Weld pool surface is believed to contain sufficient information to determine the weld penetration from which skilled welders are able to control the welding process to desired penetration states. However, it is unclear how human welders extract the weld penetration from the observed weld pool surface. In this paper, a novel method is proposed to determine the weld penetration based on the dynamic change of the weld pool surface. This study observes/measures/analyzes the development of a weld pool from partial to full penetration and correlates such change to the weld penetration. Ideally, similar trends in the weld pool surface should be observed when the weld penetration changes from partial to full penetration despite the amperage used and material welded. Correlating the weld pool surface reflecting grayness and the development of the weld penetration from experiments shows: (1) the weld pool reflection intensity will increase while the weld penetration is increasing; (2) the increasing trends of weld pool reflection intensity will decrease when the full penetration is achieved; (3) the weld pool reflection intensity will increase after the full penetration is achieved. Such trend in the weld pool surface reflection intensity when the weld penetration develops can facilitate future work on automatically detecting and control the weld penetration using the weld pool surface as the feedback.