Cornish
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cornish clear sky chart from 2005-11-06 to 2024-02-15.

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	16% 8% 3 7% 6% 3 5% 5% 5% 7% 35%	78172
	Night	16% 9% 4 7% 6% 3 4 5% 5% 6% 35%	80904
Transparency	Day	49% 7% 12% 20% 12% 2	77877
	Night	49% 7% 11% 20% 11% 1	80733
Seeing	Day	24% 18% 30% 18% 8% 2	13820
	Night	22% 16% 29% 25% 8% 1	22928

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	12% 11% 4 8% 8% 4 5% 5% 6% 6% 32%	2517
	11th to 20th	Day	15% 12% 5% 9% 10% 4 4 5% 4 5% 27%	2520
	21st to end	Day	15% 15% 6% 9% 7% 4 5% 4 5% 6% 24%	2772
February	1st to 10th	Day	13% 12% 4 8% 7% 4 5% 5% 4 8% 29%	1697
	11th to 20th	Day	11% 10% 6% 10% 8% 4 4 5% 5% 6% 31%	1788
	21st to end	Day	17% 12% 6% 8% 5% 4 4 4 4 5% 30%	1480
March	1st to 10th	Day	13% 10% 4 9% 6% 4 6% 5% 6% 5% 32%	1797
	11th to 20th	Day	17% 11% 4 7% 6% 3 4 5% 5% 5% 34%	1853
	21st to end	Day	16% 10% 4 7% 6% 3 5% 4 4 6% 36%	2361
April	1st to 10th	Day	18% 9% 4 8% 4 3 4 4 5% 6% 35%	2160
	11th to 20th	Day	16% 7% 3 7% 5% 3 5% 5% 5% 7% 36%	2134
	21st to end	Day	22% 10% 3 6% 6% 3 3 5% 5% 5% 32%	2326
May	1st to 10th	Day	23% 9% 3 6% 6% 3 4 4 4 6% 31%	2340
	11th to 20th	Day	17% 6% 3 6% 5% 3 5% 4 5% 6% 38%	2455
	21st to end	Day	16% 6% 3 5% 5% 4 5% 4 4 8% 40%	2725
June	1st to 10th	Day	18% 6% 3 7% 7% 5% 6% 5% 6% 7% 31%	2520
	11th to 20th	Day	18% 7% 2 5% 5% 3 4 5% 5% 8% 37%	2520
	21st to end	Day	18% 7% 3 7% 7% 3 6% 6% 6% 7% 31%	2520
July	1st to 10th	Day	15% 6% 3 4 6% 3 5% 5% 6% 8% 38%	2520
	11th to 20th	Day	12% 5% 3 7% 6% 4 6% 5% 7% 10% 35%	2520
	21st to end	Day	14% 5% 2 5% 6% 3 6% 6% 7% 9% 36%	2744
August	1st to 10th	Day	13% 5% 3 6% 7% 4 5% 6% 7% 9% 36%	2449
	11th to 20th	Day	13% 5% 2 6% 6% 4 6% 7% 7% 8% 35%	2242
	21st to end	Day	13% 4 3 5% 5% 4 4 4 7% 8% 43%	2376
September	1st to 10th	Day	13% 5% 2 5% 5% 3 5% 5% 5% 9% 43%	2160
	11th to 20th	Day	14% 5% 2 5% 5% 3 4 4 4 6% 49%	2023
	21st to end	Day	15% 6% 2 5% 4 3 4 5% 5% 7% 44%	1980
October	1st to 10th	Day	22% 9% 2 6% 6% 3 4 4 5% 5% 35%	1962
	11th to 20th	Day	14% 8% 3 6% 5% 3 5% 5% 6% 8% 38%	1731
	21st to end	Day	24% 9% 4 7% 6% 3 4 4 4 5% 30%	1782
November	1st to 10th	Day	13% 7% 3 6% 5% 3 5% 4 5% 8% 41%	1671
	11th to 20th	Day	18% 9% 3 6% 5% 4 4 4 5% 6% 38%	1710
	21st to end	Day	19% 10% 3 6% 5% 3 5% 5% 5% 6% 34%	1619
December	1st to 10th	Day	19% 12% 5% 9% 7% 3 4 5% 5% 6% 25%	1520
	11th to 20th	Day	22% 12% 3 6% 5% 3 4 4 4 5% 33%	1906
	21st to end	Day	24% 13% 5% 10% 5% 3 5% 3 3 4 25%	2772
January	1st to 10th	Night	16% 9% 4 8% 6% 3 5% 5% 6% 7% 33%	4429
	11th to 20th	Night	12% 12% 5% 11% 9% 4 5% 6% 5% 6% 26%	4440
	21st to end	Night	18% 13% 5% 10% 7% 4 4 4 4 5% 24%	4884
February	1st to 10th	Night	13% 9% 5% 8% 7% 4 6% 7% 5% 6% 28%	2480
	11th to 20th	Night	10% 9% 4 7% 7% 4 6% 6% 7% 8% 30%	2153
