Lake Afton Public Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Lake Afton Public Observatory clear sky chart from 2005-11-06 to 2026-03-18.

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	9% 5% 2 5% 3 3 4 3 5% 7% 53%	131569
	Night	9% 5% 2 5% 3 3 3 3 5% 7% 56%	130249
Transparency	Day	30% 5% 10% 27% 22% 5%	135273
	Night	28% 4 10% 27% 24% 7%	134609
Seeing	Day	11% 9% 29% 31% 17% 3	17324
	Night	11% 9% 33% 35% 11% 1	27363

By Month

Percentage Cloud Cover SE (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	10% 6% 2 7% 4 4 4 3 5% 7% 48%	5289
	11th to 20th	Day	5% 7% 2 6% 4 4 4 3 5% 6% 54%	5310
	21st to end	Day	10% 9% 3 8% 3 3 2 3 4 5% 50%	5812
February	1st to 10th	Day	9% 10% 3 6% 4 4 3 3 4 7% 47%	1921
	11th to 20th	Day	7% 7% 2 7% 4 4 4 3 5% 6% 51%	2049
	21st to end	Day	7% 6% 2 5% 3 4 4 3 6% 8% 53%	1730
March	1st to 10th	Day	7% 7% 2 5% 3 3 4 2 3 6% 58%	4887
	11th to 20th	Day	8% 5% 1 3 3 3 3 3 5% 8% 59%	5057
	21st to end	Day	8% 6% 2 6% 4 4 4 4 7% 8% 46%	5462
April	1st to 10th	Day	11% 6% 2 5% 3 3 3 3 5% 8% 52%	2200
	11th to 20th	Day	13% 7% 2 6% 4 4 4 3 5% 6% 47%	2460
	21st to end	Day	17% 6% 2 5% 4 4 4 3 5% 6% 45%	2600
May	1st to 10th	Day	17% 8% 2 6% 3 3 3 3 5% 5% 45%	5862
	11th to 20th	Day	9% 5% 2 5% 3 3 3 3 4 6% 57%	5862
	21st to end	Day	18% 7% 3 6% 4 4 3 3 5% 6% 41%	6434
June	1st to 10th	Day	5% 4 1 5% 4 4 5% 4 6% 10% 53%	2600
	11th to 20th	Day	6% 4 1 4 4 4 4 4 7% 8% 54%	2600
	21st to end	Day	6% 3 1 4 3 3 5% 4 6% 9% 57%	2600
July	1st to 10th	Day	7% 4 2 5% 4 4 4 4 7% 8% 51%	5900
	11th to 20th	Day	7% 4 2 5% 4 3 4 4 6% 8% 53%	5900
	21st to end	Day	5% 3 1 4 3 4 4 4 7% 10% 53%	6422
August	1st to 10th	Day	6% 5% 2 5% 4 4 5% 4 6% 9% 51%	4234
	11th to 20th	Day	7% 3 2 5% 3 4 4 3 6% 9% 54%	4171
	21st to end	Day	4 3 1 4 2 3 4 3 6% 9% 61%	4496
September	1st to 10th	Day	9% 5% 2 5% 3 3 4 3 4 7% 56%	2200
	11th to 20th	Day	9% 4 2 5% 4 4 4 4 5% 7% 53%	2200
	21st to end	Day	8% 4 1 5% 3 3 4 3 5% 7% 57%	2200
October	1st to 10th	Day	10% 4 2 4 3 3 4 3 5% 5% 58%	2150
	11th to 20th	Day	10% 4 1 3 2 3 3 2 4 4 64%	2000
	21st to end	Day	12% 6% 2 4 3 3 3 2 3 5% 57%	2200
November	1st to 10th	Day	8% 4 2 3 2 2 3 3 4 7% 63%	1883
	11th to 20th	Day	8% 4 2 4 3 2 3 3 5% 6% 60%	1890
