A large pool of hot air will sweep across Australia this week, sending the mercury soaring in multiple states and territories. While this type of heat is common in Australia during December, some members of the community are likely to find the upcoming weather challenging.

So, what makes this week's spell of sweaty weather a heatwave?

Generally speaking, heatwaves are prolonged periods of unusually warm daytime and nighttime temperatures for a specific location.

In Australia, heatwaves are defined as three or more days and nights where temperatures remain unusually high, relative to a location's long-term average at that time of year and the temperatures observed there during the past 30 days.

Because this definition is location-dependent, the temperature threshold for heatwaves differs across Australia. This is why a single temperature value can't be used to define heatwaves across the country.

The three types of heatwave classifications in Australia are: Low-intensity, Severe and Extreme.

Low-intensity heatwaves generally aren't dangerous and most people will be able to cope with these conditions.

Severe heatwaves start to place more stress on vulnerable groups of the community, such as the elderly, pregnant women, young babies and those with chronic illness.

Extreme heatwaves are the most intense type and may impact infrastructure, such as transport and power networks, and pose a threat to the health and wellbeing of regular members of the community.

Heatwaves can also pose a threat to pets and livestock, so it's important to consider their wellbeing alongside your own.

While heatwaves may seem benign compared to other types of extreme weather, such as thunderstorms or tropical cyclones, they are the most deadly natural hazard in Australia.

More than 370 people lost their lives in the heatwave that preceded the Black Saturday bushfires in Victoria, while the fires themselves claimed just over 170 lives directly.