Welcome to You Ask Andy

Douglas Irwin, age 11, of Mulvane, Kansas, for his question:

Why are snowflakes six‑sided?

Snowflakes are made from air and water. Arid water is a mineral, just as quartz is a mineral. We tend to forget this because water is a liquid at normal temperatures. A small fraction of the world's water is in the form of vapor and about three percent in the form of solid glacial ice. Water is liquid because the particles of which it is made have just so much energy and no more. With less energy they gel into solid ice. With more energy they fly off into the air as gassy vapor.

The particles of which ice, water and water vapor are made are all alike. Each is composed of two atoms of oxygen and four atoms of hydrogen. The hydrogen atom is the smallest of all atoms. The oxygen atom is eight times as large as the hydrogen atom. A particle of water, then, is composed of two sizeable atoms and four smaller atoms. Water is built from countless numbers of these tiny structures. And it is the structure of these tiny particles which causes water vapor to build six‑sided snowflakes.

 A snowflake is the crystallization of water vapor. Quartz and other minerals also form crystals, tend the crystal of each mineral depends upon the tiny particles of which it is made. Quartz, like water vapor, also forms six‑sided, or hexagonal, crystals. The mineral galena forms cube­ shaped crystals and the mineral calcite forms perfect rhombs, or sloping cubes. The crystal form of each mineral depends upon the structure of its particles.

A deck of cards is composed of 52 smooth faced rectangles. If you push the cards from one side the smooth surfaces slide parallel with one another. On a much smaller scale, a particle of vapor also tends to slide in one direction. It tends to slide along a plane parallel with the base of a hexagon.

A snowflake is composed of trillions of these small particles, each with a tendency to slide into a hexagonal position. The size of these mites is too small for our minds to grasp. Atoms are measured in angstroms and an angstrom is about four billionths of an inch. The smallest atom is less than on angstrom and the largest is about four angstroms. And each water vapor particle is composed of two medium sized oxygen atoms and four hydrogen atoms, smallest of all atoms.

Snow is made in air below freezing point. At such temperatures, the vapor particles may gel into solid ice without passing through the watery liquid stage. As snow forms, myriad vapor particles become tiny fragments of ice.

We do not know for certain, but the experts suspect that a snowflake must have a little core around which to build. The nucleus may be a bit of dust, salt or soot. The nucleus gathers icy fragments as it is tossed hither and thither in the turbulent air. Trillions of particles slide inter position and the hexagonal crystal begins to grow. The number of possible different designs is endless.

Countless snowflakes have been photographed but no two have ever been found exactly alike. They are among the most beautiful and the most fragile of all the crystals in the world. If you catch a falling snowflake you have but a minute to study it under a magnifying glass. Then it melts. Large, fluffy snowflakes are bundles of many individual six‑sided snowflakes.

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