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 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	16% 9% 3 8% 5% 5% 5% 4 5% 7% 34%	108789
	Night	16% 9% 3 8% 5% 5% 4 4 5% 6% 34%	110554
Transparency	Day	49% 6% 12% 20% 12% 1	112344
	Night	50% 6% 11% 20% 12% 2	115163
Seeing	Day	24% 18% 30% 18% 8% 2	15260
	Night	22% 16% 29% 24% 8% 1	24347

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	10% 9% 6% 19% 7% 5% 4 3 5% 5% 27%	3597
	11th to 20th	Day	11% 12% 5% 14% 6% 5% 4 3 5% 6% 30%	3570
	21st to end	Day	12% 12% 5% 9% 5% 5% 5% 3 5% 6% 33%	3960
February	1st to 10th	Day	13% 12% 4 9% 7% 5% 5% 4 5% 7% 29%	1787
	11th to 20th	Day	10% 11% 5% 11% 7% 6% 3 4 5% 6% 32%	1948
	21st to end	Day	16% 13% 4 9% 5% 5% 3 3 5% 5% 31%	1650
March	1st to 10th	Day	20% 16% 4 10% 5% 4 4 3 4 4 26%	3216
	11th to 20th	Day	14% 14% 5% 7% 5% 5% 4 4 6% 6% 30%	3358
	21st to end	Day	18% 12% 3 9% 5% 5% 4 3 4 6% 32%	4222
April	1st to 10th	Day	19% 11% 3 8% 4 4 4 3 5% 5% 34%	2280
	11th to 20th	Day	16% 9% 3 7% 4 5% 4 4 5% 7% 35%	2254
	21st to end	Day	21% 11% 3 7% 5% 4 3 4 5% 5% 33%	2456
May	1st to 10th	Day	23% 11% 3 8% 5% 5% 4 3 5% 6% 27%	4106
	11th to 20th	Day	18% 8% 3 9% 5% 5% 5% 4 6% 7% 30%	4361
	21st to end	Day	12% 6% 2 6% 4 5% 5% 4 6% 8% 41%	4837
June	1st to 10th	Day	18% 7% 3 8% 6% 6% 5% 4 6% 7% 30%	2660
	11th to 20th	Day	17% 7% 2 6% 5% 4 5% 4 6% 8% 36%	2660
	21st to end	Day	18% 7% 3 8% 6% 4 6% 4 6% 7% 31%	2660
July	1st to 10th	Day	13% 6% 2 6% 5% 5% 5% 5% 7% 10% 36%	4440
	11th to 20th	Day	11% 5% 2 7% 5% 6% 5% 5% 9% 11% 35%	4440
	21st to end	Day	17% 6% 2 6% 5% 5% 5% 4 6% 8% 36%	4814
August	1st to 10th	Day	16% 5% 3 7% 6% 5% 6% 5% 7% 8% 32%	4311
	11th to 20th	Day	15% 7% 3 7% 5% 5% 6% 4 7% 7% 33%	3927
	21st to end	Day	11% 4 2 6% 5% 5% 5% 4 7% 10% 42%	4180
September	1st to 10th	Day	12% 5% 2 6% 4 4 6% 4 5% 8% 44%	2280
	11th to 20th	Day	13% 5% 2 6% 4 4 4 3 4 6% 49%	2135
	21st to end	Day	16% 6% 2 6% 4 4 3 4 5% 7% 43%	2090
October	1st to 10th	Day	21% 9% 2 7% 5% 4 4 3 5% 5% 36%	2071
	11th to 20th	Day	15% 8% 3 6% 5% 4 4 4 6% 7% 39%	1827
	21st to end	Day	22% 10% 3 8% 5% 4 4 3 4 5% 32%	1881
November	1st to 10th	Day	13% 8% 3 6% 4 4 5% 4 5% 8% 41%	1761
	11th to 20th	Day	16% 10% 3 7% 4 4 3 3 5% 5% 39%	1800
	21st to end	Day	19% 12% 3 6% 5% 4 4 3 5% 6% 34%	1704
December	1st to 10th	Day	15% 15% 5% 11% 6% 4 5% 4 6% 6% 24%	2600
	11th to 20th	Day	21% 14% 4 6% 4 3 3 3 4 5% 32%	2986
	21st to end	Day	20% 12% 5% 11% 4 4 4 3 3 5% 30%	3960
January	1st to 10th	Night	13% 8% 6% 17% 6% 4 4 3 5% 6% 27%	6349
	11th to 20th	Night	10% 12% 5% 14% 6% 6% 5% 5% 5% 6% 26%	6315
	21st to end	Night	13% 10% 4 10% 6% 5% 5% 4 5% 6% 31%	6996
February	1st to 10th	Night	13% 10% 5% 10% 6% 6% 5% 5% 6% 6% 29%	2610
	11th to 20th	Night	10% 10% 4 8% 6% 6% 5% 5% 7% 8% 30%	2338
	21st to end	Night	14% 10% 2 8% 6% 5% 5% 4 7% 5% 33%	1980
