The process you are referring to is called "coalescence." Coalescence is the merging of small water droplets in the atmosphere to form larger droplets. This process occurs within clouds when water vapor condenses onto tiny particles known as cloud condensation nuclei (CCN). If the surrounding air contains a sufficient amount of water vapor and the droplets collide and stick together, they will coalesce and grow in size. Eventually, these larger droplets may become heavy enough to fall as precipitation, such as rain or snow.
