Cornwall
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cornwall clear sky chart from 2005-11-06 to 2025-11-03.

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	17% 11% 4 8% 5% 5% 5% 3 5% 6% 31%	127060
	Night	18% 13% 4 9% 5% 4 4 3 5% 6% 28%	122968
Transparency	Day	53% 6% 12% 20% 9% 1	130500
	Night	57% 6% 11% 17% 9% 1	127614
Seeing	Day	26% 17% 29% 18% 8% 2	15824
	Night	26% 14% 28% 23% 8% 1	24819

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	16% 22% 7% 15% 6% 5% 4 2 3 4 16%	3597
	11th to 20th	Day	12% 17% 8% 16% 7% 5% 4 3 4 4 18%	3570
	21st to end	Day	13% 20% 7% 12% 6% 6% 5% 3 4 5% 19%	4029
February	1st to 10th	Day	15% 17% 6% 12% 7% 5% 4 3 5% 5% 22%	1800
	11th to 20th	Day	16% 14% 5% 12% 6% 6% 4 3 4 6% 24%	1818
	21st to end	Day	20% 17% 4 10% 6% 4 4 2 5% 5% 23%	1650
March	1st to 10th	Day	19% 13% 5% 10% 6% 4 5% 3 4 5% 25%	4476
	11th to 20th	Day	12% 12% 4 10% 6% 5% 5% 4 5% 6% 29%	4837
	21st to end	Day	19% 10% 3 10% 5% 4 4 3 4 4 35%	5873
April	1st to 10th	Day	21% 13% 3 7% 5% 4 3 3 4 6% 32%	2400
	11th to 20th	Day	21% 10% 4 7% 4 4 3 3 4 5% 35%	2374
	21st to end	Day	23% 11% 3 5% 4 4 4 3 5% 5% 32%	2505
May	1st to 10th	Day	26% 11% 3 9% 5% 4 4 3 5% 5% 24%	6346
	11th to 20th	Day	18% 9% 4 9% 5% 5% 5% 3 5% 6% 30%	6318
	21st to end	Day	22% 9% 3 6% 4 4 4 3 5% 7% 32%	6949
June	1st to 10th	Day	22% 8% 3 6% 6% 5% 5% 4 6% 7% 30%	2800
	11th to 20th	Day	15% 6% 2 7% 5% 4 5% 5% 7% 8% 37%	2800
	21st to end	Day	21% 7% 2 6% 5% 5% 4 4 6% 7% 32%	2800
July	1st to 10th	Day	12% 7% 3 8% 6% 5% 6% 4 7% 9% 34%	6360
	11th to 20th	Day	11% 5% 2 6% 5% 5% 5% 4 7% 8% 41%	6360
	21st to end	Day	11% 5% 2 5% 4 4 5% 4 6% 10% 44%	6926
August	1st to 10th	Day	15% 8% 3 7% 5% 4 5% 4 6% 8% 34%	4563
	11th to 20th	Day	15% 7% 2 6% 4 4 5% 4 6% 8% 40%	4036
	21st to end	Day	16% 6% 2 6% 5% 4 4 4 6% 8% 38%	4312
September	1st to 10th	Day	14% 6% 2 5% 4 4 5% 4 5% 7% 44%	2400
	11th to 20th	Day	13% 6% 2 5% 4 3 4 3 4 7% 48%	2355
	21st to end	Day	18% 6% 2 6% 5% 3 5% 4 5% 7% 39%	2200
October	1st to 10th	Day	20% 8% 2 6% 5% 4 5% 3 5% 6% 36%	2005
	11th to 20th	Day	19% 11% 3 8% 6% 5% 5% 4 4 6% 30%	2000
	21st to end	Day	26% 11% 4 8% 4 3 4 4 4 4 28%	1993
November	1st to 10th	Day	16% 11% 4 9% 5% 5% 5% 4 5% 7% 30%	1779
	11th to 20th	Day	17% 13% 3 8% 5% 5% 5% 3 5% 7% 29%	1683
	21st to end	Day	22% 16% 4 9% 6% 5% 5% 4 5% 5% 19%	1600
December	1st to 10th	Day	20% 21% 6% 13% 6% 4 4 3 5% 4 13%	2600
	11th to 20th	Day	19% 18% 6% 11% 5% 4 4 3 5% 5% 21%	2986
	21st to end	Day	22% 16% 4 11% 5% 5% 4 3 4 4 20%	3960
January	1st to 10th	Night	15% 24% 8% 14% 5% 4 3 2 3 4 16%	6349
	11th to 20th	Night	11% 21% 9% 16% 7% 6% 5% 4 4 4 14%	6315
	21st to end	Night	16% 19% 7% 11% 7% 6% 5% 2 4 5% 18%	6925
February	1st to 10th	Night	15% 16% 6% 14% 6% 5% 4 3 5% 4 20%	2600
	11th to 20th	Night	15% 14% 5% 13% 7% 5% 4 3 4 4 25%	2467
