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 2024-07-08.

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	21% 12% 4 9% 5% 5% 4 4 5% 6% 24%	89717
	Night	22% 12% 4 10% 6% 5% 4 4 5% 6% 23%	89696
Transparency	Day	28% 56% 6% 7% 3	89435
	Night	29% 55% 6% 7% 3	89533
Seeing	Day	25% 14% 28% 23% 8% 1	17177
	Night	27% 13% 28% 23% 7% 1	24610

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	14% 12% 6% 21% 7% 6% 4 3 4 5% 16%	2526
	11th to 20th	Day	14% 14% 6% 16% 7% 6% 5% 3 4 7% 18%	2530
	21st to end	Day	16% 13% 8% 14% 8% 5% 5% 3 4 5% 18%	2883
February	1st to 10th	Day	20% 11% 4 14% 9% 6% 6% 3 6% 6% 14%	1710
	11th to 20th	Day	17% 13% 5% 12% 7% 7% 6% 4 5% 6% 18%	1663
	21st to end	Day	12% 16% 4 10% 8% 4 4 4 4 5% 28%	1565
March	1st to 10th	Day	25% 11% 3 8% 6% 5% 3 3 3 4 28%	3114
	11th to 20th	Day	15% 19% 6% 13% 7% 6% 4 4 4 5% 17%	3479
	21st to end	Day	30% 14% 6% 10% 5% 4 3 2 4 4 18%	4026
April	1st to 10th	Day	23% 13% 4 7% 4 4 5% 4 5% 6% 26%	2280
	11th to 20th	Day	23% 12% 3 7% 4 4 4 2 3 5% 33%	2254
	21st to end	Day	24% 12% 3 8% 4 4 3 3 4 6% 28%	2429
May	1st to 10th	Day	25% 15% 4 8% 4 4 4 4 4 6% 22%	4425
	11th to 20th	Day	22% 12% 3 8% 5% 4 4 3 5% 7% 28%	4398
	21st to end	Day	22% 10% 3 8% 5% 5% 5% 4 6% 8% 25%	4836
June	1st to 10th	Day	30% 11% 2 7% 5% 5% 4 4 6% 7% 21%	2660
	11th to 20th	Day	20% 10% 3 7% 4 5% 4 3 5% 8% 31%	2660
	21st to end	Day	30% 12% 3 7% 4 4 4 3 5% 6% 21%	2660
July	1st to 10th	Day	18% 9% 2 7% 5% 5% 4 4 6% 8% 32%	2622
	11th to 20th	Day	19% 10% 3 6% 5% 5% 5% 5% 6% 8% 28%	2520
	21st to end	Day	22% 10% 3 7% 5% 4 5% 4 7% 8% 25%	2745
August	1st to 10th	Day	17% 9% 4 7% 5% 4 4 4 5% 8% 33%	2479
	11th to 20th	Day	19% 9% 3 8% 5% 4 4 4 5% 8% 32%	2287
	21st to end	Day	17% 8% 3 6% 5% 5% 5% 3 7% 8% 34%	2376
September	1st to 10th	Day	18% 8% 3 7% 4 3 4 4 6% 8% 36%	2160
	11th to 20th	Day	20% 9% 3 6% 5% 5% 4 4 5% 6% 33%	2160
	21st to end	Day	22% 8% 3 6% 3 3 4 3 5% 5% 38%	1923
October	1st to 10th	Day	18% 8% 3 7% 5% 5% 5% 4 6% 7% 31%	1800
	11th to 20th	Day	20% 11% 4 8% 5% 5% 4 3 6% 7% 27%	1800
	21st to end	Day	22% 12% 4 10% 5% 5% 6% 3 5% 6% 21%	1833
November	1st to 10th	Day	24% 11% 4 8% 6% 5% 5% 5% 5% 6% 22%	1665
	11th to 20th	Day	23% 17% 4 9% 6% 5% 5% 3 4 5% 18%	1520
	21st to end	Day	16% 15% 7% 12% 7% 6% 5% 5% 5% 6% 17%	1520
December	1st to 10th	Day	24% 18% 6% 11% 8% 6% 4 4 3 5% 13%	1520
	11th to 20th	Day	30% 15% 5% 15% 7% 6% 4 3 5% 4 8%	1906
	21st to end	Day	21% 17% 7% 14% 8% 6% 5% 4 5% 4 11%	2783
January	1st to 10th	Night	14% 12% 7% 22% 7% 6% 5% 4 5% 4 14%	4429
	11th to 20th	Night	21% 12% 5% 16% 8% 6% 5% 3 5% 6% 14%	4440
	21st to end	Night	15% 12% 5% 15% 8% 7% 6% 4 5% 5% 19%	4779
February	1st to 10th	Night	20% 12% 4 13% 7% 7% 6% 5% 6% 5% 16%	2470
	11th to 20th	Night	17% 14% 4 11% 9% 6% 5% 4 5% 5% 18%	2403
