According to the BOM website, a trough is a line of lowest atmospheric pressure, so that at a point either side of the trough the pressure is higher than on the trough line. This makes sense to me when there are 2 highs next to each other, not separated by a low, then there must be a line of lowest pressure (ie trough line) between the two. Looking at today's weather map a trough over NSW separates two highs. It joins two 1012 isobars and runs parallel between two 1016 isobars. It is a line of lowest pressure (probably around 1012 hPa)separating two 1016 isobars. So it meets the above definition.
www.seabreeze.com.au/gallery/gallery.asp?imageid=1390

But there is also a trough drawn running down the west coast. Unlike the one over NSW this one does not join two isobars of the same pressure. It runs at right angles to isobars of increasing pressure. Where it crosses an isobar (ie a line of equal pressure) there cannot be a higher pressure on either side of the trough as the BOM definition requires. So why is there a trough here ?

Also why does a trough over the WA coast often stay there for a few days, why does it bring hot unsettled weather and why does it kill the seabreeze ?

I used to live in SE QLD and there troughs never seemed to affect the weather and were never talked about but in WA they seem very important.

Any answers appreciated
holgs

A big fat high in the Bight will often trough-out the West Coast,
but then Esperance is nukin'.
It's all good.

Interesting, I hadn't seen that definition of a trough.
That's a classic trough shape for W.A. That's what I thought a trough was, more of a distortion of the lines.
You can see that if you took the bend out of the lines, and stood on that undistorted line where it's crossed by the trough line, then it's at a lower pressure when the distortion's in place, so altough the lines are the same pressure, the bend is actually a lower pressure area.

Think it's life's got something to do with the passage of the high pressure systems.
The wind associated with it are NE, can't help but be HOT! why they're humid though is another question?
It's heat that drives the seabreeze so why does hotter mean less? Somebody did explain it once, but I seem to have forgotten.

Thanks decrepit, I see your point. Reminds me of topographic maps. If this were a topographic map, then the area of distortion where the trough is would be a gully. I can see why this is a trough now. I still hope that someone can explain the humidity and lack of seabreeze and why the trough often stays right on the coast for several days

Just had another look at the map, and a penny dropped. The breeze is SW, when the trough is on the coast the wind is NE, in direct opposition. when the trough moves inland, the wind is SE, not so opposite!

Your theory sounds convincing decrepit. Hadn't really thought of that.
Thanks

feels a tad too simplisttic, think the truth may be more complicated, (it usually is)

Here's my take on it... www.seabreeze.com.au/info/wind.asp

Fantastic laurie, but it doesn't explain the exact mechanism that stops the breeze.

Found this excellent article for you which gives some good insights..

http://www.abc.net.au/northcoast/stories/s1055784.htm

quote:

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I used to live in SE QLD and there troughs never seemed to affect the weather and were never talked about but in WA they seem very important.

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I'd suggest that Perth is generally very dry compared to some parts of Qld .. when a trough "comes down" from the north it brings with it a bunch of water vapor, which drives up the humidity. Because troughs reduce the effect of the seabreeze, it means that there's no cooling wind to stop the maximum temperature at something less than 30 degrees .. so it drives up to 35 deg's+ along with high humidity (like yesterday/today! .. 37 deg's)

Thanks laurie !!!