The following steps are considered to proceed in the shaft: i) When the fan at the top of the shaft is turned on and an updraft is generated, the ascending air gets cooled by expansion. At a certain height from the bottom of the shaft, the air becomes cool enough so that water vapor in the air starts to form droplets. The air in the updraft is continually cooled, while the droplet temperature follows the cooling through the heat exchange process. Therefore the droplet surface temperature, T L stays higher than the air temperature, T 1 :