Parts of southern Australia just sweltered through their hottest Christmas in more than 70 years. Now, some areas in the east are preparing to usher in the new year amid a severe to extreme heatwave. Is this heat worth all the hype? Is it really an extreme heatwave, or just some of Australia's classic hot summer days? In short, it's both. Australia is one of the hottest countries on Earth. Temperatures exceeding 50 degrees have previously been recorded in two states (SA and WA). It's common knowledge amongst locals around the country that heatwaves are part and parcel of summer Down Under. While heatwaves are an annual occurrence in Australia, they are dangerous and should not be ignored or played down. Many of us have lived through heatwaves before, although some people haven't. Heatwaves are responsible for more deaths in Australia than any other natural hazard, including bushfires, tropical cyclones and floods. Victoria's Black Saturday bushfires caused 173 deaths, although the extreme heatwave preceding this event claimed more than 350 lives. In addition to people's health, heatwaves also impact infrastructure, power and water supplies, transport, healthcare, emergency services, agricultural and bushfire risk. Recent studies have shown that extreme heat in Australia is responsible for the deaths of more than 1000 people aged over 65 each year. Pregnant women, young children, the elderly and chronically ill are most vulnerable to heatwaves, although the most intense events can pose a threat to everyone. So, what is a heatwave? Identifying a heatwave has been a contentious issue in the past because there are many definitions around the world. In general, a heatwave is an extended period of excessive heat. In Australia, a heatwave is calculated by looking at the combined maximum and minimum temperatures over a three day period and comparing this to the conditions observed during the last 30 days. This method calculates what is called the Excess Heat Factor (EHF) for any given three day period, and this value is used to determine the severity of a heatwave. Low intensity heatwaves have little effect on people's health or infrastructure and occur frequently during summer. Severe heatwaves are more dangerous, particularly for vulnerable community groups. Extreme heatwaves are rarer and can impact the whole community and a wide range of infrastructure and services. The EHF used to identify heatwaves in Australia may sound complicated, although it effectively identifies which days are likely to pose the greatest threat to communities in Australia. The Bureau of Meteorology provides a heatwave forecast service covering the next seven days. This can be found at http://www.bom.gov.au/australia/heatwave/. If you are worried about your health during a heatwave, consult your doctor or local health authorities for advice. Information is also available through your state's public health website.