Question:
Dose humid warm air rise faster then warm dry air?
Replies:
Mike,
Buoyancy is dependent on a difference in density between
an air parcel and the surrounding air.
Assuming that you have two air parcels of the same volume,
the warmer parcel will be less dense and more buoyant.
This also implies that the warmer air parcel exerts more
pressure outwardly than does a cooler air parcel (this effect
causes hot air balloons to expand as they warm).
Increasing the amount of water vapor in an air parcel
(assuming that no condensation of water is taking place)
makes it less dense, as water molecules (which have a
lower mass than air) displace air molecules.
Therefore, humid air would rise faster than dry air, assuming
that they both started with the same volume and temperature.
As both parcels rise, they will both cool at the same rate with
height (the dry adiabatic lapse rate); both will continue to
rise until they become cooler than the surrounding air.
However, if the humid air cools to the point of becoming
saturated (where condensation of water droplets begins),
at that point it would cool more slowly with height (since
heat is released as water is condensed out from the air). This
reduction in cooling with height means that it can have a higher
temperature and an even lower density than the dry parcel.
The humid parcel can then rise even more rapidly than it did
before, while the dry parcel is still rising at it's original
rate.
David R. Cook
Meteorologist
Climate Research Section
Environmental Science Division
Argonne National Laboratory
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