Cloud formation

[Bob Yarwood]

When a cloud forms on a rising column of moist air, why does the cloud always remain at that height and stop moving?  Presumably the air, now dry, keeps on rising for a time, because I see no reason why it would stop at that height just because the moisture has condensed out.   But if the air does continue to rise, why doesn't it carry the cloud up with it?  I have a physics degree but I have never studied meteorology and it seems to me that clouds are odd things - almost a fourth state of matter to go with solid, liquid and gas.

Edited September 30, 2016 by Bob Yarwood
letter missing

[chrisbell-nottheweatherman]

Hi Bob.  As none of the more expert meteorologists have responded, I thought I might offer a few thoughts.  Generally-speaking, a parcel of air will undergo convection, or lift, until such a point at which its own temperature drops to that of the air surrounding it.  If that parcel temperature is at or below the dew point (the temperature at which the parcel is fully saturated), then moisture will condense to form clouds, and precipitation may occur.  As can be gathered from this, the point at which the parcel ceases to rise has nothing (AFAIK) to do with cloud formation, save for the release of latent heat into the air parcel as condensation occurs, which may warm it suchthat it continues to convect. 

[Bob Yarwood]

Exactly - you have basically just repeated what I said.   But if  the air continues to rise I would think that it would carry the cloud up with it but that never seems to happen.  It's a point which the textbooks I have read have not mentioned, but that happens in textbooks in many subjects - they all just repeat the same things.   I call it "tramline thinking", where everybody just repeats what they hear from other people.   

[knocker]

I'm afraid I'm not following this. If we are talking Cumulus formation then initially the air will rise along the DALR until it reaches the condensation level (saturated) when it will it will then rise along SALR unless inhibited say by an inversion. It doesn't dump all of it's water content at the CL and the upward convection currents (thermals) continue, and specifically the cloud formation in them, are maintained, not by thermals of surface origin, but ones set up within the cloud as a result of the release of latent heat by condensation. Thus you get the towers of Cumulus. So they do 'carry' the cloud up except I wouldn't have used this terminology. I'm obviously missing something..And just to remember all of this is virtually impossible in clean air.

Edited October 19, 2016 by knocker

[knocker]

A brief follow up to the above to put in a nutshell what I think is the answer to your question

The thermal updraft will continue above the condensation level (where the cloud base has formed) and cloud will continue to be formed as the air cools (as mentioned above) until such a time as the thermal ceases and/or there are changes in air mass characteristics

What will not happen is the condensation level also rising because the air above is already saturated (as mentioned above) thus the cloud base remains at the same height. Again changes in air mass characteristics will change the condensation level which can be easily observed by changes in the height of a Cumulus base, particularly in summer.

Just to add this is a very simplistic attempt at an answer and cloud physics is an extremely interesting and complicated subject and is a scientific discipline in it's own right and way above my pay grade.

 

Edited October 20, 2016 by knocker

[Bob Yarwood]

Yes, fluid dynamics is a complex subject and one which does not come into the undergraduate physics syllabus.   Also, meteorology is a chaotic process as was discovered some years ago.