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#625837 - 17/07/2001 10:17 Troughs, Ridges and their Highs and Lows
MareebaWeather Offline
Weatherzone Addict

Registered: 15/12/2003
Posts: 1820
Loc: Mareeba 17 S 145.2 E
Hi all,

I thought I'd post this in a different topic since, more so because I think this is a common question.

The original question was posted by O-3 and Weathergirl asking what a trough was.

A trough is an elongated area of relatively lower pressure to its surroundings. However, unlike its older brother the low - it doesn't have its low pressure completely encompassed by a circle of lower pressure like a low does.

The cousins of the trough and low are the ridge and high. Basically it's the opposite - a ridge is an elongated area of relatively higher pressure than its surroundings. And its older brother the high is an area of relatively high pressure but has an area of high pressure completely encompassed by a circle of high pressure.

The key here is relative to its surroundings. For example, in the tropics the pressure is often generally quite low, so 1015hPa might be considered a high if its sitting in between two monsoonal depressions. However, further south, say during winter. If there's too big ugly 1030+hPa highs, and a pressure of 1015hPa between it that is fully encompassed by a circle of lower pressure, then it might be considered as a low.

I've marked an MSL below with some ridges and troughs (the highs and lows are already marked) - I wanted to use different colours, but my computer is cranky and won't let me...



You can see here the troughs (marked with a 'T') and the ridges (marked with a 'R'). One of the troughs was already marked in by the BoM - this is represented by the dashed lines over QLD. As you can see, all the regions are elongated regions, sometimes stemming from high and low pressures themselves. Other times you will naturally get troughs between two highs, sometimes attached to cold fronts.

Troughs also often mark wind changes, because of this there is often an airmass change. A cold front is an airmass change - but it is a very significant airmass change where the air behind the cold front is significantly colder than the air ahead of it. Often in Australia you have warm to hot NW'lies ahead of a cold front, with a SW change. Often a trough (or several troughs) are responsible for this.

Hope this helps...if there's any other questions don't hesitate to ask! The best way is to just keeping a look at the BoM Mean Sea Level images and getting familiar with them!

AC
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Mareeba - Queensland - Australia
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#625838 - 17/07/2001 13:20 Re: Troughs, Ridges and their Highs and Lows
filski Offline
Weather Freak

Registered: 07/06/2001
Posts: 233
Loc: Brisvegas
Wow, thanks Cyclone - I was just coming in to ask about this.

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#625839 - 17/07/2001 20:36 Re: Troughs, Ridges and their Highs and Lows
TrenthamStormchasers Offline
Meteorological Motor Mouth

Registered: 15/04/2001
Posts: 6258
Loc: Trentham 705m
OK Cyc, now can you go on to explain 2 things for us - why troughs & ridges appear 'reversed' at higher level of the atmosphere eg: 300hPa, to those at the surface, and also at what height do they 'reverse'.

By 'reversed' I mean that a trough at 300hPa will have its axis to the north and be open to the south - I don't think I'm making myself very clear - help!!!!!!

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#625840 - 17/07/2001 23:25 Re: Troughs, Ridges and their Highs and Lows
MareebaWeather Offline
Weatherzone Addict

Registered: 15/12/2003
Posts: 1820
Loc: Mareeba 17 S 145.2 E
Hi Jane,

Different types of troughs and ridges can occur. IE - you can have summer surface troughs that point to the south, but these generally occur in warmer areas (and in summer) in the southern hemisphere. These are often associated with heat troughs in central Australia (due to the very strong heating, the air mass rises and an area of low pressure is formed).

Below is an example of a surface summer trough (Nov 5, 06z, 2000).



This type of trough wouldn't occur much down in your parts of the woods.

The types of troughs that you guys would be used to would be winter troughs, they point to the north, and are often associated with low pressure systems further to their south, and sometimes have cold fronts associated with them.

Below is an example of a winter surface trough (June 12, 2001, 18Z)



So why do we see an increasing number of "winter like surface troughs" (ie, their axis points northward) as you ascend higher into the atmosphere? Summer troughs, even at 300mb still exist though, an example is below: (May 06, 2001 00z)



Well - we know that troughs are a region of lower pressure. So what causes lower pressure in the upper atmosphere? Cold air does - it is more dense that warm air, thus it takes up less space than warm air. So there's lower pressure heights, and the pressure is lower.

So - where does the cold air come from? Well, this tends to come back to a fundemental question - what is weather? Weather is the ongoing balance of the uneven surface heating on the Earth. The Equatorial regions receive high amounts of surface heating, and the Polar regions receive small amounts. Essentially, the cold air from the poles move towards the Equator (in the north in the Southern Hemisphere), and the warm air from the Equator moves polewards (to the south in the Southern Hemisphere).

So if the upper level troughs require cold air, then it would make sense that the bases of the upper level troughs in the Southern Hemisphere commence on towards the south, and point to the north. (IE, the cold air intrustion is to the north, with its base, or open end towards the south).

Upper level ridges would be the opposite of this, as they would require warm air.

The question then arises why do the winter troughs (northward pointing ones) compared to summer troughs (southward pointing ones) occur more often as you ascend into the atmosphere?

