Heat air up enough and it turns into plasma. Plasma's opaque. That's going to do terrible things to your beam focus. I remember hearing that this is a problem

Heat air up enough and it turns into plasma. Plasma's opaque. That's going to do terrible things to your beam focus. I remember hearing that this is a problem with high-energy laser weaponry, which is why it tends to be pulsed, to give the hot air time to dissipate. But I can't find a reference now.