What visual indicator suggests an unstable atmosphere during wildfire conditions?

The presence of dust devils and fire whirls serves as a clear visual indicator of an unstable atmosphere during wildfire conditions. These phenomena occur when the air near the surface is warmed significantly, causing it to rise quickly. As the warm air ascends, it can create strong updrafts that lead to the formation of dust devils, which are small, rotating columns of air that lift debris and dust from the ground.

Fire whirls, or fire tornadoes, are similar in that they are formed as intense heat from a wildfire creates a powerful updraft. The combination of rapid vertical air movement and other environmental factors, such as wind shear, can produce these swirling columns that can be quite dangerous, both for the fire behavior and for firefighting efforts.

In contrast, good visibility, heavy rain, and stable air conditions do not indicate instability. Good visibility suggests a more stable atmosphere, where air is moving steadily without much turbulence. Heavy rain often stabilizes the atmosphere, leading to cooler conditions that dampen fire behavior. Likewise, stable air conditions are characterized by a lack of vertical movement, making it less likely for phenomena like dust devils or fire whirls to develop.