fluid dynamics - What is the microscopic picture for warm air rising?

The usual explanation for warm fluids rising past cooler ones is that the warmer fluid has a lower density. I'm trying to understand what this looks like at a molecular scale. The density seems to be a large-scale phenomenon, and I don't understand how it can affect whether a particular molecule rises or falls.

Consider a cylinder of fluid that is being heated at the bottom. The molecules at the bottom have a higher average energy. How does this result in the tendency of the warmer molecules at the bottom to move upwards past the cooler ones?