Australia Weather News

What causes heatwaves and where do they come from? - ABC

Heat waves are Australia's deadliest natural disaster. They have killed more people than bushfires, cyclones and floods put together.

The Bureau of Meteorology defines them as "three days or more of high maximum and minimum temperatures that is unusual for that location".

During the extreme heatwave that hit south east Australia in 2009, 374 people died in Victoria and evidence exists for up to 500 heatwave-related deaths all together.

Events like 2009 are rare — the last comparable event in south east Australia occurred in 1939.

But it doesn't have to be a once in a generation event to put people at risk.

So what are heatwaves exactly, and why do they happen?

Australian heatwaves are usually noticed when hot, dry air from the outback moves over populated coastal areas.

They can occur anywhere across Australia, but we have seen some whoppers across southern and eastern states in recent years.

Why is central Australia so hot in the first place?

The outback gets so hot and dry because a high pressure ridge sits over it most of the time. This high pressure ridge is the result of the relationship between the earth and the sun.

Because our planet is a sphere, more of the sun's energy is focussed around the equator than at the poles. And because hot air rises, this causes warm most air to rise around the equator. As it goes up into the cooler, upper atmosphere the moisture condenses to form tropical rain which is why the tropics are so hot and rainy.

But what goes up must come down. Once this air has lost its moisture it travels away from the equator and falls back down to earth about at roughly 30 degrees of latitude north and south.
This descending air forms a high pressure ridge.

The dominant high pressure prevents rain because it blocks air rising and the air doesn't have enough moisture for raindrops to form. This band of falling air is the reason why there are deserts all over the world about 30 degrees north and south, including over Australia.

The centre's hot, dry conditions are helped by its location so far from the sea.

Moist sea breezes don't make it so far inland and most rain-bearing systems have dropped their rain by the time they make it to the outback.
The high pressure ridge is why high pressure systems are so common at this latitude.

You can spot these systems when you watch the weather report at the end of the nightly news. They are the H's on the weather maps. They spin anti-clockwise in the southern hemisphere.

These high pressure systems along with low pressure systems (clockwise spin and rising air — bringing rain), fronts (which occur where warm and cool air collide, also bring rain) and troughs (areas of low pressure with no spin, yet more rain), are responsible for moving the air around at a continental scale and drive our day-to-day weather.

But back to the heatwaves

Australia's worst heatwave death tolls have historically occurred in the south east of the country and take place when a high pressure system moves across the continent and pushes the hot, dry air from the outback down towards the coast.


When you watch a weather report you will see that highs move across Australia from west to east every week or so, and yet we don't have a heatwave every week — so what makes a normal weather pattern turn into a heatwave?

There are three ways a normal weather week can turn on an extremely dangerous heatwave:

1. If there is a drought in central Australia the air being pushed towards the southern and eastern states will be extra hot. Usually part of the sun's energy is consumed evaporating moist soils, it's called evaporative cooling. But if there isn't any soil moisture around then the energy hangs around in the form of heat. So the air that's dragged away from these dry areas is extra hot and dry.

2. If the high pressure system stalls over the east coast or in the Tasman sea, it will pull more hot air down towards the south east for longer, making the heatwave worse. If it stalls right over the south east the falling air can warm things up too, just like the high pressure ridge that heats up central Australia.

3. If there is a cyclone over the north west of WA it can push tropical heat into the upper atmosphere which is then sucked down over central Australia, making it even hotter out there, which in turn makes the heatwaves in south east Australia even bigger.

Even more epic

If two or even all three of these separate elements join up, we could be looking at an even more intense heatwave.

The Bureau of Meteorology uses three categories to describe heatwave severity and issue warnings.

One way of thinking about the severity of a heatwave would be to consider a game of cricket — during alow intensityheatwave, the whole family can be out playing and enjoying the sun, grandparents included, providing you take normal conditions like drinks and hats. But when it becomessevere, it is not a good idea for the grandparents to be outside. And when you get to anextremeheatwave, why are you even playing cricket?

In Alice Springs, 40 degrees Celsius is just another summer day. But down in Tassie, a few days over 30C can be enough to be dangerous.

It's all about what you're used to. The bureau's heatwave service is sensitive to both the local climate and the recent history of heat.
It also takes into account the overnight temperatures. If it stays hot overnight it will heat up faster and stay hotter for longer the next day. Warm nights can also make it harder for those struggling to recover because they don't get a break.

What's next?

There is evidence that heatwaves are becoming more common in the changing climate. Heatwaves have long been a part of Australian life and they will continue to be for the foreseeable future.

So, keep an eye on the heatwave warnings and the next time it's hot, take it easy if you're in the sun, and keep an eye on the vulnerable. If in doubt seek professional medical advice.
Most of the time heatwaves will be easy enough to shrug off but occasionally a few won't be so lucky and every so often we will all need to take care.

ABC