	21st to end	Night	15% 9% 3 8% 7% 3 5% 5% 7% 5% 33%	1776
March	1st to 10th	Night	14% 9% 4 7% 7% 4 5% 4 5% 7% 34%	2150
	11th to 20th	Night	16% 8% 5% 6% 7% 3 5% 5% 5% 6% 33%	2105
	21st to end	Night	15% 9% 3 6% 5% 3 5% 5% 5% 5% 39%	1994
April	1st to 10th	Night	18% 8% 3 6% 6% 3 5% 4 6% 7% 35%	1800
	11th to 20th	Night	18% 8% 3 6% 4 2 5% 5% 5% 6% 38%	1775
	21st to end	Night	20% 8% 4 7% 6% 3 4 3 6% 5% 35%	1634
May	1st to 10th	Night	21% 9% 3 7% 6% 4 4 3 4 6% 32%	1620
	11th to 20th	Night	18% 7% 2 5% 6% 3 5% 4 6% 6% 37%	1453
	21st to end	Night	16% 5% 3 5% 6% 3 5% 5% 6% 8% 39%	1550
June	1st to 10th	Night	19% 5% 2 7% 6% 3 4 5% 5% 8% 34%	1440
	11th to 20th	Night	19% 5% 3 6% 5% 2 3 5% 5% 8% 39%	1440
	21st to end	Night	16% 5% 3 5% 6% 4 6% 5% 7% 8% 36%	1440
July	1st to 10th	Night	15% 4 2 4 5% 4 4 5% 6% 7% 43%	1440
	11th to 20th	Night	14% 4 2 4 5% 4 4 5% 6% 10% 42%	1440
	21st to end	Night	12% 5% 2 5% 4 3 4 5% 7% 9% 44%	1574
August	1st to 10th	Night	11% 5% 2 4 6% 3 4 5% 6% 8% 45%	1471
	11th to 20th	Night	12% 6% 2 6% 5% 4 5% 5% 6% 8% 41%	1655
	21st to end	Night	14% 6% 2 5% 4 2 4 4 5% 8% 45%	1980
September	1st to 10th	Night	13% 5% 2 4 4 3 3 4 5% 7% 51%	1800
	11th to 20th	Night	14% 6% 2 4 5% 2 3 3 5% 5% 50%	1936
	21st to end	Night	14% 7% 2 4 4 2 3 4 5% 7% 47%	1980
October	1st to 10th	Night	20% 8% 3 6% 6% 3 4 4 5% 6% 37%	1996
	11th to 20th	Night	16% 7% 2 6% 6% 4 5% 5% 6% 7% 37%	2227
	21st to end	Night	22% 9% 3 6% 6% 3 4 5% 5% 5% 31%	2574
November	1st to 10th	Night	14% 8% 2 6% 6% 4 4 4 5% 7% 40%	2407
	11th to 20th	Night	16% 8% 3 6% 6% 3 4 5% 5% 6% 38%	2470
	21st to end	Night	15% 8% 4 7% 6% 4 5% 5% 5% 6% 35%	2559
December	1st to 10th	Night	19% 11% 5% 8% 7% 3 6% 4 4 6% 27%	2660
	11th to 20th	Night	21% 10% 5% 7% 5% 3 4 5% 4 6% 31%	3288
	21st to end	Night	23% 14% 7% 9% 6% 3 3 4 4 5% 24%	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	49% 6% 12% 26% 6% 1	2518
	11th to 20th	Day	58% 8% 10% 17% 6% 1	2520
	21st to end	Day	60% 5% 11% 16% 7% 1	2772
February	1st to 10th	Day	54% 6% 13% 17% 9% 1	1697
	11th to 20th	Day	54% 6% 11% 18% 10% 2	1797
	21st to end	Day	56% 5% 10% 16% 12% 1	1480
March	1st to 10th	Day	53% 6% 11% 17% 11% 3	1800
	11th to 20th	Day	52% 5% 12% 13% 14% 3	1853
	21st to end	Day	50% 6% 9% 16% 15% 4	2361
April	1st to 10th	Day	51% 6% 9% 20% 12% 3	2160
	11th to 20th	Day	47% 7% 10% 19% 15% 2	2134
	21st to end	Day	55% 6% 8% 16% 12% 3	2326
May	1st to 10th	Day	56% 5% 7% 19% 11% 2	2340
	11th to 20th	Day	47% 6% 9% 21% 15% 3	2459
	21st to end	Day	44% 7% 14% 22% 11% 2	2725
June	1st to 10th	Day	52% 7% 13% 19% 9% 1	2520
	11th to 20th	Day	46% 7% 11% 19% 15% 2	2520
	21st to end	Day	51% 9% 12% 17% 10% 1	2520
July	1st to 10th	Day	42% 9% 12% 21% 15% 1	2450
	11th to 20th	Day	42% 9% 15% 24% 9% 1	2445
	21st to end	Day	42% 9% 13% 24% 11% 1	2591
August	1st to 10th	Day	42% 10% 15% 22% 11% 1	2435
	11th to 20th	Day	43% 9% 15% 21% 10% 1	2242
