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 2024-01-15.

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	9% 3 1 3 3 2 3 3 4 6% 64%	74766
	Night	11% 4 2 3 3 2 3 3 4 6% 60%	75043
Transparency	Day	23% 4 9% 19% 24% 21%	74480
	Night	27% 4 9% 19% 22% 19%	74850
Seeing	Day	10% 6% 18% 31% 28% 7%	14042
	Night	12% 3 13% 38% 30% 5%	23469

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	24% 7% 2 3 5% 3 5% 3 5% 7% 36%	1519
	11th to 20th	Day	17% 5% 2 4 4 2 3 4 4 6% 49%	1617
	21st to end	Day	18% 7% 2 5% 3 3 4 5% 4 8% 41%	1782
February	1st to 10th	Day	19% 5% 2 4 4 2 4 4 4 6% 48%	1620
	11th to 20th	Day	16% 5% 2 3 4 2 4 3 4 7% 49%	1687
	21st to end	Day	17% 6% 3 4 3 2 3 3 4 6% 49%	1480
March	1st to 10th	Day	18% 7% 3 5% 5% 3 5% 5% 5% 6% 38%	1858
	11th to 20th	Day	21% 6% 2 5% 4 2 3 4 5% 7% 41%	1980
	21st to end	Day	15% 6% 2 4 5% 2 4 4 4 7% 46%	2259
April	1st to 10th	Day	15% 6% 2 3 4 2 4 4 5% 7% 47%	2160
	11th to 20th	Day	12% 3 2 3 4 2 3 4 5% 6% 57%	2129
	21st to end	Day	8% 3 2 3 2 2 3 4 4 7% 63%	2160
May	1st to 10th	Day	6% 3 1 3 3 2 3 3 3 6% 66%	2206
	11th to 20th	Day	4 3 1 3 3 2 4 4 4 7% 67%	2477
	21st to end	Day	5% 2 2 3 3 2 3 3 5% 7% 65%	2723
June	1st to 10th	Day	5% 3 1 2 3 2 3 3 4 7% 67%	2520
	11th to 20th	Day	1 1 2 2 1 2 2 3 5% 81%	2520
	21st to end	Day	2 1 1 1 1 1 1 2 3 3 85%	2520
July	1st to 10th	Day	1 1 1 1 2 3 3 88%	2520
	11th to 20th	Day	1 1 1 1 1 1 1 2 3 87%	2520
	21st to end	Day	1 1 1 1 1 2 2 3 3 84%	2557
August	1st to 10th	Day	2 1 1 1 1 2 2 2 4 84%	2328
	11th to 20th	Day	1 1 1 1 1 2 3 4 86%	2161
	21st to end	Day	1 1 1 1 1 1 1 3 92%	2376
September	1st to 10th	Day	2 1 1 1 1 1 1 2 3 88%	2160
	11th to 20th	Day	3 1 1 2 2 2 2 3 4 4 77%	2160
	21st to end	Day	4 2 1 2 2 1 1 2 3 5% 79%	1946
October	1st to 10th	Day	4 3 1 2 3 2 2 3 3 6% 72%	1800
	11th to 20th	Day	6% 3 1 2 2 2 2 4 4 6% 70%	1800
	21st to end	Day	8% 3 2 3 3 2 3 4 3 5% 65%	1980
November	1st to 10th	Day	12% 3 2 3 3 3 3 4 6% 6% 56%	1712
	11th to 20th	Day	13% 3 2 3 4 3 4 3 5% 7% 53%	1710
	21st to end	Day	12% 3 1 4 3 3 3 5% 5% 7% 55%	1577
December	1st to 10th	Day	16% 5% 2 4 4 3 4 4 4 7% 48%	1520
	11th to 20th	Day	30% 7% 3 5% 4 2 3 3 4 7% 31%	1928
	21st to end	Day	16% 8% 3 4 5% 3 5% 5% 7% 8% 35%	2794
January	1st to 10th	Night	22% 6% 3 4 4 2 3 4 4 8% 40%	2646
	11th to 20th	Night	15% 6% 2 4 4 3 4 5% 5% 6% 46%	2443
	21st to end	Night	17% 5% 2 5% 4 2 4 4 5% 6% 46%	2574
February	1st to 10th	Night	18% 5% 2 3 3 2 3 3 4 5% 52%	2340
	11th to 20th	Night	14% 4 2 4 4 3 4 3 5% 8% 49%	2160
	21st to end	Night	18% 5% 2 3 3 2 3 4 4 5% 51%	1776
March	1st to 10th	Night	21% 7% 3 5% 5% 3 4 3 5% 5% 40%	2099
	11th to 20th	Night	20% 5% 2 3 3 3 4 4 4 8% 45%	1980
	21st to end	Night	18% 5% 2 4 4 2 5% 4 4 6% 47%	2096
