Collision/Coalescence

• Collision/Coalescence - cloud droplet growth by collision

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

• 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??