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 2024-01-31.

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	18% 10% 4 7% 6% 3 5% 5% 5% 6% 32%	74896
	Night	19% 11% 4 7% 6% 3 4 4 4 6% 30%	75234
Transparency	Day	52% 7% 11% 19% 9% 1	74601
	Night	56% 7% 11% 17% 9% 1	75063
Seeing	Day	26% 17% 29% 18% 8% 2	13482
	Night	26% 15% 28% 23% 8% 1	22685

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	20% 19% 7% 11% 8% 3 4 3 3 4 18%	1517
	11th to 20th	Day	20% 16% 7% 10% 8% 3 5% 5% 3 5% 19%	1520
	21st to end	Day	13% 17% 6% 10% 7% 5% 5% 5% 5% 6% 21%	1691
February	1st to 10th	Day	16% 15% 7% 11% 8% 3 5% 5% 5% 5% 20%	1620
	11th to 20th	Day	17% 14% 5% 10% 6% 4 5% 4 4 6% 25%	1631
	21st to end	Day	20% 16% 5% 8% 8% 3 4 3 4 5% 23%	1480
March	1st to 10th	Day	16% 14% 6% 9% 7% 3 5% 3 4 4 29%	1797
	11th to 20th	Day	20% 11% 4 8% 5% 3 5% 4 4 6% 30%	1921
	21st to end	Day	22% 10% 3 6% 5% 2 4 4 4 5% 35%	2335
April	1st to 10th	Day	22% 11% 4 6% 6% 3 3 4 4 6% 32%	2160
	11th to 20th	Day	21% 8% 4 7% 5% 3 3 3 4 5% 36%	2134
	21st to end	Day	24% 11% 3 4 4 3 4 3 5% 6% 32%	2254
May	1st to 10th	Day	21% 8% 2 7% 4 3 4 4 5% 6% 36%	2520
	11th to 20th	Day	16% 6% 3 6% 5% 3 5% 4 5% 6% 40%	2478
	21st to end	Day	15% 7% 3 6% 6% 4 5% 5% 5% 7% 37%	2725
June	1st to 10th	Day	23% 7% 3 6% 6% 3 4 5% 5% 7% 30%	2520
	11th to 20th	Day	16% 5% 3 6% 6% 3 4 5% 7% 8% 37%	2520
	21st to end	Day	21% 7% 3 4 6% 4 5% 5% 6% 7% 32%	2520
July	1st to 10th	Day	12% 6% 3 5% 5% 3 5% 5% 6% 8% 42%	2520
	11th to 20th	Day	13% 5% 3 5% 5% 4 6% 7% 7% 9% 38%	2520
	21st to end	Day	12% 5% 2 5% 5% 4 6% 6% 7% 10% 38%	2744
August	1st to 10th	Day	13% 6% 3 6% 5% 4 5% 6% 6% 8% 38%	2506
	11th to 20th	Day	12% 5% 2 5% 5% 4 6% 6% 7% 8% 40%	2239
	21st to end	Day	14% 6% 2 6% 6% 3 4 5% 7% 8% 39%	2376
September	1st to 10th	Day	15% 6% 2 5% 5% 3 4 6% 5% 7% 43%	2160
	11th to 20th	Day	15% 6% 3 4 4 3 4 4 5% 7% 47%	2120
	21st to end	Day	18% 6% 2 5% 6% 3 4 5% 5% 8% 39%	1980
October	1st to 10th	Day	22% 8% 2 5% 6% 3 5% 4 5% 6% 33%	1805
	11th to 20th	Day	20% 10% 4 7% 8% 4 5% 5% 4 6% 27%	1800
	21st to end	Day	28% 10% 4 7% 5% 3 4 4 4 4 27%	1795
November	1st to 10th	Day	17% 9% 5% 7% 6% 4 5% 5% 5% 7% 30%	1671
	11th to 20th	Day	19% 12% 4 8% 5% 4 5% 4 5% 7% 28%	1599
	21st to end	Day	22% 14% 5% 8% 7% 3 6% 4 5% 5% 20%	1520
December	1st to 10th	Day	25% 17% 5% 11% 7% 3 4 4 4 5% 15%	1520
	11th to 20th	Day	22% 18% 6% 8% 5% 3 4 4 5% 5% 21%	1906
	21st to end	Day	26% 19% 5% 9% 6% 3 4 3 3 3 18%	2772
January	1st to 10th	Night	19% 20% 6% 11% 7% 4 4 3 4 4 17%	2649
	11th to 20th	Night	18% 19% 6% 11% 7% 4 6% 4 4 5% 18%	2660
	21st to end	Night	14% 16% 7% 9% 9% 5% 5% 4 4 5% 24%	2890
February	1st to 10th	Night	16% 15% 8% 13% 8% 3 6% 4 5% 4 18%	2340
	11th to 20th	Night	16% 13% 5% 12% 8% 4 4 4 4 4 27%	2214
	21st to end	Night	18% 14% 6% 8% 8% 2 4 4 4 4 28%	1776
