I was speaking with my brother who is a mechanical engineer working with HVAC design in large buildings. As we discussed some of the physics involved with his work he told me that one of the factors that must be considered when designing ventilation systems is that humid air tends to rise to the ceiling. As I pondered why this may be a few questions came to mind that I am having difficulty answering on my own. The first comes directly from the conversation with my brother.
1) Why should humid air in a building tend to rise to the ceiling? The thought that comes to my mind is that the source of humidity is generally people breathing hot air. Hot air rises and therefor the humidity goes along for the ride. However, I am not satisfied with this answer because it would seem that the air breathed from people would mix with the room air too quickly to cause much convection movement. Is there more that I am missing. It seems like there must be.
2) Why is hot air less dense than cool air? I basically understand but my ideas need a little polishing. In a balloon the hotter air would collide with the balloon surface with more kinetic energy and therefore stretch the balloon to a larger size. Larger size and same # of particles = less density. Less density = greater buoyant lifting force than gravitational force. However when hot air is unconfined in the atmosphere what acts as the surface of the balloon?
3) If the air I am breathing right now was trapped into a well insulated tightly sealed container and was allowed to sit on a shelf for a period of time would that various molecules settle into layers. Heavy molecules on the bottom and light on the top?
Any answers pointing me in a useful direction would be appreciated.
Cliff Parker
Never express yourself more clearly than you can think. Niels Bohr
This posting is the position of the writer, not that of SUNY-BSC, NAU or the AAPT.