Lake Tahoe facts, myths & legends, Part II: Does Tahoe really enjoy 300 days of sunshine?

Graph compiled by Mark McLaughlin

The Tahoe Sierra is awe-inspiring, renowned for its natural beauty, breathtaking landscapes and dynamic climate features. Its ski resorts, blessed with frequent Pacific storms, are among the snowiest locations in the United States. In the summer, when everyone wants to recreate outside, it is one of the sunniest and driest places. It’s an unbeatable combination that elevates Lake Tahoe to the worldclass, year-round playground that promoters dream about.

You have probably seen the statement in a brochure or magazine: Lake Tahoe enjoys 300 days of sunshine a year. It has become a common selling point for the area’s chambers of commerce, realtors and agents in group sales. But is it true?

According to research by hydrologist Randall Osterhuber, retired lead scientist for U.C. Berkeley’s Central Sierra Snow Lab near Donner Pass, the lab annually has 105 days of measurable precipitation, meaning rain or snow can be expected there nearly 30 percent of the 365 calendar days in a year. That leaves 260 possible days to be considered sunny (not partly cloudy, etc.). However, when it’s storming on Donner Pass, the region is generally not experiencing a typical sunny day. And that does not include cloudy or overcast weather preceding or following the storm itself. Considering the data, getting to 300 days of clear skies seems like a heavy lift.

When talking about weather and climate facts of Tahoe, it all comes down to which of the many microclimates you’re considering. The basin’s weather is dramatically different depending on location, even within short distances. Areas on the eastern perimeter of Big Blue such as Incline Village, Sand Harbor, Glenbrook or Zephyr Cove on the Nevada side annually receive about 26 inches of precipitation (rain plus the water content of snow). That’s half of what West Shore weather stations measure at Homewood, Tahoma and D.L. Bliss State Park. (There is significantly more precipitation in the higher elevations at all locations.) Under a modified Mediterranean climate, wet weather dominates the winter months while sunshine prevails through the summer. Regionwide, only 5 percent of yearly precipitation falls from June through September.

What about our much drier neighbors, Reno and Sacramento? According to the National Oceanic Atmospheric Administration (NOAA), Reno receives less than 8 inches of precipitation a year in about 50 rain or snow days. That’s 50 percent less than Donner Pass. Storm-related overcast conditions reduce that to about 290 mostly, cloud-free skies in Reno. The Biggest Little City basks in sunshine 80 percent of the time and is ranked the eighth sunniest city in the United States.

Sacramento is considered the 10th sunniest city in the country, with 265 clear sky days. That ranking would be higher if it wasn’t for extended tulefog inversions in the Central Valley during winter months. Both these cities receive just a fraction of the Sierra’s annual precipitation yet neither experience 300 days of sun.

 

Calculating sunny days

Turns out that based on NOAA’s methodology, it’s easy to get an official sunny day. Locations are evaluated based on the fraction of sunshine that reaches the Earth’s surface expressed as the percentage of the maximum amount possible from sunrise to sunset. Broken into three stages, “clear” denotes up to 3/10 percent average sky cover for daylight hours, followed by partly cloudy at 4/10 percent to 7/10 percent and cloudy at 8/10 percent to 10/10 percent. Therefore, a day with up to 33 percent cloud cover is designated as clear.

But in the Tahoe Sierra don’t be surprised if you experience rain or snow on a sunny dayespecially after a winter storm when, despite a perfectly clear morning, secondary waves of unstable moisture surge in behind the cold front or when summer thunderstorms quickly blow up in the afternoon.

 

Why doesn’t Lake Tahoe freeze?

The surface of Lake Tahoe has never been known to freeze, but why is that? The typical reasons given are depth, volume and wind. When the top layer gets cold enough to freeze its density increases so it sinks and is replaced by warmer water from below. True enough.

The volume of water in the basin is nearly 40 trillion gallons and although much of it is in the sub-40-degree Fahrenheit range, it still generates heat during the cold winter months when air temperatures contacting the surface fall below freezing for extended periods of timealbeitless than they used to. In winter, frequent storms followed by rebuilding atmospheric high pressure generate wind that agitates the lake’s surface, inhibiting ice formation.

Limnology is an integrative science where physics, chemistry and biology interact, specifically in inland bodies of water like lakes, streams and rivers. In this discipline, Lake Tahoe has a sister lake, Baikal, in Siberia. While there is no doubt that Tahoe has many profound attributes, Lake Baikal truly exceeds Big Blue in every category, except perhaps scenery and even that is debatable.

Lake Baikal was formed in a continental rift zone, where the Earth’s crust is pulling apart, making it an extraordinarily deep basin at 5,387 feet. Lake Tahoe has a maximum depth of 1,625 feet, making it less than a third as deep.

Not only is Baikal the world’s deepest lake, it’s also the oldest and boasts the cleanest water. Its colossal basin contains 23 percent of the planet’s unfrozen freshwater and is incredibly biologically diverse. It’s home to at least 1,700 plant species and 2,500 varieties of animals and fish80 percent are endemic and found nowhere else.

I have only scratched the surface of Lake Baikal’s geographic and ecological accolades, but let’s get back to our topic. Despite its massive size and incredible depth, Lake Baikal is exposed to average winterair temperatures of minus 6 degrees Fahrenheit, which in January turns its surface to ice 3 to 5 feet thick. It doesn’t thaw until May or June, meaning that if Lake Tahoe was exposed to a severe Siberian winter, it, too, would freeze and become one of the world’s most amazing ice-skating rinks.

The primary reason that Big Blue doesnt ice over is because it’s a mere 200 miles atmospherically downstream from the Pacific Ocean. As the largest ocean in the world, it has a profound modifying influence on our climate that includes relatively mild winters accompanied by heavy snow a skier’s delight.