Penantly Creek Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Penantly Creek Observatory clear sky chart from 2005-11-06 to 2024-01-14.

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	11% 6% 3 6% 6% 4 5% 5% 6% 7% 42%	75002
	Night	12% 6% 2 5% 5% 3 4 4 5% 8% 46%	74726
Transparency	Day	40% 7% 11% 18% 18% 6%	74735
	Night	36% 6% 11% 18% 20% 8%	74514
Seeing	Day	15% 9% 21% 24% 22% 10%	13714
	Night	15% 8% 22% 29% 22% 5%	23442

By Month

Percentage Cloud Cover SE (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	18% 9% 2 5% 3 3 3 3 4 6% 44%	1678
	11th to 20th	Day	20% 7% 4 5% 4 2 3 3 5% 4 43%	1713
	21st to end	Day	18% 8% 1 4 3 3 3 4 4 5% 46%	1980
February	1st to 10th	Day	17% 10% 4 5% 5% 3 5% 4 4 5% 39%	1800
	11th to 20th	Day	19% 7% 3 5% 5% 3 3 4 4 5% 42%	1748
	21st to end	Day	17% 8% 3 6% 6% 4 4 4 3 7% 38%	1480
March	1st to 10th	Day	15% 6% 3 5% 5% 3 5% 4 5% 6% 43%	1800
	11th to 20th	Day	15% 7% 4 6% 6% 3 5% 5% 5% 7% 38%	1934
	21st to end	Day	12% 6% 3 5% 5% 3 5% 6% 5% 6% 44%	2348
April	1st to 10th	Day	12% 5% 3 6% 6% 3 5% 5% 6% 7% 44%	2160
	11th to 20th	Day	14% 7% 2 5% 6% 4 4 4 4 6% 44%	2134
	21st to end	Day	8% 5% 2 4 5% 3 5% 5% 5% 8% 50%	2160
May	1st to 10th	Day	12% 5% 2 5% 5% 3 5% 4 5% 8% 46%	2160
	11th to 20th	Day	8% 6% 2 6% 6% 5% 5% 7% 6% 7% 41%	2181
	21st to end	Day	8% 5% 3 7% 7% 4 7% 5% 7% 9% 39%	2531
June	1st to 10th	Day	10% 6% 3 6% 7% 4 6% 7% 7% 9% 35%	2340
	11th to 20th	Day	5% 4 2 5% 7% 4 8% 6% 9% 10% 39%	2469
	21st to end	Day	6% 5% 2 6% 8% 5% 8% 7% 7% 11% 36%	2520
July	1st to 10th	Day	8% 5% 3 8% 9% 6% 8% 7% 8% 10% 29%	2483
	11th to 20th	Day	9% 5% 2 6% 7% 6% 8% 9% 9% 10% 29%	2278
	21st to end	Day	7% 5% 3 7% 8% 5% 7% 7% 8% 10% 32%	2350
August	1st to 10th	Day	4 4 3 6% 8% 5% 7% 7% 9% 10% 36%	2148
	11th to 20th	Day	8% 5% 3 8% 8% 5% 7% 7% 8% 9% 32%	2125
	21st to end	Day	12% 5% 2 6% 7% 5% 7% 6% 6% 9% 35%	2376
September	1st to 10th	Day	9% 6% 3 5% 7% 5% 6% 7% 7% 7% 38%	2160
	11th to 20th	Day	13% 6% 3 6% 5% 3 5% 5% 5% 6% 43%	2050
	21st to end	Day	10% 5% 2 5% 5% 3 5% 4 5% 5% 52%	1980
October	1st to 10th	Day	7% 5% 2 4 4 3 4 4 4 4 58%	1980
	11th to 20th	Day	10% 5% 2 6% 4 3 3 5% 5% 7% 51%	1814
	21st to end	Day	12% 5% 2 4 4 2 3 3 3 5% 56%	1980
November	1st to 10th	Day	10% 6% 2 4 4 2 3 3 4 6% 55%	1746
	11th to 20th	Day	13% 6% 2 4 4 2 3 3 4 5% 53%	1710
	21st to end	Day	15% 5% 3 5% 5% 2 4 4 4 6% 48%	1710
December	1st to 10th	Day	15% 8% 3 5% 5% 2 4 4 5% 6% 43%	1710
	11th to 20th	Day	15% 9% 4 7% 6% 3 4 3 4 4 43%	2153
	21st to end	Day	14% 8% 3 5% 4 3 5% 4 5% 6% 42%	3113
January	1st to 10th	Night	18% 8% 2 4 3 2 3 3 4 6% 47%	2482
	11th to 20th	Night	16% 9% 4 6% 5% 3 3 3 4 5% 43%	2311
	21st to end	Night	19% 8% 3 6% 4 2 3 3 3 5% 45%	2376
February	1st to 10th	Night	17% 8% 3 5% 6% 3 4 5% 5% 5% 40%	2160
	11th to 20th	Night	21% 7% 3 4 5% 2 4 3 4 5% 43%	2098
