Meteorology (Weather)/Pressure


Pressure Diagram
Pressure Diagram  
Hi David,

I asked a question the other day but I hope going back to basics a bit will help me.

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? (and likewise try to rise from high to low in a anti cyclone)

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".


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.



Hi Dylan

I think the better way for me to answer these questions is with the following:

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Donald Rosenfeld


Any questions (except private) answered from the 1st grade level on up pertaining to any aspect of Weather. I am a 20 year member of the American Meteorological society and a long time forecaster of eastern United States snow storms and Hurricanes.

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