Snow begins its journey high in the atmosphere when water vapor cools and condenses around tiny particles, forming ice crystals.

These crystals grow by clinging to one another, creating the intricate snowflakes we see falling to the ground.

For this process to happen, temperatures in the clouds must be below freezing, and the air must be moist enough to supply water vapor.

During massive winter storms like the one threatening 27 U.S. states right now, conditions are primed for heavy snow and ice. Arctic air masses collide with warmer, moist air, fueling intense precipitation.

When the ground temperatures hover near freezing, this precipitation can shift between snow, sleet, and freezing rain - compounding the chaos with slick roads, power outages, and transportation standstills.