Land's End
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Land's End clear sky chart from 2005-11-06 to 2025-03-27.

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	12% 6% 2 6% 4 4 4 4 6% 8% 44%	108862
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 44%	110440
Transparency	Day	36% 7% 13% 23% 17% 4	112497
	Night	38% 6% 10% 23% 18% 4	114945
Seeing	Day	15% 6% 20% 32% 21% 6%	15649
	Night	17% 6% 25% 33% 18% 3	25335

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% 4 1 4 3 3 3 2 4 5% 57%	3781
	11th to 20th	Day	11% 7% 2 5% 4 3 3 3 4 6% 53%	4060
	21st to end	Day	12% 7% 3 8% 5% 4 4 3 4 6% 44%	4466
February	1st to 10th	Day	13% 7% 3 6% 5% 4 5% 4 6% 7% 41%	1800
	11th to 20th	Day	20% 6% 2 4 3 3 4 3 4 6% 45%	1894
	21st to end	Day	15% 8% 2 6% 4 5% 4 4 6% 6% 39%	1650
March	1st to 10th	Day	20% 9% 2 7% 5% 4 3 4 5% 6% 36%	3417
	11th to 20th	Day	10% 4 2 4 3 3 4 3 5% 7% 55%	3590
	21st to end	Day	18% 8% 2 5% 4 3 3 3 5% 7% 42%	3998
April	1st to 10th	Day	11% 6% 2 5% 4 3 4 3 5% 7% 50%	2280
	11th to 20th	Day	9% 5% 2 6% 4 4 4 3 5% 7% 50%	2254
	21st to end	Day	14% 6% 3 5% 4 4 4 3 5% 8% 44%	2280
May	1st to 10th	Day	15% 7% 3 8% 5% 4 5% 4 6% 9% 33%	3788
	11th to 20th	Day	18% 7% 3 7% 5% 4 5% 4 6% 7% 34%	4260
	21st to end	Day	9% 4 2 5% 4 4 5% 4 7% 10% 47%	4823
June	1st to 10th	Day	10% 4 1 5% 4 4 5% 5% 8% 11% 42%	2660
	11th to 20th	Day	10% 4 2 5% 5% 4 5% 4 8% 10% 43%	2660
	21st to end	Day	6% 4 3 5% 5% 5% 5% 4 7% 11% 44%	2660
July	1st to 10th	Day	6% 4 1 5% 4 5% 6% 6% 9% 13% 42%	4440
	11th to 20th	Day	12% 7% 3 7% 4 5% 5% 5% 8% 10% 36%	4234
	21st to end	Day	8% 6% 3 7% 6% 6% 6% 5% 8% 10% 34%	4466
August	1st to 10th	Day	14% 8% 3 8% 5% 5% 6% 6% 8% 10% 29%	4090
	11th to 20th	Day	10% 6% 2 8% 6% 6% 6% 5% 6% 9% 37%	3730
	21st to end	Day	8% 4 2 5% 3 3 4 4 8% 12% 47%	4180
September	1st to 10th	Day	14% 5% 2 5% 4 4 5% 3 6% 8% 44%	2280
	11th to 20th	Day	13% 6% 2 7% 5% 3 6% 3 6% 7% 43%	2280
	21st to end	Day	19% 8% 2 6% 4 4 3 3 6% 7% 37%	2031
October	1st to 10th	Day	15% 6% 2 5% 4 4 4 3 5% 6% 46%	1900
	11th to 20th	Day	14% 6% 2 5% 4 3 3 3 5% 6% 51%	1900
	21st to end	Day	12% 4 2 6% 4 4 3 3 5% 6% 52%	2090
November	1st to 10th	Day	13% 5% 2 5% 3 3 3 3 4 7% 50%	1855
	11th to 20th	Day	13% 7% 2 4 4 3 4 3 4 7% 50%	1800
	21st to end	Day	14% 5% 1 5% 3 3 3 4 4 6% 52%	1715
December	1st to 10th	Day	11% 6% 2 5% 4 3 3 3 4 6% 55%	2600
	11th to 20th	Day	16% 8% 2 6% 3 4 2 3 3 5% 47%	2990
	21st to end	Day	12% 6% 2 5% 3 3 3 4 5% 6% 50%	3960
January	1st to 10th	Night	12% 6% 3 5% 4 3 4 3 4 5% 52%	6148
	11th to 20th	Night	12% 6% 2 6% 3 3 3 2 4 5% 53%	5900
	21st to end	Night	14% 8% 3 6% 3 3 3 2 4 5% 49%	6490
February	1st to 10th	Night	16% 7% 2 6% 3 4 4 4 5% 7% 42%	2600
	11th to 20th	Night	20% 7% 2 5% 3 3 4 2 5% 6% 41%	2389
	21st to end	Night	17% 8% 1 5% 3 3 3 2 5% 5% 47%	1980
