I have just noticed that several texts describe warm fluids rising relative to cooler fluids due to their lower density. I think there may be a problem with this hypothesis. Low relative density causes solids to rise because of a sufficient surface area to mass ratio which allows the greater pressure of fluid below the solid (comparted to the pressure of the fluid above the object) to overcome the gravitational force acting down on the solid. The problem with using low density to describe warm fluids rising, is that fluids like air, do not have a "surface" for the fluid below it to push on. I think it is the fact that the particles of the warm fluid are moving faster, which allows them to simply bounce harder and better overcome gravity than particles that are not. This is one of those cases, where I am almost sure I am correct over the text. So in reality, I'm probably wrong. What do you guys think?