Who What Why: What is blood rain?

Weird weather is predicted in the UK this week and could include "blood rain". What is it?

If current weather forecasts turn out to be right, the weather will be a very mixed bag in the UK this week.

Unseasonably warm temperatures reaching 20C (68F) are predicted, followed by rain and possibly snow - and all over the next few days.

There is also a possibility of something called "blood rain", but what is it?

"Blood rain" a term used for rain carrying sand from deserts. When the rain dries off it leaves a dusty residue which can sometimes be a reddish colour, hence the name. It is capable of coating houses, cars and garden furniture.

"It is a rather grandiose term for fine desert sand particles that are whipped up by winds and mix with the moisture in clouds," says a Met Office spokesman.

Storms in the Sahara desert are usually responsible for stirring up dust that is blown towards the UK, say weather experts.

The current winds are part of the band of warm air which is predicted to bring unseasonably warm temperatures over the next few days, followed by rain.

The fine layer of dust that is left after it falls can be reddish, among other colours.

"The different coloured sands in the Sahara mean the coating the rain leaves can vary in colour," says weather expert Philip Eden.

"It can be reddish, but it is quite rare. It is more likely to be a sandy colour or brown. It's not as spectacular as it sounds."

"Blood rain" happens a few times a year in the UK, say experts.

It is more common in southern European countries like Spain and southern France, which are closer to the Sahara. But it can travel longer distances and fall in areas like Scandinavia.

For "blood rain" to leave a residue it needs to be a brief shower.

"This is because there is a higher concentration of sand in a short shower," says Eden. "Heavier, more prolonged rainfall simply ends up washing away the residue."

There are very early recordings of "blood rain" in historical texts. It is mentioned in Homer's Iliad, thought to have been written in the 8th Century.

In earlier times it was believed the rain was actually blood and it was considered a bad omen.

With the spread of modern scientific method in the 17th Century, it started to be explained in terms of rational causes.

By the 19th Century, the idea of dust being to blame started to dominate.