Today Victoria was under a high pressure cell
I was expecting clearing skies and some warmth.
I was just amazed today that the sky was overcast with generally 100% mid to low level cloud. In fact it looked like a low pressure cell was over us today We only saw the sun for an hour and there was no wind.
When l went to look on the synoptic chart to see what had happened, l noticed we were still under the influence of the high
Barom pressure about 1024 hecta pascals
The only thing l saw that could of caused it was a short trough to our west near Adelaide,marked on the synoptic chart posted below
( we are located in western section of Vic). I would like to know how trough lines are marked on the synoptic chart. What is the criteria for the position of them on the synoptic chart?
The trough lines appear as different shapes and lengths and l just can't figure out why they are positioned on the charts as they appear.
I have tried 'googling' but have found nothing so far. Are there any good links l could go to to learn how the trough lines are marked. Thanks
I think this is a description of the weather that day:
"Sometimes, the region between two high pressure centers may assume the character of a trough when there is a detectable wind shift noted at the surface. In the absence of a wind shift, the region is designated a col, akin to a geographic saddle between two mountain peaks."
That's from wikipedia, so maybe not the most reliable source.
Thanks Jayman. I read the link you suggested and very informative especially the sub-links.
Very surprised at how little info there was on that topic especially seeing the term trough is so widely used in weather talk. I read the wikopedia occassionally and generally find it very informative. I'm still grappling with how you would mark in the lines of a trough. Lets us know if you come across anything. I just haven't been able to find info' on the how to..
cheers
crikey wrote:Thanks Jayman. I read the link you suggested and very informative especially the sub-links.
Very surprised at how little info there was on that topic especially seeing the term trough is so widely used in weather talk. I read the wikopedia occassionally and generally find it very informative. I'm still grappling with how you would mark in the lines of a trough. Lets us know if you come across anything. I just haven't been able to find info' on the how to..
cheers
Trough lines are marked as " dashes ". The monsoon trough up our way which does not appear till December usually is marked with " Dashes and Dots " to disguish it from the normal southern troughs. If you have similar Q's, ask me, I was in BoM for 30 years...
Absolutely awesome retiredweatherman!!
I did notice from the N/East Queensland thread the first post indicated you are an experienced meteorologist. AWF is truly blessed with up to at least 2 professional contributors
Yep.. I have Lots of questions. I have not wanted to annoy you previously, so thanks for your welcome in inquiring about the weather..
I guess firstly you mention there are tropical troughs from December onwards up your way in Nth Queensland and the others are from mid and polar latitudes further south.
I noticed you have had a few troughs of the mid-latitude 'variety' ( dashed lines) on the synoptic chart recently.?
Why is there a need to distinguish between the two. Is it just a latitude definition or is there a difference in dynamics etc?
I believe a trough is an elongated area where the pressure is lower? I noticed often storms and rain associated in these regions? No doubt a convective zone? When you look at the BOM observations for the area l can never find lower pressure recorded on the ground observations. Does that mean the trough is located at higher altitudes ( mid-level or upper trough)
Too many questions already. LOL
Noticed there is a particularly long trough over Queensland today( sept 8th 2011)
A trough is basically the dividing line between 2 areas of high pressure. 2 opposing air masses meet - the rear side of one high sees a flow from the north, the forward side of the next high sees a flow from the south ( southern hemisphere ). The pressure is at its lowest between the 2 highs - along the line of demarkation, namely the trough. On a 3 dimensional basis, the flows from the highs are descending. The flows have to go somewhere, so it is basically upwards - again along the trough line. Depending on the middle/upper conditions ( winds/temps ) above the trough line, the forced upward flow may be relatively gentle ( hence not much associated weather ) or more rapid/violent ( hence TS development up to tornado stage sometimes ).
Trough lines are mostly N/S orientated, with variations.
Sometimes a closed low forms along the trough line and its development again depends largely on the upper environment.
The monsoon trough is another name for the Inter Tropic Convergence Zone ( ITCZ ). Its orientation is basically E/W, again with variations.
The ITCZ is basically a line of demarkation between the Equator bound southern hemisphere SE trade flow and the northern hemisphere NE trade flow. If the Earth did not tilt 23 1/2 deg, the ITCZ would be more or less permanently along the Equator, the constant area of maximum heating, with variations over land masses.
However the Earth does tilt, so the area of maximum heating shifts north and south during the respective summers. This allows trade flows from one hemisphere to cross the Equator to the other one, toward the area of maximum heating, this heating accentuated over land masses.
As Asia is a large continent compared to Australia, and therefore more available heating, the ITCZ normally reaches the 30 deg north latitude in their summer, sometimes further. In Australia a smaller continent, with a lesser area of available heating, the ITCZ usually aligns about the 15/18 deg south latitude, and again sometimes further.
In our summer when a large high, say 1045 + hPa persists over northern China and our own southern highs weaken ( due more heating ) and are south of the continent, the NE trade flow from the northern hemisphere pushes south over the Equator, turning more NW as it crosses with massive amounts of tropical moisture, and so the ITCZ can be pushed further south, say down to 25 deg, very occasionally further, and this massive moisture can penetrate way south. As the environment cools with southern penetration there is huge condensation of this moisture, flooding rains result. Depending on the local presence of other troughs and more favourable upper conditions - the flooding rains can be enhanced further over local areas. And of course local topography also plays a part.
During La Nina conditions like during our last summer, the higher sea temperatures off our northern and eastern coastlines further add to the available moisture.
TC's usually ( but not always ) form along the ITCZ and their initial movement is usually along the ITCZ. Sometimes a TC breaks off the ITCZ and is more at the mercy of the higher level air flow which can push it more southwards and can also " drag " the ITCZ with it for a while - with the resultant flooding etc..
The above is very broadly described in relatively simple terms to give a rough understanding between the 2 types of troughs...Hope this helps your understanding.
Thankyou so much weatherman. I learnt heaps from your post.
I am sure l am not the only one to benefit. I think we have at least 3 young regular forum members wanting to train to become meteorologists and everyone likes to progress and learn a little more.
Meteorology is a science that seems to have no boundaries in the realm of knowledge.
Just really enjoying atm . Maybe just a phase. Who knows.
I have lots of questions so l will just ask bit by bit. I have often heard forum members talk about long wave troughing. What exactly is that?
I did notice the ITCZ( inter tropical convergence zone is well into the northern hemisphere atm. Is the zone at its furthest most latitude there atm. I noticed that the Atlantic and pacific cyclones are emerging there quite 'fluently' at the moment. Does the peak hurricane/typhoon season coincide with its maximum migration to the south?
