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 2026-07-31.

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% 11% 4 9% 5% 4 4 3 5% 6% 32%	138111
	Night	14% 13% 5% 12% 6% 5% 4 3 5% 6% 26%	134943
Transparency	Day	34% 35% 8% 15% 7% 1	141645
	Night	39% 35% 7% 12% 6% 1	139498
Seeing	Day	18% 14% 29% 25% 11% 3	17480
	Night	21% 14% 26% 25% 12% 2	26638

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% 19% 11% 18% 12% 5% 4 3 4 3 8%	4126
	11th to 20th	Day	9% 23% 11% 27% 12% 6% 3 2 1 2 3	4694
	21st to end	Day	11% 15% 9% 20% 11% 8% 6% 4 5% 4 9%	5780
February	1st to 10th	Day	12% 21% 6% 17% 10% 5% 5% 2 4 4 14%	1890
	11th to 20th	Day	11% 16% 8% 13% 10% 6% 5% 4 5% 5% 17%	1843
	21st to end	Day	13% 17% 5% 12% 8% 6% 6% 4 5% 5% 19%	1688
March	1st to 10th	Day	16% 12% 4 10% 5% 5% 5% 3 5% 6% 29%	4859
	11th to 20th	Day	11% 15% 6% 12% 6% 6% 5% 4 5% 6% 25%	5090
	21st to end	Day	18% 13% 5% 10% 5% 4 4 3 4 5% 28%	5586
April	1st to 10th	Day	18% 12% 4 7% 5% 5% 4 3 5% 7% 30%	2431
	11th to 20th	Day	18% 11% 2 7% 4 3 3 3 4 5% 39%	2702
	21st to end	Day	18% 9% 3 6% 4 3 4 4 5% 8% 35%	2730
May	1st to 10th	Day	12% 8% 3 6% 4 4 4 4 6% 8% 40%	7642
	11th to 20th	Day	11% 9% 3 7% 4 4 4 4 5% 8% 41%	7959
	21st to end	Day	17% 7% 2 6% 4 3 4 3 5% 6% 44%	9060
June	1st to 10th	Day	13% 7% 2 6% 4 5% 4 4 7% 7% 41%	3134
	11th to 20th	Day	11% 6% 2 6% 5% 4 5% 5% 6% 8% 43%	3150
	21st to end	Day	14% 7% 2 6% 4 3 5% 4 6% 8% 41%	3150
July	1st to 10th	Day	12% 7% 3 7% 4 4 5% 4 6% 8% 41%	6969
	11th to 20th	Day	10% 5% 2 5% 3 3 3 3 5% 8% 53%	6844
	21st to end	Day	6% 5% 2 5% 3 4 4 3 5% 8% 54%	6609
August	1st to 10th	Day	10% 6% 2 5% 3 4 4 4 7% 9% 45%	4234
	11th to 20th	Day	14% 6% 2 5% 4 4 4 4 6% 8% 42%	4174
	21st to end	Day	9% 6% 2 5% 4 4 5% 4 6% 8% 46%	4657
September	1st to 10th	Day	12% 7% 3 7% 5% 4 5% 3 6% 8% 42%	2366
	11th to 20th	Day	11% 6% 1 6% 4 4 5% 4 6% 7% 46%	2200
	21st to end	Day	17% 8% 3 8% 4 4 4 4 5% 6% 37%	2200
October	1st to 10th	Day	17% 9% 3 7% 5% 4 4 3 5% 7% 36%	2033
	11th to 20th	Day	23% 14% 4 9% 6% 5% 3 3 5% 6% 24%	2000
	21st to end	Day	25% 15% 4 9% 5% 5% 5% 3 4 5% 19%	2105
November	1st to 10th	Day	18% 16% 5% 10% 7% 5% 4 4 5% 6% 21%	1805
	11th to 20th	Day	18% 16% 5% 11% 7% 6% 5% 4 5% 6% 17%	1680
	21st to end	Day	19% 26% 6% 12% 5% 4 4 4 4 5% 10%	1680
December	1st to 10th	Day	14% 28% 10% 15% 6% 4 4 3 4 4 8%	2680
	11th to 20th	Day	19% 24% 6% 17% 7% 4 4 3 3 5% 8%	2808
	21st to end	Day	26% 21% 7% 17% 5% 6% 3 2 3 3 6%	3553
January	1st to 10th	Night	13% 19% 9% 20% 9% 5% 4 4 5% 4 8%	8794
	11th to 20th	Night	10% 22% 10% 22% 10% 5% 4 3 3 3 7%	8204
	21st to end	Night	9% 15% 9% 20% 11% 8% 4 3 3 4 12%	8401
February	1st to 10th	Night	11% 20% 6% 17% 9% 6% 4 3 4 5% 15%	2730
	11th to 20th	Night	12% 16% 6% 14% 8% 6% 5% 3 5% 5% 18%	2663
	21st to end	Night	14% 15% 6% 12% 7% 4 6% 3 5% 6% 22%	2113
