Lake Sundown
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Lake Sundown clear sky chart from 2005-11-06 to 2026-02-01.

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	12% 8% 3 8% 5% 4 5% 4 6% 7% 40%	127769
	Night	12% 9% 3 8% 4 4 4 3 5% 6% 43%	124532
Transparency	Day	41% 6% 12% 24% 15% 2	131370
	Night	41% 5% 12% 24% 16% 3	129007
Seeing	Day	15% 10% 30% 26% 14% 4	17439
	Night	15% 10% 32% 30% 11% 1	27399

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% 13% 5% 14% 6% 6% 6% 4 5% 6% 27%	3697
	11th to 20th	Day	7% 9% 3 11% 6% 6% 8% 4 5% 5% 34%	4141
	21st to end	Day	11% 13% 6% 14% 6% 4 4 3 5% 5% 29%	4587
February	1st to 10th	Day	13% 12% 4 10% 7% 5% 6% 4 5% 7% 29%	1800
	11th to 20th	Day	13% 11% 3 9% 5% 5% 5% 3 6% 6% 35%	1754
	21st to end	Day	14% 10% 3 7% 6% 4 3 4 4 7% 39%	1640
March	1st to 10th	Day	17% 10% 4 6% 3 3 2 2 3 4 46%	4883
	11th to 20th	Day	9% 6% 2 7% 5% 4 4 3 5% 8% 47%	4980
	21st to end	Day	17% 13% 4 9% 5% 4 5% 3 6% 7% 28%	5523
April	1st to 10th	Day	16% 11% 2 7% 4 4 4 4 5% 6% 37%	2400
	11th to 20th	Day	15% 10% 3 8% 5% 4 4 4 5% 5% 37%	2374
	21st to end	Day	20% 8% 3 9% 5% 4 4 3 5% 6% 33%	2446
May	1st to 10th	Day	14% 7% 3 7% 5% 4 5% 4 6% 8% 37%	5862
	11th to 20th	Day	14% 6% 3 6% 4 4 4 3 5% 7% 43%	6268
	21st to end	Day	15% 7% 2 7% 5% 4 5% 4 6% 7% 39%	6935
June	1st to 10th	Day	8% 5% 2 6% 4 4 5% 5% 7% 9% 44%	2800
	11th to 20th	Day	8% 5% 2 7% 5% 4 5% 5% 7% 8% 45%	2800
	21st to end	Day	9% 5% 2 6% 5% 5% 5% 5% 7% 8% 43%	2800
July	1st to 10th	Day	7% 7% 2 7% 4 5% 5% 4 7% 9% 42%	6360
	11th to 20th	Day	8% 7% 2 7% 4 5% 5% 4 7% 9% 42%	6325
	21st to end	Day	8% 5% 2 6% 5% 5% 5% 5% 7% 10% 41%	6422
August	1st to 10th	Day	5% 4 2 6% 4 4 6% 4 8% 10% 45%	4234
	11th to 20th	Day	13% 6% 2 6% 4 5% 5% 4 6% 9% 41%	4146
	21st to end	Day	7% 5% 2 5% 4 4 4 4 6% 8% 51%	4312
September	1st to 10th	Day	12% 4 2 4 3 4 3 3 5% 7% 53%	2400
	11th to 20th	Day	10% 5% 2 5% 3 3 5% 4 6% 9% 49%	2398
	21st to end	Day	10% 6% 2 4 4 4 4 4 4 6% 51%	2085
October	1st to 10th	Day	14% 6% 2 5% 5% 3 3 2 5% 6% 50%	2000
	11th to 20th	Day	14% 7% 2 6% 4 3 3 3 4 6% 48%	2000
	21st to end	Day	15% 9% 3 7% 4 4 3 3 5% 6% 42%	2200
November	1st to 10th	Day	10% 7% 2 5% 4 4 4 4 6% 8% 46%	1922
	11th to 20th	Day	13% 10% 2 8% 3 4 4 3 5% 6% 42%	1747
	21st to end	Day	16% 10% 4 8% 3 3 3 2 6% 6% 39%	1680
December	1st to 10th	Day	7% 8% 3 8% 4 3 3 3 4 6% 52%	2690
	11th to 20th	Day	13% 9% 3 15% 6% 5% 5% 3 5% 7% 29%	3088
	21st to end	Day	21% 18% 8% 12% 5% 4 4 3 4 5% 17%	4070
January	1st to 10th	Night	11% 11% 5% 18% 6% 5% 4 3 4 6% 27%	6488
	11th to 20th	Night	8% 12% 5% 12% 6% 5% 6% 4 6% 5% 31%	6053
	21st to end	Night	13% 12% 5% 12% 5% 4 4 3 4 5% 33%	6633
February	1st to 10th	Night	13% 11% 4 10% 5% 4 4 4 5% 6% 33%	2601
	11th to 20th	Night	12% 11% 3 8% 5% 4 4 3 4 6% 40%	2533
	21st to end	Night	12% 10% 4 7% 5% 4 4 2 4 4 43%	1990
