Immediately after turning on the fan at the top to generate an updraft, it was found by the visual observation that cloud formation started at a height of 35 m from the bottom. This was regarded as the cloud base height. There was no appendant horizontal tunnel opening on the shaft walls up to the height of 35 m from the bottom; hence there was no water vapor supply to the updraft after it left the bottom. Therefore the cloud formation was solely due to a rise in relative humidity caused by the temperature drop, which was brought about by the adiabatic expansion in the updraft.