What is an atmospheric river?
Rivers in the sky
Atmospheric rivers — sometimes called "flying rivers" or "rivers of vapor" — are long, narrow corridors in the atmosphere that carry enormous amounts of water as vapor over long distances. A single one is typically narrow (300–800 km wide) but very long (1,500–3,000 km), stretching from the tropics and subtropics toward colder regions, and it usually lasts one to three days.
How they turn into rain
As long as that vapor stays aloft, nothing dramatic happens. But when an atmospheric river runs into a cold air mass moving down from the north, or a mountain range that forces it to rise, the vapor cools, condenses, and falls — as rain or snow. Depending on the conditions and how long it lingers, the result can be intense, "atypical" precipitation that reshapes local weather, alters the water supply, and can do real damage on the ground.
Why they matter
Atmospheric rivers are a major part of the planet's water cycle. On the U.S. West Coast they deliver much of the year's water — and many of its worst floods. They redistribute where and how much it rains, which makes them central both to water security and to flood risk. That dual role is exactly why they're a major subject of study in atmospheric science: the better we understand them, the better we can anticipate and prepare for their impacts.
A warming climate is changing them
Global warming changes the timing and intensity of many weather phenomena, and atmospheric rivers are no exception. Studies project that their frequency may rise by roughly 30%, and that the amount of vapor they transport may increase by up to about 37%, with rivers that are, on average, longer and wider. More water, arriving more often, makes the activities that depend on stable weather — agriculture especially — more vulnerable.
The view from a coffee farm
That last point isn't abstract for me. My family grows coffee in Nayarit, on Mexico's Pacific slope, and the coffee harvest runs from roughly November to March — right through the season when these winter rivers arrive. In February 2024, a single atmospheric river dropped about 60 mm of rain in two days across the Pacific coffee states — Nayarit, Jalisco, Colima, and Guerrero. In Nayarit it knocked roughly half the harvest to the ground, and about a quarter of the cherries split and dried on the branch. Excess moisture rots fruit, splits cherries, saturates soils, and stalls the sun-drying that natural and honey-processed coffees depend on.
I wrote about that event, and what growers can do to soften the next one, in a Spanish-language feature for a coffee magazine.
Global climate change, deforestation, and soil degradation may make atmospheric rivers more frequent and intense — while also lowering the resilience of coffee agro-ecosystems to their effects.
Read the full piece in El Café de Mi Tierra ↗
How we study them
Part of my research is about a deceptively hard question: how big is an atmospheric river? The answer depends on how you choose to detect it, so I've worked on ways to measure their size and intensity that don't depend on any single detection algorithm — and on quantifying the uncertainty that comes with the choice. Getting that right is what lets us say something defensible about how they'll change.