Paul Plishner Radio Astronomy and Science Center
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Paul Plishner Radio Astronomy and Science Center clear sky chart from 2005-11-06 to 2024-03-01.

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	5% 4 2 4 4 2 4 4 6% 8% 59%	78559
	Night	5% 5% 2 4 4 3 4 4 5% 7% 57%	80901
Transparency	Day	23% 5% 11% 25% 25% 11%	78264
	Night	26% 5% 11% 24% 24% 10%	80719
Seeing	Day	7% 6% 23% 33% 24% 8%	14515
	Night	7% 6% 30% 41% 15% 1	24687

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	2 5% 2 6% 5% 2 4 4 5% 7% 58%	2538
	11th to 20th	Day	3 4 2 4 4 3 6% 6% 6% 8% 54%	2540
	21st to end	Day	4 5% 2 5% 3 3 3 4 5% 7% 59%	3096
February	1st to 10th	Day	4 6% 3 5% 4 3 4 4 5% 7% 54%	1867
	11th to 20th	Day	2 5% 2 6% 6% 2 4 4 5% 6% 56%	1849
	21st to end	Day	3 4 2 5% 6% 3 4 4 5% 6% 59%	1570
March	1st to 10th	Day	3 4 2 4 4 2 4 4 5% 8% 60%	1800
	11th to 20th	Day	5% 5% 1 4 4 2 3 4 5% 7% 60%	1842
	21st to end	Day	6% 5% 2 4 4 2 4 4 6% 8% 53%	2376
April	1st to 10th	Day	5% 4 2 3 3 2 3 4 6% 7% 59%	2160
	11th to 20th	Day	8% 5% 2 4 5% 3 3 4 6% 8% 52%	2131
	21st to end	Day	7% 5% 2 5% 4 3 4 5% 6% 7% 52%	2168
May	1st to 10th	Day	10% 5% 1 4 4 2 4 5% 5% 8% 53%	2340
	11th to 20th	Day	6% 5% 2 4 4 2 4 5% 7% 9% 52%	2302
	21st to end	Day	6% 3 2 3 4 2 4 5% 6% 9% 57%	2674
June	1st to 10th	Day	4 2 1 2 3 2 3 5% 8% 12% 57%	2520
	11th to 20th	Day	4 2 1 3 3 2 4 4 6% 8% 63%	2520
	21st to end	Day	3 3 2 3 4 2 4 5% 7% 9% 58%	2520
July	1st to 10th	Day	4 3 1 3 4 3 5% 5% 7% 10% 54%	2520
	11th to 20th	Day	2 1 1 2 2 2 4 5% 6% 11% 63%	2520
	21st to end	Day	6% 3 1 3 4 2 5% 5% 8% 11% 52%	2742
August	1st to 10th	Day	4 2 1 3 4 3 5% 6% 8% 12% 52%	2208
	11th to 20th	Day	5% 3 1 3 3 2 4 5% 6% 9% 58%	2123
	21st to end	Day	3 2 1 3 3 2 3 4 6% 9% 64%	2376
September	1st to 10th	Day	4 3 1 3 3 2 4 4 6% 7% 62%	2160
	11th to 20th	Day	5% 2 1 2 3 2 3 3 4 6% 67%	2049
	21st to end	Day	6% 3 2 3 2 2 3 4 4 6% 67%	1980
October	1st to 10th	Day	8% 3 2 3 3 2 3 3 5% 7% 61%	1974
	11th to 20th	Day	3 3 1 2 4 1 3 3 4 5% 71%	1800
	21st to end	Day	6% 6% 1 3 3 2 2 2 3 4 67%	1833
November	1st to 10th	Day	3 3 2 2 3 1 2 3 4 6% 71%	1673
	11th to 20th	Day	3 3 2 3 3 2 2 3 4 6% 69%	1710
	21st to end	Day	5% 5% 2 4 3 2 2 3 4 6% 64%	1710
December	1st to 10th	Day	2 6% 3 5% 5% 3 3 4 4 6% 59%	1648
	11th to 20th	Day	4 5% 2 4 2 2 3 4 5% 9% 59%	1926
	21st to end	Day	5% 8% 2 6% 4 3 4 4 5% 6% 52%	2794
January	1st to 10th	Night	3 5% 2 4 4 3 5% 5% 6% 8% 55%	4417
	11th to 20th	Night	2 4 2 4 4 2 3 5% 6% 8% 59%	4430
	21st to end	Night	3 6% 2 4 5% 3 3 4 5% 7% 58%	4588
February	1st to 10th	Night	5% 5% 3 6% 5% 3 3 3 4 7% 57%	2310
	11th to 20th	Night	5% 6% 3 7% 4 2 4 5% 6% 8% 51%	2218
	21st to end	Night	4 5% 2 4 4 2 3 4 5% 7% 60%	1884
March	1st to 10th	Night	5% 6% 2 4 3 3 4 5% 5% 7% 58%	2166
