Lows and Highs

[Avain]

In several posts here in the learning centre and on the forum in general I have read a description of an event, the event in question being that a high blocks a low, in other words, an easterly moving low pressure system will tend to drift north or south of a high, skirting round it's boundary, so the high effectively seems to "Block" the easterly progress of the Low from it's original course at any rate.

But is this what is actually happening? Why does the low do this, the high consisting as it does of a mass of cold descending air, albeit less cold probably on the borders than at centre, it seems that the warm aired Low (at the frontal zone at least) air should ride over the cooler air, or mix in with it and disperse if the gradient is close, how is it that the low maintains it's structure and exists almost as an entity in it's own right rather than an amophous mass of different temp air?

I am learning such a stack in this learning centre, thanks to all who have contributed the variuos articles etc!

[SP1986]

Highs can 'block lows' but they don't techincally block them otherwise they'd start bouncing off each other, and there would be lots of col (areas of no discernible high or low pressure) on the Earth

It's all due to the different airmasses really. Air from one air mass cannot mix with another it can only override, and at the points of borders between low and high pressure systems, this means that rather than welding into each other they move around each other.

[Avain]

Thanks for that input Steve, and I am sure it is smack on, but still, the question remains, why?

Why can the air from one airmass not mix with the other? For sure the answer lies with the physical nature of the airmasses, but I can't picture just what it is that forms the "membrane" so to speak.

[full_frontal_occlusion]

Hi Avain.

The 'membrain' as you put it, is exactly where the mixing is taking place:-

The key point to remember is that both pressure systems have enormous mass and therefore corresponding inertia. Like two colliding cars, the mixing takes place at the point of collision until the energy difference between them is equalised when no further mixing takes place.

The boundary between the contrasting air masses is the 'front' where literally collisions between the different velocity air molecules is taking place. This is a sharp demarcation with air on either side of the contrasting air masses experiencing mixing over the length of the front and height extending to tens of thousands of feet.

The rate of mixing is determined by the air pressure gradient between the air masses i.e. the pressure difference between the high and low pressure systems. The bigger the difference, the higher the wind speeds generated.

The colder heavier air associated with the high pressure air mass sinks below the lighter warmer air associated with the low pressure air at the boundary (Heat rises, cold sinks). Because the low is advancing in relation to the receding high, the front is formed at an angle rising above and towards the centre of the low.

Warm air with a high moisture content (picked up over the oceans) is forced to rise at the front, and in so doing the moisture condenses into clouds and then rain in a relatively narrow but long band.

All this to say that mixing occurs over a large surface area at the point of collision between the different pressure air masses.

Because of intertia, the low pressure air behind the collision front is deflected (think of billiard balls glancing off each other). The higher energy content of the high pressure system winning out and thus forces the warmer low pressure air to change direction. i.e. cars never win an argument with a tank.

Keep in mind though, that all the while mixing is taking place at the frontal boundary. Eventually the energy difference and exchange between the two systems is equalised - the low pressure system increases in pressure fed with energy from the high, whilst the high pressure system weakens somewhat sapped by the low. When that happens, the pressure gradient eases and the winds drop - mixing and deflection has stopped.

Hope this helps.

ffO

[Avain]

Thanks FFO, that does help a good deal, I had to read it several times to picture the actual processes, and admit that although I understand it a good deal better, I could not explain it (I think being able to explain something is the true test of understanding)

So, despite two excellent presentations, anyone with a slightly different angle is equally welcome to add their own explanation to build as full a picture as possible (Graphics welcome!!)

Cheers!

[full_frontal_occlusion]

Lets try this one again Avian!

The thing is to think about what's happening with individual atoms / molecules at the micro scale, then once that's in your head, try and imagine what it would look like at the macro - weather system scale.

Atomic scale:

Atoms of gas (Oxygen, Nitrogen, Hydrogen etc.), and molecules (Carbon Dioxide, Water Vapour etc.) all have very different masses and are very small in comparison to the distances between the atoms. They rush around at very high speeds colliding with each other or the ground and any objects in their path. Each time a collision between individual atoms occurs, the atoms exert a force and deflect each other changing speed and direction in the process. Very similar to billiard balls on a table.

Some atoms colliding head on will bounce off each other and head off in the direction from which they came. Others will glance off each other, rebounding at different angles. All speed up or slow down depending on the relative mass / velocities of the atoms involved in each collision.

Other forces are also acting on individual atoms; namely the pull of gravity, and the induced Coriolis force (see description later on) due to the Earths rotation imparting a constant change in direction as atoms collide with the surface objects of the Earth.

At this point, introduce the concept of pressure. Pressure, is just a way of describing the collective force imparted by all the atomic collisions happening over a given area. Gravity pulls the atoms down towards the Earths centre of mass so that many more atoms congregate close to the surface than do higher up. As a result of this gravitational pull, more collisions between atoms occur since there are many more atoms to collide with - hence the higher pressure at the Earths surface than high up in the atmosphere.

Temperature also affects pressure in that the individual atoms speed up with higher temperatures. This means that more collisions between atoms/molecules happen over the same time span than for a cooler gas. With greater numbers of collisions, the force exerted collectively has increased and hence the pressure within the gas is higher.

Because of the sheer number of atoms in the atmosphere, no atom will make the journey from one side of a weather system to the other (not impossible, just highly improbable) without soon colliding with another atom.

However, momentum can be exchanged between individual atoms and therefore energy propagates throughout the weather system because of the fundamental physical law of conservation of energy.

This effect is observed every time a drop of water hits the surface of a pond - the drop soon vanishes into the water but the momentum imparted ripples outwards in concentric waves. Note however that the waves propagate at a certain finite speed.

Macro scale:

As previously explained (last post) at the collision boundary between the different pressure air masses (weather front) momentum between atoms is constantly exchanged and ripples through both systems (as described above) at finite speed because of the sheer number of atoms involved even though individual atoms do not make the journey from one side of a system to the other.

The higher pressure system comprises on average, higher momentum atoms than the low pressure system and so collisions at the weather front will tend to ripple further and more quickly into the low pressure system than into the high.

However, because at the atomic scale momentum is conserved, the original 'bias' of the bulk atoms of each weather system must also be conserved. The collisions at the weather front imparts a new bulk 'bias' as they ripple throughout the weather system. But so many collisions are happening, it's impossible to pick out one ripple from the next. It's the NET effect that's important because this is what we observe and comprehend at the human scale.

At the atomic level, mingling and exchange of momentum between atoms occurs constantly at the weather front - with exchanges of energy rippling throughout both weather systems. The intial momentum conservation (rotation etc) as observed at the bulk level is also maintained and therefore 'appears' as if the air masses are skirting around each other.

ffO