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

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