When people say snowflake, they often mean snow crystal. The latter is a single crystal of ice, within which the water molecules are all lined up in a precise hexagonal array. Snow crystals display that characteristic six-fold symmetry we are all familiar with. The picture below shows a snow crystal.
A snowflake, on the other hand, is a more general term. It can mean an individual snow crystal, but it can also mean just about anything that falls from the winter clouds. Often hundreds or even thousands of snow crystals collide and stick together in mid-air as they fall, forming flimsy puff-balls we call snowflakes. Calling a snow crystal a snowflake is fine, like calling a tulip a flower.
The life of a snowflake begins high in Earth’s atmosphere and if the snowflake is very lucky it might reach the ground.
A Tiny Particle High in Earth’s Atmosphere
A snowflake begins when a tiny dust or pollen particle encounters water vapor high in Earth’s atmosphere. The water vapor coats the tiny particle and freezes into a tiny crystal of ice. This tiny crystal will be the “seed” from which a snowflake will grow.
Hexagonal “Mineral” Crystals
The molecules of water that form each tiny ice crystal naturally arrange themselves into a hexagonal (six sided) structure. The result will be a snowflake with six sides or six arms. Ice crystals are “minerals” because they are naturally occurring solids with a definite chemical composition and an ordered internal structure.
The Snowflake Grows as it Falls
The newly-formed ice crystal (snowflake) is heavier than the surrounding air and it begins falling. As it falls towards Earth through humid air more water vapor freezes onto the surface of the tiny crystal. This freezing process is very systematic. The water molecules of the vapor arrange themselves so that the hexagonal crystal structure of ice is repeated. The snowflake grows larger and larger as it falls, enlarging the hexagonal pattern.
Every Snowflake is Different!
Although all snowflakes have a hexagonal shape other details of their geometry can vary. These variations are produced by different temperature and humidity conditions through which the snowflake falls. Some temperature/humidity combinations produce flakes with long needle-like arms. Other conditions produce flakes with wide flat arms. Other conditions produce thin, branching arms.
These different shapes have an unlimited number of variations, each representing the conditions of temperature and humidity and water vapor the snowflake encountered during its fall.
Notice the wide variety of shapes.
Will They Reach the Ground as Snow?
The formation of snowflakes high in Earth’s atmosphere does not guarantee snowfall on Earth’s surface. That will only happen if air temperatures are below freezing all the way to the ground as shown in the cartoon at right.