% 8% 1	4085
	21st to end	Day	36% 8% 11% 22% 19% 4	4834
June	1st to 10th	Day	41% 7% 13% 25% 12% 2	2660
	11th to 20th	Day	39% 7% 12% 23% 17% 2	2825
	21st to end	Day	38% 9% 12% 24% 16% 1	2850
July	1st to 10th	Day	37% 7% 13% 25% 14% 3	4686
	11th to 20th	Day	34% 7% 14% 31% 13% 1	4128
	21st to end	Day	37% 9% 12% 21% 19% 1	4324
August	1st to 10th	Day	41% 8% 13% 23% 14% 1	4091
	11th to 20th	Day	41% 8% 15% 26% 9% 1	4083
	21st to end	Day	37% 8% 14% 28% 12% 1	4473
September	1st to 10th	Day	40% 8% 14% 24% 13% 1	2227
	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	1699
	11th to 20th	Day	54% 6% 17% 14% 8% 1	1559
	21st to end	Day	66% 6% 13% 12% 3	1520
December	1st to 10th	Day	65% 6% 15% 11% 3	1520
	11th to 20th	Day	70% 6% 10% 11% 2	1906
	21st to end	Day	78% 5% 11% 6% 1	2770
January	1st to 10th	Night	69% 6% 12% 10% 3	4230
	11th to 20th	Night	75% 6% 9% 7% 3	4120
	21st to end	Night	74% 5% 9% 9% 3 1	4532
February	1st to 10th	Night	64% 6% 11% 14% 5%	2470
	11th to 20th	Night	60% 5% 12% 16% 7%	2405
	21st to end	Night	62% 5% 10% 17% 7%	1884
March	1st to 10th	Night	53% 5% 13% 23% 6%	3493
	11th to 20th	Night	45% 7% 15% 21% 12% 1	3480
	21st to end	Night	49% 6% 9% 20% 14% 2	3817
April	1st to 10th	Night	50% 6% 10% 19% 13% 2	2020
	11th to 20th	Night	47% 5% 9% 20% 17% 2	1720
	21st to end	Night	47% 6% 11% 19% 15% 3	1710
May	1st to 10th	Night	44% 7% 10% 19% 15% 5%	2840
	11th to 20th	Night	43% 8% 12% 25% 10% 2	2813
	21st to end	Night	30% 8% 12% 23% 23% 4	2739
June	1st to 10th	Night	37% 6% 11% 28% 16% 3	1520
	11th to 20th	Night	34% 8% 13% 25% 18% 2	1349
	21st to end	Night	35% 7% 13% 25% 20% 1	1330
July	1st to 10th	Night	28% 6% 12% 29% 18% 7%	2175
	11th to 20th	Night	29% 6% 13% 31% 19% 2	2704
	21st to end	Night	30% 9% 16% 22% 19% 3	2988
August	1st to 10th	Night	37% 7% 12% 26% 17% 1	2814
	11th to 20th	Night	35% 6% 14% 31% 12% 1	2812
	21st to end	Night	31% 7% 12% 31% 17% 1	3150
September	1st to 10th	Night	35% 7% 13% 25% 17% 2	1950
	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% 14% 25% 12% 1	2452
	11th to 20th	Night	53% 7% 13% 18% 7% 1	2618
	21st to end	Night	61% 6% 17% 12% 3 1	2660
December	1st to 10th	Night	65% 5% 14% 11% 4	2660
	11th to 20th	Night	64% 6% 13% 13% 4	3288
	21st to end	Night	77% 5% 10% 7% 1	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	21% 22% 32% 18% 6% 1	238
	11th to 20th	Day	20% 21% 41% 14% 3 2	270
	21st to end	Day	26% 15% 33% 22% 3 1	297
February	1st to 10th	Day	24% 21% 36% 14% 3 1	270
	11th to 20th	Day	24% 20% 39% 14% 2	270
	21st to end	Day	26% 27% 35% 12% 1	225
March	1st to 10th	Day	25% 12% 38% 20% 4 1	406
	11th to 20th	Day	21% 17% 34% 23% 5% 1	460
