For the first time, the Bureau of Meteorology has provided a national definition of a heatwave.

Many Australians have kicked off the new year in sweltering heatwave conditions, with the mercury soaring to record-breaking temperatures in several states.

The bureau says heatwaves have taken more Australian lives than any other natural hazard in the past 200 years, but until now it had not given a national definition of just what constitutes a heatwave.

A heatwave is now defined by three or more days of unusually high maximum and minimum temperatures in any area.

ABC weather guru Graham Creed says there are three grades of heatwave, with severe and extreme posing the most serious risk.

“Standard heatwave will only have slight effects on the general population,” he said.

“As we move into severe, that’s when we start to see on the elderly and also people with debilitating illnesses.

“Then as we move into extreme heatwave conditions, that’s when the general public and also infrastructure can be affected.”

Extreme levels of heat will also coincide with dangerous fire weather conditions across the southern states.

The definition was enabled by the bureau’s new heatwave forecast service, which allows it to map areas that are expected to have unusually hot conditions over a period of four days.

Creed says the service provides a “very interesting” forecast using a variety of factors.

“The severity is a function of the maximum and minimum temperatures,” he said.

“So it’s not just looking at a run of a couple of hot days – it also takes into account what the temperatures have been like in the mornings.

“So if you have a really hot morning, it means that your high maximum temperatures are going to be felt for a longer period through the day.

“That accumulated over three days becomes a real strain on the body, particularly if you’re young, elderly or ill.”

The forecast also uses local climate averages, and examines how the temperatures have changed over the past 30 days.

Creed says that provides an insight into “whether or not the population will be acclimatised to warmer conditions”.

“For areas such as the Pilbara, the Kimberley, even Central Australia, you can have ongoing hot conditions and a rise of a couple of degrees won’t have a huge impact,” he said.

“But get a big run of hot temperatures across the south-east, where maximums usually sit around the low to mid 20s for the summer, then it’s going to have a much bigger impact because the population is just not acclimatised to that heat.”

Health warnings over ‘silent killer’

The bureau and emergency services describe heatwaves as the “silent killer” as they can adversely affect the health of vulnerable people.

Outdoor workers, athletes and the homeless are also considered to be particularly vulnerable during heatwaves.

Steven Clarke from Queensland Ambulance says drinking plenty of water is key.

Mr Clarke says families should ensure loved ones are coping with the heat, with the elderly particularly vulnerable.

“For the elderly and people with chronic illnesses, especially with kidney problems, diabetic problems, obese people, they need to take extra care,” he said.

“Probably a critical point to make is family need to check their loved ones – grandad and grandma living on their own – they need to ring them up, they need to go around and visit them and make sure they are doing the right thing.

“Especially with the elderly. They’ll lock their windows to make themselves feel safe; they live on their own. Family need to spend the time and go visit them.”

At the conclusion of summer, the Bureau of Meteorology will evaluate the accuracy of the heatwave forecast maps and work with the health and emergency services sector on developing a heatwave warning system.