Arc Blow in Welding refers to the phenomenon in which the arc does not trace the shortest path between the electrode and the workpiece. The arc gets deflected away from the desired path. This is due to the interaction of the magnetic field of the welding current with residual magnetic field which may be present in the metal.
Arc Blow occurs only in DC welding. In AC welding, since the current and the magnetic field of the arc reverse direction many times a second, there is no net deflection of the arc.