Could someone tell me what does the Theta A index measure and is used for?

--
Joe
Wolverhampton
175m asl

"Joe Egginton" wrote in message
...
Could someone tell me what does the Theta A index measure and is used
for?

.... in what context have you come across Theta-A? I have only heard of
Theta-E and Theta-W in the world of meteorology, but that doesn't mean
it doesn't exist ;-)

Martin.

--
FAQ & Glossary for uk.sci.weather at:-
http://homepage.ntlworld.com/booty.weather/uswfaqfr.htm

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

Could someone tell me what does the Theta A index measure and is used
for?


... in what context have you come across Theta-A? I have only heard of
Theta-E and Theta-W in the world of meteorology, but that doesn't mean
it doesn't exist ;-)

Martin.


Hello Martin,

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


--
Joe
Wolverhampton
175m asl

I think you will find, Joe, that the Theta-A you refer to is actually
straight Theta, or the potential temperature. Other suffixes to theta are
Theta-w for wet bulb potential temperature, Theta-V for Virtual potential
temperature, Theta-E for equivalent potential temperature and Theta-L for
liquid water potential temperature.

--
Bernard Burton
Wokingham, Berkshire, UK.

Satellite images at:
www.btinternet.com/~wokingham.weather/wwp.html
"Joe Egginton" wrote in message
...
Could someone tell me what does the Theta A index measure and is used for?

--
Joe
Wolverhampton
175m asl

"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.

HTH

Martin.

"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.

"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.

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.

Bernard Burton wrote:
I think you will find, Joe, that the Theta-A you refer to is actually
straight Theta, or the potential temperature. Other suffixes to theta are
Theta-w for wet bulb potential temperature, Theta-V for Virtual potential
temperature, Theta-E for equivalent potential temperature and Theta-L for
liquid water potential temperature.

--
Bernard Burton
Wokingham, Berkshire, UK.


Do'h!! Thanks for the info Bernard. :-)

--
Joe
Wolverhampton
175m asl

"Bernard Burton" wrote in message
...
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.

Got it! To work out the buoyancy (instability) you have to use the virtual
temperature, not the potential temperature.

Thanks Martin and Bernard,

Cheers, Alastair.