	21st to end	Day	36% 8% 15% 26% 14% 1	2376
September	1st to 10th	Day	38% 7% 14% 26% 14% 1	2160
	11th to 20th	Day	37% 7% 8% 26% 20% 2	2023
	21st to end	Day	38% 6% 13% 25% 15% 2	1980
October	1st to 10th	Day	52% 5% 10% 19% 13% 2	1962
	11th to 20th	Day	43% 7% 14% 27% 8% 1	1731
	21st to end	Day	58% 5% 8% 17% 9% 2	1782
November	1st to 10th	Day	41% 7% 14% 23% 14% 1	1671
	11th to 20th	Day	48% 5% 12% 18% 15% 2	1710
	21st to end	Day	51% 6% 16% 18% 8% 1	1619
December	1st to 10th	Day	59% 6% 14% 13% 7% 1	1520
	11th to 20th	Day	53% 7% 11% 16% 12% 1	1906
	21st to end	Day	63% 5% 9% 13% 9%	2772
January	1st to 10th	Night	48% 8% 13% 22% 9% 1	4429
	11th to 20th	Night	57% 8% 11% 16% 7% 1	4440
	21st to end	Night	60% 8% 8% 15% 8% 1	4884
February	1st to 10th	Night	53% 9% 11% 17% 10% 1	2480
	11th to 20th	Night	49% 8% 11% 18% 12% 1	2155
	21st to end	Night	49% 8% 11% 20% 9% 3	1776
March	1st to 10th	Night	50% 6% 11% 19% 12% 2	2160
	11th to 20th	Night	52% 6% 10% 18% 12% 3	2106
	21st to end	Night	47% 5% 10% 16% 18% 4	1994
April	1st to 10th	Night	50% 5% 11% 20% 13% 2	1800
	11th to 20th	Night	47% 5% 10% 22% 15% 1	1775
	21st to end	Night	51% 5% 10% 17% 13% 3	1634
May	1st to 10th	Night	55% 6% 7% 16% 15% 2	1620
	11th to 20th	Night	48% 7% 9% 20% 13% 3	1454
	21st to end	Night	44% 8% 13% 22% 11% 2	1551
June	1st to 10th	Night	49% 7% 13% 19% 11%	1440
	11th to 20th	Night	45% 7% 14% 19% 12% 2	1440
	21st to end	Night	45% 9% 13% 22% 11%	1440
July	1st to 10th	Night	38% 8% 16% 24% 13% 1	1400
	11th to 20th	Night	36% 9% 17% 26% 12%	1392
	21st to end	Night	34% 8% 15% 30% 11% 2	1486
August	1st to 10th	Night	36% 9% 14% 28% 13% 1	1461
	11th to 20th	Night	40% 9% 16% 23% 11% 1	1655
	21st to end	Night	36% 7% 13% 28% 15%	1980
September	1st to 10th	Night	34% 7% 15% 28% 16% 1	1800
	11th to 20th	Night	36% 6% 10% 28% 19% 1	1936
	21st to end	Night	36% 6% 12% 28% 16% 2	1980
October	1st to 10th	Night	48% 7% 11% 21% 11% 2	1996
	11th to 20th	Night	45% 7% 14% 24% 9%	2227
	21st to end	Night	55% 6% 11% 17% 9% 1	2574
November	1st to 10th	Night	45% 7% 12% 21% 13% 2	2407
	11th to 20th	Night	46% 7% 10% 21% 14% 2	2470
	21st to end	Night	50% 7% 14% 21% 8% 1	2559
December	1st to 10th	Night	59% 6% 12% 18% 6%	2660
	11th to 20th	Night	54% 7% 11% 15% 12% 1	3288
	21st to end	Night	63% 6% 9% 17% 6%	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	29% 24% 27% 15% 5%	180
	11th to 20th	Day	32% 27% 28% 12% 1 1	180
	21st to end	Day	20% 30% 33% 11% 5%	198
February	1st to 10th	Day	28% 34% 27% 9% 1	285
	11th to 20th	Day	28% 31% 28% 12% 2	432
	21st to end	Day	29% 25% 31% 12% 2	346
March	1st to 10th	Day	28% 21% 35% 13% 3	420
	11th to 20th	Day	31% 26% 27% 11% 4	420
	21st to end	Day	31% 21% 27% 12% 5% 3	462
April	1st to 10th	Day	35% 19% 25% 13% 6% 1	420
	11th to 20th	Day	32% 20% 27% 12% 7% 2	412
	21st to end	Day	31% 22% 30% 10% 7% 1	420
