Ponte di Legno Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ponte di Legno Observatory clear sky chart from 2005-11-06 to 2025-03-27.

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	14% 12% 4 10% 5% 4 4 3 5% 6% 32%	108740
	Night	14% 14% 5% 12% 6% 5% 4 4 5% 6% 26%	110636
Transparency	Day	31% 43% 7% 13% 6%	112317
	Night	36% 41% 6% 11% 5%	115223
Seeing	Day	19% 13% 29% 24% 11% 3	15607
	Night	21% 13% 26% 25% 12% 2	25027

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	12% 18% 12% 17% 13% 5% 4 3 4 3 7%	3176
	11th to 20th	Day	10% 21% 10% 28% 11% 5% 4 2 2 2 4	3604
	21st to end	Day	14% 17% 9% 20% 9% 7% 5% 3 4 4 9%	4466
February	1st to 10th	Day	13% 21% 6% 18% 10% 5% 4 2 4 4 13%	1800
	11th to 20th	Day	11% 16% 8% 13% 10% 6% 5% 4 5% 5% 17%	1753
	21st to end	Day	13% 18% 5% 12% 8% 6% 6% 4 5% 5% 19%	1610
March	1st to 10th	Day	16% 13% 4 9% 5% 5% 5% 3 5% 5% 30%	3415
	11th to 20th	Day	11% 16% 6% 13% 7% 5% 4 4 5% 5% 25%	3590
	21st to end	Day	14% 12% 5% 9% 5% 5% 4 4 5% 6% 31%	3881
April	1st to 10th	Day	19% 12% 4 7% 4 5% 4 3 5% 7% 31%	2199
	11th to 20th	Day	18% 10% 2 7% 4 3 4 3 4 5% 38%	2442
	21st to end	Day	19% 9% 3 6% 4 4 4 3 6% 7% 36%	2470
May	1st to 10th	Day	15% 8% 3 6% 4 3 4 4 5% 7% 41%	4108
	11th to 20th	Day	12% 8% 2 6% 4 4 4 3 6% 8% 44%	4260
	21st to end	Day	14% 6% 2 5% 4 3 4 3 5% 7% 47%	4837
June	1st to 10th	Day	13% 6% 2 6% 4 5% 4 4 7% 7% 40%	2836
	11th to 20th	Day	10% 6% 2 6% 5% 4 5% 4 6% 8% 44%	2850
	21st to end	Day	14% 7% 3 6% 4 3 5% 4 6% 8% 41%	2850
July	1st to 10th	Day	13% 7% 3 7% 5% 4 5% 4 6% 7% 40%	4760
	11th to 20th	Day	10% 5% 2 5% 3 3 4 3 6% 8% 51%	4678
	21st to end	Day	8% 5% 2 5% 3 3 4 3 5% 8% 52%	4506
August	1st to 10th	Day	10% 6% 2 5% 3 4 4 4 7% 9% 45%	4104
	11th to 20th	Day	14% 6% 2 5% 4 4 4 4 6% 8% 42%	4044
	21st to end	Day	9% 6% 2 5% 4 4 5% 4 6% 8% 45%	4514
September	1st to 10th	Day	12% 7% 3 7% 5% 4 4 3 6% 7% 43%	2248
	11th to 20th	Day	11% 6% 2 6% 5% 4 5% 4 6% 7% 46%	2090
	21st to end	Day	16% 7% 3 8% 4 4 4 4 6% 6% 37%	2090
October	1st to 10th	Day	18% 10% 3 7% 5% 4 4 3 5% 7% 34%	1932
	11th to 20th	Day	23% 14% 4 9% 6% 5% 3 3 5% 6% 23%	1900
	21st to end	Day	26% 15% 4 9% 5% 5% 5% 3 4 5% 18%	2000
November	1st to 10th	Day	18% 16% 5% 10% 7% 5% 4 4 5% 6% 22%	1717
	11th to 20th	Day	18% 17% 5% 11% 7% 6% 5% 4 5% 6% 18%	1600
	21st to end	Day	19% 26% 6% 12% 6% 4 4 4 4 5% 10%	1600
December	1st to 10th	Day	15% 28% 10% 15% 6% 4 4 3 4 4 8%	2600
	11th to 20th	Day	19% 24% 6% 17% 7% 4 4 3 3 5% 9%	2734
	21st to end	Day	26% 21% 7% 17% 5% 6% 3 2 3 3 6%	3476
January	1st to 10th	Night	13% 19% 9% 20% 9% 4 4 4 6% 4 7%	6785
	11th to 20th	Night	12% 19% 9% 23% 9% 6% 5% 3 4 3 8%	6298
	21st to end	Night	11% 18% 9% 19% 8% 7% 4 3 4 4 11%	6490
February	1st to 10th	Night	11% 20% 6% 18% 9% 6% 4 3 4 5% 14%	2600
	11th to 20th	Night	12% 17% 6% 15% 8% 6% 4 3 5% 5% 18%	2533
