What is an Atmospheric River?

Did you know there's a river in the sky that can carry twice the amount of water of the Amazon Rainforest? It's called an atmospheric river, and it's important we know when they're coming.

Atmospheric rivers are characterized as narrow, flowing columns of water vapor in the sky. When they reach land, they can produce tremendous, and sometimes dangerous, amounts of rainfall.

In the Pacific Ocean, atmospheric rivers originate over the warm tropical waters near the equator. There, water evaporates into water vapor and is swept up by large Pacific low-pressure systems. As the water vapor is transported north, it's concentrated and often driven towards the West Coast. Once the moisture reaches land, elevated topography along the coast acts to 'squeeze' moisture out of the air producing heavy rain—also known as orographic lifting, or mountain snow.

Heavy rain and meltwater can lead to very serious and sometimes life-threatening impacts. Flash flooding and debris flows enhanced by topography are among the most frequent hazards. Fast-flowing water can carry away cars, wash away roads and even homes. Loose soil or rocks combined with very steep or vegetation-less terrain can exacerbate these risks.

Fortunately, atmospheric rivers are not all bad news. The phenomenon is responsible for 50 percent of California's yearly precipitation. Without them, it would be impossible to replenish natural water reservoirs fully. They also add to the Sierra Nevada and the Rocky Mountain snowpack, which supplies a constant stream of melt-water to cities, crops, and ecosystems below.