	21st to end	Night	16% 16% 5% 9% 7% 4 4 3 4 4 27%	1980
March	1st to 10th	Night	22% 14% 4 9% 6% 4 4 3 3 4 27%	5364
	11th to 20th	Night	16% 15% 3 7% 5% 4 4 3 5% 7% 30%	5120
	21st to end	Night	18% 14% 3 9% 5% 4 4 3 4 4 32%	5059
April	1st to 10th	Night	23% 12% 3 9% 4 4 3 3 4 5% 30%	2000
	11th to 20th	Night	22% 11% 3 7% 4 4 4 3 4 5% 33%	1975
	21st to end	Night	21% 11% 3 6% 5% 3 3 4 4 6% 34%	1900
May	1st to 10th	Night	26% 10% 4 8% 5% 5% 5% 4 5% 6% 22%	3605
	11th to 20th	Night	21% 9% 3 8% 5% 4 4 3 5% 6% 32%	3575
	21st to end	Night	18% 8% 3 8% 5% 5% 4 4 5% 8% 30%	3926
June	1st to 10th	Night	18% 7% 3 7% 6% 5% 5% 5% 7% 9% 29%	1600
	11th to 20th	Night	17% 6% 2 7% 4 3 5% 4 5% 9% 39%	1600
	21st to end	Night	18% 8% 3 7% 5% 5% 4 4 6% 7% 33%	1600
July	1st to 10th	Night	14% 6% 2 6% 5% 5% 5% 4 7% 10% 35%	3600
	11th to 20th	Night	15% 6% 3 6% 4 4 3 4 5% 8% 43%	3600
	21st to end	Night	11% 6% 3 7% 4 5% 5% 4 6% 8% 43%	3926
August	1st to 10th	Night	16% 8% 3 7% 3 4 4 3 5% 8% 38%	2586
	11th to 20th	Night	16% 7% 3 7% 5% 5% 4 3 5% 6% 38%	2965
	21st to end	Night	14% 6% 3 7% 5% 4 5% 4 6% 8% 39%	3586
September	1st to 10th	Night	13% 6% 3 7% 4 4 4 4 5% 7% 42%	2000
	11th to 20th	Night	14% 5% 2 5% 4 3 4 2 5% 7% 50%	2044
	21st to end	Night	18% 6% 2 6% 4 4 5% 3 6% 6% 41%	2200
October	1st to 10th	Night	20% 7% 3 5% 4 4 5% 4 6% 6% 36%	2392
	11th to 20th	Night	19% 8% 4 8% 5% 5% 4 3 4 6% 34%	2400
	21st to end	Night	26% 10% 3 7% 5% 4 5% 3 4 7% 28%	2846
November	1st to 10th	Night	20% 11% 3 8% 5% 4 5% 4 4 6% 32%	2564
	11th to 20th	Night	18% 12% 3 8% 5% 4 4 3 4 6% 32%	2715
	21st to end	Night	22% 15% 5% 9% 5% 5% 5% 3 4 5% 23%	2800
December	1st to 10th	Night	20% 22% 6% 11% 7% 5% 4 3 4 4 13%	4580
	11th to 20th	Night	24% 16% 5% 10% 5% 4 4 3 5% 4 21%	5208
	21st to end	Night	26% 17% 5% 10% 4 4 4 4 5% 4 17%	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	74% 5% 10% 9% 2	3598
	11th to 20th	Day	70% 7% 9% 8% 6% 1	3600
	21st to end	Day	68% 6% 9% 10% 7% 1	4029
February	1st to 10th	Day	66% 6% 7% 16% 4 1	1800
	11th to 20th	Day	63% 6% 11% 13% 7%	1827
	21st to end	Day	64% 5% 11% 12% 8% 1	1650
March	1st to 10th	Day	61% 5% 9% 15% 8% 3	4479
	11th to 20th	Day	55% 5% 16% 14% 8% 1	4837
	21st to end	Day	54% 4 7% 17% 17% 2	5873
April	1st to 10th	Day	55% 4 9% 19% 11% 2	2400
	11th to 20th	Day	52% 5% 8% 18% 15% 2	2374
	21st to end	Day	54% 5% 9% 15% 13% 3	2505
May	1st to 10th	Day	61% 6% 8% 16% 7% 1	6357
	11th to 20th	Day	54% 7% 12% 16% 10% 1	6322
	21st to end	Day	51% 7% 11% 21% 9% 1	6949
June	1st to 10th	Day	52% 7% 11% 18% 11% 1	2800
	11th to 20th	Day	44% 7% 12% 25% 11% 2	2800
	21st to end	Day	50% 8% 12% 20% 10% 1	2800
July	1st to 10th	Day	46% 4 13% 27% 9%	6290
	11th to 20th	Day	39% 6% 14% 29% 13%	6285
	21st to end	Day	35% 7% 13% 37% 8%	6728
August	1st to 10th	Day	47% 7% 15% 23% 8%	4549
