Precipitation measurements from the TRMM satellite indicate that the southeastern area of Kalimantan (Borneo) Island receives much less rainfall than elsewhere on the island during July to October. Results from surface meteorological observations show that the diurnal cycle of rainfall differs greatly between the eastern and western coasts of the island. Rainfall at the western coast of the island is frequent in the afternoon and evening, whereas almost all rainfall at the eastern coast occurs in the morning. Meanwhile, the GPS-derived precipitable water (PW) at the eastern coast shows a substantial decrease of moisture in the afternoon and evening. Numerical experiments with a mesoscale model reveal that gravity waves driven by diurnal heating of the elevated land surface of the mountains on Sulawesi Island, which lies approximately 300 kilometers to the east of Kalimantan Island, significantly affect the diurnal cycle of rainfall over southeast Kalimantan Island.