Atmospheric rivers are inundating California – but what are they?

Severe storms brought on by this weather phenomenon have dumped 30tn gallons of water across the state

Atmospheric rivers, the severe storms that have doused California during its extremely wet winter, have dumped 30tn gallons of water across the state, buried mountain towns in snow and caused widespread flooding - and they aren't over yet, with more expected to continue into spring.

But what exactly are they and why are they causing so much damage? Here's what you need to know.