Atmospheric Rivers: The Basics

By Asha Kannan

Though California’s rainy season is over, our winter rain was generated not just by the average storm, but rather through a phenomenon known as atmospheric rivers. Atmospheric rivers (ARs) are long, narrow areas in the atmosphere that carry water vapor. They are the largest carriers of freshwater in the world. When the rivers reach land, the water vapor is usually released as rain or snow. About half of California’s water supply comes directly from atmospheric rivers, with about 5-6 ARs occurring each winter. Typically, California falls into droughts with a lack of ARs, with the drought only ending when an atmospheric river provides rainfall.

While much of California’s rain comes from ARs, this state isn’t the only place that is affected by this phenomenon. Atmospheric rivers typically occur outside the tropics in the Pacific and Atlantic Oceans, releasing water vapor on the west coasts of North and South America. Other areas that experience rainfall from atmospheric rivers are Greenland, Antarctica, and the southern US.

The mechanisms behind atmospheric rivers are a bit more complex. When ARs move inland, cooled water vapor rises to create increased precipitation. The air is lifted under conditions such as intense humidity and strong (but low-level) winds. These heavy rains are moved over mountains, where the sloping geography of the landscape can cause floods and mudslides easily. ARs can be present on Earth at any given time since they move according to the weather.

Due to the prevalence of atmospheric rivers, especially in the western United States, scientists have been able to successfully collect information critical for projecting future weather patterns. One way scientists can predict how much rain or snow will result from an AR is by studying fossil fuels. Nuclei centers in rain droplets can form when exposed to high levels of salinity, or human-made substances like fossil fuels. Observing the sources of the nuclei, such as if they were found on a man-made substance versus a natural environment, can be a determining factor. For example, if the source of the particle is man-made, scientists can generally predict a lesser amount of precipitation from the AR.

Although ARs can be considered deadly weather patterns, wiping out towns with aggressive flooding and winds, the scientific implications of this phenomenon can actually be considered beneficial, not just for research purposes, but also for California’s water cycle.

Sources:

• https://www.noaa.gov/stories/what-are-atmospheric-rivers

• https://ghrc.nsstc.nasa.gov/home/micro-articles/atmospheric-rivers

• https://water.ca.gov/News/Blog/2020/October/Atmospheric-Rivers-and-Their-Impact-on-California-Reservoirs

Written by Asha Kannan