Climate Fact: Atmospheric Rivers and Precipitation in the West Coast

Did you know that rivers exist in the air? Atmospheric rivers—the Amazons of the air—are vast and unbroken streams of wind that carry water vapor from tropical oceans, moving thousands of miles through the sky. Scientists discovered them in 1998, when they noticed that water vapor traveled from the tropics into mid-latitudes in narrow and intense bands of air. Atmospheric rivers are massive not only in their geographical extent, but in the amount of water vapor they transport. A typical atmospheric river is about 250 miles wide and one mile high, and carries as much moisture as seven to 15 Mississippi Rivers or one Amazon River. You may have heard of the Pineapple Express, a subset of atmospheric rivers originating in the waters near Hawaii that travels to the Pacific coast of North America.

Atmospheric rivers can rain themselves out over the oceans, but many travel over continents, bringing moisture. When atmospheric rivers sweep warm and moist air through a mountainous coastal area, such as the Coast Range and Sierra Nevada, air rises and cools and water vapor condenses into precipitation. Atmospheric rivers are one of the most important sources of precipitation, stream flow and flooding in the West Coast. The Sierra Nevada runoff is used for drinking water, agriculture and hydropower. California obtains one-third to half of its water supply from precipitation due to atmospheric rivers. Atmospheric rivers are also responsible for a great portion of wintertime extreme precipitation in the West Coast. They contribute 30 to 40 percent to the seasonal water equivalent, the amount of water contained within the snowpack. The amount of precipitation falling as snow or rain depends on air temperatures in the mountains; during winter, colder surface air temperatures increase the amount of snow accumulation. As far as scientists can tell, climate change affects atmospheric rivers in two opposing ways: in a warming planet, the difference in temperature between the poles and tropics is getting smaller, which can result in weaker storms. But, warmer air holds more water vapor, which can make atmospheric rivers even moister.

Today atmospheric rivers are easy to spot. Atmospheric rivers became visible to scientists when they started using microwave imagers in satellites. Microwaves are not absorbed by water vapor to the same extent as infrared radiation, facilitating their visualization. Despite their importance and impact, atmospheric rivers don’t generate as much publicity, evacuations or early-warning efforts. The scientific community is currently working on better understanding atmospheric rivers and the mechanisms to predict major weather events associated with them, to establish an emergency preparedness program.

Watch NASA’s animation of atmospheric rivers and NOAA’s video of an atmospheric river moving across the Pacific Ocean below.

NASA: Animation of atmospheric rivers

NOAA: A River of Atmospheric Moisture

(Source: Guan, B., N.P. Molotch, D.E. Waliser, E.J. Fetzer, P.J. Neiman. 2010. Extreme Snowfall Events Linked to Atmospheric Rivers and Surface Air Temperature via Satellite Measurements. Geophysical Research Letters, Volume 37, L20401, doi:10.1029/2010GL044696. and Mackenzie, D. 2013. Atmospheric Rivers: Amazons of the Air. New Scientist 217:38-41. and National Aeronautics and Space Administration. 2013. Study Finds Climate Link to ‘Atmospheric River’ Storms. Accessed online 9 January 2014. and National Oceanic and Atmospheric Administration. Atmospheric River Q&A. Accessed online 9 January 2013.

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