	21st to end	Night	14% 15% 6% 11% 7% 5% 6% 3 5% 6% 22%	2015
March	1st to 10th	Night	12% 11% 3 9% 6% 5% 4 3 5% 6% 35%	3631
	11th to 20th	Night	10% 13% 4 10% 5% 4 4 3 6% 6% 35%	3589
	21st to end	Night	12% 10% 5% 10% 5% 5% 5% 3 5% 6% 33%	3890
April	1st to 10th	Night	20% 11% 4 7% 4 4 4 3 4 6% 32%	1997
	11th to 20th	Night	18% 9% 2 7% 3 4 4 4 6% 7% 36%	1687
	21st to end	Night	19% 10% 3 8% 5% 4 4 4 5% 6% 33%	1710
May	1st to 10th	Night	14% 7% 3 7% 5% 5% 5% 4 5% 7% 38%	2833
	11th to 20th	Night	12% 7% 3 7% 6% 4 4 4 6% 7% 41%	2631
	21st to end	Night	11% 5% 3 7% 4 4 4 4 6% 7% 44%	2738
June	1st to 10th	Night	12% 9% 2 7% 5% 5% 5% 3 5% 7% 39%	1340
	11th to 20th	Night	9% 6% 2 5% 5% 3 6% 5% 6% 10% 43%	1330
	21st to end	Night	11% 8% 2 7% 4 4 5% 4 6% 8% 40%	1330
July	1st to 10th	Night	10% 8% 3 6% 4 3 4 3 5% 7% 46%	2210
	11th to 20th	Night	11% 5% 2 6% 4 4 4 4 5% 7% 50%	2289
	21st to end	Night	7% 6% 2 5% 4 4 4 4 6% 8% 50%	3046
August	1st to 10th	Night	10% 8% 2 6% 4 4 4 4 6% 8% 44%	2825
	11th to 20th	Night	12% 7% 3 7% 5% 4 5% 4 5% 7% 41%	2785
	21st to end	Night	10% 6% 2 6% 4 4 4 4 6% 8% 47%	3124
September	1st to 10th	Night	10% 6% 2 5% 5% 4 4 4 6% 8% 47%	1914
	11th to 20th	Night	10% 5% 2 6% 4 4 5% 3 5% 7% 48%	2090
	21st to end	Night	16% 7% 3 8% 4 4 4 4 5% 6% 37%	2090
October	1st to 10th	Night	18% 8% 3 9% 5% 5% 5% 3 5% 6% 34%	2247
	11th to 20th	Night	23% 11% 4 10% 5% 5% 4 4 5% 6% 23%	2280
	21st to end	Night	24% 14% 4 9% 5% 6% 4 4 5% 4 20%	2595
November	1st to 10th	Night	19% 14% 4 9% 6% 5% 5% 4 5% 6% 25%	2579
	11th to 20th	Night	17% 18% 5% 12% 7% 4 5% 3 5% 6% 18%	2800
	21st to end	Night	22% 22% 5% 11% 6% 5% 4 3 4 5% 12%	2800
December	1st to 10th	Night	16% 24% 7% 15% 7% 5% 5% 4 5% 5% 8%	4580
	11th to 20th	Night	15% 24% 7% 17% 7% 5% 5% 3 4 4 8%	5453
	21st to end	Night	22% 22% 8% 17% 7% 6% 4 2 3 3 7%	7502

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	61% 28% 3 6% 1	3177
	11th to 20th	Day	69% 26% 2 3 1	3635
	21st to end	Day	57% 30% 6% 6% 2	4466
February	1st to 10th	Day	30% 61% 2 6% 1	1800
	11th to 20th	Day	25% 60% 4 6% 4	1762
	21st to end	Day	25% 61% 3 7% 3	1610
March	1st to 10th	Day	37% 38% 9% 9% 7%	3418
	11th to 20th	Day	41% 38% 5% 12% 3	3590
	21st to end	Day	35% 39% 7% 11% 8% 1	3881
April	1st to 10th	Day	21% 64% 3 8% 4	2199
	11th to 20th	Day	19% 64% 2 10% 5% 1	2442
	21st to end	Day	17% 64% 5% 9% 5% 1	2470
May	1st to 10th	Day	27% 39% 6% 14% 13% 1	4120
	11th to 20th	Day	24% 38% 11% 20% 6% 1	4264
	21st to end	Day	25% 38% 7% 12% 14% 3	4837
June	1st to 10th	Day	15% 65% 3 10% 7% 1	2836
	11th to 20th	Day	16% 65% 4 9% 6% 1	2850
	21st to end	Day	15% 63% 4 10% 7% 1	2850
July	1st to 10th	Day	30% 38% 10% 14% 8% 1	4686
	11th to 20th	Day	18% 36% 10% 26% 9%	4597
	21st to end	Day	18% 36% 7% 25% 14% 1	4363
August	1st to 10th	Day	20% 38% 10% 20% 10% 1	4091
