A rainy winter season in California tends to surface the phrase, but what exactly is an atmospheric river and what does it mean for the region? NBC 7 Meteorologist Brooke Martell explains. What is an atmospheric river? The term atmospheric river is defined as narrow areas in the atmosphere that transport significant amounts of moisture to areas outside of the tropics. It’s like a river in the sky, Martell said. While atmospheric rivers can vary greatly in size and strength, the average atmospheric river carries an amount of water vapor roughly equivalent to the average flow of water at the mouth of the Mississippi River, according to the National Oceanic Atmospheric Administration. When the atmospheric rivers make landfall, they often release this water vapor in the form of rain or snow. One particular atmospheric river you may have heard of on the West Coast is sometimes referred to as a Pineapple Express. The jet stream in the sky transports humid air from the Hawaiian Islands toward California. Because the stream is filled with moisture, it can bring torrential downpours to California and heavy snow to the Sierra Nevada in the winter months. The science behind atmospheric rivers. How do atmospheric rivers contribute to annual rainfall? Extratropical storms combined with high wind and high concentrations of water vapor can result in the heavy rainfall that happens when atmospheric rivers make landfall, which can also be associated with flooding, the NOAA said. Dr. Anna Wilson with The Center for Western Weather and Water Extremes at Scripps Institution of Oceanography told NBC 7 via email that rainfall can also depend on the location. On the West Coast, particularly in California, most of the incoming rainfall is associated with atmospheric rivers, and it falls during only a few storms a year, Wilson said. NOAA said about 30-50% of annual rainfall in the West Coast states accumulates with just a few atmospheric river events. Like NOAA, Wilson said atmospheric rivers can provide 50% more of California’s water supply and can cause the most flood-related damages — up to 84% in the 11 western states and up to 99% at the coast. How do atmospheric rivers contribute to climate change? While Atmospheric Rivers are a natural part of our hydrology, NOAA says their frequency and intensity could change because of climate change. According to NOAA research, managing the water produced by atmospheric rivers could become more challenging, plus, researchers also found that the result could mean less rain in mountainous regions, but more rain at lower elevations. Kelly Mahoney, a research meteorologist with NOAA told NBC 7 that yes, climate change does indicate the potential for more intense precipitation events stemming from atmospheric river events. “The changes in frequency of such events tend to depend on what part of the west coast we’re talking about, and isn’t quite as confidently projected due to variability around patterns like El Nino and La Nina — i.e., patterns that affect the placement of large-scale weather patterns like those associated with atmospheric rivers,” Mahoney said via email. Mahoney went on to say that through assessment, NOAA can say that the trends stated above are already taking place. Images courtesy of NOAA Image courtesy of NOAA Are atmospheric rivers common? Yes, atmospheric rivers are very common. “ARs move with the weather and are present somewhere on Earth at any given time,” according to NOAA. Is there a rating system for atmospheric rivers? Yes. Atmospheric rivers are rated using the atmospheric river scale, which has five categories. AR 1: Weak (primarily beneficial) AR 2: Moderate (mostly beneficial but also somewhat hazardous) AR 3: Strong (balance of beneficial and hazardous) AR 4: Extreme (mostly hazardous but also beneficial) AR 5: Exceptional (primarily hazardous) The scale focuses on total expected water transport and how long that available water will be over the same area. This story uses functionality that may not work in our app. Click here to open the story in your web browser.