How hot does it need to be for a heatwave to be declared?

• Will there be a heatwave in Wales this week?

Temperatures are set to increase this week as warm, sunny weather is predicted for parts of Wales.

The Met Office has predicted that temperatures in parts of Wales will reach 27ºC towards the beginning of the week.

But does this mean Wales is set to experience a heatwave?

Certain criteria needs to be met for an official heatwave to be declared.

What is a heatwave?

According to the Met Office, a heatwave is declared after at least three consecutive days of temperatures above what would usually be expected for that time of year.

The threshold is different for each county, depending on where they are in the country, and it is usually accompanied by high humidity.

In most parts of Wales, temperatures must breach 25ºC to be considered a heatwave. However, areas in south-east Wales must breach 26ºC.

The different threshold temperatures between counties reflect the differences in climate across the UK.

They have been calculated using the 1991-2020 climatology of daily maximum temperature at the mid-point of the meteorological summer (July 15).

Why do heatwaves happen?

Heatwaves are more common in the summer due to higher temperatures. Because the UK is centred below a jet stream, high pressure can develop over the UK, resulting in persistent dry and settled weather.

How hot can it get during a heatwave?

Wales recorded its highest temperature on record during the heatwaves of 2022, hitting 37.1ºC at Hawarden Airport, Flintshire, on 18 July that year.

The following day, England recorded a new record-high temperature for the UK at 40.3ºC in Coningsby, Lincolnshire. This became the hottest day on record.

How hot does it need to be for a weather warning?

The Met Office is responsible for issuing weather warnings for the impact caused by severe weather.

It can issue Extreme Heat Warnings to highlight the potential impacts of extreme heat to lives, infrastructure and property.

In July 2022, it issued its first ever red warning for exceptional heat when temperatures of 40°C were forecast in the UK for the first time.

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