Penryn
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Penryn clear sky chart from 2005-11-06 to 2026-03-18.

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	8% 3 1 3 2 2 2 2 3 5% 67%	139987
	Night	11% 4 1 4 2 2 3 2 4 5% 61%	138933
Transparency	Day	22% 3 7% 18% 25% 24%	134925
	Night	27% 4 8% 18% 23% 20%	134980
Seeing	Day	10% 6% 19% 31% 27% 7%	16876
	Night	12% 3 14% 38% 29% 4	26298

By Month

Percentage Cloud Cover SW (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	25% 7% 1 3 3 3 3 2 4 7% 41%	4722
	11th to 20th	Day	13% 5% 1 5% 3 2 2 2 3 4 59%	5202
	21st to end	Day	16% 6% 2 5% 4 4 4 4 5% 7% 44%	5843
February	1st to 10th	Day	20% 6% 2 5% 3 3 4 3 4 5% 46%	1890
	11th to 20th	Day	18% 6% 2 4 4 4 4 3 4 6% 45%	1977
	21st to end	Day	16% 7% 2 5% 3 3 3 3 5% 6% 48%	1730
March	1st to 10th	Day	17% 10% 3 7% 4 4 4 3 5% 6% 37%	4770
	11th to 20th	Day	17% 6% 2 5% 3 3 3 3 4 6% 48%	5078
	21st to end	Day	16% 7% 2 6% 4 4 4 3 5% 8% 38%	5696
April	1st to 10th	Day	14% 7% 2 3 3 3 4 4 5% 7% 48%	2400
	11th to 20th	Day	11% 4 2 3 3 3 3 3 5% 6% 57%	2369
	21st to end	Day	7% 4 1 3 2 2 3 3 4 7% 62%	2400
May	1st to 10th	Day	5% 2 1 2 1 2 2 1 2 4 77%	5577
	11th to 20th	Day	3 1 1 2 1 2 2 2 3 4 79%	6277
	21st to end	Day	2 1 1 2 1 1 1 1 3 5% 81%	6902
June	1st to 10th	Day	5% 3 1 3 3 2 2 3 4 6% 69%	2800
	11th to 20th	Day	1 1 2 1 2 1 2 3 5% 81%	2800
	21st to end	Day	2 1 1 1 1 1 1 1 3 3 85%	2800
July	1st to 10th	Day	1 1 1 1 1 1 94%	6319
	11th to 20th	Day	1 1 1 1 1 1 1 2 3 89%	6320
	21st to end	Day	1 1 2 1 2 2 2 3 4 84%	6413
August	1st to 10th	Day	1 1 1 1 1 1 1 2 4 85%	5777
	11th to 20th	Day	1 1 1 1 1 2 3 90%	5453
	21st to end	Day	2 1 1 1 1 1 1 2 4 85%	5984
September	1st to 10th	Day	2 1 1 1 1 1 2 4 88%	2400
	11th to 20th	Day	3 2 1 2 2 2 2 2 4 4 77%	2400
	21st to end	Day	4 2 1 2 1 2 1 1 2 4 79%	2160
October	1st to 10th	Day	2 2 1 3 2 2 2 2 4 5% 75%	3280
	11th to 20th	Day	4 2 1 2 1 2 2 2 3 5% 74%	3261
	21st to end	Day	5% 3 2 4 3 3 3 3 4 7% 64%	3558
November	1st to 10th	Day	11% 3 1 3 3 3 3 3 6% 7% 57%	1896
	11th to 20th	Day	15% 4 2 4 4 4 3 3 5% 7% 50%	1890