	21st to end	Day	11% 5% 1 3 3 2 2 2 4 6% 60%	1890
December	1st to 10th	Day	5% 4 2 4 2 2 3 2 4 6% 66%	3030
	11th to 20th	Day	7% 5% 2 5% 3 3 3 2 4 6% 61%	3481
	21st to end	Day	13% 10% 4 8% 5% 4 4 3 5% 6% 39%	4587
January	1st to 10th	Night	12% 6% 2 6% 3 3 4 3 5% 6% 50%	7634
	11th to 20th	Night	4 5% 2 5% 4 4 4 3 4 5% 61%	7680
	21st to end	Night	13% 8% 3 5% 3 2 3 3 4 5% 52%	8402
February	1st to 10th	Night	9% 8% 3 6% 4 3 3 2 5% 6% 49%	2718
	11th to 20th	Night	10% 9% 3 5% 4 3 3 3 5% 7% 49%	2458
	21st to end	Night	8% 5% 2 6% 3 3 3 3 4 6% 57%	2076
March	1st to 10th	Night	7% 5% 2 5% 3 2 3 2 5% 5% 61%	5194
	11th to 20th	Night	6% 4 1 4 3 4 4 3 5% 6% 61%	5057
	21st to end	Night	9% 6% 2 6% 4 4 4 3 5% 7% 51%	5467
April	1st to 10th	Night	11% 5% 2 4 3 4 3 3 5% 7% 53%	2200
	11th to 20th	Night	14% 5% 2 4 4 3 3 3 6% 8% 49%	1883
	21st to end	Night	15% 6% 2 5% 3 4 4 3 5% 6% 47%	1800
May	1st to 10th	Night	16% 6% 2 4 3 4 3 3 4 6% 48%	4053
	11th to 20th	Night	10% 3 1 5% 2 3 3 2 5% 8% 57%	4053
	21st to end	Night	16% 5% 2 5% 2 2 3 3 4 7% 51%	4443
June	1st to 10th	Night	5% 4 2 5% 3 3 5% 4 7% 11% 53%	1800
	11th to 20th	Night	5% 3 1 3 3 4 5% 4 7% 11% 54%	1800
	21st to end	Night	5% 2 2 4 3 3 4 3 6% 8% 61%	1800
July	1st to 10th	Night	5% 3 1 4 3 4 4 5% 7% 10% 53%	4078
	11th to 20th	Night	4 3 2 5% 4 4 4 4 6% 11% 53%	4077
	21st to end	Night	5% 4 2 6% 4 4 4 3 7% 10% 51%	4449
August	1st to 10th	Night	4 3 2 4 4 3 5% 4 6% 9% 57%	2926
	11th to 20th	Night	6% 4 2 5% 4 5% 5% 4 6% 8% 52%	2888
	21st to end	Night	4 3 1 4 3 3 4 3 5% 8% 64%	3394
September	1st to 10th	Night	7% 5% 2 4 4 3 3 3 4 6% 60%	2200
	11th to 20th	Night	7% 4 2 6% 3 3 3 4 5% 8% 55%	2200
	21st to end	Night	6% 3 1 5% 3 3 3 3 5% 6% 61%	2200
October	1st to 10th	Night	9% 5% 2 4 3 3 3 2 3 7% 58%	2230
	11th to 20th	Night	9% 4 1 4 2 2 2 2 4 5% 66%	2400
	21st to end	Night	11% 5% 1 3 2 2 3 2 3 5% 62%	2640
November	1st to 10th	Night	9% 3 2 4 3 2 2 2 4 5% 64%	2630
	11th to 20th	Night	8% 4 1 3 3 3 3 4 4 6% 62%	2730
	21st to end	Night	12% 5% 1 4 2 3 3 3 3 6% 58%	2730
December	1st to 10th	Night	5% 4 1 3 3 3 3 2 4 6% 66%	4380
	11th to 20th	Night	7% 5% 1 5% 3 3 3 2 4 6% 61%	4946
	21st to end	Night	15% 7% 3 6% 4 4 3 3 4 6% 47%	6633

