collision-coalescence

The collision-coalescence process is a fundamental mechanism in meteorology responsible for the formation of precipitation. This process explains how small cloud droplets grow large enough to fall to the Earth's surface as rain.1

In warm clouds, where temperatures are above freezing, cloud droplets initially grow through condensation. However, this process becomes very slow once droplets reach a radius of about 10 micrometers. For further growth, droplets must collide with each other. If a collision is forceful enough to overcome surface tension, the droplets will merge, a process known as coalescence.1

Precipitation particles grow significantly through this collision-coalescence mechanism. As these larger particles fall, they collide with smaller cloud droplets in their path, continuing to grow. This process is particularly common in shallow cumulus clouds over tropical oceans where the concentration of condensation nuclei is low, allowing for more efficient droplet growth.21