How Snow Forms

When shoveling 10” of snow from the driveway before setting out for the office, most of us take very little time to wonder at the beauty and complexity of snowflakes. But if you get the chance to examine some of them, you will see incredibly intricate designs. How they come into being has been the study of several scientists, beginning with Ukichiro Nakaya.

A nuclear physicist, Nakaya never intended to study snow. But in 1932, he was assigned a professorship in Hokkaido, the Northern Island of Japan. There were no facilities available for nuclear research, but there was a lot of snow. He began to study and classify snowflakes, in nature and in the laboratory. His work is the basis for our understanding of the morphology of snowflakes.

Snow is formed when super saturated air at cold temperatures comes into contact with particles on which crystals can form. This is the simple explanation. But the more you know about cloud science, the more complicated this subject becomes. It is a complex and dynamic specialty and there is no way to describe the process that is both simple and precise.

Snow is one of four types of solid precipitation. The others are sleet, hail, and graupel. Each is formed in a different way. The problem with snow is that the crystals can take one trillion, trillion, trillion different forms, depending on the amount of available moisture, temperature and air currents. These factors interact in a myriad of different patterns to produce the ice crystals we know as snow.

We first need to define the term “super saturated.” When air is full of water vapor to the point that it cannot hold more if the vapor comes into contact with a solid surface, the air is said to be “saturated,” or at 100% humidity. Super saturated air is saturated air that has cooled but not dropped any precipitation. When the droplets in super saturated air come into contact with solid particles such as dust or bits of bacteria, they cling to the solid surface.

If the temperature is at or below freezing, ice crystals will form. These take a characteristic hexagonal shape due to the shape of the molecules. Air currents within the cloud move these crystals constantly up and down. They come into contact with super-cooled water droplets (still liquid but below the temperature at which ice forms), which accumulate on the 6 sides of the ice crystal.

In temperatures between -5 and 0 degrees Centigrade, snowflakes will form from these ice crystals. Most snowflakes are branched and may have a light, open structure.  Small flakes up to 4 or 5 mm in diameter often have a star shape and are truly beautiful. Larger flakes are usually composed of several small flakes and may not readily show the structure.

The temperature within a cloud varies, so the developing snowflakes are constantly swirled around on moving currents of air. They collide with other flakes and, if the temperature is between -4 and 0 degrees Celsius, they carry a thin film of super-cooled water. They easily freeze to other flakes.

The snowflakes also collide with tiny droplets of super cooled water, which cling to them in a process known as riming. This appears as tiny bumps on the surface of the snowflake. If the flake is heavily covered with rime, it becomes graupel, which resembles a very tiny snowball.

When the snowflakes become too heavy to resist the pull of gravity, they fall to earth. Colder than -4 degrees Celsius, individual flakes will fall.

Knowing how complex the process of producing snow is may not make you like clearing it away any better. But sometimes you might like watching the white stuff fall.

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