I recently moved from the UK, where I had a basic "user's" level
understanding of the weather, to Perth, Western Australia. Things are a bit
different here. )

It took me a while to work out that a "trough" is a long narrow area of low
pressure, sandwiched between 2 areas of high pressure. In the UK, low
pressure tends to arrive in a spinning circle, and a long narrow area was
new to me. I tell you this so you understand the level of knowledge I'm
at... }

The way it seems to work here is that a trough forms down the western
coastline, which causes the temperature in Perth to rocket. It's doing it
now, and the forecast is for 38C in a few days time. When this happens, we
all steam for a day or two, then the trough moves inland, taking the heat
with it. We then get a humid day, followed by a milder spell until the
cycle starts again. I don't understand the principles which cause this
behaviour, so perhaps someone would be kind enough to give me layman's
answers to the following:

Why does a trough produce heat? It seems a given a here; weather presenters
on TV say "a heat trough will form..." but no one ever explains why a
trough means such heat!

Why do troughs form down the coastline? Not so important this, but they
invariably appear right down the coast, so there must be a reason.

Why do they go inland? Again, they always move that way so there must be a
reason.

Why does it then become humid?

Apologies for the Geography-101 type questions, and thanks to anyone who can
help educate this overheating Pom... )

Derek Fountain wrote:
I recently moved from the UK, where I had a basic "user's" level
understanding of the weather, to Perth, Western Australia. Things are
a bit different here. )

It took me a while to work out that a "trough" is a long narrow area
of low pressure, sandwiched between 2 areas of high pressure. In the
UK, low pressure tends to arrive in a spinning circle, and a long
narrow area was new to me. I tell you this so you understand the
level of knowledge I'm at... }

The way it seems to work here is that a trough forms down the western
coastline, which causes the temperature in Perth to rocket. It's
doing it now, and the forecast is for 38C in a few days time. When
this happens, we all steam for a day or two, then the trough moves
inland, taking the heat with it. We then get a humid day, followed by
a milder spell until the cycle starts again. I don't understand the
principles which cause this behaviour, so perhaps someone would be
kind enough to give me layman's answers to the following:

Why does a trough produce heat? It seems a given a here; weather
presenters on TV say "a heat trough will form..." but no one ever
explains why a trough means such heat!

It's the heat that produces the trough - warm air expands and rises,
resulting in net lower pressure.

Why do troughs form down the coastline? Not so important this, but
they invariably appear right down the coast, so there must be a
reason.

Not sure if the mechanism is the same, but here in the Pacific Northwest
USA, we get similar patterns during the summer. Higher pressure farther
inland causes easterly winds to blow down off the mountains - as the air
sinks toward the coast, it warms even further due to adiabatic heating
(like a bicycle tire pump gets warm as you pump up a tire, compressing
the air causes it to warm.) This often leaves the coastal areas as the
hottest in the region for a day or so.

Why do they go inland? Again, they always move that way so there must
be a reason.

As the air continues to warm, the 'thermal trough' that results tends to
expand, then move inland as the cooler air over the Pacific to our west
begins to move eastward to fill the lower pressure just inland. After a
few hot days, we will get a wonderfully refreshing marine push, with
temps often dropping 10 to 20C from one day to the next.

Why does it then become humid?

This cwuld be the moist ocean air being drawn inland. Not sure what the
water temperatures are like off your coast, but here they are seldom
more than 10-15C. If they are warmer in your area, you will get the
humidity increase without the cooling we see.

Apologies for the Geography-101 type questions, and thanks to anyone
who can help educate this overheating Pom... )

Hope I have been of some help, and/or marginally successful in being
understandable.

Bob ^,,^

On Fri, 07 Nov 2003 16:19:26 +0800, Derek Fountain
wrote:

I recently moved from the UK, where I had a basic "user's" level
understanding of the weather, to Perth, Western Australia. Things are a bit
different here. )

It took me a while to work out that a "trough" is a long narrow area of low
pressure, sandwiched between 2 areas of high pressure. In the UK, low
pressure tends to arrive in a spinning circle, and a long narrow area was
new to me. I tell you this so you understand the level of knowledge I'm
at... }

The way it seems to work here is that a trough forms down the western
coastline, which causes the temperature in Perth to rocket. It's doing it
now, and the forecast is for 38C in a few days time. When this happens, we
all steam for a day or two, then the trough moves inland, taking the heat
with it. We then get a humid day, followed by a milder spell until the
cycle starts again. I don't understand the principles which cause this
behaviour, so perhaps someone would be kind enough to give me layman's
answers to the following:

Why does a trough produce heat? It seems a given a here; weather presenters
on TV say "a heat trough will form..." but no one ever explains why a
trough means such heat!

Why do troughs form down the coastline? Not so important this, but they
invariably appear right down the coast, so there must be a reason.

Why do they go inland? Again, they always move that way so there must be a
reason.

Why does it then become humid?

Apologies for the Geography-101 type questions, and thanks to anyone who can
help educate this overheating Pom... )

Weather systems move from west to east in the southern half of Australia. They
usually take about three and a half days to go from Perth to Sydney.

If Perth get 40C, Sydney can look forward to a heatwave four days later.

In article , RB@.. wrote:
[snipped previous topic]

Weather systems move from west to east in the southern half of Australia. They
usually take about three and a half days to go from Perth to Sydney.

If Perth get 40C, Sydney can look forward to a heatwave four days later.

Have the bloody things speeded up?

When I did a brief course in "Climatology" at UQ 40 something years
ago, we were told that the high/low systems coming across the
continent were roughly 5 days/2 days. The upshot of this convenient
division was that if you had a wet weekend at the beginning of the
season, then every weekend for some time after was likely to be wet.

The hoi polloi tended to assign this disagreeble state of affairs to
Murphy (or, for the more devout, a sign of God's disfavour).

However, the redoubtable Mr Dick assured us it was just a matter of
prevailing meteorological conditions.

Mind you, Queensland usually only barely gets the tail end of these
systems, and the main effect of them often peters out by Toowoomba (or
did back then) -- so maybe it's just that the tail moves slower than
the head. (Fascinating thought. 8-)


Cheers, Phred.

--
LID

On Tue, 18 Nov 2003 14:08:56 GMT, (Phred) wrote:

In article , RB@.. wrote:
[snipped previous topic]

Weather systems move from west to east in the southern half of Australia. They
usually take about three and a half days to go from Perth to Sydney.

If Perth get 40C, Sydney can look forward to a heatwave four days later.

Have the bloody things speeded up?

When I did a brief course in "Climatology" at UQ 40 something years
ago, we were told that the high/low systems coming across the
continent were roughly 5 days/2 days. The upshot of this convenient
division was that if you had a wet weekend at the beginning of the
season, then every weekend for some time after was likely to be wet.

Well, that's basically what I said. Highs and Lows alternate. So both the highs
and the lows are about seven days apart. The weather is likely to stay about
the same from one weekend to the next.


The hoi polloi tended to assign this disagreeble state of affairs to
Murphy (or, for the more devout, a sign of God's disfavour).

However, the redoubtable Mr Dick assured us it was just a matter of
prevailing meteorological conditions.

Yes it is unfortunate but true.


Mind you, Queensland usually only barely gets the tail end of these
systems, and the main effect of them often peters out by Toowoomba (or
did back then) -- so maybe it's just that the tail moves slower than
the head. (Fascinating thought. 8-)

It certainly does. The weather systems actually move the opposite way across
northern Australia. In between there is not as much movement. Just have a look
at the animated satellite images on the b.o.m website..



Cheers, Phred.