April	1st to 10th	Night	13% 5% 3 5% 4 3 4 4 5% 6% 49%	1800
	11th to 20th	Night	11% 4 1 4 3 2 3 3 5% 7% 56%	1779
	21st to end	Night	8% 3 1 2 2 2 3 4 4 8% 64%	1800
May	1st to 10th	Night	7% 3 1 3 3 2 3 3 4 5% 66%	1751
	11th to 20th	Night	6% 2 1 2 3 1 3 3 5% 7% 67%	1416
	21st to end	Night	5% 2 1 2 3 2 3 3 4 7% 67%	1552
June	1st to 10th	Night	5% 3 1 2 2 2 3 4 4 6% 68%	1440
	11th to 20th	Night	1 1 2 2 1 2 2 3 5% 81%	1440
	21st to end	Night	2 1 1 1 1 2 2 3 3 83%	1440
July	1st to 10th	Night	1 1 1 1 1 1 2 2 3 89%	1440
	11th to 20th	Night	1 1 1 1 1 1 1 1 3 87%	1440
	21st to end	Night	1 1 1 1 2 1 2 2 3 5% 81%	1758
August	1st to 10th	Night	2 1 1 2 2 2 2 3 3 4 79%	1611
	11th to 20th	Night	1 1 1 2 1 2 2 3 4 83%	1741
	21st to end	Night	1 1 1 1 1 1 2 1 2 2 88%	1980
September	1st to 10th	Night	1 1 1 1 1 1 1 2 2 89%	1800
	11th to 20th	Night	3 2 1 2 2 1 1 2 2 5% 79%	1800
	21st to end	Night	2 1 1 1 2 2 2 3 3 4 80%	2012
October	1st to 10th	Night	5% 2 1 2 2 1 3 3 3 3 74%	2160
	11th to 20th	Night	6% 2 1 2 2 1 2 3 3 6% 72%	2160
	21st to end	Night	6% 2 1 2 3 1 2 2 4 6% 70%	2376
November	1st to 10th	Night	12% 5% 2 3 3 2 3 3 5% 7% 56%	2364
	11th to 20th	Night	11% 4 2 3 3 2 3 4 5% 7% 57%	2470
	21st to end	Night	13% 4 2 3 3 2 3 3 4 6% 58%	2601
December	1st to 10th	Night	19% 5% 1 3 4 2 3 4 3 7% 50%	2660
	11th to 20th	Night	29% 7% 3 4 3 2 3 3 5% 7% 34%	3276
	21st to end	Night	20% 6% 3 6% 6% 4 5% 5% 4 6% 35%	4862

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	48% 7% 10% 20% 11% 3	1519
	11th to 20th	Day	38% 6% 9% 17% 17% 13%	1617
	21st to end	Day	42% 5% 11% 15% 18% 8%	1782
February	1st to 10th	Day	39% 5% 10% 20% 17% 10%	1620
	11th to 20th	Day	36% 5% 11% 20% 21% 8%	1696
	21st to end	Day	39% 5% 10% 22% 17% 7%	1480
March	1st to 10th	Day	46% 7% 11% 16% 17% 4	1858
	11th to 20th	Day	43% 4 9% 23% 16% 5%	1980
	21st to end	Day	39% 7% 12% 21% 17% 4	2259
April	1st to 10th	Day	36% 6% 11% 24% 16% 6%	2160
	11th to 20th	Day	28% 5% 12% 26% 22% 6%	2132
	21st to end	Day	22% 5% 13% 28% 27% 4	2160
May	1st to 10th	Day	20% 4 12% 25% 30% 10%	2206
	11th to 20th	Day	19% 5% 9% 29% 28% 10%	2480
	21st to end	Day	19% 6% 14% 29% 24% 7%	2724
June	1st to 10th	Day	18% 5% 10% 26% 28% 13%	2520
	11th to 20th	Day	9% 3 8% 20% 34% 26%	2520
	21st to end	Day	8% 1 3 14% 36% 38%	2520
July	1st to 10th	Day	5% 1 3 10% 26% 55%	2451
	11th to 20th	Day	6% 1 4 12% 25% 52%	2445
	21st to end	Day	8% 1 4 7% 24% 56%	2415
August	1st to 10th	Day	8% 1 4 11% 29% 47%	2312
	11th to 20th	Day	5% 1 4 10% 33% 48%	2161
	21st to end	Day	4 1 2 11% 36% 47%	2376
September	1st to 10th	Day	5% 1 3 8% 29% 55%	2160
	11th to 20th	Day	12% 3 6% 20% 37% 22%	2160
	21st to end	Day	12% 2 4 20% 33% 29%	1946
October	1st to 10th	Day	16% 3 7% 18% 28% 27%	1800
