What is an Atmospheric River? A River in the Sky?

What is an Atmospheric River?

An atmospheric river (AR) is a band of tropical water vapor caught on the edge of a cold front and carried toward the poles. These 250 to 350-mile-wide bands carry as much as 300,000 tons of water, 7 to 15 times as much as the mouth of the Mississippi River!

Most atmospheric rivers do not damage; most are weak systems that often provide beneficial rain or snow that is crucial to the water supply—from 30 to 50% of precipitation along the West Coast, according to NOAA!

If you live on the West Coast, some folks, especially farmers, are pleased that these giant atmospheric rivers bring rain to a parched state. So much of the food for Americans (and the world) is grown in California. 

Of course, no one embraces landslides, power outages, accidents, and mudslides. The really bad atmospheric-river-driven storms will cause extensive flooding as the water runs out of places to go.

When Did the Term “Atmospheric River” Start?

We are only beginning to understand these rivers. Discovered by Reginald Newell and Yong Zhu in the 1990s, they were originally connected with West Coast flooding, particularly the infamous “Pineapple Express” that flows through Hawaii. 

We now know they have a central role in the global water cycle. On any given day, these relatively narrow bands of water vapor account for over 90% of the movement of tropical moisture north and south. Most of the time, they bring warm, welcome rainfall in moderate amounts.

The good news is that scientists can increasingly warn people where and when the worst atmospheric river floods or snows will hit. They have also learned that these AR events can happen throughout the world. The floods that have hit England are often river storms. Some of the worst Nor’easters, like 2010’s Snowmageddon, were caused by an atmospheric river flooding the chilly East Coast.

So look up in the skies! It’s a bird, a plane—no—an atmospheric river coming at you. You may not enjoy the soggy mess, but the flowers this spring will be grateful.