As WT blades rotate, they experience constantly changing angles of attack due to wind shear and yaw. Under these conditions, a phenomenon known as dynamic stall (as opposed to static stall) occurs. The angle of attack will increase beyond the point at which the blade would normally have stalled in a quasi-static situation in a wind tunnel, but the delayed stall is very sudden and hard when it occurs at some large angle of attack, causing large loads in the blades and significant fatigue.