	21st to end	Night	11% 15% 5% 12% 6% 4 5% 4 5% 6% 25%	1888
March	1st to 10th	Night	20% 13% 4 10% 7% 5% 4 3 5% 4 26%	3726
	11th to 20th	Night	22% 15% 5% 10% 6% 4 4 3 4 5% 23%	3479
	21st to end	Night	30% 12% 6% 11% 6% 4 3 2 3 3 20%	3605
April	1st to 10th	Night	24% 14% 3 6% 5% 4 5% 4 6% 6% 26%	1900
	11th to 20th	Night	21% 11% 2 6% 4 5% 3 4 6% 6% 31%	1876
	21st to end	Night	25% 11% 2 8% 4 4 3 3 4 5% 32%	1747
May	1st to 10th	Night	28% 11% 3 6% 4 4 5% 4 4 6% 24%	2525
	11th to 20th	Night	21% 14% 4 7% 4 4 4 3 4 6% 32%	2506
	21st to end	Night	20% 14% 4 9% 5% 4 4 4 5% 7% 24%	2749
June	1st to 10th	Night	33% 13% 2 6% 5% 4 3 2 4 5% 22%	1520
	11th to 20th	Night	24% 11% 2 5% 4 3 3 3 4 6% 33%	1520
	21st to end	Night	31% 13% 4 7% 4 4 4 3 4 5% 21%	1520
July	1st to 10th	Night	19% 12% 3 9% 5% 4 5% 3 6% 8% 27%	1504
	11th to 20th	Night	23% 12% 3 8% 5% 4 4 3 6% 6% 25%	1440
	21st to end	Night	24% 13% 3 8% 5% 4 5% 4 6% 6% 24%	1573
August	1st to 10th	Night	17% 9% 3 8% 5% 5% 5% 5% 5% 7% 31%	1458
	11th to 20th	Night	19% 11% 3 8% 4 5% 4 4 6% 8% 28%	1612
	21st to end	Night	16% 8% 2 5% 5% 4 5% 4 5% 9% 37%	1980
September	1st to 10th	Night	18% 6% 2 5% 5% 5% 4 4 5% 7% 38%	1800
	11th to 20th	Night	23% 8% 3 6% 3 4 4 3 3 7% 37%	1800
	21st to end	Night	20% 9% 4 7% 4 3 3 3 5% 5% 37%	2036
October	1st to 10th	Night	18% 7% 2 6% 4 5% 5% 5% 5% 7% 36%	2160
	11th to 20th	Night	23% 9% 3 6% 4 4 4 3 5% 7% 32%	2160
	21st to end	Night	24% 12% 4 8% 5% 4 4 3 5% 6% 25%	2522
November	1st to 10th	Night	22% 11% 4 9% 5% 4 5% 4 5% 7% 25%	2412
	11th to 20th	Night	22% 13% 4 9% 6% 5% 5% 4 6% 6% 20%	2660
	21st to end	Night	22% 14% 5% 10% 6% 6% 6% 4 6% 6% 17%	2660
December	1st to 10th	Night	25% 16% 5% 10% 6% 4 5% 4 5% 5% 13%	2660
	11th to 20th	Night	31% 13% 5% 12% 6% 5% 4 3 4 5% 12%	3293
	21st to end	Night	28% 15% 5% 13% 6% 5% 4 3 3 4 16%	4884

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	38% 45% 10% 7% 1	2527
	11th to 20th	Day	34% 42% 11% 9% 3	2530
	21st to end	Day	39% 44% 6% 3 7% 1	2883
February	1st to 10th	Day	22% 67% 4 5% 2	1710
	11th to 20th	Day	22% 66% 5% 5% 2	1671
	21st to end	Day	18% 65% 3 6% 6% 1	1565
March	1st to 10th	Day	34% 41% 6% 11% 7%	3118
	11th to 20th	Day	38% 47% 7% 5% 3	3479
	21st to end	Day	47% 41% 5% 3 2 1	4026
April	1st to 10th	Day	22% 65% 3 6% 3 1	2280
	11th to 20th	Day	20% 66% 4 7% 4	2254
	21st to end	Day	19% 66% 5% 5% 6%	2429
May	1st to 10th	Day	36% 45% 8% 8% 2 1	4440
	11th to 20th	Day	32% 44% 10% 10% 4	4402
	21st to end	Day	32% 46% 10% 9% 2	4836
June	1st to 10th	Day	22% 65% 4 6% 3	2660
	11th to 20th	Day	16% 66% 7% 7% 3 1	2660
	21st to end	Day	23% 63% 3 7% 3	2660
July	1st to 10th	Day	21% 61% 6% 8% 3	2552
	11th to 20th	Day	22% 63% 5% 7% 3	2444
	21st to end	Day	22% 63% 6% 6% 3	2592
