Re: Adiabatic wet bulb temperature as defined by the AMS

Thanks for this.

In reading further, it appears that the "lifted condensation level" is
the elevation associated with the air parcel being "cooled
adiabatically to saturation".

The "moist adiabatic compression" process sounds complicated -- the
latent heat of vaporization for water varies with pressure. So the
properties of the droplets that are magically injected into the parcel
would seem to vary throughout the process of compression.

So I gather that the AMS "adiabatic wet-bulb temperature":
1. Equals the "isobaric wet-bulb temperature" if the ground-level
air parcel starts out saturated.
2. Is less than the "isobaric wet-bulb temperature" if the air parcel
starts out less than saturated -- with the difference between the two
values increasing as the starting ground-level relative humidity of
the air parcel decreases.

The discrepancy between the values seems to arise because:
1. For the isobaric wet-bulb temperature, all moisture added to the
air has the same latent heat of vaporization -- as determined only by
the pressure at ground level.
2. For the adiabatic wet-bulb temperature, the latent heat of
vaporization for the moisture added to the air has an "average" value
somewhere between the value at ground level (where the latent heat is
lowest) and the value at the "lifted condensation level" (where it is
highest).

Am I understanding things correctly?

Thanks,
Adam



On Thu, 27 Mar 2008 11:19:10 -0400, Brent Lofgren wrote:

A "moist adiabatic process" is technically not adiabatic. What you are
supposed to imagine in this moist adiabatic process is that as the
parcel is compressed, enough water comes from somewhere (cloud/fog
droplets were magically injected into the parcel) that vaporization
occurs, keeping the water vapor at saturation. Thus there is a cooling
from vaporization (diabatic) that partially offsets the adiabatic
heating from the compression. Terms that are used for the curves on a
temperature-pressure graph along which a saturated parcel would travel
are "moist adiabats" and "pseudo-adiabats," the latter being more accurate.

Actually, a more real-world thing to visualize would be the reverse
process: a saturated parcel starting at the adiabatic wet bulb
temperature, being transported up to a specified level (saturation point
of the original parcel), leaving behind the water that had condensed
out, then warming adiabatically to the temperature and pressure of the
original parcel.

Brent

Adam wrote:
Hello,

Please help me to understand the definition of adiabatic wet bulb
temperature, as given here:

http://amsglossary.allenpress.com/glossary/browse?s=w&p=23

"Adiabatic wet-bulb temperature (or pseudo wet-bulb temperature): the
temperature an air parcel would have if cooled adiabatically to
saturation and then compressed adiabatically to the original pressure
in a moist-adiabatic process."

I am an engineer. I was taught to use the term "adiabatic wet bulb
temperature" to refer to what is called the "isobaric wet-bulb
temperature" by the AMS. It is the isobaric wet-bulb temperature that
is shown on psychrometric charts for the air-conditioning industry.

What is accomplished by the process in the AMS definition quoted
above?

It sounds like the air cools adiabatically as the pressure reduces
(such as when an air parcel moves to a higher elevation), but no
condensation happens.

Is some other kind of "adiabatic cooling" possible?

Is the "adiabatic wet-bulb temperature" equal to the dew point at this
cooled and saturated condition?

Then the air is re-compressed adiabatically to the original pressure.

What is the point of describing the re-compression in the definition?
Wouldn't adiabatic re-compression simply return the air to exactly its
original state?

Thanks for any clarity you may bring to the AMS defintion of the
adiabatic wet bulb temperature.

Adam



--
Posted via a free Usenet account from http://www.teranews.com

.

Relevant Pages

  • Re: Adiabatic wet bulb temperature as defined by the AMS
    ... What you are supposed to imagine in this moist adiabatic process is that as the parcel is compressed, enough water comes from somewhere that vaporization occurs, keeping the water vapor at saturation. ... Actually, a more real-world thing to visualize would be the reverse process: a saturated parcel starting at the adiabatic wet bulb temperature, being transported up to a specified level, leaving behind the water that had condensed out, then warming adiabatically to the temperature and pressure of the original parcel. ...
    (sci.geo.meteorology)
  • Re: More on Theta E
    ... > Does it measure the temperature of an airmass, ... parcel of moist air would achieve if it were lifted cooling at the saturated ... down to the same level warming at the dry adiabatic lapse rate. ... unstable layer, an increase with height, a stable layer. ...
    (uk.sci.weather)