March	1st to 10th	Night	13% 11% 4 11% 6% 5% 5% 3 6% 7% 30%	5187
	11th to 20th	Night	11% 12% 4 10% 5% 5% 5% 4 6% 7% 31%	5089
	21st to end	Night	18% 11% 5% 9% 5% 5% 4 3 4 5% 31%	5588
April	1st to 10th	Night	19% 10% 4 7% 4 4 4 3 4 7% 32%	2206
	11th to 20th	Night	18% 9% 2 7% 4 4 5% 4 5% 7% 36%	1867
	21st to end	Night	19% 9% 3 8% 5% 4 4 4 5% 5% 33%	1890
May	1st to 10th	Night	13% 7% 3 8% 5% 5% 5% 4 6% 7% 37%	5257
	11th to 20th	Night	14% 7% 3 7% 5% 3 4 3 5% 7% 42%	4885
	21st to end	Night	17% 6% 2 7% 4 4 4 3 5% 7% 42%	5114
June	1st to 10th	Night	12% 8% 2 6% 5% 5% 5% 3 5% 7% 41%	1481
	11th to 20th	Night	9% 6% 2 6% 5% 3 6% 5% 7% 10% 41%	1470
	21st to end	Night	11% 8% 3 7% 5% 4 5% 4 6% 8% 39%	1470
July	1st to 10th	Night	11% 8% 3 6% 4 4 4 3 6% 9% 43%	3230
	11th to 20th	Night	11% 5% 2 6% 4 4 4 4 5% 7% 48%	3339
	21st to end	Night	8% 7% 2 5% 4 3 4 3 5% 8% 49%	4462
August	1st to 10th	Night	10% 8% 2 6% 4 4 4 4 6% 8% 44%	2915
	11th to 20th	Night	12% 7% 3 7% 5% 4 4 4 5% 7% 41%	2875
	21st to end	Night	10% 6% 2 6% 4 4 4 4 6% 8% 47%	3223
September	1st to 10th	Night	10% 6% 2 5% 5% 4 5% 4 6% 8% 46%	2015
	11th to 20th	Night	10% 5% 2 6% 4 4 5% 3 5% 7% 49%	2200
	21st to end	Night	16% 8% 3 8% 5% 4 4 4 5% 6% 37%	2200
October	1st to 10th	Night	17% 7% 3 8% 5% 5% 5% 3 5% 7% 35%	2366
	11th to 20th	Night	23% 11% 4 10% 5% 5% 4 4 5% 6% 23%	2400
	21st to end	Night	24% 13% 4 9% 5% 5% 4 4 5% 5% 21%	2732
November	1st to 10th	Night	18% 15% 4 8% 6% 5% 5% 4 5% 6% 24%	2711
	11th to 20th	Night	16% 18% 5% 12% 7% 4 5% 3 5% 6% 18%	2940
	21st to end	Night	22% 23% 5% 11% 6% 5% 4 3 4 5% 12%	2940
December	1st to 10th	Night	15% 24% 7% 16% 7% 5% 5% 4 5% 5% 8%	4720
	11th to 20th	Night	15% 24% 7% 17% 7% 5% 5% 3 4 4 8%	5599
	21st to end	Night	22% 22% 8% 17% 7% 6% 4 2 3 3 7%	7667

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	64% 23% 6% 6% 2	4127
	11th to 20th	Day	74% 21% 2 3 1	4725
	21st to end	Day	61% 25% 6% 6% 2	5780
February	1st to 10th	Day	30% 59% 2 7% 2	1890
	11th to 20th	Day	28% 58% 5% 6% 4	1852
	21st to end	Day	27% 59% 4 7% 3	1688
March	1st to 10th	Day	42% 29% 10% 8% 10% 1	4862
	11th to 20th	Day	46% 29% 7% 13% 4 1	5090
	21st to end	Day	45% 28% 8% 12% 6% 1	5586
April	1st to 10th	Day	24% 59% 3 10% 4	2431
	11th to 20th	Day	21% 59% 3 11% 6% 1	2702
	21st to end	Day	19% 59% 6% 11% 5% 1	2730
May	1st to 10th	Day	31% 25% 10% 20% 12% 2	7651
	11th to 20th	Day	32% 24% 9% 21% 14% 1	7963
	21st to end	Day	35% 22% 7% 21% 13% 2	9061
June	1st to 10th	Day	17% 59% 3 12% 8% 1	3134
	11th to 20th	Day	18% 59% 4 11% 6% 1	3150
	21st to end	Day	18% 58% 5% 12% 7% 1	3150
July	1st to 10th	Day	31% 27% 13% 19% 9% 1	6896
	11th to 20th	Day	21% 26% 10% 32% 11%	6763
	21st to end	Day	20% 26% 10% 30% 13% 1	6424
August	1st to 10th	Day	21% 37% 10% 21% 10% 1	4221
