Download Understanding weather - fronts, troughs and ridges

MAY 2007
PRIMEFACT 608
(REPLACES AGNOTE ET-11)
UNDERSTANDING WEATHER
Understanding weather – fronts, troughs
and ridges
Paul Carberry
Advisory Officer Climatology, Extensive Industries
Development, Tamworth
Fronts
A front is the boundary between two air masses of
different temperatures. It may be moving, in which
case the front is named for the advancing air mass,
cold or warm, or it may be stationary. A simplified
model of a front is shown in figure 1. The
advancing air mass, cold in this case, pushes into
the existing air mass, causing the air at the
boundary to rise and consequently form cloud and
rain.
This classic model was developed in Norway
during the middle of the 20th century and is often
referred to as the Norwegian Frontal Model.
Although cold fronts often can be described in
terms of this model, observational studies have
shown many frontal systems to be far more
complex.
Figure 1. The Norwegian Frontal Model
At the boundary, or front, there is a marked drop in
temperature, increase in humidity, sudden wind
change and a marked pressure rise. This marked
discontinuity lends support to the theory of two
separate air masses with the front being the
boundary between the two.
Cold fronts are more relevant over the southern
half of Australia. This classic picture occurs in parts
of Western Australia, South Australia and Victoria
and in New South Wales during winter and spring
but seldom applies in New South Wales in summer.
Features that can cause variations from the classic
case include the properties of the existing air mass:
• it may not be moist or unstable enough to form
cloud or rain even when forced to rise
• the depth of the advancing air mass varies – in
NSW during summer it is often very shallow and
can result in just a change of wind direction and
a drop in temperature.
Troughs
Troughs are regions of relatively low pressure
which often precede a cold front. A dashed line on
Figure 2. Cold front, prefrontal trough and high pressure ridge
the weather map indicates the location of a weak
pressure trough. These troughs form during the
warmer months of the year over the southern part
of the Australian continent and waters to the south.
Troughs have the potential to intensify rapidly,
generally at the expense of the cold front, resulting
in strengthening of the northerly winds ahead of a
cool change. In summer the major significant wind
change is associated with prefrontal troughs.
Figure 2 shows a cold front and prefrontal trough
through SA and Victoria. An easterly dip (not shown
here – see What drives NSW weather?) is a
specific type of inland trough that extends into
NSW from Queensland. These areas of relatively
low pressure are unstable and tend to have high
moisture associated with them. Consequently, they
are good sources of thunderstorms.
A high pressure ridge may be associated with
coastal showers when it brings onshore winds
along the east coast in advance of the ridge itself.
These onshore winds can produce widespread
coastal showers.
The zone of interaction of the ridge with nearby
areas of low pressure or troughs can be unstable
and produce storms or rain in any area.
© State of New South Wales through NSW Department of
Primary Industries 2007. You may copy, distribute and
otherwise freely deal with this publication for any purpose,
provided that you attribute NSW Department of Primary
Industries as the owner.
ISSN 1832-6668
Replaces Agnote ET-11
Check for updates of this Primefact at:
www.dpi.nsw.gov.au/primefacts
Ridges
A ridge is a line of relatively high pressure forming
an arm out of a defined high, but not forming a
closed loop. Figure 2 shows a ridge pushing out
into South Australia from the high positioned below
Perth.
Ridges, being areas of high pressure, generally
result in dry conditions in their immediate vicinity.
Disclaimer: The information contained in this publication is
based on knowledge and understanding at the time of writing
(May 2007). However, because of advances in knowledge,
users are reminded of the need to ensure that information
upon which they rely is up to date and to check currency of
the information with the appropriate officer of New South
Wales Department of Primary Industries or the user’s
independent adviser.
Job number 7751
PRIMEFACT 608, UNDERSTANDING WEATHER - FRONTS, TROUGHS AND RIDGES
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