The thought experiment: What would happen if the UK's annual rainfall fell in one giant drop?
As the old saying goes: "When it rains, it... incinerates the impact site and floods the entire country."
1. Massive drop
The UK receives around 1,200mm of rain per year. That equates to 291 cubic kilometres in total – more than four times the volume of every lake and reservoir in the UK! If you could form it into a single drop, it would be 8km across (see image above). From a typical cloud height of 2,000m, this raindrop would take about 30 seconds to fall and would be travelling at 300km/h when it hit the ground.
2. Initial impact
The kinetic energy of 291 billion tonnes of water impacting at this speed is about one terrajoule. This is equivalent to 250 megatons of TNT, or almost half as much as the combined energy of every nuclear weapons test in history. If it fell in the middle of the country, Leicester would be instantly incinerated and then smashed flat by a superheated bow wave of compressed air beneath the water bomb.
As the drop hits the ground, its upper edge is still at an altitude of 8,000m, and as the water falls inexorably downwards it is forced sideways to form a supersonic tidal wave over a kilometre high. Every tree and building in its path for at least 100km in every direction is torn up. The whole of the Midlands, East Anglia and much of the south of England is flooded to a depth of at least a metre.