Yale professor Ron Smith continues his course on the atmosphere, ocean, and environmental change (GG 140) with a lecture on the Ideal Gas Law and its role in atmospheric physics. He works through the SI system of units, then derives the relationship between pressure, temperature, and density that governs how gas parcels behave. The lecture explains buoyancy force in fluids, showing why warm air rises and cool air sinks and why a helium balloon floats, then applies these principles to the atmosphere's composition and how density and pressure change with altitude. Chapters run from unit conventions through pressure and the gas law, buoyancy, atmospheric composition, and altitude effects, giving a full derivation-based treatment rather than a qualitative overview. Recorded at Yale in Fall 2011 as part of the Open Yale Courses series.