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 2025-05-11.

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	4 4 1 4 3 3 3 3 5% 8% 61%	117634
	Night	5% 5% 2 5% 4 4 4 3 5% 7% 56%	120159
Transparency	Day	22% 5% 10% 24% 27% 12%	117376
	Night	26% 5% 10% 23% 25% 11%	120008
Seeing	Day	7% 6% 23% 33% 23% 8%	16161
	Night	7% 7% 30% 41% 14% 1	26265

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	3 7% 3 7% 3 3 4 3 5% 8% 55%	3638
	11th to 20th	Day	2 5% 2 6% 4 4 5% 4 5% 6% 57%	3640
	21st to end	Day	3 6% 2 5% 3 3 3 3 4 6% 61%	4446
February	1st to 10th	Day	4 6% 2 5% 4 5% 4 3 5% 7% 54%	1966
	11th to 20th	Day	2 6% 2 7% 5% 4 4 3 5% 6% 55%	1949
	21st to end	Day	3 5% 2 6% 5% 3 3 4 5% 5% 60%	1650
March	1st to 10th	Day	2 4 2 4 3 4 4 3 4 6% 65%	4500
	11th to 20th	Day	6% 5% 1 5% 3 3 3 3 5% 8% 58%	4663
	21st to end	Day	4 3 1 4 3 3 4 4 6% 9% 59%	5969
April	1st to 10th	Day	5% 5% 1 4 3 3 4 3 6% 7% 59%	2400
	11th to 20th	Day	7% 4 2 5% 4 4 3 4 6% 8% 54%	2371
	21st to end	Day	7% 6% 2 5% 4 4 4 4 6% 7% 53%	2410
May	1st to 10th	Day	8% 5% 1 4 3 3 3 3 5% 6% 57%	4250
	11th to 20th	Day	5% 4 1 5% 3 3 4 3 6% 8% 56%	4083
	21st to end	Day	4 3 1 3 3 3 3 3 6% 8% 63%	4749
June	1st to 10th	Day	4 2 1 3 3 3 4 4 8% 12% 57%	2660
	11th to 20th	Day	5% 3 1 3 3 3 3 3 6% 7% 63%	2660
	21st to end	Day	3 3 1 5% 3 4 4 4 7% 9% 57%	2660
July	1st to 10th	Day	4 3 1 4 4 4 5% 4 7% 9% 55%	4430
	11th to 20th	Day	2 1 1 3 2 3 4 4 6% 11% 63%	4430
	21st to end	Day	4 2 1 3 3 3 4 4 7% 10% 60%	4802
August	1st to 10th	Day	6% 4 2 4 3 4 4 4 7% 10% 52%	3871
	11th to 20th	Day	4 2 1 3 3 3 4 3 6% 9% 61%	3726
	21st to end	Day	3 2 1 3 3 3 4 4 7% 11% 59%	4180
September	1st to 10th	Day	4 3 1 3 3 3 4 3 6% 7% 63%	2280
	11th to 20th	Day	5% 2 1 3 2 3 3 3 4 6% 67%	2162
	21st to end	Day	6% 3 2 3 2 2 3 3 4 6% 67%	2090
October	1st to 10th	Day	4 2 1 2 1 2 1 2 3 4 77%	3461
	11th to 20th	Day	5% 2 1 3 3 2 4 2 5% 7% 67%	3150
	21st to end	Day	5% 4 1 4 2 2 1 2 3 4 73%	3169
November	1st to 10th	Day	4 4 2 3 2 2 2 2 5% 6% 68%	1763
	11th to 20th	Day	3 4 2 4 2 3 2 3 4 6% 68%	1800
	21st to end	Day	5% 5% 2 4 3 3 2 2 4 6% 64%	1800
December	1st to 10th	Day	2 6% 2 3 2 2 2 2 3 4 70%	2826
	11th to 20th	Day	3 4 2 3 2 2 3 3 6% 9% 64%	3026
	21st to end	Day	4 7% 2 6% 3 4 4 3 5% 6% 57%	4004
January	1st to 10th	Night	3 7% 3 7% 4 4 4 3 5% 7% 53%	6327
	11th to 20th	Night	2 5% 2 6% 4 4 3 3 5% 7% 59%	6340
	21st to end	Night	3 6% 2 5% 3 3 3 3 4 6% 63%	6563
February	1st to 10th	Night	5% 5% 3 6% 5% 4 3 2 4 6% 57%	2431
	11th to 20th	Night	5% 7% 3 7% 4 3 4 3 6% 8% 49%	2338
	21st to end	Night	3 5% 2 5% 3 3 3 3 5% 7% 60%	1980
March	1st to 10th	Night	3 5% 2 4 3 3 4 3 4 6% 64%	5374
	11th to 20th	Night	6% 6% 2 5% 3 3 4 3 5% 7% 55%	5302
	21st to end	Night	7% 6% 2 6% 4 4 4 3 6% 8% 49%	4984
