What is a heatwave?

Heatwave definition

A heatwave is when the maximum and minimum temperatures are unusually hot over 3 days. This is compared to the local climate and past weather.

It takes more than a high daily maximum temperature to make a heatwave. It's also about how much it cools down overnight.

Cooler nights allow some recovery from each day's heat. A high overnight temperature can mean that the next day heats up quickly. There may be more hours of extreme heat.

In heatwaves, hot nights make it harder to recover from the heat of the day. This puts more stress on the body.

A run of unusually high night and daytime temperatures can:

• lead to heat stress, a critical factor in human health
• affect infrastructure such as public transport and electricity supply.

How we classify heatwaves

We use the excess heat factor (EHF) in Australian heatwave monitoring and forecasting. Using this index, we classify heatwaves by intensity.

The EHF combines:

• a comparison of the average temperatures for a 3-day period with what would be considered hot at that location
• the observed temperatures at that location over the past 30 days.

In simple terms, the EHF measures how much of a shock to the body the forecast temperatures will be, compared to the weather over the past month.

Heatwave intensity

Low-intensity heatwaves are frequent during summer. Most people can cope during these heatwaves.

Severe heatwaves are less frequent. They are likely to be more challenging for vulnerable people. This can include older people, particularly those with medical conditions.

Extreme heatwaves are rare. They are a problem for people who don't take precautions to keep cool – even for healthy people. Anyone who works or exercises outdoors can be at risk.

Heatwave forecasts and warnings

View our Heatwave Service for Australia.