<p>Well, there's something else to consider here: Zaheer isn't just moving the air around him. He has, for all intents and purposes, <i>become</i> the air around him. He doesn't produce a tidal wave of air around him; from the shots I've seen, it's simply reacting like he's a gust of wind. My hypothesis, then, is that airspeed and drag as we know it wouldn't affect him, and while it defies what we know of even Avatarasya physics, it still seems relatively reasonable.
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