At 6:15 p.m. Monday, a fire at the Chevron Refinery in Richmond sent huge black plumes of smoke billowing into the air, forcing thousands of nearby residents to stay in their homes.

While Marin residents saw the dark, ominous plumes less than 12 miles away, the incident was more a curiosity than immediate cause for concern.

Marin was helped by the predominant winds that blow from west to east, said Bob Benjamin, meteorologist with the National Weather Service.

"That's why Marin didn't have to deal with gaseous fumes coming over the county," Benjamin said.

The meteorologist said the primary reason winds blow from west to east is the Pacific high pressure system over the ocean that exists throughout most of the year. Air molecules from the high pressure rush to low pressure areas on land, producing winds moving east. Along the way the winds bend a little to the right because of the Earth's rotation, a phenomenon known as the Coriolis Effect.

"Every once in awhile, when it gets warm along the coast, it will flow the other way, but that only happens a few times a year," Benjamin said.

And that wasn't the case Monday evening, when winds were blowing at about 15 mph east and a little to the north.

A Bay Area Air Quality Management District air quality monitor in San Rafael showed minimal impacts from the fire, with pollution levels below the federal health standards, said Kristine Roselius, a district spokeswoman.

And

closer to the fire, weather conditions were favorable at the time of the incident with the light surface winds pushing the smoke upwards, where stronger winds aloft helped to disperse the smoke, she said.

While some people reported smelling smoke in Marin, Roselius noted human noses are far more sensitive than air monitors.

"The smoke was not a threat to Marin," she said. "You can thank the wind for that."

Contact Mark Prado via email at mprado@marinij.com

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©2012 The Marin Independent Journal (Novato, Calif.)

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