	11th to 20th	Day	17% 5% 7% 20% 23% 28%	1800
	21st to end	Day	22% 4 8% 19% 26% 21%	1980
November	1st to 10th	Day	27% 7% 9% 22% 22% 12%	1712
	11th to 20th	Day	32% 5% 11% 22% 21% 8%	1710
	21st to end	Day	29% 7% 12% 19% 23% 11%	1577
December	1st to 10th	Day	38% 5% 12% 18% 18% 9%	1520
	11th to 20th	Day	55% 5% 10% 12% 14% 5%	1928
	21st to end	Day	42% 9% 18% 18% 10% 3	2794
January	1st to 10th	Night	43% 5% 13% 22% 11% 6%	2646
	11th to 20th	Night	38% 7% 11% 16% 15% 15%	2443
	21st to end	Night	40% 4 11% 14% 21% 10%	2574
February	1st to 10th	Night	37% 3 7% 22% 21% 11%	2340
	11th to 20th	Night	35% 5% 8% 22% 22% 9%	2162
	21st to end	Night	35% 5% 9% 23% 21% 7%	1776
March	1st to 10th	Night	47% 5% 9% 17% 16% 5%	2099
	11th to 20th	Night	39% 5% 9% 21% 19% 7%	1980
	21st to end	Night	39% 6% 11% 21% 17% 6%	2096
April	1st to 10th	Night	37% 5% 13% 18% 20% 7%	1800
	11th to 20th	Night	28% 6% 12% 25% 17% 11%	1779
	21st to end	Night	21% 5% 12% 31% 25% 6%	1800
May	1st to 10th	Night	20% 6% 12% 21% 29% 13%	1751
	11th to 20th	Night	18% 4 12% 31% 23% 13%	1416
	21st to end	Night	17% 4 17% 30% 25% 7%	1552
June	1st to 10th	Night	18% 6% 11% 27% 27% 12%	1440
	11th to 20th	Night	9% 2 8% 21% 33% 27%	1440
	21st to end	Night	10% 2 3 12% 33% 40%	1440
July	1st to 10th	Night	5% 1 3 11% 28% 53%	1400
	11th to 20th	Night	7% 1 4 10% 22% 56%	1392
	21st to end	Night	8% 1 5% 8% 23% 54%	1660
August	1st to 10th	Night	11% 1 4 13% 26% 44%	1602
	11th to 20th	Night	7% 1 3 10% 31% 47%	1741
	21st to end	Night	7% 1 2 10% 30% 51%	1980
September	1st to 10th	Night	6% 1 3 7% 30% 53%	1800
	11th to 20th	Night	11% 2 6% 18% 39% 24%	1800
	21st to end	Night	10% 2 6% 17% 30% 34%	2012
October	1st to 10th	Night	17% 3 5% 17% 27% 30%	2160
	11th to 20th	Night	16% 3 6% 19% 26% 31%	2160
	21st to end	Night	18% 3 6% 21% 26% 25%	2376
November	1st to 10th	Night	28% 5% 10% 22% 19% 16%	2364
	11th to 20th	Night	28% 4 11% 25% 22% 10%	2470
	21st to end	Night	30% 4 8% 22% 22% 14%	2601
December	1st to 10th	Night	35% 5% 11% 19% 18% 12%	2660
	11th to 20th	Night	51% 4 8% 15% 15% 6%	3276
	21st to end	Night	48% 8% 12% 17% 11% 4	4862

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	41% 4 17% 21% 14% 2	180
	11th to 20th	Day	23% 7% 16% 27% 22% 5%	311
	21st to end	Day	20% 7% 20% 25% 22% 6%	374
February	1st to 10th	Day	21% 8% 17% 25% 24% 4	340
	11th to 20th	Day	20% 12% 22% 24% 18% 4	387
	21st to end	Day	21% 9% 26% 26% 15% 3	346
March	1st to 10th	Day	25% 12% 26% 22% 12% 3	420
	11th to 20th	Day	23% 12% 23% 22% 14% 6%	420
	21st to end	Day	18% 12% 30% 23% 15% 2	462
April	1st to 10th	Day	19% 10% 26% 25% 16% 4	420
	11th to 20th	Day	15% 10% 30% 24% 15% 5%	412
	21st to end	Day	9% 10% 22% 30% 22% 8%	420
