PNRO
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the PNRO clear sky chart from 2005-11-06 to 2025-09-19.

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	7% 6% 3 7% 4 4 4 3 5% 6% 51%	135373
	Night	9% 8% 3 8% 5% 4 4 4 5% 6% 44%	130984
Transparency	Day	21% 39% 7% 15% 13% 6%	130091
	Night	25% 40% 8% 14% 10% 4	127268
Seeing	Day	10% 9% 21% 27% 23% 10%	15484
	Night	13% 7% 22% 34% 21% 3	24516

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	16% 18% 8% 16% 6% 4 5% 4 5% 5% 13%	3639
	11th to 20th	Day	14% 14% 5% 8% 5% 5% 5% 5% 7% 6% 27%	3640
	21st to end	Day	13% 12% 5% 12% 6% 5% 5% 4 5% 5% 27%	4004
February	1st to 10th	Day	15% 15% 4 11% 7% 6% 4 3 6% 5% 23%	1730
	11th to 20th	Day	14% 13% 5% 9% 5% 5% 5% 3 5% 5% 31%	1843
	21st to end	Day	10% 10% 4 9% 7% 5% 6% 4 8% 7% 30%	1650
March	1st to 10th	Day	6% 8% 3 7% 6% 7% 6% 5% 7% 11% 35%	4518
	11th to 20th	Day	6% 9% 3 8% 5% 4 4 3 5% 7% 46%	4707
	21st to end	Day	9% 9% 4 9% 6% 7% 6% 5% 7% 8% 30%	5962
April	1st to 10th	Day	8% 8% 2 6% 6% 4 7% 5% 7% 9% 37%	2400
	11th to 20th	Day	6% 7% 2 6% 5% 4 5% 5% 5% 8% 47%	2369
	21st to end	Day	8% 6% 2 5% 4 4 4 4 6% 7% 50%	2566
May	1st to 10th	Day	7% 4 1 4 3 3 3 3 6% 8% 58%	6266
	11th to 20th	Day	6% 4 2 6% 4 5% 5% 3 5% 6% 56%	6277
	21st to end	Day	4 4 2 4 3 4 4 4 6% 8% 56%	6902
June	1st to 10th	Day	10% 5% 2 6% 4 4 5% 4 5% 7% 49%	2800
	11th to 20th	Day	7% 5% 2 5% 4 4 4 3 5% 7% 56%	2800
	21st to end	Day	3 3 1 4 3 3 3 3 6% 6% 67%	2800
July	1st to 10th	Day	2 2 1 2 1 2 2 2 3 3 80%	6319
	11th to 20th	Day	1 1 1 2 2 2 2 1 2 3 84%	6320
	21st to end	Day	2 1 1 2 1 2 2 2 3 6% 79%	6880
August	1st to 10th	Day	3 3 2 4 3 2 3 2 4 6% 68%	6210
	11th to 20th	Day	2 2 2 1 2 3 2 4 5% 76%	5810
	21st to end	Day	3 4 2 4 3 3 3 3 4 5% 65%	5984
September	1st to 10th	Day	2 2 1 2 2 3 3 3 4 6% 73%	2400
	11th to 20th	Day	5% 4 1 5% 3 4 4 3 6% 7% 59%	2320
	21st to end	Day	6% 5% 2 5% 4 4 3 3 5% 8% 56%	2090
October	1st to 10th	Day	6% 4 2 4 3 3 3 2 4 6% 64%	3250
	11th to 20th	Day	4 5% 2 6% 4 6% 5% 5% 8% 9% 47%	3111
	21st to end	Day	10% 7% 3 7% 5% 6% 5% 4 6% 7% 40%	3101
November	1st to 10th	Day	13% 9% 3 9% 5% 5% 5% 5% 5% 9% 33%	1764
	11th to 20th	Day	14% 14% 4 9% 6% 7% 5% 5% 6% 6% 25%	1689
	21st to end	Day	11% 14% 5% 13% 7% 7% 6% 5% 7% 6% 20%	1600
December	1st to 10th	Day	8% 9% 5% 19% 10% 8% 6% 5% 6% 5% 18%	2620
	11th to 20th	Day	16% 17% 9% 16% 8% 6% 5% 3 4 4 14%	3028
	21st to end	Day	15% 12% 6% 16% 9% 6% 6% 3 6% 5% 16%	4004
January	1st to 10th	Night	18% 21% 8% 15% 6% 5% 4 3 4 4 11%	6306
	11th to 20th	Night	12% 11% 4 9% 4 4 4 4 6% 7% 34%	6320
	21st to end	Night	11% 11% 5% 12% 6% 5% 5% 4 5% 5% 30%	6952
February	1st to 10th	Night	14% 13% 5% 10% 6% 6% 5% 4 5% 6% 27%	2665
	11th to 20th	Night	11% 12% 4 9% 5% 4 5% 4 6% 7% 33%	2442
	21st to end	Night	11% 11% 4 9% 7% 5% 5% 4 5% 7% 32%	1980
