The physics behind clouds can actually be a whole profession. It turns out clouds are much more complicated then most people may think. At the base of it, clouds are made up of microscopic water droplets and or tiny crystals of ice. These droplets are initially formed from condensation onto the nuclei then the air is supersaturated. This happens when the air exceeds a critical value of condensation known as the Kohler Theory. These condensation nuclei are a big part and are necessary for cloud droplets to form. In warm clouds, larger cloud droplets fall at a higher terminal velocity; because at a given velocity, the drag force per unit of droplet weight on smaller droplets is larger than on large droplets.