Snow is formed when water vapor is super-cooled inside a cloud and becomes crystallized. These ice crystals begin to clump together to form snowflakes, which eventually fall to the ground as they grow larger. Meteorologists have the unenviable task of predicting the onset of precipitation, since even the slightest degree of variance in temperature can result in sleet or hail, instead of snow.
Air temperatures must be near or below freezing (0° Celsius or 32° Fahrenheit), both at cloud level and near the ground, otherwise the snow would simply melt on the way to the ground. As a cold front approaches, ice crystals will likely form and clump together. Once these crystals become heavy enough, they descend to the ground as snowflakes.
Water vapor is often sucked into the air from nearby bodies of water and is super-cooled to form ice crystals. This process is integral to the formation of clouds, and later, precipitation such as snow and rain. Water vapor forms snow through two distinct processes: coalescence, where ice crystals in a cloud collide and combine with the water droplets; and deposition, when water vapor molecules freeze onto the ice crystals.
Clouds are formed when warm, humid air rises, through a process called expansion, to meet cool dry air. Water vapor, or moisture in the air, begins to cool at the confluence of these two fronts and mixes with dust particles. Finally, condensation takes hold and the outline of a cloud begins to form.
