This model demonstrates the effect of a gravity on gas molecules. It is one in a series of GasLab models that use the same basic rules for what happens when molecules run into each other. In this model, a gaseous "atmosphere" is placed above the surface of a "planet", which exerts a gravitational pull on each molecule. The molecules "escape" if they reach the top of the screen. The initially uniform energy and density of the gas becomes nonuniform as the model runs.
For detailed instructions on using the program, take a look at the info window from the Atmosphere model.
Click on one of the pictures to see a quicktime movie of the model: