The cloud chamber is filled with isopropyl alcohol vapor. The chamber is warm at the top, extremely cold at the bottom. At a certain height, the temperature is low enough that the vapor will condense to make droplets, a cloud, if there is something for a droplet to form on, a nucleation site. This can be a dust particle, or the ions produced by ionizing radiation traversing the chamber. This is the source of the trails of cloud that appear in the chamber; these are the tracks of high-energy charged particles. If you compared the track of an alpha particle and a beta particle, the track of the alpha particle would be , but the track of the beta particle would be .