How Atmospheric Rivers Shape Our Weather

For many weeks, California has been pummeled by torrential rains from atmospheric rivers. But what are these rivers in the sky and where did they come from? Two ingredients are needed for the formation of an atmospheric river—a plume of concentrated moisture in the atmosphere and strong, fast winds to move it around.

In California’s case, the moisture is coming from the tropical Pacific, where water evaporates from the warm ocean surface into the atmosphere. Winds concentrate the water vapor into long, narrow bands of extra-wet air. When the wet air hits the mountains of California’s west coast, it’s forced upward into colder air and the water vapor is squeezed out in a deluge of rain.

Atmospheric rivers aren’t a new phenomenon. More than 90% of precipitation in Earth’s midlatitudes is transported by these sky rivers. The average atmospheric river is about 1200 miles long and 500 miles wide, carrying 25 times as much water as the Mississippi River. And they’re everywhere. They’ve dumped record-breaking rain on California, driven flooding in Europe and Australia and caused numerous nor’easters in the eastern U.S. About 50% of the most intense, extreme storms have involved atmospheric rivers.

Air temperatures have been rising steadily over the last century. As air gets warmer, it evaporates more water from the ocean. Air can hold 7% more water vapor for each additional degree of warmth. In a warmer future, scientists expect that atmospheric rivers will get correspondingly wetter and stronger.

The good news for now is that 64% of California is currently drought-free (up from 1% in October 2022). Most reservoirs are in good shape, but groundwater levels are still low. Hydrogeologists are studying ways to utilize storm water runoff to recharge aquifers. Two possibilities are managed flooding and infiltration basins, both of which would divert some of the runoff to areas where water could percolate into the ground. There are many challenges to overcome but meeting the water needs of a growing population is crucial to our future.