March	1st to 10th	Night	17% 10% 2 6% 3 3 3 2 3 5% 46%	5012
	11th to 20th	Night	11% 7% 2 5% 3 3 3 3 4 6% 52%	4980
	21st to end	Night	16% 11% 4 10% 5% 5% 4 4 4 6% 30%	5404
April	1st to 10th	Night	12% 8% 2 6% 5% 3 4 4 5% 6% 44%	2000
	11th to 20th	Night	12% 9% 2 6% 5% 4 5% 3 5% 6% 42%	1975
	21st to end	Night	17% 8% 3 6% 4 4 4 3 5% 6% 40%	1950
May	1st to 10th	Night	14% 7% 3 6% 4 4 5% 4 6% 7% 42%	4053
	11th to 20th	Night	12% 6% 2 5% 4 4 5% 3 6% 8% 46%	3647
	21st to end	Night	14% 8% 3 7% 4 4 3 4 4 6% 44%	3942
June	1st to 10th	Night	7% 5% 2 5% 4 4 5% 4 7% 10% 49%	1600
	11th to 20th	Night	7% 5% 2 4 4 4 5% 3 6% 10% 50%	1600
	21st to end	Night	7% 5% 2 5% 5% 5% 5% 4 6% 8% 49%	1600
July	1st to 10th	Night	9% 5% 2 5% 4 3 5% 4 7% 9% 48%	3618
	11th to 20th	Night	5% 4 2 6% 4 4 5% 3 7% 9% 51%	3634
	21st to end	Night	10% 5% 2 6% 4 4 4 4 6% 9% 46%	4449
August	1st to 10th	Night	4 3 1 4 3 3 3 4 6% 9% 59%	2926
	11th to 20th	Night	10% 6% 1 4 3 3 4 3 4 7% 55%	2910
	21st to end	Night	6% 3 2 5% 3 3 4 4 6% 9% 56%	3586
September	1st to 10th	Night	10% 4 1 5% 3 3 4 3 5% 7% 56%	2000
	11th to 20th	Night	10% 5% 2 6% 4 4 4 3 5% 6% 52%	2000
	21st to end	Night	9% 4 2 4 3 3 4 3 5% 6% 58%	2292
October	1st to 10th	Night	11% 5% 2 5% 3 4 3 3 4 5% 54%	2400
	11th to 20th	Night	14% 6% 2 6% 3 3 3 3 3 6% 52%	2400
	21st to end	Night	16% 7% 3 6% 4 4 3 3 4 6% 45%	2640
November	1st to 10th	Night	9% 6% 2 5% 3 3 3 3 5% 7% 55%	2596
	11th to 20th	Night	12% 8% 2 7% 4 4 3 3 6% 6% 45%	2871
	21st to end	Night	16% 11% 4 7% 4 3 3 3 4 5% 42%	2940
December	1st to 10th	Night	7% 10% 3 8% 4 4 4 3 4 6% 49%	4720
	11th to 20th	Night	13% 10% 4 10% 5% 4 4 3 4 6% 38%	5339
	21st to end	Night	20% 17% 7% 11% 5% 4 3 3 3 4 21%	7150

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	56% 6% 13% 15% 9% 1	3698
	11th to 20th	Day	50% 5% 9% 18% 15% 3	4141
	21st to end	Day	57% 4 10% 21% 7% 1	4587
February	1st to 10th	Day	53% 6% 12% 17% 10% 1	1800
	11th to 20th	Day	49% 5% 13% 21% 11% 1	1763
	21st to end	Day	46% 4 12% 23% 13% 2	1640
March	1st to 10th	Day	44% 4 8% 24% 20% 1	4883
	11th to 20th	Day	35% 5% 10% 26% 22% 2	4980
	21st to end	Day	54% 7% 16% 18% 4 1	5525
April	1st to 10th	Day	47% 5% 11% 26% 10%	2400
	11th to 20th	Day	47% 6% 10% 23% 13% 1	2374
	21st to end	Day	51% 5% 12% 20% 12% 1	2446
May	1st to 10th	Day	43% 7% 13% 18% 18% 1	5900
	11th to 20th	Day	40% 6% 10% 28% 14% 3	6272
	21st to end	Day	42% 6% 11% 25% 15% 1	6949
June	1st to 10th	Day	34% 8% 17% 28% 12% 2	2800
	11th to 20th	Day	33% 8% 13% 24% 20% 2	2800
	21st to end	Day	35% 8% 15% 24% 14% 2	2800
July	1st to 10th	Day	36% 7% 14% 28% 14% 1	6291
	11th to 20th	Day	37% 7% 14% 25% 15% 1	6250
	21st to end	Day	34% 10% 16% 28% 11% 1	6240
August	1st to 10th	Day	31% 8% 15% 32% 14% 1	4220
	11th to 20th	Day	39% 6% 14% 25% 16% 1	4185
