Alpine Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Alpine Observatory clear sky chart from 2005-11-06 to 2026-10-14.

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	4 5% 2 6% 4 4 5% 4 5% 6% 55%	157701
	Night	2 4 1 4 3 4 4 3 6% 7% 61%	151692
Transparency	Day	23% 7% 9% 15% 26% 21%	152618
	Night	20% 6% 10% 16% 28% 20%	147782
Seeing	Day	12% 17% 28% 25% 14% 4	17175
	Night	6% 8% 24% 36% 23% 3	27178

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	1 5% 3 5% 5% 4 3 3 4 6% 61%	5318
	11th to 20th	Day	1 2 2 3 2 2 2 3 3 4 76%	5340
	21st to end	Day	3 8% 2 4 3 3 4 3 5% 5% 61%	5844
February	1st to 10th	Day	2 6% 2 5% 3 3 4 3 4 6% 63%	1890
	11th to 20th	Day	3 7% 2 5% 4 2 5% 3 5% 5% 59%	1926
	21st to end	Day	2 7% 2 5% 3 2 3 4 5% 6% 62%	1903
March	1st to 10th	Day	1 7% 2 4 3 3 4 3 5% 6% 63%	5064
	11th to 20th	Day	3 10% 2 5% 4 4 4 3 5% 6% 53%	5110
	21st to end	Day	2 6% 3 5% 4 4 4 3 5% 6% 57%	5607
April	1st to 10th	Day	2 3 1 4 3 3 3 4 4 8% 64%	2404
	11th to 20th	Day	2 4 1 4 4 3 3 3 4 6% 66%	2491
	21st to end	Day	3 3 1 4 3 4 3 3 3 6% 65%	2520
May	1st to 10th	Day	5% 6% 2 5% 3 3 4 3 4 4 61%	7331
	11th to 20th	Day	2 3 1 4 3 3 3 3 4 4 69%	7596
	21st to end	Day	3 3 1 4 3 3 3 2 4 5% 70%	8370
June	1st to 10th	Day	3 3 1 4 3 3 4 4 5% 7% 64%	2730
	11th to 20th	Day	4 3 1 3 3 3 3 3 4 5% 67%	2730
	21st to end	Day	5% 5% 2 7% 6% 5% 6% 4 6% 5% 50%	2730
July	1st to 10th	Day	5% 4 2 7% 6% 5% 6% 4 6% 6% 48%	7649
	11th to 20th	Day	4 4 3 9% 9% 9% 9% 6% 8% 9% 30%	7650
	21st to end	Day	5% 5% 2 9% 8% 9% 8% 6% 8% 9% 31%	8304
August	1st to 10th	Day	4 4 2 7% 7% 8% 8% 6% 8% 9% 36%	7549
	11th to 20th	Day	5% 4 3 9% 7% 7% 8% 6% 8% 8% 35%	7287
	21st to end	Day	5% 5% 2 9% 6% 7% 8% 6% 8% 10% 34%	7751
September	1st to 10th	Day	8% 5% 2 8% 6% 6% 5% 5% 6% 7% 42%	2520
	11th to 20th	Day	8% 4 2 7% 4 4 4 4 5% 5% 52%	2320
	21st to end	Day	6% 4 2 5% 4 5% 4 4 5% 7% 55%	2305
October	1st to 10th	Day	5% 4 2 5% 4 4 4 3 4 5% 61%	3370
	11th to 20th	Day	6% 3 1 3 2 3 2 2 4 6% 68%	3280
	21st to end	Day	1 2 1 2 1 2 1 1 3 5% 80%	3548
November	1st to 10th	Day	2 4 1 3 2 2 2 2 4 4 73%	2059
	11th to 20th	Day	1 3 1 4 3 2 3 2 4 6% 69%	2100
	21st to end	Day	4 6% 2 3 4 3 3 2 4 6% 63%	1957
December	1st to 10th	Day	2 3 1 3 2 2 3 3 5% 5% 72%	3040
	11th to 20th	Day	2 7% 2 5% 2 2 2 2 5% 7% 62%	3499
	21st to end	Day	1 5% 1 5% 3 3 4 3 4 5% 66%	4609
January	1st to 10th	Night	2 5% 3 6% 3 4 5% 4 6% 7% 56%	7605
	11th to 20th	Night	1 2 1 2 1 1 2 1 3 4 82%	7650
	21st to end	Night	2 6% 2 5% 3 4 4 3 5% 7% 58%	8370
February	1st to 10th	Night	1 5% 1 4 4 3 3 3 5% 7% 63%	2730
	11th to 20th	Night	2 6% 2 4 4 4 4 3 6% 6% 60%	2575
	21st to end	Night	1 5% 1 3 3 2 3 3 4 7% 67%	1903
March	1st to 10th	Night	1 3 2 4 3 3 3 3 5% 7% 68%	5050
	11th to 20th	Night	3 10% 2 5% 3 3 3 2 4 6% 61%	5110
