South Lake Tahoe
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the South Lake Tahoe clear sky chart from 2005-11-06 to 2026-10-29.

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	1 3 1 4 3 3 3 3 5% 7% 67%	157937
	Night	1 3 2 4 3 3 3 3 5% 7% 65%	151592
Transparency	Day	18% 3 8% 21% 29% 22%	152913
	Night	19% 3 10% 22% 27% 19%	147600
Seeing	Day	16% 17% 31% 23% 11% 3	17682
	Night	10% 10% 25% 31% 19% 5%	26984

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 4 2 5% 3 4 5% 4 8% 10% 55%	4722
	11th to 20th	Day	2 1 3 2 3 4 4 6% 8% 66%	4920
	21st to end	Day	1 6% 3 8% 4 4 5% 5% 6% 11% 46%	5843
February	1st to 10th	Day	1 5% 2 6% 3 4 5% 5% 7% 9% 53%	1896
	11th to 20th	Day	1 4 3 7% 5% 5% 5% 4 6% 9% 50%	2050
	21st to end	Day	1 4 2 6% 5% 4 4 4 7% 10% 53%	1730
March	1st to 10th	Day	1 7% 4 9% 5% 5% 5% 4 7% 9% 44%	4618
	11th to 20th	Day	4 2 5% 3 3 4 4 6% 10% 59%	5054
	21st to end	Day	1 8% 2 6% 3 4 5% 4 7% 8% 52%	5958
April	1st to 10th	Day	3 4 2 6% 4 4 4 4 6% 10% 53%	2520
	11th to 20th	Day	2 4 2 5% 4 4 5% 4 7% 9% 56%	2489
	21st to end	Day	2 6% 2 4 3 3 4 4 5% 9% 59%	2520
May	1st to 10th	Day	4 5% 1 4 2 2 2 2 4 6% 67%	7357
	11th to 20th	Day	1 2 1 2 2 2 2 2 4 5% 77%	8007
	21st to end	Day	3 4 1 4 3 3 3 2 4 6% 67%	8992
June	1st to 10th	Day	2 3 1 4 3 3 4 4 6% 7% 64%	2940
	11th to 20th	Day	1 2 1 2 2 2 2 2 4 5% 78%	2940
	21st to end	Day	1 1 1 2 2 2 2 2 4 5% 77%	2940
July	1st to 10th	Day	1 1 1 2 1 2 1 2 3 86%	8219
	11th to 20th	Day	2 2 1 3 2 2 3 2 4 5% 73%	8220
	21st to end	Day	1 1 1 2 2 2 3 3 4 5% 77%	8665
August	1st to 10th	Day	1 1 1 2 1 1 2 2 3 4 83%	7444
	11th to 20th	Day	1 1 1 2 2 2 2 3 5% 81%	7001
	21st to end	Day	2 2 1 4 3 3 3 2 4 5% 71%	7750
September	1st to 10th	Day	1 1 1 2 2 1 3 3 4 4 79%	2520
	11th to 20th	Day	2 2 1 3 3 2 3 2 3 6% 74%	2489
	21st to end	Day	2 2 1 3 2 2 2 2 3 4 77%	2310
October	1st to 10th	Day	2 3 1 3 2 3 3 2 5% 6% 70%	3424
	11th to 20th	Day	1 3 1 3 2 1 2 1 3 4 78%	3361
	21st to end	Day	1 2 1 3 3 3 4 3 4 8% 69%	3562
November	1st to 10th	Day	1 3 2 4 4 4 4 4 6% 9% 60%	1854
	11th to 20th	Day	2 4 1 3 3 3 4 3 7% 9% 60%	1890
	21st to end	Day	3 1 4 3 3 4 4 7% 10% 58%	1832
December	1st to 10th	Day	1 3 1 2 3 3 3 4 6% 9% 64%	2700
	11th to 20th	Day	1 5% 3 6% 5% 6% 5% 7% 9% 11% 43%	3108
	21st to end	Day	1 5% 3 8% 4 5% 6% 5% 8% 13% 42%	4092
January	1st to 10th	Night	1 4 2 5% 4 4 4 4 7% 12% 52%	8172
	11th to 20th	Night	2 1 4 3 3 3 4 5% 7% 66%	8032
	21st to end	Night	1 6% 3 7% 4 4 5% 4 6% 8% 50%	8371
February	1st to 10th	Night	1 4 2 7% 4 4 4 4 8% 9% 53%	2722
	11th to 20th	Night	4 2 6% 4 4 5% 6% 9% 10% 49%	2458
	21st to end	Night	4 1 6% 4 3 4 4 8% 10% 54%	2076
March	1st to 10th	Night	1 7% 5% 8% 4 4 5% 5% 7% 10% 45%	5496
	11th to 20th	Night	4 2 7% 4 4 4 4 7% 8% 55%	5161
