A heatwave is a period of unusual or exceptionally hot weather for a given location. In most jurisdictions it is defined as a minimum sequence of consecutive days where daily maximum temperatures reach a designated threshold.
In Western Australia, the WA Health Department has defined it as three or more days of the daily average temperature (maximum and minimum for each day) is above 32 degrees celsius. This definition was developed largely for the metropolitan area.
Heatwaves are caused by combinations of temperature, humidity and air movements that cause unusually high and sustained temperatures. There are parts of WA (e.g. Marble Bar) that experience significant (+40 degrees celsius) sustained temperatures during the year, but these are not classified as heatwaves because such temperatures are not unusual and individuals living in the area have had time to acclimatise to the heat. Therefore, it is the temperature variability that is important.
A heatwave intensity index has been created by Nairn and Fawcett and piloted in 2014. The index measures excess heat and heat stress. Excess heat refers to unusually high heat that is built up during the day from high temperatures which is not sufficiently discharged overnight due to unusually high overnight temperature. Normally there is a sufficient drop in temperature at night that heat is able to be discharged.
High overnight temperatures however lead to heat retention and there is no break from the heat. The excess heat index is determined by comparing the average daily temperature (maximum and minimum) for three consecutive days to general climate values. Similarly, the heat stress index compares the average three day temperatures to the preceding 30 days, to account for acclimatisation. The combination of excess heat index and heat stress index create the excess heat factor (EHF).
Depending on the level of moisture in the air, the temperature can feel hotter than it would in an arid climate. High humidity tends to lead to high minimum temperatures, and low humidity to low minimum temperatures. Therefore humidity is taken into account by the minimum temperature rather than as a separate factor when defining heatwaves.
On average all areas in Western Australia receive at least one severe heatwave per year. Most towns average about two to three severe heatwaves per year and the greatest number of severe heatwaves occur in the inland Mid West region.
Heatwave impacts can include:
The impacts of heatwaves for humans can be minimised by:
The white line overlying is the observed Australian annual temperature. The warmest year on record currently is 2013. The annual temperatures experienced in 2013 are projected to become the average temperatures of 2030, and in 70 years (2083), such temperatures would be considered a cool year.
Heatwave factsheet (pdf, 343KB)
More information about Heatwaves can be found at: