Destiny of nephology’s lucky drops

Here in the UK we are all too familiar with rain, and this winter we seem to be having more than our fair share. In recent weeks it has been frontal rain - formed where cold air meets warm tropical air - that has repeatedly flooded northern areas. Its process of formation is well understood.

Other kinds of rain are less straightforward. In particular, the sudden downpours that spring from innocent looking fluffy cumulus clouds don't seem to obey the laws of physics. Inside a typical cumulus cloud the average droplets are about 20 micrometres wide. But to become raindrops that can fall to the ground they need to grow to about 2mm - which equates to a millionfold increase in volume.