PPRuNe Forums - Stability of air

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Stability of air

Can anyone explain why the answer to the below question is b) unstable please?

The air between 1000ft and 2000ft at 55N 0730W (attached) can be described as:
a) Stable
b) Unstable
c) Neutrally stable
d) Conditionally unstable

I thought it is a) stable, because the parcel of air has 'lapsed' at 4c per 1000ft. This is the dry lapse rate. The environmental lapse rate for the same 1000ft is 1.98c. Because 4c is greater than 1.98c, the parcel of air will sink relative to the environment. Therefore it is stable. Or so I thought...

No, you have to think of a parcel of air that is at 1,000' and 11c, you then raise THAT parcel and it cools at the DALR (3/1,000) if dry or SALR (1.5/1,000') so if dry then raising it 1,000' it will cool by 3 degrees to 8C which is warmer than 7 so will continue rising i.e. Unstable, if Saturated it will cool even less so still be unstable, if the temp at 2,000' was say 9C then it would be conditionally unstable.

I think you are getting your lapse rates mixed up.
ELR is the ACTUAL lapse rate - 4C/1000' in this example
1.98C/1000' is the International standard atmosphere figure
DALR and SALR I think you know, though you did get it mixed up at the start of this thread.

The environmental lapse rate is whatever is actually measured in the atmosphere. The ISA assumes an ELR of -1.98 C/1,000', an actual measured ELR won't necessarily be the same.

An ELR greater than the DALR is so unstable as to be unlikely in real life, except at ground level.

Quote:

An ELR greater than the DALR is so unstable as to be unlikely in real life, except at ground level.

Not the case at all, certainly in hot countries you will often find this happens and is how Cbs start, ELR greater than DALR so the air rises until it reaches the dewpoint at which point it forms the base of the cloud and then rises at the SALR and goes up even faster!
In fact you can even have an inversion - say 10C at the surface, 8 at 1,000' and at 2,000 it goes back to 10, as the day goes on the surface warms to 16C and so will rise at the DALR to 2,000' and is thus also at 10C, any further surface warming will thus cause it to break through the inversion and Cbs are extremely likely!

foxmoth, take your second example. What doesn't happen is that the air at 1,000' remains at 8 C until the surface temp reaches 16 C and then suddenly everything pops off. What happens is that once the surface layer temp reaches 11 C you have an unstable layer below the inversion. Any warm air that detaches from the surface will rise until it hits the inversion, introducing warmer air to the bottom of the inversion. The cold air that sinks to replace it is soon warmed by contact with the ground. This is an ongoing process.

Once the surface temp reaches 12 C, the 1,000' temp will be 9 C, when the surface reaches 14 C the 1,000' temp will be 11 C & since the temp within the inversion is 10 C the unstable layer is now up to 1,330'. When the surface temp reaches 15 C the unstable layer reaches up to 1,660'. So the ELR below the inversion remains at 3 C/1,000' as the day warms.

Hot air rises. One thing that stops unsaturated warm air rising is if the ELR < the DALR, once the ELR = the DALR the warm air below swaps places with the cold air above. Hot air next to the ground can't lift off in one go leaving a vacuum behind it, so you can get a super adiabatic layer close to the ground as the air heats up faster than it can lift off. I have never, ever seen a sounding with a layer of air detached from the surface where the ELR > the DALR.

Here's a link to a screenshot of yesterday's 1100Z sounding from Herstmonceaux. You can see the super adiabatic layer close to the surface & an unstable layer from about 400' to 2,000', nowhere else does the ELR come close to the DALR.

Yes, see what you are saying.

...nonetheless:

Quote:

A super-adiabatic lapse rate is common in the Southwest U.S. in the summer, but can occur in most regions of the U.S. in the summer when the skies are clear (maximum insolation), wind speeds are low (limited vertical mixing) and soils are dry (no evaporational cooling).

The super-adiabatic layer is generally shallow and located near the earth's surface.

Another situation a super-adiabatic lapse rate can occur is over a warm lake.

In heavy lake-effect snow situations there will often be a super-adiabatic lapse rate above the lake.

A downsloping wind is another situation it can occur. With a downsloping wind, air is warmed at the dry adiabatic lapse rate as it sinks. This combined with surface heating can produce a super-adiabatic lapse rate in the lower troposphere in the afternoon.

SUPERADIABATIC LAPSE RATE

That’s a really good explanation from Prop swinger.

Pretty much the only times you get a lapse rate > DALR is a) in the afore-mentioned superadiabatic layer near the ground or b) when an atmospheric phenomenon advects warm air under cold, like in some rotor flow.

Thanks for the explanations all! I had not appreciated that ELR is a guide and this data was showing the actual ELR.

Not so much a guide but an assumption that some calculations may be based on.

An average ELR of 1.98°/1000' is as accurate as a temperature of 15°C and a sea level pressure of 1013.2 hpa. They all simply represent a defined standard against which you can compare the actual conditions and in no way should be taken as conforming to the real world.