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 2025-07-06.

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% 66%	123774
	Night	1 3 2 4 3 3 4 3 6% 8% 63%	123861
Transparency	Day	19% 3 9% 21% 29% 19%	123521
	Night	19% 3 11% 23% 28% 16%	123701
Seeing	Day	16% 17% 31% 23% 10% 3	15916
	Night	10% 9% 24% 31% 20% 5%	25291

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 3 1 4 3 4 5% 4 8% 10% 58%	3639
	11th to 20th	Day	3 2 4 3 4 4 4 6% 8% 63%	3778
	21st to end	Day	2 7% 3 9% 4 4 5% 5% 5% 9% 47%	4510
February	1st to 10th	Day	1 5% 2 6% 3 4 5% 5% 7% 9% 53%	1805
	11th to 20th	Day	1 4 3 6% 5% 5% 5% 4 7% 9% 51%	1950
	21st to end	Day	1 4 2 6% 5% 4 4 4 7% 10% 52%	1650
March	1st to 10th	Day	1 7% 4 9% 5% 5% 5% 4 7% 9% 43%	4518
	11th to 20th	Day	4 2 5% 3 3 4 4 6% 10% 58%	4946
	21st to end	Day	1 8% 2 6% 3 4 5% 4 7% 8% 52%	5829
April	1st to 10th	Day	3 4 2 6% 5% 4 4 4 6% 10% 53%	2400
	11th to 20th	Day	2 4 2 5% 4 4 5% 4 6% 9% 56%	2369
	21st to end	Day	2 6% 2 3 3 3 4 4 5% 9% 60%	2400
May	1st to 10th	Day	2 5% 1 3 2 2 2 2 4 6% 69%	5676
	11th to 20th	Day	1 2 1 2 2 2 3 3 4 6% 73%	6161
	21st to end	Day	2 3 1 3 2 2 3 2 4 6% 70%	6902
June	1st to 10th	Day	2 3 1 4 3 3 4 4 6% 7% 64%	2800
	11th to 20th	Day	1 2 1 2 2 2 2 2 4 5% 78%	2800
	21st to end	Day	2 2 1 2 2 2 2 2 4 5% 76%	2800
July	1st to 10th	Day	1 1 1 1 2 1 2 2 89%	4491
	11th to 20th	Day	2 3 1 4 2 3 3 3 4 5% 71%	4420
	21st to end	Day	1 1 1 2 2 2 3 3 3 4 79%	4650
August	1st to 10th	Day	2 2 1 3 2 2 2 2 3 4 76%	4038
	11th to 20th	Day	1 1 1 1 1 2 1 2 4 86%	3724
	21st to end	Day	1 1 1 1 1 2 1 2 2 87%	4180
September	1st to 10th	Day	1 1 2 1 3 2 3 4 81%	2280
	11th to 20th	Day	2 1 1 3 3 2 3 2 4 6% 74%	2252
	21st to end	Day	2 2 1 3 2 2 2 2 3 4 78%	2090
October	1st to 10th	Day	2 3 1 3 2 3 3 2 5% 6% 70%	3221
	11th to 20th	Day	1 3 1 3 1 1 2 2 3 4 78%	3161
	21st to end	Day	1 1 1 3 3 3 4 3 4 8% 69%	3373
November	1st to 10th	Day	1 3 2 4 4 4 4 4 6% 8% 59%	1764
	11th to 20th	Day	2 3 1 3 3 3 4 3 7% 10% 62%	1800
	21st to end	Day	1 3 1 4 3 4 5% 4 8% 10% 57%	1745
December	1st to 10th	Day	1 3 1 2 3 3 3 4 6% 9% 64%	2620
	11th to 20th	Day	1 5% 3 6% 6% 6% 5% 7% 9% 11% 42%	3028
	21st to end	Day	1 5% 3 8% 4 5% 6% 5% 8% 13% 42%	4004
January	1st to 10th	Night	1 3 2 5% 3 4 4 4 7% 12% 54%	6306
	11th to 20th	Night	3 2 5% 4 3 4 4 6% 7% 61%	6171
	21st to end	Night	1 7% 3 7% 4 4 5% 4 7% 8% 51%	6468
February	1st to 10th	Night	1 4 2 7% 5% 4 4 4 8% 9% 52%	2593
	11th to 20th	Night	4 2 5% 4 4 5% 6% 9% 10% 51%	2338
	21st to end	Night	4 1 6% 4 3 4 4 8% 10% 53%	1980
March	1st to 10th	Night	1 7% 5% 8% 4 4 4 5% 7% 10% 45%	5376
	11th to 20th	Night	5% 2 8% 4 4 4 4 7% 9% 55%	5050
	21st to end	Night	1 6% 3 7% 5% 5% 5% 5% 8% 11% 46%	5144
