As of the end of December 2022. Californians are suffering from recurring storms fueled by torrents of moisture, known as the “wet weather. “atmospheric rivers” that flood their roads and homes. This winter wave of precipitation has killed more than a dozen people and ordered tens of thousands to evacuate.

What are atmospheric rivers?

Traditionally, a river is water flowing over land; by analogy, an atmospheric river is a stream of water vapor moving across the sky. Atmospheric rivers are formed when water evaporates and, carried by the wind, forms long currents. They look like streaks of delicate clouds that can extend for hundreds of kilometers. Typical atmospheric rivers are 400 to 600 kilometers wide and can reach more than 1,600 kilometers in length.

Atmospheric rivers and their occurrence in California

Atmospheric rivers, due to their origin in tropical parts of the Pacific, are commonly called “pineapple expresses.” Under the influence of high temperatures, ocean water evaporates and rises into the atmosphere. Strong winds carry water vapor. After crossing the Pacific Ocean, atmospheric rivers encounter mountain ranges along the California coast, such as Santa Cruz, which force them to move upward. The water vapor rises higher into the colder layers of the atmosphere, cooling and forming droplets of water that return to earth as precipitation.

Winds are capable of continuing to push rain from atmospheric rivers into the California Valley, where they can cause widespread flooding. The rains often become “trapped” in the valley and can fall for several hours or even days. Stronger atmospheric rivers sometimes flow further east into the Sierra Nevada. Streams of torrential rain then rise to the tops of the mountains, where the cool air turns them into snow.

Do atmospheric rivers help or hurt?

In most cases, atmospheric rivers are weak and do not cause damage. Instead, they can provide much-needed rain or snow. During the rainy season, they help replenish depleted reservoirs and provide water for agriculture or cities. Such storms also create a snow cover that supplies California with water after the rainy season ends. However, they happen to be very intense and dangerous, as extreme rainfall and wind can cause catastrophic flooding.

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