Cold Air Inversion

This is a view of the far western end of Williams Lake on 16 Dec 2009, taken from the road leading up to Fox Mountain.

The low clouds evident in the picture are not natural clouds, but rather industrial emissions from the Epcor bioenergy plant and Soda Creek lumber mills, trapped in the valley by a cold air inversion.

Inversions occur when a warm front pushes over a layer of underlying cold air. Emissions from industrial activity rise until they hit the boundary between the two layers where the density difference between the warm and cold air block circulation between the two, and thus the emissions flatten out. This occurs frequently in the small valleys of British Columbia where many towns are located.

Today in Williams Lake the temperature above this inversion is approximately 13 °C warmer than the air below. Thus, while the recorded temperature early this morning was -4 °C at the airport (above the inversion), it was -17 °C in town. Quite a difference.

Characteristic of an inversion is a near complete absence of air movement in the cooler pooled air. When inversions last for days, significant deteriorations in air quality can occur, especially in communities where industrial activity is juxtaposed with residential areas, and when you have many people burning wood for heat, as in Williams Lake.

The changing density of warm air over cold can also lead to interesting visual affects. If the inversion lasts through the day, you may notice, from town, a distortion in the shape of the sun as it sets. With longer views of the horizon than we normally get in British Columbia, the refractive differences between the cold and warm air can cause a green flash the moment the sun sets. You might recall this phenomena noted in the Pirates of the Caribbean III.

"Ever gazed upon the green flash, Master Gibbs?"
"I reckon I've seen my fair share. Happens on rare occasion; the last glimpse of sunset, a green flash shoots up into the sky. Some go their whole lives without ever seeing it. Some claim to have seen it who ain't. And some say—"
"It signals when a soul comes back to this world, from the dead!"

Or it could mean that a layer of warm air overlays a layer of cold.

Photo Credit:Me