	21st to end	Night	1 6% 3 7% 4 5% 5% 5% 8% 11% 46%	5257
April	1st to 10th	Night	2 4 1 5% 4 5% 4 5% 7% 9% 53%	2100
	11th to 20th	Night	1 5% 2 4 4 3 3 3 7% 9% 58%	2079
	21st to end	Night	3 5% 1 3 3 2 3 3 6% 8% 63%	2100
May	1st to 10th	Night	4 3 2 4 2 2 3 2 3 5% 69%	5603
	11th to 20th	Night	1 3 1 2 2 2 2 2 3 6% 76%	4844
	21st to end	Night	2 4 1 3 2 2 3 3 4 6% 69%	5181
June	1st to 10th	Night	3 3 1 2 3 3 4 3 5% 6% 68%	1680
	11th to 20th	Night	1 1 1 1 3 2 2 3 4 82%	1680
	21st to end	Night	2 2 1 1 2 2 2 2 3 4 81%	1680
July	1st to 10th	Night	1 1 1 1 2 2 2 3 88%	4740
	11th to 20th	Night	2 3 1 3 3 3 3 2 3 3 73%	4740
	21st to end	Night	1 2 1 3 2 2 3 2 3 5% 76%	5407
August	1st to 10th	Night	2 1 1 2 1 1 2 2 3 4 83%	5368
	11th to 20th	Night	1 1 1 1 1 2 2 3 6% 82%	5886
	21st to end	Night	2 3 1 3 2 2 3 2 3 5% 74%	6499
September	1st to 10th	Night	1 1 2 1 1 1 1 3 4 84%	2100
	11th to 20th	Night	2 2 1 2 2 2 2 2 3 4 79%	2131
	21st to end	Night	1 2 1 3 2 2 2 2 4 4 77%	2310
October	1st to 10th	Night	2 2 1 2 2 1 2 2 4 4 78%	3991
	11th to 20th	Night	1 2 1 2 2 1 2 2 3 4 80%	4017
	21st to end	Night	2 1 4 3 2 4 3 5% 7% 68%	4411
November	1st to 10th	Night	1 3 1 4 2 3 3 4 8% 9% 62%	2665
	11th to 20th	Night	2 3 1 2 2 3 4 4 8% 10% 61%	2730
	21st to end	Night	2 4 2 4 3 2 3 3 6% 9% 62%	2783
December	1st to 10th	Night	1 2 1 3 3 3 4 4 6% 8% 66%	4700
	11th to 20th	Night	2 4 2 5% 4 4 6% 6% 9% 14% 45%	5316
	21st to end	Night	1 5% 3 7% 6% 5% 6% 6% 9% 11% 41%	7106

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% 19% 29% 22% 2	4723
	11th to 20th	Day	18% 3 10% 19% 29% 22%	4920
	21st to end	Day	31% 5% 9% 21% 25% 10%	5843
February	1st to 10th	Day	27% 5% 13% 25% 22% 9%	1896
	11th to 20th	Day	31% 2 11% 27% 24% 5%	2060
	21st to end	Day	26% 4 17% 28% 19% 7%	1730
March	1st to 10th	Day	35% 4 11% 23% 22% 5%	4618
	11th to 20th	Day	20% 4 13% 36% 24% 3	5054
	21st to end	Day	27% 4 16% 28% 22% 2	5958
April	1st to 10th	Day	27% 4 17% 26% 21% 5%	2520
	11th to 20th	Day	25% 3 13% 30% 23% 6%	2492
	21st to end	Day	23% 3 14% 31% 26% 4	2520
May	1st to 10th	Day	20% 2 7% 27% 38% 6%	7357
	11th to 20th	Day	11% 2 5% 24% 46% 11%	8010
	21st to end	Day	18% 3 5% 26% 39% 7%	8994
June	1st to 10th	Day	19% 3 10% 24% 33% 10%	2940
	11th to 20th	Day	11% 2 5% 22% 38% 23%	2940
	21st to end	Day	12% 2 2 13% 37% 34%	2940
July	1st to 10th	Day	6% 2 1 5% 23% 63%	8151
	11th to 20th	Day	14% 3 2 8% 14% 58%	8145
	21st to end	Day	10% 4 3 12% 27% 45%	8471
August	1st to 10th	Day	10% 2 1 9% 28% 50%	5895
	11th to 20th	Day	10% 3 2 20% 21% 44%	5517
	21st to end	Day	12% 1 1 21% 35% 29%	6077
September	1st to 10th	Day	10% 2 3 10% 36% 40%	2520
	11th to 20th	Day	14% 2 3 18% 40% 22%	2489
	21st to end	Day	14% 1 3 20% 38% 24%	2310
October	1st to 10th	Day	16% 2 4 12% 37% 29%	3424