March	1st to 10th	Night	18% 15% 5% 8% 7% 4 5% 3 3 4 27%	2150
	11th to 20th	Night	19% 13% 4 7% 7% 3 3 4 4 5% 31%	2036
	21st to end	Night	20% 12% 4 7% 5% 2 4 4 3 4 36%	2021
April	1st to 10th	Night	23% 10% 4 9% 5% 3 4 4 4 5% 31%	1800
	11th to 20th	Night	22% 9% 3 7% 5% 3 5% 3 4 6% 34%	1775
	21st to end	Night	22% 10% 3 5% 5% 3 3 5% 4 6% 34%	1710
May	1st to 10th	Night	23% 7% 3 5% 5% 3 5% 5% 5% 6% 33%	1451
	11th to 20th	Night	18% 7% 3 6% 6% 3 4 4 5% 5% 40%	1415
	21st to end	Night	14% 5% 2 5% 5% 3 4 5% 5% 9% 41%	1550
June	1st to 10th	Night	19% 5% 3 6% 6% 4 5% 5% 7% 9% 30%	1440
	11th to 20th	Night	17% 5% 2 5% 5% 3 4 5% 5% 9% 39%	1440
	21st to end	Night	17% 7% 3 6% 5% 3 5% 5% 6% 8% 34%	1440
July	1st to 10th	Night	11% 3 1 5% 5% 4 6% 5% 7% 9% 44%	1440
	11th to 20th	Night	13% 4 3 4 5% 4 5% 5% 6% 10% 42%	1440
	21st to end	Night	12% 5% 3 5% 4 3 6% 5% 7% 9% 40%	1574
August	1st to 10th	Night	12% 5% 3 5% 4 3 6% 5% 6% 9% 42%	1432
	11th to 20th	Night	14% 6% 3 6% 6% 3 4 4 5% 6% 43%	1642
	21st to end	Night	14% 6% 2 5% 6% 3 3 4 6% 8% 42%	1980
September	1st to 10th	Night	14% 6% 3 6% 5% 3 4 5% 5% 7% 41%	1800
	11th to 20th	Night	15% 5% 2 5% 4 3 3 4 5% 7% 48%	1839
	21st to end	Night	18% 6% 3 5% 5% 3 4 5% 5% 6% 41%	1980
October	1st to 10th	Night	21% 7% 3 5% 5% 3 5% 5% 6% 6% 34%	2152
	11th to 20th	Night	20% 8% 4 7% 6% 4 4 4 4 6% 32%	2160
	21st to end	Night	26% 9% 3 7% 6% 3 5% 4 4 6% 27%	2560
November	1st to 10th	Night	20% 10% 4 7% 5% 3 5% 5% 4 6% 32%	2407
	11th to 20th	Night	19% 12% 3 7% 6% 3 5% 4 4 6% 31%	2579
	21st to end	Night	23% 13% 5% 8% 6% 3 5% 4 4 6% 23%	2660
December	1st to 10th	Night	23% 18% 7% 9% 7% 4 4 5% 4 4 16%	2660
	11th to 20th	Night	24% 17% 5% 8% 5% 3 4 4 3 3 23%	3288
	21st to end	Night	32% 16% 5% 9% 5% 3 3 3 3 3 17%	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	73% 4 9% 10% 3	1518
	11th to 20th	Day	69% 6% 8% 9% 7% 1	1520
	21st to end	Day	64% 7% 11% 12% 5% 1	1691
February	1st to 10th	Day	67% 6% 7% 15% 4 1	1620
	11th to 20th	Day	62% 6% 11% 14% 7% 1	1640
	21st to end	Day	65% 5% 10% 11% 8% 1	1480
March	1st to 10th	Day	61% 5% 10% 14% 9% 1	1800
	11th to 20th	Day	57% 5% 12% 13% 10% 2	1921
	21st to end	Day	53% 4 9% 17% 14% 4	2335
April	1st to 10th	Day	55% 4 9% 19% 11% 2	2160
	11th to 20th	Day	51% 5% 8% 19% 15% 2	2134
	21st to end	Day	54% 6% 9% 14% 13% 3	2254
May	1st to 10th	Day	49% 6% 8% 19% 15% 2	2520
	11th to 20th	Day	45% 6% 10% 20% 16% 2	2482
	21st to end	Day	44% 9% 13% 19% 13% 2	2725
June	1st to 10th	Day	53% 8% 11% 17% 11% 1	2520
	11th to 20th	Day	43% 7% 12% 24% 11% 2	2520
	21st to end	Day	50% 8% 12% 19% 10% 1	2520
July	1st to 10th	Day	39% 7% 11% 28% 13% 1	2450
	11th to 20th	Day	39% 10% 15% 28% 8%	2445
	21st to end	Day	40% 11% 17% 23% 9%	2591
August	1st to 10th	Day	42% 10% 16% 24% 8%	2492
	11th to 20th	Day	39% 10% 17% 25% 10%	2239
