What the heck are atmospheric rivers and why should we care? A simple explainer.

The normally drought-ridden California experienced yet another atmospheric river earlier this week, the twelfth of an incredibly wet and perplexing winter of extreme weather. Or, as some people call it, the new normal.

Most of those atmospheric rivers hit the state in 2023. The latest deluge brought heavy rain, snowfall, and hurricane-force winds across the state, including a bomb cyclone. A tornado ripped off a roof in east Los Angeles, injuring two people. It was the strongest to hit the city since 1983.

This carnage comes after years of drought and, unexpectedly, a series of powerful storms that drenched the state from the end of 2022 until late January. It resulted in severe flooding and record amounts of snow and caused at least 22 fatalities.

It all gave atmospheric rivers a really bad name.

But what exactly are they?

In short, it’s a river in the sky.

“It’s a stream of water vapor in the atmosphere that stays in the lowest few kilometers,” said Bin Guan, a scientist at the Joint Institute for Regional Earth System Science and Engineering in Southern California. “It’s called a river because the amount of water vapor transported is similar to the amount of water in rivers on the ground.”

The rivers start life in the tropics, which are extremely hot areas between the Tropic of Cancer to the north and the Tropic of Capricorn to the south. The equator is in the middle. Because it’s so hot in those areas, ocean water evaporates and sends moisture into the atmosphere, where it forms clouds, according to an explanation by the National Oceanic and Atmospheric Administration.

Once in the atmosphere, strong winds push the newly formed and vapor-packed clouds on a journey.

“If the winds are strong enough, they can reach inland to places like Yellowstone National Park,” added Guan, whose institute is a collaboration between the University of California, Los Angeles, and NASA’s Jet Propulsion Laboratory in Southern California. “We saw that happen last year.”

Mountains usually empty atmospheric rivers of their moisture because of how high up they are. In January, one atmospheric river lasted until Minnesota, dumping a foot and a half of snow on the state. That’s how the atmospheric river is born.

Read More: After weeks of downpour, California is ‘dangerously unprepared’ for a wetter future

The ones that hit California are often called the “Pineapple Express,” referring to the tropical climates in which pineapples thrive. After the atmospheric river hits land, it pushes even higher into the atmosphere and the clouds rapidly cool down. The clouds expand and can no longer hold the vapor. It falls as rain.

But don’t forget that the atmospheric rivers are accompanied by the strong winds that helped them get there. That adds to the potentially chaotic and dangerous weather.

Guan is the author of a recent study into the atmospheric river rating system, created in 2019 and ranks the storms from one to five. The hope is that scientists can differentiate between helpful storms and dangerous ones in the future.

As part of his findings, Guan found that atmospheric rivers can be found all over the world, not just in the western United States.

“The weakest atmospheric rivers can be found anywhere, but the strongest tend to be found in clustered in certain areas, mostly along the eastern boundaries of the ocean basins, like northeastern pacific or the Atlantic.”

Why should we care?

One reason we see frequent and powerful ones is that the oceans are warming, causing more water to evaporate into the atmosphere. That’s a direct result of climate change. These storms keep heading toward California because of the wind current, like the jet stream. It carries the weather from east to west. They are somewhat influenced by El Niño and La Niña - large climate patterns in the Pacific Ocean. El Niño is wet, and La Niña is dry. We’re in the third year of La Niña.

The severe weather is not ideal in places like California and surrounding states where drought has dried out the ground. You’d think they could do with a bit of rain. It’s true; they could. But when heavy rain meets the arid ground, it doesn’t hydrate it. It creates a phenomenon known as hydrophobic soil – where water flows across soil rather than into it. That means dry California is prone to flash flooding.

In short, strong atmospheric rivers can be deadly.

“Some people on the west coast equate them [atmospheric rivers] to hurricanes because of the amount of rain and wind they produce. It can be very impactful and very dangerous.”

But it’s not all bad news.

Atmospheric rivers are a crucial part of global weather patterns.

“In many instances, when the atmospheric river is weak, it produces rain and snow, which helps replenish reservoirs, build the snowpack, which California relies on during the dry summer,” he said. “It’s very beneficial.”