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Weather: This heatwave could be more dangerous than the one last month as UK under heat dome

The UK is under a heat dome and this heatwave could be more dangerous than the last, according to Sky News' weather presenter.

A Met Office amber extreme heat warning is in place for much of England and Wales until Sunday and although temperatures will not hit the record-breaking highs of 40.3C seen last month, the current heatwave is expected to be longer than July's.

Sky's Kirsty McCabe explains that the gradual build-up of heat over several days this time could actually be more dangerous than the conditions seen previously.

Experts from the UK Health Security Agency (UKHSA) are advising people to look out for those who are older or with existing health conditions, as well as young children.

McCabe said: "It's the duration of the heatwave that will allow dangerous levels of heat to build up, especially in buildings and urban areas. The nights will be increasingly warm too, making it difficult to sleep.

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"By Thursday lunchtime, parts of the UK had exceeded 30C for 4 days in a row, and it's likely somewhere in the UK will be above 30C for a least a week".

She said that while temperatures are set to peak at 36C across southern England this weekend, heatwave conditions will be met widely across the UK and Ireland.

The Met Office defines a heatwave as at least three consecutive days of temperatures above a certain level.

July's extreme heatwave lasted three successive days, while this extreme heat warning is expected to last at least four days, according to the weather agency. Alongside this, the UKHSA has a six-day heat-health alert in place, which also ends on Sunday.

UK under a heat dome

McCabe says we are currently under a heat dome, as temperatures have risen even though we have not reached the peak.

A heat dome is when an area of high pressure stays over a large part of a region for days, or even weeks.

Like a lid on a saucepan, it traps hot air underneath, and can cause heatwaves with temperatures well above the norm.

Hot air expands vertically into the atmosphere then high pressure from above means it has nowhere to escape and pushes that warm air down.

As the warm air sinks, it compresses and heats up, which then traps more heat underneath. The ground then heats up and loses moisture which makes it heat up even more, and means it is ripe for fires to start.

The dome of high pressure also pushes the clouds around it, keeping the heat in even more.

Usually, winds can move the high pressure around but as the dome stretches high into the atmosphere, the high pressure system becomes very slow moving, almost stationary.

McCabe says the intense heat last month came from "very hot, dry air being dragged up from North Africa on strong southerly winds".

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However this time, she says the current hot weather is "home grown" from high pressure sitting over the British Isles.

What's happening next week?

After a hot weekend, cooler, more changeable conditions are forecast to move in from the west next week.

McCabe says the heat will only slowly relinquish its grip, and at the moment any showers are unlikely to bring much relief to those areas struggling with drought.

Sky News understands that a drought is expected to be declared on Friday in the South West, meaning people living there can expect to see a series of restrictions on domestic and commercial use of water - including hosepipe bans.