A look at atmospheric rivers, the long bands of water vapor that form over oceans and fuel storms
The storm that hit California this week was fueled by an atmospheric river, a plume of moisture that stretched across the Pacific Ocean to near Hawaii. Here’s a look at the phenomenon:
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WHAT IS AN ATMOSPHERIC RIVER?
Atmospheric rivers are long and relatively narrow bands of water vapor that form above an ocean and flow through the air, transporting much of the moisture from the tropics to northern latitudes.
They occur worldwide but are especially important on the west coast of the United States, where they produce 30% to 50% of annual precipitation and are vital to water supplies, but can also generate storms that cause flooding and mudslides, according to the National Oceanic and Atmospheric Administration.
Formed by winds associated with cyclones, atmospheric rivers are typically 400 to 600 kilometers wide and move under the influence of different weather.
Many atmospheric river events are weak. But the powerful ones can transport extraordinary amounts of moisture. Studies have shown that they can carry seven to 15 times the average amount of water drained daily by the Mississippi River, according to the US Geological Survey.
Forty-six atmospheric rivers made landfall on the U.S. West Coast in the 2023 water year, according to the Center for Western Weather and Water Extremes at the Scripps Institution of Oceanography. Nine of these were categorized as strong, two were extreme and one was exceptional. California suffered extensive flooding and massive snowfall.
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WHAT HAPPENS WHEN AN ATMOSPHERIC RIVER REACHES LAND?
When the moisture-laden air moves over mountain ranges like the Sierra Nevada along the California-Nevada border, the water vapor rises and cools, becoming heavy precipitation that falls as rain or snow, according to NOAA.
While traditional cold winter storms from the North Pacific build the Sierra snowpack, atmospheric rivers are often warm. Snow can still fall at the highest elevations, but at lower elevations rain usually falls on snowpack. That could quickly lead to melting, runoff and flooding and reduce the amount of snow needed to supply California’s water supply.
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WHAT IS A PINEAPPLE EXPRESS?
It is a nickname for a strong atmospheric river in the tropical Pacific Ocean near Hawaii.
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WHERE DOES THE TERM ATMOSPHERIC RIVER COME FROM?
The name comes from research published in the 1990s by scientists Yong Zhu and Reginald E. Newell of the Massachusetts Institute of Technology. Atmospheric rivers are often called ARs.