The present paper describes the upward motion, the interaction and the impingement heat transfer to the upper wall of two thermal plumes from two heated sections on a horizontal plate. Numerical analysis was performed for dimensionless pitch Pi = 0.26, 0.50, 0.76 and 1.0, Grashof number Gr = 10(3)-10(6) and Pr = 170. Numerical results show that the upward motion is divided into three patterns depending on the pitch of two heated sections. The first pattern is the unification of two thermal plumes like one for short pitch. The second one is the separate upward motion after the unification. The third one is the independent motion for long pitch. These patterns were investigated by visualization of thermosensitive liquid crystal suspension. The impingement local heat transfer on the upper wall is evaluated transiently for various Grashof numbers. Mean Nusselt number depends on Gr(-0.16). (c) 2005 Elsevier Ltd. All rights reserved.