This apparatus, warm on top and cool on the bottom, is called a diffusion chamber . Now let's look at it in more detail. Saturated air . If I take a container, add a bit of water, and close the top, then the air in the container soon becomes saturated with water vapor. Saturated air has a relative humidity of 100 percent.
Saturated air is the equilibrium state whenever there's lots of water around.
Thus when it's raining, or foggy, the humidity of the air outside is close to 100 percent. Supersaturated air . In physics we call this a non-equilibrium state, or a metastable state. Rather they are crystals that grow in supersaturated air that is below freezing. This only happens when there is some nucleation site on which condensation can occur.