St. Peter's
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the St. Peter's clear sky chart from 2005-11-06 to 2026-04-02.

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	20% 12% 4 9% 5% 5% 4 3 5% 6% 25%	131727
	Night	21% 12% 4 10% 6% 5% 4 3 5% 5% 24%	130380
Transparency	Day	37% 39% 8% 11% 5%	135200
	Night	39% 38% 8% 11% 5%	135004
Seeing	Day	25% 14% 28% 23% 8% 1	19500
	Night	27% 14% 28% 23% 7% 1	27059

By Month

Percentage Cloud Cover NE (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	13% 20% 9% 19% 7% 5% 4 3 4 4 13%	4690
	11th to 20th	Day	19% 13% 11% 14% 6% 6% 4 3 4 5% 15%	4681
	21st to end	Day	13% 11% 6% 13% 11% 8% 5% 3 5% 6% 20%	5361
February	1st to 10th	Day	19% 12% 4 15% 9% 6% 6% 4 6% 6% 14%	1890
	11th to 20th	Day	16% 13% 5% 12% 7% 8% 6% 4 5% 6% 18%	1843
	21st to end	Day	13% 15% 4 10% 8% 4 4 4 4 6% 28%	1725
March	1st to 10th	Day	24% 11% 4 10% 8% 5% 3 3 3 4 25%	4574
	11th to 20th	Day	14% 17% 6% 12% 7% 5% 4 3 4 5% 23%	5089
	21st to end	Day	28% 13% 6% 12% 5% 4 4 2 4 4 17%	5901
April	1st to 10th	Day	23% 13% 4 8% 4 4 5% 4 5% 5% 25%	2412
	11th to 20th	Day	23% 12% 3 7% 4 4 4 2 4 5% 32%	2374
	21st to end	Day	24% 12% 3 8% 4 4 3 3 4 6% 28%	2557
May	1st to 10th	Day	26% 16% 4 7% 4 4 4 3 4 6% 23%	6345
	11th to 20th	Day	22% 13% 3 7% 4 4 4 3 5% 7% 29%	6318
	21st to end	Day	22% 12% 3 7% 5% 4 4 3 5% 6% 30%	6948
June	1st to 10th	Day	29% 11% 2 7% 5% 5% 4 4 6% 7% 22%	2800
	11th to 20th	Day	20% 10% 3 7% 4 5% 4 4 5% 8% 31%	2800
	21st to end	Day	29% 12% 3 7% 4 4 4 3 6% 6% 22%	2800
July	1st to 10th	Day	19% 11% 4 10% 6% 5% 5% 4 5% 8% 24%	6360
	11th to 20th	Day	17% 9% 3 8% 5% 5% 5% 4 6% 8% 31%	6360
	21st to end	Day	16% 7% 2 6% 4 4 5% 4 7% 8% 37%	6927
August	1st to 10th	Day	20% 11% 4 8% 5% 4 5% 4 5% 7% 27%	4507
	11th to 20th	Day	20% 8% 3 7% 5% 4 4 4 6% 9% 30%	4143
	21st to end	Day	12% 8% 2 6% 4 5% 5% 3 6% 7% 41%	4312
September	1st to 10th	Day	18% 8% 2 6% 4 3 4 4 6% 8% 36%	2400
	11th to 20th	Day	18% 8% 3 6% 5% 5% 4 4 6% 7% 34%	2400
	21st to end	Day	20% 8% 3 6% 4 4 4 4 5% 5% 38%	2135
October	1st to 10th	Day	17% 8% 3 7% 6% 5% 5% 4 6% 7% 32%	2000
	11th to 20th	Day	20% 11% 3 7% 5% 5% 5% 3 7% 7% 28%	2000
	21st to end	Day	22% 12% 4 10% 5% 5% 5% 3 6% 6% 22%	2036
November	1st to 10th	Day	23% 13% 3 8% 5% 5% 5% 5% 5% 6% 21%	1843
	11th to 20th	Day	23% 19% 4 10% 6% 5% 5% 3 4 5% 17%	1680
	21st to end	Day	16% 17% 6% 12% 7% 6% 5% 4 4 5% 16%	1680
December	1st to 10th	Day	16% 16% 9% 12% 7% 6% 4 4 4 5% 17%	2690
	11th to 20th	Day	28% 14% 5% 16% 5% 5% 4 3 4 3 13%	3076
	21st to end	Day	20% 14% 6% 14% 7% 5% 5% 4 4 5% 17%	4070
January	1st to 10th	Night	17% 17% 7% 17% 8% 6% 4 3 4 4 13%	8234
	11th to 20th	Night	23% 13% 7% 17% 7% 5% 5% 3 4 4 12%	8234
	21st to end	Night	11% 12% 5% 15% 12% 10% 5% 4 5% 5% 17%	8839
February	1st to 10th	Night	20% 12% 4 13% 7% 7% 6% 5% 6% 5% 16%	2730
	11th to 20th	Night	17% 14% 4 12% 9% 6% 5% 4 5% 5% 20%	2663
	21st to end	Night	12% 15% 5% 13% 6% 4 6% 4 5% 6% 25%	2080
March	1st to 10th	Night	23% 13% 4 9% 8% 5% 4 3 4 4 23%	5486
	11th to 20th	Night	21% 14% 5% 9% 5% 4 4 3 4 5% 27%	5089
	21st to end	Night	31% 13% 6% 12% 6% 4 3 2 2 3 19%	5272
April	1st to 10th	Night	24% 14% 3 6% 5% 4 4 4 5% 6% 25%	2011
	11th to 20th	Night	21% 11% 3 7% 4 5% 3 4 6% 6% 31%	1976
	21st to end	Night	25% 11% 2 8% 4 4 3 3 4 5% 32%	1839
May	1st to 10th	Night	29% 13% 4 6% 4 4 5% 3 4 5% 23%	3615
	11th to 20th	Night	21% 16% 4 6% 4 4 3 3 4 6% 29%	3596
	21st to end	Night	18% 15% 4 8% 5% 4 4 3 5% 7% 28%	3948
June	1st to 10th	Night	32% 13% 2 6% 5% 4 3 2 4 5% 25%	1600
	11th to 20th	Night	24% 11% 2 5% 4 3 3 3 4 6% 33%	1600
	21st to end	Night	30% 13% 4 7% 4 4 4 3 4 5% 21%	1600
July	1st to 10th	Night	24% 13% 4 10% 5% 4 5% 4 6% 7% 20%	3619
	11th to 20th	Night	23% 13% 3 6% 3 3 3 3 4 6% 33%	3620
	21st to end	Night	21% 11% 3 7% 4 4 4 4 6% 6% 31%	3948
August	1st to 10th	Night	20% 13% 3 7% 4 4 4 4 5% 6% 28%	2649
	11th to 20th	Night	20% 10% 3 7% 4 5% 4 4 6% 8% 29%	2903
	21st to end	Night	15% 6% 2 5% 4 4 4 4 5% 8% 45%	3586
September	1st to 10th	Night	19% 6% 2 5% 5% 4 4 4 5% 7% 39%	2000
	11th to 20th	Night	21% 8% 3 6% 3 4 4 3 3 7% 39%	2000
	21st to end	Night	20% 9% 3 7% 4 3 4 3 5% 6% 38%	2264
October	1st to 10th	Night	18% 7% 2 6% 4 5% 5% 4 5% 7% 37%	2400
	11th to 20th	Night	22% 8% 3 6% 4 4 4 3 5% 7% 33%	2400
	21st to end	Night	24% 11% 3 8% 5% 4 4 3 5% 6% 26%	2803
November	1st to 10th	Night	21% 12% 3 8% 5% 4 5% 4 5% 7% 24%	2673
	11th to 20th	Night	24% 14% 4 8% 6% 5% 5% 4 6% 5% 19%	2940
	21st to end	Night	22% 15% 5% 9% 6% 6% 6% 4 6% 6% 16%	2940
December	1st to 10th	Night	18% 13% 7% 13% 6% 5% 5% 3 5% 5% 20%	4720
	11th to 20th	Night	27% 12% 6% 13% 5% 5% 4 3 4 4 18%	5353
	21st to end	Night	24% 14% 6% 12% 6% 5% 4 3 4 4 20%	7150