	11th to 20th	Day	12% 1 3 20% 49% 15%	3361
	21st to end	Day	15% 2 7% 27% 36% 12%	3562
November	1st to 10th	Day	21% 4 10% 26% 29% 10%	1854
	11th to 20th	Day	20% 3 16% 24% 30% 8%	1890
	21st to end	Day	20% 3 12% 27% 27% 9%	1832
December	1st to 10th	Day	17% 3 12% 26% 31% 10%	2700
	11th to 20th	Day	32% 6% 16% 19% 19% 7%	3108
	21st to end	Day	29% 7% 19% 24% 17% 3	4092
January	1st to 10th	Night	23% 5% 19% 30% 22% 2	8172
	11th to 20th	Night	18% 2 11% 19% 27% 23%	8032
	21st to end	Night	31% 3 11% 18% 27% 10%	8371
February	1st to 10th	Night	27% 5% 12% 26% 21% 11%	2722
	11th to 20th	Night	25% 3 14% 30% 23% 5%	2459
	21st to end	Night	24% 5% 15% 29% 20% 7%	2076
March	1st to 10th	Night	32% 4 12% 22% 24% 6%	5496
	11th to 20th	Night	24% 4 14% 40% 15% 3	5161
	21st to end	Night	29% 5% 21% 28% 14% 4	5257
April	1st to 10th	Night	25% 4 18% 28% 19% 6%	2100
	11th to 20th	Night	22% 4 16% 31% 21% 7%	2079
	21st to end	Night	19% 4 16% 35% 23% 3	2100
May	1st to 10th	Night	19% 3 7% 28% 35% 8%	5601
	11th to 20th	Night	12% 2 6% 25% 43% 13%	4844
	21st to end	Night	15% 3 9% 28% 39% 6%	5182
June	1st to 10th	Night	18% 4 10% 26% 32% 11%	1680
	11th to 20th	Night	9% 2 5% 24% 36% 25%	1680
	21st to end	Night	10% 1 3 15% 37% 34%	1680
July	1st to 10th	Night	4 2 1 6% 22% 65%	4700
	11th to 20th	Night	17% 3 2 7% 15% 56%	4692
	21st to end	Night	12% 2 3 12% 22% 49%	5290
August	1st to 10th	Night	11% 2 2 8% 27% 50%	4238
	11th to 20th	Night	8% 2 2 19% 26% 44%	4636
	21st to end	Night	12% 1 1 22% 32% 31%	5092
September	1st to 10th	Night	7% 1 2 10% 39% 41%	2100
	11th to 20th	Night	10% 2 5% 21% 39% 22%	2131
	21st to end	Night	13% 2 3 22% 37% 24%	2310
October	1st to 10th	Night	12% 1 4 13% 38% 32%	3991
	11th to 20th	Night	10% 2 4 19% 48% 17%	4017
	21st to end	Night	15% 3 7% 24% 39% 13%	4411
November	1st to 10th	Night	17% 5% 10% 28% 28% 11%	2665
	11th to 20th	Night	16% 6% 12% 29% 29% 8%	2730
	21st to end	Night	19% 4 11% 29% 28% 10%	2783
December	1st to 10th	Night	17% 4 12% 26% 29% 12%	4700
	11th to 20th	Night	26% 6% 18% 23% 21% 6%	5316
	21st to end	Night	32% 6% 18% 22% 17% 5%	7106

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	33% 20% 23% 15% 8% 2	220
	11th to 20th	Day	25% 13% 28% 21% 10% 4	283
	21st to end	Day	18% 18% 27% 21% 13% 4	473
February	1st to 10th	Day	26% 20% 26% 18% 8% 2	434
	11th to 20th	Day	27% 21% 28% 19% 6% 1	535
	21st to end	Day	24% 19% 33% 18% 5%	446
March	1st to 10th	Day	31% 23% 26% 15% 5% 1	540
	11th to 20th	Day	25% 24% 24% 17% 9% 1	540
	21st to end	Day	27% 24% 26% 18% 5%	594
April	1st to 10th	Day	22% 21% 29% 18% 9% 1	540
	11th to 20th	Day	23% 25% 29% 15% 6% 2	532
	21st to end	Day	19% 20% 31% 19% 8% 4	540
May	1st to 10th	Day	19% 23% 30% 18% 7% 3	543
	11th to 20th	Day	19% 18% 32% 19% 8% 4	568
	21st to end	Day	15% 15% 29% 24% 12% 5%	617
