The answer is yes, Alistair. Moist air is less dense than dry air at the
same temperature, and thus more bouyant. As Martin says, using the virtual
temperature in your calculations will take this into account.

--
Bernard Burton
Wokingham, Berkshire, UK.

Satellite images at:
www.btinternet.com/~wokingham.weather/wwp.html
"Alastair McDonald" k wrote
in message ...

"Martin Rowley" wrote in
message
...

"Joe Egginton" wrote in message
...
Martin Rowley wrote:

I've noticed Theta A in the RAOB soundings labeling.
PRES HGHT TEMP DWPT RELH MIXR DRCT SKNT **THTA** THTE
hPa m C C % g/kg deg knot K K

... that is just 'Theta' (i.e. no subscript): it is the Potential
Temperature (degrees Kelvin), defined as the temperature of an air
sample that it would have if it was brought adiabatically (dry adiabatic
lapse rate) to a standard pressure of 1000mbar. It can be used as a
tracer in much the same way as Theta W, Theta E etc., but the latter
have the advantage (in the lower troposphere) that they include the
measure of absolute humidity.

And before you ask, THTV is as for THTA, but using the *virtual*
temperature of the air parcel: you'll see that the two are very close,
and the same at high altitudes where the humidity content is low: from a
book on my shelf (because I can never remember the details) ...

" Virtual temperature is the temperature that dry air must have in order
to have the same density as the moist air at the same pressure. Moist
air is less dense than dry air; therefore the virtual temperature is
always greater than the actual temperature. " (and hence, THTV is
greater than or equal to THTA).

For practical meteorology (that is, idiots like me) we ignore the
difference: for the real number-crunchers though it is important.

Martin,

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?

Or is there a better way of phrasing the question?

TIA,

Cheers, Alastair.