What is an atmospheric river?

MYRNA BROWN, HOST: Coming up next on The World and Everything in It: the weather.

For several weeks now, California’s been slammed with a weather phenomenon. Maybe you’ve heard of it? Here’s audio from CBS on Tuesday.

CBS: Tonight, California is being slammed by the 12th atmospheric river of the season. Heavy rain, strong winds and flooding are already causing problems across southern California.

MARY REICHARD, HOST: So what exactly is an atmospheric river? Well, WORLD’s Brian Basham, better known as the Big Bash of World Watch, is a meteorologist, and he’s here now with the weather.

BRIAN BASHAM, REPORTER: The easiest way to think about an atmospheric river is to just think about it as a river in the sky. Instead of flowing water, it’s water vapor, which if you go back to the science classes of your childhood, you know it’s the gaseous state of water.

It may seem strange to think we get that much rain out of small droplets of water floating through our atmosphere. But I want you to picture this river stretching all the way from the tropics or subtropics—like say Hawaii—to the coast of California. That’s a huge distance to travel over the Pacific Ocean, picking up moisture all along the way. It’s possible for an atmospheric river to transport as much as 15 times the average flow of water at the mouth of the Mississippi River. Imagine that.

Now, you have all that water vapor rushing towards the Sierra Nevadas, the enormous mountain ranges stretching down the western United States. Again, going back to science class, we get rain when moisture in the atmosphere is forced high enough into cold air to condense, becoming too heavy to float, and then falling back to the surface.

So, what we get with these weather patterns is massive amounts of water vapor piling up against those mountain ranges. That moisture has to go somewhere. It can’t go through land, so it’s forced upward. That’s called orographic lifting. If you’ve ever lived around a mountain—especially the western side since most weather patterns move from west to east—you’ve probably experienced this. I like to imagine this as taking a sopping wet sponge and smooshing it against a wall. You know how much water is going to pour out of that thing.

Like most weather events, there are both positive and negative effects. For instance, California gets about 30 to 50 percent of their water for the year from atmospheric events. That’s good. It can fill the aquifers, lakes, and rivers. It’s bad when it rains too much at a time or for too long, then we see the dangerous flooding and mudslides. Plus, excessive rain just runs off into the ocean, and not that much gets stored.

California has had drought conditions off and on—more on than off—for the last 100 years, so they can use SOME rain. But let’s pray it comes more frequently and in smaller amounts.

I’m Brian Basham.

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