Weather warnings are given a colour – yellow, amber or red - depending on a combination of two things; the impact the weather is likely to have and the likelihood of it happening.

A matrix system is used to help decide on the severity of the warnings, which then helps decide the colour.

Yellow is the lowest tier of warning, amber is a middle-tier warning and red is the most serious type of warning possible. The warning is determined by what best matches the impact of the weather against the likelihood of it happening.

Eight different types of weather are covered: rain, snow, wind, thunderstorms, lightning, fog, ice, and extreme heat.

Image source, Getty Images

The Met Office's red warning for extreme heat has just come into effect, meaning an "exceptional spell" of hot and humid weather can be expected across the affected region.

This is only the second time a red warning has been issued since extreme heat warnings began in 2021.

Among the areas included are the East Midlands, East of England, London & South East England, South West England, Wales and the West Midlands.

The Met Office says to expect population-wide impacts, affecting not just those who are most vulnerable to extreme heat, including serious illness or danger to life.

The service says "substantial changes" in working practices and daily routines will be required, while there is also a high risk of heat-sensitive systems and equipment failing.

This could result in power cuts and the loss of other essential services such as water, electricity, gas or mobile phone signal.

Furthermore, delays on roads and road closures are expected alongside delays and cancellations to rail and air travel. Those who experience even moderate delays on such services are at risk of significant welfare issues.

There is also an increased risk of water safety incidents, due to significantly more people visiting coastal areas, lakes and rivers.

A rare red weather warning has just come into effect and will remain in place until 21:00 BST on Thursday.

It covers parts of southern England, as well as the Midlands and south Wales, with people in those areas warned there is a risk of serious illness or danger to life.

Marthe de Ferrer
Senior journalist

For parents with young babies, keeping them cool can be a major concern - especially as infants cannot regulate their body temperature properly until they are nearer nine to 12-months-old.

The NHS advises that babies be kept out of direct sunlight, but says to avoid covering prams with a blanket, as this can reduce airflow and lead to overheating.

When it comes to sleep, it’s recommended to keep babies’ rooms at 16-20C - which can be impossible during a heatwave.

Safer sleep charity The Lullaby Trust suggests placing a fan pointing away from the baby while they sleep, to help cool the room and using a room thermometer so you have an accurate idea of temperature.

The Lullaby Trust says to use lighter, or even no bedding at night to keep babies safer while sleeping in hot conditions.

Chris Partridge
Reporting from Newbury

Image source, BBC/Chris Partridge

Train services seem to be running normally on GWR through Newbury into London Paddington right now. Fewer people than usual are at the station, with many heeding the advice not to travel.

Carriages are quieter too, indicating fewer passengers boarding further back along the line.

This is a busy commuter route into London, and many of the trains are electric with air conditioning as standard.

Temperatures could reach up to 36 or 37C today - but that's only part of the story.

The "feels like" temperature is likely to be much hotter, reaching into the 40s in parts of England.

BBC Weather presenter Matt Taylor explains why that is and how it might lead to an uncomfortable day for some.

Pet owners have been advised not to take their dogs out for walks during the red extreme heat warning.

Lauren Bennett from the RSPCA tells BBC Breakfast that while dogs should still be brought outside briefly to go to the toilet, it's about "being sensible" and keeping to shaded areas while minimising high impact exercise.

She says: "Certainly, with this red weather warning, actually we would advise probably not to walk your dogs at all because there isn't actually a cooler time of day."

She says dogs could be taken out very early in the morning or very late at night, as well as being given plenty of water and keeping cool in homes through air conditioning and fans.

She adds: "Just like people, it can be fatal, which is why we tell people to change their behaviour because our pets are completely under our control so the responsibility is on us."

Image source, EPA

Temperatures on Tuesday peaked at 34.6C in Wisley, Surrey, while highs of 32.2C were reached in Wales.

Ahead of the red warning coming in to force later this morning in some parts, here are some pictures showing how people coped with the heat yesterday:

Image source, Getty Images

Image source, Getty Images

Image source, EPA/Shutterstock

Image source, Reuters

Katy Austin
Transport correspondent

Rail passengers in England and Wales are being advised to only travel if absolutely necessary today and Thursday.

Lots of train operators have asked that people only make essential journeys, and have put in place reduced timetables: South Western Railway, Thameslink, Northern and Avanti West Coast to name just a few.

Gatwick Express won’t run on Wednesday or Thursday afternoons, and LNER “strongly recommends” its customers don’t travel.

Running fewer trains puts less stress on the railway infrastructure. The idea is also to minimise the number of people caught up in any disruption, and create breathing space in the timetable so any delays cause less of a knock-on impact.

If you decide not to travel and already have a ticket, you’re probably able to use it another day or get a refund. Check with your operator.

Image source, Getty Images

Image source, Getty Images

The short answer is... very hot.

Scotland and Northern Ireland recorded their hottest days of the year so far, and while it was still a scorcher in England, overnight storms meant the temperature wasn’t as hot as predicted.

The maximum temperatures on Tuesday across the UK were:

• England: 34.6C at Wisley, Surrey
• Scotland: 29.0C at Dyce, Aberdeen
• Wales: 32.2C at Cardiff Bute Park
• Northern Ireland: 28.1C at Katesbridge, County Down

The rare red weather warning affects London, parts of the Midlands, south-east Wales and southern England, and will be in place from 09:00 BST on Wednesday through to 21:00 BST on Thursday.

The Met Office says there is a risk of serious illness or danger to life in those impacted areas.

Loss of power and essential services like water are also likely, due to a greater risk of failure of heat-sensitive systems and equipment.

The Met Office also warns of delays on roads, trains and plains.

Temperatures are forecast to rise in to the high 30s, in southern England, and a maximum of 39C on Wednesday is not out of the question.

Less severe amber weather warnings are also in place across much of southern and central England and most of Wales until the end of Thursday.

Image source, Getty Images

It's predicted to be an exceptionally hot day across the UK, with the Met Office's red weather warning for extreme heat for parts of England and Wales due to kick in at 09:00.

This means the heat is likely to bring impacts to health and could be a danger to life, but also a risk to infrastructure such as power supplies and transport.

The warning covers parts of central and southern England, and parts of south Wales, lasting until 21:00 on Thursday.

The red warning is an upgrade in severity from yesterday's amber warning for extreme heat, which also covered parts of England and Wales. Those in Wisley, Surrey were feeling the heat the most, where it reached 34.6C.

Expect delays on roads, tracks and in the air, the Met Office has warned.

We'll be covering the ongoing impact of the heat throughout the day, as well as sharing some tips for dealing with the weather.