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Atmospheric rivers get talked about a lot during winter in California, and for good reason.
The invisible rivers in the sky play a significant role in how much rain the state receives and can contribute to some of California’s wettest winters and most damaging storms on record.
But what exactly are they?
What is an atmospheric river?
Atmospheric rivers are long, powerful portions of the atmosphere that carry lots of water from the tropical regions near the Earth’s equator towards the poles.
They’re invisible to the naked eye. The water is moved over the ocean in the form of water vapor, not a “river” in the way we think of them on land. They tend to move through the atmosphere in streams between 250 and 375 miles wide.
That’s about the distance between LA and San Francisco.
That stretch moves an astonishing amount of water. Just one atmospheric river can move an average of 10.5 trillion gallons of water per day.
The strongest atmospheric rivers can move anywhere between seven and 25 times as much water as the flow of the Mississippi River, which is the second longest river in North America and has a watershed that reaches 32 states, according to the national park service.
According to the National Oceanic and Atmospheric Administration, atmospheric rivers “are a primary feature in the entire global water supply and flood risks, particularly in the western US”
How do atmospheric rivers form?
A view of the Earth using a water vapor satellite shows that moisture in the atmosphere is concentrated over the equator.
Temperatures at the equator tend to run hotter, and just like in grade-school science lessons, hotter temperatures cause water to evaporate into the atmosphere. Warmer air can also hold more water vapor.
The circulation of the atmosphere will pull streams of moisture away from the equator forming “atmospheric rivers.” These “rivers” of moisture can be pushed towards land by weather systems.
Atmospheric rivers play a major role in California’s rain season. Twenty five to 50 percent of the state’s annual precipitation is produced by atmospheric rivers. Rain and snow amounts can vary widely depending on the exact location, timing and moisture content.
Why are storms fueled by atmospheric rivers so powerful?
Atmospheric rivers form and move fairly close to the surface of Earth at below 10,000 feet.
In contrast, planes spend most of their time traveling at altitudes of 30,000 feet or higher.
This means that, once atmospheric rivers reach land, especially the coastal mountains in California, they move up in altitude from their starting point as they travel.
As the water vapor that makes up the rivers moves up, it cools with the temperature of the atmosphere, turning into water droplets and — eventually — lots and lots of precipitation.
Atmospheric rivers come in all shapes and sizes around the globe, and while some tend to point in a mostly consistent direction — there’s one that, according to NOAA, generally comes in from Hawaii and moves towards the West Coast of the US — they do get moved around with the winds like other weather phenomena.
But as the atmospheric river moves south, the storm will bring more and more rain to Southern California.
“Now the one note with atmospheric rivers is, because they’re kind of narrow, and they’re very dependent on the moisture coming in off the Pacific, any slight shift in position or timing could have a big impact on how much rainfall we ‘re looking at,” NBCLA forecaster David Biggar said.
The atmospheric rivers that produce the biggest impacts, such as flooding or mudslides, tend to be the kind that move slow or linger. This means the river of moisture remains pointed at the same area for an extended period.
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