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Why troughs point to equator and ridges to poles?


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Hello! I'm reading a portuguese (I'm brazilian) Meteorology book, and it says troughs (low pressure elongations) always point to equator and ridges (high pressure elongations) always point to poles. I don't understand why.

The book says too that cyclones (low pressure areas) rotate counter-clockwise in the Northern Hemisphere and clockwise in the Southern, and that the opposite occurs for high pressure areas. I understood that after spending some time analising the processes, considering Earth spin, Coriolis force and pressure centers attraction/pulling act on the moving masses. But I could'n understand why the elongations are the way they are using this knowledge neither thinking about the moving masses cooling/heating (changing their pressure as they approaches or leaves equator proximities. None of my analysis results in a answer that corresponds to reality (what book says).

Thank you for any help!

 

 

 

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Posted
  • Location: Eastbourne, East Sussex (work in Mid Sussex)
  • Location: Eastbourne, East Sussex (work in Mid Sussex)

    Thank you for any help!

     

    Welcome Eduardo!

     

    I'm not sure I understand quite why yet, but this may point you in the right direction:

     

    http://www.theweatherprediction.com/charts/500/basics/

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    Posted
  • Location: Nuneaton,Warks. 128m asl
  • Weather Preferences: Snow then clear and frosty.
  • Location: Nuneaton,Warks. 128m asl

    Welcome Eduardo,

    In simple terms the jetstream in the N.Hemisphere marks the boundary between the cold Polar air moving south and the warm Tropical air moving north.

    The jetstream never runs flat and undulates or meanders because of the spinning of the Earth around it's axis and the torque effects of Mountain ranges.

    This creates waves or troughs and ridges in the general flow sometimes called Rossby waves..

    The troughs contain the cold upper air which remember is coming south and will point towards the equator in trying to undercut the warm Tropical air moving north.

    Ridges forming ahead of the trough will then of course point northwards.

    The corolis effect inducing cyclonic activity along the boundary.

    This site here goes into more detail and has graphics that may make things easier to understand

    http://www.skepticalscience.com/news.php?n=1967#trop

     

    I hope i have explained it Ok but have a read of the link which probably explains things much better than i can.Posted Image

     

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    Hello my friends! Thank you very much for your help! That was good materials. I'm knowing a lot more now. I'm not sure if it's tottaly clear yet for me, but I think it is quite enough.

    I'm just starting metheorology studies, but I have a reasonable knowledge about physics, and I'm trying to understand the more I can about the physical processes that determines metheorology while I study this new science area.

    It was very interesting reading about the vorticity and the forces that create it. But I couldn't inicially get rid of my inicial doubt (maybe bad explained by be) "why cyclones have troughs that point to equator and anticyclones have ridges that point to the pole". (I kept studying and writing this message for a long time today, and now I think my error was to associate ridges/troughs to cyclones/anticyclones. I will talk about this below)

    I didn't know about these jet streams, I've spent some hours yesterday and most part of the day today studying and trying to understand them and another related processes before posting here again. I didn't fully understand the jet streams, but I'm pretty convincend that at least should exist one stream (per hemiphere) of fast wind going east at some latitude because of the equilibrium there between coriolis and pressure gradient forces, and because disturbances in the flow would change coriolis force in a way that the wind tends to get back to the "jet stream place" (and the jet stream exists just because of this stability, that concentrates the wind at a narrow place).

    Well, some irregularities (always existing on real world) on the pressures will change the jet stream path. If we call trough a low pressure lobe, and ridge a high pressure lobe, it becomes clear that, on the jet stream path, the trough must be to equator and the ridge must be to the pole, because the jet stream is separating a low pressure area at the higher altitude from a high pressure area at the lower latitude. But this becomes a little more complicated when there's closed isobars, and we have the pole at the north and the equator at the south.

    Maybe because the way my book presented the subject "troughs and ridges", basically drawing circular isobars forming a low/high pressure spot with a little elongation to the south/north that was the trough/ridge, and doing this just after talking about cyclones and anticyclones, that was the same thing without the elongations, I thought this troughts and ridges were all about cyclones/anticyclones and that I should explain them by the same processes (coriolis forces and stuff) I used to explain how cyclones arise.