April	1st to 10th	Night	6% 6% 2 5% 3 3 4 3 5% 8% 55%	2000
	11th to 20th	Night	8% 5% 2 5% 3 3 5% 3 6% 7% 52%	1977
	21st to end	Night	10% 6% 2 6% 4 4 4 4 6% 7% 49%	1988
May	1st to 10th	Night	11% 7% 2 6% 4 3 4 3 5% 6% 49%	2940
	11th to 20th	Night	11% 6% 2 5% 4 4 4 4 5% 8% 47%	2828
	21st to end	Night	9% 5% 2 6% 5% 5% 5% 5% 7% 7% 46%	2835
June	1st to 10th	Night	4 3 1 3 4 4 5% 5% 8% 9% 54%	1520
	11th to 20th	Night	4 4 2 4 3 4 4 5% 7% 8% 56%	1520
	21st to end	Night	5% 6% 2 4 3 4 5% 5% 7% 9% 50%	1520
July	1st to 10th	Night	6% 4 2 5% 4 5% 6% 5% 8% 10% 45%	2540
	11th to 20th	Night	4 4 2 6% 4 4 6% 4 7% 10% 50%	2540
	21st to end	Night	5% 4 2 5% 4 5% 5% 5% 7% 7% 51%	2761
August	1st to 10th	Night	11% 5% 2 8% 6% 6% 7% 5% 7% 9% 33%	3061
	11th to 20th	Night	6% 4 2 7% 5% 5% 6% 5% 8% 10% 41%	3113
	21st to end	Night	3 2 1 6% 5% 6% 7% 6% 10% 11% 44%	3487
September	1st to 10th	Night	6% 3 1 4 3 4 3 3 6% 9% 57%	1900
	11th to 20th	Night	5% 3 1 3 3 3 4 3 5% 9% 60%	2016
	21st to end	Night	6% 3 2 4 3 2 3 2 4 5% 64%	2090
October	1st to 10th	Night	5% 2 1 4 3 3 3 3 4 5% 67%	3498
	11th to 20th	Night	7% 3 1 3 2 2 3 3 5% 7% 63%	3778
	21st to end	Night	4 5% 1 4 3 2 2 2 3 4 69%	4356
November	1st to 10th	Night	3 3 1 3 2 3 3 2 4 6% 67%	2535
	11th to 20th	Night	3 4 1 4 2 2 3 2 3 4 70%	2600
	21st to end	Night	4 5% 2 4 2 3 3 3 5% 6% 63%	2600
December	1st to 10th	Night	2 4 1 4 3 2 3 2 4 5% 70%	4362
	11th to 20th	Night	3 4 2 4 3 3 4 3 4 7% 62%	5181
	21st to end	Night	4 6% 2 6% 4 4 4 3 6% 7% 53%	6974

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	28% 4 8% 32% 20% 8%	3639
	11th to 20th	Day	27% 4 10% 22% 27% 11%	3640
	21st to end	Day	24% 3 9% 26% 28% 10%	4446
February	1st to 10th	Day	29% 5% 9% 25% 22% 10%	1966
	11th to 20th	Day	28% 4 9% 26% 25% 8%	1959
	21st to end	Day	26% 3 7% 19% 28% 17%	1650
March	1st to 10th	Day	22% 3 5% 16% 34% 20%	4500
	11th to 20th	Day	24% 4 6% 20% 36% 9%	4663
	21st to end	Day	20% 5% 10% 38% 22% 4	5969
April	1st to 10th	Day	23% 5% 9% 24% 25% 14%	2400
	11th to 20th	Day	27% 4 10% 25% 20% 13%	2374
	21st to end	Day	28% 5% 11% 24% 23% 8%	2410
May	1st to 10th	Day	26% 5% 11% 26% 19% 14%	4250
	11th to 20th	Day	24% 6% 15% 26% 23% 7%	4087
	21st to end	Day	18% 5% 14% 23% 24% 15%	4749
June	1st to 10th	Day	18% 6% 16% 29% 22% 9%	2660
	11th to 20th	Day	19% 4 9% 22% 25% 20%	2660
	21st to end	Day	21% 5% 11% 23% 25% 15%	2660
July	1st to 10th	Day	23% 7% 9% 18% 30% 13%	4361
	11th to 20th	Day	14% 6% 15% 23% 31% 12%	4355
	21st to end	Day	18% 6% 13% 21% 24% 17%	4651
August	1st to 10th	Day	25% 7% 18% 29% 15% 5%	3857
	11th to 20th	Day	19% 6% 14% 23% 27% 10%	3762
	21st to end	Day	18% 6% 12% 31% 22% 11%	4179
September	1st to 10th	Day	20% 5% 9% 24% 26% 17%	2280
	11th to 20th	Day	18% 4 8% 19% 29% 22%	2162
