As the growing snow crystal is tossed by the air currents within a cloud, it encounters areas with different atmospheric conditions. Even subtle shifts in air pressure, temperature, and humidity can cause a radical shift in the crystal's growth pattern.

To make a spool-shaped flake, a crystal begins to form into a hexagonal pillar in a region that's about 20 degrees Fahrenheit before it drifts into another zone and "has a crystal midlife crisis," Libbrecht says. If the temperature gets either a little warmer or cooler the crystal growth shifts and it begins to form flat plates on the top and bottom of the pillar.