March	1st to 10th	Night	6% 7% 3 7% 5% 5% 6% 4 7% 10% 39%	5376
	11th to 20th	Night	7% 9% 4 8% 5% 5% 5% 3 6% 6% 43%	5291
	21st to end	Night	7% 8% 3 9% 7% 7% 7% 5% 8% 8% 29%	5011
April	1st to 10th	Night	12% 9% 2 8% 5% 4 5% 4 6% 8% 37%	2000
	11th to 20th	Night	10% 7% 2 6% 4 5% 4 3 4 6% 47%	1979
	21st to end	Night	9% 6% 3 7% 5% 4 5% 4 6% 7% 45%	1832
May	1st to 10th	Night	6% 4 1 5% 3 4 5% 3 5% 6% 57%	3692
	11th to 20th	Night	7% 4 2 6% 4 5% 5% 4 5% 6% 51%	3616
	21st to end	Night	5% 5% 1 5% 3 4 4 4 6% 8% 54%	3972
June	1st to 10th	Night	9% 5% 2 5% 4 4 5% 4 7% 8% 47%	1600
	11th to 20th	Night	8% 7% 2 5% 5% 4 4 4 5% 6% 51%	1600
	21st to end	Night	4 4 1 5% 4 4 4 3 5% 5% 62%	1600
July	1st to 10th	Night	2 2 1 2 2 2 2 2 4 5% 77%	3640
	11th to 20th	Night	1 1 1 1 1 2 2 2 4 85%	3640
	21st to end	Night	2 2 2 1 2 2 2 4 7% 77%	3970
August	1st to 10th	Night	3 2 1 4 3 3 4 3 6% 7% 64%	3582
	11th to 20th	Night	1 2 1 3 2 3 3 3 4 6% 71%	4035
	21st to end	Night	3 3 2 4 3 3 4 3 5% 6% 65%	5015
September	1st to 10th	Night	3 2 1 3 2 2 3 2 3 5% 73%	2000
	11th to 20th	Night	4 4 1 5% 4 3 4 2 6% 6% 61%	2036
	21st to end	Night	6% 3 1 4 3 3 4 4 5% 7% 59%	2090
October	1st to 10th	Night	5% 3 2 4 3 3 3 3 4 4 67%	3728
	11th to 20th	Night	7% 5% 2 5% 4 4 5% 5% 6% 8% 51%	3824
	21st to end	Night	10% 6% 3 6% 5% 5% 5% 4 6% 8% 42%	4457
November	1st to 10th	Night	15% 9% 3 8% 5% 5% 4 4 5% 6% 36%	2535
	11th to 20th	Night	16% 10% 3 9% 6% 5% 5% 4 5% 6% 29%	2710
	21st to end	Night	13% 11% 4 10% 6% 5% 5% 4 7% 7% 29%	2800
December	1st to 10th	Night	8% 8% 4 11% 7% 6% 5% 5% 7% 7% 32%	4560
	11th to 20th	Night	16% 19% 7% 15% 7% 5% 5% 4 5% 5% 12%	5176
	21st to end	Night	15% 12% 8% 13% 7% 6% 5% 3 5% 6% 20%	6952

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	54% 30% 9% 5% 2	3639
	11th to 20th	Day	39% 33% 12% 14% 2	3640
	21st to end	Day	39% 34% 7% 14% 5%	4004
February	1st to 10th	Day	28% 63% 4 3 2	1730
	11th to 20th	Day	25% 63% 5% 6% 2	1852
	21st to end	Day	20% 64% 6% 6% 4 1	1650
March	1st to 10th	Day	28% 31% 8% 20% 13%	4518
	11th to 20th	Day	25% 30% 9% 25% 11%	4707
	21st to end	Day	35% 34% 12% 14% 5%	5962
April	1st to 10th	Day	16% 63% 8% 9% 4	2400
	11th to 20th	Day	13% 63% 7% 10% 7% 1	2372
	21st to end	Day	11% 63% 5% 12% 9% 1	2566
May	1st to 10th	Day	16% 30% 5% 19% 25% 5%	6266
	11th to 20th	Day	19% 29% 7% 22% 17% 5%	6280
	21st to end	Day	14% 31% 13% 23% 18% 2	6904
June	1st to 10th	Day	10% 62% 4 11% 11% 3	2800
	11th to 20th	Day	10% 62% 3 10% 12% 4	2800
	21st to end	Day	6% 60% 4 11% 14% 4	2800
July	1st to 10th	Day	9% 25% 2 9% 23% 32%	6251
	11th to 20th	Day	6% 25% 2 7% 25% 36%	6245
	21st to end	Day	7% 25% 5% 20% 26% 17%	6681
August	1st to 10th	Day	10% 35% 3 7% 22% 23%	4524
	11th to 20th	Day	7% 36% 4 27% 21% 5%	4212
	21st to end	Day	14% 36% 5% 19% 20% 7%	4311
September	1st to 10th	Day	6% 57% 6% 14% 13% 4	2400
