With increasing peak voltages, surface discharge patterns are formed successively on a dielectric target, which include diffuse spots, single rings and concentric triple rings in a system where an argon column hit the dielectric vertically. During the evolution of these patterns, the number of positive discharge increases while the number of negative discharges remains one per voltage cycle. The mechanism of pattern formation is studied using fast photography. It is shown that?the positive and negative discharges in each pattern are caused by streamer mechanism, and the formation of these patterns is originated from the temporal superposition of positive and negative surface discharges. In each voltage cycle, the first positive streamer results in a central point of each pattern; the last positive streamer leads to the formation of diffuse background of each pattern; other positive streamers finally forms rings in patterns. The negative streamers corresponding to each pattern are always diffuse. The propagation characteristics of these positive streamers are qualitatively explained by analyzing the influence of applied electric fields, residual charges and air diffusions.