Collision/Coalescence

  • Collision/Coalescence - cloud droplet growth by collision

  • this is a dominant process for precipitation formation in warm clouds (tops warmer than about -15C)

  • some cloud droplets will grow large enough and will start to fall in the cloud -->>

  • since the bigger drops fall faster than the smaller drops, they will "collect" the smaller drops - the bigger drop grows

  • droplet fall speed is called its terminal velocity

  • need droplets of different sizes for this process to really work

  • Q: what determines the droplets fall speed relative to the ground??