cloud plume of an atmospheric river on the west coast

What Is an Atmospheric River?

An atmospheric river is a long and narrow band of water vapor that forms over an ocean and flows through the sky.  Well, that should be simple to see.  But what are these fast moving abundant
plumes of moisture really up to?

An atmospheric river occurs in several areas around the world.  They occur in western Canada, northern Europe, New Zealand, and southern South America.  In these locWest coast with an arrow showing the flow of an atmospheric riverations, they can occur on average 30 to 35 days per year.   On the west coast, they create 30 percent to 50 percent of the annual precipitation and are linked to flooding and mudslides.  These rivers are caused by winds from cyclones and can typically range from 250 miles to 375 miles in width.  A powerful atmospheric river can transport an amount of water vapor equal to 15 times the average flow of water that flows out of the mouth of the Mississippi River.


Who Ordered the Pineapple?

The Pineapple Express has become a famous atmospheric river.  This type of weather feature carries atmospheric moisture and associated heavy precipitation from the waters adjacent to the Hawaiian Islands.  Winds cross over warm bands of tropical water vapor to form this “river,” which then travels across the Pacific as part of the global conveyor belt.  When an atmospheric river such as this reaches the west coast, it can dump as much as five inches of rain on California in one day.  The Pineapple Express is driven by a strong, southern branch of the polar jet stream.   A surface frontal boundary will mark its presence, which is either stationary or slowly moving.  Then waves of low pressure will be traveling along its axis.


How to Turn Moisture into a Flood

Mountain showing the condensing of moisture in an atmospheric river

When moisture-laden air from an atmospheric river moves over mountain ranges, the orographic effect occurs.  Air passing over a mountain oscillates up and down as it moves downstream.   If the air lifts upward and cools through expansion then it will reach a saturation temperature causing the water vapor to condense.  Now one thing they have out west is mountains.  The Sierra Mountains produce heavy rains in the lower elevations and during cold winters deliver a healthy snow pack.  Moisture quickly cools down adiabatically when it gains altitude, which can raise the relative humidity to 100 percent. This, in turn, creates clouds and, under the right conditions, precipitation.   The orographic effect does have a negative side.  It causes the rain shadow effect.  This happens when an area receives little precipitation due to the effect of a topographic barrier, especially a mountain range.  The barrier causes the prevailing winds to lose their moisture on the windward side, making the leeward side to be dry.

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