MountainSim models the adiabatic process of a rising and falling air mass. An animated air mass, whose temperature and vapor pressure are displayed numerically and graphically, passes over a mountain. The student's goal is either to cause precipitation at a given altitude or to produce a specified temperature increase when the air mass descends. To reach these goals, the student must set the initial temperature and vapor pressure values for the air mass. When set in motion, the simulation animates the air movement and any precipitation that occurs. Two graphs are also displayed. One graph plots temperature and vapor pressure for the air mass and the other shows temperature and altitude. A notebook that records all trials is also provided. (Review by author)