straight isobars?
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straight isobars?
Here's a question that came up when I was teaching CPL Met.
Background: Geostrophic wind is defined as a theoretical wind that would occur above 3000' (the friction layer) when two forces, gradient force and coriolis force, are in balance along straight isobars. In other words, geostrophic force acts parallel to straight isobars.
There's a good animated demo of this at: http://ww2010.atmos.uiuc.edu/(Gh)/gu...w/gifs/geo.mov
It's been said that few things in nature are ever perfectly straight. Without straight isobars, there can be no perfectly geostrophic wind.
The question is, when/where would isobars even come close to being straight?
Background: Geostrophic wind is defined as a theoretical wind that would occur above 3000' (the friction layer) when two forces, gradient force and coriolis force, are in balance along straight isobars. In other words, geostrophic force acts parallel to straight isobars.
There's a good animated demo of this at: http://ww2010.atmos.uiuc.edu/(Gh)/gu...w/gifs/geo.mov
It's been said that few things in nature are ever perfectly straight. Without straight isobars, there can be no perfectly geostrophic wind.
The question is, when/where would isobars even come close to being straight?
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This weekend's forecast Arctic blast in the UK might be a starter for ten. Not perfectly straight I grant you but not bad....
Last edited by Streety; 19th Nov 2008 at 08:26.
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I think you are getting confused over the nature of an Isobar. Forget wind and geostrophic forces - an Isobar is merely a line of equal pressure. It can be almost any contour it wishes!
Yes, wind flows from high to low pressure, so it 'tends' to flow from one isobar to the next, but is 'deflected' by GF. GF itself has nothing to do with wind. It is an apparent force which acts on any moving particle, be it a cricket ball or a lump of air, and is the result of the earth's rotation.. Whenever that particle moves, depending on its latitude, of course, GF will appear to deflect it to the side (being careful here 'cos of your 'origin'
). Forget acting "parallel to straight isobars" - that is the definition of a pure Geostrophic Wind. Consider it to act at 90 deg to the particle motion and in practical terms tangentially to the curved isobars, except, of course, other effects such as centripetal (oh, ok, centrifugal) forces modify the wind from the theoretical to that experienced in curved isobaric systems.
The answer to your final question is, as 'streety' says, never in real terms due to orographic and other disturbances but sometimes pretty close, and often between two 'matched' pressure systems as per the pic.
Yes, wind flows from high to low pressure, so it 'tends' to flow from one isobar to the next, but is 'deflected' by GF. GF itself has nothing to do with wind. It is an apparent force which acts on any moving particle, be it a cricket ball or a lump of air, and is the result of the earth's rotation.. Whenever that particle moves, depending on its latitude, of course, GF will appear to deflect it to the side (being careful here 'cos of your 'origin'
). Forget acting "parallel to straight isobars" - that is the definition of a pure Geostrophic Wind. Consider it to act at 90 deg to the particle motion and in practical terms tangentially to the curved isobars, except, of course, other effects such as centripetal (oh, ok, centrifugal) forces modify the wind from the theoretical to that experienced in curved isobaric systems.The answer to your final question is, as 'streety' says, never in real terms due to orographic and other disturbances but sometimes pretty close, and often between two 'matched' pressure systems as per the pic.
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Streety,
Thanks for the MSL chart. Where I live we don't see a lot of those.
BOAC,
Thanks for you input, too. I take your point about isobars being any shape...
Question...
When you say 'GF', do you mean Gradient Force? If so, my understanding was that it was not GF but Coriolis Force that deflects wind (or any moving particle) left/right (depending on southern/northern hemisphere). I understood Gradient Force as being the force which accelerates air particles from a high pressure to a low.
So, considering a southern hemisphere example...
Gradient force accelerates air from a high to a low pressure. It is then subjected to Coriolis Force which causes it to bear left. the faster the air travels, the more Coriolis Force pushes it left until, eventually, it the forces balance out and the air particles/wind being to travel parallel to curved isobars. Is this correct?
So, not quite sure how centripetal/centrifugal force affects life in a pressure system. Centrifugal force is defined as the tendency of a body in curved motion to travel in a straight line once released from it 'orbit'...but that doesn't really help me too much...
What I'm after is a vector diagram-type understanding/'picture' for all the forces that act on an average pressure system, assuming it is above the friction layer...
The student was happy with a pretty basic answer, it's just me that's over-thinking the topic
Cheers,
Maudlin.
Thanks for the MSL chart. Where I live we don't see a lot of those.
BOAC,
Thanks for you input, too. I take your point about isobars being any shape...
Question...
When you say 'GF', do you mean Gradient Force? If so, my understanding was that it was not GF but Coriolis Force that deflects wind (or any moving particle) left/right (depending on southern/northern hemisphere). I understood Gradient Force as being the force which accelerates air particles from a high pressure to a low.
