What condition is indicated by the third common denominator relevant to fire behavior?

The third common denominator relevant to fire behavior is indicated by a sudden change in wind direction or speed. This condition is critical because wind is a primary factor that can significantly influence the spread and intensity of a fire. A sudden shift in wind dynamics can lead to drastic changes in fire behavior, making it unpredictable and potentially more dangerous, as it can cause flames to spread rapidly in new directions or increase the intensity of the blaze.

When a sudden change in wind occurs, it can also lead to spot fires or cause embers to be carried further away from the main fire, which can complicate control measures. Understanding and monitoring wind patterns is essential for fire control workers, as it plays a crucial role in determining fire line strategies and safety measures.

The other conditions mentioned, such as a consistent wind speed and direction, the absence of any wind, and steady rain, all relate to fire behavior, but they do not carry the same level of immediate threat and unpredictability as a sudden change in wind. A consistent wind can still help predict fire spread, while the absence of wind or rain generally aids in controlling fires.