"Rends air" means to tear or rip the atmosphere. Synonyms for this phrase include "pierces the air," "cuts through the air," "slices through the air," "severs the atmosphere," and "rips the sky." These phrases can be used to describe a variety of situations, from the sound of a gunshot to the roar of a fast-moving vehicle. Each of these synonyms captures the sense of a disturbance in the air, either through sheer force or through the sudden movement of an object. They can be used in writing to create a more vivid and evocative image of a scene.