Why is it so hot?

This article was written in 2019.

Thursday 25 July is set to be the hottest day ever recorded in Britain, and my goodness are we feeling it.

The hottest July temperature ever recorded in the UK was 36.7C in 2015, however this week it could reach a whopping 39C.

But it’s not just here: records are set to be smashed all over Europe, with the mercury predicted to rise to the low forties.

But why is it happening, and is it something we should expect to see more often?

What is a heatwave?

Let’s start with the basics. A heatwave is defined by the Met Office as an uninterrupted period of exceptionally high temperatures.

There’s a threshold temperature for it to be described as a heatwave, and it varies in different parts of the country:

• In parts of Scotland, Northern Ireland, Wales, North and South West England, the threshold is 25C.
• From Lincolnshire across to Cheshire and down to Dorset, the threshold rises to 26C.
• For parts of the Midlands, East Anglia and for much of the Home Counties, it’s 27C.
• It goes up to 28C in London.

These thresholds take into account what the temperature is like on average at the hottest part of summer in each area of the UK, and then they’re set to be quite a bit higher.

The threshold has to be reached or exceeded for three days in a row for it to be called a heatwave - if it doesn’t last three days, it’s considered a hot spell.

It’s definitely a heatwave right now, so why is it happening?

Much of the UK’s hot weather comes from the jet stream, which is a narrow band of high speed winds. On either side of this, there will be slower warm and cool winds.

At the moment, the jet stream is sitting to the west of the UK, meaning we’re on the warmer side of it, and it’s been in the same position for about a week. The warm air that’s being brought up to us is originating in northern Africa, and this week the winds will change and bring it through Europe and up to us from France, meaning the air we’re getting is exceptionally hot.

It’s also linked to the fact that there’s an area of high pressure sat to the east of the UK, which is drawing up the warmer flow.

Will this happen more often, and should we be worried it?

A lot of people worry that heatwaves such as this one are due to climate change. We spoke to Grahame Madge, a Met Office climate spokesman, who told us that whilst the frequency of extreme events such as these likely hasn’t changed much over the years, the severity has definitely increased.

“Extreme events in themselves are nothing new… what’s interesting from a climate point of view is when you look at those extremes we’re adding far more hot extremes than we are cold extremes.

“What’s different is we’re effectively super charging the atmosphere with all this background warmth. Planetary temperatures have raised by about one degree or so since pre-industrial periods so we’re adding that spike to all of the extreme events that we have, which is getting us to a point where we’re seeing more warm extremes and less cool ones.

“These would have always been warm events in the absence of climate change but they wouldn’t have been as extreme.”

So should we be concerned about this?

“We’re seeing signs of climate change happening now, we are experiencing new extremes… in terms of temperature, and obviously this week we may see the all-time record temperature broken… these are worrying signals," said Mr Madge.

He added we’re also seeing extreme events in terms of extreme rainfall and, when you combine that with other indicators of climate change, such as Arctic sea ice melt, glacier melt, sea level rise, there is compelling evidence that our climate is changing rapidly.

"We recognise we are going to be committed to a certain level of climate change… [but] it should encourage every attempt humanity can make to reduce the impact of further climate change,” warned Mr Madge.