	11th to 20th	Night	7% 5% 2 4 4 2 3 3 5% 7% 57%	2116
	21st to end	Night	7% 6% 2 4 4 3 4 5% 5% 7% 54%	1980
April	1st to 10th	Night	6% 5% 2 4 3 2 4 4 5% 8% 57%	1800
	11th to 20th	Night	8% 5% 2 5% 3 3 5% 4 6% 7% 52%	1777
	21st to end	Night	10% 5% 2 5% 4 2 4 4 6% 8% 50%	1790
May	1st to 10th	Night	10% 5% 2 4 4 2 4 5% 5% 7% 51%	1620
	11th to 20th	Night	10% 4 3 4 5% 3 4 6% 6% 8% 47%	1593
	21st to end	Night	10% 5% 2 3 4 3 6% 6% 6% 9% 46%	1599
June	1st to 10th	Night	4 3 1 3 4 3 4 6% 8% 10% 55%	1440
	11th to 20th	Night	4 3 2 3 3 3 5% 5% 6% 8% 57%	1440
	21st to end	Night	5% 5% 2 3 4 2 5% 6% 7% 9% 51%	1440
July	1st to 10th	Night	5% 4 2 5% 6% 4 8% 7% 8% 8% 44%	1440
	11th to 20th	Night	4 3 2 4 4 3 6% 5% 7% 10% 52%	1440
	21st to end	Night	7% 4 3 5% 6% 4 6% 6% 7% 9% 44%	1576
August	1st to 10th	Night	7% 4 2 6% 6% 5% 7% 6% 7% 10% 41%	1724
	11th to 20th	Night	6% 3 2 3 5% 3 5% 7% 7% 9% 49%	1778
	21st to end	Night	4 2 1 3 4 3 5% 6% 8% 9% 55%	1980
September	1st to 10th	Night	7% 3 1 3 4 3 3 5% 5% 9% 56%	1800
	11th to 20th	Night	5% 3 1 3 3 2 4 4 5% 9% 60%	1909
	21st to end	Night	6% 3 2 4 4 2 3 3 4 6% 63%	1980
October	1st to 10th	Night	8% 3 1 3 4 2 4 4 5% 6% 59%	1984
	11th to 20th	Night	4 2 1 3 2 2 3 3 4 6% 71%	2160
	21st to end	Night	5% 6% 1 3 4 2 3 3 3 5% 64%	2503
November	1st to 10th	Night	3 2 1 3 2 2 3 3 4 6% 70%	2405
	11th to 20th	Night	3 4 1 3 3 2 3 2 3 5% 70%	2470
	21st to end	Night	4 5% 2 3 3 2 3 4 5% 6% 63%	2470
December	1st to 10th	Night	3 4 2 4 4 2 3 4 5% 7% 60%	2530
	11th to 20th	Night	4 5% 2 5% 4 3 4 4 5% 7% 58%	3271
	21st to end	Night	4 7% 3 6% 4 3 4 4 6% 7% 53%	4873

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	26% 5% 9% 30% 24% 6%	2539
	11th to 20th	Day	26% 5% 13% 25% 22% 9%	2540
	21st to end	Day	26% 3 12% 30% 24% 4	3096
February	1st to 10th	Day	29% 5% 10% 25% 22% 9%	1867
	11th to 20th	Day	28% 4 8% 26% 26% 8%	1859
	21st to end	Day	26% 3 8% 20% 27% 16%	1570
March	1st to 10th	Day	23% 4 9% 25% 23% 15%	1800
	11th to 20th	Day	24% 5% 9% 23% 31% 8%	1842
	21st to end	Day	28% 5% 13% 25% 23% 7%	2376
April	1st to 10th	Day	23% 5% 10% 23% 26% 14%	2160
	11th to 20th	Day	28% 5% 11% 25% 20% 12%	2134
	21st to end	Day	29% 5% 11% 23% 23% 8%	2168
May	1st to 10th	Day	28% 5% 11% 27% 22% 7%	2340
	11th to 20th	Day	27% 5% 18% 29% 17% 4	2306
	21st to end	Day	23% 6% 16% 27% 19% 9%	2674
June	1st to 10th	Day	18% 6% 16% 29% 22% 9%	2520
	11th to 20th	Day	19% 4 10% 22% 25% 20%	2520
	21st to end	Day	21% 5% 11% 23% 26% 15%	2520
July	1st to 10th	Day	24% 8% 10% 20% 27% 11%	2451
	11th to 20th	Day	12% 7% 12% 24% 33% 11%	2445
	21st to end	Day	24% 8% 16% 29% 19% 4	2590
August	1st to 10th	Day	21% 9% 21% 26% 14% 9%	2193
	11th to 20th	Day	22% 6% 10% 26% 25% 11%	2123
	21st to end	Day	17% 6% 10% 27% 27% 13%	2376
September	1st to 10th	Day	20% 5% 10% 24% 26% 16%	2160
