Atmospheric rivers cause serious damage. They occur when water evaporates into the air and is carried along by the wind, forming long currents that flow in the sky like rivers flow on land. They can cause severe rains and mountain snow. Atmospheric rivers are partly to blame for the torrential rains in California last week, and another one is expected to hit on Monday. …Once in the air, winds swiftly carry along the vapour, which, if lifted by a front or passing over mountains, condenses and falls as rain or snow. …But the meteorological plumes of moisture affecting California recently have also overlapped with other severe weather, including “bomb cyclones”, which is a term for a rapidly deepening area of low pressure. …Severe droughts in the West have also affected the region’s ability to absorb the water, which makes flooding much more likely. Forest fires have also destabilised areas, as burn scars make landslides more likely.