Why?

[annita]

Hi all.
Had PoF today and our instructor said that an increase of temp. will lead to reduce pressure.
Then we had Met. and the other instructor said that increase of temp. will increase the pressure and that is why we connect High Pressure sytems with high temp.:ugh:

Half of us in the class are confused now and would like to know the answer to this please.

[paco]

The met guy has it the wrong way round. The increase in pressure in a high as the air is sinking increases the temperature due to compression.

The other guy wasn't much better, though - an increase in temperature leads to an increase in pressure, if the volume remains constant. Charles' law.

What he probably meant is that, at any particular level, if the air around you gets warmer, the column expands and the altimeter will read less.

Phil

[GearDownFlaps]

where Mr T Goes Mr P goes with him .
remember with the atpls the sky is always blue ....except when it isnt

[TheBeak]

Because met is describing a text book, contained 'bike pump' example of temp and pressure. In the real world, pof, the air molecules condense and move less (become more dense) as they cool and actually cause an increase in atmospheric pressure. Whilst hot, excited molecules move off, diffuse (and become less dense) and cause a reduction in pressure.

For the sake of the ATPLs, they are both correct and you will have to accept them and learn them. That is the rule to ATPLs, don't ask why, just learn the answer. The real learning comes afterwards.

[bucket_and_spade]

Both statements can be true depending on whether or not you're talking about a fixed volume or not. For a fixed volume (e.g. a sealed metal box) pressure is propotional to temperature i.e. increasing one increases the other. If you're a bit more mathematically minded, the relationship can be expressed as:-

PV = kT where P = pressure, V = volume, T = temperature and k = some constant

This can be rearranged to PV/T = a constant.

From this second equation you can see that, for a constant volume, if you double the temperature you'll double the pressure in order to satisfy the relationship.

B&S :ok:

[gfunc]

Bucket and Spade has hit the nail on the head - there are three terms in the relationship: Temperature, Pressure and Volume (or density, depending on the formulation). You need to know what volume is doing in order to know what T and P are up to.

Putting on my weather geek hat, your Met guy is oversimplifying - yes we expect the low-level temperature to increase as a result of adiabatic compression that is driven by descending air, but in the mid-latitudes the association between high pressure and hot weather tends to be due to the increase in solar heating due to the suppression of cloud cover. If you look for the highest surface pressures in the world, they occur in Siberia during winter - definitely not associated with high temperature. Maybe this is a bit too geeky for the ATPLs.

Good luck with all your subjects,

Gareth.

[TheBeak]

High pressure and hot weather are associated because first of all you get the 'piston' effect (adiabatic compression) from high pressure like with a subsidence inversion. Secondly, in high pressure the air is descending and warming at the DALR, which is always greater that the SALR.

[gfunc]

Adiabatic compression and warming at the DALR are the same thing - the clue is in the abbreviation: Dry adiabatic lapse rate.

I think the notion that the adiabatic warming (i.e. descent and compression) accounts for the warm temperatures experienced at the ground is a bit of an old wive's tale. The mean vertical motion in an anticyclone is of the order 1cm per second or 36 metres per hour. The DALR is 9.8K per km (0.0098 K per m), so in one hour the change in temperature due to the adiabatic compression is (0.0098 * 36) = 0.35 K. Much of this heating occurs above the surface (at the top of the boundary layer); this can be seen as a well marked inversion in temperature profiles during anticyclonic conditions.

Very, very little of this temperature change actually impacts the surface - see the example of Siberia having both the highest mean MSLP and lowest surface temperatures. The warm weather under anticyclonic conditions is predominately due to the supression of cloud cover and the increase in solar radiation. Compare the 0.35K/hr temperature change from the adiabatic heating to the difference you might expect between sitting outside under clear skies for an hour versus 2/8 cloud cover - I'm arguing it's going to be much larger than 0.35K!.

As I say, all far too geeky for the ATPLs, but Met is the one thing I'm half decent at!

Hope this helps,

Gareth.