When asked to think about dangerous weather events, you may picture strong winds, heavy rain or storms — but did you know that heatwaves are the most dangerous environmental hazard in Australia?
Heatwaves result in more deaths nationally than bushfires, floods, cyclones and severe storms combined.
Extreme heat can cause a range of health problems, even in otherwise healthy people. Heatwaves can also affect the transport, agriculture and energy sectors and associated infrastructure.
So, when is it a heatwave, rather than just hot? And how will you know if one is on the way?
What is a heatwave?
A heatwave occurs when maximum and minimum temperatures are unusually hot over a three-day period. This is considered in relation to the local climate and past weather at the location.
It takes more than just a high daily maximum temperature to define a heatwave. It's also about how much it cools down overnight.
Hot days without hot nights allow some recovery from each day's heat, but if the temperature stays high overnight, the maximum will be reached earlier the following day and will potentially last longer.
When unusually high night and daytime temperatures persist, heat stress becomes a critical factor in human health and whether infrastructure functions properly.
In heatwaves, hot nights make it harder to recover from the heat of the day and this puts more stress on the body.
Heatwave intensity
For each part of the country, the Bureau compares the forecast maximum and minimum temperatures for each three-day period in the coming week (e.g. Monday-Wednesday, Tuesday-Thursday) to what would be considered hot for that location, and also to observed temperatures over the last 30 days.
Put simply, it is a measure of the forecast compared to what is considered hot for that location, and how much of a shock to the body the heat will be compared to recent temperatures.
Heatwaves are classified into three types, based on intensity.
- Low-intensity heatwaves are more frequent during summer. Most people can cope during these heatwaves.
- Severe heatwaves are less frequent and are likely to be more challenging for vulnerable people such as the elderly, 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 people who are healthy. People who work or exercise outdoors are also at greater risk of being affected.
Heatwaves in Tasmania
Tasmania doesn't often get long-lived "hot" spells like other parts of Australia.
This is largely because the state is surrounded by water and doesn't have sprawling deserts that bake during long summer days and hold onto the heat overnight.
Experiencing somewhat-regular cold fronts also limits hot spells because they bring cooler air from over the Southern Ocean.
But that doesn't mean it can't get hot for Tasmania. It is relatively common to get a few days reaching the high 20s or low 30s during summer, with some days likely to reach into the 40s.
Hobart's hottest day was 41.8C on January 4, 2013, and Launceston reached 39.0C on January 30, 2009. The highest temperature ever recorded in Tasmania was 42.2C at Scamander on January 30, 2009.
Even if the maximum temperature remains in the 20s for a few days, you can still experience a heatwave.
This could occur if that is unusual for your location, or if there are a few warm days after a cool spell.
Heatwave calculations take overnight minimum temperatures into account too — because if it's a hotter day than what you're used to, and you can't cool down overnight because it stays unusually warm, you're in a heatwave.
New heatwave warning service
Through collaboration with partner health and emergency service agencies across states and territories, the Bureau now provides a nationally consistent public warning service for heatwave events.
Released on October 4, 2022, the heatwave warning service allows the public to take steps to prepare and mitigate the impacts of heatwaves.
The heatwave warning service directly alerts Australians, no matter where they are in the country, that an upcoming severe or extreme heatwave is forecast within the coming four days.
The alerts will include heatwave warnings issued by the Bureau with heat health messages provided by state and territory agencies, equipping the community and health, emergency and community services to better prepare for and minimise the effects of heatwave events.
The Bureau has worked closely with partner health and emergency services agencies over several years to agree on a nationally consistent approach for heatwaves. In late 2021, the Bureau launched a trial heatwave warning product.
Based on feedback from this trial, several changes have been made to the heatwave warning product for this season including issuing the warning earlier in the day and listing agreed action statements for different heatwave severities.
A list of locations in each state and territory is now included in the weather situation section of the warning.
Additionally, the safety advice section will appear before the weather situation section in the warning to align with best practice warning communication.
Heatwave warnings will be listed on the Tasmanian Warnings Summary page, where you can find all other warnings current for our state. The warnings will also be available via the Bureau's Weather app.
Further information about the new heatwave service can be found in the Bureau's heatwave knowledge centre.
Luke Johnston is the Hazard and Preparedness Manager with the Bureau of Meteorology in Tasmania
