Mirage
The shimmering, wavy distortion you see through a scope over hot ground or a heated barrel. It carries wind direction and speed for those who can read it.
When mirage runs at an angle across your scope, it’s telling you wind direction. When mirage boils straight up, the wind is calm, or coming straight at you. When mirage tilts severely and flattens, the wind is faster than you think.
The trick noted in every long-range manual: read mirage at half-distance, not at the target. The shimmer halfway out tells you what the wind is doing where the bullet is most exposed.