Atmospheric Rivers Explained

Parts of California, including San Francisco and Oakland, are reeling after record rainfall fell over the New Years holiday – and it’s not over yet. More rain is forecast to hit the region in the coming days.

But what’s powering all this rain? There’s a meteorological term for it: atmospheric river.

According to NPR, this same part of north-central California has seen three atmospheric river events since just Dec. 26, 2022. At least one death has already been linked to the ongoing storm.

The term atmospheric river was coined based on its function: transporting great quantities of water and feeding different ecosystems on its way.

Atmospheric rivers are long and narrow “rivers in the sky” that transport large amounts of water vapor from the tropics to other regions of Earth. Their extension can go from 400 to 600 kilometers in width, and they can carry as much water on average, in the form of steam, as 25 Mississippi Rivers.

When an atmospheric river reaches land, especially in mountainous terrain, or interacts with a storm system, it releases much of its vapor as rain or snow. This can result in extreme rain or snowfall, leading to flooding and other deadly weather disasters.

“Atmospheric rivers are a key feature in the global water cycle and are closely tied to both water supply and flood risks — particularly in the western United States,” according to NOAA.

We’re seeing this now in north-central California. And the climate crisis could only be making matters worse.

As global temperatures rise, more and more water evaporates from our oceans. At the same time, warmer air also holds more moisture. Seven percent more moisture for every degree Celsius of warming, in fact. Moisture that the atmosphere sucks up and carries over land, falling as rain or snow.

So while the climate crisis may not have created the atmospheric river itself, it’s making more moisture available to these massive rivers in the sky – to be carried to land, most recently to California, where it can cause catastrophic flooding.

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