When using hydraulic ventilation, the fog pattern should fill _________ of the window opening to be effective.

The effectiveness of hydraulic ventilation hinges on the proper deployment of water in a fog pattern. When aiming for an optimal flow to control smoke and heat from a structure, filling 85%-90% of the window opening creates sufficient coverage to enhance ventilation efficiency.

This coverage allows the fog pattern to effectively mix and interact with the hot gases and smoke, pushing them out of the structure while drawing in cooler air. It ensures that the water droplets are widespread enough to absorb heat and create a cooling effect. At this percentage, there's a balance that prevents excessive turbulence and enables smooth airflow through the opening, maximizing the overall ventilation process.

Filling less than this range may lead to inadequate air movement and may not effectively remove heat and smoke, while going above this range could lead to oversaturation and reduced effectiveness of the method. Thus, utilizing a fog pattern that fills 85%-90% of the window opening is key to implementing successful hydraulic ventilation during firefighting operations.