	21st to end	Day	13% 3 1 4 2 3 3 4 5% 7% 55%	1743
December	1st to 10th	Day	9% 3 1 4 2 3 4 3 4 8% 58%	2700
	11th to 20th	Day	29% 7% 2 5% 3 3 4 4 5% 7% 32%	3108
	21st to end	Day	27% 7% 2 5% 3 4 4 4 6% 8% 28%	4092
January	1st to 10th	Night	20% 7% 3 5% 3 4 4 3 5% 8% 40%	8172
	11th to 20th	Night	13% 4 1 3 2 2 3 3 4 5% 60%	7764
	21st to end	Night	16% 6% 2 7% 4 3 4 3 4 6% 45%	8371
February	1st to 10th	Night	19% 5% 2 4 3 3 3 3 4 5% 50%	2730
	11th to 20th	Night	16% 5% 2 5% 3 4 4 2 5% 7% 45%	2530
	21st to end	Night	17% 5% 1 4 3 3 3 3 4 5% 51%	2076
March	1st to 10th	Night	20% 9% 2 6% 4 4 4 3 4 6% 38%	5328
	11th to 20th	Night	22% 6% 2 4 3 3 2 2 3 5% 49%	5077
	21st to end	Night	20% 7% 2 5% 3 3 4 3 6% 7% 39%	5278
April	1st to 10th	Night	12% 5% 2 5% 4 4 4 3 4 6% 50%	2000
	11th to 20th	Night	10% 4 1 5% 3 3 3 3 5% 7% 56%	1979
	21st to end	Night	7% 3 1 3 2 2 3 3 4 7% 64%	2000
May	1st to 10th	Night	4 3 1 3 2 2 2 2 3 5% 75%	4399
	11th to 20th	Night	3 2 1 2 2 2 2 2 3 5% 77%	3616
	21st to end	Night	2 1 1 1 1 2 2 3 4 83%	3972
June	1st to 10th	Night	5% 3 1 3 2 3 3 3 5% 6% 68%	1600
	11th to 20th	Night	1 1 2 2 1 2 1 3 4 83%	1600
	21st to end	Night	2 1 1 1 2 2 2 3 3 84%	1600
July	1st to 10th	Night	1 1 1 1 1 2 93%	3640
	11th to 20th	Night	1 1 1 1 1 1 1 1 3 88%	3640
	21st to end	Night	1 1 1 2 1 2 2 2 3 5% 80%	4443
August	1st to 10th	Night	2 1 1 2 1 1 1 2 3 4 82%	4035
	11th to 20th	Night	1 1 1 1 1 1 3 91%	4413
	21st to end	Night	2 1 1 2 1 2 2 1 2 3 83%	5015
September	1st to 10th	Night	1 1 1 1 1 1 1 2 3 88%	2000
	11th to 20th	Night	3 2 1 2 2 1 1 2 2 5% 78%	2000
	21st to end	Night	2 1 1 2 1 2 2 2 3 4 80%	2238
October	1st to 10th	Night	3 2 1 3 2 2 2 2 3 3 78%	3920
	11th to 20th	Night	4 2 1 2 1 1 2 2 3 5% 78%	3897
	21st to end	Night	7% 2 1 3 3 3 3 3 4 5% 66%	4253
November	1st to 10th	Night	12% 5% 1 3 2 2 3 3 5% 7% 56%	2620
	11th to 20th	Night	12% 5% 2 3 3 3 4 3 5% 7% 54%	2730
	21st to end	Night	13% 4 1 3 2 3 3 2 4 6% 57%	2875
December	1st to 10th	Night	11% 3 1 3 2 2 4 3 4 7% 61%	4700
	11th to 20th	Night	27% 7% 2 5% 3 3 4 4 6% 9% 30%	5316
	21st to end	Night	27% 7% 3 7% 5% 4 4 3 4 6% 31%	7106

