View Full Version : Atmospheric Stability - Confused!
Greg2041
7th Mar 2009, 14:18
Hmmm. Very confused!
Atmospheric Stability: The tendency of air when displaced to either return to its original position or increase its displacement.
If air which is raised through the atmosphere is colder than its surroundings, it will naturally descend to its original position because it is denser and heavier than the surrounding air.
This makes sense but my question is, as the cold air descends to its original position, will its temperature increase?
I think I am getting confused between this and ISA. Can someone explain?
Yours struggling
Greg
P.Pilcher
7th Mar 2009, 14:43
Yes- its temperature will increase. Provided this temperature increase does not cause the air to get warmer than the air surrounding it, the descending air will continue to descend until it hits the earth's surface.
When you compress air, which is what you doing to this parcel because it's pressure is increasing with the surrounding air, you do work on it and the work done has to be stored. This work appears as heat energy which causes the temperature of the air to rise. Such air is said to be stable, because if you push such a "parcel" of air upwards, it will cool more rapidly than its surroundings and immediately sink back down again.
P.P.
Greg2041
7th Mar 2009, 14:56
Ah that now makes perfect sense. One more thing, in my OAT DVD it says the rate it falls will increase the colder it is (not the exact words). Is this right?
Yes, it will fall faster when colder, as the colder it gets the more dense it is. There will be a bigger difference in density with the environment.
There is an equation that predicts the acceleration of an air parcel based on the density differences:
acceleration = 9.81 x ((density difference / density of air parcel))
there's more on Wikipedia:
Air parcel - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Air_parcel)
Hope this helps!
Gareth.
Greg2041
7th Mar 2009, 15:23
Of course. It all makes sense now. What would I do without PPRuNe.
Thanks again
Greg
Greg2041
7th Mar 2009, 15:33
Such air is said to be stable, because if you push such a "parcel" of air upwards, it will cool more rapidly than its surroundings and immediately sink back down again.
Quick last question. What can cause the parcel of air to be 'pushed' upwards?
Greg
Greg2041
7th Mar 2009, 15:48
Got it..... finally!
Many thanks
Greg
This got me thinking - wouldn't a lava lamp be a rather useful teaching aid?
If anything it should brighten up some of the grumpy old men around the club :}
Gareth.
effortless
8th Mar 2009, 10:50
his is because fluids (including air) of different densities do not readily tend to mix (think oil and water in a bottle).
We call this stratification. It can cause problems to all sorts of disciplines from meteorologists to process engineers, plumbers and chefs.
Greg2041
8th Mar 2009, 11:39
Sorry guys, another question.
My OAT notes say that:
Atmospheric Stability: The tendency of air when displaced to either return to its original position or increase its displacement.
But if its displacement increases, I'm assuming because ELR is greater than DALR, then the air would be unstable so the above definition wouldn't hold true.
Am I right or I have got this caught-up around my neck?
Greg
increase its displacement. - semantically, Greg, they should have added 'INstability' for that. Also don't ignore SALR in case the rising air reaches Dew point on the way up.
Greg2041
8th Mar 2009, 16:30
Thanks BOAC. I'm probably over-doing the study thing for PPL and I know I'm over doing detail but it is very interesting.
Greg
Not really - it depends on the level you are studying for. I've no doubt you have Googled your heart out, but this (http://www.indiana.edu/%7Egeog109/topics/08_stability/stabili.htm) looks useful?