Hello and thanks for joining us live at the Bureau of Meteorology for our Facebook Live event.
I'm Adam Morgan; I'm a meteorologist here at the Bureau.
And I'm Jonno How—I'm also a meteorologist at the Bureau—and we're broadcasting to you
live from our Melbourne office, right here from the National Operations Centre.
Yeah, it's kind of like the nerve centre of weather in Australia, isn't it Jonno?
It's been pretty cold over the last few days, hasn't it? Especially over southern Australia.
Yes, we had that cold front move through
southeastern Australia over the weekend and I don't know about you but on
Friday evening I was pretty warm, it was quite mild across Melbourne, and
Saturday morning: Bam. damn cold all over again.
It was also thanks to a series of cold fronts that moved through
and that's exactly why we're here with you today. We're here to answer all of your questions about 'What is a cold front?'
It's all part of our video series 'AskBOM'.
We get asked lots of questions about the weather, so we've put together this series of videos
to answer some of them; and the first one we're going to address today is
'What is a cold front?' So over the next 15 minutes or so we'd love you to get
into the comments section and type away—ask us all your questions and we'll do
our best to answer them. Absolutely. And on that one we'll go to the first question
from Dianne: How does a cold front form, Jonno?
Yeah, so a cold front in simplest terms
is basically a transition zone between a warm air mass and a cold air mass
and the cold front actually marks the boundary of where that
cold air is pushing out that warmer air mass, so that's why we see with a cold front—
we see warmer conditions ahead of the cold front
and behind the cold front it's just basically just cooler air coming through.
Yeah, that's right, and when we look at a weather map we often will get those blue lines that you see with
little triangles poking out from the front— that indicates a cold front.
And it is warmer ahead of the cold front and colder behind it.
But why do we get that difference in the warm air in front and the cold air behind? How does it actually form?
Yeah. The best way to think about it to think of the atmosphere as being a liquid
and it's in a constant state of just trying to re-balance itself; and I remember at
university my lecturer actually taught me that the best way to kind of talk
about the atmosphere is you think about a cup of tea: So you're making a hot cup of,
you know, English breakfast / Earl Grey— it's hot and it's black and you add that
cold milk and just as you add it you see the white milk kind of swirl around and
while the tea's just trying to get that consistent colour and so it's ready to drink,
and that's what the atmosphere's basically trying to do.
The sun causes a lot more heating at the equator and it's a lot cooler at the poles,
and so the Earth needs to find a way to kind of balance that all out
and a cold front is one of those mechanisms of how the Earth just
tries to achieve that equilibrium, I guess. Yeah you talked about cups of tea.
When I think about cold fronts I like to think of
when you jump in a bath and it's too hot so you turn that cold tap on and you ...
when you mix that cold water with the warm water there's all sorts of cold fronts and warm fronts
even within the water in the bath and you swirl around with your hands to try
and mix the temperature around to get it to a nice even temperature ready for you
to jump in. That's essentially what the atmosphere is doing we the cold fronts.
All right, from Gowri we've got another question now: Why do you most of the cold fronts
approach from the west, Jonno? Yeah so that basically just comes down
to the Earth's rotation: So the Earth rotates from the west to the east and
that's pretty much the main reason why we see our weather
systems come through from the west and to the east so if you're living in
southwest WA, they're normally the first place Australia to get those
really strong cold fronts right off the Indian Ocean, so they're pretty much slammed
straight away from the cold front and a couple of days later we'll see that
cold front move through to the southeast of Australia—so it has to do the Earth's rotation, basically.
Yeah, if we didn't have any rotation of the Earth we actually wouldn't get these swirling weather systems
like low pressure systems and high pressure systems and the cold fronts and warm fronts that define
the boundaries between them— and you talked about how quickly fronts move or
kind of getting from Western Australia through to the east. I remember when I was little my nan told me once—
living in Melbourne—she told me that whatever weather Perth was having, Melbourne
would get it two days later and there is actually an element of truth to that, isn't there?
Yeah, so the thing with weather forecasting is that it's full of these little rules of the
trade and little tips and tricks and rules of thumb and they do exist for a reason.
So as you were saying, you know, that Perth will get a cold front, just say, on a
Monday and it is fairly, with fair confidence you can say that it would
come through like Adelaide and Victoria probably a day to two days
later; and these kind of rules and tricks are what we used to look at before
we had the high-tech satellites and observations and so they do exist for a reason:
because we've been forecasting these for many decades before even satellites were around.
That's right, I mean satellite information has really only
been around since about the 1960s, and back then we'd only get satellite images every, well, six hours or so
and then gradually every three hours and then every one hour during the 90s and 2000s
and then only a couple of years ago now we started getting satellite images every ten minutes.