June	1st to 10th	Day	9% 19% 32% 25% 10% 5%	570
	11th to 20th	Day	13% 19% 35% 20% 8% 4	570
	21st to end	Day	8% 15% 39% 23% 9% 6%	570
July	1st to 10th	Day	3 15% 42% 25% 13% 4	566
	11th to 20th	Day	4 10% 37% 29% 16% 4	564
	21st to end	Day	5% 10% 35% 32% 13% 5%	619
August	1st to 10th	Day	6% 14% 31% 30% 14% 4	567
	11th to 20th	Day	4 13% 35% 29% 14% 4	557
	21st to end	Day	5% 11% 37% 31% 14% 3	627
September	1st to 10th	Day	4 15% 38% 27% 15% 2	570
	11th to 20th	Day	9% 17% 31% 30% 10% 3	570
	21st to end	Day	9% 14% 35% 30% 10% 2	570
October	1st to 10th	Day	10% 19% 26% 28% 16% 2	570
	11th to 20th	Day	15% 14% 24% 31% 14% 3	570
	21st to end	Day	16% 16% 25% 25% 14% 4	575
November	1st to 10th	Day	19% 15% 23% 19% 19% 5%	330
	11th to 20th	Day	20% 12% 28% 23% 11% 5%	330
	21st to end	Day	19% 14% 25% 24% 13% 4	300
December	1st to 10th	Day	20% 14% 25% 18% 15% 8%	220
	11th to 20th	Day	25% 15% 26% 17% 11% 5%	220
	21st to end	Day	33% 9% 26% 19% 12% 2	242
January	1st to 10th	Night	16% 11% 28% 21% 16% 7%	840
	11th to 20th	Night	13% 9% 25% 34% 14% 5%	840
	21st to end	Night	11% 9% 31% 28% 16% 6%	924
February	1st to 10th	Night	15% 9% 29% 28% 16% 4	840
	11th to 20th	Night	15% 13% 27% 24% 16% 4	824
	21st to end	Night	17% 14% 25% 29% 12% 3	692
March	1st to 10th	Night	15% 18% 29% 23% 12% 3	840
	11th to 20th	Night	14% 14% 27% 23% 19% 4	840
	21st to end	Night	15% 16% 29% 26% 12% 2	924
April	1st to 10th	Night	11% 12% 29% 30% 16% 2	840
	11th to 20th	Night	14% 15% 26% 27% 14% 4	831
	21st to end	Night	11% 11% 27% 29% 17% 4	840
May	1st to 10th	Night	11% 13% 25% 30% 17% 4	730
	11th to 20th	Night	8% 9% 27% 35% 18% 3	469
	21st to end	Night	9% 6% 25% 33% 25% 2	455
June	1st to 10th	Night	7% 5% 28% 36% 21% 3	420
	11th to 20th	Night	4 9% 28% 33% 26% 1	420
	21st to end	Night	4 5% 28% 39% 21% 3	420
July	1st to 10th	Night	2 3 25% 45% 22% 2	410
	11th to 20th	Night	2 3 29% 41% 24% 1	408
	21st to end	Night	3 3 22% 37% 29% 6%	548
August	1st to 10th	Night	4 2 22% 42% 26% 4	657
	11th to 20th	Night	3 4 26% 39% 24% 4	832
	21st to end	Night	4 5% 24% 39% 25% 4	924
September	1st to 10th	Night	3 7% 23% 35% 28% 5%	840
	11th to 20th	Night	5% 8% 21% 32% 27% 7%	840
	21st to end	Night	5% 7% 23% 31% 27% 7%	840
October	1st to 10th	Night	8% 7% 21% 35% 21% 6%	840
	11th to 20th	Night	10% 7% 18% 38% 23% 5%	840
	21st to end	Night	9% 9% 21% 29% 20% 11%	912
November	1st to 10th	Night	12% 10% 19% 29% 22% 8%	820
	11th to 20th	Night	12% 11% 25% 32% 15% 6%	840
	21st to end	Night	14% 11% 21% 30% 18% 5%	840
December	1st to 10th	Night	13% 10% 24% 33% 14% 5%	840
	11th to 20th	Night	16% 12% 24% 30% 13% 6%	840
	21st to end	Night	18% 14% 28% 25% 12% 3	924

Page generated at 2026-10-26 01:37:51. Version ac07

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