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 2026-07-31.

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	16% 9% 3 8% 5% 5% 5% 4 5% 7% 33%	138035
	Night	16% 9% 3 8% 5% 5% 4 4 5% 6% 33%	135003
Transparency	Day	50% 5% 11% 20% 12% 1	141544
	Night	50% 5% 11% 20% 12% 2	139583
Seeing	Day	25% 18% 30% 17% 8% 2	17040
	Night	22% 16% 29% 25% 8% 1	25924

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	9% 9% 6% 20% 7% 5% 5% 3 5% 5% 25%	4661
	11th to 20th	Day	10% 14% 5% 13% 7% 5% 4 3 5% 6% 29%	4650
	21st to end	Day	10% 12% 5% 9% 6% 6% 5% 3 5% 6% 34%	5124
February	1st to 10th	Day	12% 12% 4 9% 7% 5% 6% 4 5% 8% 29%	1877
	11th to 20th	Day	10% 11% 5% 11% 7% 6% 3 4 5% 6% 32%	2048
	21st to end	Day	16% 13% 5% 9% 5% 5% 4 3 5% 5% 31%	1730
March	1st to 10th	Day	19% 14% 4 11% 5% 5% 4 3 4 4 29%	4576
	11th to 20th	Day	14% 11% 4 7% 5% 5% 4 4 5% 6% 34%	4760
	21st to end	Day	22% 12% 3 8% 4 4 4 3 4 6% 31%	6072
April	1st to 10th	Day	19% 11% 3 8% 4 4 4 3 4 6% 34%	2520
	11th to 20th	Day	16% 9% 3 7% 4 5% 5% 4 5% 7% 35%	2494
	21st to end	Day	21% 10% 3 7% 5% 4 3 4 5% 5% 34%	2714
May	1st to 10th	Day	29% 9% 3 8% 5% 5% 4 3 5% 6% 23%	7639
	11th to 20th	Day	20% 7% 3 8% 5% 5% 5% 4 5% 7% 31%	8157
	21st to end	Day	19% 8% 3 7% 4 5% 5% 4 6% 8% 31%	9060
June	1st to 10th	Day	17% 7% 2 8% 5% 6% 5% 4 6% 7% 31%	2940
	11th to 20th	Day	17% 7% 2 6% 5% 4 5% 4 6% 9% 36%	2940
	21st to end	Day	18% 7% 3 8% 6% 4 6% 4 6% 7% 32%	2940
July	1st to 10th	Day	14% 6% 2 6% 6% 5% 5% 4 7% 10% 34%	6500
	11th to 20th	Day	10% 5% 2 7% 5% 6% 6% 5% 8% 10% 37%	6500
	21st to end	Day	15% 5% 2 6% 4 4 5% 4 6% 9% 41%	7066
August	1st to 10th	Day	16% 5% 3 7% 6% 5% 5% 5% 7% 8% 33%	4448
	11th to 20th	Day	15% 7% 3 7% 5% 5% 6% 4 7% 7% 34%	4053
	21st to end	Day	11% 4 2 6% 5% 5% 5% 4 7% 10% 42%	4312
September	1st to 10th	Day	12% 5% 2 6% 4 4 6% 4 5% 8% 45%	2400
	11th to 20th	Day	12% 5% 2 6% 4 4 4 3 4 6% 49%	2247
	21st to end	Day	16% 6% 2 6% 4 4 4 4 5% 7% 43%	2200
October	1st to 10th	Day	20% 8% 2 6% 4 4 3 3 4 5% 38%	2180
	11th to 20th	Day	15% 8% 3 6% 5% 4 4 3 5% 7% 39%	1923
	21st to end	Day	22% 10% 4 8% 5% 4 4 3 4 5% 32%	1980
November	1st to 10th	Day	13% 8% 3 6% 4 4 5% 4 5% 8% 40%	1851
	11th to 20th	Day	16% 10% 3 7% 4 4 3 3 5% 5% 39%	1890
	21st to end	Day	19% 12% 3 7% 5% 4 4 3 5% 5% 33%	1789
December	1st to 10th	Day	15% 15% 5% 10% 5% 4 5% 4 6% 6% 25%	2680
	11th to 20th	Day	20% 14% 4 6% 4 3 3 3 4 5% 32%	3066
	21st to end	Day	20% 12% 5% 11% 4 4 4 3 4 5% 30%	4048
January	1st to 10th	Night	12% 10% 6% 16% 7% 5% 5% 3 5% 5% 26%	8234
	11th to 20th	Night	11% 13% 5% 14% 6% 6% 5% 5% 5% 6% 23%	8235
	21st to end	Night	11% 9% 4 9% 6% 7% 6% 4 6% 7% 31%	9057
February	1st to 10th	Night	12% 10% 5% 10% 6% 5% 6% 5% 6% 6% 30%	2740
	11th to 20th	Night	9% 10% 4 9% 6% 6% 5% 5% 7% 8% 30%	2458
	21st to end	Night	14% 10% 2 8% 6% 5% 5% 4 7% 5% 33%	2076
