The Pineapple Express: A River in the Sky That Can Drench California

Some rivers flow through the sky. Atmospheric rivers are narrow bands of the atmosphere that carry much of the moisture from the tropics toward the north. The best-known strong one is the Pineapple Express — named because its moisture builds up in the tropical Pacific around Hawaii — and when it reaches land it can dump as much as five inches of rain on California in a single day.

An atmospheric river event in January 2017, in an animation from NOAA's Earth System Research Laboratory. Credit: NOAA/ESRL/PSD.

What an atmospheric river is

Atmospheric rivers are relatively narrow regions of the atmosphere responsible for most of the horizontal movement of water vapor outside the tropics. They are part of the Earth's water cycle, and they are closely tied to both water supply and flood risk.

How the Pineapple Express forms

Prevailing winds pass over warm bands of tropical water vapor and form a "river" that travels across the Pacific. When it reaches the West Coast, it can hit the U.S. and Canada with heavy rain and snow.

Floods — or water supply

Atmospheric rivers come in all shapes and sizes. The ones carrying the most water vapor and the strongest winds cause extreme rainfall and the flooding that follows, and they can affect the entire west coast of North America, disrupting travel and damaging property.

But not all are disruptive. Many are weak and bring welcome rain, or snow at high elevations, that supply crucial water to communities across the West.

Sources

Based on "What is the Pineapple Express?" from NOAA's National Ocean Service, a work of the United States government in the public domain.