Please select your home edition
Edition
T Clewring One Design

Southern Hemisphere Cold Fronts Explained

by Ken Batt, Australian Bureau of Meteorology on 13 Apr 2006
Approaching Cold Front . .
For the maritime community the cold front is one of the most significant features that can be seen every day throughout the year on the Bureau’s weather map.

These fronts mostly affect the southern waters surrounding Australia, but from time to time, especially in the winter and spring months, can penetrate well north into tropical areas. Unfortunately few seafarers fully understand what these fronts are and what to expect when they are encountered when sailing around the Australian coast.

A front is simply the way forecasters represent the boundary between two different air masses. It may be moving, in which case the front is named by the advancing air mass – cold or warm – or it can be stationary, with neither air mass advancing. In Australia cold fronts are far more common than warm fronts.

For those who have looked at the classic textbook descriptions of cold fronts, you may remember the simple explanation given was of an advancing cold air mass pushing under an existing region of warmer air. This lifting of the warm air frequently produces a band of cloud with accompanying showers or rain.
The textbook description, although accurate in broad terms, does not fully explain the sometimes-severe wind changes experienced along the NSW coastline, particularly during the warmer months.

Two important factors of the weather situation largely determine the nature of these changes. The first of these relates to the properties of the air mass ahead of the change – it may not be moist or unstable enough to form cloud or rain, even when forced to rise.

The second factor is the depth of the advancing cold air. In NSW during summer the advancing cold air is often very shallow, with sailors experiencing effects that differ greatly from those that would be expected, based upon the earlier mentioned simple description of the passage of a cold front.

The idea that a front is a single boundary is also frequently incorrect with the concept of a transition zone or frontal zone being more appropriate.

These frontal boundaries are seldom constant in their characteristics. They are continually progressing through a sequence of intensification, then decay, and will periodically completely dissipate or regenerate from an initially very weak change as they move across the waters.

Winds on each side of a front can increase the temperature difference across the front, which leads to a rapid strengthening of the front. Alternatively, a front may run up against a large slow-moving high-pressure system, known as a blocking high, which causes the front to stall then quickly dissipate.

These and other more complicated factors make forecasting the arrival of a wind change (although vague and ambiguous, arguably a better term than front for the NSW coastline) a more complex matter than simply calculating its speed and extrapolating its movement across land and ocean.

South Australia and Victoria
The structure of the cold fronts that affect South Australia (SA) and Victoria during the summer was investigated during a field study called the Cold Fronts Research Program (CFRP) during the years 1980, 1982 and 1984. Two types of cold fronts were identified from this study.

The first was a cold front with an associated frontal transition zone. This was the most common type. A model of the structure of the frontal transition zone has been developed and is shown in Fig. 1.

The transition zone typically extends for approximately 300k ahead of the front and contains discontinuity in wind, pressure temperature and humidity. This discontinuity is usually associated with bands of convective cloud (cumulus clouds, sometimes appearing from the ground as a series of lines of showers that would rapidly move through).

CFRP studies indicate that; the speed of movement of the most significant frontal change depends on the orientation and strength of the winds behind the front and the extent to which the winds ahead of the front blow against the direction from which the cold front is approaching.

The second type of front has a structure similar to a shallow layer of cold air sometimes only about 900 metres thick – which slides under warmer air ahead. It can be thought of as an airflow whose speed depends on the density difference between the cold and warm air and the depth of the cold air, referred to as gravity currents in scientific circles. Winds ahead of the front also influence the front’s speed.



A very strong pressure gradient occurs in the air immediately behind the front and usually there is little or no precipitation with this type of change. This type of front may often be experienced when sailing across Bass Strait during the warmer months, sometimes accompanied by a roll cloud and nearly always with strong winds. At times it will precede the first kind of front mentioned above by a number of hours, typically three to six hours.

Fronts over NSW
The second type of front described above is often not the result of a front originating in the Southern Ocean. Many of them form ahead of a major frontal system somewhere over central southern Australia, even as far east as Victoria or close to the NSW coast and are sometimes associated with small low pressure systems.

These shallow fronts, besides being the most common summer-time front in NSW are also the most difficult cool change to forecast, as they may not exist 24 hours before the change arrives.



A significant problem in forecasting the movement of shallow fronts across NSW is that the mountains of south-eastern Australia retard their progress. Because of this, the fronts develop a distorted shape as they move northwards along the NSW coastline – see the surface synoptic chart in Fig. 2.

In these situations locations close to the coast receive the wind change long before the air behind the front becomes deep enough to move inland as far as the mountains.

Southerly Buster
The most famous of the shallow fronts affecting NSW is the Southerly Buster (also known as the Southerly Burster) which is a change characterised by the sudden onset of strong southerly wind squalls as it speeds along the NSW coast.

Numerous Sydney to Hobart yacht race fleets have experienced a Southerly Buster a short time into the race. The southerly winds are cool, originating from over the sea and usually replace warm to hot north-westerly to north-easterly winds ahead of the front. The winds behind the change are very gusty and frequently reach at least 30 knots, with wind squalls up to 72 knots having been recorded. The strong and gusty winds may last for several hours.

Temperature changes with the southerly buster can be dramatic. A fall in temperature of 10º to 15ºC in a few minutes is common. Southerly Busters are most intense during the afternoon as this is when the temperature difference between the pre-frontal and post-frontal air will be greatest and this is one of the contributing factors to the speed and strength of the front.