March	1st to 10th	Night	21% 10% 3 8% 5% 5% 5% 4 5% 6% 29%	5484
	11th to 20th	Night	15% 11% 4 8% 6% 5% 5% 4 4 6% 34%	5418
	21st to end	Night	22% 12% 3 8% 4 4 4 3 4 5% 30%	5087
April	1st to 10th	Night	19% 9% 3 7% 5% 5% 4 3 5% 6% 33%	2100
	11th to 20th	Night	18% 10% 3 7% 4 3 5% 4 5% 6% 36%	2075
	21st to end	Night	18% 8% 3 7% 5% 3 4 3 6% 5% 38%	1906
May	1st to 10th	Night	30% 9% 3 6% 4 5% 4 3 5% 6% 25%	5259
	11th to 20th	Night	18% 9% 3 8% 5% 4 5% 4 5% 7% 33%	4759
	21st to end	Night	21% 6% 2 7% 5% 5% 4 4 6% 8% 32%	5114
June	1st to 10th	Night	18% 6% 2 7% 5% 4 5% 4 6% 8% 36%	1680
	11th to 20th	Night	18% 6% 2 7% 4 3 4 4 5% 7% 38%	1680
	21st to end	Night	16% 5% 3 6% 5% 6% 5% 5% 7% 7% 36%	1680
July	1st to 10th	Night	12% 4 2 6% 5% 5% 5% 4 7% 8% 41%	3680
	11th to 20th	Night	16% 5% 3 6% 4 5% 4 4 6% 8% 39%	3680
	21st to end	Night	13% 5% 2 5% 4 3 4 3 5% 6% 48%	4006
August	1st to 10th	Night	21% 8% 3 6% 4 4 4 3 5% 7% 35%	2669
	11th to 20th	Night	16% 7% 2 6% 4 4 4 4 6% 7% 40%	2991
	21st to end	Night	9% 4 2 5% 4 4 4 3 6% 9% 49%	3586
September	1st to 10th	Night	13% 6% 2 4 3 4 3 3 5% 6% 51%	2000
	11th to 20th	Night	13% 5% 2 4 4 3 3 3 5% 5% 52%	2152
	21st to end	Night	15% 7% 2 6% 3 4 3 3 5% 7% 45%	2200
October	1st to 10th	Night	19% 8% 2 7% 5% 4 3 3 5% 6% 39%	2218
	11th to 20th	Night	16% 7% 2 6% 5% 5% 4 4 6% 7% 38%	2475
	21st to end	Night	22% 9% 3 7% 5% 4 4 4 5% 5% 33%	2860
November	1st to 10th	Night	14% 8% 2 7% 5% 4 4 3 5% 7% 40%	2667
	11th to 20th	Night	15% 9% 3 7% 5% 4 4 4 5% 6% 38%	2730
	21st to end	Night	14% 10% 4 8% 5% 5% 5% 4 6% 6% 34%	2829
December	1st to 10th	Night	16% 12% 4 12% 6% 6% 5% 3 5% 6% 26%	4720
	11th to 20th	Night	23% 12% 4 8% 4 3 4 3 4 4 31%	5348
	21st to end	Night	20% 15% 5% 9% 5% 4 3 3 4 5% 26%	7150

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	60% 4 10% 20% 6%	4662
	11th to 20th	Day	56% 5% 9% 16% 11% 2	4680
	21st to end	Day	50% 4 9% 21% 13% 2	5124
February	1st to 10th	Day	53% 5% 12% 18% 10% 1	1877
	11th to 20th	Day	54% 6% 10% 19% 9% 2	2057
	21st to end	Day	54% 5% 10% 17% 12% 1	1730
March	1st to 10th	Day	61% 4 7% 12% 10% 6%	4579
	11th to 20th	Day	48% 6% 16% 19% 9% 2	4760
	21st to end	Day	55% 5% 6% 14% 17% 4	6072
April	1st to 10th	Day	52% 5% 8% 19% 13% 3	2520
	11th to 20th	Day	48% 7% 10% 18% 15% 2	2494
	21st to end	Day	53% 6% 8% 18% 14% 3	2714
May	1st to 10th	Day	62% 5% 9% 15% 8% 1	7651
	11th to 20th	Day	53% 6% 13% 18% 9% 1	8161
	21st to end	Day	50% 6% 11% 23% 10% 1	9061
June	1st to 10th	Day	50% 7% 12% 20% 10%	2940
	11th to 20th	Day	46% 7% 12% 19% 15% 2	2940
	21st to end	Day	50% 8% 12% 18% 11% 1	2940
July	1st to 10th	Day	46% 5% 14% 21% 13%	6430
	11th to 20th	Day	41% 5% 17% 24% 13% 1	6425
	21st to end	Day	42% 4 12% 28% 12% 1	6868
August	1st to 10th	Day	47% 7% 14% 21% 11%	4434
	11th to 20th	Day	47% 6% 16% 24% 7% 1	4092
