Snowflakes form in the atmosphere when extremely cold water droplets form frozen crystals around tiny particles of dust or pollen. When these ice crystals fall closer to the ground, water vapor freezes onto the primary crystal and snowflakes grow larger with more crystals. This frozen water creates six-sided crystals.
These six-sided crystals form different shapes depending upon the temperature of the surrounding air. Long, needle-shaped crystals prevail at 23 degrees Fahrenheit, while flat, plate-like crystals form at 5 degrees Fahrenheit. Individual arms of a snowflake may grow differently as the snowflake falls to Earth. If the atmosphere changes from one layer to the next, a snowflake's arms grow one way for a time and then grow in a different pattern later.
No two snowflakes are exactly alike because each crystal follows slightly different paths from the atmosphere to the surface. Even two snowflakes next to each other do not follow the exact same downward pathway. All snowflakes have six sides due to the way in which water forms crystal patterns at the molecular level, but no two flakes are precisely the same. Most snowflakes are less than ½ inch across, although some reach 2 inches in diameter in the right conditions.
Most snow occurs when the temperature is 15 degrees Fahrenheit or warmer, although it cannot be too cold to snow, according to the National Snow & Ice Data Center. Warmer air holds more water vapor, so extremely cold air is unlikely to have enough moisture to create snow.