Perth Geography and Weather

Geography

The geography of a locality plays a significant role in the design of airspace and traffic handling procedures.  The prevailing winds and their intensity dictate runway direction and length, while the location of population centres, other airports, vegetation, hills and mountains affect approach and departure procedures.  

The city of Perth is located on a narrow coastal plain, set against an escarpment, the Darling Range, which runs parallel to the coast.  Perth airport is east of the city, close to the escarpment.  Significant turbulence is caused by the passage of easterly winds over the escarpment.  

RAAF Base Pearce is located in close proximity to the north of Perth, which impacts on approaches and departures at both airfields, while Jandakot is located under the flight path to the southwest.


Weather

Perth enjoys a Mediterranean style climate of mild wet winters and hot, dry summers.  Operations at Perth are affected by the passage of fronts in winter, low pressure troughs in summer, easterly winds, the sea breeze, thunderstorms and cyclones.


Sea Breeze - the “Fremantle Doctor”

The South-West area of WA is subject to significant sea breeze activity during the summer months. The prevailing wind is a strong, gusty, hot easterly, but during the day the land heats rapidly and a strong sea breeze crosses the coast.

The sea breeze occurs over two thirds of the days between the months of November to February, and can reach wind speeds of more than 20knots.  As the easterly wind and the sea breeze battle for supremacy, there may occur several closely spaced runway changes at Perth.  A significant low-level wind shear can develop as the cooler air cuts under the warmer easterly.  

Because it is closer to the coast, Jandakot Tower can give 30 minutes or so notice to Perth Tower of the sea breeze’s arrival.  Passed onto Perth TMA and Melbourne Centre, this is a significant benefit for traffic planning. The easterly wind, as it passes over the Darling Range, causes significant turbulence at Perth airport.


Thunderstorm Development

The major weather influences during summer in Perth are the sea breeze and the development of a low-pressure trough down the west coast.  A typical summer situation is the formation of a large high-pressure cell in the Great Australian Bight, which directs hot and gusty easterly winds from inland Australia to Perth.  As a  trough starts to develop down the west coast, temperatures rise into the 30'sC° and a late weak sea breeze develops.

During the next 24 hours the trough of low pressure down the west coast starts to deepen, the wind backs to the northeast and the mornings are hot, with an earlier sea breeze bringing some relief.

The position of the trough is the deciding factor in how hot it gets and what time the sea breeze reaches the city. It can mean the difference of a 40 C° day or a much cooler day of 25 C° - 30C°.

However, the trough can become active and produce thunderstorms and extreme rainfall, particularly to the north and east of Perth.

 

Photo: Frank Bilki, The West Australian

Cyclones

A "tropical cyclone" is a tropical low-pressure system intense enough to produce sustained gale force winds (at least 63 km/h). A "severe tropical cyclone" produces sustained hurricane force winds (at least 118 km/h), and corresponds to the hurricanes or typhoons of other parts of the world.

In the average cyclone season, 10 tropical cyclones develop over Australian waters, of which six cross the coast, mostly over northwest Australia (between Exmouth and Broome), and northeast Queensland (between about Mossman and Maryborough).

In April 1978, Cyclone Alby passed over Perth, killing 5 people and causing severe storm damage and bushfires to the southwest of WA.


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