	11th to 20th	Day	18% 4 8% 19% 28% 22%	2049
	21st to end	Day	19% 4 7% 18% 36% 16%	1980
October	1st to 10th	Day	22% 4 10% 21% 30% 13%	1974
	11th to 20th	Day	17% 2 7% 19% 35% 19%	1800
	21st to end	Day	23% 2 7% 24% 28% 16%	1833
November	1st to 10th	Day	16% 3 9% 25% 29% 17%	1673
	11th to 20th	Day	18% 3 7% 29% 31% 12%	1710
	21st to end	Day	24% 3 7% 18% 33% 16%	1710
December	1st to 10th	Day	26% 4 10% 25% 28% 7%	1648
	11th to 20th	Day	22% 3 9% 24% 34% 7%	1924
	21st to end	Day	32% 4 9% 23% 25% 7%	2794
January	1st to 10th	Night	25% 6% 11% 29% 25% 4	4417
	11th to 20th	Night	21% 5% 15% 27% 23% 9%	4430
	21st to end	Night	26% 4 12% 28% 25% 4	4588
February	1st to 10th	Night	30% 3 10% 23% 26% 9%	2310
	11th to 20th	Night	29% 5% 10% 26% 23% 7%	2218
	21st to end	Night	24% 3 7% 22% 29% 15%	1884
March	1st to 10th	Night	26% 4 8% 21% 27% 14%	2166
	11th to 20th	Night	26% 6% 11% 24% 23% 11%	2116
	21st to end	Night	28% 5% 12% 24% 24% 7%	1980
April	1st to 10th	Night	26% 6% 9% 23% 25% 13%	1800
	11th to 20th	Night	29% 6% 12% 25% 19% 9%	1777
	21st to end	Night	31% 5% 14% 23% 20% 6%	1790
May	1st to 10th	Night	32% 7% 13% 21% 20% 7%	1620
	11th to 20th	Night	34% 7% 18% 20% 15% 6%	1594
	21st to end	Night	33% 8% 16% 20% 14% 9%	1600
June	1st to 10th	Night	21% 8% 22% 26% 16% 7%	1440
	11th to 20th	Night	23% 6% 13% 23% 19% 16%	1440
	21st to end	Night	27% 7% 11% 21% 23% 11%	1440
July	1st to 10th	Night	33% 9% 13% 22% 17% 6%	1400
	11th to 20th	Night	25% 8% 12% 26% 22% 8%	1392
	21st to end	Night	37% 8% 16% 22% 14% 2	1488
August	1st to 10th	Night	34% 9% 17% 22% 10% 7%	1715
	11th to 20th	Night	28% 8% 15% 27% 17% 6%	1778
	21st to end	Night	23% 7% 14% 24% 23% 10%	1980
September	1st to 10th	Night	25% 5% 12% 22% 22% 14%	1800
	11th to 20th	Night	20% 5% 11% 21% 24% 19%	1909
	21st to end	Night	23% 4 6% 19% 34% 14%	1980
October	1st to 10th	Night	25% 4 12% 19% 26% 13%	1984
	11th to 20th	Night	15% 4 7% 22% 29% 23%	2160
	21st to end	Night	23% 4 8% 20% 27% 18%	2503
November	1st to 10th	Night	16% 2 8% 25% 29% 19%	2405
	11th to 20th	Night	19% 3 6% 25% 34% 14%	2470
	21st to end	Night	23% 3 7% 20% 30% 16%	2470
December	1st to 10th	Night	23% 4 12% 28% 25% 9%	2530
	11th to 20th	Night	26% 4 7% 22% 35% 6%	3272
	21st to end	Night	30% 4 9% 24% 27% 5%	4873

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	6% 6% 23% 46% 17% 4	270
	11th to 20th	Day	11% 8% 25% 37% 16% 3	270
	21st to end	Day	8% 5% 25% 38% 21% 4	297
February	1st to 10th	Day	7% 8% 36% 36% 13% 1	270
	11th to 20th	Day	4 4 38% 39% 13% 1	270
	21st to end	Day	4 10% 38% 39% 8% 1	225
March	1st to 10th	Day	5% 10% 27% 40% 15% 3	240
	11th to 20th	Day	10% 10% 32% 25% 17% 6%	260
	21st to end	Day	13% 14% 32% 30% 10% 1	462
April	1st to 10th	Day	9% 11% 35% 27% 13% 4	420
	11th to 20th	Day	11% 17% 30% 27% 12% 2	412