	21st to end	Day	41% 9% 14% 25% 11%	2376
September	1st to 10th	Day	40% 8% 13% 27% 13%	2160
	11th to 20th	Day	38% 6% 13% 31% 12% 1	2120
	21st to end	Day	43% 9% 13% 26% 8%	1980
October	1st to 10th	Day	52% 5% 12% 20% 11%	1805
	11th to 20th	Day	57% 8% 13% 15% 6%	1800
	21st to end	Day	61% 6% 10% 15% 7% 1	1795
November	1st to 10th	Day	53% 7% 13% 20% 7% 1	1671
	11th to 20th	Day	57% 6% 12% 15% 8% 1	1599
	21st to end	Day	66% 8% 11% 11% 4	1520
December	1st to 10th	Day	72% 7% 10% 8% 2	1520
	11th to 20th	Day	67% 6% 11% 12% 3	1906
	21st to end	Day	72% 6% 7% 12% 2	2772
January	1st to 10th	Night	71% 7% 9% 9% 4	2649
	11th to 20th	Night	70% 6% 9% 9% 5%	2660
	21st to end	Night	63% 7% 9% 13% 8% 1	2890
February	1st to 10th	Night	70% 6% 8% 10% 5%	2340
	11th to 20th	Night	62% 5% 10% 16% 6%	2216
	21st to end	Night	60% 5% 10% 14% 10%	1776
March	1st to 10th	Night	63% 5% 9% 13% 9% 1	2160
	11th to 20th	Night	57% 5% 10% 17% 8% 3	2037
	21st to end	Night	55% 4 7% 18% 13% 4	2021
April	1st to 10th	Night	56% 5% 8% 18% 11% 1	1800
	11th to 20th	Night	53% 5% 10% 17% 14% 1	1775
	21st to end	Night	52% 5% 9% 16% 14% 4	1710
May	1st to 10th	Night	51% 7% 8% 14% 16% 4	1451
	11th to 20th	Night	46% 6% 8% 20% 17% 3	1416
	21st to end	Night	40% 9% 13% 20% 16% 2	1551
June	1st to 10th	Night	49% 10% 13% 17% 11% 1	1440
	11th to 20th	Night	43% 6% 13% 20% 17% 1	1440
	21st to end	Night	45% 9% 14% 21% 11%	1440
July	1st to 10th	Night	33% 10% 14% 24% 18% 1	1400
	11th to 20th	Night	36% 8% 13% 30% 13%	1392
	21st to end	Night	37% 12% 14% 23% 13% 1	1486
August	1st to 10th	Night	37% 10% 18% 23% 11%	1422
	11th to 20th	Night	42% 6% 15% 24% 12% 1	1642
	21st to end	Night	40% 8% 15% 25% 11% 1	1980
September	1st to 10th	Night	42% 8% 12% 25% 13% 1	1800
	11th to 20th	Night	36% 7% 12% 33% 11% 1	1839
	21st to end	Night	43% 8% 10% 26% 12% 1	1980
October	1st to 10th	Night	49% 7% 11% 22% 10% 1	2152
	11th to 20th	Night	53% 8% 13% 19% 8%	2160
	21st to end	Night	59% 7% 13% 13% 7% 1	2560
November	1st to 10th	Night	53% 7% 13% 17% 8% 1	2407
	11th to 20th	Night	54% 8% 12% 18% 7% 2	2579
	21st to end	Night	64% 7% 11% 13% 4	2660
December	1st to 10th	Night	73% 6% 9% 10% 2	2660
	11th to 20th	Night	67% 5% 9% 14% 5%	3288
	21st to end	Night	73% 6% 8% 11% 2	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	33% 17% 27% 18% 4	180
	11th to 20th	Day	35% 24% 31% 7% 3	180
	21st to end	Day	25% 21% 34% 18% 2	208
February	1st to 10th	Day	43% 25% 27% 5%	240
	11th to 20th	Day	33% 24% 36% 6% 1	298
	21st to end	Day	36% 20% 31% 12% 1	346
March	1st to 10th	Day	31% 22% 29% 14% 4	420
	11th to 20th	Day	33% 21% 30% 11% 3 1	420
	21st to end	Day	35% 23% 21% 13% 8% 1	462
April	1st to 10th	Day	35% 17% 29% 15% 3 1	420
	11th to 20th	Day	36% 16% 25% 14% 7% 1	412
	21st to end	Day	34% 18% 28% 12% 7% 1	420