March	1st to 10th	Night	23% 8% 2 6% 4 4 3 3 5% 6% 36%	3638
	11th to 20th	Night	15% 5% 2 5% 3 3 4 3 5% 6% 49%	3590
	21st to end	Night	16% 8% 3 7% 4 3 3 2 3 5% 47%	3773
April	1st to 10th	Night	14% 7% 2 5% 5% 4 4 3 6% 7% 42%	1900
	11th to 20th	Night	11% 7% 2 5% 4 4 4 2 5% 8% 49%	1875
	21st to end	Night	15% 5% 2 5% 5% 4 4 4 5% 7% 45%	1900
May	1st to 10th	Night	19% 7% 3 6% 5% 4 5% 4 6% 8% 32%	3130
	11th to 20th	Night	22% 8% 3 7% 4 4 4 4 5% 6% 34%	2638
	21st to end	Night	10% 5% 2 6% 4 5% 4 3 6% 10% 44%	2732
June	1st to 10th	Night	12% 4 2 7% 5% 4 5% 5% 7% 10% 39%	1520
	11th to 20th	Night	10% 4 2 5% 5% 5% 5% 5% 7% 10% 41%	1520
	21st to end	Night	8% 5% 2 6% 6% 6% 6% 4 7% 9% 43%	1520
July	1st to 10th	Night	8% 5% 2 6% 6% 5% 6% 5% 6% 11% 39%	2520
	11th to 20th	Night	9% 7% 3 5% 4 5% 7% 5% 8% 11% 36%	2708
	21st to end	Night	13% 8% 3 8% 6% 5% 5% 4 7% 9% 31%	3087
August	1st to 10th	Night	15% 8% 3 8% 5% 5% 5% 4 6% 10% 28%	2825
	11th to 20th	Night	9% 6% 3 8% 5% 4 4 4 7% 10% 40%	3067
	21st to end	Night	9% 5% 2 6% 4 4 4 3 6% 10% 47%	3476
September	1st to 10th	Night	13% 4 2 5% 4 5% 5% 4 6% 9% 44%	1900
	11th to 20th	Night	11% 6% 2 6% 4 4 5% 3 5% 7% 46%	1900
	21st to end	Night	20% 8% 3 7% 4 4 4 3 5% 6% 36%	2144
October	1st to 10th	Night	15% 6% 2 6% 4 3 4 3 5% 7% 45%	2280
	11th to 20th	Night	15% 6% 2 5% 3 3 3 3 4 6% 51%	2280
	21st to end	Night	14% 5% 2 5% 3 4 4 3 4 5% 52%	2508
November	1st to 10th	Night	14% 6% 2 6% 3 4 4 3 4 6% 49%	2441
	11th to 20th	Night	16% 6% 2 5% 3 3 4 3 5% 6% 48%	2600
	21st to end	Night	11% 5% 2 5% 4 4 4 4 5% 6% 51%	2680
December	1st to 10th	Night	10% 5% 2 5% 4 4 4 3 4 6% 54%	4580
	11th to 20th	Night	17% 8% 3 6% 5% 4 4 4 5% 6% 39%	5205
	21st to end	Night	11% 6% 3 6% 4 3 4 3 6% 8% 46%	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	31% 3 5% 27% 22% 12%	3782
	11th to 20th	Day	35% 3 6% 21% 28% 8%	4060
	21st to end	Day	42% 5% 8% 28% 15% 2	4466
February	1st to 10th	Day	42% 5% 10% 26% 14% 3	1800
	11th to 20th	Day	42% 4 9% 22% 19% 5%	1903
	21st to end	Day	44% 6% 11% 18% 16% 5%	1650
March	1st to 10th	Day	46% 6% 11% 17% 16% 4	3417
	11th to 20th	Day	28% 6% 10% 24% 25% 8%	3590
	21st to end	Day	42% 6% 12% 18% 19% 3	3998
April	1st to 10th	Day	34% 5% 11% 23% 21% 6%	2280
	11th to 20th	Day	33% 5% 13% 24% 20% 5%	2254
	21st to end	Day	38% 5% 12% 24% 15% 5%	2280
May	1st to 10th	Day	44% 11% 15% 20% 9% 2	3823
	11th to 20th	Day	45% 9% 15% 22% 7% 1	4264
	21st to end	Day	28% 10% 16% 26% 17% 2	4837
June	1st to 10th	Day	32% 9% 18% 27% 13% 2	2660
	11th to 20th	Day	33% 8% 18% 25% 14% 2	2660
	21st to end	Day	32% 9% 18% 24% 16% 2	2660
July	1st to 10th	Day	30% 12% 25% 19% 11% 3	4370
	11th to 20th	Day	41% 9% 23% 20% 6% 1	4161
	21st to end	Day	41% 10% 17% 17% 14% 1	4324
August	1st to 10th	Day	47% 8% 17% 22% 5% 1	4077
	11th to 20th	Day	41% 8% 14% 25% 12%	3766