	21st to end	Day	12% 14% 31% 29% 11% 3	428
May	1st to 10th	Day	10% 12% 27% 30% 18% 2	603
	11th to 20th	Day	11% 8% 27% 30% 18% 6%	625
	21st to end	Day	8% 10% 28% 29% 20% 4	679
June	1st to 10th	Day	5% 5% 22% 29% 29% 11%	630
	11th to 20th	Day	5% 6% 26% 34% 25% 4	630
	21st to end	Day	5% 4 21% 32% 31% 7%	630
July	1st to 10th	Day	5% 2 15% 33% 34% 11%	621
	11th to 20th	Day	3 10% 33% 40% 14%	618
	21st to end	Day	6% 2 8% 28% 39% 17%	684
August	1st to 10th	Day	4 2 11% 33% 38% 13%	496
	11th to 20th	Day	4 3 17% 36% 30% 12%	437
	21st to end	Day	3 1 15% 34% 32% 14%	495
September	1st to 10th	Day	5% 1 17% 36% 27% 14%	450
	11th to 20th	Day	4 2 16% 31% 33% 15%	339
	21st to end	Day	9% 3 17% 24% 27% 20%	270
October	1st to 10th	Day	10% 5% 20% 26% 21% 18%	270
	11th to 20th	Day	3 4 23% 31% 27% 11%	270
	21st to end	Day	12% 6% 23% 30% 21% 8%	297
November	1st to 10th	Day	4 4 24% 38% 21% 9%	270
	11th to 20th	Day	4 7% 21% 37% 26% 4	270
	21st to end	Day	9% 9% 24% 33% 20% 6%	270
December	1st to 10th	Day	4 6% 24% 44% 19% 3	270
	11th to 20th	Day	9% 5% 26% 38% 21% 1	270
	21st to end	Day	8% 9% 33% 37% 11% 1	297
January	1st to 10th	Night	6% 5% 28% 46% 14% 2	950
	11th to 20th	Night	6% 5% 31% 43% 15% 1	950
	21st to end	Night	6% 5% 28% 49% 11% 1	877
February	1st to 10th	Night	7% 5% 34% 44% 9%	760
	11th to 20th	Night	7% 8% 40% 36% 9%	744
	21st to end	Night	7% 9% 39% 39% 6%	628
March	1st to 10th	Night	7% 7% 38% 38% 9% 1	722
	11th to 20th	Night	9% 9% 33% 37% 12% 1	698
	21st to end	Night	10% 12% 41% 31% 6%	594
April	1st to 10th	Night	10% 12% 44% 29% 5%	540
	11th to 20th	Night	12% 12% 43% 29% 4	533
	21st to end	Night	13% 11% 34% 32% 10%	540
May	1st to 10th	Night	13% 9% 36% 31% 10%	540
	11th to 20th	Night	11% 9% 33% 35% 12% 1	531
	21st to end	Night	12% 10% 38% 32% 8%	584
June	1st to 10th	Night	4 6% 32% 44% 13% 1	540
	11th to 20th	Night	5% 5% 42% 40% 8%	540
	21st to end	Night	6% 4 31% 47% 12% 1	540
July	1st to 10th	Night	5% 2 23% 47% 21% 1	525
	11th to 20th	Night	5% 1 16% 53% 25% 1	522
	21st to end	Night	9% 2 14% 45% 28% 2	591
August	1st to 10th	Night	7% 3 19% 38% 32% 1	537
	11th to 20th	Night	6% 2 23% 49% 20% 1	534
	21st to end	Night	4 1 18% 50% 25% 2	594
September	1st to 10th	Night	7% 1 24% 47% 20% 1	540
	11th to 20th	Night	6% 3 24% 45% 21% 1	540
	21st to end	Night	6% 5% 28% 42% 19% 1	540
October	1st to 10th	Night	11% 7% 29% 37% 16% 1	544
	11th to 20th	Night	4 7% 28% 41% 19% 2	720
	21st to end	Night	8% 8% 25% 38% 19% 2	919
November	1st to 10th	Night	4 4 26% 44% 20% 1	925
	11th to 20th	Night	6% 7% 25% 44% 18% 1	950
	21st to end	Night	8% 7% 24% 43% 17% 1	950
December	1st to 10th	Night	5% 6% 30% 45% 13% 1	950
	11th to 20th	Night	7% 7% 28% 43% 15% 1	950
	21st to end	Night	8% 10% 30% 41% 11%	1045

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