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	58% 25% 8% 7% 2	4691
	11th to 20th	Day	53% 24% 11% 9% 3	4710
	21st to end	Day	48% 26% 6% 10% 8% 1	5361
February	1st to 10th	Day	26% 61% 5% 5% 2	1890
	11th to 20th	Day	25% 60% 6% 6% 2	1851
	21st to end	Day	22% 59% 4 8% 6% 1	1725
March	1st to 10th	Day	46% 28% 7% 10% 9% 1	4578
	11th to 20th	Day	42% 34% 12% 8% 4	5089
	21st to end	Day	53% 29% 7% 7% 3 1	5901
April	1st to 10th	Day	24% 61% 3 6% 4 1	2412
	11th to 20th	Day	21% 63% 5% 7% 4	2374
	21st to end	Day	21% 63% 5% 5% 6%	2557
May	1st to 10th	Day	44% 32% 8% 12% 4	6357
	11th to 20th	Day	38% 32% 12% 12% 6%	6322
	21st to end	Day	38% 32% 9% 17% 4	6948
June	1st to 10th	Day	23% 62% 4 7% 4	2800
	11th to 20th	Day	18% 63% 7% 8% 3 1	2800
	21st to end	Day	24% 61% 4 7% 4	2800
July	1st to 10th	Day	47% 30% 9% 9% 5%	6290
	11th to 20th	Day	38% 30% 10% 17% 5%	6284
	21st to end	Day	30% 28% 16% 20% 6%	6730
August	1st to 10th	Day	38% 39% 8% 11% 4	4493
	11th to 20th	Day	32% 40% 11% 15% 2	4182
	21st to end	Day	26% 36% 9% 20% 8%	4312
September	1st to 10th	Day	19% 58% 7% 11% 6%	2400
	11th to 20th	Day	23% 58% 6% 10% 4	2400
	21st to end	Day	16% 58% 6% 13% 7%	2135
October	1st to 10th	Day	35% 37% 9% 13% 7%	3260
	11th to 20th	Day	25% 38% 7% 19% 10%	3240
	21st to end	Day	33% 37% 12% 15% 4	3270
November	1st to 10th	Day	29% 56% 7% 6% 2	1843
	11th to 20th	Day	35% 56% 5% 4 1	1680
	21st to end	Day	33% 56% 5% 4 2	1680
December	1st to 10th	Day	42% 37% 6% 11% 4	2690
	11th to 20th	Day	50% 34% 6% 6% 4	3075
	21st to end	Day	47% 31% 9% 7% 5%	4070
January	1st to 10th	Night	55% 26% 10% 6% 3	8234
	11th to 20th	Night	59% 24% 8% 7% 2	8280
	21st to end	Night	52% 26% 7% 8% 7%	8839
February	1st to 10th	Night	28% 61% 4 4 3	2730
	11th to 20th	Night	26% 59% 5% 7% 2	2666
	21st to end	Night	23% 60% 4 8% 5%	2080
March	1st to 10th	Night	44% 30% 5% 11% 9% 1	5495
	11th to 20th	Night	43% 32% 10% 8% 7% 1	5090
	21st to end	Night	56% 28% 4 8% 3 1	5272
April	1st to 10th	Night	23% 63% 4 6% 3 1	2011
	11th to 20th	Night	20% 63% 5% 7% 4 1	1976
	21st to end	Night	21% 62% 4 7% 4 1	1839
May	1st to 10th	Night	45% 33% 7% 8% 6% 1	3620
	11th to 20th	Night	40% 32% 10% 15% 4	3597
	21st to end	Night	40% 33% 8% 15% 4	3949
June	1st to 10th	Night	23% 62% 4 7% 4	1600
	11th to 20th	Night	18% 62% 4 11% 5%	1600
	21st to end	Night	23% 60% 4 8% 4 1	1600
July	1st to 10th	Night	51% 27% 7% 10% 4	3581
	11th to 20th	Night	40% 27% 8% 16% 8% 1	3572
	21st to end	Night	38% 27% 12% 19% 4	3831
August	1st to 10th	Night	36% 38% 9% 13% 3	2638
	11th to 20th	Night	32% 40% 13% 13% 2	2930
	21st to end	Night	23% 37% 10% 21% 9%	3585
September	1st to 10th	Night	19% 59% 6% 11% 5%	2000
	11th to 20th	Night	22% 57% 6% 12% 3	2000
	21st to end	Night	17% 58% 6% 14% 5%	2264
October	1st to 10th	Night	33% 37% 8% 14% 8%	3920
	11th to 20th	Night	24% 38% 8% 21% 9%	3899
	21st to end	Night	32% 37% 11% 14% 5%	4528
November	1st to 10th	Night	28% 56% 7% 7% 2	2673
	11th to 20th	Night	31% 57% 5% 5% 2	2940
	21st to end	Night	31% 57% 6% 5% 2	2940
December	1st to 10th	Night	38% 37% 8% 14% 3	4720
	11th to 20th	Night	46% 32% 6% 11% 4 1	5355
	21st to end	Night	48% 28% 10% 10% 4	7150

