Temperatures soaring above 49°C in multiple states have been making headlines this week as a prolonged and intense heatwave grips a broad area of southeastern Australia. But while extreme maximum temperatures are a dangerous part of any Australian heatwave, high overnight minimums also have a big impact on people’s health.

What is a heatwave?

There is no globally accepted definition of a heatwave. The World Meteorological Organization defines a heatwave as “a period where local excess heat accumulates over a sequence of unusually hot days and nights.”

The Australian definition uses the average temperatures (minimums and maximums) over a 3-day period combined with observed temperatures over the past 30 days. This approach aims to measure how much of a shock to the body the forecast heat will be relative to the weather experienced over the past few weeks.

Australia’s heatwave classification is split into three categories based on intensity:

A low-intensity heatwave is a run of warm weather that most people can cope with. These happen each summer.

Severe heatwaves may be challenging for vulnerable people, particularly older people with medical conditions.

Extreme heatwaves are rare bouts of heat that may cause problems for people who don’t take steps to stay cool, even those who are healthy. Working or exercising outdoors can be dangerous in these heatwaves.

Overnight temperatures make recovery harder

Hot days are common across Australia during summer. However, when these hot days happen in succession and the nights in between them also stay very warm, it becomes harder to recover from the daytime heat.

Warm nights can be dangerous in two main ways:

A high overnight minimum temperature means the day starts off at a higher temperature, which may allow more hours of extreme heat during the daytime.

Hot nights make it harder for humans and animals to recover from intense daytime heat, which can put stress on the body.

Days and nights abnormally hot this week

Daytime temperatures have been remarkably high in southeastern Australia this week, reaching above 49°C in parts of South Australia and New South Wales and over 48°C in Victoria and Queensland.

This extreme daytime heat has been bookended by some exceptionally warm nights too. In SA, Marree registered two consecutive overnight minimum temperatures of 34.2°C on Wednesday and Thursday mornings. These hot nights both followed daytime maximums above 48°C.

Adelaide also had a very hot night earlier this week, with the temperature sitting on 36.2°C at midnight on Monday night. This midnight heat was more than 7°C warmer than the city’s average daytime maximum temperature for January.

This week’s heatwave will continue to cause maximum and minimum temperatures to stay well above average over parts of NSW, SA, Qld and Vic until at least Saturday.

Image: Forecast for Marree, SA showing temperatures staying above 30°C for the next few days and nights. Source: Weatherzone.

How to stay safe during a heatwave

NSW Health has the following advice to stay safe during a heatwave:

If you have air-conditioning that is used for both heating and cooling, make sure it is set to cool.

Stock up on food and drinks for your household and pets, and medicines to last the period of forecast hot weather so that you can avoid going out in the hottest part of the day.

Fill ice trays in your freezer or put some cool-packs in the refrigerator or freezer.

Check in with family and friends who may be more at risk during hot weather to see if there is anything they might need to help them prepare.

Consider how changes in weather might impact planned daily activities and exercise routines.

Check alerts for roads and transport during extreme weather events and bushfires.