	11th to 20th	Day	28% 38% 11% 17% 6%	4213
	21st to end	Day	20% 38% 11% 25% 7%	4657
September	1st to 10th	Day	18% 57% 8% 11% 5%	2366
	11th to 20th	Day	13% 57% 5% 18% 7%	2200
	21st to end	Day	18% 57% 7% 13% 5%	2200
October	1st to 10th	Day	22% 37% 9% 21% 11%	3306
	11th to 20th	Day	31% 36% 8% 19% 6%	3240
	21st to end	Day	35% 37% 9% 14% 4	3381
November	1st to 10th	Day	34% 52% 5% 7% 2	1805
	11th to 20th	Day	31% 54% 6% 7% 2	1680
	21st to end	Day	34% 55% 5% 5% 1	1680
December	1st to 10th	Day	52% 38% 6% 4 1	2680
	11th to 20th	Day	55% 32% 5% 7% 1	2808
	21st to end	Day	65% 26% 5% 4 1	3553
January	1st to 10th	Night	62% 24% 6% 5% 2	8794
	11th to 20th	Night	67% 21% 5% 5% 1	8248
	21st to end	Night	59% 24% 6% 7% 3	8401
February	1st to 10th	Night	29% 58% 3 7% 2	2730
	11th to 20th	Night	25% 57% 4 10% 4	2665
	21st to end	Night	26% 59% 4 6% 4	2113
March	1st to 10th	Night	39% 29% 10% 12% 8% 2	5197
	11th to 20th	Night	38% 28% 12% 16% 5% 1	5090
	21st to end	Night	45% 27% 3 15% 9% 1	5588
April	1st to 10th	Night	26% 59% 3 8% 4	2206
	11th to 20th	Night	22% 59% 2 10% 6% 1	1867
	21st to end	Night	22% 59% 4 8% 6% 1	1890
May	1st to 10th	Night	36% 23% 5% 18% 14% 4	5264
	11th to 20th	Night	35% 23% 5% 19% 17% 1	4886
	21st to end	Night	35% 22% 5% 22% 13% 2	5115
June	1st to 10th	Night	14% 58% 2 12% 11% 2	1481
	11th to 20th	Night	15% 59% 5% 13% 7% 1	1470
	21st to end	Night	17% 57% 4 13% 7% 1	1470
July	1st to 10th	Night	29% 27% 11% 20% 10% 3	3195
	11th to 20th	Night	25% 26% 7% 27% 14% 1	3297
	21st to end	Night	25% 26% 7% 24% 17% 1	4333
August	1st to 10th	Night	24% 36% 9% 19% 11% 1	2904
	11th to 20th	Night	30% 38% 11% 13% 7%	2902
	21st to end	Night	22% 37% 8% 24% 8%	3223
September	1st to 10th	Night	14% 57% 8% 14% 6%	2015
	11th to 20th	Night	14% 56% 4 15% 10%	2200
	21st to end	Night	20% 56% 6% 11% 7%	2200
October	1st to 10th	Night	19% 38% 6% 24% 12%	3873
	11th to 20th	Night	30% 35% 10% 18% 7%	3899
	21st to end	Night	31% 38% 11% 16% 4	4406
November	1st to 10th	Night	32% 53% 5% 8% 2	2711
	11th to 20th	Night	30% 54% 6% 7% 2	2940
	21st to end	Night	36% 54% 4 4 2	2940
December	1st to 10th	Night	50% 38% 7% 4 1	4720
	11th to 20th	Night	54% 32% 6% 5% 2	5598
	21st to end	Night	63% 26% 6% 4 1	7667

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% 26% 36% 14% 4 1	224
	11th to 20th	Day	22% 25% 33% 15% 5%	330
	21st to end	Day	24% 18% 34% 20% 5%	363
February	1st to 10th	Day	22% 27% 33% 15% 2	330
	11th to 20th	Day	22% 27% 34% 15% 2	330
	21st to end	Day	24% 29% 35% 12% 1	273
March	1st to 10th	Day	20% 22% 35% 18% 4	456
	11th to 20th	Day	22% 20% 33% 19% 4 1	540
	21st to end	Day	27% 16% 31% 21% 4 1	594
April	1st to 10th	Day	23% 16% 34% 19% 8% 1	540
	11th to 20th	Day	26% 17% 27% 15% 10% 4	532
	21st to end	Day	25% 12% 35% 19% 8% 1	540
May	1st to 10th	Day	18% 11% 36% 24% 9% 2	543