March	1st to 10th	Night	23% 12% 3 7% 5% 5% 4 3 5% 6% 27%	3844
	11th to 20th	Night	13% 11% 4 8% 6% 5% 5% 4 5% 6% 33%	3820
	21st to end	Night	19% 12% 3 8% 4 4 4 3 4 5% 33%	3547
April	1st to 10th	Night	18% 9% 3 7% 5% 5% 4 3 6% 7% 34%	1900
	11th to 20th	Night	18% 10% 3 7% 3 3 5% 4 5% 6% 37%	1875
	21st to end	Night	20% 8% 3 8% 5% 3 4 3 6% 5% 36%	1724
May	1st to 10th	Night	25% 10% 3 7% 4 5% 4 3 6% 7% 27%	2834
	11th to 20th	Night	17% 9% 3 9% 7% 5% 5% 4 5% 6% 28%	2561
	21st to end	Night	15% 5% 2 6% 5% 4 4 4 6% 8% 40%	2738
June	1st to 10th	Night	19% 6% 2 7% 6% 4 5% 4 6% 8% 34%	1520
	11th to 20th	Night	18% 6% 2 7% 4 3 3 4 5% 7% 39%	1520
	21st to end	Night	17% 5% 3 6% 5% 6% 5% 5% 7% 7% 36%	1520
July	1st to 10th	Night	12% 4 2 6% 6% 5% 5% 5% 7% 8% 40%	2520
	11th to 20th	Night	16% 5% 2 5% 4 5% 4 4 5% 8% 42%	2520
	21st to end	Night	18% 6% 2 5% 4 3 5% 4 6% 7% 41%	2739
August	1st to 10th	Night	21% 8% 3 7% 4 4 4 3 5% 7% 34%	2587
	11th to 20th	Night	17% 7% 2 6% 4 4 4 4 6% 7% 39%	2897
	21st to end	Night	9% 4 2 5% 4 4 4 3 6% 9% 49%	3476
September	1st to 10th	Night	13% 6% 2 4 3 4 3 3 5% 6% 51%	1900
	11th to 20th	Night	14% 6% 2 5% 4 3 3 3 5% 5% 51%	2044
	21st to end	Night	14% 7% 2 6% 3 3 3 3 5% 7% 46%	2090
October	1st to 10th	Night	19% 8% 2 7% 5% 4 3 3 5% 6% 36%	2107
	11th to 20th	Night	15% 8% 2 7% 5% 5% 4 4 6% 7% 37%	2351
	21st to end	Night	22% 9% 3 7% 5% 4 4 4 5% 5% 32%	2717
November	1st to 10th	Night	14% 8% 2 7% 5% 4 4 3 5% 7% 40%	2537
	11th to 20th	Night	15% 9% 3 6% 5% 4 4 4 5% 6% 38%	2600
	21st to end	Night	15% 10% 3 8% 5% 5% 5% 4 6% 6% 34%	2694
December	1st to 10th	Night	17% 12% 4 12% 6% 6% 5% 3 5% 6% 25%	4580
	11th to 20th	Night	24% 12% 4 8% 4 3 4 3 4 4 31%	5208
	21st to end	Night	21% 15% 5% 9% 5% 4 3 3 4 5% 26%	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	59% 4 9% 21% 6%	3598
	11th to 20th	Day	54% 6% 10% 17% 10% 3	3600
	21st to end	Day	53% 4 8% 18% 15% 3	3960
February	1st to 10th	Day	54% 6% 13% 17% 9% 2	1787
	11th to 20th	Day	54% 6% 11% 18% 10% 2	1957
	21st to end	Day	54% 6% 10% 17% 12% 1	1650
March	1st to 10th	Day	63% 5% 8% 12% 10% 2	3219
	11th to 20th	Day	49% 8% 15% 15% 10% 2	3358
	21st to end	Day	53% 6% 6% 14% 18% 3	4222
April	1st to 10th	Day	51% 6% 8% 20% 12% 3	2280
	11th to 20th	Day	48% 7% 10% 19% 15% 2	2254
	21st to end	Day	54% 6% 8% 16% 13% 3	2456
May	1st to 10th	Day	57% 7% 10% 18% 7% 1	4120
	11th to 20th	Day	51% 9% 14% 16% 8% 2	4365
	21st to end	Day	37% 9% 13% 26% 14% 1	4837
June	1st to 10th	Day	50% 8% 13% 19% 9% 1	2660
	11th to 20th	Day	45% 7% 12% 19% 15% 2	2660
	21st to end	Day	52% 8% 12% 17% 10% 1	2660
July	1st to 10th	Day	42% 6% 14% 23% 15% 1	4370
	11th to 20th	Day	41% 6% 18% 25% 10%	4365
	21st to end	Day	47% 6% 13% 21% 13% 1	4661
August	1st to 10th	Day	48% 7% 14% 20% 10%	4297
	11th to 20th	Day	47% 6% 16% 24% 6%	3966
