A snow crystal can become heavy enough to fall by growing as water vapor sublimates onto it. In some cases, this water vapor comes from evaporating cloud droplets. Cloud droplets can also freeze onto a snow crystal. In addition, snow crystals can collide with, and stick to, other snow crystals to form snowflakes that are heavy enough to fall. If the snow crystal or snowflake falls through warm enough air, it melts and falls to the ground as rain.
Clouds commonly form in rising air.
As air rises, it expands and cools. If the expanding air contains so much water vapor that it can cool this vapor to the dew point, clouds will form. The air can rise in several ways. When the sun heats the ground, the air next to the ground is warmed. Because warmer air is lighter than cooler air, the warm air rises.