	21st to end	Day	36% 6% 16% 27% 13% 1	4312
September	1st to 10th	Day	36% 7% 13% 28% 16% 1	2400
	11th to 20th	Day	35% 6% 9% 28% 21% 1	2247
	21st to end	Day	39% 6% 13% 26% 15% 2	2200
October	1st to 10th	Day	47% 4 8% 23% 17% 1	3557
	11th to 20th	Day	43% 4 12% 24% 16% 1	3113
	21st to end	Day	41% 4 8% 31% 13% 2	3180
November	1st to 10th	Day	41% 6% 14% 23% 14% 1	1851
	11th to 20th	Day	47% 5% 11% 18% 17% 2	1890
	21st to end	Day	53% 6% 15% 18% 7% 1	1789
December	1st to 10th	Day	59% 4 14% 17% 5%	2680
	11th to 20th	Day	56% 5% 9% 17% 13% 1	3066
	21st to end	Day	57% 4 7% 16% 13% 3	4048
January	1st to 10th	Night	59% 5% 10% 17% 8% 1	8234
	11th to 20th	Night	61% 6% 9% 13% 9% 2	8280
	21st to end	Night	51% 5% 8% 18% 14% 4	9057
February	1st to 10th	Night	51% 8% 10% 19% 11% 1	2740
	11th to 20th	Night	49% 8% 11% 19% 12% 1	2460
	21st to end	Night	50% 7% 11% 20% 9% 3	2076
March	1st to 10th	Night	56% 5% 8% 14% 13% 3	5494
	11th to 20th	Night	51% 6% 16% 16% 9% 2	5419
	21st to end	Night	56% 5% 5% 13% 18% 3	5087
April	1st to 10th	Night	52% 5% 10% 18% 13% 1	2100
	11th to 20th	Night	49% 5% 10% 21% 14% 1	2075
	21st to end	Night	49% 5% 10% 17% 16% 3	1906
May	1st to 10th	Night	60% 6% 11% 13% 10% 1	5265
	11th to 20th	Night	52% 6% 13% 20% 8% 1	4760
	21st to end	Night	51% 6% 11% 23% 8% 1	5115
June	1st to 10th	Night	46% 8% 12% 22% 12%	1680
	11th to 20th	Night	46% 6% 14% 19% 12% 1	1680
	21st to end	Night	46% 8% 13% 22% 10% 1	1680
July	1st to 10th	Night	40% 4 18% 26% 11%	3640
	11th to 20th	Night	45% 6% 14% 22% 13%	3632
	21st to end	Night	37% 4 13% 34% 12% 1	3889
August	1st to 10th	Night	50% 7% 12% 21% 10% 1	2659
	11th to 20th	Night	43% 6% 18% 22% 10%	3018
	21st to end	Night	31% 5% 19% 31% 13% 1	3586
September	1st to 10th	Night	34% 6% 14% 29% 17% 1	2000
	11th to 20th	Night	34% 5% 10% 29% 20% 1	2152
	21st to end	Night	38% 6% 12% 27% 16% 2	2200
October	1st to 10th	Night	46% 4 13% 24% 11% 1	3621
	11th to 20th	Night	40% 5% 12% 24% 18% 2	4024
	21st to end	Night	45% 4 10% 30% 11% 1	4610
November	1st to 10th	Night	45% 6% 11% 22% 14% 2	2667
	11th to 20th	Night	47% 6% 10% 21% 14% 2	2730
	21st to end	Night	51% 6% 14% 20% 8%	2829
December	1st to 10th	Night	61% 4 12% 19% 4	4720
	11th to 20th	Night	58% 5% 8% 13% 15% 1	5348
	21st to end	Night	60% 4 7% 15% 10% 3	7150

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	30% 24% 26% 15% 5%	220
	11th to 20th	Day	32% 25% 30% 11% 1 1	220
	21st to end	Day	24% 30% 31% 10% 5%	242
February	1st to 10th	Day	30% 32% 28% 9% 2	345
	11th to 20th	Day	31% 29% 27% 12% 1	536
	21st to end	Day	30% 25% 30% 12% 2	446
March	1st to 10th	Day	31% 21% 33% 13% 2	540
	11th to 20th	Day	33% 26% 26% 11% 3	540
	21st to end	Day	35% 22% 25% 10% 4 3	594
April	1st to 10th	Day	39% 19% 24% 11% 5% 1	540
	11th to 20th	Day	35% 20% 28% 10% 6% 2	532
	21st to end	Day	30% 22% 30% 10% 7% 1	540