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	36% 3 10% 27% 20% 4	5290
	11th to 20th	Day	31% 3 10% 27% 23% 6%	5310
	21st to end	Day	38% 2 7% 24% 25% 3	5812
February	1st to 10th	Day	39% 4 10% 26% 17% 4	1921
	11th to 20th	Day	34% 4 13% 26% 17% 5%	2059
	21st to end	Day	29% 3 10% 24% 26% 8%	1730
March	1st to 10th	Day	30% 2 6% 18% 26% 17%	4887
	11th to 20th	Day	25% 3 10% 19% 28% 15%	5057
	21st to end	Day	34% 7% 11% 33% 14% 2	5464
April	1st to 10th	Day	31% 4 12% 25% 21% 7%	2200
	11th to 20th	Day	38% 5% 10% 24% 18% 5%	2460
	21st to end	Day	40% 5% 11% 28% 14% 2	2600
May	1st to 10th	Day	42% 5% 9% 19% 20% 4	5900
	11th to 20th	Day	28% 5% 9% 22% 28% 8%	5866
	21st to end	Day	43% 5% 10% 20% 18% 4	6447
June	1st to 10th	Day	25% 9% 17% 30% 16% 4	2600
	11th to 20th	Day	27% 6% 13% 27% 22% 5%	2600
	21st to end	Day	22% 7% 15% 27% 23% 7%	2600
July	1st to 10th	Day	29% 6% 13% 24% 24% 5%	5836
	11th to 20th	Day	28% 6% 11% 31% 22% 3	5831
	21st to end	Day	23% 7% 15% 37% 17% 1	6240
August	1st to 10th	Day	28% 6% 13% 27% 22% 3	4220
	11th to 20th	Day	27% 6% 18% 27% 19% 3	4210
	21st to end	Day	20% 6% 14% 35% 19% 5%	4495
September	1st to 10th	Day	30% 5% 10% 26% 21% 9%	2200
	11th to 20th	Day	31% 5% 9% 27% 23% 4	2200
	21st to end	Day	27% 5% 10% 26% 25% 8%	2200
October	1st to 10th	Day	20% 3 6% 18% 44% 8%	3501
	11th to 20th	Day	18% 3 7% 28% 36% 9%	3240
	21st to end	Day	26% 3 7% 26% 29% 9%	3538
November	1st to 10th	Day	22% 4 9% 30% 28% 8%	1883
	11th to 20th	Day	27% 4 8% 28% 27% 6%	1890
	21st to end	Day	28% 3 10% 29% 24% 5%	1890
December	1st to 10th	Day	22% 3 7% 42% 24% 2	3030
	11th to 20th	Day	26% 3 6% 35% 25% 3	3479
	21st to end	Day	45% 3 8% 29% 13% 2	4587
January	1st to 10th	Night	36% 3 10% 27% 22% 3	7635
	11th to 20th	Night	26% 3 11% 27% 26% 7%	7680
	21st to end	Night	37% 2 8% 22% 27% 4	8402
February	1st to 10th	Night	36% 4 12% 25% 18% 5%	2718
	11th to 20th	Night	35% 5% 11% 26% 18% 5%	2458
	21st to end	Night	29% 3 8% 22% 26% 11%	2076
March	1st to 10th	Night	26% 3 6% 21% 29% 14%	5194
	11th to 20th	Night	24% 3 7% 19% 29% 18%	5057
	21st to end	Night	33% 6% 12% 34% 13% 2	5467
April	1st to 10th	Night	30% 5% 11% 22% 24% 8%	2200
	11th to 20th	Night	32% 5% 13% 24% 20% 6%	1883
	21st to end	Night	37% 5% 11% 23% 20% 4	1800
May	1st to 10th	Night	37% 6% 12% 18% 21% 5%	4080
	11th to 20th	Night	27% 6% 12% 26% 19% 10%	4054
	21st to end	Night	34% 5% 14% 22% 21% 4	4452
June	1st to 10th	Night	26% 7% 19% 28% 17% 3	1800
	11th to 20th	Night	23% 6% 16% 29% 21% 5%	1800
	21st to end	Night	22% 6% 13% 29% 23% 8%	1800
July	1st to 10th	Night	24% 6% 17% 26% 22% 4	4035
	11th to 20th	Night	25% 5% 11% 32% 24% 3	4026
	21st to end	Night	28% 6% 13% 36% 17% 1	4320
August	1st to 10th	Night	24% 6% 12% 28% 23% 7%	2916
	11th to 20th	Night	28% 7% 15% 25% 21% 4	2915
	21st to end	Night	19% 5% 15% 37% 19% 5%	3394
September	1st to 10th	Night	27% 4 10% 24% 24% 11%	2200
	11th to 20th	Night	28% 5% 10% 26% 26% 6%	2200
	21st to end	Night	23% 5% 9% 23% 30% 9%	2200
October	1st to 10th	Night	20% 3 6% 14% 46% 12%	3646
	11th to 20th	Night	16% 2 7% 26% 34% 15%	3899
	21st to end	Night	22% 3 7% 23% 38% 7%	4252
November	1st to 10th	Night	23% 3 10% 27% 28% 9%	2630
	11th to 20th	Night	24% 3 8% 29% 29% 7%	2730
	21st to end	Night	29% 4 9% 27% 24% 8%	2730
December	1st to 10th	Night	21% 3 10% 42% 22% 2	4380
	11th to 20th	Night	26% 3 7% 33% 25% 6%	4947
	21st to end	Night	39% 4 10% 30% 14% 4	6633