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	44% 6% 15% 21% 11% 4	4723
	11th to 20th	Day	32% 3 4 14% 26% 21%	5202
	21st to end	Day	40% 4 8% 18% 20% 11%	5843
February	1st to 10th	Day	40% 5% 9% 19% 17% 10%	1890
	11th to 20th	Day	40% 5% 10% 20% 18% 7%	1986
	21st to end	Day	38% 5% 11% 22% 17% 7%	1730
March	1st to 10th	Day	46% 7% 8% 18% 19% 2	4770
	11th to 20th	Day	37% 5% 11% 28% 11% 8%	5078
	21st to end	Day	42% 8% 14% 24% 11% 1	5696
April	1st to 10th	Day	35% 6% 11% 26% 17% 5%	2400
	11th to 20th	Day	28% 5% 13% 26% 21% 7%	2372
	21st to end	Day	22% 5% 14% 29% 26% 4	2400
May	1st to 10th	Day	14% 2 8% 26% 33% 17%	5577
	11th to 20th	Day	11% 3 6% 27% 42% 11%	6280
	21st to end	Day	9% 3 8% 30% 42% 8%	6904
June	1st to 10th	Day	17% 5% 10% 26% 30% 12%	2800
	11th to 20th	Day	8% 2 8% 19% 36% 26%	2800
	21st to end	Day	8% 1 3 13% 36% 39%	2800
July	1st to 10th	Day	3 1 1 7% 18% 71%	6251
	11th to 20th	Day	5% 2 5% 11% 76%	6245
	21st to end	Day	7% 2 4 14% 26% 48%	6227
August	1st to 10th	Day	9% 1 3 7% 23% 57%	4206
	11th to 20th	Day	4 1 2 7% 35% 51%	3946
	21st to end	Day	6% 1 2 9% 38% 45%	4311
September	1st to 10th	Day	5% 1 3 9% 29% 53%	2400
	11th to 20th	Day	13% 3 6% 20% 37% 21%	2400
	21st to end	Day	12% 2 4 18% 33% 30%	2160
October	1st to 10th	Day	13% 2 5% 12% 25% 44%	3280
	11th to 20th	Day	14% 3 5% 20% 38% 18%	3261
	21st to end	Day	21% 3 8% 18% 35% 15%	3558
November	1st to 10th	Day	26% 6% 10% 23% 23% 12%	1896
	11th to 20th	Day	35% 5% 12% 22% 20% 7%	1890
	21st to end	Day	29% 7% 12% 20% 22% 12%	1743
December	1st to 10th	Day	26% 4 10% 26% 25% 9%	2700
	11th to 20th	Day	51% 5% 10% 16% 14% 5%	3108
	21st to end	Day	51% 7% 16% 15% 8% 2	4092
January	1st to 10th	Night	43% 6% 16% 20% 10% 6%	8172
	11th to 20th	Night	28% 4 7% 14% 29% 17%	7764
	21st to end	Night	41% 3 8% 15% 21% 12%	8371
February	1st to 10th	Night	38% 3 7% 21% 21% 10%	2730
	11th to 20th	Night	38% 5% 8% 22% 19% 7%	2532
	21st to end	Night	35% 5% 10% 24% 20% 7%	2076
March	1st to 10th	Night	46% 7% 7% 19% 15% 5%	5328
	11th to 20th	Night	39% 5% 9% 23% 16% 8%	5077
	21st to end	Night	43% 6% 13% 22% 13% 3	5278
April	1st to 10th	Night	35% 6% 12% 20% 21% 6%	2000
	11th to 20th	Night	28% 6% 13% 26% 16% 11%	1979
	21st to end	Night	21% 5% 12% 31% 24% 6%	2000
May	1st to 10th	Night	14% 4 12% 24% 27% 19%	4396
	11th to 20th	Night	13% 2 7% 29% 38% 11%	3616
	21st to end	Night	8% 2 9% 32% 41% 8%	3972
June	1st to 10th	Night	17% 5% 11% 28% 28% 11%	1600
	11th to 20th	Night	8% 2 7% 20% 34% 28%	1600
	21st to end	Night	9% 2 3 11% 34% 41%	1600
July	1st to 10th	Night	3 1 1 7% 23% 66%	3600
	11th to 20th	Night	6% 2 5% 10% 76%	3592
	21st to end	Night	10% 1 4 14% 26% 45%	4317
August	1st to 10th	Night	12% 1 3 8% 25% 51%	2929
	11th to 20th	Night	5% 1 2 8% 37% 47%	3188
	21st to end	Night	8% 1 2 9% 34% 46%	3608
September	1st to 10th	Night	6% 1 2 8% 30% 53%	2000
	11th to 20th	Night	13% 2 6% 18% 38% 23%	2000
	21st to end	Night	11% 2 6% 16% 30% 35%	2238
October	1st to 10th	Night	14% 2 3 11% 20% 50%	3920
	11th to 20th	Night	12% 2 6% 25% 32% 23%	3897
	21st to end	Night	21% 3 6% 18% 32% 20%	4253
November	1st to 10th	Night	27% 5% 10% 23% 20% 16%	2620
	11th to 20th	Night	30% 4 12% 25% 21% 9%	2730
	21st to end	Night	29% 4 8% 23% 22% 14%	2875
December	1st to 10th	Night	24% 3 9% 26% 24% 12%	4700
	11th to 20th	Night	51% 5% 8% 16% 16% 4	5316
	21st to end	Night	55% 7% 11% 15% 9% 3	7106

