Amber alert marks longest heatwave in June

The Met Office has issued an amber heatwave warning – one level behind a national emergency – as another day of high temperatures sweeps the UK.

But the weather is set to turn cooler towards the weekend, with temperatures projected to dip nearer 21C in London by Friday.

The Met Office has said Britain is experiencing its longest spell of hot weather in June in decades. It is the first time since 1996 that temperatures reached more than 30C for five consecutive days.

Monday was the hottest day of the year so far, with the temperature reaching 32.5C at Hampton Water Works in the South East.

Peak temperatures of as high as 33C are anticipated as the week continues.

But the weather will turn cooler and cloudier towards the weekend.

"It will be hot for a couple of days in the South," Sky News weather producer Chris England said.

"Then it will be cooler and generally more unsettled, possibly turning more settled and hotter towards July."

Temperatures on Tuesday were pegged at 29C in the South, but further north were lower at around 18C.

Wednesday is set to see continued high temperatures of up to 31C in central London, but on Friday they will drop to around 21C.

Grahame Madge, a spokesperson for the Met Office, said the hot weather was unusual.

"It’s fairly rare. We’ve not really experienced anything like this for four years," he said.

But the Met Office will not label the run of warm days as a heatwave.

Instead it compared the current weather to previous spells when temperatures have topped 30C for more than five days.

Globally, a heatwave is defined as five consecutive days when the temperature is 5C warmer than the monthly average.

But by that calculation the last time Britain experienced a heatwave was in December 2016.

"It’s challenging to come to up with a definition that fits with people’s experience of a heatwave and is meaningful to people who aren’t meteorologists," Mr Madge told Sky News.

Factors like humidity or how hot it feels at night can also be taken into account.

This week’s amber health warning is based on a Department of Health and Met Office calculation. It is activated when there’s an average temperature of 30C in the day and 15C at night.

The NHS has urged people to spend time in the shade between 11am and 3pm, keep rooms cool and to stay hydrated.

(c) Sky News 2017: Amber alert marks longest heatwave in June