Alpenglow Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Alpenglow Observatory clear sky chart from 2005-11-06 to 2025-07-21.

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	6% 11% 4 7% 4 4 4 3 5% 6% 48%	123918
	Night	7% 11% 3 7% 4 3 4 3 5% 6% 48%	124124
Transparency	Day	36% 4 7% 21% 21% 11%	123684
	Night	37% 4 8% 21% 21% 10%	123945
Seeing	Day	17% 17% 33% 24% 7% 1	15905
	Night	14% 9% 25% 32% 17% 3	26351

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	6% 19% 8% 11% 6% 3 3 4 5% 6% 30%	4098
	11th to 20th	Day	5% 16% 7% 17% 6% 5% 2 3 3 4 33%	4100
	21st to end	Day	4 16% 6% 9% 3 3 3 2 4 5% 44%	4510
February	1st to 10th	Day	11% 19% 5% 10% 6% 3 4 2 3 4 32%	1800
	11th to 20th	Day	8% 20% 6% 11% 6% 4 4 3 4 6% 29%	1825
	21st to end	Day	6% 20% 6% 13% 4 4 4 4 5% 5% 29%	1650
March	1st to 10th	Day	4 18% 8% 11% 4 5% 4 3 4 4 33%	4826
	11th to 20th	Day	4 14% 7% 12% 4 4 4 3 5% 6% 36%	5000
	21st to end	Day	7% 27% 7% 11% 4 4 4 3 4 4 26%	5486
April	1st to 10th	Day	9% 15% 5% 9% 5% 4 4 4 5% 5% 36%	2220
	11th to 20th	Day	9% 17% 4 8% 5% 3 4 3 4 6% 37%	2521
	21st to end	Day	9% 15% 3 8% 4 3 4 4 5% 6% 40%	2600
May	1st to 10th	Day	8% 15% 3 5% 4 4 4 3 5% 6% 42%	5880
	11th to 20th	Day	7% 13% 3 5% 3 3 4 3 6% 7% 45%	5841
	21st to end	Day	6% 7% 2 6% 4 4 4 4 6% 7% 51%	6423
June	1st to 10th	Day	4 5% 2 6% 4 4 4 4 6% 7% 53%	2655
	11th to 20th	Day	6% 5% 2 4 3 3 3 2 4 6% 60%	2790
	21st to end	Day	2 3 1 4 3 3 3 2 4 4 70%	2600
July	1st to 10th	Day	2 2 1 2 3 2 3 2 4 4 75%	4240
	11th to 20th	Day	3 3 1 4 4 4 6% 4 7% 8% 56%	4240
	21st to end	Day	3 2 1 4 4 4 4 4 5% 6% 62%	4455
August	1st to 10th	Day	4 3 1 4 3 4 5% 3 5% 6% 61%	4092
	11th to 20th	Day	3 3 1 5% 4 4 4 3 5% 5% 63%	4028
	21st to end	Day	3 3 1 3 3 3 3 3 5% 5% 68%	4474
September	1st to 10th	Day	3 3 1 3 2 3 4 3 4 5% 67%	2147
	11th to 20th	Day	5% 4 1 5% 4 3 4 4 5% 5% 61%	2090
	21st to end	Day	7% 4 1 4 3 2 2 2 3 5% 66%	2090
October	1st to 10th	Day	6% 6% 2 4 3 3 2 2 3 4 66%	3330
	11th to 20th	Day	9% 8% 2 3 3 3 3 3 5% 7% 55%	3150
	21st to end	Day	6% 13% 4 5% 2 2 3 2 3 5% 54%	3424
November	1st to 10th	Day	10% 12% 3 6% 5% 3 3 3 4 5% 45%	1764
	11th to 20th	Day	12% 12% 3 7% 3 3 4 3 4 5% 44%	1800
	21st to end	Day	8% 14% 5% 9% 4 5% 3 2 3 5% 41%	1661
December	1st to 10th	Day	6% 9% 2 7% 3 2 3 2 4 7% 55%	2620
	11th to 20th	Day	9% 14% 5% 10% 5% 4 5% 3 6% 9% 30%	3026
	21st to end	Day	10% 18% 5% 11% 5% 4 5% 3 4 5% 29%	4462
January	1st to 10th	Night	6% 14% 7% 14% 5% 3 3 3 4 6% 34%	5867
	11th to 20th	Night	7% 18% 5% 15% 4 3 2 2 3 4 36%	5880
	21st to end	Night	3 12% 5% 10% 4 3 3 3 4 5% 47%	6468
February	1st to 10th	Night	10% 15% 5% 10% 5% 4 4 4 4 6% 33%	2600
	11th to 20th	Night	7% 16% 4 10% 5% 4 3 3 4 5% 38%	2458
	21st to end	Night	6% 14% 5% 13% 4 4 4 3 4 5% 37%	1980
March	1st to 10th	Night	5% 17% 5% 10% 5% 5% 4 4 5% 5% 36%	5055