	11th to 20th	Day	12% 58% 6% 11% 11% 2	2320
	21st to end	Day	12% 61% 6% 14% 6% 1	2090
October	1st to 10th	Day	12% 37% 6% 27% 16% 2	3250
	11th to 20th	Day	19% 40% 14% 18% 9%	3111
	21st to end	Day	27% 39% 8% 19% 7% 1	3101
November	1st to 10th	Day	21% 58% 9% 9% 3	1764
	11th to 20th	Day	28% 58% 6% 7% 1 1	1689
	21st to end	Day	26% 60% 7% 5% 3	1600
December	1st to 10th	Day	43% 44% 7% 6% 1	2620
	11th to 20th	Day	54% 33% 6% 6% 1	3028
	21st to end	Day	49% 32% 9% 9% 1	4004
January	1st to 10th	Night	59% 30% 7% 4 1	6306
	11th to 20th	Night	34% 30% 17% 15% 4	6320
	21st to end	Night	37% 34% 10% 12% 7%	6952
February	1st to 10th	Night	26% 64% 6% 4 1	2665
	11th to 20th	Night	21% 64% 5% 8% 2	2444
	21st to end	Night	21% 63% 3 8% 3 1	1980
March	1st to 10th	Night	25% 32% 6% 21% 16% 1	5376
	11th to 20th	Night	27% 30% 7% 25% 10%	5291
	21st to end	Night	34% 34% 12% 17% 3	5011
April	1st to 10th	Night	17% 63% 7% 9% 5%	2000
	11th to 20th	Night	14% 62% 4 10% 8% 2	1979
	21st to end	Night	11% 63% 6% 10% 8% 2	1832
May	1st to 10th	Night	18% 30% 7% 13% 28% 4	3689
	11th to 20th	Night	22% 30% 7% 26% 14% 2	3616
	21st to end	Night	16% 31% 14% 25% 13% 1	3972
June	1st to 10th	Night	8% 63% 3 12% 11% 3	1600
	11th to 20th	Night	11% 62% 3 10% 11% 3	1600
	21st to end	Night	10% 60% 3 10% 11% 6%	1600
July	1st to 10th	Night	8% 25% 2 14% 17% 34%	3600
	11th to 20th	Night	4 24% 2 6% 25% 39%	3592
	21st to end	Night	8% 24% 3 18% 27% 20%	3857
August	1st to 10th	Night	11% 35% 2 6% 18% 27%	2601
	11th to 20th	Night	9% 36% 4 26% 19% 6%	2909
	21st to end	Night	10% 36% 5% 21% 17% 12%	3608
September	1st to 10th	Night	8% 57% 5% 11% 13% 5%	2000
	11th to 20th	Night	11% 58% 6% 12% 8% 5%	2036
	21st to end	Night	10% 60% 5% 13% 10% 1	2090
October	1st to 10th	Night	11% 36% 6% 22% 23% 2	3728
	11th to 20th	Night	17% 39% 12% 17% 15%	3824
	21st to end	Night	25% 38% 6% 23% 7%	4457
November	1st to 10th	Night	21% 57% 7% 11% 4	2535
	11th to 20th	Night	22% 59% 7% 8% 3 1	2710
	21st to end	Night	21% 59% 8% 8% 3	2800
December	1st to 10th	Night	28% 43% 17% 9% 2	4560
	11th to 20th	Night	53% 35% 8% 3 1	5176
	21st to end	Night	44% 32% 10% 10% 3	6952

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	24% 11% 22% 23% 17% 5%	200
	11th to 20th	Day	20% 8% 25% 29% 14% 5%	200
	21st to end	Day	18% 8% 21% 29% 20% 3	220
February	1st to 10th	Day	22% 13% 24% 25% 12% 3	330
	11th to 20th	Day	23% 13% 21% 24% 16% 3	440
	21st to end	Day	15% 14% 22% 25% 21% 3	414
March	1st to 10th	Day	17% 14% 28% 22% 15% 4	500
	11th to 20th	Day	12% 12% 22% 28% 18% 8%	500
	21st to end	Day	13% 14% 29% 27% 14% 3	550
April	1st to 10th	Day	13% 16% 26% 22% 14% 8%	500
	11th to 20th	Day	10% 15% 32% 22% 15% 7%	492
	21st to end	Day	7% 13% 27% 28% 18% 7%	500
May	1st to 10th	Day	8% 10% 24% 27% 24% 7%	503
	11th to 20th	Day	12% 8% 26% 20% 24% 11%	528