	11th to 20th	Day	28% 38% 11% 16% 6%	4083
	21st to end	Day	20% 39% 10% 24% 7%	4514
September	1st to 10th	Day	17% 60% 7% 11% 5%	2248
	11th to 20th	Day	13% 59% 4 16% 7%	2090
	21st to end	Day	17% 59% 7% 12% 4	2090
October	1st to 10th	Day	22% 38% 9% 20% 11%	3205
	11th to 20th	Day	31% 37% 8% 18% 6%	3140
	21st to end	Day	35% 38% 9% 14% 4	3276
November	1st to 10th	Day	32% 55% 4 7% 2	1717
	11th to 20th	Day	30% 57% 5% 7% 2	1600
	21st to end	Day	32% 57% 5% 4 1	1600
December	1st to 10th	Day	52% 39% 6% 4 1	2600
	11th to 20th	Day	55% 33% 4 7% 1	2734
	21st to end	Day	64% 26% 5% 4 1	3476
January	1st to 10th	Night	59% 29% 5% 5% 1	6785
	11th to 20th	Night	61% 27% 6% 4 2	6342
	21st to end	Night	55% 29% 6% 6% 3	6490
February	1st to 10th	Night	28% 61% 2 6% 2	2600
	11th to 20th	Night	23% 60% 3 9% 4	2535
	21st to end	Night	24% 61% 4 6% 5%	2015
March	1st to 10th	Night	31% 38% 10% 13% 8%	3641
	11th to 20th	Night	31% 37% 9% 16% 6% 1	3590
	21st to end	Night	35% 38% 4 17% 6% 1	3890
April	1st to 10th	Night	24% 64% 2 7% 4	1997
	11th to 20th	Night	18% 65% 2 8% 6% 1	1687
	21st to end	Night	20% 64% 4 6% 5% 1	1710
May	1st to 10th	Night	28% 39% 5% 14% 13% 1	2840
	11th to 20th	Night	28% 38% 6% 16% 11% 1	2632
	21st to end	Night	27% 38% 6% 16% 11% 2	2739
June	1st to 10th	Night	13% 64% 2 10% 9% 1	1340
	11th to 20th	Night	12% 64% 5% 12% 7% 1	1330
	21st to end	Night	14% 63% 3 12% 7% 1	1330
July	1st to 10th	Night	26% 37% 9% 14% 10% 4	2175
	11th to 20th	Night	22% 36% 6% 27% 9%	2247
	21st to end	Night	22% 35% 7% 15% 20% 1	2949
August	1st to 10th	Night	23% 37% 9% 19% 11% 1	2814
	11th to 20th	Night	30% 39% 11% 13% 7%	2812
	21st to end	Night	22% 38% 8% 24% 9%	3124
September	1st to 10th	Night	13% 60% 7% 14% 6%	1914
	11th to 20th	Night	14% 59% 4 13% 9%	2090
	21st to end	Night	19% 59% 6% 10% 6%	2090
October	1st to 10th	Night	20% 39% 6% 23% 12%	3754
	11th to 20th	Night	30% 36% 10% 18% 6%	3779
	21st to end	Night	31% 39% 11% 15% 3	4269
November	1st to 10th	Night	30% 55% 4 8% 2	2579
	11th to 20th	Night	29% 57% 6% 7% 2	2800
	21st to end	Night	34% 57% 4 4 2	2800
December	1st to 10th	Night	49% 39% 7% 4 1	4580
	11th to 20th	Night	54% 33% 6% 5% 2	5452
	21st to end	Night	62% 26% 6% 4 1	7502

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	18% 25% 36% 15% 5% 1	203
	11th to 20th	Day	23% 21% 34% 16% 5%	300
	21st to end	Day	25% 16% 35% 18% 5%	330
February	1st to 10th	Day	24% 28% 31% 15% 2	300
	11th to 20th	Day	22% 28% 35% 15% 1	300
	21st to end	Day	25% 29% 34% 10% 1	249
March	1st to 10th	Day	21% 20% 34% 19% 5%	420
	11th to 20th	Day	21% 19% 34% 21% 5% 1	500
	21st to end	Day	25% 14% 32% 23% 5% 1	530
April	1st to 10th	Day	23% 14% 35% 19% 8% 1	460
	11th to 20th	Day	28% 16% 27% 15% 9% 5%	452
	21st to end	Day	27% 12% 34% 18% 8% 2	460
May	1st to 10th	Day	21% 10% 34% 23% 10% 2	463
