Thermals

Thermals are columns of moving air driven by uneven heating of the ground. Sun-baked rock, dark fields, and pavement warm the air above them until it rises as an updraft, while shaded draws and water can chill air into a sinking current. These vertical motions are easy to overlook because they do not show on a flag, yet over a long flight they can nudge a bullet up or down enough to miss a small target.

Thermals interact with horizontal flow, so the wind gradient you read near the muzzle may not describe the air the bullet crosses downrange. The best visual cue is mirage, which rises and shimmers vertically when thermal activity is strong, and the same heating that drives thermals also shifts density altitude through the day. Folding thermal activity into your wind reading is part of explaining vertical surprises that pure crosswind cannot.