K-4 Content Standard D . Rising warm air carries water vapor high into the sky where it cools, forming water droplets around tiny bits of dust in the air.
Some vapor freezes into tiny ice crystals which attract cooled water drops.
The drops freeze to the ice crystals, forming larger crystals we call snowflakes. When the snowflakes become heavy they fall. When the snowflakes meet warmer air on the way down, they melt into raindrops. In tropical climates, cloud droplets combine together around dust or sea salt particles. They bang together and grow in size until they're heavy enough to fall.