“Atmospheric rivers” of water vapor in the sky

Dear Tom,
I was in the Bay Area last week and the weather news was all about an atmospheric river driving rain across the area. Can you tell us more about it?
— Michael Nolan Chicago
Dear Michael,
NOAA, the parent agency of the National Weather Service, describes atmospheric rivers as flowing columns of water vapor that are identified by satellites. They are usually about 250 to 375 miles wide and found in the lower levels of the atmosphere, often around the 5,000-foot level. They are a major feature of the global water cycle and are found throughout the world. In the U.S., they are responsible for up to 50 percent of the annual precipitation on the West Coast, which is produced by orographic lift when the column of moisture moves onshore and is lifted and cooled as it interacts with area’s mountain ranges. The strongest atmospheric rivers can produce extreme amounts of rain and snow.

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