May	1st to 10th	Day	8% 9% 29% 27% 21% 5%	423
	11th to 20th	Day	7% 8% 25% 28% 25% 6%	448
	21st to end	Day	6% 7% 20% 29% 28% 11%	485
June	1st to 10th	Day	5% 8% 19% 28% 31% 9%	450
	11th to 20th	Day	3 6% 21% 36% 29% 5%	450
	21st to end	Day	1 3 15% 38% 34% 9%	450
July	1st to 10th	Day	1 10% 45% 40% 4	446
	11th to 20th	Day	7% 43% 44% 5%	444
	21st to end	Day	1 10% 40% 40% 9%	487
August	1st to 10th	Day	2 1 12% 40% 38% 6%	447
	11th to 20th	Day	1 1 12% 38% 39% 9%	437
	21st to end	Day	1 10% 36% 46% 6%	495
September	1st to 10th	Day	2 1 9% 43% 35% 9%	450
	11th to 20th	Day	5% 4 17% 35% 32% 7%	450
	21st to end	Day	4 2 20% 36% 29% 8%	450
October	1st to 10th	Day	2 7% 22% 29% 29% 10%	450
	11th to 20th	Day	6% 5% 15% 27% 35% 12%	450
	21st to end	Day	9% 7% 16% 29% 28% 11%	495
November	1st to 10th	Day	14% 8% 11% 29% 30% 8%	308
	11th to 20th	Day	12% 3 19% 28% 30% 9%	270
	21st to end	Day	11% 4 19% 32% 27% 7%	207
December	1st to 10th	Day	21% 7% 17% 25% 22% 8%	180
	11th to 20th	Day	19% 8% 20% 24% 19% 9%	180
	21st to end	Day	20% 10% 19% 27% 19% 4	198
January	1st to 10th	Night	25% 3 13% 30% 22% 7%	760
	11th to 20th	Night	18% 4 11% 39% 23% 5%	740
	21st to end	Night	21% 2 15% 33% 24% 6%	792
February	1st to 10th	Night	20% 3 13% 35% 25% 4	720
	11th to 20th	Night	16% 5% 18% 35% 22% 5%	704
	21st to end	Night	19% 5% 19% 37% 17% 3	592
March	1st to 10th	Night	23% 7% 15% 31% 20% 4	720
	11th to 20th	Night	21% 5% 14% 31% 26% 3	720
	21st to end	Night	18% 5% 20% 34% 20% 2	792
April	1st to 10th	Night	15% 3 16% 39% 24% 2	720
	11th to 20th	Night	12% 5% 20% 34% 25% 5%	711
	21st to end	Night	8% 3 17% 38% 29% 4	720
May	1st to 10th	Night	7% 4 21% 37% 27% 3	671
	11th to 20th	Night	7% 2 14% 41% 33% 3	354
	21st to end	Night	7% 2 13% 39% 38% 1	389
June	1st to 10th	Night	6% 2 18% 39% 34% 2	360
	11th to 20th	Night	2 1 14% 49% 32% 2	360
	21st to end	Night	2 1 9% 46% 39% 2	360
July	1st to 10th	Night	1 7% 52% 40% 1	350
	11th to 20th	Night	2 4 56% 36% 2	348
	21st to end	Night	1 5% 41% 48% 4	576
August	1st to 10th	Night	3 1 7% 46% 42% 3	537
	11th to 20th	Night	1 6% 45% 44% 4	673
	21st to end	Night	1 5% 43% 48% 3	792
September	1st to 10th	Night	2 1 5% 38% 49% 6%	720
	11th to 20th	Night	4 1 9% 36% 43% 7%	720
	21st to end	Night	2 2 13% 38% 35% 10%	720
October	1st to 10th	Night	6% 1 12% 41% 32% 8%	720
	11th to 20th	Night	7% 2 10% 36% 39% 6%	720
	21st to end	Night	7% 2 13% 35% 31% 12%	792
November	1st to 10th	Night	13% 3 11% 34% 31% 8%	740
	11th to 20th	Night	13% 2 13% 40% 27% 6%	760
	21st to end	Night	14% 2 14% 38% 26% 6%	760
December	1st to 10th	Night	20% 4 12% 35% 21% 7%	760
	11th to 20th	Night	28% 3 13% 32% 21% 4	760
	21st to end	Night	24% 3 18% 33% 18% 3	836

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