Snow is formed when water vapor in the air freezes into ice crystals, typically inside clouds, and then falls to the ground as precipitation when those crystals grow too large to be suspended. Temperature and moisture conditions in the atmosphere determine whether snow forms, and where it falls. Key points

• Formation process: Water vapor in clouds cools and condenses directly into ice crystals around microscopic particles (nuclei). As these crystals grow, they combine to become snowflakes. If the air is cold enough all the way from the cloud to the ground, snow reaches the surface instead of melting into rain.
• Conditions that favor snow:
  • Atmospheric temperatures at or below 0°C (32°F) within the cloud and through the lower atmosphere.
  • Sufficient atmospheric moisture to form ice crystals and allow growth.
  • Weather systems such as mid-latitude cyclones, cold fronts, lake- or ocean-effect snowfall, or orographic lifting over mountains can enhance snowfall by supplying moisture and lift.
• Common scenarios:
  • Frontal snowfall: A cold air mass moves in with a warm air mass above; the vertical motion and cold temperatures produce snow along the front.
  • Lake- or sea-effect snow: Warm, moist air moving over cooler water and then over colder land can dump heavy snow on downwind regions.
  • Mountain snow: Rising air over terrain cools and condenses, leading to snow on the windward slopes.

Simple explanation for everyday understanding

• Snow happens when it’s cold enough for water vapor to turn directly into ice crystals before reaching the ground. If there’s enough moisture and lift, those crystals stick together to form snowflakes and fall as snow.

If you’d like, I can tailor this to a specific region or season and include typical weather patterns and signs to watch for.