The experimental facilities used have been described elsewhere in detail ; therefore only an outline is given here. The experiments were carried out in a shaft in a mine operated by Kamaishi Mining Co. in Iwate Prefecture, Japan. A schematic diagram of the shaft is shown in Fig. . It was formerly used to mine metal ores, but had been abandoned for many years until we started the project. The length of the shaft was 430 m; the top was at an altitude of 850 m above sea level. The cross-section was a rectangle with sides of 5.5 × 2.8 m. There was a horizontal tunnel leading to the bottom of the shaft (the lowest tunnel), while at the top the shaft was connected to another horizontal tunnel that extended out (the highest tunnel). Along the lowest tunnel, there were streams of abundant underground water, so that the relative humidity of the air stayed very close to 100%. There were a few appendant horizontal tunnels opening on the shaft walls up to the top. No underground water flow was found in any of these tunnels; therefore, water vapor was supplied only in the lowest tunnel. A 30 kW electric fan was placed in the highest horizontal tunnel to create an updraft with a velocity of about 0.7 m/s, and the updraft created artificial clouds in the shaft. The fan was located about 10 m from the top of the shaft. A series of ladders were mounted on one of the walls from the bottom up to the top; these ladders allowed us to observe the cloud formation at any height. The height of the cloud base was measured by visual observation from the ladders. The droplet size, temperature, and humidity measurements were taken at the top of the shaft to investigate the conditions for cloud formation. The thermometer was placed about 22 cm from one of the walls. The hygrometer was placed about 1 m from the wall, and the droplet size measurement was also taken at this location.