The science behind dust devils

pgeweather@pge.com.July 20, 2013 

The dark asphalt playground at Mark West Elementary School in Santa Rosa, Calif. was a great spot for dust devils. As a young student, I would chase these vortexes of spinning dust with the hope of becoming airborne. It was like the scene from the Wizard of Oz as Dorothy’s home is lifted by a tornado and transported to Munchkin Country.

These energetic swirls that look like small tornadoes can reach more than 3,000 feet in height with winds of more than 60 mph, and can last on the order of an hour or more. Typically though, they are short-lived and generally don’t extend much more than 100 feet into the air with winds around 25 mph.

Dust devils most often develop when you have uneven heating of the Earth’s surface, such as the interface between pavement and dirt roads or irrigated fields and dry land.

Calm to light winds and clear and sunny skies are also important ingredients.

As the sun rises, it begins to heat the ground and the air immediately above it becomes warmer. As the air warms, it becomes less dense and, like a hot air balloon, begins to rise.

When the updrafts of warmer air move toward the sky, the cooler air around it rushes in to fill the void.

This rush of cooler air can cause a spin and form a vortex. As the warmer air continues to rush upwards, it often elongates and narrows the dust devil. That causes it to spin faster, like the increased spinning of ice skaters as they bring their arms in toward their bodies.

The faster-moving air in the center of the dust devil produces lower air pressure and causes even more air to be drawn in. As the air rushes in along the ground, it picks up dust and debris, causing visible whirlwinds.

It turns out that dust devils are much more common than we originally thought.  The eruption of Mount St. Helens in Washington spread a layer of volcanic ash over a large area. When the weather turned sunny, hundreds of dust devils could be observed as they picked up the light, powdery ash. Dust devils have even been observed on Mars. NASA’s Mars Exploration Rover Spirit took a series of photographs of a dust devil on the Martian surface.

The City of San Luis Obispo has begun partnering with PG&E this week on a citywide effort to replace its streetlights with LED fixtures. The city will save more than $100,000 in energy costs, receive nearly $150,000 in PG&E rebates and reduce energy consumption equivalent to reducing carbon dioxide by 400,000 pounds a year. To lean more, visit www.pgecurrents.com

John Lindsey’s column is special to The Tribune. He is a media relations representative for PG&E and a longtime local meteorologist. If you have a question, send him an email at pgeweather@pge.com.

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