Regular drop formation (humping) along the weld root during laser welding was studied by high speed imaging of the melt pool underneath the workpiece. The formation of droplets towards the rear of this weld pool was seen to be primarily caused by the pumping of melt from the bottom of the keyhole and the influence of gravity drawing melt into a sagging hump. This is a different process from the one which creates humps on the top surface of welds.