	21st to end	Day	36% 6% 16% 27% 13% 1	4180
September	1st to 10th	Day	36% 7% 13% 27% 15% 1	2280
	11th to 20th	Day	36% 6% 9% 27% 20% 2	2135
	21st to end	Day	39% 6% 13% 25% 15% 2	2090
October	1st to 10th	Day	48% 4 8% 22% 16% 1	3448
	11th to 20th	Day	42% 4 12% 24% 16% 1	3017
	21st to end	Day	41% 4 8% 31% 13% 2	3081
November	1st to 10th	Day	40% 7% 15% 23% 14% 1	1761
	11th to 20th	Day	47% 5% 12% 18% 17% 2	1800
	21st to end	Day	52% 6% 15% 19% 8% 1	1704
December	1st to 10th	Day	60% 4 14% 17% 5% 1	2600
	11th to 20th	Day	56% 5% 9% 17% 13% 1	2986
	21st to end	Day	57% 4 7% 16% 13% 3	3960
January	1st to 10th	Night	57% 6% 10% 18% 8% 1	6349
	11th to 20th	Night	58% 6% 11% 14% 9% 1	6360
	21st to end	Night	53% 6% 8% 14% 15% 4	6996
February	1st to 10th	Night	52% 8% 11% 18% 10% 1	2610
	11th to 20th	Night	49% 8% 12% 18% 12% 1	2340
	21st to end	Night	49% 8% 11% 21% 8% 3	1980
March	1st to 10th	Night	57% 6% 9% 16% 10% 2	3854
	11th to 20th	Night	50% 8% 11% 19% 10% 1	3821
	21st to end	Night	52% 6% 7% 13% 19% 4	3547
April	1st to 10th	Night	50% 5% 11% 19% 13% 2	1900
	11th to 20th	Night	48% 5% 10% 21% 14% 1	1875
	21st to end	Night	50% 5% 10% 17% 14% 3	1724
May	1st to 10th	Night	55% 9% 11% 15% 9% 1	2840
	11th to 20th	Night	54% 10% 13% 15% 8% 2	2562
	21st to end	Night	40% 9% 15% 21% 13% 2	2739
June	1st to 10th	Night	48% 8% 13% 20% 11%	1520
	11th to 20th	Night	45% 7% 15% 20% 12% 1	1520
	21st to end	Night	46% 9% 13% 21% 10% 1	1520
July	1st to 10th	Night	41% 6% 17% 23% 13% 1	2480
	11th to 20th	Night	41% 6% 15% 26% 13%	2472
	21st to end	Night	44% 6% 11% 28% 12% 1	2651
August	1st to 10th	Night	50% 7% 12% 21% 9%	2577
	11th to 20th	Night	44% 7% 18% 21% 9%	2924
	21st to end	Night	31% 5% 19% 30% 13% 1	3476
September	1st to 10th	Night	34% 7% 15% 29% 16%	1900
	11th to 20th	Night	35% 5% 10% 29% 19% 1	2044
	21st to end	Night	37% 6% 12% 28% 15% 2	2090
October	1st to 10th	Night	48% 5% 13% 23% 10% 1	3510
	11th to 20th	Night	40% 5% 11% 24% 18% 2	3900
	21st to end	Night	44% 4 10% 30% 11% 1	4467
November	1st to 10th	Night	44% 7% 12% 22% 14% 2	2537
	11th to 20th	Night	46% 7% 10% 21% 15% 2	2600
	21st to end	Night	50% 6% 14% 21% 8%	2694
December	1st to 10th	Night	61% 4 12% 18% 4	4580
	11th to 20th	Night	57% 5% 8% 13% 15% 1	5208
	21st to end	Night	60% 4 7% 15% 10% 3	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	31% 25% 26% 14% 5%	200
	11th to 20th	Day	31% 27% 29% 12% 1 1	200
	21st to end	Day	23% 30% 32% 10% 5%	220
February	1st to 10th	Day	31% 33% 27% 8% 1	315
	11th to 20th	Day	32% 29% 26% 11% 1	496
	21st to end	Day	30% 25% 30% 12% 2	414
March	1st to 10th	Day	31% 22% 33% 12% 2	500
	11th to 20th	Day	32% 26% 27% 12% 4	500
	21st to end	Day	33% 22% 26% 12% 5% 3	530
April	1st to 10th	Day	36% 19% 25% 13% 6% 1	460
	11th to 20th	Day	34% 20% 27% 11% 6% 2	452
	21st to end	Day	30% 22% 30% 10% 7% 1	460
