IOWA CITY, Iowa, July 25 (UPI) -- Atmospheric "rivers" of moisture flowing above the oceans have caused heavy rainfall and flooding in places such as California and Britain, researchers say.
These "rivers," unknown just 20 years ago, could become more common -- and more severe -- as global temperatures rise, they said.
Atmospheric rivers are up to 180 miles wide and can be as long as 1,200 miles, flowing invisibly more than a mile above the surface of the ocean.
One such atmospheric river is believed to have been behind the violent flooding that hit Cumbria in Britain in 2009, scientists said.
They calculated at its peak it was transporting almost 300,000 tons of moisture every second; by comparison, the River Thames carries about 65 tons of water through London in the same amount of time.
When atmospheric rivers make landfall, and if they encounter rising terrain, they are forced upwards where they cool and give up their massive amounts of moisture as rainfall, often for long periods.
The world's most closely studied atmospheric river, linked to a number of extremely damaging storms along the U.S. West Coast, has been dubbed the "Pineapple Express" because it usually originates from the region of Hawaii, scientists said.
Britain has seen an average of nine to 11 strong atmospheric river events every year for the past three decades, researchers said.
A warming climate, which would see the atmosphere capable of holding much more moisture, would make more severe atmospheric rivers likely, they said.
"One of the big things is that these are the most relevant feature of winter flooding in Britain and the work is certainly suggesting an increase in strength and frequency," study leader David Lavers of the University of Iowa told the BBC.