When you picture a river, you probably imagine water winding between banks of earth. But some of the largest rivers on our planet flow far overhead, invisible, through the sky.

Atmospheric rivers are long, narrow bands of concentrated water vapour, typically a few hundred kilometres wide and thousands of kilometres long. A single strong one can carry more water than 20 Mississippi Rivers , not as liquid, but as vapour, swept along by powerful winds. When one of these airborne rivers strikes a mountain range, the air is forced upward, cools, and releases its cargo as drenching rain or heavy snow.

Atmospheric rivers are neither purely good nor purely bad. In places like California and parts of Australia, a handful of them can deliver up to half of the year's water supply, ending droughts and filling reservoirs. The same storms, arriving too strongly or too often, cause devastating floods and landslides.

Scientists now track these sky rivers with satellites and rank them on a scale, much like cyclones, from AR1 (weak) to AR5 (exceptional). Better forecasts give farmers, dam operators and emergency services precious days to prepare , to store the water where it is wanted, and to move people out of the way where it is not.