Snowflakes arrived on Owl Acres to welcome winter on the Saturday after Thanksgiving. They piled up to about three inches of fluffy white snow. Each one of those trillions of snowflakes was a unique work of art. In spite of the fact that they all formed in the same cloud, at the same time and took similar journeys to the ground, no two were identical. Never mind that they all formed under the same laws of physics. Each one was unique. How does that happen? It goes something like this.
A mass of warm, moist air collides with a mass of colder air to form a weather front.
The warmer air (and warmer is a relative term here) rises about a mile into the sky.
It’s colder up there, and dust, pollen and other tiny bits are circulating around collecting water to form water droplets. A cloud made up of these water droplets forms, holding as much as a million tons of water droplets suspended in the cloud. As the temperature drops, water droplets begin to freeze into ice crystals. These crystals take on specific shapes depending on just how cold and humid it is up there in the location in the cloud where the crystal froze. Some of the crystals are flat plates with six arms. If it’s a bit colder, the crystals may take on a six-sided columnar shape or form into very thin needles. Either way, the number six is critical. Every ice crystal has six facets or six arms. It’s six because of the way the atoms in the water molecules are arranged and the way the water molecules fit together in what is known as a crystal lattice. In the case of ice crystals, the positive and negative charges on the water molecules cause them to fit together in hexagonal rings. These initial crystals are very tiny, measured in nanometers.
The crystals begin to grow into more complex shapes. They grow as water vapor in the cloud touches the crystal and freezes. The added growth will follow the six-sided scheme but will attach to the growing snowflake in various ways depending on what’s happening around it. Turbulence in the cloud will toss our snowflake around, knocking off some bits, adding others and sometimes causing it to collide with and stick to another crystal. Temperature changes will cause crystals to grow in different shapes. Humidity differences will affect where and how much new crystal material will freeze onto the snowflake. If our snowflake is a star with six arms, Water vapor in the cloud freezes onto the arms, creating fern-like additions and making the snowflake bigger and heavier as it grows. If our snowflake is made up of the columnar crystals instead of the stars, its shape will cluster new columnar crystals, sometimes forming a rosette of little faceted columns. But it might also add a disk at the top of a column that will grow outward as the snowflake travels through the cloud. That added disk may sprout arms which in turn may add fern-like features along their edges. Aside from the six-sided requirement, our snowflake can grow in any direction It all depends on the individual journey of that particular snowflake. The number of water molecules in a single snowflake is estimated to be 1 with 18 zeros after it. With that many building blocks to play with, the variety would seem to be endless.
Eventually our snowflake gets too heavy to stay in the cloud, so it tumbles or floats down toward the ground along with trillions of its sisters. No two snowflakes take the exact same journey to the ground, either. Pockets of warmer or colder air in the atmosphere, wind and humidity all continue to impact each snowflake as it falls to the ground. And that is why it is true that no two snowflakes are exactly alike.
When our snowflake hits the ground, it becomes part of the clean white landscape. If the sun hits its facets just right, it might twinkle in the morning sun. Welcome to winter.
Photo from Wikimedia.org by: Adrian Tync. Alt text: Snowflake with six strong, hexagon-based arms. It’s suspended in mid-air, caught on the gossamer strand of a barely visible spider web.