    Reading more about troughs and ridges, now I think it was an error to think this concepts would be strictly associated with cyclones and anticyclones. If I think at a ridge just as an elongation of the isobars of a high pressure region, and knowing that the poles have low pressures that attracts these "high pressures isobars" (attracts the high pressure masses, changing the isobars path), it's clear that the ridge has to be towards north, because equator has higher pressures than the pole, and all the high pressure lines will tend to go to the north. All the high pressure masses will try to squeeze themselves to find a way to go northward. For the low pressure centers, it works the opposite way: the equator high pressures will try to find a way to "inject" masses towards north, and it will "squeeze" the low pressure center starting at its south part (equivalent to a southward elongation).

    I don't know if my thought are quite right, but at least now I have a understanding that makes some sense and corresponds to what the books says.

    Thank you Coast and phil nw. Anything else you want to add saying I'm right or I'm wrong I'll be greatful.

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    Posted
  • Location: Ireland
  • Location: Ireland

    Oi Eduardo

     

    The reason why troughs normally point to the equator (not always) is that they are areas of colder air, therefore they normally originate nearer to the Poles. Conversely, ridges are areas of warm air, therefore originate nearer to the equator.

     

    When I say cold or warm air I mean low or high geopotential, respectively. Geopotential (the height of a certain pressure level) is directly proportional to the virtual temperature (which in turn is proportional to the temperature and humidity) of an airmass. As cold air is denser than warmer air, the spacing between pressure levels is less in cold air. This means that all the pressure levels in cold airmasses are at a lower height than in warmer airmasses, therefore leading to the troughs and ridges.

     

    I wrote this article which might explain it a bit better. http://irishweatheronline.wordpress.com/how-weather-works/our-atmosphere-and-how-it-makes-weather/

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    Hello Su Campu!

    Nice article!

    Rethinking what I last said, actually I don't know if it was a good interpretation.

    Let's see if I get you: It's clear to me that we have closer to the ground and less spaced pressure levels for colder air masses. But it's not quite clear how it leads to troughs and ridges. Do hot masses tends to "go down the pressure level lean" at high altitudes to the poles or to other low pressure areas where the atmosphere is lower, therefore forming the ridges, and cool masses to go "by the ground" (low altitudes) to the high pressure areas, therefore forming the troughs? So as we ordinarily have lower pressure areas to the poles and higher pressure areas to the equator, troughs and ridges will normally point that way. 

    If that's right, I think we should feel more cool winds at low troposphere (where we live) coming from poles that hot winds coming from equator (because the hot masses would circulate at higher altitudes). Is that what actually happens?

    Thank you very much!

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    Posted
  • Location: Ireland
  • Location: Ireland

    Basically the global circulation's job is balancing out the large temperature difference between the equator and the Poles. Warm air flows poleward and cold air equatorward. Throw in the effect of the Earth's rotation (Coriolis force), and we get large wave patterns (Rossby waves) in the flow, with the jet stream forming due to the large thermal (and hence geopotential) difference where the two airmasses meet (the Polar Front).

     

    What is the book you are reading? I would recommend Tim Vasquez' books as good starting material, but if you already have a good grasp of physics then Roland Stuhl's "Meteorology for Scientists and Engineers" or James. R. Holton's "An Introduction to Dynamic Meteorology" (but you'd better be good at vector calculus for that one!).

    Edited by Su Campu
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    I'm reading "Meteorologia e Oceanografia - Usuário Navegante" by Paulo Roberto Valgas Lobo and Carlos Alberto Soares. It's part of the bibliografy for a maritime pilot application (test / recruitment.. I don't know the english best word for that) here in Brazil. It's important that I read this book, even if it's not a good one (I don't really know if it is or it is not, but It seems to be away from being a "best one").

    But of course I want to learn how things are the right and best way, so I would try to find one of the books you suggested! By the way, I'm an Engeneer, then I liked the idea of "Meteorology for Scientists and Engineers". I think I will guide my study by the brazilian book and use these others to actually learn things.

    Thank you again!

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    Ok then! Maybe I'm going to deep trying to understand things beyond the application scope. It's just that I like it =) ... I will move on with the study by the book since I understood enough about troghs and ridges to not mix it up months later when I won't remember so much about it.

    Obrigado!

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