	21st to end	Day	18% 4 7% 18% 36% 17%	2090
October	1st to 10th	Day	13% 2 6% 17% 37% 25%	3461
	11th to 20th	Day	18% 3 10% 24% 35% 11%	3150
	21st to end	Day	18% 2 5% 23% 36% 17%	3169
November	1st to 10th	Day	18% 3 9% 24% 29% 16%	1763
	11th to 20th	Day	18% 3 6% 28% 32% 13%	1800
	21st to end	Day	23% 2 7% 19% 33% 16%	1800
December	1st to 10th	Day	19% 3 7% 26% 37% 9%	2826
	11th to 20th	Day	18% 3 7% 29% 37% 7%	3024
	21st to end	Day	29% 3 7% 26% 27% 8%	4004
January	1st to 10th	Night	29% 5% 9% 31% 21% 5%	6327
	11th to 20th	Night	24% 4 11% 23% 25% 14%	6340
	21st to end	Night	24% 3 10% 24% 27% 12%	6563
February	1st to 10th	Night	29% 3 9% 24% 26% 10%	2431
	11th to 20th	Night	31% 5% 10% 25% 22% 7%	2338
	21st to end	Night	24% 3 7% 21% 29% 16%	1980
March	1st to 10th	Night	21% 4 5% 17% 33% 20%	5374
	11th to 20th	Night	28% 4 9% 16% 26% 17%	5302
	21st to end	Night	30% 6% 11% 30% 19% 4	4984
April	1st to 10th	Night	26% 5% 8% 23% 25% 12%	2000
	11th to 20th	Night	29% 6% 11% 24% 20% 10%	1977
	21st to end	Night	32% 5% 14% 24% 19% 6%	1988
May	1st to 10th	Night	32% 8% 17% 17% 16% 10%	2940
	11th to 20th	Night	34% 7% 16% 22% 14% 7%	2829
	21st to end	Night	31% 8% 13% 20% 18% 9%	2836
June	1st to 10th	Night	21% 8% 22% 26% 16% 7%	1520
	11th to 20th	Night	24% 6% 13% 23% 19% 15%	1520
	21st to end	Night	28% 7% 11% 21% 22% 11%	1520
July	1st to 10th	Night	30% 8% 10% 23% 25% 4	2500
	11th to 20th	Night	28% 7% 16% 19% 25% 5%	2492
	21st to end	Night	31% 6% 11% 18% 23% 11%	2674
August	1st to 10th	Night	44% 8% 14% 20% 9% 5%	3052
	11th to 20th	Night	34% 9% 15% 22% 16% 4	3143
	21st to end	Night	29% 8% 13% 26% 17% 7%	3487
September	1st to 10th	Night	24% 5% 12% 23% 22% 15%	1900
	11th to 20th	Night	20% 5% 10% 21% 25% 19%	2016
	21st to end	Night	22% 4 6% 19% 33% 15%	2090
October	1st to 10th	Night	19% 4 8% 17% 30% 23%	3498
	11th to 20th	Night	20% 4 7% 26% 27% 16%	3778
	21st to end	Night	21% 3 5% 21% 35% 15%	4356
November	1st to 10th	Night	19% 2 8% 24% 28% 18%	2535
	11th to 20th	Night	18% 3 6% 24% 34% 15%	2600
	21st to end	Night	22% 3 8% 21% 31% 16%	2600
December	1st to 10th	Night	17% 2 7% 34% 30% 10%	4362
	11th to 20th	Night	22% 3 5% 27% 34% 8%	5182
	21st to end	Night	30% 4 8% 24% 27% 7%	6974

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% 16% 4	300
	11th to 20th	Day	11% 8% 26% 36% 16% 3	300
	21st to end	Day	7% 6% 25% 40% 19% 3	330
February	1st to 10th	Day	6% 8% 39% 35% 12% 1	300
	11th to 20th	Day	5% 5% 39% 38% 13% 1	300
	21st to end	Day	4 10% 37% 39% 9% 2	249
March	1st to 10th	Day	4 11% 27% 38% 16% 4	300
	11th to 20th	Day	11% 9% 32% 25% 19% 6%	323
	21st to end	Day	11% 13% 31% 32% 10% 2	550
April	1st to 10th	Day	10% 12% 34% 28% 13% 3	500
	11th to 20th	Day	10% 16% 32% 27% 12% 2	492
	21st to end	Day	11% 15% 30% 29% 12% 3	510