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	39% 6% 18% 21% 13% 3	220
	11th to 20th	Day	20% 7% 16% 27% 23% 7%	383
	21st to end	Day	19% 8% 20% 26% 22% 5%	473
February	1st to 10th	Day	22% 9% 18% 25% 22% 4	430
	11th to 20th	Day	22% 13% 24% 24% 14% 3	497
	21st to end	Day	20% 8% 25% 30% 15% 2	446
March	1st to 10th	Day	24% 14% 26% 22% 13% 2	540
	11th to 20th	Day	21% 13% 23% 21% 16% 5%	529
	21st to end	Day	19% 13% 30% 22% 14% 3	550
April	1st to 10th	Day	17% 10% 26% 25% 18% 4	500
	11th to 20th	Day	14% 9% 30% 25% 17% 5%	492
	21st to end	Day	8% 10% 22% 31% 23% 7%	500
May	1st to 10th	Day	8% 9% 31% 27% 21% 5%	503
	11th to 20th	Day	7% 9% 23% 28% 27% 7%	528
	21st to end	Day	5% 6% 20% 32% 28% 10%	573
June	1st to 10th	Day	4 7% 18% 32% 31% 8%	530
	11th to 20th	Day	3 6% 22% 37% 28% 4	530
	21st to end	Day	1 4 15% 38% 33% 9%	530
July	1st to 10th	Day	1 1 13% 42% 39% 5%	526
	11th to 20th	Day	1 7% 44% 42% 6%	524
	21st to end	Day	1 10% 41% 39% 9%	575
August	1st to 10th	Day	2 1 13% 40% 37% 7%	527
	11th to 20th	Day	1 2 13% 38% 37% 9%	517
	21st to end	Day	1 1 10% 35% 45% 8%	583
September	1st to 10th	Day	2 2 13% 42% 33% 9%	530
	11th to 20th	Day	5% 5% 17% 34% 31% 8%	530
	21st to end	Day	4 2 20% 36% 29% 9%	530
October	1st to 10th	Day	3 7% 21% 30% 29% 11%	530
	11th to 20th	Day	6% 6% 16% 26% 33% 12%	530
	21st to end	Day	9% 8% 17% 28% 28% 10%	583
November	1st to 10th	Day	13% 7% 13% 29% 29% 9%	372
	11th to 20th	Day	16% 3 19% 29% 25% 8%	330
	21st to end	Day	12% 4 18% 32% 25% 8%	253
December	1st to 10th	Day	17% 6% 17% 27% 23% 10%	220
	11th to 20th	Day	22% 7% 19% 24% 20% 9%	220
	21st to end	Day	25% 10% 18% 26% 18% 3	242
January	1st to 10th	Night	25% 4 14% 30% 21% 6%	840
	11th to 20th	Night	17% 4 12% 39% 24% 4	840
	21st to end	Night	20% 2 15% 36% 22% 5%	924
February	1st to 10th	Night	22% 3 14% 35% 22% 3	840
	11th to 20th	Night	18% 6% 18% 35% 19% 4	824
	21st to end	Night	18% 4 20% 39% 16% 2	692
March	1st to 10th	Night	22% 7% 17% 32% 19% 3	840
	11th to 20th	Night	21% 5% 14% 32% 25% 3	828
	21st to end	Night	19% 5% 20% 34% 20% 2	880
April	1st to 10th	Night	15% 3 17% 39% 24% 2	800
	11th to 20th	Night	11% 5% 20% 35% 25% 4	791
	21st to end	Night	8% 4 17% 39% 29% 4	800
May	1st to 10th	Night	6% 5% 23% 36% 27% 3	743
	11th to 20th	Night	7% 2 16% 41% 31% 2	394
	21st to end	Night	6% 2 15% 40% 36% 1	433
June	1st to 10th	Night	5% 2 17% 42% 32% 2	400
	11th to 20th	Night	2 1 16% 49% 31% 2	400
	21st to end	Night	2 1 10% 48% 38% 2	400
July	1st to 10th	Night	1 8% 52% 38% 1	390
	11th to 20th	Night	2 5% 56% 35% 2	388
	21st to end	Night	1 6% 43% 46% 4	641
August	1st to 10th	Night	3 1 7% 46% 42% 2	597
	11th to 20th	Night	1 7% 47% 41% 4	749
	21st to end	Night	2 5% 43% 47% 3	880
September	1st to 10th	Night	2 1 6% 40% 47% 5%	800
	11th to 20th	Night	4 2 10% 37% 42% 6%	800
	21st to end	Night	3 2 13% 40% 34% 9%	800
October	1st to 10th	Night	6% 2 12% 41% 32% 7%	800
	11th to 20th	Night	7% 3 10% 37% 38% 5%	800
	21st to end	Night	7% 3 13% 36% 30% 11%	880
November	1st to 10th	Night	13% 3 11% 34% 32% 7%	820
	11th to 20th	Night	14% 2 15% 38% 25% 6%	840
	21st to end	Night	15% 2 14% 39% 25% 6%	840
December	1st to 10th	Night	18% 4 13% 35% 23% 7%	840
	11th to 20th	Night	28% 3 12% 31% 22% 4	840
	21st to end	Night	27% 3 18% 32% 17% 3	924

Page generated at 2026-03-14 23:23:50. Version ac07

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