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Rain is formed within warm clouds when a particle, such as a grain of dust, attracts a drop of moisture to itself. In a next step, this initial cloud droplet will…

  • A
    be taken higher by updrafts in the cloud, thereby freezing and growing in size due to collisions with other ice crystals.
  • B
    collide with other droplets, thereby growing in size and eventually forming a rain drop.
  • C
    merge with other droplets, a process which is greatly improved if the colliding drops feature the same electrical charge.
  • D
    leave the cloud, eventually forming the final rain drop at and below the cloud base, as this is where the humidity is highest.

Refer to figure.
Unlike the Bergeron Process, where precipitation forms under supercooled conditions, the COALESCENCE PROCESS typically occurs within relatively warm clouds with tops warmer than -15°C. As you can tell by the name of this process, the collision of falling and rising droplets is what allows them to grow large enough to fall to the ground as precipitation.

In simple words, Cloud droplets of different sizes will fall at different velocities and will collide with droplets of different radii. If the collision is hard enough to overcome the surface tension between the two colliding droplets, coalescence will occur and result in a new and larger single droplet. This process of cloud-droplet growth is referred to as collision-coalescence. Warm-cloud rain results when the droplets attain a sufficient size to fall to the ground.

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