LAKE TAHOE, Calif./Nev. – The blanket of snow that covers Tahoe in the winter months is both a major draw for snowboarders and skiers, and a picturesque seasonal scene for the otherwise fairly sunny California and Nevada. And on the microscopic scale, snowflakes can tell their own story of how they formed and the storm patterns of the Sierra Nevada.
Winter weather
Weather patterns in the Sierra Nevada are reliant on the atmospheric conditions known as the North Pacific High and the Aleutian Gulf of Alaska Low. While the North Pacific High helps make the fair weather of spring and summer, the Aleutian Gulf of Alaska Low causes air streams to converge and collide, which starts to create cloudy conditions and precipitation.
The mountains themselves are also responsible for these weather patterns. As Pacific Ocean air travels up the mountain, the altitude gain cools it down and raises the relative humidity. This process, known as orographic lift, is also responsible for forming clouds. And once the temperatures get cold enough, then it’s a snowstorm.
So, humidity and temperature are the driving factors of winter weather. And on the small scale, it’s also what changes the formation and shapes of snowflakes.
Flurries of flakes
As the saying goes, every snowflake is special—which is true. No two snowflakes are exactly alike, but many of them form specific kinds of shapes based on humidity, temperature and distance traveled to the ground.
Every snowflake starts as a speck, either of pollen or dust, that a water droplet freezes onto. As the crystal tumbles through the sky, water vapor will freeze into different shapes and build more arms of the snowflake. The arrangement of water molecules in an ice crystal lattice is hexagonal, which is why we see snowflakes with six arms.
The simplest form of a snow crystal is a flat hexagonal plate. As humidity increases in the cloud, the speed of growth and complexity of the crystals increase too, as there’s more water to latch onto the original crystal.
As temperatures drop to 20 degrees Fahrenheit, needles, prisms and hollow columns start to form. The most photogenic snowflakes form between 10 to -10 degrees Fahrenheit, where you see large dendrites (branch and tree-like shapes) at high humidity, stellar plates, thin plates and solid plates. Then, as the temperature drops below -15 degrees Fahrenheit, plates and columns become the norm.
But all these conditions are just in the cloud. As a snowflake drops from the sky, it experiences all sorts of differing temperatures and humidities, as well as colliding with the trillions of other snow crystals that make up a storm. This is what makes every snowflake unique.
The farther snowflakes fall, the more time they have to form different shapes—but the more likely those shapes are going to be broken off. These irregular snowflakes are some of the most common.
Snowflakes can also become covered in rime, which are small droplets that freeze onto the crystal and cause a dimpled appearance. The rime makes these snowflakes look extra frosty rather than clear like glass. When a snowflake is covered in rime, that’s what called graupel, or soft hail.
Rime forms because of collisions with water droplets in the air. These interactions can happen in supercooled clouds, which contain water that does not freeze at below freezing temperatures. The formation of clouds through orographic lift often produces supercooled clouds, and places with relatively warm winter weather like Tahoe tend to have more rime on snowflakes.
Artificial snow
Resorts often use artificial snow blowers to help produce snow, its formation is very different than Mother Nature. Snow machines use compressed air and water, which are then shot out of a nozzle as fine droplets. As the air decompresses, it cools, which causes the droplets to freeze. Then, a fan blows these particles onto the slopes.
It’s pretty easy to tell artificial snow from real snow. Under a microscope, they look like frozen water droplets, like the kind you might see in your freezer. Since they’re not interacting with humidity, temperature and distance, they don’t create the intricate structures you see in falling snow.
Sorting snowflakes
Snowflakes can be sorted with all kinds of classification systems. The most widely used is the International Classification System, which describe seven snow crystal types: plates, stellar crystals, columns, needles, spatial dendrites, capped columns and irregular forms.
Physicist Ukichiro Nakaya created his own snowflakes in a lab, entering the data into the Nakaya diagram, which relates snow crystal shapes to temperature and humidity. He also created a classification with 41 different shapes of falling snow, and he famously declared that “each snowflake is a letter sent from heaven.”
Choji Magono and Chung Woo Lee expanded on Nakaya’s work, which was then expanded again by Katushiro Kikuchi and a further group working on a global classification. Because Nakaya, Magono and Lee all studied snow in Japan, scientists who have expanded the classification have made observations in polar regions, where special particles form due to the extremely low temperature.
Now, 121 classifications have been made for precipitation, ranging from descriptions like “stellar crystal with needles” to “bullet with dendrites” to graupel, hail and snow pellets.
You too can also observe the wonder of snowflakes in Tahoe with a simple magnifying glass. The best times for viewing snowflakes are typically the coldest: either in the early morning or later at night during a snowfall. Snow can be collected onto glass plates and even rotated with a fine paintbrush, or you can simply observe them on a dark surface, like a coat or black sheet of paper. Any way you look at them, you’ll now understand the science of snowflakes.