	21st to end	Night	1 3 2 3 3 3 4 3 5% 8% 64%	5610
April	1st to 10th	Night	1 2 1 3 3 2 3 4 6% 6% 68%	2215
	11th to 20th	Night	2 2 1 3 3 3 3 3 4 5% 71%	2077
	21st to end	Night	1 2 1 2 2 2 2 2 4 5% 78%	2100
May	1st to 10th	Night	3 3 1 4 3 3 3 2 4 5% 68%	5611
	11th to 20th	Night	1 2 1 2 2 2 3 3 4 4 76%	5296
	21st to end	Night	1 1 2 2 2 3 2 3 4 78%	5837
June	1st to 10th	Night	2 3 1 4 3 3 3 2 4 6% 67%	1890
	11th to 20th	Night	2 1 1 3 2 3 3 2 3 4 76%	1890
	21st to end	Night	3 4 1 5% 4 4 5% 4 7% 8% 56%	1890
July	1st to 10th	Night	2 3 2 6% 5% 5% 6% 5% 6% 8% 54%	5340
	11th to 20th	Night	2 4 2 8% 6% 6% 8% 7% 10% 12% 35%	5340
	21st to end	Night	3 5% 2 7% 6% 6% 7% 6% 11% 14% 34%	5805
August	1st to 10th	Night	2 2 1 5% 5% 5% 7% 6% 11% 12% 44%	5273
	11th to 20th	Night	3 4 2 7% 6% 6% 7% 6% 11% 13% 36%	5565
	21st to end	Night	4 5% 2 8% 6% 7% 8% 7% 11% 12% 30%	6465
September	1st to 10th	Night	5% 5% 2 6% 5% 5% 5% 4 6% 8% 49%	2100
	11th to 20th	Night	5% 4 1 4 3 4 4 3 4 6% 61%	2295
	21st to end	Night	4 3 1 4 3 3 4 3 5% 5% 66%	2312
October	1st to 10th	Night	3 3 1 4 3 3 3 3 4 5% 69%	4040
	11th to 20th	Night	4 3 1 3 2 2 2 2 4 5% 72%	3934
	21st to end	Night	1 2 2 1 2 2 2 3 5% 81%	4252
November	1st to 10th	Night	3 3 1 2 2 2 3 2 5% 5% 74%	2460
	11th to 20th	Night	2 3 1 3 2 3 3 2 5% 4 72%	2520
	21st to end	Night	4 5% 1 3 3 3 3 3 4 7% 63%	2660
December	1st to 10th	Night	3 3 1 3 2 2 3 2 4 7% 72%	4370
	11th to 20th	Night	2 5% 2 5% 3 3 4 3 5% 6% 63%	4941
	21st to end	Night	2 4 1 3 3 3 4 3 4 5% 67%	6611

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	23% 5% 6% 18% 37% 12%	5321
	11th to 20th	Day	12% 4 5% 16% 29% 35%	5340
	21st to end	Day	23% 5% 5% 10% 27% 30%	5844
February	1st to 10th	Day	20% 4 8% 17% 26% 24%	1890
	11th to 20th	Day	24% 4 8% 18% 28% 19%	1935
	21st to end	Day	20% 5% 6% 16% 29% 24%	1903
March	1st to 10th	Day	16% 6% 6% 12% 33% 27%	5064
	11th to 20th	Day	22% 9% 8% 13% 34% 14%	5110
	21st to end	Day	22% 6% 7% 14% 38% 13%	5607
April	1st to 10th	Day	18% 4 5% 16% 35% 22%	2404
	11th to 20th	Day	18% 4 6% 13% 31% 29%	2494
	21st to end	Day	17% 4 6% 17% 36% 21%	2520
May	1st to 10th	Day	20% 5% 8% 14% 31% 24%	7331
	11th to 20th	Day	13% 6% 5% 12% 34% 30%	7599
	21st to end	Day	13% 3 5% 8% 25% 46%	8371
June	1st to 10th	Day	16% 5% 6% 12% 28% 32%	2730
	11th to 20th	Day	15% 3 6% 9% 21% 46%	2730
	21st to end	Day	29% 7% 10% 12% 21% 22%	2730
July	1st to 10th	Day	29% 8% 8% 10% 17% 28%	7586
	11th to 20th	Day	39% 14% 20% 15% 9% 4	7581
	21st to end	Day	38% 14% 22% 15% 9% 3	8120
August	1st to 10th	Day	35% 13% 18% 22% 10% 2	5978
	11th to 20th	Day	35% 12% 16% 22% 12% 2	5746
	21st to end	Day	35% 11% 16% 21% 15% 3	6078
September	1st to 10th	Day	34% 9% 12% 16% 18% 11%	2520
	11th to 20th	Day	27% 8% 9% 14% 28% 15%	2320
	21st to end	Day	24% 6% 8% 17% 27% 18%	2305
