What is a heatwave? Understanding extreme heat

A heatwave can have an adverse impact on all our lives. It's important to understand what extreme heat is and who it may affect.

What is a heatwave and what causes it?

The word “heatwave” is often used to describe a period of hot and sunny weather however, this isn’t always correct. A heatwave is a meteorological definition and is officially declared when certain thresholds are met.

The Met Office defines a heatwave as “an extended period of hot weather relative to the expected conditions of the area at that time of year, which may be accompanied by high humidity.”

In the UK, a heatwave is declared when the daily temperature of a certain location meets or exceeds the heatwave temperature threshold for at least three days.

There are four heatwave thresholds in the UK reflecting the differences in climate. The lowest, covering places like Wales and Scotland, is 25°c, and the highest, covering London and parts of the East Midlands, is 28°c.

How long does a heatwave last?

A heatwave lasts a minimum of three consecutive days, though it can be much longer. The longest official heatwave in the UK was in 1976 when the heatwave threshold was met for 15 consecutive days.

A heatwave can feel much longer because it usually comes within a period of dry and hot weather.

When was the last heatwave in the UK?

The last official heatwave in the UK was in the summer of 2022 when there were three heatwaves from 15 to 17 June, 17 to 19 July, and 9 to 15 August. This came at the same time as several heatwaves across Europe and North Africa.

In the UK, the Met Office issued its first-ever red extreme heat weather warning for parts of England on 15 July 2022 and increased the Heatwave Alert was increased to Level 4 meaning there was a threat of illness and death among fit and healthy people – not just high-risk groups.

The highest-ever temperature for the UK was recorded in Coningsby in Lincolnshire on 19 July 2022 and was a record-breaking 40.3°C. Records were also set in Scotland and Wales. Many areas of the UK declared drought and there was an increase in wildfires in urban and rural areas.

Is there another heatwave coming in the UK in 2024?

The Met Office can give long-term forecasts, but it’s impossible to predict exactly when a heatwave may occur. Usually, when a hot weather warning is given, people will only have a few weeks or even days' notice.

The UK experiences its warmest temperatures in the summer months of June, July, and August, but it’s not unusual to also have hot weather in May and September as well. However, the Met Office cannot predict when the next UK heatwave will be a long time in advance.

Are heatwaves becoming more common?

Heatwaves are extreme weather events and because of climate change, they are becoming longer and more intense. Heatwaves and periods of hot weather have increased in the UK in recent years and are only projected to get hotter and longer.

By 2050, the UK will be 50 per cent more likely to experience hot summers, while heat-related deaths could triple to around 7,000 per year. You can learn more about the impact of Heatwaves in our report Feeling the Heat.

While there was no official heatwave in the UK in 2023, the Met Office confirmed in January 2024 that it was the second warmest year on record for the UK and globally was the hottest year on record with eight months out of the year warmer than average. They also predicted this pattern to continue.

Met Office Senior Scientist Mike Kendon said: “While our climate will remain variable, with periods of cold and wet weather, what we have observed over recent decades is a number of high temperature records tumbling. We expect this pattern to continue as our climate continues to change in the coming years as a result of human-induced climate change.”

Close-up shot of a male construction worker wiping the sweat from his forehead with his arm while holding his construction hat

PHOTO: GETTY

Who is most affected by a heatwave?

A heatwave can affect anyone, but some groups are affected more than others. As heatwaves become longer and hotter, it’s important that these groups are prepared for extreme heat.

According to the NHS, the people most affected by heatwaves are:

older people – especially those over 75 and female
those who live on their own people who have illnesses such as heart or lung conditions, diabetes, and kidney disease
people who are less able to cool themselves such as young children, the bed-bound, those with drug or alcohol addictions or Alzheimer's disease
people who are more exposed to hot temperatures such as those who live in high-rise buildings, the homeless, or those whose jobs are outside

If you fall into one of these groups, or you know someone who does, please make sure you follow our heatwave advice.

Even if you're not in one of these groups, it's important to take extra care of your health during periods of hot weather and heatwaves.