A snowflake forms inside a cloud when a droplet of water freezes around a tiny particle of dust, salt, bacteria, smog, or some other substance. Most snow forms in supercooled water-droplet clouds of the middle and upper atmosphere, such as nimbostratus, cumulus congestus, or cumulonimbus. (Supercooled water can exist at subfreezing temperatures without freezing, but will freeze when it comes into contact with a particle.) In a supercooled atmosphere within a cloud, liquid water droplets and free ice crystals cannot coexist for long periods of time. The ice crystals rob the liquid droplets of their moisture and thereby grow continuously and rapidly. Some of these sizable ice crystals stick to each other to create a collection of ice crystals known as a snowflake.
The shape that an ice crystal takes depends on the temperature and moisture content in the cloud. The colder the air, the less water it contains. Warm air contains more moisture than cold air, allowing larger crystals to grow. Crystals that grow from the meager water supply at -20 degrees F or below form pencil-shaped hexagonal (six-sided) columns. At temperatures from -10 to 0 F, most crystals are flat, hexagonal plates. At temperatures of 0 to 20 degrees, crystals become large, delicate, six-pointed shapes called dendrites, a word derived from the Greek word for "branched." Warmer cloud-level temperatures (20-32 degrees) yield splinter-shaped crystals called needles.
Most ice crystals form as six-sided plates, as they replicate the shape of water molecules, but all snowflakes don't retain this shape. During their "lifetime" they may crash into other ice crystals and break; partially melt and refreeze; or they may be misshapen by dirt or other impurities. By the time the millions of snowflakes in a snowstorm reach the ground, they may be shattered, half-melted, or fused with others. Under such circumstances it would be difficult to find two that are completely identical. But considering the billions upon billions that have fallen in the history of the world, there remains a chance that over all of time, two identical snowflakes may have fallen. In 1986, a scientist with the National Center for Atmospheric Research is reported to have photographed a pair of column-shaped snowflakes that looked exactly alike. But whether they were truly identical remains the question.