Hydrodynamics of cloud and precipitation particles

Abstract

In the previous chapters, we have discussed the basic thermodynamics of cloud and precipitation particles and how they are initiated by nucleation, but we have not discussed their motion. None of these particles are truly stationary in the cloud – they are heavier than air so that they are always falling relative to the air. If a cloud appears to be floating in the air without apparent motion, it is only because the updraft in the cloud supports the fall of the cloud particles and gives us the illusion that the cloud is not moving vertically. The motion of cloud and precipitation particles has great impact on the behavior of the cloud system. It influences how fast vapor can be accumulated on these particles, how fast these particles evaporate when they fall, how they collide and coalesce with each other and aerosol particles, and how they break up when acted on by sufficient force. All these processes ultimately determine how fast a cloud will develop or dissipate and how much precipitation will be produced by the cloud. The motion of a cloud or precipitation particle in a viscous medium – the air – causes a flow field around the particle. The study of the flow fields around such particles belongs to the discipline of hydrodynamics, which is the main subject of this chapter.