	21st to end	Day	9% 6% 22% 24% 25% 13%	573
June	1st to 10th	Day	9% 10% 19% 23% 25% 14%	530
	11th to 20th	Day	8% 12% 29% 27% 18% 6%	530
	21st to end	Day	3 5% 20% 28% 30% 13%	530
July	1st to 10th	Day	2 2 16% 33% 39% 9%	526
	11th to 20th	Day	1 4 18% 35% 30% 12%	524
	21st to end	Day	1 3 13% 32% 33% 18%	575
August	1st to 10th	Day	3 6% 12% 30% 31% 20%	527
	11th to 20th	Day	2 3 14% 27% 31% 23%	517
	21st to end	Day	5% 6% 17% 27% 28% 16%	583
September	1st to 10th	Day	3 3 15% 29% 33% 18%	530
	11th to 20th	Day	7% 6% 18% 26% 27% 17%	523
	21st to end	Day	8% 8% 17% 28% 27% 12%	490
October	1st to 10th	Day	11% 6% 18% 27% 26% 11%	490
	11th to 20th	Day	10% 10% 19% 23% 26% 12%	465
	21st to end	Day	16% 11% 23% 28% 16% 6%	330
November	1st to 10th	Day	16% 13% 25% 24% 19% 3	300
	11th to 20th	Day	21% 11% 24% 25% 15% 4	244
	21st to end	Day	13% 10% 23% 24% 26% 5%	200
December	1st to 10th	Day	13% 10% 21% 27% 16% 14%	200
	11th to 20th	Day	22% 13% 22% 31% 11% 3	200
	21st to end	Day	20% 8% 28% 30% 11% 3	220
January	1st to 10th	Night	28% 9% 21% 25% 15% 2	800
	11th to 20th	Night	19% 9% 19% 33% 17% 3	800
	21st to end	Night	19% 6% 17% 35% 21% 3	880
February	1st to 10th	Night	20% 11% 21% 32% 15% 2	800
	11th to 20th	Night	17% 9% 20% 33% 18% 3	784
	21st to end	Night	17% 14% 23% 33% 12% 2	660
March	1st to 10th	Night	14% 10% 26% 31% 17% 2	800
	11th to 20th	Night	12% 10% 24% 33% 20% 2	800
	21st to end	Night	13% 10% 31% 32% 13% 1	880
April	1st to 10th	Night	15% 9% 23% 35% 16% 2	800
	11th to 20th	Night	12% 10% 29% 28% 17% 4	791
	21st to end	Night	10% 9% 28% 32% 20% 2	632
May	1st to 10th	Night	7% 6% 27% 37% 20% 2	426
	11th to 20th	Night	10% 8% 18% 37% 26% 2	394
	21st to end	Night	8% 6% 24% 35% 24% 3	433
June	1st to 10th	Night	11% 6% 27% 34% 19% 3	400
	11th to 20th	Night	10% 10% 33% 34% 12% 1	400
	21st to end	Night	5% 5% 27% 37% 23% 3	400
July	1st to 10th	Night	4 3 24% 41% 27% 2	390
	11th to 20th	Night	2 3 29% 43% 21% 2	388
	21st to end	Night	2 4 22% 39% 32% 2	438
August	1st to 10th	Night	4 2 20% 41% 30% 3	398
	11th to 20th	Night	3 2 17% 35% 34% 8%	574
	21st to end	Night	3 3 18% 38% 32% 6%	880
September	1st to 10th	Night	4 2 18% 38% 34% 5%	800
	11th to 20th	Night	5% 3 18% 33% 33% 8%	792
	21st to end	Night	7% 5% 19% 32% 31% 7%	760
October	1st to 10th	Night	10% 4 18% 38% 26% 4	760
	11th to 20th	Night	11% 8% 18% 33% 25% 6%	760
	21st to end	Night	12% 7% 25% 31% 19% 5%	836
November	1st to 10th	Night	18% 6% 24% 34% 17% 1	780
	11th to 20th	Night	20% 10% 23% 29% 15% 3	800
	21st to end	Night	17% 8% 16% 35% 21% 4	800
December	1st to 10th	Night	16% 9% 17% 35% 19% 5%	800
	11th to 20th	Night	22% 8% 20% 35% 13% 2	800
	21st to end	Night	26% 10% 22% 29% 12% 1	880

Page generated at 2025-09-15 22:49:13. Version ac07

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