	11th to 20th	Day	16% 10% 29% 29% 13% 4	487
	21st to end	Day	14% 7% 30% 30% 15% 5%	529
June	1st to 10th	Day	13% 7% 31% 33% 13% 2	666
	11th to 20th	Day	11% 8% 31% 34% 13% 3	680
	21st to end	Day	17% 6% 28% 33% 12% 3	680
July	1st to 10th	Day	9% 6% 27% 33% 22% 3	671
	11th to 20th	Day	12% 5% 26% 33% 17% 6%	668
	21st to end	Day	9% 6% 27% 34% 18% 5%	559
August	1st to 10th	Day	13% 5% 22% 32% 21% 6%	486
	11th to 20th	Day	12% 5% 18% 36% 19% 9%	479
	21st to end	Day	12% 9% 27% 27% 19% 5%	539
September	1st to 10th	Day	16% 10% 22% 27% 18% 6%	490
	11th to 20th	Day	11% 7% 25% 25% 24% 8%	490
	21st to end	Day	18% 12% 22% 27% 17% 4	490
October	1st to 10th	Day	22% 16% 24% 22% 12% 3	490
	11th to 20th	Day	22% 22% 30% 20% 6% 1	490
	21st to end	Day	29% 21% 28% 18% 2 1	449
November	1st to 10th	Day	27% 22% 31% 16% 4	277
	11th to 20th	Day	22% 26% 33% 15% 4 1	200
	21st to end	Day	27% 26% 32% 14% 2	200
December	1st to 10th	Day	28% 25% 30% 12% 6%	200
	11th to 20th	Day	30% 26% 30% 12% 3	200
	21st to end	Day	31% 26% 31% 8% 2	220
January	1st to 10th	Night	24% 20% 33% 19% 4 1	1000
	11th to 20th	Night	26% 21% 27% 19% 5% 1	995
	21st to end	Night	20% 16% 30% 24% 8% 1	880
February	1st to 10th	Night	21% 22% 33% 20% 5%	800
	11th to 20th	Night	20% 17% 35% 21% 8%	783
	21st to end	Night	21% 18% 35% 22% 4	660
March	1st to 10th	Night	17% 17% 34% 24% 7% 1	800
	11th to 20th	Night	18% 18% 34% 22% 8% 1	800
	21st to end	Night	19% 13% 28% 28% 11%	863
April	1st to 10th	Night	25% 13% 29% 26% 6% 1	753
	11th to 20th	Night	22% 13% 27% 24% 12% 2	563
	21st to end	Night	25% 10% 24% 26% 14% 1	570
May	1st to 10th	Night	17% 5% 25% 34% 16% 2	570
	11th to 20th	Night	16% 6% 22% 35% 19% 2	458
	21st to end	Night	15% 3 24% 37% 18% 2	411
June	1st to 10th	Night	17% 6% 27% 35% 14% 2	380
	11th to 20th	Night	12% 6% 23% 40% 19% 1	380
	21st to end	Night	13% 5% 22% 36% 23% 1	380
July	1st to 10th	Night	9% 4 14% 50% 21% 2	370
	11th to 20th	Night	10% 3 22% 36% 25% 4	408
	21st to end	Night	9% 4 17% 42% 25% 3	624
August	1st to 10th	Night	11% 3 18% 38% 26% 3	567
	11th to 20th	Night	11% 6% 15% 31% 30% 7%	564
	21st to end	Night	14% 4 22% 30% 26% 4	627
September	1st to 10th	Night	12% 7% 21% 32% 21% 6%	746
	11th to 20th	Night	13% 6% 19% 31% 23% 7%	760
	21st to end	Night	19% 9% 20% 27% 19% 6%	760
October	1st to 10th	Night	23% 10% 25% 24% 17% 2	760
	11th to 20th	Night	28% 15% 23% 23% 9% 1	760
	21st to end	Night	32% 14% 28% 21% 4 1	836
November	1st to 10th	Night	28% 16% 25% 23% 7% 1	780
	11th to 20th	Night	25% 22% 25% 19% 8% 1	800
	21st to end	Night	32% 19% 27% 18% 4	800
December	1st to 10th	Night	32% 21% 28% 13% 6%	800
	11th to 20th	Night	28% 22% 32% 14% 4	919
	21st to end	Night	34% 17% 29% 15% 4	1100

Page generated at 2025-03-24 09:04:25. Version ac07

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