	21st to end	Day	30% 6% 15% 30% 15% 4	4311
September	1st to 10th	Day	31% 5% 12% 26% 22% 4	2400
	11th to 20th	Day	31% 6% 11% 30% 19% 3	2398
	21st to end	Day	33% 6% 10% 29% 21% 1	2085
October	1st to 10th	Day	24% 3 5% 18% 47% 3	3260
	11th to 20th	Day	25% 3 8% 25% 31% 8%	3240
	21st to end	Day	39% 5% 9% 26% 19% 2	3538
November	1st to 10th	Day	34% 5% 15% 23% 19% 3	1922
	11th to 20th	Day	43% 4 10% 25% 17% 1	1747
	21st to end	Day	47% 4 14% 22% 13%	1680
December	1st to 10th	Day	36% 4 16% 31% 13% 1	2690
	11th to 20th	Day	54% 5% 11% 19% 11%	3085
	21st to end	Day	70% 5% 9% 12% 4 1	4070
January	1st to 10th	Night	59% 5% 10% 18% 7% 1	6488
	11th to 20th	Night	53% 4 12% 14% 15% 3	6053
	21st to end	Night	55% 4 10% 17% 12% 2	6633
February	1st to 10th	Night	51% 4 12% 20% 12% 1	2601
	11th to 20th	Night	46% 5% 11% 24% 13% 2	2534
	21st to end	Night	45% 4 9% 22% 17% 4	1990
March	1st to 10th	Night	43% 3 7% 24% 21% 1	5012
	11th to 20th	Night	34% 4 8% 29% 20% 6%	4980
	21st to end	Night	54% 6% 15% 16% 7% 2	5404
April	1st to 10th	Night	39% 6% 12% 25% 16% 2	2000
	11th to 20th	Night	42% 5% 10% 24% 16% 3	1975
	21st to end	Night	45% 5% 13% 22% 14% 2	1950
May	1st to 10th	Night	41% 7% 12% 22% 16% 2	4080
	11th to 20th	Night	35% 7% 14% 24% 16% 4	3648
	21st to end	Night	40% 7% 10% 20% 21% 2	3950
June	1st to 10th	Night	29% 9% 16% 27% 17% 2	1600
	11th to 20th	Night	28% 7% 14% 28% 20% 4	1600
	21st to end	Night	30% 7% 15% 25% 20% 4	1600
July	1st to 10th	Night	30% 7% 13% 30% 17% 4	3580
	11th to 20th	Night	28% 7% 17% 27% 19% 1	3589
	21st to end	Night	32% 8% 18% 27% 14% 1	4320
August	1st to 10th	Night	21% 7% 18% 35% 17% 1	2916
	11th to 20th	Night	30% 5% 14% 32% 19% 1	2937
	21st to end	Night	24% 6% 15% 31% 17% 6%	3586
September	1st to 10th	Night	28% 5% 10% 27% 25% 6%	2000
	11th to 20th	Night	33% 4 10% 29% 19% 4	2000
	21st to end	Night	27% 4 10% 30% 25% 3	2292
October	1st to 10th	Night	23% 4 6% 19% 45% 4	3920
	11th to 20th	Night	27% 3 8% 23% 30% 9%	3899
	21st to end	Night	41% 4 8% 22% 21% 3	4252
November	1st to 10th	Night	29% 4 16% 27% 20% 4	2596
	11th to 20th	Night	39% 4 11% 28% 15% 2	2871
	21st to end	Night	47% 3 10% 25% 13% 1	2940
December	1st to 10th	Night	37% 4 11% 37% 11% 1	4720
	11th to 20th	Night	48% 5% 12% 24% 11%	5341
	21st to end	Night	68% 4 9% 14% 5% 1	7150

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	23% 12% 33% 25% 8%	330
	11th to 20th	Day	21% 20% 42% 16% 2	330
	21st to end	Day	19% 18% 42% 18% 2	363
February	1st to 10th	Day	21% 19% 39% 17% 4	300
	11th to 20th	Day	21% 17% 38% 18% 5%	300
	21st to end	Day	19% 16% 46% 14% 4	249
March	1st to 10th	Day	20% 16% 46% 14% 3	300
	11th to 20th	Day	22% 17% 34% 23% 4	300
	21st to end	Day	35% 14% 34% 14% 3 1	353
April	1st to 10th	Day	23% 15% 35% 20% 5% 1	500
	11th to 20th	Day	21% 18% 33% 20% 8% 1	492
	21st to end	Day	22% 16% 37% 18% 6% 1	546