Well - we can see that the surface is much more variable than the upper levels. We can readily get summer surface troughs. And they are often reflected in the lower layers (ie, 925mb, 850mb and 700mb). Often they are the result of heating lows and troughs (the summer troughs at those levels). But to then answer your question of where does the change from a mixture of winter and summer troughs, to generally only winter troughs (with the exceptions to the rules) occur? Generally around 700mb - after this, the winter troughs tend to dominate. And the only reason why you'll get a summer trough, is if a cut-off upper level develops (ie, the example I showed before at 300mb), and extends colder air to its south towards the poles due to it being completely enclosed by warmer surrounding air.

Hope this helps answer your question!

AC
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Mareeba - Queensland - Australia
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#625841 - 17/07/2001 23:37 Re: Troughs, Ridges and their Highs and Lows
MareebaWeather Offline
Weatherzone Addict

Registered: 15/12/2003
Posts: 1820
Loc: Mareeba 17 S 145.2 E
Hi all,

There is one other pressure region that I forgot to mention. That is a col. This is a broad area of equal pressure (ie, no highs or lows) - the area is often very still as there are no pressure gradients.

Just thought I'd mention that...

AC
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Mareeba - Queensland - Australia
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#625842 - 18/07/2001 02:48 Re: Troughs, Ridges and their Highs and Lows
O-3 Offline
Weather Freak

Registered: 07/06/2001
Posts: 368
Loc: SA-AUSTRALIA
Well cyclone, i think i learned more from those posts than i have since ive been coming to this board!
I'm glad i asked that question....turns out it wasn't AS stupid of a question as i first thought it was... thanks buddy.

Ozone.

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#625843 - 18/07/2001 11:26 Re: Troughs, Ridges and their Highs and Lows
Kirk Offline
Member

Registered: 23/05/2001
Posts: 2239
Loc: Natalia Cooper Fan Club - Pert...
Another question

What is it that makes a trough active,Can a trough also be known as a convergance zone.
Often with cold fronts we get a Ne wind change to a Nw.Sometimes twice a year we will get a thunderstorm from this.Why doesn't this happen all the time??

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#625844 - 18/07/2001 13:14 Re: Troughs, Ridges and their Highs and Lows
Dave Williams Offline
Member

Registered: 07/03/2001
Posts: 845
Loc: Sydney, Australia
Surface pressure is not simply related to the temperature of the air mass at all. It was my intention to get around to this with regards to what I was doing with the thicknesses. But it seemed that too few got involved in it.

Hence ridges and troughs are not simply related to the warmth and coolness of the air. Sure this is a factor but the primary reason for the variations are related to the dynamics of the atmosphere.

Regions of higher pressure are usually associated with regions of the atmosphere in which there is an accumulation of air, through the process of convergence and subsidence etc. Regions of low pressure are regions in which air is being 'lost' through mass divergence.

I won't say much more seeing I'm short of time, but one of the interesting aspects with low pressure systems is that at the surface they usually have air converging into them. Why the lower pressure when they actually have a convergence of air? It is because the air is diverging out higher up in the atmosphere at a greater rate than it is converging at the surface. Hence there is an overall loss of air..... Don't forget tropical cyclones have very low air pressure at their centre and these guys/girls are pretty warm. It is more than the temperature alone that determines whether it is a region of high pressure or a region of low pressure..... if only the weather was that easy.

Dave.

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#625845 - 18/07/2001 13:22 Re: Troughs, Ridges and their Highs and Lows
MareebaWeather Offline
Weatherzone Addict

Registered: 15/12/2003
Posts: 1820
Loc: Mareeba 17 S 145.2 E
Hi Dave,

I hope that wasn't the impression I gave in my post - because that is not the case, I was referring to upper levels, and not the surface pressure when I was referring to temperatures.

I thought I made this clear when I talked about heat troughs (ie, hot air rising and causing a broad region of low pressure) but perhaps I didn't.

AC
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Mareeba - Queensland - Australia
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#625846 - 18/07/2001 13:27 Re: Troughs, Ridges and their Highs and Lows
MareebaWeather Offline
Weatherzone Addict

Registered: 15/12/2003
Posts: 1820
Loc: Mareeba 17 S 145.2 E
Hi Kirk,

A trough is simply a trigger (ie, convergence line), by which it can cause convection (eg, thunderstorms).

We need heat and moisture as well to assist in convection. How much heat and moisture? That is highly dependent on the coldness of the atmosphere above the trough. But if you have a very dry trough (which can often be the case with the inland WA heat troughs), where you get NE'lies, they're very dry. So if there's a strong upper level ridge over you, the temperatures over you will be very warm, so potential instability will be decreased.

There's a lot more to it than this...ie, if there's an upper level ridge it'll cause subsidence over the area and further assist in surpressing the potential convection, or if there's an upper trough, then it'll enhance convection.

Then there's the wind shear and the processes that occur there too...

Not to mention whether or not the cap will break!

AC
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Mareeba - Queensland - Australia
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Live Weather Data & Lightning Tracker

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