Heatwaves now need to be hotter as Met Office changes rules in England - but only in certain areas

Eight counties are set to need hotter temperatures to qualify for heatwaves

Heatwave thresholds are being updated across eight counties in England (Getty Images)

The Met Office is raising the bar for what is considered a heatwave - but only in certain parts of the UK.

Heatwaves are defined as happening when a location records at least three days in a row with daily maximum temperatures meeting or exceeding a certain threshold.

But different parts of the country have different temperatures to reach before a heatwave is officially declared.

The national weather body said it is making changes to those numbers to match the UK’s current climate - which is different from the conditions the existing system is based on.

The head of the Met Office National Climate Information Centre said there had been an “undeniable warming trend” across the country, but with temperatures rising at different rates.

Here is what we know about the changes and why they are happening:

Although many people refer to ‘heatwaves’ when the weather is warm, there are official criteria that need to be met (Getty Images)

What is a heatwave threshold?

The Met Office says heatwaves happen when somewhere experiences at least three days in a row with the highest temperatures meeting or surpassing a certain threshold.

This threshold already varied across the UK due to the different weather patterns across the country.

What are the changes and where is affected?

East Riding of Yorkshire will see it rise from from 25C to 26C, while Lincolnshire’s will increase from 26C to 27C.

Six areas - Surrey, Berkshire, Buckinghamshire, Bedfordshire, Hertfordshire, Cambridgeshire - will see their threshold increase from 27C to 28C.

The UK experienced a late summer heatwave last year (Getty Images)

Why are the changes happening?

The heatwave definitions are designed to be relative to the current climate.

Original temperature thresholds were based on a reference climate period for 1981-2010, according to the Met Office.

The heatwave definition is meant to be flexible so it can adapt in line with changes to climate - as is happening now.

How has the UK’s climate been changing?

Average temperatures are rising in the UK as a result of global warming.

“Warming across the country has, however, not been even with some regions experiencing more rapid change than others,” the Met Office said.

Officials say temperature rise has been greatest across parts of central and eastern England, which is why the thresholds are increasing for the eight counties in a band from Surrey to east Yorkshire.

Data released by the Met Office last year shows how temperatures have risen at different rates across the country over the past 60 years.

The 1C increase is to reflect changes in climate (Getty Images)

What do the experts say?

Dr Mark McCarthy said: “Climate statistics over time reveal an undeniable warming trend for the UK.

“Temperature rise has been greatest across parts of central and eastern England where they have increased by more than 1.0C in some locations, while further north areas of Scotland and Northern Ireland have seen temperatures rise by closer to 0.7C.

“Although heatwaves are extreme weather events, research shows that climate change is making these events more likely.

“A scientific study by the Met Office into the Summer 2018 heatwave in the UK showed that it was 30 times more likely to occur now than in 1750 because of the higher concentration of carbon dioxide, a greenhouse gas, in the atmosphere.

“As greenhouse gas concentrations increase, heatwaves of similar intensity are projected to become even more frequent, perhaps occurring as regularly as every other year.”