It seems almost counterintuitive that wet air should be less dense than dry air and float in the sky. But the beauty of the cirrus and cumulus attest to this as we gaze at the myriad shapes and forms above us. Cloud formation reveals to us properties of the environment as well as properties of gases. As we notice that the western slopes of the Rockies are moist while the eastern slopes are deserts, we deduce that the air currents must be coming from the Pacific Ocean. This brief introduction provides the background for this chapter concerning cloud formation on the side of a mountain.