In the 1862, during the American Civil War Battle of Iuka, an acoustic shadow, believed to have been enhanced by a northeast wind, kept two divisions of Union soldiers out of the battle, because they could not hear the sounds of battle only six miles downwind. Wind engineering is a field of engineering devoted to the analysis of wind effects on the natural and built environment. It includes strong winds which may cause discomfort as well as extreme winds such as tornadoes, hurricanes and storms which may cause widespread destruction. Wind engineering draws upon meteorology, aerodynamics and a number of specialist engineering disciplines. The tools used include climate models, atmospheric boundary layer wind tunnels and numerical models. It involves, among other topics, how wind impacting buildings must be accounted for in engineering. Wind turbines are affected by wind shear. Vertical wind-speed profiles result in different wind speeds at the blades nearest to the ground level compared to those at the top of blade travel, and this in turn affects the turbine operation. This low level wind shear can create a large bending moment in the shaft of a two bladed turbine when the blades are vertical. The reduced wind shear over water means shorter and less expensive wind turbine towers can be used in shallow seas.