Effects due to wind shear
The speed of sound varies with temperature. Since temperature and sound velocity normally decrease with increasing altitude, sound is refracted upward, away from listeners on the ground, creating an acoustic shadow at some distance from the source. Wind shear of 4 m·s?1·km?1 can produce refraction equal to a typical temperature lapse rate of 7.5 °C/km. Higher values of wind gradient will refract sound downward toward the surface in the downwind direction, eliminating the acoustic shadow on the downwind side. This will increase the audibility of sounds downwind. This downwind refraction effect occurs because there is a wind gradient; the sound is not being carried along by the wind. For sound propagation, the exponential variation of wind speed with height can be defined as follows: where: = speed of the wind at height , and is a constant = exponential coefficient based on ground surface roughness, typically between 0.08 and 0.52 = expected wind gradient at height In the 1862 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 10 km (six miles) downwind. An acoustic shadow is an area through which sound waves fail to propagate, due to topographical obstructions or disruption of the waves via phenom na such as wind currents. As one website refers to it, "an acoustic shadow is to sound what a mirage is to light". For example, at the Battle of Iuka, a northerly wind prevented General Ulysses S. Grant from hearing the sounds of battle and sending more troops. Many other instances of acoustic shadowing were prevalent during the American Civil War, including the Battles of Seven Pines, Gaines' Mill, Perryville and Five Forks. Indeed, this is addressed in the Ken Burns's documentary The Civil War, produced by Florentine Films and aired on PBS in September 1990. Observers of nearby battles would sometimes see the smoke and flashes of light from cannon but not hear the corresponding roar of battle, while those in more distant locations would hear the sounds distinctly. The American Civil War, also known as the War between the States or simply the Civil War (see naming), was a civil war fought from 1861 to 1865 between the United States (the "Union" or the "North") and several Southern slave states that had declared their secession and formed the Confederate States of America (the "Confederacy" or the "South"). The war had its origin in the fractious issue of slavery, and, after four years of bloody combat (mostly in the South), the Confederacy was defeated, slavery was abolished, and the difficult Reconstruction process of restoring unity and guaranteeing rights to the freed slaves began.