	11th to 20th	Night	5% 14% 6% 9% 4 4 5% 5% 5% 7% 37%	5000
	21st to end	Night	9% 20% 5% 8% 4 4 3 3 4 5% 35%	5489
April	1st to 10th	Night	11% 13% 4 6% 4 3 3 3 4 6% 43%	2180
	11th to 20th	Night	7% 12% 3 7% 4 4 4 3 6% 6% 43%	1823
	21st to end	Night	10% 13% 3 7% 4 4 4 4 4 5% 43%	1800
May	1st to 10th	Night	8% 12% 3 5% 4 4 4 3 5% 6% 48%	4100
	11th to 20th	Night	6% 9% 2 5% 3 3 4 3 5% 7% 54%	4074
	21st to end	Night	7% 5% 2 5% 3 4 4 3 6% 6% 56%	4474
June	1st to 10th	Night	7% 6% 2 5% 4 3 3 3 6% 7% 54%	1732
	11th to 20th	Night	6% 4 2 4 2 3 4 3 5% 6% 63%	1600
	21st to end	Night	4 4 1 4 3 3 3 3 5% 6% 65%	1794
July	1st to 10th	Night	5% 3 1 3 3 2 3 2 3 4 71%	2950
	11th to 20th	Night	3 4 2 6% 4 4 5% 5% 7% 7% 52%	2950
	21st to end	Night	4 5% 2 5% 4 4 4 4 5% 6% 58%	3109
August	1st to 10th	Night	4 4 1 5% 3 4 5% 4 6% 7% 58%	2847
	11th to 20th	Night	5% 3 2 6% 4 5% 4 3 4 6% 59%	2811
	21st to end	Night	4 3 1 3 3 3 3 3 4 5% 68%	3191
September	1st to 10th	Night	4 4 1 5% 3 3 4 2 3 5% 66%	2029
	11th to 20th	Night	5% 3 2 3 3 3 4 3 4 6% 63%	2090
	21st to end	Night	7% 4 1 4 2 2 2 2 3 4 69%	2090
October	1st to 10th	Night	6% 5% 1 3 3 2 2 2 3 4 69%	3643
	11th to 20th	Night	9% 7% 2 4 3 3 3 3 4 5% 58%	3778
	21st to end	Night	7% 11% 3 4 3 3 4 3 5% 6% 52%	4120
November	1st to 10th	Night	9% 11% 3 5% 3 3 3 4 6% 7% 47%	2535
	11th to 20th	Night	10% 10% 2 6% 3 3 3 4 5% 7% 47%	2600
	21st to end	Night	8% 12% 5% 6% 4 3 4 3 5% 5% 46%	2714
December	1st to 10th	Night	6% 8% 3 5% 3 2 2 2 4 6% 58%	4570
	11th to 20th	Night	9% 15% 4 7% 4 4 4 3 5% 6% 40%	5181
	21st to end	Night	10% 17% 4 11% 5% 4 4 3 5% 6% 31%	6542

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	54% 5% 12% 15% 11% 2	4099
	11th to 20th	Day	56% 4 9% 16% 11% 3	4100
	21st to end	Day	43% 5% 12% 18% 17% 5%	4510
February	1st to 10th	Day	56% 5% 12% 16% 10% 2	1800
	11th to 20th	Day	56% 5% 10% 18% 10% 1	1834
	21st to end	Day	52% 6% 9% 19% 11% 2	1650
March	1st to 10th	Day	50% 6% 6% 20% 13% 5%	4826
	11th to 20th	Day	46% 5% 8% 29% 11% 1	5000
	21st to end	Day	59% 5% 13% 16% 6% 1	5486
April	1st to 10th	Day	48% 4 9% 20% 14% 4	2220
	11th to 20th	Day	46% 5% 10% 21% 16% 2	2524
	21st to end	Day	43% 4 8% 26% 18% 1	2600
May	1st to 10th	Day	38% 6% 6% 25% 21% 4	5880
	11th to 20th	Day	36% 4 4 30% 24% 3	5844
	21st to end	Day	28% 6% 4 31% 28% 3	6424
June	1st to 10th	Day	28% 4 6% 24% 29% 9%	2655
	11th to 20th	Day	25% 3 4 20% 29% 19%	2790
	21st to end	Day	18% 3 1 12% 33% 33%	2600
July	1st to 10th	Day	13% 3 2 7% 19% 57%	4176
	11th to 20th	Day	23% 4 2 13% 28% 29%	4171
	21st to end	Day	20% 4 4 11% 30% 31%	4315
August	1st to 10th	Day	23% 3 3 23% 27% 21%	4078
	11th to 20th	Day	22% 3 3 18% 29% 24%	4067
	21st to end	Day	18% 3 3 15% 36% 25%	4473
September	1st to 10th	Day	18% 4 4 18% 30% 27%	2147