	21st to end	Night	19% 9% 3 7% 4 2 4 3 5% 6% 38%	1776
March	1st to 10th	Night	13% 7% 3 4 4 3 4 4 4 8% 46%	2160
	11th to 20th	Night	15% 8% 3 6% 4 3 4 4 5% 7% 41%	2039
	21st to end	Night	12% 5% 2 6% 4 3 4 4 4 8% 47%	2008
April	1st to 10th	Night	11% 6% 3 5% 5% 3 5% 5% 5% 6% 47%	1800
	11th to 20th	Night	13% 5% 2 4 5% 3 4 4 6% 7% 47%	1775
	21st to end	Night	7% 4 2 4 4 2 4 5% 5% 8% 54%	1800
May	1st to 10th	Night	8% 5% 2 4 4 2 3 3 5% 7% 56%	1800
	11th to 20th	Night	7% 4 2 6% 6% 3 5% 5% 6% 9% 46%	1711
	21st to end	Night	6% 4 2 5% 5% 4 5% 5% 7% 12% 45%	1744
June	1st to 10th	Night	7% 4 3 5% 6% 4 6% 5% 8% 12% 40%	1620
	11th to 20th	Night	4 3 3 4 5% 4 5% 6% 8% 11% 48%	1484
	21st to end	Night	5% 4 2 6% 5% 3 6% 6% 8% 12% 43%	1440
July	1st to 10th	Night	5% 5% 3 7% 6% 5% 6% 7% 8% 11% 37%	1471
	11th to 20th	Night	5% 4 2 4 5% 3 6% 8% 8% 12% 42%	1676
	21st to end	Night	3 4 3 5% 6% 4 6% 8% 9% 12% 41%	1968
August	1st to 10th	Night	3 2 2 4 5% 4 7% 6% 8% 12% 46%	1790
	11th to 20th	Night	6% 4 2 5% 6% 4 6% 6% 8% 14% 38%	1776
	21st to end	Night	10% 4 2 5% 6% 4 6% 5% 7% 10% 42%	1980
September	1st to 10th	Night	8% 4 2 5% 7% 3 6% 6% 7% 9% 42%	1800
	11th to 20th	Night	9% 6% 2 4 5% 3 4 5% 6% 8% 47%	1890
	21st to end	Night	8% 4 1 5% 4 3 4 5% 6% 6% 53%	1980
October	1st to 10th	Night	7% 4 1 4 4 2 4 4 6% 7% 56%	1980
	11th to 20th	Night	9% 5% 2 4 4 2 3 3 4 7% 55%	2142
	21st to end	Night	13% 6% 2 5% 4 2 2 3 2 4 57%	2376
November	1st to 10th	Night	10% 4 2 3 4 2 3 3 4 5% 59%	2329
	11th to 20th	Night	13% 5% 2 3 3 2 4 4 4 6% 54%	2470
	21st to end	Night	14% 6% 3 4 4 3 3 4 4 7% 48%	2470
December	1st to 10th	Night	18% 6% 2 4 5% 2 4 4 4 6% 46%	2470
	11th to 20th	Night	16% 7% 3 6% 5% 3 3 3 4 5% 45%	3042
	21st to end	Night	17% 9% 3 5% 4 2 3 3 5% 6% 42%	4532

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	43% 3 4 20% 24% 7%	1679
	11th to 20th	Day	44% 4 5% 15% 24% 8%	1713
	21st to end	Day	42% 3 5% 15% 27% 8%	1980
February	1st to 10th	Day	48% 4 7% 16% 17% 9%	1800
	11th to 20th	Day	46% 4 6% 16% 21% 8%	1758
	21st to end	Day	48% 5% 7% 14% 15% 11%	1480
March	1st to 10th	Day	42% 5% 8% 15% 20% 10%	1800
	11th to 20th	Day	45% 6% 9% 13% 20% 7%	1934
	21st to end	Day	40% 6% 8% 20% 21% 5%	2348
April	1st to 10th	Day	39% 7% 9% 18% 20% 7%	2160
	11th to 20th	Day	41% 5% 7% 17% 23% 7%	2134
	21st to end	Day	32% 6% 10% 22% 25% 4	2160
May	1st to 10th	Day	37% 6% 10% 19% 23% 5%	2160
	11th to 20th	Day	39% 10% 12% 22% 15% 2	2185
	21st to end	Day	39% 9% 13% 22% 16% 1	2531
June	1st to 10th	Day	42% 9% 14% 22% 13% 1	2340
	11th to 20th	Day	35% 11% 17% 21% 13% 3	2469
	21st to end	Day	39% 11% 16% 21% 11% 2	2520
July	1st to 10th	Day	46% 12% 16% 17% 8% 1	2416
	11th to 20th	Day	43% 14% 20% 17% 6% 1	2209
	21st to end	Day	42% 12% 18% 19% 7% 2	2219
August	1st to 10th	Day	38% 12% 21% 22% 8%	2136
	11th to 20th	Day	45% 12% 17% 15% 8% 3	2125