May	1st to 10th	Day	32% 22% 24% 13% 6% 2	423
	11th to 20th	Day	21% 20% 36% 18% 3 2	447
	21st to end	Day	21% 15% 32% 23% 8% 1	485
June	1st to 10th	Day	21% 17% 38% 15% 8% 1	450
	11th to 20th	Day	22% 16% 33% 18% 9% 3	450
	21st to end	Day	19% 19% 33% 19% 9% 2	450
July	1st to 10th	Day	16% 13% 32% 23% 13% 3	445
	11th to 20th	Day	14% 9% 37% 27% 10% 2	444
	21st to end	Day	12% 11% 39% 28% 7% 2	488
August	1st to 10th	Day	11% 13% 33% 26% 15% 3	446
	11th to 20th	Day	15% 10% 33% 22% 13% 7%	439
	21st to end	Day	14% 14% 28% 24% 13% 5%	495
September	1st to 10th	Day	17% 12% 26% 25% 16% 5%	450
	11th to 20th	Day	18% 14% 27% 19% 15% 8%	450
	21st to end	Day	18% 15% 29% 21% 12% 6%	450
October	1st to 10th	Day	25% 14% 27% 18% 11% 4	450
	11th to 20th	Day	21% 16% 30% 23% 8% 2	415
	21st to end	Day	35% 17% 22% 18% 7%	396
November	1st to 10th	Day	24% 18% 26% 20% 11% 1	365
	11th to 20th	Day	28% 23% 25% 18% 6% 1	370
	21st to end	Day	33% 19% 27% 16% 3 1	279
December	1st to 10th	Day	28% 27% 25% 17% 3	180
	11th to 20th	Day	34% 27% 29% 6% 3	180
	21st to end	Day	35% 20% 30% 12% 4	198
January	1st to 10th	Night	23% 23% 33% 20% 3	760
	11th to 20th	Night	26% 20% 33% 18% 3	760
	21st to end	Night	19% 22% 35% 20% 3	836
February	1st to 10th	Night	23% 22% 36% 17% 2	760
	11th to 20th	Night	21% 23% 37% 17% 2	722
	21st to end	Night	25% 21% 32% 19% 3	592
March	1st to 10th	Night	27% 20% 31% 21% 2	720
	11th to 20th	Night	25% 21% 28% 20% 5%	720
	21st to end	Night	28% 18% 28% 20% 6%	792
April	1st to 10th	Night	27% 19% 27% 22% 4	720
	11th to 20th	Night	22% 23% 27% 20% 7%	711
	21st to end	Night	27% 16% 30% 23% 4	554
May	1st to 10th	Night	28% 14% 28% 24% 5%	540
	11th to 20th	Night	20% 15% 31% 29% 6%	392
	21st to end	Night	17% 10% 30% 33% 10% 1	389
June	1st to 10th	Night	20% 8% 29% 31% 12%	360
	11th to 20th	Night	19% 13% 26% 30% 12% 1	360
	21st to end	Night	15% 9% 25% 40% 10% 1	360
July	1st to 10th	Night	15% 6% 25% 40% 13% 1	350
	11th to 20th	Night	13% 3 24% 41% 17% 2	348
	21st to end	Night	13% 5% 27% 40% 15% 1	394
August	1st to 10th	Night	12% 4 23% 43% 18% 1	397
	11th to 20th	Night	13% 7% 29% 32% 17% 3	591
	21st to end	Night	16% 6% 22% 35% 18% 2	792
September	1st to 10th	Night	15% 8% 26% 30% 19% 1	720
	11th to 20th	Night	16% 10% 25% 28% 18% 4	720
	21st to end	Night	18% 9% 26% 29% 16% 2	720
October	1st to 10th	Night	26% 12% 20% 27% 14% 1	720
	11th to 20th	Night	20% 14% 27% 26% 11% 1	720
	21st to end	Night	28% 17% 27% 22% 5%	792
November	1st to 10th	Night	20% 18% 27% 26% 9% 1	740
	11th to 20th	Night	22% 19% 29% 21% 7% 1	760
	21st to end	Night	24% 20% 29% 22% 6%	760
December	1st to 10th	Night	24% 22% 33% 17% 4	760
	11th to 20th	Night	27% 22% 30% 18% 3	760
	21st to end	Night	28% 22% 33% 16% 2	836

Page generated at 2024-02-12 06:35:36. Version ac07

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