What Is An Atmospheric River, or Pineapple Express?

Read about this oddly-named, but common weather occurrence that can pack quite a punch!

“Atmospheric River” is in the news—this type of storm has taken aim at the west coast and it’s bringing heavy winds, heavy rain, with a potential for flooding. These storms are also referred to as a Pineapple Express (Seth Rogen jokes aside). But what is it, and what causes it?

Powerful Storms

In early December 2014, a powerful storm system pummeled the West Coast of the United States, bringing two weeks of heavy rain, as well as blizzards, high winds, flooding, and mudslides. An incredibly rare tornado even touched down in Los Angeles, a city that’s never been prone to thunderstorms.

The storm was a double-edged sword, leaving destruction in its wake—including highway closures, property damage, and widespread power outages—but also bringing some measure of relief to a region that has been beset by drought for the last three years.

The storm also brought to light a weather term that may have been unfamiliar to those living east of the Rocky Mountains: Pineapple Express, a name that may sound incongruously whimsical given the phenomenon’s devastating capabilities.

What’s With The Name?

The name is a slang term for one specific type of atmospheric river—a moving band of concentrated moisture in the Earth’s atmosphere. A Pineapple Express is, more specifically, an atmospheric river that travels from Hawaii to the Western U.S.

Atmospheric rivers transport large amounts of water vapor along flowing currents of air. They are generally a few thousand miles long and around a hundred miles wide. And though they consist of water vapor—water in its gaseous, rather than liquid, form—a single atmospheric river can carry more water than the Amazon River, the Earth’s largest river, making its name apropos.

Although a West Coast storm of the severity of that wrought by the 2014 Pineapple Express is pretty rare— occurring perhaps once or twice in a decade—atmospheric rivers are actually pretty common. At any one time, there are 6-10 of them present across the face of the Earth.

Atmospheric rivers play a crucial part in transporting water around the planet. While they cover only about 10% of the Earth’s surface, they are responsible for as much as 90% of global water vapor transport. In most cases, this happens without much turmoil.

In the case of a Pineapple Express, though, when a river of warm, moist tropical air moves in, colliding with the drier and, in winter, cooler air of the mainland, it results in turbulence in the atmosphere and rapid condensation of the water vapor. That means high winds, flooding, blizzards, and generally unpleasant conditions.

For a weather system that sounds like it was named after a fruity, tropical drink, a Pineapple Express can pack quite the punch!

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Jaime McLeod

Jaime McLeod is a longtime journalist who has written for a wide variety of newspapers, magazines, and websites, including MTV.com. She enjoys the outdoors, growing and eating organic food, and is interested in all aspects of natural wellness.

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