Wind Gradient
The change in wind speed with height above the ground or across terrain, meaning the wind at the target can differ from the wind at the muzzle.
Wind gradient is the way wind speed varies from place to place along the bullet’s path, most notably with height above the ground. Friction slows the air near the surface, so wind is typically lighter close to the dirt and stronger higher up, and a bullet arcing through its trajectory passes through layers moving at different speeds. The same gradient appears horizontally where terrain channels or shelters the flow, so a valley, ridge, or treeline can hold a different wind than open ground.
The practical lesson is that the wind value you measure at your position is only a sample, and the air the bullet actually crosses may be faster or slower downrange. Good wind reading builds a picture across the whole course rather than trusting one flag, using cues like mirage at distance and watching how vegetation responds at different ranges. Vertical air movement from thermals layers on top of the horizontal gradient to complicate the read further.