	21st to end	Day	42% 11% 15% 16% 11% 4	2376
September	1st to 10th	Day	41% 9% 13% 16% 15% 5%	2160
	11th to 20th	Day	40% 9% 10% 16% 18% 7%	2050
	21st to end	Day	33% 5% 10% 22% 21% 8%	1980
October	1st to 10th	Day	29% 4 9% 19% 27% 11%	1980
	11th to 20th	Day	32% 4 6% 17% 29% 11%	1814
	21st to end	Day	31% 3 5% 17% 30% 13%	1980
November	1st to 10th	Day	32% 3 8% 16% 29% 12%	1746
	11th to 20th	Day	34% 4 6% 14% 27% 15%	1710
	21st to end	Day	37% 5% 8% 16% 22% 11%	1710
December	1st to 10th	Day	42% 5% 9% 18% 18% 9%	1710
	11th to 20th	Day	46% 3 6% 14% 23% 8%	2150
	21st to end	Day	42% 4 8% 21% 21% 4	3113
January	1st to 10th	Night	40% 4 7% 16% 27% 7%	2482
	11th to 20th	Night	45% 3 6% 17% 21% 8%	2311
	21st to end	Night	44% 3 4 14% 27% 8%	2376
February	1st to 10th	Night	45% 4 8% 15% 19% 9%	2160
	11th to 20th	Night	45% 4 6% 13% 22% 10%	2099
	21st to end	Night	49% 4 7% 13% 15% 12%	1776
March	1st to 10th	Night	38% 5% 7% 15% 22% 12%	2160
	11th to 20th	Night	43% 6% 11% 15% 18% 8%	2039
	21st to end	Night	35% 6% 10% 19% 23% 7%	2008
April	1st to 10th	Night	39% 5% 11% 15% 21% 9%	1800
	11th to 20th	Night	36% 5% 9% 19% 21% 9%	1775
	21st to end	Night	28% 5% 10% 23% 27% 7%	1800
May	1st to 10th	Night	28% 5% 9% 28% 23% 7%	1800
	11th to 20th	Night	33% 9% 15% 23% 18% 2	1712
	21st to end	Night	31% 9% 14% 21% 22% 2	1745
June	1st to 10th	Night	36% 9% 16% 24% 13% 2	1620
	11th to 20th	Night	27% 11% 17% 29% 13% 3	1484
	21st to end	Night	31% 11% 19% 22% 14% 3	1440
July	1st to 10th	Night	37% 12% 21% 19% 7% 3	1430
	11th to 20th	Night	29% 15% 27% 20% 8% 1	1622
	21st to end	Night	31% 15% 24% 21% 7% 2	1858
August	1st to 10th	Night	26% 13% 29% 23% 8% 1	1778
	11th to 20th	Night	34% 16% 19% 16% 11% 4	1776
	21st to end	Night	37% 12% 19% 18% 10% 5%	1980
September	1st to 10th	Night	37% 9% 15% 17% 16% 6%	1800
	11th to 20th	Night	34% 10% 13% 17% 18% 9%	1890
	21st to end	Night	31% 6% 10% 23% 22% 9%	1980
October	1st to 10th	Night	27% 7% 11% 17% 23% 16%	1980
	11th to 20th	Night	29% 5% 7% 18% 27% 15%	2142
	21st to end	Night	33% 3 5% 15% 26% 18%	2376
November	1st to 10th	Night	28% 4 8% 17% 28% 15%	2329
	11th to 20th	Night	32% 3 6% 15% 28% 15%	2470
	21st to end	Night	36% 4 7% 20% 19% 13%	2470
December	1st to 10th	Night	39% 5% 10% 18% 20% 7%	2470
	11th to 20th	Night	43% 4 6% 12% 28% 7%	3044
	21st to end	Night	43% 3 7% 16% 27% 4	4532

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	27% 21% 31% 17% 3 1	270
	11th to 20th	Day	25% 16% 34% 16% 7% 2	249
	21st to end	Day	23% 25% 34% 15% 3	264
February	1st to 10th	Day	31% 20% 30% 15% 5%	240
	11th to 20th	Day	31% 20% 33% 13% 3	240
	21st to end	Day	24% 24% 38% 13% 1	198
March	1st to 10th	Day	23% 20% 37% 15% 5%	240
	11th to 20th	Day	22% 23% 31% 19% 5%	240
	21st to end	Day	22% 16% 30% 23% 8%	434
April	1st to 10th	Day	18% 14% 29% 27% 11% 1	420
	11th to 20th	Day	22% 16% 25% 23% 12% 2	412