April	1st to 10th	Night	2 5% 1 5% 4 5% 4 6% 7% 9% 52%	2000
	11th to 20th	Night	1 4 2 4 4 3 3 3 7% 9% 59%	1979
	21st to end	Night	3 5% 1 3 3 2 3 3 6% 8% 63%	2000
May	1st to 10th	Night	2 3 1 3 2 2 2 2 3 5% 74%	4312
	11th to 20th	Night	1 3 1 3 2 2 2 2 4 6% 73%	3720
	21st to end	Night	2 3 1 2 2 2 2 2 4 6% 75%	3972
June	1st to 10th	Night	3 3 1 2 3 3 4 3 5% 6% 67%	1600
	11th to 20th	Night	1 1 1 1 2 2 2 3 5% 82%	1600
	21st to end	Night	2 2 1 1 2 2 2 2 3 4 80%	1600
July	1st to 10th	Night	1 1 1 1 1 2 2 90%	2580
	11th to 20th	Night	2 4 2 4 3 2 2 2 3 3 74%	2540
	21st to end	Night	1 2 1 2 2 1 2 2 3 4 81%	2903
August	1st to 10th	Night	3 2 1 2 2 2 3 2 4 5% 75%	2901
	11th to 20th	Night	1 1 1 1 1 1 1 1 2 3 87%	3126
	21st to end	Night	1 1 1 1 1 1 1 2 2 88%	3498
September	1st to 10th	Night	1 2 1 1 1 1 2 4 87%	1900
	11th to 20th	Night	2 2 1 2 1 2 2 2 3 5% 80%	1928
	21st to end	Night	2 2 1 3 2 2 2 2 4 4 77%	2090
October	1st to 10th	Night	2 2 1 2 2 1 2 2 4 4 78%	3755
	11th to 20th	Night	1 2 1 2 2 1 2 2 3 4 80%	3777
	21st to end	Night	2 1 4 3 2 4 3 5% 8% 68%	4181
November	1st to 10th	Night	1 3 2 4 2 3 4 4 8% 10% 61%	2535
	11th to 20th	Night	1 3 1 2 2 2 4 4 8% 11% 62%	2600
	21st to end	Night	2 3 2 4 3 2 4 4 6% 9% 62%	2650
December	1st to 10th	Night	1 2 1 3 3 3 4 4 6% 8% 65%	4560
	11th to 20th	Night	2 4 2 5% 4 4 6% 6% 9% 14% 44%	5176
	21st to end	Night	1 5% 3 7% 6% 5% 6% 6% 9% 11% 41%	6952

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	20% 5% 22% 27% 24% 2	3639
	11th to 20th	Day	21% 3 12% 22% 31% 11%	3778
	21st to end	Day	33% 5% 10% 17% 24% 10%	4510
February	1st to 10th	Day	27% 5% 13% 26% 21% 8%	1805
	11th to 20th	Day	30% 2 12% 27% 24% 6%	1960
	21st to end	Day	27% 4 15% 29% 19% 7%	1650
March	1st to 10th	Day	36% 4 11% 23% 22% 5%	4518
	11th to 20th	Day	21% 4 14% 36% 23% 3	4946
	21st to end	Day	27% 4 16% 28% 22% 2	5829
April	1st to 10th	Day	28% 4 17% 26% 21% 5%	2400
	11th to 20th	Day	25% 3 13% 30% 23% 7%	2372
	21st to end	Day	22% 3 15% 31% 25% 4	2400
May	1st to 10th	Day	18% 2 6% 31% 37% 6%	5676
	11th to 20th	Day	13% 2 6% 25% 43% 9%	6164
	21st to end	Day	16% 2 5% 23% 45% 8%	6904
June	1st to 10th	Day	20% 3 10% 25% 32% 10%	2800
	11th to 20th	Day	11% 2 5% 22% 37% 23%	2800
	21st to end	Day	12% 2 2 13% 35% 35%	2800
July	1st to 10th	Day	6% 1 1 5% 16% 72%	4422
	11th to 20th	Day	16% 3 2 8% 17% 53%	4345
	21st to end	Day	10% 3 2 11% 26% 49%	4503
August	1st to 10th	Day	13% 2 2 8% 28% 47%	4023
	11th to 20th	Day	7% 1 1 5% 22% 64%	3760
	21st to end	Day	7% 1 1 11% 39% 41%	4179
September	1st to 10th	Day	9% 2 2 9% 35% 43%	2280
	11th to 20th	Day	14% 2 4 19% 39% 22%	2252
	21st to end	Day	14% 1 2 19% 39% 25%	2090
October	1st to 10th	Day	15% 2 4 12% 37% 29%	3221
	11th to 20th	Day	12% 1 3 20% 50% 13%	3161
