Severe and extreme heat waves have claimed more lives than any other natural hazard in Australia. Sometimes they don't seem as dramatic as floods, fires or tropical cyclones. So it can be easy to underestimate the risks. But heat waves significantly threaten our health, infrastructure such as power and transport, agriculture and the natural environment.

So what exactly is a heatwave? While we all expect hot weather in the warmer months, it's not classified as heatwave until specific thresholds are reached. A heatwave is when daytime and overnight temperatures are unusually high for three or more days, compared to what's typical for the location and the recent temperatures there. So not every hot day or period of hot weather qualifies as a heatwave.

In Australia, heatwaves start as we approach spring and the end of the northern dry season. The sun and weather systems increase temperatures over northern parts of the country. Clear skies, dry ground and light winds allow that heat to build up and become trapped.

Into the summer months our more significant heatwaves start in the Pilbara in Western Australia. This region is sometimes referred to as Australia's heat engine.

Weather systems such as troughs and cold fronts can drag these hot air masses over southern and eastern Australia. Under certain conditions they can linger for days or even weeks. This heat tends to recirculate anticlockwise and back over central and northern Australia, where the cycle starts again.

Heatwaves aren't the same everywhere. For example, a heatwave in Darwin often occurs at different temperatures than one in Hobart. But both can be dangerous because the temperatures are higher than what is typical, and what the community has experienced recently.

During heatwaves the hot nights have a big impact. When temperatures don't drop overnight our bodies, homes and infrastructure don't get a chance to cool down and recover.

We classify heatwaves into 3 levels of intensity.
- Low intensity, which are relatively frequent during summer. And while they can still pose risks, most people can cope.
- Severe heatwaves are less frequent. These can be challenging. Particularly for vulnerable people, including older Australians, people with medical conditions and young children.
- Extreme heatwaves are rare. And they can affect anyone who doesn't take the steps to stay cool – including healthy people. Anyone who works or exercises outdoors can especially be at risk.

If severe or extreme heatwave conditions are occurring, or forecast within the next 4 days, we issue heatwave warnings. You can check the latest warnings on our website or on the BOM Weather app. And don't forget to turn on notifications.

Understanding heatwaves is the first step to staying safe, and you can find out more on heatwaves on our website. And for further safety information, head to the Red Cross website or your Health and Emergency services website.