Explainer: what is a heatwave and why are they so dangerous?

As the first heatwave of the season drifts across Australia, the man behind heatwave warnings reminds us that heatwaves kill more people than all other natural phenomena put together.

Dr John Nairn – formerly with the BoM but now working as a consultant to the World Meteorological Office in Geneva – created the heatwave warning system and helped BoM implement it.

See Dr Nairn in a video describing heatwaves

It’s getting on in February and Australia’s heatwave season has been going for some time, although southern Australia could be forgiven for thinking that the heat has only just begun.

The heatwave developing across southern Australia today has a significant warning area impacting Western and South Australia, which will slowly weaken as it moves east. By the time it moves into Tasmania and southern Victoria, it is likely to have low impact.

On this occasion the warnings for Western and South Australia are unlikely to be extended east.

Severe to extreme heatwaves were present in spring across the Top End, creating incredibly hot days and nights during October whilst the continent’s east coast was still being drenched in rain. 

But now the heat that has been building in Western Australia is being drawn down and across the southern states.

The BoM definition of a heatwave is: “three or more days in a row when both daytime and night-time temperatures are unusually high – in relation to the local long-term climate and the recent past.” The context is that people adapt over the hot season.

The BoM says there is no single temperature threshold for a heatwave in Australia. For each part of the country, the Bureau compares the forecast maximum and minimum temperatures for each three-day period in the coming week to the ‘normal’ temperatures expected for that location at that time of year, and to observed temperatures over the last 30 days.

Heatwaves are classified into three types, based on intensity.

1. Low-intensity heatwaves are the most common: most people are able to cope with this level of heat.

2. Severe heatwaves are less frequent and are challenging for vulnerable people such as the elderly: particularly those with pre-existing medical conditions.

3. Extreme heatwaves are the rarest kind. They affect the reliability of infrastructure, like power and transport, and are dangerous for anyone who does not take precautions to keep cool, even those who are healthy. People who work or exercise outdoors are particularly at risk.

Also in Cosmos: Heatwaves and climate change

Nairn says the important words to focus on are “heat” and, surprisingly, “night.”

“When people are asked to describe a heatwave, they will usually focus on how it will impact their daily activities or business interests due to the high day-time temperature. This is an adaptive description.

“The focus on the impacted activity provides clues for potential adaptive strategies. It is extremely rare for a person to reflect on an adaptive description that includes risk to their health, which is one of the reasons why so many people are injured or die in heatwaves.

“Heatwaves locally accumulate heat during a sequence [waves] of unusually hot days and nights.

“During a heatwave the minimum temperature is more significant that the maximum when assessing how much heat is being accumulated. If the heating cycle starts from a higher base, very high temperatures can be reached by mid-morning and be sustained long into the evening.

“A reduced overnight cooling cycle results in less discharge of the day’s heat, allowing more of that day’s heat to be carried over to accumulate in the next heating cycle.  It’s a vicious cycle that can build heat rapidly.

“Fortunately, most [85% of] heatwaves at any location are low-intensity events and pose little threat. These are the heatwaves we call normal in summer, and it is reasonable to expect that local adaptation strategies will keep most people protected. “

The Australian Institute for Disaster Resilience says heatwaves can also affect the transport, agriculture and energy sectors and associated infrastructure.

Nairn says the normally much rarer and more extreme heatwaves are becoming more frequent due to global warming.

“Research has shown that vulnerable people can be expected to die during these events, and need to mitigate their activities to avoid exposure.

“This is also the class of heatwave which has shown the highest casualty rate in outdoor workers.

“It isn’t easy to judge the crossover into the more dangerous heatwaves, because the heat accumulates over a three-day period, and the pressure to finish job lots in fine weather can be very high.”

In Australia January and February are the most dangerous months of the year due to the length of day v the shorter nights, and thermal lag from accumulated heat.