Atmospheric stability

Stability refers to the resistance of the atmosphere to vertical (up and down) movement. The chance of extreme fire behavior increases as the atmosphere becomes unstable. An unstable atmosphere allows the fire to develop “vertically” and grow rapidly.

Another important indicator of changing conditions is the breaking down of an inversion layer. An inversion layer exists when the atmosphere is stable, and a warmer layer of air tops a colder air mass. When an inversion layer is present, fire behavior below it will normally be suppressed, like the damper is closed on a stove flue. However, conditions will be very smoky for crews who must work below the inversion. As the day begins to warm and the air becomes unstable, the inversion layer will begin to break. As the inversion breaks, fire activity will increase, as if the damper on the stove is opened. This is the time to be alert and cautious.