	11th to 20th	Day	42% 6% 17% 28% 6%	4075
	21st to end	Day	43% 6% 17% 23% 10%	4312
September	1st to 10th	Day	38% 7% 13% 29% 12%	2400
	11th to 20th	Day	36% 6% 13% 31% 13% 1	2355
	21st to end	Day	43% 8% 13% 25% 10%	2200
October	1st to 10th	Day	47% 4 12% 28% 10%	3265
	11th to 20th	Day	49% 5% 15% 24% 6% 1	3240
	21st to end	Day	49% 5% 17% 23% 6%	3193
November	1st to 10th	Day	53% 7% 12% 20% 7% 1	1779
	11th to 20th	Day	56% 6% 12% 16% 9% 1	1683
	21st to end	Day	66% 8% 11% 11% 4	1600
December	1st to 10th	Day	76% 5% 10% 8% 1	2600
	11th to 20th	Day	67% 5% 8% 14% 6%	2986
	21st to end	Day	67% 5% 8% 15% 4	3960
January	1st to 10th	Night	73% 6% 10% 8% 2	6349
	11th to 20th	Night	74% 7% 8% 8% 3	6360
	21st to end	Night	70% 6% 7% 8% 9% 1	6925
February	1st to 10th	Night	68% 6% 8% 12% 5%	2600
	11th to 20th	Night	63% 5% 10% 16% 6%	2469
	21st to end	Night	61% 5% 10% 14% 9%	1980
March	1st to 10th	Night	61% 4 6% 16% 11% 1	5374
	11th to 20th	Night	54% 5% 16% 15% 7% 3	5121
	21st to end	Night	56% 4 6% 21% 11% 2	5059
April	1st to 10th	Night	57% 6% 7% 17% 11% 1	2000
	11th to 20th	Night	54% 5% 10% 17% 13% 1	1975
	21st to end	Night	51% 5% 9% 16% 14% 3	1900
May	1st to 10th	Night	60% 8% 9% 14% 7% 1	3611
	11th to 20th	Night	52% 6% 10% 19% 12% 1	3576
	21st to end	Night	51% 6% 11% 19% 12% 1	3927
June	1st to 10th	Night	49% 10% 12% 17% 11% 1	1600
	11th to 20th	Night	43% 6% 13% 21% 16% 1	1600
	21st to end	Night	47% 9% 12% 20% 11%	1600
July	1st to 10th	Night	44% 5% 17% 22% 12%	3560
	11th to 20th	Night	40% 4 15% 28% 12%	3552
	21st to end	Night	39% 7% 13% 29% 12%	3809
August	1st to 10th	Night	45% 6% 15% 25% 9%	2576
	11th to 20th	Night	47% 5% 17% 23% 8% 1	2991
	21st to end	Night	43% 6% 15% 23% 12% 1	3586
September	1st to 10th	Night	41% 7% 11% 28% 13% 1	2000
	11th to 20th	Night	35% 7% 13% 33% 13%	2044
	21st to end	Night	43% 8% 11% 24% 13% 1	2200
October	1st to 10th	Night	45% 5% 13% 27% 9% 1	3912
	11th to 20th	Night	47% 5% 14% 24% 9% 1	3899
	21st to end	Night	50% 5% 17% 20% 8%	4596
November	1st to 10th	Night	54% 6% 13% 18% 8% 1	2564
	11th to 20th	Night	54% 7% 12% 18% 8% 2	2715
	21st to end	Night	64% 7% 11% 13% 4	2800
December	1st to 10th	Night	76% 4 10% 8% 2	4580
	11th to 20th	Night	68% 4 7% 13% 7%	5208
	21st to end	Night	69% 6% 6% 12% 6%	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	33% 17% 30% 17% 4	200
	11th to 20th	Day	34% 24% 32% 8% 3	200
	21st to end	Day	24% 23% 33% 18% 2	235
February	1st to 10th	Day	39% 25% 27% 8% 1	300
	11th to 20th	Day	32% 27% 35% 5% 1	365
	21st to end	Day	35% 21% 31% 11% 1	414
March	1st to 10th	Day	32% 23% 28% 14% 4	500
	11th to 20th	Day	32% 22% 30% 12% 3 1	500
	21st to end	Day	36% 23% 22% 12% 6% 1	550
April	1st to 10th	Day	38% 18% 27% 15% 2 1	500
	11th to 20th	Day	37% 18% 24% 13% 7% 1	492
	21st to end	Day	34% 19% 29% 11% 7% 1	500
