Fog, smog, and smoke, their impact on us is determined by an invisible presence in our atmosphere.
In the troposphere, the layer of our atmosphere closest to the surface & where most of our weather takes place, temperature decreases the higher you go. This trend sometimes flips with warmer air above cooler air to create a stable layer of the atmosphere called an inversion. Air is no longer able to freely move throughout the troposphere as this inversion acts like a lid and traps everything below it.
Let’s do a quick demonstration.
As a fluid, both water and air behave similarly so in this experiment we can use water-which I’ve color coded. Yellow for warm. Blue for cold. Take a tube and slowly pour the ice cold blue water to the bottom of the colored warm water and watch what happens.
Each layer stays separate, stagnant as the less dense warm water sits on top of the denser cold water! This is what happens when we’re under an inversion as the cooler air at the surface, with all the pollutants, smoke, and fog is unable to mix throughout the atmosphere- all because of a change in temperature with height!
In extreme cases, especially out west, you can actually see this normally invisible inversion layer as the dividing line between a dense cloud of smog with cleaner, clearer skies above. Here in the Lowcountry, with much less traffic, smoke is the bigger issue with forest fires sometimes causing a haze over the area. Officials in charge of controlled blazes know all too well of this possibility, so they make sure to manage a burn with inversions in mind.
Sean Bowers, a Clemson forestry and wildlife extension agent, explains,
“If we did have an inversion layer we’d take a little more care to mop up, keep any of these hot spots out. It’s important to keep that smoke going up and away so it doesn’t interfere with driver’s visibility or road safety.”
Timing a burn is everything as every day a temperature inversion begins to form around sundown as the ground cools while the air above remains warmer.
This nighttime inversion lasts for only a few hours after sunrise as the sun warms the surface- eroding that temperature inversion and the fog that sometimes stays locked under it.
We can see that beginning to happen in our experiment as the cool water warms. That’s the slow way to get rid of an inversion, there’s another way that’s a lot quicker. Manually mixing it as winds pick up or a weather system approaches.
Understanding when and how long these inversions will last is incredibly important for making air quality forecasts, as well as predicting whether or not severe weather will occur. But that’s a story for another time.
Storm Team 2 Meteorologist David Dickson