Link to newsround

Why is it suddenly so cold?

A girl wears a wooly hat, gloves and scarf and a coat in a field in Autumn.Getty Images

The sudden drop in temperature may mean we are scrambling for hats, gloves and scarves a bit earlier than usual...

Have you suddenly had to dig out your 'BIG' coat this week?

Well that's because the outside temperature has suddenly dropped due to a blast of Arctic air moving down the UK.

Overnight temperatures are expected to fall to -5C in some rural parts of England.

Yellow cold weather health alerts have been put in place by the UK Health Security Agency (UKHSA) in the West Midlands and the North West of England.

These will be in place from 09:00 GMT on Thursday until 12:00 GMT on Saturday.

This means vulnerable people could be more at risk, so they should take extra care to keep warm.

But why is it suddenly so chilly? Read on to find out more.

What is a polar vortex?

graphic showing the polar vortex and polar jet stream. On the left, strong westerly vortex winds circle the Arctic with cold air kept in place with a slightly meandering jet stream. On the right, the polar vortex is disrupted and split in two, the jet stream meanders more southward allowing colder air to filter to the lower latitudes.

Much like a spinning top, when something disrupts the polar vortex, it can 'wobble' and change its course, therefore changing the weather conditions further south

Earlier this week, many people in the UK saw foggy mornings, but now you may have spotted frost forming on grass, cars and pavements. Some parts of Northern Scotland could even see light flurries of snow.

Sometimes, weather like this is caused by something called a polar vortex.

This is when a huge area of very cold air spins high up in the atmosphere around the North Pole.

Most of the time, the polar vortex stays locked in place by powerful winds. But sometimes those winds can weaken or become disrupted.

When that happens, some of the cold Arctic air can escape southwards towards places that are normally much milder, including parts of Europe and the UK.

Will it snow soon?

A frost covered spider's web on a fence post. Getty Images

Some of you may be waking up to frosty scenes outside your windows over the next few days

Experts at the Met Office, (the UK's national weather service), say it is very early in the year for this to occur.

Lead climate scientist Jeff Knight told the BBC: "It would appear to be the earliest on record looking back over the 75 years we've been observing…with most experts agreeing this is a very unusual event."

But they stress that it is too early to know whether recent changes in the polar vortex will change the overall winter outlook.

A polar vortex does not create cold weather on its own, and not every chilly spell in the UK is caused by one.

So this doesn't mean we are about to get an influx of snow across the UK.

In fact, by the weekend, milder weather will head over from the Atlantic Ocean, so you may not want to put away your lighter coats just yet.