Understanding heatwaves

What is a heatwave?

A heatwave occurs when the maximum and the minimum temperatures are unusually hot over a three-day period at a location. 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 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, we compare 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.

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.

Past heatwave reports