Pressure Systems - Surface and Aloft

[Dylfish]

Hi Guys, 


I have simplified the model, dividing the surface level and “air aloft”.

 

 

My Understandings

-       Air at all levels flows from High pressure to Low pressure. This allows pressure to even out, thus creating winds due to the pressure force gradient.

-       Cold / Cool Air is denser than warm air, and therefore exerts more pressure at the surface.

-       Air Masses of high pressure are denser than normal air, this “Pushes” down exerting a higher surface pressure.

-       Air Masses of low pressure as less dense than normal air, which allows the molecules to rise and have a relatively lower surface pressure.

 

What I think happens

1)    At the surface, high pressure air from the surface High pressure system blows outwards towards the Low pressure system. Here air is rushing from all directions converges and with nowhere to go, it pushes upwards.

2)    Since at the surface pressure is low there are relatively more molecules aloft of the surface low. This creates a relative high pressure aloft of the Low.

3)    High pressure aloft blows from the high above the Low to the low aloft of the surface high. This is because the cold air is dense and therefore there are more molecules at the surface and not a lot aloft.

4)    Due to divergence, the cold air aloft of the High sink. This keeps the pressure high at the surface and the weather clear.

 

My questions

1)    Is this basically correct?

2)    If above the surface low is relatively high, why doesn’t the air try to sink from the high aloft to the low at the surface?

3)    Is the relative Low & High aloft idea correct (surface high = more molecules at the surface & less aloft, hence lower aloft pressure)

4)    I know that winds aloft in both hemispheres are usually West -> East. If a high pressure system is to the East of a Low, shouldn’t it go East -> West at times?

 

I’ve been reading Essentials of Meteorology by C. Donald Aherns, but I got confused after I saw this section on thermal lows and it had an image on this link below where the air is in a "loop".

 

http://oceanservice.noaa.gov/education/yos/resource/JetStream/ocean/sb_circ.htm

I’m assuming all pressure systems (Cyclone & Anti-Cyclone’s) work in the same principle as this image would show. Any help would be great.

 

Cheers,

 

Dylan

[Black Hole]

I'll get back to you on this tonight or tomorrow. 

[Black Hole]

 

1)    Is this basically correct?

2)    If above the surface low is relatively high, why doesn’t the air try to sink from the high aloft to the low at the surface?

3)    Is the relative Low & High aloft idea correct (surface high = more molecules at the surface & less aloft, hence lower aloft pressure)

4)    I know that winds aloft in both hemispheres are usually West -> East. If a high pressure system is to the East of a Low, shouldn’t it go East -> West at times?

1.) You are on the right track with most of it. The most important thing to think about involves the dynamics which I guess I will get into some below. Also, aloft the air in high pressure tries to move towards low pressure but usually something called geostrophic balance prevents it from reaching the low pressure. (Do you know what that is?) Instead, it blows along the isobars aloft.

 

2.) This one involves dynamics. You have a low at the surface and a high aloft because of the pattern in place. Something is causing the air to rise in that location. There is a surface low because air is being removed through lift faster than it is being replaced by the converging flow. Similarly aloft the air diverges but not as quickly as it is being replaced from below. 

 

I need to add here that your conceptual graphic is often not the real state of the atmosphere. You can have stacked systems of low pressure under low pressure. Also the most often instance of a surface low results from divergent flow aloft between upper level lows and highs.

 

3.) Often times the pressure centers aloft are somewhat a consequence of what is going on beneath them. You will often have a very strong arctic high with an upper level low near it aloft. This is because a very cold airmass is dense as you suggested, which means that pressure drops off quickly with height. This results in relatively lower pressure aloft. Hopefully that helps. 

 

4.) The most often direction is west to east but it may go any direction if the pattern is favorable. Normally it is west to east because winds blow along the isobars, and with colder air at the poles pressure is less towards the poles. Geostrophic balance demands the air move west to east in such a case.