What is an atmospheric river?

A channel of water vapour that is taken up near Hawaii and then carried by atmospheric wind directly into the West is known technically as an atmospheric river, or AR. from Getty Images

Have you ever heard of the term "atmospheric river" to describe a weather pattern? The intense and protracted situation is preparing to spread over the western part of the United States until January 4.

According to the U.S. Geological Survey, an atmospheric river is essentially a conveyor belt of moisture from the Pacific Ocean. Although they are not on land like typical rivers, they do contain enough water to be categorised as rivers.

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A channel of water vapour that is taken up near Hawaii and then carried by atmospheric wind directly into the West is known technically as an atmospheric river, or AR.

The National Oceanic and Atmospheric Administration says that another name for an AR is "Pineapple Express," which you may have heard.

Water is carried along atmospheric rivers by atmospheric wind. (AP Photos)

When they come into contact with land, the moisture in the AR is released and carried higher into the sky, where it turns into heavy rain or snow. This is especially noticeable in the western highland regions. While certain landscapes might benefit from AR moisture—for example, the Sierra Nevada mountain range receives between 30 and 50 percent of its annual precipitation from ARs—too much of it can be problematic.

An atmospheric river earlier this week caused a chain reaction of problems in parts of California and Oregon.

William Churchill, a forecaster and meteorologist for the National Weather Service, told The New York Times that although California often benefits from the additional precipitation, "too much all at once" raises risks in places affected by wildfires. According to Churchill, charred debris here raises the danger of mudslides.

USGS elaborates on this idea by stating that soil that has been damaged by fire dries up and becomes less porous. Water immediately slides off surfaces during heavy downpours, raising the possibility of excess runoff and flash floods. Additionally, excess moisture can cause dry, weedy vegetation to grow where it shouldn't, giving wildfires even more dry foliage to burn through, according to USGS scientists. This can present risks for subsequent fires.

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Although many people may not be familiar with the phrase "atmospheric river," its influence on the weather is rather consistent: USGS notes that there is always one occurring somewhere in the world.