	21st to end	Day	13% 13% 29% 30% 14% 1	420
May	1st to 10th	Day	14% 11% 26% 23% 21% 4	423
	11th to 20th	Day	9% 6% 24% 30% 19% 11%	447
	21st to end	Day	10% 2 20% 32% 22% 14%	485
June	1st to 10th	Day	15% 3 14% 28% 28% 12%	450
	11th to 20th	Day	8% 2 11% 32% 35% 12%	579
	21st to end	Day	8% 2 13% 30% 30% 17%	630
July	1st to 10th	Day	8% 1 8% 27% 36% 19%	621
	11th to 20th	Day	8% 1 5% 21% 41% 24%	556
	21st to end	Day	7% 1 9% 23% 35% 24%	488
August	1st to 10th	Day	7% 1 8% 19% 35% 31%	446
	11th to 20th	Day	9% 1 6% 23% 35% 26%	439
	21st to end	Day	14% 2 8% 19% 36% 21%	495
September	1st to 10th	Day	9% 1 11% 29% 34% 16%	450
	11th to 20th	Day	13% 3 10% 24% 36% 13%	450
	21st to end	Day	12% 3 13% 30% 31% 10%	450
October	1st to 10th	Day	8% 6% 23% 31% 25% 7%	450
	11th to 20th	Day	9% 7% 27% 32% 21% 4	284
	21st to end	Day	20% 12% 23% 31% 11% 3	297
November	1st to 10th	Day	19% 9% 24% 34% 9% 5%	270
	11th to 20th	Day	16% 14% 35% 24% 9% 1	270
	21st to end	Day	14% 18% 37% 17% 11% 3	270
December	1st to 10th	Day	23% 15% 35% 17% 9%	270
	11th to 20th	Day	23% 16% 38% 15% 7% 1	270
	21st to end	Day	28% 16% 31% 20% 4 1	297
January	1st to 10th	Night	23% 15% 31% 25% 5%	950
	11th to 20th	Night	22% 14% 30% 26% 7%	843
	21st to end	Night	24% 14% 35% 24% 3	792
February	1st to 10th	Night	22% 13% 30% 30% 5%	720
	11th to 20th	Night	26% 16% 28% 25% 5%	703
	21st to end	Night	23% 16% 38% 21% 2	592
March	1st to 10th	Night	18% 15% 32% 28% 8%	720
	11th to 20th	Night	18% 11% 37% 27% 7% 1	599
	21st to end	Night	15% 11% 34% 27% 11% 1	594
April	1st to 10th	Night	16% 11% 33% 25% 15%	540
	11th to 20th	Night	15% 11% 32% 27% 14% 1	533
	21st to end	Night	8% 7% 31% 40% 14% 1	540
May	1st to 10th	Night	10% 5% 27% 35% 21% 2	540
	11th to 20th	Night	9% 2 21% 43% 23% 3	531
	21st to end	Night	9% 1 14% 35% 33% 9%	584
June	1st to 10th	Night	10% 1 12% 34% 38% 5%	540
	11th to 20th	Night	5% 1 10% 29% 44% 12%	540
	21st to end	Night	7% 1 10% 31% 38% 13%	540
July	1st to 10th	Night	6% 1 7% 27% 45% 15%	525
	11th to 20th	Night	5% 1 6% 21% 45% 23%	522
	21st to end	Night	5% 1 6% 29% 44% 16%	591
August	1st to 10th	Night	4 4 22% 51% 20%	537
	11th to 20th	Night	9% 1 6% 21% 44% 18%	534
	21st to end	Night	11% 2 6% 22% 42% 17%	594
September	1st to 10th	Night	10% 2 7% 29% 42% 10%	540
	11th to 20th	Night	14% 2 8% 27% 42% 7%	630
	21st to end	Night	11% 1 12% 33% 39% 4	720
October	1st to 10th	Night	9% 2 19% 31% 33% 6%	720
	11th to 20th	Night	12% 5% 21% 33% 26% 3	720
	21st to end	Night	16% 7% 20% 33% 20% 3	792
November	1st to 10th	Night	12% 7% 19% 36% 21% 4	740
	11th to 20th	Night	17% 8% 27% 32% 15% 1	760
	21st to end	Night	16% 10% 27% 31% 14% 2	760
December	1st to 10th	Night	20% 12% 28% 29% 11%	760
	11th to 20th	Night	22% 9% 32% 27% 10%	760
	21st to end	Night	26% 14% 27% 29% 5%	836

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