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	26% 24% 33% 14% 3	430
	11th to 20th	Day	26% 21% 29% 16% 6% 1	430
	21st to end	Day	21% 22% 36% 18% 3	473
February	1st to 10th	Day	26% 20% 34% 16% 4	430
	11th to 20th	Day	23% 28% 29% 17% 3	426
	21st to end	Day	19% 20% 38% 18% 5%	355
March	1st to 10th	Day	22% 18% 38% 19% 3	430
	11th to 20th	Day	18% 21% 34% 21% 6% 1	540
	21st to end	Day	27% 20% 31% 15% 6% 1	594
April	1st to 10th	Day	30% 21% 31% 16% 3	504
	11th to 20th	Day	25% 15% 35% 17% 7% 1	492
	21st to end	Day	25% 14% 31% 23% 7% 1	557
May	1st to 10th	Day	32% 11% 30% 20% 5% 2	703
	11th to 20th	Day	25% 7% 27% 27% 11% 3	724
	21st to end	Day	24% 11% 28% 28% 8% 1	790
June	1st to 10th	Day	35% 9% 22% 25% 9% 1	730
	11th to 20th	Day	23% 8% 29% 30% 9% 2	730
	21st to end	Day	35% 7% 23% 25% 9% 1	730
July	1st to 10th	Day	25% 5% 23% 34% 13% 1	720
	11th to 20th	Day	24% 4 23% 34% 14% 1	718
	21st to end	Day	25% 5% 20% 35% 13% 2	795
August	1st to 10th	Day	20% 3 24% 34% 16% 2	694
	11th to 20th	Day	20% 7% 23% 32% 14% 3	519
	21st to end	Day	19% 7% 26% 31% 16% 2	583
September	1st to 10th	Day	18% 8% 28% 30% 13% 2	530
	11th to 20th	Day	21% 12% 26% 25% 13% 2	530
	21st to end	Day	24% 10% 26% 23% 14% 3	423
October	1st to 10th	Day	20% 10% 27% 28% 14% 1	420
	11th to 20th	Day	23% 14% 29% 20% 11% 3	420
	21st to end	Day	24% 19% 31% 18% 8%	462
November	1st to 10th	Day	25% 18% 30% 20% 4 2	425
	11th to 20th	Day	31% 23% 27% 16% 4 1	430
	21st to end	Day	27% 22% 33% 12% 5% 1	430
December	1st to 10th	Day	27% 22% 29% 17% 4	430
	11th to 20th	Day	31% 28% 27% 12% 2	430
	21st to end	Day	21% 24% 35% 15% 3 1	473
January	1st to 10th	Night	27% 24% 29% 16% 3	950
	11th to 20th	Night	28% 19% 34% 15% 4	950
	21st to end	Night	20% 24% 36% 16% 4	1006
February	1st to 10th	Night	27% 17% 32% 20% 4	840
	11th to 20th	Night	25% 24% 33% 15% 3	823
	21st to end	Night	18% 19% 35% 22% 7%	692
March	1st to 10th	Night	22% 18% 36% 21% 4	840
	11th to 20th	Night	22% 15% 29% 25% 8% 1	840
	21st to end	Night	28% 16% 28% 22% 5%	790
April	1st to 10th	Night	29% 13% 31% 21% 5%	604
	11th to 20th	Night	26% 10% 30% 28% 7%	594
	21st to end	Night	30% 8% 26% 26% 9%	600
May	1st to 10th	Night	34% 7% 26% 21% 10% 2	600
	11th to 20th	Night	28% 6% 24% 29% 12% 1	592
	21st to end	Night	25% 7% 28% 30% 9% 1	649
June	1st to 10th	Night	38% 7% 19% 29% 7%	600
	11th to 20th	Night	29% 5% 24% 32% 10%	600
	21st to end	Night	37% 5% 20% 31% 7% 1	600
July	1st to 10th	Night	26% 4 21% 35% 13% 1	586
	11th to 20th	Night	30% 3 20% 35% 11%	582
	21st to end	Night	30% 5% 20% 35% 9% 1	656
August	1st to 10th	Night	22% 4 21% 35% 18% 1	598
	11th to 20th	Night	22% 4 21% 34% 17% 2	592
	21st to end	Night	19% 4 22% 37% 18% 1	660
September	1st to 10th	Night	20% 6% 24% 33% 16% 2	600
	11th to 20th	Night	26% 7% 24% 27% 16% 1	600
	21st to end	Night	26% 8% 29% 28% 9% 1	669
October	1st to 10th	Night	23% 8% 29% 28% 11% 1	800
	11th to 20th	Night	26% 10% 29% 24% 10% 2	800
	21st to end	Night	30% 15% 30% 20% 5%	971
November	1st to 10th	Night	27% 15% 31% 20% 5% 1	930
	11th to 20th	Night	30% 23% 26% 17% 5%	950
	21st to end	Night	29% 19% 32% 16% 4	950
December	1st to 10th	Night	31% 21% 29% 14% 4	950
	11th to 20th	Night	29% 25% 31% 12% 3	950
	21st to end	Night	28% 21% 33% 15% 3 1	1045

Page generated at 2026-03-30 03:18:46. Version ac07

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