A firefighter is directing a foam stream over an aboveground open-top storage tank, letting the foam gently fall onto the surface of the liquid. Which foam application method does this describe?

The method being described is known as the rain-down method. In this application technique, the firefighter directs a foam stream over the aboveground open-top storage tank, allowing the foam to gently fall onto the liquid surface. This technique is effective because it minimizes turbulence and helps to create a blanket of foam that covers the flammable liquid, thereby suppressing vapors and controlling potential fire hazards.

The gentle application of foam in this manner is particularly advantageous because it prevents the foam from being disrupted or broken apart upon hitting the surface, maintaining its effectiveness in forming a barrier against ignition. This contrasts with other methods that might use more aggressive application techniques that could disturb the surface of the liquid and reduce the foam's effectiveness.