	21st to end	Day	15% 2 7% 27% 36% 13%	3373
November	1st to 10th	Day	22% 4 10% 26% 29% 10%	1764
	11th to 20th	Day	18% 3 16% 24% 31% 8%	1800
	21st to end	Day	21% 3 12% 27% 27% 10%	1745
December	1st to 10th	Day	18% 3 12% 25% 32% 10%	2620
	11th to 20th	Day	33% 7% 16% 19% 19% 6%	3028
	21st to end	Day	29% 7% 20% 24% 17% 3	4004
January	1st to 10th	Night	21% 5% 21% 28% 23% 2	6306
	11th to 20th	Night	22% 3 14% 23% 29% 9%	6171
	21st to end	Night	30% 4 13% 18% 26% 9%	6468
February	1st to 10th	Night	27% 5% 12% 26% 20% 10%	2593
	11th to 20th	Night	25% 3 14% 30% 23% 5%	2339
	21st to end	Night	24% 5% 13% 31% 19% 7%	1980
March	1st to 10th	Night	32% 4 12% 22% 24% 6%	5376
	11th to 20th	Night	25% 4 14% 40% 15% 2	5050
	21st to end	Night	29% 5% 21% 28% 13% 4	5144
April	1st to 10th	Night	25% 4 19% 28% 19% 5%	2000
	11th to 20th	Night	21% 4 16% 31% 21% 7%	1979
	21st to end	Night	20% 4 16% 35% 22% 4	2000
May	1st to 10th	Night	15% 3 7% 34% 37% 5%	4309
	11th to 20th	Night	14% 2 8% 28% 39% 9%	3720
	21st to end	Night	12% 2 8% 25% 45% 8%	3972
June	1st to 10th	Night	18% 4 10% 27% 32% 9%	1600
	11th to 20th	Night	9% 2 5% 25% 36% 24%	1600
	21st to end	Night	10% 2 3 15% 37% 34%	1600
July	1st to 10th	Night	4 1 1 5% 18% 71%	2540
	11th to 20th	Night	16% 3 2 8% 15% 56%	2492
	21st to end	Night	9% 2 2 8% 25% 54%	2812
August	1st to 10th	Night	14% 2 2 9% 29% 45%	2892
	11th to 20th	Night	7% 1 1 7% 21% 63%	3156
	21st to end	Night	7% 1 1 11% 39% 42%	3498
September	1st to 10th	Night	6% 1 1 9% 38% 44%	1900
	11th to 20th	Night	10% 2 5% 22% 38% 22%	1928
	21st to end	Night	13% 2 3 21% 37% 25%	2090
October	1st to 10th	Night	12% 1 4 13% 38% 32%	3755
	11th to 20th	Night	9% 2 4 19% 49% 16%	3777
	21st to end	Night	15% 3 7% 24% 39% 13%	4181
November	1st to 10th	Night	18% 5% 10% 28% 28% 11%	2535
	11th to 20th	Night	15% 6% 12% 30% 30% 8%	2600
	21st to end	Night	20% 4 11% 28% 27% 10%	2650
December	1st to 10th	Night	18% 4 12% 25% 29% 13%	4560
	11th to 20th	Night	27% 6% 18% 23% 20% 5%	5176
	21st to end	Night	32% 6% 18% 22% 18% 5%	6952

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	32% 20% 23% 16% 8% 2	200
	11th to 20th	Day	28% 14% 28% 20% 8% 3	260
	21st to end	Day	18% 19% 28% 20% 11% 4	440
February	1st to 10th	Day	27% 20% 27% 17% 7% 2	403
	11th to 20th	Day	26% 20% 28% 19% 6% 1	495
	21st to end	Day	24% 19% 33% 19% 6%	414
March	1st to 10th	Day	32% 22% 26% 14% 5% 1	500
	11th to 20th	Day	27% 24% 24% 15% 9% 1	500
	21st to end	Day	28% 24% 25% 19% 4	550
April	1st to 10th	Day	22% 22% 29% 18% 8% 1	500
	11th to 20th	Day	22% 26% 29% 15% 6% 2	492
	21st to end	Day	18% 20% 32% 19% 8% 4	500
May	1st to 10th	Day	19% 23% 30% 18% 6% 3	503
	11th to 20th	Day	19% 17% 32% 19% 8% 4	528
	21st to end	Day	15% 14% 29% 24% 12% 5%	573
