In order to elucidate water vapour behaviour in soil in arid regions due to the diurnal cycle of meteorological conditions, column experiments using a saline sandy soil from Western Australia were undertaken under controlled atmospheric conditions. The upper surface of the sand column was exposed to conditions of constant temperature (25 °C) and constant relative humidity changed stepwise from 20 % to 65 % and vice versa with or without applied radiated heat. Furthermore, experimental cases in which the heat application only was given stepwise and stopped under the 65 % relative humidity atmospheric condition, corresponding to daytime and night-time, were also conducted.
 When only atmospheric humidity was changed stepwise, the soil vapour density, ρ_v, followed a function of x/√t(x : depth, t : elapsed time). This characteristic, and related results were explained through a modified vapour diffusion equation in ρ_v under an isothermal condition proposed in this paper. Comparison of experimentally determined vapour diffusivity with a model calculation showed that evaporation and condensation within the profile significantly retarded the movement of water vapour density profile as a breakthrough curve.
 The above obtained characteristic behaviours of vapour water are important in understanding the water transfer between the ground surface and atmosphere as well as that in the soil profile, in arid areas.