	21st to end	Day	28% 7% 16% 29% 19% 2	4180
September	1st to 10th	Day	37% 9% 16% 17% 17% 4	2280
	11th to 20th	Day	39% 7% 11% 22% 18% 2	2280
	21st to end	Day	46% 6% 9% 21% 15% 3	2031
October	1st to 10th	Day	31% 5% 10% 26% 25% 3	3160
	11th to 20th	Day	25% 3 7% 21% 36% 8%	3140
	21st to end	Day	22% 3 8% 29% 25% 12%	3428
November	1st to 10th	Day	35% 5% 9% 28% 21% 4	1855
	11th to 20th	Day	36% 4 11% 20% 23% 6%	1800
	21st to end	Day	34% 4 9% 27% 21% 4	1715
December	1st to 10th	Day	32% 4 9% 29% 21% 5%	2600
	11th to 20th	Day	42% 4 12% 21% 17% 5%	2986
	21st to end	Day	34% 5% 13% 30% 17% 1	3960
January	1st to 10th	Night	35% 3 6% 21% 26% 9%	6148
	11th to 20th	Night	35% 4 5% 23% 27% 6%	5900
	21st to end	Night	38% 4 6% 28% 20% 4	6490
February	1st to 10th	Night	43% 4 11% 23% 16% 3	2600
	11th to 20th	Night	44% 4 7% 22% 18% 6%	2391
	21st to end	Night	40% 4 8% 22% 18% 7%	1980
March	1st to 10th	Night	48% 6% 8% 20% 16% 3	3638
	11th to 20th	Night	36% 5% 7% 21% 23% 8%	3590
	21st to end	Night	42% 4 7% 24% 17% 6%	3773
April	1st to 10th	Night	40% 6% 10% 16% 23% 4	1900
	11th to 20th	Night	34% 5% 11% 24% 21% 6%	1875
	21st to end	Night	39% 6% 11% 24% 17% 4	1900
May	1st to 10th	Night	47% 9% 14% 19% 9% 2	3160
	11th to 20th	Night	47% 11% 12% 22% 7% 1	2639
	21st to end	Night	34% 9% 16% 25% 15% 1	2739
June	1st to 10th	Night	38% 10% 15% 22% 14% 1	1520
	11th to 20th	Night	35% 10% 17% 23% 13% 2	1520
	21st to end	Night	36% 9% 14% 23% 14% 3	1520
July	1st to 10th	Night	38% 14% 17% 21% 10% 2	2480
	11th to 20th	Night	38% 11% 22% 23% 5% 1	2659
	21st to end	Night	49% 9% 12% 18% 12%	2988
August	1st to 10th	Night	49% 8% 17% 20% 5%	2814
	11th to 20th	Night	38% 8% 16% 24% 13%	3097
	21st to end	Night	32% 6% 11% 27% 23% 1	3476
September	1st to 10th	Night	36% 8% 16% 20% 16% 4	1900
	11th to 20th	Night	38% 5% 11% 23% 21% 2	1900
	21st to end	Night	49% 6% 10% 16% 17% 2	2144
October	1st to 10th	Night	36% 6% 10% 22% 22% 4	3800
	11th to 20th	Night	26% 4 7% 22% 32% 9%	3779
	21st to end	Night	22% 3 6% 33% 25% 11%	4120
November	1st to 10th	Night	38% 5% 10% 22% 22% 4	2441
	11th to 20th	Night	38% 5% 9% 21% 19% 9%	2600
	21st to end	Night	34% 4 9% 27% 22% 4	2680
December	1st to 10th	Night	32% 4 8% 27% 25% 4	4580
	11th to 20th	Night	46% 4 12% 18% 14% 5%	5208
	21st to end	Night	36% 5% 13% 29% 15% 1	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% 13% 32% 29% 7%	240
	11th to 20th	Day	17% 14% 39% 26% 4	300
	21st to end	Day	16% 9% 36% 33% 6%	330
February	1st to 10th	Day	17% 11% 32% 34% 5% 1	300
	11th to 20th	Day	26% 9% 40% 22% 3	442
	21st to end	Day	20% 12% 34% 30% 5%	414
March	1st to 10th	Day	15% 11% 35% 32% 5% 1	500
	11th to 20th	Day	17% 10% 36% 28% 8% 1	500
	21st to end	Day	16% 10% 31% 34% 8% 1	530
April	1st to 10th	Day	13% 13% 32% 32% 10% 1	460
	11th to 20th	Day	11% 6% 27% 36% 17% 4	452