	11th to 20th	Day	24% 4 5% 19% 28% 20%	2090
	21st to end	Day	21% 3 4 23% 34% 16%	2090
October	1st to 10th	Day	25% 2 3 19% 37% 15%	3330
	11th to 20th	Day	30% 2 5% 31% 26% 6%	3150
	21st to end	Day	34% 3 7% 30% 23% 3	3424
November	1st to 10th	Day	40% 5% 10% 21% 19% 4	1764
	11th to 20th	Day	41% 4 9% 29% 15% 1	1800
	21st to end	Day	45% 4 9% 26% 14% 3	1661
December	1st to 10th	Day	31% 3 10% 38% 16% 2	2620
	11th to 20th	Day	50% 4 12% 21% 12% 1	3024
	21st to end	Day	55% 6% 8% 16% 14%	4462
January	1st to 10th	Night	51% 4 11% 18% 13% 4	5867
	11th to 20th	Night	53% 4 8% 19% 15% 2	5880
	21st to end	Night	39% 5% 13% 20% 19% 5%	6468
February	1st to 10th	Night	49% 5% 13% 20% 10% 2	2600
	11th to 20th	Night	47% 3 9% 21% 16% 2	2459
	21st to end	Night	46% 5% 8% 23% 13% 4	1980
March	1st to 10th	Night	48% 5% 5% 21% 15% 6%	5055
	11th to 20th	Night	45% 4 8% 27% 15% 1	5000
	21st to end	Night	50% 5% 12% 19% 13% 1	5489
April	1st to 10th	Night	41% 5% 10% 21% 17% 6%	2180
	11th to 20th	Night	39% 5% 8% 23% 21% 4	1823
	21st to end	Night	42% 4 9% 24% 20% 1	1800
May	1st to 10th	Night	32% 7% 8% 23% 27% 3	4097
	11th to 20th	Night	29% 4 4 31% 27% 4	4075
	21st to end	Night	26% 4 5% 29% 32% 2	4474
June	1st to 10th	Night	28% 4 6% 23% 30% 9%	1732
	11th to 20th	Night	22% 3 5% 18% 30% 21%	1600
	21st to end	Night	19% 3 1 10% 31% 35%	1794
July	1st to 10th	Night	19% 3 2 7% 13% 57%	2905
	11th to 20th	Night	28% 3 2 12% 26% 29%	2896
	21st to end	Night	27% 3 4 12% 24% 30%	3011
August	1st to 10th	Night	25% 3 5% 18% 26% 23%	2838
	11th to 20th	Night	28% 3 4 13% 29% 24%	2838
	21st to end	Night	20% 2 4 10% 35% 28%	3191
September	1st to 10th	Night	21% 3 3 15% 26% 31%	2029
	11th to 20th	Night	22% 2 5% 20% 27% 23%	2090
	21st to end	Night	22% 2 3 18% 35% 21%	2090
October	1st to 10th	Night	21% 2 4 19% 35% 18%	3643
	11th to 20th	Night	31% 2 3 30% 27% 8%	3778
	21st to end	Night	33% 3 5% 29% 25% 5%	4120
November	1st to 10th	Night	35% 4 8% 25% 21% 6%	2535
	11th to 20th	Night	35% 4 11% 28% 18% 3	2600
	21st to end	Night	39% 4 8% 28% 17% 4	2714
December	1st to 10th	Night	28% 3 8% 36% 22% 4	4570
	11th to 20th	Night	45% 3 15% 25% 11%	5182
	21st to end	Night	52% 5% 12% 17% 12% 2	6542

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	28% 18% 22% 21% 12%	300
	11th to 20th	Day	22% 19% 31% 18% 9% 2	300
	21st to end	Day	19% 19% 29% 25% 7% 1	330
February	1st to 10th	Day	33% 18% 31% 13% 5%	300
	11th to 20th	Day	22% 22% 33% 16% 4 1	370
	21st to end	Day	23% 22% 36% 16% 4	414
March	1st to 10th	Day	25% 19% 34% 16% 5% 1	500
	11th to 20th	Day	22% 19% 33% 20% 5%	500
	21st to end	Day	28% 22% 32% 15% 2	550
April	1st to 10th	Day	26% 19% 29% 21% 4 1	500
	11th to 20th	Day	25% 25% 30% 15% 4 1	492
	21st to end	Day	28% 20% 26% 20% 6% 1	500
May	1st to 10th	Day	19% 22% 33% 19% 6% 1	503
	11th to 20th	Day	21% 20% 31% 20% 7% 2	528