May	1st to 10th	Day	21% 14% 34% 20% 10% 1	703
	11th to 20th	Day	20% 12% 28% 25% 12% 3	725
	21st to end	Day	15% 9% 30% 28% 14% 4	789
June	1st to 10th	Day	8% 5% 24% 34% 23% 6%	730
	11th to 20th	Day	8% 7% 27% 34% 20% 4	730
	21st to end	Day	9% 8% 35% 30% 15% 3	730
July	1st to 10th	Day	7% 5% 18% 32% 27% 12%	721
	11th to 20th	Day	9% 3 25% 34% 23% 7%	718
	21st to end	Day	7% 3 21% 37% 27% 5%	794
August	1st to 10th	Day	8% 3 21% 37% 26% 5%	726
	11th to 20th	Day	11% 4 18% 32% 27% 7%	703
	21st to end	Day	10% 2 17% 36% 26% 9%	583
September	1st to 10th	Day	13% 6% 17% 32% 24% 8%	530
	11th to 20th	Day	10% 3 21% 35% 23% 8%	528
	21st to end	Day	12% 9% 27% 25% 21% 7%	330
October	1st to 10th	Day	19% 8% 29% 24% 13% 7%	330
	11th to 20th	Day	16% 19% 35% 21% 7% 1	330
	21st to end	Day	22% 20% 33% 19% 6% 1	363
November	1st to 10th	Day	18% 11% 41% 24% 5% 2	330
	11th to 20th	Day	20% 18% 36% 19% 6% 2	330
	21st to end	Day	23% 16% 36% 20% 5%	330
December	1st to 10th	Day	14% 15% 44% 19% 6% 2	330
	11th to 20th	Day	20% 17% 36% 20% 6% 1	330
	21st to end	Day	26% 21% 33% 16% 3	363
January	1st to 10th	Night	16% 14% 42% 21% 7%	1050
	11th to 20th	Night	16% 18% 41% 19% 7%	1050
	21st to end	Night	17% 14% 41% 23% 4	1155
February	1st to 10th	Night	20% 14% 39% 23% 4	1001
	11th to 20th	Night	18% 14% 38% 26% 3	979
	21st to end	Night	19% 15% 39% 22% 3 1	670
March	1st to 10th	Night	18% 13% 39% 22% 7% 1	645
	11th to 20th	Night	20% 12% 34% 27% 7%	600
	21st to end	Night	25% 12% 36% 21% 5%	660
April	1st to 10th	Night	16% 15% 40% 26% 4	600
	11th to 20th	Night	18% 15% 34% 27% 5% 1	593
	21st to end	Night	20% 11% 33% 28% 8% 1	600
May	1st to 10th	Night	16% 8% 35% 33% 7% 1	600
	11th to 20th	Night	17% 9% 32% 28% 14% 1	591
	21st to end	Night	14% 6% 28% 34% 17% 1	650
June	1st to 10th	Night	9% 3 26% 41% 20% 2	600
	11th to 20th	Night	8% 5% 25% 43% 18% 1	600
	21st to end	Night	8% 4 31% 41% 15% 1	600
July	1st to 10th	Night	8% 2 14% 40% 33% 2	585
	11th to 20th	Night	5% 3 19% 44% 27% 3	582
	21st to end	Night	9% 2 16% 44% 28% 1	657
August	1st to 10th	Night	6% 3 16% 46% 27% 2	597
	11th to 20th	Night	9% 2 13% 42% 29% 4	594
	21st to end	Night	7% 2 13% 44% 30% 3	660
September	1st to 10th	Night	11% 4 19% 37% 27% 2	600
	11th to 20th	Night	12% 2 20% 39% 25% 4	600
	21st to end	Night	11% 5% 25% 34% 22% 2	694
October	1st to 10th	Night	13% 7% 30% 31% 17% 2	800
	11th to 20th	Night	16% 11% 35% 30% 7%	800
	21st to end	Night	18% 12% 35% 27% 7%	880
November	1st to 10th	Night	14% 10% 38% 31% 7%	820
	11th to 20th	Night	16% 15% 40% 23% 5%	981
	21st to end	Night	22% 13% 36% 23% 6%	1050
December	1st to 10th	Night	17% 15% 39% 23% 5% 1	1050
	11th to 20th	Night	19% 13% 37% 25% 5%	1050
	21st to end	Night	22% 16% 36% 23% 3	1155

Page generated at 2026-01-29 00:21:40. Version ac07

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