The Formation of Rain Droplets Due to Collision and Coalescence Credit: COD Meteorology/Plymouth State Meteorology/ (Wallace & Hobbs, 2006)

​In order for rain to precipitate from warm clouds, individual cloud droplets must first undergo a growth process in order to grow and form rain droplets. There are two processes by which this growth and cloud formation can occur. These two processes include growth by condensation or diffusion, and growth by collection, also known as collision and coalescence. 

​Collision and coalescence typically occurs in warm clouds with cloud tops reaching temperatures greater than -15 degrees Celsius. In order for this process to occur within the cloud, there must be a high liquid water content, strong and consistent updrafts, a large range of droplet sizes, and a cloud layer that is thick enough for the droplets to have time to grow to larger sizes. Before a cloud droplet can grow to these large sizes and become a rain droplet, the cloud droplet itself must form! This occurs when there are aerosols or other pollutants within the air that can act as cloud condensation nuclei. Cloud condensation nuclei or CCN, act as a particle on which the water can condense onto, as pure water cannot condense and form a cloud droplet on its own. 

Credit: Plymouth State Meteorology (Diagram Made Based on (Lutgens, 1992))

​The process of collision and coalesce occurs following the formation of the warm cloud. Within the warm cloud, convergence occurs near the surface causing rising motion within the cloud as this air rises in the updraft. Small cloud droplets can be caught within this updraft and rise up into the cloud as larger droplets within the cloud may be falling to the surface. As these water droplets rise and fall within the cloud, some collide into each other due to their different fall speeds. As more collisions occur over time, these droplets can coalesce together forming larger droplets. These larger droplets falling from the cloud are known as “collector drops”. Because larger cloud droplets have a higher terminal velocity, these larger collector drops fall faster all the while coalescing with smaller droplets on the way down. This causes the rain droplets to sometimes become so large that they break apart due to instability as they fall within the cloud.  

The formation and growth of rain droplets by collection or collision and coalescence is necessary in order to explain the formation of warm rain in warm clouds, and it is a very efficient way for cloud droplets to become rain drops.

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©2019 Weather Forecaster Christina Talamo