Understanding weather - fronts, troughs and ridges

[Pages:3]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.

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 1. The Norwegian Frontal Model

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.

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.

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: dpi..au/primefacts

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 2

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