Trace
Also: Vapor trail
The visible streak of disturbed air a bullet leaves behind in flight. Visible through a spotting scope under the right light conditions.
Trace is one of a spotter’s most useful tells. By watching where the bullet’s path curves against the background, an experienced spotter can read wind in real time. The line of trace is the bullet’s actual path, and any deviation from straight tells the wind story between shooter and target.
Trace reads best against a uniform dark background, in low-angle sunlight, with crisp atmospheric conditions. Heavy heat shimmer can wash out the trace and make it hard to follow.