So we're getting lots of information now and in terms of the speed of cold fronts
and how quickly they move look, if we look at a whole bunch of cold fronts
over the course of a year, I mean, they all vary at different speeds
—some are faster than others—but if you average them all out, that time frame
—say a couple days from the west to the east of Australia—it's a pretty
reliable rule of thumb, isn't it? Yeah, not too bad, yeah. All right, let's move on to the next
question, from Yunus. Hi Yunus, how are you going? Why do cold fronts mean bad weather?
It's a very good question and so when a cold front actually approaches
what happens is the cold air is pushing up
the warm air and as you probably learnt from primary school science, hot air rises
and as the air rises it forms clouds, and so sometimes these clouds
produce like things like rain, sometimes we get storms, and behind the cold front
we get these kind of cold showers so it has to do with just the air being pushed up.
Yeah, the cold front kind of wedges in, doesn't it?
Cold fronts aren't sort of straight up and down in the atmosphere.
Cold fronts actually wedge in from the west and they slope upwards with height, so as they
wedge in they lift that warm air up and if that warm air is moist then that warm air will reach its
condensation point and we'll see cloud form and that's when we see all the rain
and then sometimes thunderstorms as well on the cold front boundary, so some
of the, I guess, most hazardous weather can happen with cold fronts.
And we usually think about cold fronts in winter and certainly they're the ones
that bring all the snow in winter to the alpine regions and heavy rain across
southern Australia that helps fill our dams and our strong winds as well, because
when there's a strong temperature difference across a cold front we get
very strong winds, but actually in summer as well, cold ... We don't necessarily
think of summertime being important for cold fronts,
but cold fronts can really do some damage in summer as well, can't they?
Yeah, so Australia gets cold fronts throughout the year and it really depends on the season as well as where
you are in Australia and that really determines the kind of impacts that
you feel, and so just talking about a summer cold front, what sticks in my
mind is back from Black Saturday 2009:
There were devastating bushfires across southeastern Australia and what we had in that
situation was very hot northwesterly winds and behind that cold front ... behind them we actually
had a cold front coming through and ahead of the cold front we get those
really strong gusty northwesterly winds, and behind the cold front that sharp
westerly change, and that's what actually happened all those years ago—well nine years ago actually—
just that really strong wind change that brought devastating
conditions and very challenging times for firefighters across southeastern Australia.
Yeah, particularly when you live in Melbourne in summer you know to expect those
days when the temperature can drop from around 40°C to 20°C within half an hour with the
passage of a cold front in summer, and we often think of the temperature
change being important in a bushfire situation, which it is,
and particularly the humidity change across the cold front as well, but it really is
that wind direction change that's important in a bushfire situation. When we think about
those northwesterly winds and northerly winds ahead of a cold front—when you
think about how that pushes the fire front, pushes it from north to south, so
you might end up with a really narrow band of the fire front pushing from
north to south, and when that wind changes around to those westerlies or
southwesterlies it then pushes that narrow band in completely the opposite direction
and creates a very long and dangerous fire front. Yeah, absolutely.
All right, let's move on and have another look at another question. Now, from—its related—
from Talla. Hi Talla. Why does the wind pick up each time it rains?
That's a very good question. Thanks for that.
Yeah, that's actually true, so just before like you get a heavy downpour or shower your senses
feel a bit of a gust, and that's basically just gravity doing its work.
So if you think about a cloud and all that rain and precipitation in the cloud,
it's very heavy. It's heavy. It's basically water.
Yeah, I mean, if you're carrying a bucket of water or something there's a lot of weight to it isn't there?
Yeah, how I think about it is if you're sitting in a Metro tunnel
then before the train comes you feel that wind coming through, it's basically
just the mass of water pushing down from above, and so you feel that kind of
gust coming through and then the rain comes through. So it really is the weight
of the water acting with gravity. Yeah, that's like that train, you're pushing
that wind through the tunnel; the water pushing down the wind / the air in the atmosphere as
the rain falls and so when you're on the ground, ahead of a shower, sometimes you
can feel ... or even a thunderstorm ... Yeah, we're in weather language about
things like gust fronts—and they're mini cold fronts, if you like. That colder air,
that colder dense air being helped by the force of gravity with those
raindrops falling from within the thunderstorm or the shower and pushing
out along the ground ahead of the shower, or the area of the shower, so you get
this mini cold front, if you like, coming out of shower or thunderstorm.
That's what we call a gust front. OK, from Tania—Hi Tanya—Why do large rain
bands seem to build up across Western Australia and then die off close
to South Australia and then build up heading to Melbourne? So I guess what Tania
is asking is how do cold fronts like this strengthen and weaken and
strengthen again? Or do they do that? So every cold front is different and the way to
think about it, I always say, is the atmosphere is very multi-layered: So you've got
what's happening at the surface—that's where we live—but kind of through the
atmosphere there's all these different things going on and there's systems
in the upper atmosphere, in the middle of the atmosphere as well, and so for
example if a cold front's coming through from the Indian Ocean and hitting Perth
what happens is that sometime is the upper kind of forcing ... the upper
kind of dynamics actually weaken off by the time they get to Melbourne. Sometimes
they do strengthen as well, so it has a lot to do with what's going on within the
cold front itself. I don't know if you have anything else to add to that?