May	1st to 10th	Day	29% 20% 28% 15% 6% 2	503
	11th to 20th	Day	20% 19% 32% 17% 10% 1	527
	21st to end	Day	22% 17% 30% 21% 9% 1	573
June	1st to 10th	Day	27% 18% 29% 19% 6% 1	530
	11th to 20th	Day	18% 16% 34% 23% 7% 2	530
	21st to end	Day	27% 15% 31% 17% 8% 2	530
July	1st to 10th	Day	13% 10% 37% 25% 13% 3	525
	11th to 20th	Day	16% 10% 35% 27% 10% 2	524
	21st to end	Day	15% 9% 36% 29% 9% 2	576
August	1st to 10th	Day	17% 8% 28% 30% 12% 3	526
	11th to 20th	Day	15% 8% 29% 30% 13% 5%	519
	21st to end	Day	17% 12% 32% 21% 13% 5%	583
September	1st to 10th	Day	17% 11% 29% 24% 14% 6%	530
	11th to 20th	Day	14% 9% 24% 26% 18% 9%	530
	21st to end	Day	20% 13% 30% 21% 13% 3	530
October	1st to 10th	Day	25% 12% 29% 17% 13% 4	530
	11th to 20th	Day	25% 25% 26% 16% 7% 1	530
	21st to end	Day	35% 16% 26% 17% 6%	468
November	1st to 10th	Day	31% 18% 26% 16% 8%	401
	11th to 20th	Day	31% 23% 30% 11% 5% 1	283
	21st to end	Day	39% 19% 28% 11% 5%	200
December	1st to 10th	Day	31% 22% 28% 16% 4 1	200
	11th to 20th	Day	34% 31% 23% 11% 2 1	200
	21st to end	Day	37% 20% 24% 14% 4	220
January	1st to 10th	Night	32% 19% 31% 15% 3	800
	11th to 20th	Night	31% 21% 27% 17% 5%	800
	21st to end	Night	24% 19% 31% 21% 5%	880
February	1st to 10th	Night	27% 21% 31% 18% 2	800
	11th to 20th	Night	26% 22% 33% 17% 2	783
	21st to end	Night	29% 17% 31% 22% 2	660
March	1st to 10th	Night	30% 18% 28% 20% 4	800
	11th to 20th	Night	30% 19% 30% 18% 3	800
	21st to end	Night	29% 16% 28% 21% 6%	880
April	1st to 10th	Night	31% 16% 29% 20% 4	800
	11th to 20th	Night	31% 17% 27% 19% 6%	791
	21st to end	Night	30% 14% 29% 21% 6%	700
May	1st to 10th	Night	30% 12% 27% 24% 7% 1	411
	11th to 20th	Night	23% 11% 31% 27% 7%	394
	21st to end	Night	17% 12% 35% 28% 9%	433
June	1st to 10th	Night	20% 10% 32% 32% 6% 1	400
	11th to 20th	Night	21% 9% 30% 31% 10% 1	400
	21st to end	Night	21% 8% 34% 26% 11% 1	400
July	1st to 10th	Night	12% 7% 31% 37% 12%	390
	11th to 20th	Night	14% 6% 29% 38% 13%	388
	21st to end	Night	14% 6% 32% 39% 8% 1	438
August	1st to 10th	Night	14% 6% 24% 39% 17% 1	398
	11th to 20th	Night	18% 5% 22% 35% 19% 1	644
	21st to end	Night	17% 6% 31% 30% 16% 1	880
September	1st to 10th	Night	17% 10% 27% 32% 14% 1	800
	11th to 20th	Night	17% 7% 23% 30% 20% 4	800
	21st to end	Night	20% 11% 25% 25% 18% 2	800
October	1st to 10th	Night	24% 12% 27% 23% 14% 1	800
	11th to 20th	Night	26% 12% 31% 23% 9%	800
	21st to end	Night	33% 15% 27% 22% 3	880
November	1st to 10th	Night	31% 18% 25% 20% 6% 1	789
	11th to 20th	Night	28% 18% 28% 18% 7% 1	800
	21st to end	Night	31% 21% 24% 19% 5%	800
December	1st to 10th	Night	34% 19% 27% 16% 3	800
	11th to 20th	Night	34% 20% 29% 14% 3 1	800
	21st to end	Night	39% 15% 27% 16% 4	880

Page generated at 2025-10-31 02:02:41. Version ac07

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