	21st to end	Day	17% 7% 23% 35% 16% 2	460
May	1st to 10th	Day	16% 7% 22% 38% 16% 2	463
	11th to 20th	Day	19% 3 19% 35% 21% 4	487
	21st to end	Day	19% 3 12% 31% 27% 9%	529
June	1st to 10th	Day	11% 3 10% 39% 29% 8%	490
	11th to 20th	Day	13% 2 11% 34% 31% 8%	490
	21st to end	Day	8% 1 11% 38% 34% 7%	490
July	1st to 10th	Day	9% 4 5% 33% 38% 12%	485
	11th to 20th	Day	9% 2 4 28% 45% 12%	484
	21st to end	Day	9% 2 6% 37% 36% 9%	532
August	1st to 10th	Day	12% 3 8% 33% 33% 12%	486
	11th to 20th	Day	15% 2 7% 30% 34% 12%	479
	21st to end	Day	10% 2 7% 28% 35% 17%	539
September	1st to 10th	Day	15% 2 9% 28% 31% 15%	490
	11th to 20th	Day	15% 4 10% 28% 32% 10%	490
	21st to end	Day	24% 4 17% 30% 20% 4	490
October	1st to 10th	Day	20% 4 13% 26% 26% 11%	490
	11th to 20th	Day	14% 6% 21% 31% 24% 3	490
	21st to end	Day	13% 6% 24% 33% 18% 5%	539
November	1st to 10th	Day	16% 6% 21% 32% 19% 5%	443
	11th to 20th	Day	17% 11% 29% 29% 14% 1	400
	21st to end	Day	16% 10% 26% 29% 17% 2	315
December	1st to 10th	Day	23% 5% 32% 32% 10%	200
	11th to 20th	Day	21% 6% 30% 32% 11% 1	200
	21st to end	Day	20% 7% 31% 28% 14%	220
January	1st to 10th	Night	17% 14% 37% 27% 6%	800
	11th to 20th	Night	18% 11% 35% 28% 8%	800
	21st to end	Night	20% 9% 37% 31% 3	880
February	1st to 10th	Night	21% 6% 37% 31% 5%	800
	11th to 20th	Night	24% 10% 42% 20% 4	783
	21st to end	Night	20% 9% 34% 32% 4	660
March	1st to 10th	Night	17% 9% 34% 32% 7%	800
	11th to 20th	Night	22% 7% 35% 28% 8% 1	800
	21st to end	Night	17% 7% 34% 34% 7%	863
April	1st to 10th	Night	18% 7% 36% 31% 7%	760
	11th to 20th	Night	14% 6% 29% 38% 12% 1	751
	21st to end	Night	16% 6% 26% 40% 12% 1	760
May	1st to 10th	Night	17% 4 26% 39% 12% 1	760
	11th to 20th	Night	15% 4 21% 40% 18% 2	465
	21st to end	Night	16% 1 11% 36% 30% 6%	411
June	1st to 10th	Night	12% 4 9% 39% 31% 5%	380
	11th to 20th	Night	10% 2 11% 46% 27% 5%	380
	21st to end	Night	7% 3 13% 40% 32% 5%	380
July	1st to 10th	Night	11% 3 7% 38% 36% 5%	370
	11th to 20th	Night	6% 2 5% 35% 46% 6%	477
	21st to end	Night	9% 3 8% 40% 36% 4	624
August	1st to 10th	Night	12% 3 6% 38% 36% 5%	567
	11th to 20th	Night	12% 3 8% 36% 36% 6%	732
	21st to end	Night	11% 2 8% 31% 39% 9%	836
September	1st to 10th	Night	15% 1 9% 32% 34% 8%	760
	11th to 20th	Night	14% 2 11% 30% 36% 7%	760
	21st to end	Night	24% 4 12% 33% 25% 3	760
October	1st to 10th	Night	19% 3 14% 30% 27% 7%	760
	11th to 20th	Night	17% 6% 23% 33% 19% 3	760
	21st to end	Night	16% 8% 24% 30% 19% 3	836
November	1st to 10th	Night	17% 6% 22% 34% 18% 4	780
	11th to 20th	Night	18% 8% 29% 31% 14% 1	800
	21st to end	Night	14% 9% 29% 32% 15% 2	800
December	1st to 10th	Night	20% 7% 34% 27% 10% 2	800
	11th to 20th	Night	18% 7% 36% 29% 9%	800
	21st to end	Night	19% 7% 32% 33% 9% 1	880

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