	11th to 20th	Day	14% 10% 31% 28% 12% 4	567
	21st to end	Day	16% 7% 31% 29% 14% 4	617
June	1st to 10th	Day	12% 7% 31% 34% 12% 2	764
	11th to 20th	Day	12% 9% 32% 33% 12% 2	780
	21st to end	Day	17% 6% 29% 34% 12% 3	780
July	1st to 10th	Day	9% 5% 28% 33% 22% 2	771
	11th to 20th	Day	11% 6% 27% 33% 17% 5%	768
	21st to end	Day	9% 6% 28% 35% 17% 5%	645
August	1st to 10th	Day	13% 5% 21% 33% 22% 6%	526
	11th to 20th	Day	13% 6% 18% 36% 19% 8%	519
	21st to end	Day	12% 9% 26% 28% 20% 5%	583
September	1st to 10th	Day	15% 11% 22% 27% 18% 6%	530
	11th to 20th	Day	10% 6% 23% 26% 25% 9%	530
	21st to end	Day	19% 12% 21% 28% 16% 4	530
October	1st to 10th	Day	21% 15% 24% 24% 13% 3	530
	11th to 20th	Day	22% 21% 31% 20% 5% 1	530
	21st to end	Day	28% 20% 30% 19% 2 1	488
November	1st to 10th	Day	28% 23% 30% 15% 4	305
	11th to 20th	Day	22% 27% 33% 15% 3	220
	21st to end	Day	29% 25% 31% 14% 1	220
December	1st to 10th	Day	29% 25% 29% 12% 5%	220
	11th to 20th	Day	30% 27% 29% 12% 2	220
	21st to end	Day	31% 26% 31% 8% 2	242
January	1st to 10th	Night	24% 20% 33% 19% 4 1	1050
	11th to 20th	Night	26% 21% 28% 19% 5% 1	1044
	21st to end	Night	20% 17% 30% 24% 7% 1	924
February	1st to 10th	Night	21% 21% 32% 21% 5%	840
	11th to 20th	Night	19% 17% 35% 21% 8%	823
	21st to end	Night	21% 18% 35% 21% 4	692
March	1st to 10th	Night	17% 17% 34% 24% 7% 1	840
	11th to 20th	Night	19% 19% 33% 22% 8% 1	840
	21st to end	Night	20% 14% 28% 28% 10%	924
April	1st to 10th	Night	25% 15% 29% 25% 5% 1	832
	11th to 20th	Night	21% 13% 27% 25% 12% 2	623
	21st to end	Night	24% 11% 24% 26% 13% 1	630
May	1st to 10th	Night	16% 7% 26% 34% 15% 2	630
	11th to 20th	Night	16% 7% 23% 34% 18% 1	506
	21st to end	Night	16% 4 25% 36% 17% 2	455
June	1st to 10th	Night	17% 5% 28% 35% 14% 2	420
	11th to 20th	Night	12% 7% 23% 39% 17% 1	420
	21st to end	Night	13% 6% 23% 36% 21% 1	420
July	1st to 10th	Night	9% 4 16% 50% 20% 2	410
	11th to 20th	Night	10% 4 22% 37% 24% 3	452
	21st to end	Night	9% 4 19% 42% 24% 3	687
August	1st to 10th	Night	11% 3 18% 39% 26% 3	597
	11th to 20th	Night	12% 6% 15% 31% 29% 7%	594
	21st to end	Night	13% 5% 21% 31% 26% 3	660
September	1st to 10th	Night	12% 8% 20% 32% 21% 6%	785
	11th to 20th	Night	12% 6% 19% 31% 24% 7%	800
	21st to end	Night	19% 9% 20% 27% 20% 6%	800
October	1st to 10th	Night	22% 9% 25% 25% 17% 2	800
	11th to 20th	Night	28% 15% 24% 24% 9% 1	800
	21st to end	Night	32% 13% 28% 22% 4 1	880
November	1st to 10th	Night	28% 16% 25% 22% 7% 1	820
	11th to 20th	Night	25% 23% 25% 19% 8% 1	840
	21st to end	Night	33% 19% 27% 17% 4	840
December	1st to 10th	Night	31% 22% 28% 13% 6%	840
	11th to 20th	Night	28% 22% 32% 14% 4	965
	21st to end	Night	34% 18% 29% 15% 4	1155

Page generated at 2026-07-28 03:25:22. Version ac07

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