Smoke from bushfires across Australia, 9 January 2020


Since the bushfires started in New South
Wales and Queensland back in September the bushfire smoke has become a real
feature of the landscape, right through the eastern seaboard, and including for
some of our major capital cities. And as more fires have started we’ve seen the
smoke become more extensive and is now even travelling around the globe. So
there’s several different factors that contribute to smoke lingering in an area:
one is the proximity of a location to the fire, another is the topography
around that area which can funnel wind in a certain direction. And then the wind
itself. So if you’ve got a light wind then it’s likely to leave the smoke
lingering in an area rather than dispersing it to other parts of the country.
Another meteorological factor that causes smoke to stay in an area is
what’s known as a ‘temperature inversion’. Normally as you go upwards in height in
the atmosphere we have the temperature decreasing, but when we have a
high-pressure system near us we have air actually descending and that creates a
little pocket of air where the temperature increases with height. And
this acts like a blanket and it traps the air underneath and in this case is
trapping the smoke. Recently Canberra and more broadly the ACT have had thick
smoke persisting and this has really been due to the proximity of the fires
through New South Wales, but also more recently smoke’s being transported from
Victoria and South Australian fires as well. Now Canberra is surrounded by
mountains to the east and normally a southerly or an easterly wind would act
to move out the smoke from the Canberra region but just given where the
fires are to the east and south, that’s just not happening at the moment. So
we’re just seeing that smoke persisting through the ACT. In Sydney smoke haze
has really become a daily feature and it’s actually the western suburbs of
Sydney that have seen the worst prolonged smoke, and that’s because
easterly winds have been coming in offshore and pushing smoke further west,
but then the Blue Mountains to the west of Sydney have been really just trapping
that smoke and not allowing it to move further inland. So again as long as we’ve
still got fires in the landscape smoke is still going to continue to be a
problem for Sydney and the Greater Sydney area. In Victoria the thickest
smoke has been in the east of the State where the worst of the fires have been
and particularly through the valleys and the mountainous areas where at
night time we see the air drain into those valleys creating what’s called a
‘nocturnal temperature inversion’ that’s really trapping the smoke in the
valleys. But of course in Melbourne as well easterly winds have dragged smoke
from the east over Melbourne and even into western Victoria and we’re likely
to see that again on Thursday with conditions really worsening as easterly
winds are bringing the smoke again from East Gippsland. So until we see the
southerly change move through Victoria on Friday we’re really likely to see
smoke linger throughout the State. Tasmania has also seen smoke over the
last few days particularly about the north of the State, and this has really
been due to the effect of two high pressure systems—one to the east and one
to the west. The easterly high pressure system has been driving northerly winds
down over Tasmania and bringing smoke from the Gippsland fires right across
Bass Strait to the northern parts. But then the high pressure system to the
west that’s been dragging southerly winds over the State so we’ve really
seen this convergence line between these two wind regimes and that’s been the
boundary for where we’ve seen the smoke. It’s likely that on Thursday again that
we’ll see smoke increasing across Tasmania but when the strong southerly
change moves through on Friday we should see that clear out and much clearer
skies return to the State. So of course the question that everybody’s asking is
when is the smoke going to clear? Well obviously as long as we’ve got fires in
the landscape and we keep seeing these peak fire weather days we are likely to
see the smoke continue to linger through parts of the country. So it
really is important for people to continue to follow advice from their
local authorities on how to stay safe in these smokey conditions. you

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