August	1st to 10th	Day	15% 65% 8% 9% 3	2465
	11th to 20th	Day	21% 65% 6% 7% 1	2287
	21st to end	Day	20% 64% 6% 7% 3	2376
September	1st to 10th	Day	16% 64% 7% 9% 4	2160
	11th to 20th	Day	22% 64% 3 8% 3	2160
	21st to end	Day	14% 63% 5% 12% 5%	1923
October	1st to 10th	Day	18% 64% 4 10% 4	1800
	11th to 20th	Day	21% 64% 6% 7% 2	1800
	21st to end	Day	22% 64% 5% 7% 3	1833
November	1st to 10th	Day	25% 61% 6% 6% 1	1665
	11th to 20th	Day	30% 62% 5% 3 1	1520
	21st to end	Day	29% 61% 5% 3 1	1520
December	1st to 10th	Day	30% 62% 4 3 1	1520
	11th to 20th	Day	40% 51% 4 4 1	1905
	21st to end	Day	44% 45% 8% 3 1	2783
January	1st to 10th	Night	38% 47% 11% 4	4429
	11th to 20th	Night	41% 43% 9% 5% 2	4440
	21st to end	Night	36% 46% 7% 5% 6%	4779
February	1st to 10th	Night	24% 67% 3 3 3	2470
	11th to 20th	Night	24% 65% 4 5% 2	2406
	21st to end	Night	19% 66% 4 7% 5%	1888
March	1st to 10th	Night	33% 43% 5% 11% 8%	3735
	11th to 20th	Night	39% 45% 6% 7% 3	3480
	21st to end	Night	47% 40% 4 6% 2 1	3605
April	1st to 10th	Night	20% 66% 4 6% 3 1	1900
	11th to 20th	Night	17% 67% 5% 6% 4 1	1876
	21st to end	Night	20% 66% 3 6% 4 1	1747
May	1st to 10th	Night	37% 46% 8% 6% 3	2530
	11th to 20th	Night	33% 44% 9% 12% 3	2507
	21st to end	Night	34% 46% 9% 9% 2	2750
June	1st to 10th	Night	22% 66% 4 6% 2	1520
	11th to 20th	Night	17% 65% 3 10% 4	1520
	21st to end	Night	22% 63% 3 8% 4	1520
July	1st to 10th	Night	24% 60% 5% 7% 4	1465
	11th to 20th	Night	25% 63% 4 5% 3	1392
	21st to end	Night	24% 63% 7% 3 2	1486
August	1st to 10th	Night	17% 65% 7% 9% 2	1447
	11th to 20th	Night	19% 67% 7% 5% 2	1612
	21st to end	Night	17% 64% 6% 9% 4	1980
September	1st to 10th	Night	15% 65% 6% 9% 4	1800
	11th to 20th	Night	21% 63% 4 10% 3	1800
	21st to end	Night	15% 64% 5% 11% 5%	2036
October	1st to 10th	Night	16% 64% 5% 10% 4	2160
	11th to 20th	Night	19% 63% 6% 10% 2	2160
	21st to end	Night	21% 63% 5% 7% 3	2522
November	1st to 10th	Night	25% 61% 6% 6% 2	2412
	11th to 20th	Night	27% 63% 5% 4 1	2660
	21st to end	Night	26% 62% 6% 4 1	2660
December	1st to 10th	Night	30% 63% 4 3 1	2660
	11th to 20th	Night	38% 51% 5% 5% 1	3295
	21st to end	Night	46% 39% 8% 6% 1	4884

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% 24% 33% 12% 3	370
	11th to 20th	Day	26% 22% 31% 16% 5%	370
	21st to end	Day	22% 19% 35% 20% 4	407
February	1st to 10th	Day	28% 21% 32% 15% 4	370
	11th to 20th	Day	23% 29% 29% 16% 2	366
	21st to end	Day	19% 21% 37% 18% 5%	307
March	1st to 10th	Day	21% 18% 38% 19% 3	370
	11th to 20th	Day	17% 20% 33% 22% 6% 1	460
	21st to end	Day	26% 21% 30% 15% 7% 1	506
April	1st to 10th	Day	29% 21% 30% 17% 3	460
	11th to 20th	Day	25% 15% 35% 17% 7% 1	452
	21st to end	Day	25% 13% 31% 23% 7% 1	514
May	1st to 10th	Day	32% 11% 30% 20% 5% 2	653
