Weather Watch

Wondering which way Alamo Fire smoke will blow? The answer may not be so simple

A pyrocumulus cloud rises from the smoke plum from the Alamo Fire east of Santa Maria.
A pyrocumulus cloud rises from the smoke plum from the Alamo Fire east of Santa Maria. Special to The Tribune

Esteemed Tribune photographer and columnist David Middlecamp made an inquisitive observation Friday afternoon in regard to his coverage of the Alamo Fire northeast of Santa Maria.

“It was interesting to feel the wind at my back at ground level from the ocean and see smoke from the top of 3,000-foot Los Coaches Mountain blowing the opposite way toward Nipomo at Alamo Fire on Friday,” he said.

Middlecamp and others have written me asking about this phenomenon. Not only can you see it occurring with smoke, but also subtropical moisture that arrives from the south and cold fronts from the northwest. In other words, cold fronts along the Central Coast often move in from the northwest despite the surface winds coming out of the southeast. It’s believed that Ben Franklin may have been the first person to understand that weather systems move in a different direction than surface winds might indicate. He was probably the first person to hypothesize the existence of the jet stream.

So why does this happen? As Bob Dylan would say, “The answer, my friend, is blowin’ in the wind.”

Nature never likes anything out of balance, and air will naturally flow from areas of high pressure to areas of low pressure. When that happens, it’s called the wind. The eastward rotation of the Earth on its axis deflects the moving air away from its initial course in the free atmosphere or planetary boundary layer. This layer lies above the frictional influence of the Earth’s surface. The apparent force responsible for turning the wind flow is called the Coriolis Effect.

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At the earth’s surface, friction with lakes, oceans, trees, buildings, canyons and mountains reduces the wind speed, which in turn decreases the Coriolis Effect, resulting in greater differences in wind velocities from those higher in the sky. Friction also causes turbulence, which can lead to even greater differences from the upper-level winds. To make matters more complex, areas of high pressure will cause air to sink downward, while low-pressure systems will cause the air to rise in the atmosphere.

Another layer completely is the fact that these giant fires can generate their own weather, producing mushroom-cloud-like formations called pyrocumulus clouds — which occurred with the Alamo Fire. Heat from a wildfire or volcano causes the air to rise in the atmosphere because it’s less dense, in much the same way a hot air balloon raises. As the air rises into the sky, it cools to the point that water vapor condenses to form visible clouds. Air rising inside the cloud can trigger thunderstorms and gusty and unpredictable winds that can be life threatening, especially for firefighters. These convective storms can contain areas of organized rotation a few miles up in the atmosphere. Under the right conditions, these thunderstorms can spin out tornadoes, which occurred in April 1926 when the Tank Farm Fire in San Luis Obispo erupted.

A rough average height of the free atmosphere for much of the Central Coast is a few thousand feet, depending upon the surface topography. Smoke, ash and embers from extremely hot wildfires can rise miles up, which can be transported for hundreds of miles depending on the atmospheric stability.

So how can you predict which way the smoke will blow?

Every morning, the 30th Weather Squadron at Vandenberg Air Force Base launches weather balloons with tiny transmitters called radiosondes. As the weather balloon climbs through the atmosphere, its transmitter broadcasts back to the receiving station readings on temperature, dew point temperature, pressure and GPS coordinates for the winds.

When they reach 100,000 feet, the balloons expand to more than 40 feet in diameter, and by 110,000 feet they usually rupture. When they pop, the radiosonde — about the size of a milk carton — comes floating back to earth on a small parachute. When hiking in some of our more remote areas, it’s not uncommon to come across one of these little white radiosondes.

The wind and other atmospheric data from these weather balloons can be viewed on the website of the National Weather Service office in Los Angeles/Oxnard at www.wrh.noaa.gov/lox on the “Upper Air” tab.

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PG&E has begun daily aerial fire detection patrols across hundreds of miles of its service area. The patrols are to assist the U.S. Forest Service, Cal Fire and local fire agencies with early fire detection and response this summer. To lean more, visit http://www.pgecurrents.com for more information.

John Lindsey’s column is special to The Tribune. He is PG&E’s Diablo Canyon marine meteorologist and a media relations representative. Email him at pgeweather@pge.com or follow him on Twitter @PGE_John.

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