So, considering a southern hemisphere example...
Gradient force accelerates air from a high to a low pressure. It is then subjected to Coriolis Force which causes it to bear left. the faster the air travels, the more Coriolis Force pushes it left until, eventually, it the forces balance out and the air particles/wind being to travel parallel to curved isobars. Is this correct?
So, not quite sure how centripetal/centrifugal force affects life in a pressure system. Centrifugal force is defined as the tendency of a body in curved motion to travel in a straight line once released from it 'orbit'...but that doesn't really help me too much...
What I'm after is a vector diagram-type understanding/'picture' for all the forces that act on an average pressure system, assuming it is above the friction layer...
The student was happy with a pretty basic answer, it's just me that's over-thinking the topic
Cheers,
Maudlin.
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Question...
When you say 'GF', do you mean Gradient Force? If so, my understanding was that it was not GF but Coriolis Force that deflects wind (or any moving particle) left/right (depending on southern/northern hemisphere). I understood Gradient Force as being the force which accelerates air particles from a high pressure to a low.
When you say 'GF', do you mean Gradient Force? If so, my understanding was that it was not GF but Coriolis Force that deflects wind (or any moving particle) left/right (depending on southern/northern hemisphere). I understood Gradient Force as being the force which accelerates air particles from a high pressure to a low.
GF is essentially a space referenced inertia effect which manifests itself as an apparent 'force' on a rotating sphere like the earth. This makes it easier to draw a 'balance of forces' picture.
the forces balance out and the air particles/wind being to travel parallel to curved isobars. Is this correct?
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In my (far too many) years working in Meteorology, there's only one useful thing I've learned: 99.9% is an approximation.
Basically geostrophic wind is a simplification that provides a ~95% fit for what is observed and is conceptually simple. In most of meteorology you throw out the smallest terms (which tend to be the most complex) and just retain the dominant ones.
In the case of geostrophic wind it is essentially describing the flow of air from high to low pressure in a rotating reference frame (i.e. the rapidly spinning planet). The assumptions that are made in the derivation of the geostrophic wind is that the motions are evolving on the same timescale as the planet's rotation. This means that it is only applicable for synoptic scale weather systems, and terms like the centrifugal force are but thrown out because they are very small relative to the other terms. If you try to employ geostrophic wind ideas at other scales (e.g. looking at the pressure variations associated with a thunderstorm) it doesn't work as the assumptions are not valid.
Even at the synoptic scale there are all sorts of other things to worry about (e.g. associated with temperature variations) that would effect how closely the wind direction matches the isobars. However as long as you recognize that geostrophic wind is a concept based on some assumptions it'll steer you in the right direction (pun intended) 95% of the time.
Hope this helps!
Gareth.
Basically geostrophic wind is a simplification that provides a ~95% fit for what is observed and is conceptually simple. In most of meteorology you throw out the smallest terms (which tend to be the most complex) and just retain the dominant ones.
In the case of geostrophic wind it is essentially describing the flow of air from high to low pressure in a rotating reference frame (i.e. the rapidly spinning planet). The assumptions that are made in the derivation of the geostrophic wind is that the motions are evolving on the same timescale as the planet's rotation. This means that it is only applicable for synoptic scale weather systems, and terms like the centrifugal force are but thrown out because they are very small relative to the other terms. If you try to employ geostrophic wind ideas at other scales (e.g. looking at the pressure variations associated with a thunderstorm) it doesn't work as the assumptions are not valid.
Even at the synoptic scale there are all sorts of other things to worry about (e.g. associated with temperature variations) that would effect how closely the wind direction matches the isobars. However as long as you recognize that geostrophic wind is a concept based on some assumptions it'll steer you in the right direction (pun intended) 95% of the time.
Hope this helps!
Gareth.
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BOAC,
Thanks. I'm definitely gonna throw the term 'space-referenced inertia effect' around with my next student...
Your explanation of Centrifugal Force (or more correctly, Centripetal Force) also makes sense in trying to picture a basic vector diagram.
Gareth,
Likewise, thanks. I guess that's why the old met texts mention centripetal force and the newer ones don't. The new ones obviously think that it's convoluting the issue too much. Plus, where I live, they've actually taken any reference to CF out of the commercial syllabus so everyone's stopped teaching that it even exists.
Cheers, all.
Thanks. I'm definitely gonna throw the term 'space-referenced inertia effect' around with my next student...
Your explanation of Centrifugal Force (or more correctly, Centripetal Force) also makes sense in trying to picture a basic vector diagram.Gareth,
Likewise, thanks. I guess that's why the old met texts mention centripetal force and the newer ones don't. The new ones obviously think that it's convoluting the issue too much. Plus, where I live, they've actually taken any reference to CF out of the commercial syllabus so everyone's stopped teaching that it even exists.
Cheers, all.
