How do heatwaves happen?
In Australia, heatwaves can form when a mass of hot air develops and persists over large areas for a long time. This hot air is caused by slow-moving weather systems.
Heatwaves can also be influenced by geography, climate, and location.
Video: Understanding heatwaves
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.
Influences on heatwave development
Weather systems
Slow-moving high pressure systems or heat lows over inland areas recirculate hot air. This creates an environment that builds up heat. High pressure systems push the air down. This creates a layer of air that acts like a lid. It can prevent deep clouds from forming and allows more sunlight to heat the land.
Wind plays a role too. Strong and persistent offshore winds limit cooling from sea breezes. This can result in heatwave conditions over coastal areas.
Slow-moving weather systems can produce light winds over inland areas, particularly in valleys. This can trap heat for several days until there is an airmass change.
Big differences in temperature at high altitudes cause strong changes in air pressure. This generates powerful winds called jet streams between the troposphere and the stratosphere. These winds can join with upper-level weather systems. Together, they can produce slow-moving high pressure systems. This pattern can result in persistent heat over an area and increase the chance of a heatwave.
Soil moisture
When soils are dry, less water evaporates. Evaporation helps cool the air so dry soils result in warmer air near the ground during the day.
Urban heat island effect
Heatwaves are more likely to occur in cities than the countryside. Cities can be several degrees hotter than nearby rural regions. This is due to the urban heat island effect.
It happens for several reasons. Cities have less vegetation and more man-made materials like buildings, concrete and roads. These materials absorb and retain heat. They are more easily heated and stay hot longer than most natural surfaces. With fewer plants and trees urban areas also get less shade and evaporative cooling.
On top of this, things like transport, factories and electricity use can also create more heat.
Climate patterns
Heatwaves typically occur during summer but can occur as early as spring or as late as early autumn. In northern Australia, the hottest days often come in late spring or early summer. This happens during the build-up period before the onset of the monsoon.
Changes in large-scale climate patterns can make heatwaves happen more often. They can also make heatwaves last longer or become more intense in Australia. These climate patterns include:
Certain phases of these patterns can favour persistent dry and stable atmospheric conditions.
In the past, El Niño events have often been linked to widespread and prolonged heatwaves. These have been most common in spring and summer.
A positive phase of the IOD can also lead to hotter and drier conditions during winter and spring. This increases the likelihood of spring heatwaves in southern Australia.
The SAM affects temperature patterns across southern and eastern Australia. A negative SAM usually brings more spring and summer heatwaves. A positive SAM can reduce the chance of extreme spring and summer heat.
Unusual warmth can still occur during the cooler times of the year. These are not heatwaves but can still have widespread effects across the landscape. They can affect the flowering of fruit trees or crops and the behaviour or movement of animals.
Impacts of sea surface temperatures (SSTs)
Heatwaves can be stronger when surrounding sea surface temperatures are above average. This is due to higher overnight temperatures limiting night-time cooling.
Warmer oceans increase evaporation and atmospheric moisture. This can lead to periods of hot and humid conditions over coastal regions. Conditions like these increase the apparent temperature and health impacts of heatwaves.
Sea surface temperatures around Australia have warmed by 1.08 °C since 1900. This has contributed to a warmer climate and increased the likelihood of heatwaves.
Lifecycle of an Australian heatwave
As spring and the end of the northern dry season approach, northern parts of the country get warmer. The higher temperatures then begin to extend southwards. This is when heatwaves usually start to happen in Australia.
Heatwaves can be localised or can occur over large areas. Each one is different in extent and intensity.
But what does the lifecycle of a typical Australian heatwave look like?
- Heat builds up in a dry inland region, such as Western Australia's Pilbara. This area is sometimes called our 'heat engine'.
- Weather patterns push or drag the hot air to the south and east.
- A slow-moving high pressure system or a heat trough forms and traps the heat in place.
- Clear skies and dry soil cause the air to heat up further.
- Cool air mass changes are blocked or delayed. This includes surface troughs or cold fronts coming from the Southern Ocean. This makes the heat last longer.
- The hot air slowly contracts north and then back to the west, where the cycle may start again.