What factor is essential for the effective expansion of foam solution?

The essential factor for the effective expansion of foam solution is aeration. Aeration refers to the incorporation of air into the foam solution, which is critical for creating a stable foam that can effectively smother fires. When a foam solution is aerated, it forms a blanket of bubbles that can cover flammable materials, preventing oxygen from reaching the fire and thus helping to extinguish it.

A well-aerated foam maintains its structure and prevents the liquid from draining too quickly, allowing it to stay on the fire surface for a longer duration and providing sustained firefighting capability. The effectiveness and characteristics of the foam, including its expansion ratio and stability, directly correlate with how well it is aerated.

Other factors like temperature, pressure, and humidity can influence firefighting operations and the performance of foam, but they do not play as critical a role in the aeration process itself, which is vital for achieving the correct foam consistency and effectiveness in fire suppression.