May	1st to 10th	Day	35% 21% 26% 12% 5% 1	543
	11th to 20th	Day	23% 20% 34% 17% 4 2	567
	21st to end	Day	23% 16% 32% 21% 7% 1	617
June	1st to 10th	Day	20% 16% 36% 17% 9% 1	570
	11th to 20th	Day	22% 16% 34% 17% 8% 2	570
	21st to end	Day	21% 18% 33% 19% 8% 2	570
July	1st to 10th	Day	16% 13% 33% 24% 11% 3	565
	11th to 20th	Day	14% 10% 37% 27% 9% 2	564
	21st to end	Day	15% 12% 36% 26% 8% 3	616
August	1st to 10th	Day	12% 12% 32% 26% 15% 3	526
	11th to 20th	Day	15% 10% 33% 21% 13% 7%	519
	21st to end	Day	14% 13% 30% 26% 13% 5%	583
September	1st to 10th	Day	16% 11% 28% 25% 15% 5%	530
	11th to 20th	Day	15% 13% 25% 20% 17% 9%	530
	21st to end	Day	19% 14% 28% 21% 12% 5%	530
October	1st to 10th	Day	24% 13% 26% 20% 12% 5%	530
	11th to 20th	Day	24% 16% 28% 22% 8% 2	487
	21st to end	Day	34% 17% 25% 17% 7% 1	462
November	1st to 10th	Day	26% 21% 25% 18% 10% 1	425
	11th to 20th	Day	28% 22% 26% 17% 6% 1	430
	21st to end	Day	36% 19% 26% 16% 3 1	329
December	1st to 10th	Day	28% 25% 26% 16% 4	220
	11th to 20th	Day	36% 27% 28% 5% 3	220
	21st to end	Day	35% 18% 31% 12% 5%	242
January	1st to 10th	Night	24% 24% 31% 19% 2	840
	11th to 20th	Night	26% 20% 33% 18% 3	840
	21st to end	Night	20% 22% 35% 19% 3 1	924
February	1st to 10th	Night	23% 23% 35% 18% 2	840
	11th to 20th	Night	22% 23% 36% 17% 2	823
	21st to end	Night	25% 20% 33% 19% 3	692
March	1st to 10th	Night	27% 20% 31% 20% 2	840
	11th to 20th	Night	26% 21% 28% 20% 4	840
	21st to end	Night	30% 18% 27% 19% 5%	924
April	1st to 10th	Night	29% 20% 26% 21% 4	840
	11th to 20th	Night	25% 23% 27% 19% 6%	831
	21st to end	Night	25% 17% 30% 24% 4	646
May	1st to 10th	Night	29% 14% 29% 23% 5%	630
	11th to 20th	Night	19% 14% 33% 29% 5%	458
	21st to end	Night	18% 9% 30% 32% 9% 1	455
June	1st to 10th	Night	19% 9% 28% 33% 12%	420
	11th to 20th	Night	19% 13% 27% 31% 10%	420
	21st to end	Night	17% 8% 24% 41% 9% 1	420
July	1st to 10th	Night	15% 6% 25% 41% 12% 1	410
	11th to 20th	Night	14% 4 25% 40% 16% 2	408
	21st to end	Night	14% 5% 27% 40% 15% 1	458
August	1st to 10th	Night	13% 4 22% 43% 18% 1	442
	11th to 20th	Night	14% 7% 29% 31% 18% 2	659
	21st to end	Night	15% 6% 23% 35% 19% 2	880
September	1st to 10th	Night	15% 9% 26% 30% 19% 2	800
	11th to 20th	Night	14% 9% 23% 29% 20% 4	800
	21st to end	Night	18% 9% 25% 31% 15% 2	800
October	1st to 10th	Night	25% 11% 21% 27% 16% 1	800
	11th to 20th	Night	22% 13% 27% 26% 11% 1	800
	21st to end	Night	27% 16% 27% 24% 5%	880
November	1st to 10th	Night	22% 18% 27% 25% 8% 1	820
	11th to 20th	Night	23% 20% 29% 21% 7% 1	840
	21st to end	Night	24% 19% 29% 22% 5%	840
December	1st to 10th	Night	24% 21% 33% 18% 4	840
	11th to 20th	Night	29% 21% 29% 18% 3	840
	21st to end	Night	29% 21% 31% 16% 2	924

Page generated at 2026-07-28 03:25:21. Version ac07

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