When lining up a shot, a sniper must calculate a number of variables, including the distance to target, and the wind speed and direction. He must then estimate the effect these variables have on the trajectory of his bullet and adjust his sights appropriately. These adjustments are based on mathematical calculations every sniper learns during training, which become almost second nature to a marksman once in the field.
Distance to target is the first major consideration for any sniper. Gravity affects even a bullet in flight, causing it to drop toward the ground as it flies. In some cases, a laser or infrared rangefinder may provide precise distance data, but in others, the sniper must estimate the distance by evaluating the relative size of his target and magnification of his scope. Then, he must adjust his aim upward to compensate for gravity's effect.
Windage is another major factor. A sniper can easily judge the wind direction and speed at his location, but the wind conditions between his position and a distant target may be substantially different. A sniper must look for clues to determine possible differences in wind speed and direction, such as blowing dust and vegetation, in order to compensate by adjusting his aim.