Thursday, November 26, 2009

The Severe Weather Season in Australia

The period from November to April each year is often a time of considerable atmospheric turmoil across Australia. Heatwaves, bushfires, tropical cyclones and severe thunderstorms are frequent visitors, producing massive damage bills and on occasion, even loss of life.

This is the severe weather season for Australia and a great deal of effort is invested in providing timely warnings to minimise the impact these events produce on the Australian public.

During this period, south-eastern Australia frequently experiences bursts of strong, hot, low humidity winds, and this, coupled with an abundance of often tinder dry eucalypt forest, can produce large and uncontrollable bushfires.

The aftermath of the Black Saturday Bushfires in Victoria, February 2009. Image - Peter Campbell, Wikipedia Commons. (Click on image to enlarge)

Three of the worst examples in recent history are:

Black Friday: Victoria
Friday 13th January 1939
71 deaths, 1300 homes destroyed

Ash Wednesday: Victoria and South Australia
Wednesday 16th February 1983
75 deaths, 2500 homes destroyed

Black Saturday: Victoria
Saturday 7th February 2009
171 deaths, 1800 homes destroyed

Severe thunderstorms:
This weather phenomenon has produced many trails of major damage across Australia in the past, with some of the most financially damaging events affecting the capital cities of Brisbane and Sydney.

Both these cities are located in an environment suitable for severe thunderstorms that are produced when warm, moist sub-tropical air interacts with cold upper atmospheric air that moves up from the higher latitudes. On occasion both cities have been blasted by large hail and strong winds, producing many millions of dollars damage.

Severe thunderstorms remain one of the main forecasting challenges because of their rapid periods of evolution and the small warning lead-time available. A severe thunderstorm can evolve from “no cloud” to a fully-fledged storm in less than an hour, leaving little time to issue weather warnings.

Developing thunderstorm towers high into the atmosphere. Image - Bidgee, Wikipedia Commons (Click on image to enlarge)

They can also produce an astonishing amount of damage in a very short period, particularly when giant hail is involved. Hail larger than golf ball size can smash roof tiles and pulverize cars, producing multi million dollar headaches for insurance companies.

Severe thunderstorms can also generate damaging wind gusts, flash flooding and in extreme cases, tornadoes, that are amongst the most destructive of all weather phenomena.

Tropical cyclones:
The tropical northern coastline of Australia is an area highly prone to tropical cyclone activity, with these often massive storm systems posing a constant threat to the area, particularly between the months of November and April – that is the cyclone season.

Tropical cyclone "Faye" off the coast of Western Australia in March 2004. NASA image - click to enlarge.

The north coasts of Western Australia and Queensland are included in this “strike zone” as is virtually the entire coastline of the Northern Territory. There have been numerous “direct hits” on these coastal areas over the years producing occasional massive destruction and loss of life when a major system collides with one of the larger tropical cities.

Heat waves:
Prolonged spells of hot weather, typically where the daytime temperatures remain five degrees or more above average, are called heat waves, and are amongst the most lethal of all weather patterns.

Mortality rates always show a pronounced peak during heat waves, with many thousands of people around the world succumbing to heat stroke or other related conditions each year.

In Australia it is believed that heat waves kill more people than any other meteorological phenomenon, and estimations indicate that more than 4000 Australians have died as a result between 1803 and 1992.

Heat waves also result in massive increases in power demand, particularly through the use of domestic air conditioning systems and swimming pool filters.

Line of air conditioning units outside a university.
Image: Ildar Sagdejev - Wikipedia Commons. (Click to enlarge)