Atmospheric rivers

An atmospheric river is an elongated region that transports a large amount of water vapour through the atmosphere. These corridors commonly occur ahead of cold fronts in extratropical cyclones and are part of broader weather systems moving heat and moisture from lower to higher latitudes. They are not rivers of liquid water in the sky: the water is carried as vapour by atmospheric flows. Their lengths, widths, moisture content and winds vary, and researchers use different measurement approaches to identify and classify events.

As moist air is carried over land, especially when it is lifted by terrain, it cools and water vapour can condense and fall as rain or snow. Atmospheric-river precipitation can contribute substantially to regional water supplies and mountain snowpack. The same systems can bring hazardous conditions when they carry exceptional moisture, stall over a watershed, or combine with vulnerable terrain and other weather factors; impacts can include flooding, landslides and travel disruption. An atmospheric river is not necessarily a damaging storm: many are relatively weak and beneficial.

Scientists monitor atmospheric rivers using satellite measurements of water vapour, winds and precipitation, alongside ground-based rain gauges and radar. Aircraft and other field observations can add measurements of the atmosphere's vertical structure. Combining these data with weather models helps forecasters estimate where a system will travel, how much precipitation it may produce and which locations could face hazards. Better observations also support research on water resources and changes in atmospheric-river behaviour.