May	1st to 10th	Day	11% 12% 28% 29% 18% 3	703
	11th to 20th	Day	10% 8% 27% 31% 19% 6%	676
	21st to end	Day	8% 10% 28% 29% 20% 4	734
June	1st to 10th	Day	4 5% 21% 30% 29% 11%	680
	11th to 20th	Day	5% 6% 26% 33% 25% 5%	680
	21st to end	Day	5% 4 21% 33% 30% 7%	680
July	1st to 10th	Day	6% 2 17% 33% 33% 10%	671
	11th to 20th	Day	3 1 10% 33% 40% 14%	668
	21st to end	Day	6% 2 9% 29% 38% 16%	739
August	1st to 10th	Day	4 2 11% 33% 37% 13%	538
	11th to 20th	Day	3 2 17% 36% 30% 11%	477
	21st to end	Day	3 1 15% 35% 31% 14%	539
September	1st to 10th	Day	4 1 17% 35% 28% 15%	490
	11th to 20th	Day	3 2 16% 31% 32% 15%	372
	21st to end	Day	9% 3 17% 24% 28% 19%	300
October	1st to 10th	Day	9% 5% 18% 26% 23% 19%	300
	11th to 20th	Day	4 4 23% 32% 26% 10%	300
	21st to end	Day	11% 7% 23% 29% 22% 7%	330
November	1st to 10th	Day	8% 4 25% 36% 20% 8%	300
	11th to 20th	Day	5% 7% 22% 38% 25% 4	300
	21st to end	Day	9% 9% 22% 34% 20% 6%	300
December	1st to 10th	Day	4 6% 22% 44% 20% 4	300
	11th to 20th	Day	8% 4 26% 40% 20% 1	300
	21st to end	Day	8% 8% 31% 40% 11% 2	330
January	1st to 10th	Night	6% 5% 28% 45% 14% 2	1000
	11th to 20th	Night	6% 5% 31% 43% 14% 1	1000
	21st to end	Night	6% 5% 29% 48% 11% 1	923
February	1st to 10th	Night	7% 6% 36% 43% 9%	800
	11th to 20th	Night	7% 8% 40% 36% 9%	784
	21st to end	Night	6% 9% 38% 40% 7%	660
March	1st to 10th	Night	7% 8% 38% 38% 9% 1	800
	11th to 20th	Night	10% 8% 34% 36% 11% 1	775
	21st to end	Night	10% 12% 42% 31% 5%	660
April	1st to 10th	Night	10% 13% 44% 29% 5%	600
	11th to 20th	Night	11% 13% 43% 29% 4	593
	21st to end	Night	13% 13% 33% 31% 9%	600
May	1st to 10th	Night	13% 11% 36% 29% 10%	600
	11th to 20th	Night	11% 9% 31% 35% 12% 1	563
	21st to end	Night	11% 11% 39% 31% 8%	617
June	1st to 10th	Night	4 6% 32% 44% 12% 1	570
	11th to 20th	Night	5% 6% 42% 40% 8%	570
	21st to end	Night	6% 4 32% 46% 12% 1	570
July	1st to 10th	Night	6% 3 24% 47% 20% 1	555
	11th to 20th	Night	5% 1 16% 53% 24% 1	552
	21st to end	Night	8% 3 15% 45% 27% 2	624
August	1st to 10th	Night	7% 3 21% 38% 31% 1	567
	11th to 20th	Night	6% 3 24% 48% 19% 1	564
	21st to end	Night	4 1 19% 50% 25% 2	627
September	1st to 10th	Night	7% 2 24% 47% 20% 1	570
	11th to 20th	Night	6% 3 25% 44% 20% 1	570
	21st to end	Night	6% 5% 27% 42% 19% 1	570
October	1st to 10th	Night	11% 7% 28% 37% 17% 1	575
	11th to 20th	Night	4 7% 28% 40% 19% 2	760
	21st to end	Night	8% 8% 25% 39% 18% 2	971
November	1st to 10th	Night	6% 5% 26% 43% 19% 1	975
	11th to 20th	Night	6% 6% 26% 44% 17% 1	1000
	21st to end	Night	8% 7% 25% 43% 16% 1	1000
December	1st to 10th	Night	5% 6% 30% 45% 14% 1	1000
	11th to 20th	Night	6% 6% 30% 43% 14% 1	1000
	21st to end	Night	8% 10% 30% 41% 11%	1100

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