Yeah, so we can think of two ways: So those upper-level systems certainly do help to weaken and
strengthen cold fronts at the surface, but we can also look at other weather
systems at the surface as well, and often when fronts weaken as they come towards
the east of the country, it's often got to do with a large high pressure
system being located in the Tasman Sea—and again meteorologists might call this
a blocking high. If you get a high pressure system in the Tasman Sea that sticks
around for a week or more then those cold fronts moving across from the west
of the country tend to just hit that, get blocked by it, and then pushed down to the south,
so where Western Australia might experience some severe weather from cold fronts,
the southeast of the country might experience nothing; and the other thing
is, too, when we think back to those analogies around cups of tea and baths with
warm water before, a cold front doesn't live forever.
Essentially, the atmosphere, all it's trying to do with fronts is to try and
re-balance its energy and its heat: So eventually it will ... cold fronts will do their
work, all the circulation around the low-pressure systems,
which is where cold fronts live, close to low pressure systems, but eventually
all that circulation will eventually mix out and so the strength or
the difference in temperature across the front won't be so stark anymore and
so that cold front will weaken over time. Yeah, all cold fronts do have a
lifespan and there's multiple factors at play—whether there are blocking highs, or whether there's
something going on in the upper atmosphere—that will cause them to eventually fade out and kind of
re-balance the kind of temperature distribution. OK, Megan's got one, and
this probably is a nice segue into how we forecast or warn for cold fronts, Jonno.
Megan says 'How do you determine the severity of a cold front?'
Yeah so as weather forecasters we use a broad range of things: So obviously we've
talked about things like satellites—so satellite is a really good way of us
kind of diagnosing how strong a cold front is. So we can kind of look at
how thick the cloud is—so thicker cloud would mean more kind of uplift and
more heavy rain and storms—but also we also use a range of different weather
models here at the Bureau: So we use about four different weather models, including ...
You're talking about computer models?
Yeah computer weather models, that's right. So we've got the Australian
model, we've got the European weather model, and different ... from other
agencies around the world; and the job of a weather forecaster is basically to analyse
these weather models and kind of look at trends of what's been going on
and try to pick out the different attributes of a cold front—or indeed
kind of any weather system, and because the atmosphere is in a constant state of
change the job of meteorologists to kind of figure out what kind of impacts there will be.
So sometimes the cold front might not bring any rain at all, sometimes they
might be really severe, so it's just kind of up to meteorologists I guess to
kind of see what's going on. Yeah, probably the most common question ...
I don't know about you, Jonno, but certainly me, when I meet new people or friends, is
'What does a weather forecaster actually do?'
and I kind of liken our job to being a storyteller: We're taking all that
information that comes in from computer models and satellites, radar, weather
stations ... and we put it all into a story and we tell that to the community to
help the community make the decisions that they need to make day-to-day
based on the weather. And going back to the issue of severity of cold fronts, often we do have
Severe Weather Warnings in winter when we see a cold front and usually
it's generally, or a lot of the time, it's because of the wind and we'll issue
Severe Weather Warnings if we expect winds
or wind gusts greater than 90 km/h to be associated with cold fronts and
also if there is very heavy rainfall expected as well. In terms of ... a lot of cold fronts
don't bring extremely heavy rain, just because they move through quite quickly,
so although you might get short, sharp bursts of heavy rain, we don't get ... or often with cold fronts
we don't often see that prolonged heavy rain that might lead
to a flooding risk. But if I just think off the top of my head, one situation where we
might see, or more commonly see, warnings for heavy rainfall and flooding
associated with cold fronts is if we get like an East Coast Low of the east coast, isn't that right Jonno?
Yeah, East Coast Lows, they bring very damaging kind of weather to the East Coast of Australia,
particularly around New South Wales.
Yeah, OK, and I think on that note Jonno, we're just about out of time. So we've had lots of fun today
talking to you all about cold fronts, and this will be the first in a series of
AskBOM Live segments. We'll try to do one every couple of weeks
to gradually answer all of your questions on different topics related to our AskBOM series.
So we'd encourage you all to go to our YouTube channel and subscribe to the
series and you'll get updates whenever we post a new one, and we'd love you to keep
asking questions in the comments section of today's video and we'll do our best to
answer them as the day goes on. So thanks for joining me Jonno. Thank you, and thanks for watching.
And we'll see you next time. Bye.