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	13% 13% 40% 24% 9% 1	330
	11th to 20th	Day	15% 17% 40% 24% 5%	330
	21st to end	Day	10% 11% 45% 28% 5%	363
February	1st to 10th	Day	16% 18% 42% 19% 4 1	330
	11th to 20th	Day	15% 17% 44% 17% 6%	330
	21st to end	Day	10% 18% 47% 20% 5%	273
March	1st to 10th	Day	14% 17% 42% 21% 5%	330
	11th to 20th	Day	19% 16% 39% 21% 6%	321
	21st to end	Day	18% 19% 41% 18% 3 1	330
April	1st to 10th	Day	18% 20% 35% 22% 5%	300
	11th to 20th	Day	17% 17% 42% 20% 4	578
	21st to end	Day	21% 15% 36% 19% 8% 1	700
May	1st to 10th	Day	16% 10% 37% 27% 8% 1	703
	11th to 20th	Day	16% 10% 30% 26% 15% 3	725
	21st to end	Day	17% 9% 29% 28% 14% 3	789
June	1st to 10th	Day	6% 4 24% 37% 23% 6%	730
	11th to 20th	Day	7% 3 28% 39% 19% 3	730
	21st to end	Day	6% 5% 27% 45% 16% 3	730
July	1st to 10th	Day	7% 2 14% 35% 33% 9%	721
	11th to 20th	Day	3 4 14% 42% 30% 7%	718
	21st to end	Day	6% 2 12% 37% 33% 9%	794
August	1st to 10th	Day	8% 2 16% 42% 27% 5%	726
	11th to 20th	Day	6% 1 16% 41% 30% 5%	715
	21st to end	Day	5% 3 16% 34% 34% 8%	702
September	1st to 10th	Day	11% 2 11% 37% 30% 9%	330
	11th to 20th	Day	5% 2 19% 42% 26% 5%	330
	21st to end	Day	14% 5% 24% 31% 20% 6%	330
October	1st to 10th	Day	11% 7% 25% 32% 19% 5%	330
	11th to 20th	Day	10% 11% 41% 25% 12% 1	330
	21st to end	Day	19% 16% 28% 24% 10% 3	363
November	1st to 10th	Day	11% 10% 34% 32% 10% 3	330
	11th to 20th	Day	13% 15% 36% 21% 13% 1	330
	21st to end	Day	17% 14% 33% 27% 8% 1	330
December	1st to 10th	Day	8% 12% 42% 26% 9% 2	330
	11th to 20th	Day	11% 13% 38% 28% 10% 1	330
	21st to end	Day	16% 17% 38% 23% 6%	363
January	1st to 10th	Night	10% 11% 38% 33% 7%	1050
	11th to 20th	Night	12% 13% 39% 29% 6% 1	1050
	21st to end	Night	12% 11% 38% 33% 5%	1155
February	1st to 10th	Night	13% 13% 40% 30% 3	1038
	11th to 20th	Night	15% 14% 39% 27% 4	824
	21st to end	Night	12% 14% 39% 31% 4	692
March	1st to 10th	Night	12% 13% 41% 30% 4	709
	11th to 20th	Night	16% 13% 38% 27% 5%	621
	21st to end	Night	15% 16% 39% 24% 5%	660
April	1st to 10th	Night	16% 13% 43% 22% 7%	600
	11th to 20th	Night	17% 16% 40% 23% 4	593
	21st to end	Night	18% 14% 36% 26% 6% 1	600
May	1st to 10th	Night	16% 10% 36% 32% 7%	600
	11th to 20th	Night	13% 7% 31% 35% 14% 1	591
	21st to end	Night	12% 7% 32% 34% 13% 2	650
June	1st to 10th	Night	6% 5% 27% 40% 21% 2	600
	11th to 20th	Night	6% 3 35% 41% 14%	600
	21st to end	Night	6% 4 33% 45% 12% 1	600
July	1st to 10th	Night	6% 2 17% 49% 25% 2	585
	11th to 20th	Night	3 2 17% 56% 20% 3	582
	21st to end	Night	6% 2 15% 47% 28% 1	657
August	1st to 10th	Night	4 1 19% 50% 25% 1	597
	11th to 20th	Night	7% 3 14% 53% 22% 1	594
	21st to end	Night	5% 1 14% 47% 30% 2	660
September	1st to 10th	Night	9% 2 25% 41% 21% 2	600
	11th to 20th	Night	9% 3 27% 42% 18% 2	600
	21st to end	Night	9% 5% 27% 40% 18% 2	600
October	1st to 10th	Night	11% 5% 30% 33% 19% 2	630
	11th to 20th	Night	11% 8% 39% 34% 8%	800
	21st to end	Night	14% 11% 34% 30% 10%	880
November	1st to 10th	Night	11% 6% 38% 33% 10%	990
	11th to 20th	Night	10% 12% 38% 31% 8%	1050
	21st to end	Night	15% 11% 30% 35% 8%	1050
December	1st to 10th	Night	11% 10% 38% 33% 8%	1050
	11th to 20th	Night	14% 11% 39% 32% 4	1050
	21st to end	Night	13% 13% 35% 33% 5%	1155

Page generated at 2026-03-15 01:23:54. Version ac07

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