What Are Atmospheric River Storms?

On Feb. 27, 2019, an atmospheric river propelled a plume of water vapor 350 miles (563 kilometers) wide and 1,600 miles (2,574 kilometers) long through the sky from the tropical North Pacific Ocean to the coast of Northern California.

Just north of San Francisco Bay, in Sonoma County’s famed wine country, the storm dumped over 21 inches (53 centimeters) of rain. The Russian River crested at 45.4 feet (13.8 meters) — 13.4 feet (4.1 meters) above flood stage.

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For the fifth time in four decades, the town of Guerneville was submerged under the murky brown floodwaters of the lower Russian River. Damages in Sonoma County alone were estimated at more than $100 million.

Events like these have drawn attention in recent years, but atmospheric rivers are not new. They have meandered through the sky for millions of years, transporting water vapor from the equator toward the poles.

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In the 1960s, meteorologists coined the phrase “Pineapple Express” to describe storm tracks that originated near Hawaii and carried warm water vapor to the coast of North America. By the late 1990s, atmospheric scientists had found that more than 90 percent of the world’s moisture from the tropics and subtropics was transported to higher latitudes by similar systems, which they named “atmospheric rivers.”

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In dry conditions, atmospheric rivers can replenish water supplies and quench dangerous wildfires. In wet conditions, they can cause damaging floods and debris flows, wreaking havoc on local economies.

atmospheric rivers

Atmospheric rivers are an important water source for the U.S. West.