A remarkable increase in moisture frequently occurred in August afternoon radio soundings in 2001 in the layer up to 1000 m above the mixed layer (ML) near the mountains of western Sumatra, Indonesia. This moisture enhancement was also apparent in the monthly mean diurnal cycle. The mixing ratio is not vertically uniform in this layer, suggesting that turbulent mixing cannot be a major mechanism of the vertical moisture transport. A climatological numerical study using a cloud-resolving model suggests that thermally-induced upslope winds converge over the mountain summits during daytime, forming a moist air band along the mountain range. Ambient winds above the mountain range then advect the moist air into the surroundings, moisturizing the air above the ML over the leeward terrain. This mechanism is important for producing diurnal mesoscale precipitation systems over a wide area of the Indian Ocean, as documented by previous studies analyzing TRMM and GMS observational data.