June	1st to 10th	Day	10% 18% 32% 26% 10% 5%	530
	11th to 20th	Day	14% 19% 35% 20% 8% 4	530
	21st to end	Day	7% 14% 39% 24% 9% 6%	530
July	1st to 10th	Day	2 13% 43% 25% 13% 4	507
	11th to 20th	Day	3 11% 36% 30% 16% 4	484
	21st to end	Day	4 10% 35% 33% 13% 5%	531
August	1st to 10th	Day	7% 14% 30% 31% 14% 4	487
	11th to 20th	Day	5% 14% 36% 29% 13% 4	477
	21st to end	Day	4 10% 38% 30% 13% 4	539
September	1st to 10th	Day	3 13% 39% 27% 16% 2	490
	11th to 20th	Day	9% 16% 33% 29% 10% 3	490
	21st to end	Day	8% 14% 36% 30% 10% 2	490
October	1st to 10th	Day	10% 19% 27% 28% 16% 2	490
	11th to 20th	Day	13% 13% 25% 32% 14% 3	490
	21st to end	Day	15% 15% 27% 25% 14% 4	500
November	1st to 10th	Day	20% 14% 23% 20% 19% 4	300
	11th to 20th	Day	18% 12% 28% 24% 12% 6%	300
	21st to end	Day	19% 14% 25% 24% 13% 4	273
December	1st to 10th	Day	21% 15% 25% 16% 15% 9%	200
	11th to 20th	Day	25% 16% 28% 17% 10% 5%	200
	21st to end	Day	31% 8% 27% 20% 12% 2	220
January	1st to 10th	Night	16% 11% 28% 22% 17% 8%	800
	11th to 20th	Night	13% 10% 27% 32% 13% 6%	800
	21st to end	Night	11% 9% 30% 28% 16% 6%	880
February	1st to 10th	Night	16% 9% 28% 28% 16% 4	800
	11th to 20th	Night	14% 13% 27% 25% 17% 5%	784
	21st to end	Night	17% 13% 24% 30% 12% 3	660
March	1st to 10th	Night	16% 18% 27% 24% 12% 3	800
	11th to 20th	Night	14% 15% 25% 23% 19% 4	800
	21st to end	Night	16% 16% 28% 26% 13% 2	880
April	1st to 10th	Night	11% 12% 29% 29% 16% 2	800
	11th to 20th	Night	13% 15% 25% 27% 15% 4	791
	21st to end	Night	11% 12% 26% 29% 18% 5%	800
May	1st to 10th	Night	10% 14% 25% 29% 17% 4	695
	11th to 20th	Night	8% 8% 25% 36% 19% 3	446
	21st to end	Night	9% 6% 24% 33% 26% 2	433
June	1st to 10th	Night	7% 5% 26% 37% 22% 3	400
	11th to 20th	Night	4 9% 27% 33% 27% 2	400
	21st to end	Night	4 5% 27% 40% 22% 3	400
July	1st to 10th	Night	2 3 22% 46% 24% 3	380
	11th to 20th	Night	2 2 27% 41% 26% 1	368
	21st to end	Night	4 2 20% 36% 31% 6%	496
August	1st to 10th	Night	4 3 20% 41% 28% 4	594
	11th to 20th	Night	3 4 23% 39% 26% 5%	752
	21st to end	Night	3 4 22% 40% 27% 4	836
September	1st to 10th	Night	2 5% 22% 36% 30% 6%	760
	11th to 20th	Night	5% 7% 20% 32% 29% 8%	760
	21st to end	Night	5% 6% 21% 30% 29% 8%	760
October	1st to 10th	Night	8% 6% 20% 36% 22% 7%	760
	11th to 20th	Night	10% 5% 16% 39% 25% 6%	760
	21st to end	Night	8% 9% 20% 29% 21% 12%	836
November	1st to 10th	Night	12% 10% 19% 29% 22% 8%	780
	11th to 20th	Night	11% 10% 24% 33% 16% 6%	800
	21st to end	Night	14% 11% 20% 31% 19% 5%	800
December	1st to 10th	Night	14% 9% 23% 34% 15% 5%	800
	11th to 20th	Night	15% 12% 24% 31% 12% 6%	800
	21st to end	Night	16% 14% 29% 26% 13% 3	880

Page generated at 2025-07-02 22:30:14. Version ac07

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