mstram,
as air is heated, the average kinetic energy of the molecules increases, thus allowing each molecule to occupy a greater volume in space. Same mass, greater volume = lower density.
In the atmosphere (or any fluid under the effect of a constant force in one direction, ie. Gravity) the denser fluid will sink to the bottom of the "container" thus PUSHING UP the less dense fluid.
So yes, when you see a thunderstorm, or even a cumulus cloud, its the colder air that sinks on the edges of the storm that pushes up the warm, less dense air in the cloud.
If on the other hand you are talking about a fluid in an enclosed container, an increase in temperature will lead to an increase in pressure, but no change in density since the overall mass (# of molecules) in the enclosed volume do not change. However, the added kinetic energy allows them to push harder onto the sides of the container, increasing the pressure.
Hope this makes sense,
palgia