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Willy Willies and other Weird Winds - Wild Weather

Monday, 15 March  2004 

A dust devil whips through the Gibson Desert

Each week Steve Symonds takes a local's look at our region's weather.

Ask Steve a weather question here.

Willy Willies and other Weird Winds

This week's column was prompted by an e-mail from David who wanted to know about willy willies - what caused them and whether they could spin in different directions.

Willy willies are more commonly known as dust devils around the world. They occur mainly over land but they have been observed at sea although they are much rarer.

Willy willies are not related to tornadoes. Tornadoes come down from cumulonimbus clouds while willy willies form under clear blue skies.

When the sun heats the ground, the ground heats the air above it. The warm air heats the air above that and so on. Wind helps to mix the air so that the heat spreads through the lower layers of the atmosphere. On hot, still days, the air close to the ground becomes superheated. It becomes very hot and expands. The expanded air is less dense than the air above it so it rises rapidly. As it does so, any sideways movement in the initial surge establishes a vortex and the twisting, rising column we know as a willy willy is established.

Most willy willies are small and short lived but occasionally they can develop into substantial columns. Grass lifted in willy willies has been observed at 8000ft in western Queensland. Willy willies are steered by the prevailing wind.

The mechanisms that set willy willies going are stronger than the Coriolis effect so willy willies can rotate in either direction in either hemisphere. When I lived in Mt Isa, there was a bend in the road near the airport. Trucks going round this bend would generate two dust devils, each rotating in the opposite direction. The two would then move across the threshold of the runway which was most disconcerting for the pilot of any aeroplane trying to land.

Willy willies can cause damage to weak structures and are of concern to aircraft close to the ground and drivers towing caravans but generally they are not dangerous.

The term "willy willy" has been used for tornadoes and even tropical cyclones but its main use is for dust devils.

Foehn winds

Air that is forced to rise expands and cools. The rate at which it changes temperature is called the lapse rate. As the air continues to rise and cool, eventually it cools to the dewpoint and cloud will form. As water vapour condenses into water droplets, latent heat is given off. This heat is added to the air and consequently the rate of cooling of the rising air slows down. Air that is forced to rise, therefore, cools at the dry lapse rate until it reaches the condensation level, then it cools at the saturated lapse rate.

On the windward side of a mountain range, we'll use the Great Dividing Range as a good example, winds coming from the west are forced to rise over the range. In winter, when westerly winds often contain moisture, the rising air produces cloud on the western side of the range. This cloud thickens and gives rain which falls in the western slopes.

When the rising air reaches the top of the range, it starts descending on the other side. Descending air becomes denser and warmer so the air starts warming at the same rate it cooled on the windward side. The air is now drier as a lot of its moisture has been lost in the rain. This means that the condensation level, the lower edge of the cloud, will be higher on the lee side than on the windward side. The descending air warms at the saturated lapse rate to the condensation level then at the dry lapse rate. The dry lapse rate is higher than the saturated lapse rate and the air is warming at the dry rate on the lee side for longer than it cooled on the windward side. This means that a place on the lee side of the range near the base of the range will be warmer than a place at the same altitude on the windward side.

The name for this warm wind off the ranges is the Foehn wind, it is the name given to the wind in Germany where it was first described. In Canada it is called the Chinook and New Zealand has one called the Canterbury Northwester. In NSW, the Foehn wind is responsible for the mild winters experienced along most of the coast.

Southerly Busters and Brickfielders

The Southerly Buster should be called the Southerly Burster as this is the name it was first given. The early name was in use for a long time until "Buster" took over. I will use "Buster" as that is the common use but the early name has precedence and describes the wind better.

Cold fronts moving up the NSW coast are held back on the ranges by friction. The ragged mountain tops slow the passage of air. Along the coast there is little friction so the front moves more rapidly. Occasionally a burst of cold air moves very rapidly up the coast. This is the Southerly Buster. Not every front produces a Buster but there are a few every year. The Southerly Buster is usually marked by squalls and very often by cloud. Showers and thunderstorms can occur with the surge of cold air and often a distinctive roll cloud - a long rolling line of cloud - accompanies the change.

I have been asked on many occasions why Sydney doesn't get Southerly Busters any more. I am told stories of Busters arriving as regular as clockwork after hot days in Sydney but you never see them these days. This is just not true. There is no evidence that there are fewer Southerly Busters today. What has changed is our 21st century lives. We have air conditioning in the car, the home, the office, the shops, the pub and the club. We exclude the weather from our lives. If we don't have to live and work in 40°C without a break, we don't notice the change roaring in in the afternoon.

In the early days of the Colony of NSW, Sydney Town was developing and needed bricks. These were made in the brickfields near what is now Haymarket. (There is still a Brickfield Hill in the area). When Southerly Busters came up the coast, the change was preceded by strengthening northwesterly winds. These northwesterlies came into town over the brick pits and picked up lots of red dust. The dusty northwesters were called Brickfielders. The name was also given to the Southerly Buster itself so there was some confusion. Brickfielder as a name has now gone but at one stage it had popularity in other parts of the country and any hot, dusty wind was called a Brickfielder. Perhaps we could revive it?


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Last Updated: 14/03/2004 7:39:00 PM AEST

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