The meteorological conditions ahead of fronts can vary considerably. Strong north-westerly winds and hot conditions are quite common. However, if the winds are not strong, sea-breezes may develop close to the coast and moderate temperatures by 10º to12ºC. A short distance inland (10–20 kilometres) the temperature may still be very high.

The moisture content of the pre-frontal air can also vary a great deal with consequent differences in the weather. The hot north-westerly winds are generally dry but when they are moist, conditions can be cloudy to overcast with some rain or scattered thunderstorms possible.

There are a few useful features of Southerly Busters affecting NSW:
· The speed of movement of the frontal wind change in the afternoon can be double that of the

Related Articles

Sailing in the Olympics beyond 2016 - A double Olympic medalist's view
Bruce Kendall takes a look at what he believes Sailing needs to do to survive beyond the 2016 Olympics. Gold and Bronze medalist and multiple world boardsailing/windsurfer champion, Bruce Kendall takes a look at what he believes Sailing needs to do to survive beyond the 2016 Olympics. A key driver is the signalled intention by the International Olympic Committee to select a basket of events that will be contested.
Posted on 29 Apr
From Olympic flag to Olympic Gold and maybe another
The Sydney Olympics was a Sailing double 470 Gold event for Australia. Having won the 420 World Championship in 2000, the feeder class to the 470, while still at school in Australia young Matt Belcher was given the honour of carrying the Olympic flag during the closing ceremony of the Sydney 2000 Olympics.
Posted on 28 Apr
The Road to Rio now 99 days short
The Road to Rio 2016 still has a few curves, bumps and potholes for teams battling to win. The Road to Rio 2016 still has a few curves, bumps and potholes for teams battling to win in Hyeres, at some World championship events and Weymouth World Cup but for many crews: 'It's 106 miles to Chicago we got a full tank of gas, half a pack of cigarettes, it's dark and we're wearing sunglasses.' Whoops wrong movie.
Posted on 28 Apr
America's Cup - Oracle Racing win in Court but with collateral damage
Oracle Racing have had another claim against them by a former crew member dismissed. Oracle Racing have had another claim against them by a former crew member dismissed. Mitchell focussed largely on the circumstances of the matter and introduced into the public arena some interesting documents to support his claims.
Posted on 23 Apr
An interview with Jake Beattie about the 2016 Race to Alaska
In 2014, Jake Beattie and a few friends envisioned the Race to Alaska. Now, it’s time this wild race’s second edition. In 2014, Jake Beattie-the executive director of the Northwest Maritime Center in Port Townsend, Washington, and a few friends hatched the bold idea of a human-powered race to Ketchikan, Alaska, took flight. They decided that their human-powered race would start in Port Townsend, Washington and run to Ketchikan, by way of the inside passage between Vancouver Island and British Columbia.
Posted on 14 Apr
Children of the Internet, Rio and Hong Kong
I have four daughters, the youngest, who in her mid 20's is a true child of the Internet. I have four daughters, the youngest, who in her mid 20's is a true child of the Internet. The kind of conversations I have with her run along these lines.... In the olden days we did not have television until I left school and they had a thing called print magazines, that reported events between two weeks and four months after they happened. And her sceptical response... Hoh! Daddy, Hoh!
Posted on 14 Apr
Go fast girls - 49er FX sailors Paris Henken and Helena Scutt
Paris Henken and Helena Scutt will be representing the USA at the Rio 2016 Olympics, which will be their first Olympiad. American’s Paris Henken (20) and Helena Scutt (23) recently won a berth to represent the USA at the Rio 2016 Olympics in the high-performance 49er FX skiff, a goal that the team has been working on for almost three years. While this is their first Games, writing them off as Olympic newbies would require ignoring their recent results and their strong teamwork.
Posted on 13 Apr
World Sailing Cup V3 - A Dead Rat in a Shoe or Spring Daffodils?
While a host of major sailing events go from strength to strength, the Sailing World Cup has very major issues. Last night my Irish better half was sitting beside me on the sofa watching an Australian version of the popular TV Cooking Program My Kitchen Rules on a tablet with her headphones while I was watching Diehard II for the seventeenth time (it’s a boy thing) on TV. She suddenly spluttered and laughed, took off her headphones and motioned for me to mute Diehard. (Seriously!!)
Posted on 9 Apr
Volvo Ocean Race appoints stadium racing pioneer as new CEO
Sail-World forecast the appointment of Mark Turner as Volvo Ocean Race CEO a month ago. We profiled Turner at that time. Sail-World forecast the appointment of Mark Turner as Volvo Ocean Race CEO a month ago. We profiled Turner at that time. Today his appointment has been confirmed.
Posted on 31 Mar
Large spectator fleet heading north for boat watching season
I’m absolutely not going to lay claim to the phrase, but it is insanely apt and hilarious all at the same time, however. I’m absolutely not going to lay claim to the phrase. It is insanely apt and hilarious, all at the same time, however. Well then, boat watching season is definitely upon us once more. The whales will soon be gathering again off the coast of Queensland to observe all manner of racing and cruising craft as they head North for a Winter in the sun.
Posted on 29 Mar
Naiad/Oracle SupplierKilwell - 1T Clewring - Generic