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• Posts: 35

### jkatz098 created the topic: Convection VS adiabatic

I’m trying to understand the relationship between convection and adiabatic.

Am I correct in saying clouds can be formed by convection where warm air will rise in cooler environment (ELR,DALR,SALR)
And then when dew point is reached the adiabatic process starts and from there it’s a cycle of convection rising the cloud and adiabatic warming and cooling the air within the cloud?

• John.Heddles
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• ATPL/consulting aero engineer
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### John.Heddles replied the topic: Convection VS adiabatic

Have a read of this link. Don't fuss too much over the equations - rather read the text and think about what it might mean.

pressbooks-dev.oer.hawaii.edu/atmo/chapt...ange%20%28neutral%29 .

After you have done that, perhaps you might come back with some more specific questions.

Basically, while there is a tie up between convection (or convective flow) and adiabatic expansion/compression processes, they are two aspects of the things which happen in fluids where temperatures and densities are varying.

The term "convection" is tied up with the bulk vertical motion of fluids when there are dissimilar densities involved (think "buoyancy").

The term "adiabatic" just means that there is no energy transfer to/from the air parcels going on while parcels of fluid are expanding or compressing. As work still has to be done to expand or compress a fluid, there must be some energy used to make that happen. If we can't get it from the surroundings, it has to come from the parcel of air - that energy change then is tied up with temperature changes - in an adiabatic expansion, the temperature of the parcel of air drops while, for compression, it increases. Think of the initial compression of air in a pushbike pump if you put your thumb over the end hole - the air gets quite hot (associated with adiabatic compression). When you let the pressure escape, the temperature drops significantly (associated with adiabatic expansion).

Adiabatic processes are not specifically related to dew point. That is just where the air becomes saturated and we observe condensation, ie cloud base, and the temperature rates change due to the effect of what is called "latent" heat.

Edit - comment added for clarification

Engineering specialist in aircraft performance and weight control.

• Posts: 35

### jkatz098 replied the topic: Convection VS adiabatic

Thanks for that John, great link and read, definitely cleared it up.

I guess it all connects in some way, but are all separate processes.

Thinking logically, because as air sinks it compresses adiabatically, so it gets warmer, does that mean convection wants to make it rise back up, assuming it is cooler than the environment. So essentially, they work against each other but are completely separate processes?

• John.Heddles
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### John.Heddles replied the topic: Convection VS adiabatic

as air sinks it compresses adiabatically, so it gets warmer,

That's what we would expect to happen.

does that mean convection wants to make it rise back up, assuming it is cooler warmer than the environment.

Convective processes are to do with buoyancy so, if the parcel of air is warmer (at the same pressure) as the surrounding air, it will be of lower density, hence buoyant and will start to move upwards. Minor correction to your text included.

So essentially, they work against each other but are completely separate processes?

More a case that they do their own thing. Compression/expansion will result in temperature and density changes. Depending on the differences between the parcel of air being considered and its surroundings, the buoyancy determines whether the parcel goes up, down, or stays where it is.

Engineering specialist in aircraft performance and weight control.