	11th to 20th	Day	25% 7% 27% 27% 11% 3	674
	21st to end	Day	25% 11% 29% 27% 7% 1	735
June	1st to 10th	Day	36% 9% 21% 24% 9% 1	680
	11th to 20th	Day	24% 7% 29% 30% 9% 2	680
	21st to end	Day	36% 8% 21% 25% 9% 1	680
July	1st to 10th	Day	25% 5% 23% 33% 13% 1	657
	11th to 20th	Day	25% 4 23% 33% 14% 1	618
	21st to end	Day	27% 4 20% 34% 13% 2	685
August	1st to 10th	Day	20% 3 25% 33% 17% 2	598
	11th to 20th	Day	22% 8% 23% 32% 12% 3	439
	21st to end	Day	19% 6% 24% 33% 16% 2	495
September	1st to 10th	Day	18% 8% 28% 31% 12% 2	450
	11th to 20th	Day	24% 12% 28% 23% 10% 2	450
	21st to end	Day	25% 11% 26% 22% 12% 3	363
October	1st to 10th	Day	21% 10% 27% 29% 13% 1	360
	11th to 20th	Day	23% 14% 29% 21% 12% 2	360
	21st to end	Day	24% 20% 31% 17% 7% 1	396
November	1st to 10th	Day	25% 17% 28% 22% 5% 2	365
	11th to 20th	Day	29% 24% 27% 16% 3 1	370
	21st to end	Day	25% 24% 33% 13% 5% 1	370
December	1st to 10th	Day	28% 22% 29% 16% 4	370
	11th to 20th	Day	31% 28% 27% 13% 1	370
	21st to end	Day	21% 25% 35% 14% 3 1	407
January	1st to 10th	Night	27% 24% 29% 17% 3	850
	11th to 20th	Night	27% 20% 36% 14% 4	850
	21st to end	Night	21% 21% 36% 17% 4	904
February	1st to 10th	Night	28% 17% 31% 20% 4	760
	11th to 20th	Night	25% 25% 33% 14% 3	743
	21st to end	Night	17% 19% 34% 22% 7%	628
March	1st to 10th	Night	20% 17% 38% 21% 4	760
	11th to 20th	Night	22% 15% 28% 26% 9% 1	760
	21st to end	Night	27% 16% 28% 23% 5%	715
April	1st to 10th	Night	29% 12% 31% 22% 6%	570
	11th to 20th	Night	25% 9% 30% 28% 7%	564
	21st to end	Night	31% 8% 25% 27% 9%	570
May	1st to 10th	Night	34% 7% 27% 21% 10% 2	570
	11th to 20th	Night	27% 6% 24% 29% 12% 1	562
	21st to end	Night	25% 7% 29% 30% 8% 1	616
June	1st to 10th	Night	39% 7% 19% 28% 7%	570
	11th to 20th	Night	29% 5% 24% 32% 11%	570
	21st to end	Night	37% 5% 20% 30% 7% 1	570
July	1st to 10th	Night	25% 4 22% 36% 12% 1	550
	11th to 20th	Night	30% 3 21% 36% 11%	522
	21st to end	Night	31% 5% 20% 34% 10% 1	590
August	1st to 10th	Night	22% 4 21% 36% 17% 1	538
	11th to 20th	Night	23% 4 22% 33% 16% 2	532
	21st to end	Night	19% 3 20% 38% 18% 1	594
September	1st to 10th	Night	19% 6% 25% 33% 15% 2	540
	11th to 20th	Night	28% 7% 25% 24% 14% 1	540
	21st to end	Night	26% 8% 30% 26% 8% 1	601
October	1st to 10th	Night	23% 8% 29% 28% 11% 1	720
	11th to 20th	Night	27% 9% 30% 23% 10% 2	720
	21st to end	Night	30% 15% 30% 20% 5%	866
November	1st to 10th	Night	27% 14% 31% 21% 6% 1	830
	11th to 20th	Night	28% 23% 26% 17% 5%	850
	21st to end	Night	28% 19% 33% 16% 4	850
December	1st to 10th	Night	32% 22% 29% 13% 4	850
	11th to 20th	Night	30% 24% 31% 12% 3	850
	21st to end	Night	28% 21% 33% 14% 3 1	935

Page generated at 2024-07-05 12:05:35. Version ac07

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