Why is wind shear dangerous to aircraft?

Wind shear, sometimes referred to as windshear or wind gradient, is a difference in wind speed and direction over a relatively short distance in the atmosphere. If the direction is downward, it can cause an abrupt drop in altitude for a plane before the pilot can compensate. Any sudden significant change in wind direction can cause a loss of control for an airplane, potentially causing it to crash. Some wind shear can be strong enough to cause structural damage to the aircraft.