Which foam application method is the least effective when burning fuel is producing a strong thermal column?

The rain-down method is the least effective foam application technique in situations where burning fuel is generating a strong thermal column. This method involves dispersing foam from above, allowing it to fall onto the fire like rain. When a thermal column is present, the intense heat creates a powerful updraft that can carry the foam away before it can effectively suffocate the flames. The rising heat above the fire can prevent the foam from reaching the burning surface, reducing its effectiveness in controlling and extinguishing the fire.

The other methods, such as the bank-down, roll-on, and liquid-foam methods, are designed to apply foam more directly onto the flaming surface or to create a blanket over the fire, which facilitates better coverage and extinguishing capability in high-temperature situations. These methods are more effective in overcoming the dynamics created by a strong thermal column, making them preferable choices in emergencies involving significant heat and fire.