adiabatic cooling

[igottafly]

How does this happen? As I understand the definition, it is the process that occurs with out the loss or gain of heat. But how??????

 

[SkyHog]

How does this happen? As I understand the definition, it is the process that occurs with out the loss or gain of heat. But how??????

First off - according to Jeppesen's Aviation Weather, 2nd Edition, Adiabatic process is defined as:

"The change of temperature of a gas (e.g. the atmosphere) by expansion of compression."

So there is a heat loss or gain.

As an air parcel ascends upwards, it expands due to the lower pressure it is entering. It tries to match the pressure of the air parcels around it. The inverse is also true - as it descends, it compresses with the higher pressure at lower altitude.

The law of conservation of energy is at place here. The parcel must use energy to expand, therefore there is less heat when the parcel expands.

Thats the basics of dry adiabatic cooling. Beyond that, I just take it as a given that it works.

 

[Anthony]

Thats the basics of dry adiabatic cooling. Beyond that, I just take it as a given that it works.

Unless there is a temperature inversion.

 

[ScottM]

First off - according to Jeppesen's Aviation Weather, 2nd Edition, Adiabatic process is defined as:

"The change of temperature of a gas (e.g. the atmosphere) by expansion of compression."

So there is a heat loss or gain.

As an air parcel ascends upwards, it expands due to the lower pressure it is entering. It tries to match the pressure of the air parcels around it. The inverse is also true - as it descends, it compresses with the higher pressure at lower altitude.

The law of conservation of energy is at place here. The parcel must use energy to expand, therefore there is less heat when the parcel expands.

Thats the basics of dry adiabatic cooling. Beyond that, I just take it as a given that it works.

Because Nick is so into math... I thought you might also like this reference that discusses the non-linear lapse rate for adiabatic cooling and demonstrates the relationship of pressure, volume, and temperature.

http://wufs.wustl.edu/pathfinder/path201/notes/notes_04_09_14.htm

 

[poadeleted20]

Actually, it does involve a change in temperature without a change in heat. It's a physics thing. If you take a volume of air and expand it (say, by lofting it to a lower pressure region) without any addition or loss of heat, the same total quantity of heat energy is spread over a larger volume, so the temperature (density of heat energy per unit volume, if you will) at any one point is lower even though the parcel of air still has the same total amount of heat energy in it.

 

[RotaryWingBob]

I don't think that's quite right, Anthony. Here's the full definition:

Adiabatic lapse rate. The rate at which air cools as it is forced upward or warms as it sinks, if no heat energy as added to it and none is taken from it.

In other words there is no transfer of heat to or from the air mass and the surrounding air (ie., an inversion has no effect on the adiabatic process). This is the basis for the lifted index which measures stability of the air by comparing the actual lapse rate (which would include your inversion) to the adiabatic lapse rate. The lifted index is negative, indicating unstable air, when the actual lapse rate is greater than the adiabatic lapse rate (meaning that a mass of air moving upwards want to keep going up because the surrounding air mass is cooler and therefore more dense than it is).

 

[Anthony]

I don't think that's quite right, Anthony. Here's the full definition:

Adiabatic lapse rate. The rate at which air cools as it is forced upward or warms as it sinks, if no heat energy as added to it and none is taken from it.

In other words there is no transfer of heat to or from the air mass and the surrounding air (ie., an inversion has no effect on the adiabatic process). This is the basis for the lifted index which measures stability of the air by comparing the actual lapse rate (which would include your inversion) to the adiabatic lapse rate. The lifted index is negative, indicating unstable air, when the actual lapse rate is greater than the adiabatic lapse rate (meaning that a mass of air moving upwards want to keep going up because the surrounding air mass is cooler and therefore more dense than it is).

Bob. I agree. I was just pointing out that while there is a formula for the air cooling as you increase altitude, there may be also situations where the air is not at the temperature you would expect due to other factors.

 

[Ed Guthrie]

How does this happen? As I understand the definition, it is the process that occurs with out the loss or gain of heat. But how??????

Heat within a gas is transferred from molecule to molecule by molecular collision. If the molecules in the warm air aren't colliding with molecules in the cold air then heat will not be transferred. That explanation being the complex & long way of saying it takes time for two neighboring air masses to mix/exchange heat/reach temperature equilibrium. Until that process occurs (and heat transfer is much slower than expansion due to orographic lifting, for example), the two air masses will maintain a temperature differential and the lifted mass will essentially undergo adiabatic expansion.

 

[Bill]

Seems like magic, but it follows basic gas laws.

If I remember correctly, PV/T must remain constant, correct?

 

[Richard]

How does this happen? As I understand the definition, it is the process that occurs with out the NET loss or gain of heat. But how??????

Statement is now correct.

BTW: Ron has it right because he mentions temp, heat, and volume. Temp is only a measure of kinetic energy. Ed discusses that quite well.