May	1st to 10th	Day	32% 21% 26% 13% 6% 2	463
	11th to 20th	Day	21% 19% 35% 19% 4 2	487
	21st to end	Day	20% 15% 32% 24% 8% 1	529
June	1st to 10th	Day	20% 18% 37% 15% 9% 1	490
	11th to 20th	Day	21% 16% 33% 19% 9% 2	490
	21st to end	Day	21% 19% 31% 18% 9% 2	490
July	1st to 10th	Day	15% 13% 33% 24% 12% 3	485
	11th to 20th	Day	14% 9% 38% 27% 10% 2	484
	21st to end	Day	13% 11% 38% 28% 7% 2	532
August	1st to 10th	Day	13% 13% 32% 26% 14% 3	486
	11th to 20th	Day	16% 10% 33% 21% 13% 7%	479
	21st to end	Day	14% 13% 29% 25% 13% 6%	539
September	1st to 10th	Day	16% 12% 28% 24% 15% 5%	490
	11th to 20th	Day	16% 13% 25% 19% 17% 9%	490
	21st to end	Day	19% 14% 29% 21% 12% 6%	490
October	1st to 10th	Day	24% 13% 27% 19% 11% 5%	490
	11th to 20th	Day	23% 16% 27% 23% 8% 2	451
	21st to end	Day	34% 17% 23% 18% 7% 1	429
November	1st to 10th	Day	24% 20% 26% 19% 10% 1	395
	11th to 20th	Day	29% 22% 25% 17% 6% 1	400
	21st to end	Day	36% 20% 26% 15% 3 1	304
December	1st to 10th	Day	27% 27% 26% 17% 4	200
	11th to 20th	Day	36% 27% 28% 6% 4	200
	21st to end	Day	33% 18% 31% 13% 5%	220
January	1st to 10th	Night	23% 24% 32% 19% 2	800
	11th to 20th	Night	25% 20% 33% 18% 3 1	800
	21st to end	Night	19% 22% 36% 20% 3	880
February	1st to 10th	Night	23% 23% 36% 17% 2	800
	11th to 20th	Night	23% 23% 36% 16% 2	783
	21st to end	Night	25% 20% 32% 20% 3	660
March	1st to 10th	Night	27% 20% 31% 20% 2	800
	11th to 20th	Night	25% 21% 29% 21% 4	800
	21st to end	Night	29% 18% 28% 20% 6%	863
April	1st to 10th	Night	27% 19% 27% 22% 4	760
	11th to 20th	Night	23% 23% 27% 20% 7%	751
	21st to end	Night	26% 17% 30% 23% 4	584
May	1st to 10th	Night	28% 15% 29% 24% 5%	570
	11th to 20th	Night	19% 14% 31% 29% 6%	414
	21st to end	Night	17% 10% 30% 33% 10% 1	411
June	1st to 10th	Night	20% 9% 29% 31% 11%	380
	11th to 20th	Night	18% 13% 26% 31% 12% 1	380
	21st to end	Night	17% 9% 25% 39% 9% 1	380
July	1st to 10th	Night	15% 6% 26% 40% 12% 1	370
	11th to 20th	Night	14% 4 23% 41% 16% 2	368
	21st to end	Night	13% 5% 27% 40% 15% 1	416
August	1st to 10th	Night	13% 4 22% 42% 17% 1	420
	11th to 20th	Night	14% 7% 29% 31% 16% 3	625
	21st to end	Night	16% 6% 22% 36% 19% 2	836
September	1st to 10th	Night	15% 8% 27% 30% 19% 2	760
	11th to 20th	Night	15% 9% 24% 27% 20% 4	760
	21st to end	Night	18% 9% 25% 30% 16% 2	760
October	1st to 10th	Night	26% 11% 20% 27% 14% 1	760
	11th to 20th	Night	21% 13% 27% 26% 11% 1	760
	21st to end	Night	27% 17% 27% 23% 5%	836
November	1st to 10th	Night	20% 18% 27% 26% 8% 1	780
	11th to 20th	Night	23% 20% 29% 21% 7% 1	800
	21st to end	Night	24% 19% 30% 21% 5%	800
December	1st to 10th	Night	24% 21% 33% 18% 4	800
	11th to 20th	Night	28% 21% 29% 18% 3	800
	21st to end	Night	28% 21% 32% 17% 2	880

Page generated at 2025-03-24 09:04:30. Version ac07

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