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I was speaking with my brother who is a mechanical engineer working w=
ith HVAC design in large buildings. As we discussed some of the phys=
ics involved with his work he told me that one of the factors that mu=
st 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 q=
uestions came to mind that I am having difficulty answering on my own=
. The first comes directly from the conversation with my brother. =
=20
1) Why should humid air in a building tend to rise to the ceiling? T=
he thought that comes to my mind is that the source of humidity is ge=
nerally people breathing hot air. Hot air rises and therefor the hum=
idity 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 b=
ut my ideas need a little polishing. In a balloon the hotter air wou=
ld collide with the balloon surface with more kinetic energy and ther=
efore stretch the balloon to a larger size. Larger size and same # o=
f particles =3D less density. Less density =3D greater buoyant lifti=
ng force than gravitational force. However when hot air is unconfine=
d 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 insula=
ted tightly sealed container and was allowed to sit on a shelf for a =
period of time would that various molecules settle into layers. Heav=
y 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=
=20
This posting is the position of the writer, not that of SUNY-BSC, NAU or the AAPT.