"Alastair McDonald" k
wrote in message ...

That raises a point that is bothering me. Can the addition of water
vapour
cause bouyancy without the air temperature rising? In other words
will a
parcel of air with a theta of 10C rise if surrounded with air that has
a
virtual theta of 10C?


.... getting a bit theoretical for me Alastair ;-) The only thing I
would risk writing is that if we accept that moist air is more buoyant
than perfectly dry air, then the addition of vapour to a dry sample
implies a more energetic medium, which implies the temperature (an
indication of the energy involved) also must rise. This is what the
equations which are used to calculate the virtual temperature seem to
imply. Can't contribute much more than that and we'll have to throw it
open to others to help us out here .....

Martin.