	21st to end	Day	16% 16% 32% 26% 8% 2	573
June	1st to 10th	Day	9% 15% 39% 29% 7% 2	585
	11th to 20th	Day	13% 17% 33% 24% 12% 1	720
	21st to end	Day	9% 19% 39% 28% 4 1	530
July	1st to 10th	Day	5% 13% 44% 29% 7% 2	526
	11th to 20th	Day	5% 11% 44% 32% 6% 2	524
	21st to end	Day	4 8% 43% 32% 11% 2	532
August	1st to 10th	Day	9% 9% 43% 29% 9% 1	487
	11th to 20th	Day	6% 11% 36% 35% 9% 2	477
	21st to end	Day	7% 13% 34% 34% 9% 3	539
September	1st to 10th	Day	7% 10% 35% 33% 12% 2	490
	11th to 20th	Day	12% 17% 29% 32% 9% 1	490
	21st to end	Day	12% 15% 29% 33% 10% 2	490
October	1st to 10th	Day	14% 17% 31% 26% 11% 1	395
	11th to 20th	Day	14% 15% 32% 25% 13% 1	300
	21st to end	Day	17% 19% 34% 23% 7%	330
November	1st to 10th	Day	26% 19% 25% 23% 6% 2	300
	11th to 20th	Day	20% 20% 33% 16% 9% 2	300
	21st to end	Day	20% 14% 25% 29% 10% 2	300
December	1st to 10th	Day	20% 13% 31% 25% 9% 2	300
	11th to 20th	Day	20% 15% 34% 23% 6% 1	300
	21st to end	Day	28% 16% 28% 22% 5%	330
January	1st to 10th	Night	19% 10% 25% 26% 17% 4	800
	11th to 20th	Night	17% 9% 28% 28% 14% 4	800
	21st to end	Night	14% 12% 24% 32% 14% 4	880
February	1st to 10th	Night	20% 8% 29% 30% 10% 3	800
	11th to 20th	Night	15% 12% 30% 28% 13% 2	784
	21st to end	Night	15% 13% 32% 28% 11% 2	660
March	1st to 10th	Night	19% 14% 29% 26% 9% 3	800
	11th to 20th	Night	14% 12% 27% 30% 14% 3	800
	21st to end	Night	18% 15% 26% 29% 9% 2	880
April	1st to 10th	Night	19% 13% 27% 28% 11% 2	780
	11th to 20th	Night	15% 12% 31% 30% 10% 2	593
	21st to end	Night	18% 10% 24% 32% 15% 2	600
May	1st to 10th	Night	14% 8% 31% 33% 13% 1	600
	11th to 20th	Night	13% 9% 24% 32% 20% 3	591
	21st to end	Night	11% 10% 24% 33% 19% 3	650
June	1st to 10th	Night	10% 9% 21% 40% 19% 2	600
	11th to 20th	Night	9% 10% 28% 38% 15% 2	600
	21st to end	Night	6% 6% 26% 37% 22% 4	600
July	1st to 10th	Night	7% 7% 23% 32% 29% 3	585
	11th to 20th	Night	6% 3 23% 44% 21% 3	582
	21st to end	Night	4 4 16% 40% 34% 2	624
August	1st to 10th	Night	6% 4 23% 40% 26% 2	567
	11th to 20th	Night	6% 5% 18% 41% 27% 4	564
	21st to end	Night	6% 4 22% 42% 23% 3	627
September	1st to 10th	Night	7% 5% 18% 43% 24% 4	699
	11th to 20th	Night	8% 6% 22% 34% 24% 5%	760
	21st to end	Night	9% 7% 21% 30% 26% 8%	760
October	1st to 10th	Night	11% 6% 27% 33% 19% 4	760
	11th to 20th	Night	12% 6% 22% 34% 21% 5%	760
	21st to end	Night	13% 8% 26% 30% 18% 4	836
November	1st to 10th	Night	17% 11% 22% 29% 17% 4	780
	11th to 20th	Night	16% 10% 28% 28% 14% 5%	800
	21st to end	Night	14% 12% 26% 30% 15% 4	914
December	1st to 10th	Night	16% 10% 27% 32% 14% 1	1000
	11th to 20th	Night	21% 10% 28% 29% 10% 2	1000
	21st to end	Night	24% 12% 25% 30% 8% 1	915

Page generated at 2025-07-17 23:10:04. Version ac07

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