The Met Office has today issued a four-day amber warning for extreme heat across parts of England and Wales.

This comes as the UK Health and Security agency (UKHSA) issued a heat health alert that covers southern and central England from midday on Tuesday until 6pm on Saturday.

The Met Office amber alert is in force from Thursday to Sunday as parts of the country will face 35C heat.

What is a heat health alert?

UK Health Security Agency (UKHSA) Heat Health Alert is an England-only service considering the impact of prolonged extreme heat on public health, especially for those with long-term health conditions.

What classes as a heatwave?

A heatwave is an extended period of hot weather, relative to the expected conditions of the area at that time of year.

The current temperatures far exceed those normally, therefore we are experiencing a heatwave.

The UK also has its own conditions to be met for a heatwave to be declared.

A UK heatwave threshold is met when a location records a period of at least three consecutive days with daily maximum temperatures meeting or exceeding the heatwave temperature threshold. The threshold varies by UK county.

What causes a heatwave?

Heatwaves are common in summer when high pressure develops across an area, the Met Office explains.

High pressure systems slowly move and can persist over many days or weeks. They can occur in the UK due to the location of the jet stream, which is usually to the north of the UK in the summer.

This can allow high pressure to develop over the UK resulting in persistent dry and settled weather.

With the latest heatwave coming after months of low rain, which have left the countryside and urban parks and gardens tinder-dry, households in some areas are being urged not to light fires or have barbecues.

The Met Office’s fire severity index (FSI), an assessment of how severe a fire could become if one were to start, is very high for most of England and Wales, and will reach “exceptional” for a swathe of England by the weekend.

Scientists warn that the likelihood of droughts occurring is becoming higher due to climate change, driven by greenhouse gas emissions from burning fossil fuels and other human activities.

Climate change is also making heatwaves more intense, frequent and likely – with last month’s record temperatures made at least 10 times more likely because of global warming, and “virtually impossible” without it, research shows.