By Michael Condon
Former Fire and Aviation Chief for the Plumas National Forest
Special to Plumas News
What does it mean when a smoke column collapses and why is it a big concern for fire fighter?
Unless you go to work wearing green and yellow Nomex clothing everyday you probably have never heard the term “column collapse,” at least not before the Dixie Fire. But now it seems to be something we are talking about every day.
So just what is a “column collapse?”
Actually there are two kinds of column collapse. There is your garden variety column collapse that occurs on most fires and then there is far more dramatic variety that occurs when a pyrocumulus cloud forms over the top of the smoke column.
Let’s consider the garden variety first. A smoke column is a column of hot air, gases, particulates, and partially burned debris pushed up into the air by the heat of the fire. The heat of the fire overcomes the forces at ground level that normally move the air laterally along the surface of the ground. At some point, the heat diminishes or the winds overcome the lifting forces of the fire and the column will lean to the side.
The first thing to fall out of the column, because of its weight, is the burning embers that were carried high into the air. Had the column continued to lift those embers, they might have stayed aloft until completely burned, or they might have been high enough in the air that they cooled off during their long decent. But with the collapse of the column, those embers will reach the ground while still burning in greater numbers than they otherwise would have.
The embers hit the ground and start spot fires; sometimes lots of spot fires. With a large column collapsing due to a strong wind, those spot fires can be a long distance; a mile or more, from the main fire.
So why is a pyrocumulus column collapse different? A pyrocumulus (the name we commonly use in place of the more proper name with way to many syllables “pyrocumulusnimbus” or “flammagenitus”) is really a lightning storm caused by a fire. The dynamics in this column are more complex and more of a concern for fire fighters.
Pyrocumulus clouds forms when an intense fire sends smoke very high into the atmosphere. Pyrocumulus clouds typically reach 20 to 40 thousand feet above the earth. Turbulence in the atmosphere causes cooler air to mix into the plume. When it reaches high enough, lower air pressure will cause it to cool and form a cloud. Water vapor condenses and causes rapid cooling which creates down-bursts.
Even though the column hasn’t even collapsed yet, it can cause problems for the firefighters. Those down-draft winds can be very strong just like any other down-burst in a lightning storm. They are unpredictable. The air is usually dry by the time it hits the ground. These down-drafts cause the fire to burn hotter and spread in unpredictable directions.
If that wasn’t enough of a problem, what goes up must come down, and eventually the pyrocumulus cloud and column collapse.
Imagine a large water balloon high in the air. The energy that is holding it up is the heat energy from the fire. Now imagine there is no longer enough energy to hold the balloon and it comes crashing to the ground. The balloon breaks upon impact and the water splashes out in every direction with a great deal of force. That is just like a pyrocumulus column collapse. Only instead of water, it is wind that hits the ground and blows rapidly in every direction. It is much more sudden and much stronger than the downdrafts that were happening before the column collapse. The increase in fire intensity and spread can be very dramatic. It can carry fire over control lines and create hazardous conditions.
Firefighters are trained to watch for these conditions and they know what to expect. Incident meteorologists and fire behavior analysts monitor these situations very carefully and alert firefighters on the ground in order to mitigate any danger. But even when they know what is coming, these column collapses can still create challenging conditions on the fire line.