The diurnal cycle of radar echo data observed at Om Koi, northern Thailand, during three rainy seasons (May to October, 1998, 1999 and 2000) are analyzed. From May to July when southwesterly monsoons blow over the observation area, the diurnal cycles in the leeward regions (inland of Thailand) show a phase delay that corresponds to the distance from the mountains. In October when the wind direction reverses and blows from the east or northeast, inland regions correspond to the upwind side of the mountains and do not show clear phase shifts. The regions near the coast of the Andaman Sea, which is the leeward in October, show a similar phase shift of the diurnal cycle as the inland area during May--July. The echo tracking analysis reveals that echoes, which are triggered near the mountains and moved to leeward regions, caused phase delays, which depended on the lee distance.