October	1st to 10th	Day	19% 5% 7% 12% 27% 31%	3370
	11th to 20th	Day	16% 3 4 11% 43% 23%	3280
	21st to end	Day	8% 2 4 13% 34% 40%	3548
November	1st to 10th	Day	14% 3 5% 21% 36% 22%	2059
	11th to 20th	Day	16% 3 5% 17% 34% 25%	2100
	21st to end	Day	22% 4 7% 14% 32% 20%	1957
December	1st to 10th	Day	12% 4 5% 17% 42% 19%	3040
	11th to 20th	Day	22% 4 5% 13% 39% 18%	3498
	21st to end	Day	19% 5% 7% 20% 30% 19%	4609
January	1st to 10th	Night	24% 6% 8% 22% 31% 9%	7605
	11th to 20th	Night	8% 3 5% 15% 38% 30%	7650
	21st to end	Night	24% 6% 5% 11% 28% 25%	8370
February	1st to 10th	Night	19% 5% 7% 19% 29% 21%	2730
	11th to 20th	Night	23% 5% 7% 19% 29% 17%	2576
	21st to end	Night	17% 4 6% 18% 30% 25%	1903
March	1st to 10th	Night	12% 4 6% 15% 41% 21%	5050
	11th to 20th	Night	20% 7% 8% 18% 31% 17%	5110
	21st to end	Night	15% 6% 8% 23% 36% 12%	5610
April	1st to 10th	Night	15% 3 7% 22% 36% 17%	2215
	11th to 20th	Night	13% 3 6% 15% 35% 26%	2077
	21st to end	Night	10% 2 5% 21% 41% 21%	2100
May	1st to 10th	Night	17% 4 8% 17% 37% 16%	5609
	11th to 20th	Night	10% 5% 5% 15% 37% 29%	5297
	21st to end	Night	10% 2 4 9% 26% 48%	5838
June	1st to 10th	Night	17% 4 6% 12% 30% 31%	1890
	11th to 20th	Night	11% 3 5% 11% 26% 44%	1890
	21st to end	Night	24% 7% 11% 11% 20% 26%	1890
July	1st to 10th	Night	25% 8% 10% 11% 16% 30%	5295
	11th to 20th	Night	34% 14% 23% 16% 11% 3	5286
	21st to end	Night	33% 14% 30% 14% 7% 3	5677
August	1st to 10th	Night	29% 12% 27% 21% 9% 3	4168
	11th to 20th	Night	32% 14% 25% 16% 10% 3	4382
	21st to end	Night	38% 12% 20% 18% 9% 2	5069
September	1st to 10th	Night	30% 9% 13% 18% 20% 10%	2100
	11th to 20th	Night	24% 6% 11% 17% 28% 15%	2295
	21st to end	Night	18% 5% 8% 20% 29% 20%	2312
October	1st to 10th	Night	16% 5% 7% 12% 31% 29%	4040
	11th to 20th	Night	15% 4 5% 14% 40% 23%	3934
	21st to end	Night	8% 2 4 13% 35% 37%	4252
November	1st to 10th	Night	13% 2 6% 19% 38% 22%	2460
	11th to 20th	Night	14% 4 6% 19% 32% 27%	2520
	21st to end	Night	20% 3 6% 19% 32% 19%	2660
December	1st to 10th	Night	14% 3 5% 25% 34% 19%	4370
	11th to 20th	Night	20% 4 6% 13% 39% 17%	4941
	21st to end	Night	18% 5% 8% 19% 31% 20%	6611

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	15% 20% 32% 23% 8% 1	330
	11th to 20th	Day	14% 19% 29% 26% 9% 2	330
	21st to end	Day	12% 18% 32% 24% 11% 2	363
February	1st to 10th	Day	12% 21% 32% 23% 10% 2	330
	11th to 20th	Day	23% 26% 29% 19% 3	372
	21st to end	Day	22% 32% 30% 13% 3	446
March	1st to 10th	Day	19% 34% 26% 16% 4 1	540
	11th to 20th	Day	23% 26% 26% 18% 6% 2	540
	21st to end	Day	20% 31% 28% 14% 5% 1	594
April	1st to 10th	Day	11% 35% 29% 17% 6% 2	540
	11th to 20th	Day	19% 34% 29% 14% 4 1	532
	21st to end	Day	19% 31% 31% 13% 5% 1	540
May	1st to 10th	Day	12% 26% 37% 17% 8%	543