May	1st to 10th	Day	28% 21% 28% 13% 7% 2	423
	11th to 20th	Day	21% 20% 32% 17% 9% 1	447
	21st to end	Day	21% 17% 31% 21% 9% 1	485
June	1st to 10th	Day	28% 17% 27% 19% 6% 2	450
	11th to 20th	Day	18% 15% 33% 25% 7% 2	450
	21st to end	Day	25% 15% 30% 18% 10% 2	450
July	1st to 10th	Day	13% 10% 33% 27% 13% 3	445
	11th to 20th	Day	16% 10% 34% 28% 11% 2	444
	21st to end	Day	15% 9% 36% 29% 8% 2	488
August	1st to 10th	Day	17% 9% 28% 30% 12% 4	446
	11th to 20th	Day	15% 8% 30% 29% 13% 5%	439
	21st to end	Day	16% 13% 33% 21% 12% 6%	495
September	1st to 10th	Day	16% 12% 29% 25% 12% 6%	450
	11th to 20th	Day	16% 10% 26% 24% 16% 7%	450
	21st to end	Day	20% 13% 30% 21% 13% 4	450
October	1st to 10th	Day	27% 13% 27% 16% 13% 4	450
	11th to 20th	Day	25% 26% 25% 15% 7% 2	450
	21st to end	Day	37% 15% 27% 17% 5%	402
November	1st to 10th	Day	30% 18% 27% 16% 8% 1	365
	11th to 20th	Day	31% 24% 29% 10% 5% 1	259
	21st to end	Day	36% 18% 29% 12% 6%	180
December	1st to 10th	Day	32% 22% 27% 16% 3 1	180
	11th to 20th	Day	34% 31% 23% 11% 1 1	180
	21st to end	Day	41% 22% 22% 13% 2 1	198
January	1st to 10th	Night	32% 19% 30% 16% 3	760
	11th to 20th	Night	31% 20% 27% 16% 5%	760
	21st to end	Night	24% 19% 31% 22% 4	835
February	1st to 10th	Night	28% 21% 32% 17% 2	720
	11th to 20th	Night	26% 21% 34% 17% 2	703
	21st to end	Night	29% 17% 31% 21% 2	592
March	1st to 10th	Night	29% 19% 27% 21% 4	720
	11th to 20th	Night	30% 19% 29% 18% 3	720
	21st to end	Night	29% 16% 27% 22% 6%	792
April	1st to 10th	Night	30% 15% 29% 21% 4	720
	11th to 20th	Night	30% 17% 27% 20% 6%	711
	21st to end	Night	30% 14% 28% 22% 6%	630
May	1st to 10th	Night	29% 11% 28% 24% 7% 1	371
	11th to 20th	Night	23% 11% 33% 26% 7%	354
	21st to end	Night	16% 12% 34% 27% 11%	389
June	1st to 10th	Night	20% 9% 32% 32% 6% 1	360
	11th to 20th	Night	21% 8% 29% 31% 10% 1	360
	21st to end	Night	19% 9% 34% 26% 12% 1	360
July	1st to 10th	Night	11% 8% 31% 38% 12%	350
	11th to 20th	Night	13% 6% 30% 38% 14%	348
	21st to end	Night	13% 6% 33% 38% 8% 1	394
August	1st to 10th	Night	13% 6% 25% 38% 17% 1	358
	11th to 20th	Night	17% 5% 23% 35% 19% 2	578
	21st to end	Night	17% 6% 31% 29% 16% 1	792
September	1st to 10th	Night	17% 9% 28% 32% 13% 1	720
	11th to 20th	Night	18% 7% 24% 30% 17% 3	720
	21st to end	Night	20% 11% 26% 24% 17% 2	720
October	1st to 10th	Night	25% 12% 28% 22% 13% 1	720
	11th to 20th	Night	27% 13% 30% 21% 9%	720
	21st to end	Night	33% 15% 28% 21% 3	792
November	1st to 10th	Night	30% 18% 25% 20% 7% 1	740
	11th to 20th	Night	28% 19% 27% 18% 8% 1	760
	21st to end	Night	31% 20% 25% 19% 6%	760
December	1st to 10th	Night	35% 19% 26% 16% 3	760
	11th to 20th	Night	34% 19% 29% 14% 3 1	760
	21st to end	Night	40% 15% 27% 15% 3	836

Page generated at 2024-01-28 05:26:32. Version ac07

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