Early risers got a good whiff of smoke in the Animas Valley this morning, an alarming smell that may have evoked fearful memories for some in La Plata County. But not to worry – fire officials said it’s no more than a sure sign of spring.
Some La Plata County residents were up as early as 5:30 a.m. this last Saturday of April to scorch land around the county, said Jeff Harris, Durango Fire Protection District battalion chief. People do controlled burns for all kinds of reasons, and not everyone has to report them when they’re lit. A lot of it is done in spring, Harris said, when the fire danger is low. It’s an almost annual occurrence.
But the fires created a haze of smoke visible from downtown Durango eerily reminiscent of the 416 Fire that burned 54,000 acres last summer. It hung above the Animas River and obscured the bluffs of Missionary Ridge – a translucent fog undisturbed by wind and kept low in the valley by the brisk morning air.
The temperatures in the Animas Valley dipped to 36 degrees Fahrenheit on Saturday morning, said Tom Renwick, forecaster with the National Weather Service in Grand Junction. A calm wind blew at less than 5 mph until 9 a.m. It picked up to about 7 mph as the sun got higher in the sky – a “light breeze,” technically, just enough to point weather vanes, wisp hair and rustle leaves.
“A lack of a real breeze in the morning and the colder temps block the smoke from rising up and away from the ground,” Harris said. “Essentially, what happens there is the atmosphere is blocking the smoke from going up and spreads out into the valley. That’s why more people notice it. It’s not going up and escaping.”
What the chief is explaining is how agricultural practices are impacted by a meteorological phenomenon called surface-based temperature inversion.
So what does that mean? To understand what a temperature inversion is, it’s important to know how the atmosphere works. Air is held to Earth by gravity, creating an atmosphere most dense and warmest closest to the surface and less dense and cooler as it reaches into space.
But air, like any form of matter, absorbs energy – it’s why an egg cooks in a frying pan over fire or the sun feels warm on your skin. Air and water share similar physical properties – cold is more dense than warm, meaning warm rises and cold sinks. That’s why deeper parts of lakes and reservoirs are colder than the surface.
When the sun rises in the morning, the mountains often block the sun from shining in the valleys. The cool air rests in the valley as the air above it was warmed by the sun, reversing the atmospheric principle that air gets cooler with altitude. Cold air doesn’t rise.
The cool air, because it’s more dense, gets trapped in the valleys until the sun rises high enough to warm it. Without any air escaping the valley, anything in it stays there, too, including smoke.
A lot of controlled burns in La Plata County means a lot of smoke. That’s why it smelled like a campfire Saturday morning.
bhauff@durangoherald.com