	11th to 20th	Day	8% 25% 36% 18% 9% 4	568
	21st to end	Day	10% 26% 36% 19% 9% 1	617
June	1st to 10th	Day	6% 15% 38% 29% 11% 2	570
	11th to 20th	Day	6% 16% 39% 24% 13% 2	570
	21st to end	Day	7% 12% 31% 32% 14% 4	570
July	1st to 10th	Day	8% 3 25% 36% 24% 5%	566
	11th to 20th	Day	8% 2 14% 30% 35% 11%	564
	21st to end	Day	7% 2 16% 29% 34% 12%	619
August	1st to 10th	Day	8% 1 13% 37% 31% 11%	567
	11th to 20th	Day	8% 4 17% 33% 28% 10%	557
	21st to end	Day	6% 4 16% 36% 29% 9%	627
September	1st to 10th	Day	9% 5% 21% 35% 24% 7%	570
	11th to 20th	Day	7% 7% 34% 33% 16% 4	570
	21st to end	Day	8% 14% 30% 30% 15% 4	565
October	1st to 10th	Day	8% 14% 29% 28% 15% 6%	360
	11th to 20th	Day	12% 15% 28% 27% 13% 5%	339
	21st to end	Day	10% 15% 26% 27% 17% 4	363
November	1st to 10th	Day	8% 20% 31% 26% 12% 2	330
	11th to 20th	Day	9% 13% 27% 31% 17% 3	330
	21st to end	Day	15% 18% 31% 24% 11% 1	330
December	1st to 10th	Day	12% 18% 23% 33% 12% 2	330
	11th to 20th	Day	10% 19% 31% 26% 13% 2	330
	21st to end	Day	20% 20% 30% 23% 6% 1	363
January	1st to 10th	Night	6% 10% 29% 36% 15% 4	840
	11th to 20th	Night	7% 8% 31% 36% 15% 3	840
	21st to end	Night	7% 12% 30% 32% 16% 3	924
February	1st to 10th	Night	5% 14% 30% 34% 14% 3	840
	11th to 20th	Night	8% 15% 37% 30% 8% 1	824
	21st to end	Night	9% 16% 36% 29% 9%	692
March	1st to 10th	Night	7% 18% 32% 33% 10% 1	840
	11th to 20th	Night	8% 13% 29% 33% 14% 3	840
	21st to end	Night	7% 13% 32% 36% 11% 1	924
April	1st to 10th	Night	4 17% 35% 34% 10%	745
	11th to 20th	Night	6% 20% 32% 32% 9% 1	623
	21st to end	Night	4 17% 37% 28% 12% 2	630
May	1st to 10th	Night	5% 9% 32% 42% 12% 1	630
	11th to 20th	Night	4 8% 29% 39% 18% 2	621
	21st to end	Night	3 9% 32% 39% 16% 1	683
June	1st to 10th	Night	4 3 22% 49% 20% 2	630
	11th to 20th	Night	3 4 25% 48% 18% 1	630
	21st to end	Night	4 2 19% 38% 34% 3	630
July	1st to 10th	Night	4 1 9% 38% 43% 5%	615
	11th to 20th	Night	3 1 6% 30% 52% 8%	612
	21st to end	Night	5% 2 5% 27% 53% 9%	690
August	1st to 10th	Night	3 1 5% 25% 55% 12%	627
	11th to 20th	Night	4 1 8% 26% 53% 8%	624
	21st to end	Night	6% 1 5% 27% 53% 8%	693
September	1st to 10th	Night	7% 1 7% 39% 42% 4	630
	11th to 20th	Night	6% 2 9% 44% 35% 5%	825
	21st to end	Night	5% 4 12% 42% 33% 4	840
October	1st to 10th	Night	3 5% 21% 45% 23% 2	840
	11th to 20th	Night	5% 6% 21% 45% 20% 4	812
	21st to end	Night	4 6% 20% 42% 23% 6%	880
November	1st to 10th	Night	4 5% 22% 43% 21% 5%	820
	11th to 20th	Night	3 8% 25% 37% 24% 3	840
	21st to end	Night	10% 11% 24% 34% 18% 4	840
December	1st to 10th	Night	8% 10% 22% 42% 15% 3	840
	11th to 20th	Night	8% 10% 30% 37% 15% 1	840
	21st to end	Night	9% 12% 31% 35% 11% 2	924

Page generated at 2026-10-11 02:06:32. Version ac07

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