Storm warning in California: What is an atmospheric river?

California residents are being advised to prepare for yet another series of torrential rains and heavy snowfalls in the coming days. The winter of 2023 has seen the Golden State hit by one atmospheric river after another bringing much needed precipitation to the parched soil, but sometimes there can be too much of a good thing.

Not all these weather events are necessarily damaging, and they form an important part of the water cycle around the globe. But when they deliver excessive moisture to soil that is too dry or saturated to soak up the buckets of water, flooding can ensue. So what exactly, is an atmospheric river?

What is an atmospheric river?

As the name suggests, this phenomenon is essentially a river in the sky, composed of water vapor. The volume of which can be the equivalent of 25 times the flow of the Mississippi River at its mouth where it empties into the Gulf of Mexico. Atmospheric rivers (AR) form out over the ocean and the concentrated band of moisture can stretch for 2,000 miles, span 500 miles in width and be 2 miles from top to bottom.They are pushed along by strong, low-altitude winds that can reach hurricane speeds. They work like a treadmill bringing the storms in a series of one after another.

Extremely powerful events don’t just affect coastal areas, but can penetrate deep inland. The monster storms that caused severe damage to Yellowstone National Park in June 2022 were the result of a particularly strong example of this weather phenomenon.

Atmospheric rivers occur around the globe, typically in the extratropical regions in Europe, Africa, New Zealand and South America, usually during their respective winter seasons. The ones that lap across the western coast of North America are called the “Pineapple Express” as they originate out by the Hawaii Islands, known for the sweet fruit.