What is the primary characteristic of fire patterns in structural members?

The primary characteristic of fire patterns in structural members is indicated by arrow patterns that show the direction of fire spread. These patterns are crucial for investigators because they can reveal how the fire traveled through the structure, providing vital clues about the fire's origin and the materials involved. Arrow patterns may develop as a result of various factors, including ventilation influences, the presence of combustibles, and airflow dynamics, which can lead to uneven burning and distinct directional markings on the surfaces.

Understanding these arrow patterns allows investigators to reconstruct the sequence of events during a fire, which is essential for determining the cause and understanding the fire's behavior within a structure. This analysis helps in pinpointing the area of origin and assessing how the fire may have progressed, which is critical for both fire investigation and fire prevention strategies in the future.

The other choices present characteristics that may be observed under specific conditions in fire scenarios, but they do not represent the primary and most significant aspect of fire patterns in structural members as clearly and importantly as the arrow patterns do.