Many factors play a role in producing extremely high rainfall totals. The wettest area in Australia on average is along the tropical north Queensland coast near Cairns. In this region the moisture-laden southeast trade winds meet the Great Dividing Range. Rainfall is also relatively high along the New South Wales coast and adjacent ranges, western Tasmania and about the Top End of the Northern Territory.
Heavy rainfall occurs through a range of mechanisms including tropical cyclones, slow moving thunderstorms and strong lows. The very heaviest falls often occur over where hills or mountains serve to enhance the rainfall. It is important to note that extremely heavy rainfall is often very localised - totals can vary by tens of millimetres or more over just one kilometre. This is particularly the case for thunderstorms.
To enable valid comparisons of extreme temperatures to be made, it is very important to have high-quality observations measured in a consistent way. Standards relating to the accuracy of the instruments, their exposure and the height (above ground level) at which measurements are taken have been in place for temperature since early in the 20th century. The Stevenson Screen became a national standard by 1910, but before then thermometers were sheltered in a variety of ways, sometimes resulting in the instrument being in direct sunlight.
For these reasons only those records taken since 1910 are included in the tables of extreme temperatures. There have been cases where Stevenson screens have been affected by bushfires and these measurements are not included. Only data that is contained in the Bureau's digital database called the Australian Data Archive for Meteorology has been included in these documents.
Most parts of the country have experienced maximum temperatures over 39°C at some point in the recorded history. Marble Bar, in the Pilbara, holds the Australian record for the longest sequence of days over the old century mark (100°F or 37.8°C). This occurred during the period from 31 October 1923 to 7 April 1924 when the maximum temperature equalled or exceeded 100°F for 160 days in a row. More recently, Marble Bar reached at least 42°C on 43 consecutive days between 16 January and 27 February 2007, and the mean maximum temperature for that month of February was 44.9°C, the highest ever recorded in Australia.
Low temperatures tend to occur in Australia when a southerly airstream transports Antarctic air rapidly northwards over the continent. On occasions this cold air can move as far north as Queensland. Very often these cold air streams are also very dry and allow rapid cooling at night giving rise to temperatures which can be well below 0°C.