Al Stahler: Atmospheric river flows into foothills |

Al Stahler: Atmospheric river flows into foothills

A snowflake. Atmospheric rivers, like the one we’re experiencing now, bring California roughly half its total supply of water.
Photo courtesy NASA

Overhead – over the Pacific – rivers flow.

Like rivers on land, these rivers flow one into another. The north, middle and south forks of the Yuba join together to form the mainstem; the mainstem Yuba is tributary to – flows into – the Feather; the Feather then joins the Sacramento.

Two major differences between rivers flowing over the land and rivers flowing through the air:

  • First, the water conveyed by the airborne rivers is not liquid, but gaseous – water vapor – the type of water we normally notice only when the air is humid … when the air is moist.
  • Second, the tributaries to the airborne river do not flow in permanent streambeds. A tributary may enter the “mainstem” over one region of the Pacific today … and then disappear; another tributary – different from the first – may feed into the “mainstem” over a different part of the Pacific tomorrow.

Some atmospheric rivers start out over the tropics; some, over the subtropics (latitude of northern Mexico/southern California); some source over more northern latitudes.

These atmospheric rivers are vitally important: they bring California roughly half its total supply of water. But only if they get here. Maybe half-a-dozen atmospheric rivers reach California in a typical winter; some winters, half that number; some winters, twice.

The atmospheric river that – Hallelujah! – now drops moisture on us is the first to reach us this winter.

To draw an atmospheric river into California, conditions have to be just right – certain patterns have to form in the sky.

Take seven coins – doesn’t matter what kind, so long as they’re the same. Place one coin on the table, then encircle it with the other six. Remove the central coin. The six coins fit together perfectly – thanks to their identical size, this is a natural pattern for them to fall into.

Six water molecules – all the same size – will also link up, into a very similar pattern. Expanding the pattern outward, the water molecules build themselves into a snowflake.

Like coins, the water molecules fall into the snowflake pattern naturally. But it’s more than just their identical size that link the waters together. It also requires electricity – the sort of electricity we see when we rub a balloon and stick it to the wall. Electricity glues water molecules together in a snowflake.

The pattern of the snowflake is similar to the pattern formed by the coins, but there’s more going on … the formation of a snowflake is more complex.

Adding motion to the mix makes for larger, and more complex, patterns yet. Given the right mix of temperature and moisture and motion, we get the atmospheric pattern known as a tornado.

Spinning once per day, planet Earth is a humongous carousel. Looking down on the north pole from space, we see that Earth spins counterclockwise.

Playing catch on a carousel, every pitch is a curve ball – the carousel’s spin makes it impossible to throw the ball straight.

Earth’s spin makes it impossible for the wind to blow straight. Winds that take on the Earth’s counterclockwise spin are described as cyclonic.

A hurricane – spinning counterclockwise – is a tropical cyclone – an atmospheric pattern constructed when the heat and moisture and motions of thunderstorms combine just right.

Non-tropical cyclones – systems of counterclockwise winds, outside the tropics – also exist.

The word “extra” is related to the word “exterior.” Latitudes outside the tropics – the latitudes in which California lies – are extratropical.

A mass of cold air, the size of a state, drifting down from the north, slams into a mass of warmer air from the south. (The cold air mass being “at fault” in this collision, the meeting place is denoted a “cold front” … “front” deriving from the concept of hostile armies, meeting each other, during World War I.).

Driven by carousel earth, the air masses swirl around each other. They swirl cyclonically – counterclockwise, seen from above – and an extratropical cyclone is born: A winter storm.

An extratropical cyclone is a pattern in the atmosphere that arises because of how warm and cold air interact, as they move. The pattern includes a region, leading the storm, that can entrain – can capture – an atmospheric river, and carry it into California … which is happening, as I write, early Wednesday.

Atmospheric rivers, long and narrow, can carry more moisture than the Mississippi. Our understanding of them is still new, and incomplete … as demonstrated by an excellent book, published in 2020. Atmospheric Rivers covers the topic world-wide; it is edited by scientists – Ralph, Dettinger, Rutz and Waliser – rooted in western North American.

Atmospheric rivers carry with them the threat of floods … even as they bring the moisture upon which we absolutely depend.

Al Stahler enjoys sharing science and nature with friends and neighbors in The Union and on KVMR-FM. He teaches classes for both kids and grown-ups, and can be reached at

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