	21st to end	Day	30% 13% 29% 21% 7% 1	506
April	1st to 10th	Day	19% 17% 33% 24% 7% 1	460
	11th to 20th	Day	27% 16% 26% 17% 10% 4	452
	21st to end	Day	24% 12% 32% 21% 8% 2	460
May	1st to 10th	Day	22% 12% 27% 24% 12% 3	463
	11th to 20th	Day	17% 10% 28% 23% 17% 6%	487
	21st to end	Day	13% 6% 26% 29% 18% 7%	529
June	1st to 10th	Day	13% 7% 33% 28% 14% 4	490
	11th to 20th	Day	12% 8% 27% 33% 15% 5%	655
	21st to end	Day	12% 8% 30% 32% 15% 4	680
July	1st to 10th	Day	10% 6% 22% 31% 23% 8%	671
	11th to 20th	Day	14% 6% 23% 28% 19% 10%	528
	21st to end	Day	9% 5% 21% 32% 23% 10%	532
August	1st to 10th	Day	11% 5% 19% 29% 23% 14%	486
	11th to 20th	Day	12% 5% 18% 28% 23% 14%	479
	21st to end	Day	11% 7% 19% 27% 25% 11%	539
September	1st to 10th	Day	12% 6% 23% 27% 21% 12%	490
	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	18% 23% 30% 18% 8% 3	279
	11th to 20th	Day	27% 15% 32% 17% 7% 3	198
	21st to end	Day	27% 16% 36% 19% 3	180
December	1st to 10th	Day	24% 23% 29% 16% 7% 1	180
	11th to 20th	Day	26% 23% 30% 17% 3	180
	21st to end	Day	30% 19% 32% 17% 2 1	198
January	1st to 10th	Night	23% 16% 38% 17% 5% 1	762
	11th to 20th	Night	29% 14% 32% 21% 4	760
	21st to end	Night	23% 11% 34% 26% 6%	836
February	1st to 10th	Night	24% 14% 35% 23% 3	760
	11th to 20th	Night	24% 14% 32% 25% 5%	743
	21st to end	Night	24% 15% 37% 21% 3	628
March	1st to 10th	Night	20% 11% 36% 27% 6%	760
	11th to 20th	Night	19% 14% 36% 24% 6% 1	760
	21st to end	Night	24% 11% 30% 26% 9% 1	836
April	1st to 10th	Night	23% 13% 33% 26% 5%	760
	11th to 20th	Night	22% 11% 32% 25% 9% 1	596
	21st to end	Night	18% 8% 34% 31% 8% 1	570
May	1st to 10th	Night	20% 6% 29% 32% 13% 1	570
	11th to 20th	Night	15% 4 27% 38% 14% 2	561
	21st to end	Night	13% 4 26% 38% 18% 1	411
June	1st to 10th	Night	15% 3 27% 43% 12%	380
	11th to 20th	Night	12% 3 30% 43% 12%	380
	21st to end	Night	11% 5% 23% 45% 16% 1	380
July	1st to 10th	Night	6% 3 19% 47% 23% 2	370
	11th to 20th	Night	11% 4 17% 40% 27% 2	541
	21st to end	Night	9% 2 16% 45% 26% 1	624
August	1st to 10th	Night	8% 4 15% 42% 29% 3	567
	11th to 20th	Night	9% 3 13% 40% 31% 4	564
	21st to end	Night	9% 3 18% 39% 27% 3	640
September	1st to 10th	Night	12% 4 21% 35% 24% 5%	760
	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	18% 12% 26% 32% 11% 1	755
	11th to 20th	Night	22% 13% 36% 21% 6% 2	760
	21st to end	Night	28% 12% 29% 26% 6%	760
December	1st to 10th	Night	29% 16% 32% 19% 5%	760
	11th to 20th	Night	24% 17% 37% 18% 4	760
	21st to end	Night	28% 14% 34% 21% 3	836

Page generated at 2024-11-01 22:47:45. Version ac07

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