A new weather observation station will be constructed at Bodega Bay to monitor low-lying streams of moisture-laden air that sweep ashore and account for half of the rain that falls in coastal California.

They are in scientific terms "atmospheric rivers," and are a well-known but little understood concept.

"Think of a river on the ground, it is a channel where the water is flowing ... this is a channel in the sky where intense water vapor is flowing," said Allen White, a researcher with the National Oceanic and Atmospheric Administration in Boulder, Colo.

The atmospheric river that typically comes to mind is the so-called Pineapple Express, which forms near Hawaii and is a veritable freeway for wet storms that can soak the coast and in some years cause the Russian River to flood.

"The really dangerous thing is when the cold front stalls, and the river stays in place for a long time," White said. "That happened this year in Southern California. The river stalled and you had 28 inches of rain in three days."

These rivers of cold air come in from the west, dropping moisture as they rise up over the coastal mountains, the reason that Cazadero gets three times more rain than Bodega Bay, White said.

The first research data on the phenomenon has been provided by an unmanned weather station operated for a dozen years by NOAA at UC Davis' Bodega Marine Lab, gathering wind speed, temperature, moisture and atmospheric pressure.

It will be replaced by a newer and more efficient station at the lab, with an enhanced ability to see rain and wind that even weather satellites can't identify.

Other stations will be built at Eureka, Point Sur and Santa Barbara, creating a picket fence of observatories along the California coast, White said.

The new stations have radar that is more sophisticated to measure what's happening in the first few miles of the earth's atmosphere, where these atmospheric rivers are found.

It will provide real-time data for weather forecasters and at the same time let NOAA track climate change.

The $3 million project is being funded by California Department of Water Resources, which is interested in the data to forecast rain for water availability.

Scientists at the Bodega Marine Lab also use the data in their studies of the Pacific Ocean.

For example, these cold streams of air as they sweep over the ocean have a profound effect on ocean upwelling, the rise of water from the ocean floor to the surface that brings up food crucial for fish and other aquatic life.

"Some of the small pelagics, like the anchovies, when upwelling is too strong the young are washed offshore, but the more upwelling, the more nutrients, the more you fertilize the ocean and there is more food for everything," said John Largier, a marine lab researcher and professor. "You can't get too much of a good thing."

The first of the weather observing stations will be built this year in Eureka, with the others, including at the Bodega Marine Lab, built by mid 2013.