Who What Why: What is blood rain?

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" in the South East, but what is it?

"Blood rain" a term used for rain carrying sand from deserts. When the rain dries off it leaves a thin layer of dust 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, which is around 2,000 miles away, are usually responsible for stirring up dust blown towards the UK, say weather experts. Watch Met Office satellite animation of such dust movement here. The current winds arriving in the country are part of the band of warm air which is predicted to bring unseasonably warm temperatures over the next few days, followed by rain in some areas.

The rain and the fine layer of dust left after it falls can be reddish, but also other colours. "The different coloured sands in the Sahara mean the rain and the coating it 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 Europe like Spain and the South of France, which are closer to the Sahara. But it can travel longer distances and fall in areas like Scandinavia.

A well-documented incident of "blood rain" happened in 2001 in the southern Indian state of Kerala. In the middle of a monsoon red rain started to fall and did so intermittently for several weeks. The colour was strong enough to stain clothes. There were also reports of green, yellow, brown and black rains.

Investigations suggested the rain was red because winds had stirred up dust from the Arabian Peninsula. Although another theory explored even suggested some sort of life form had fallen from the skies. It was reported at the time that Godfrey Louis, a physicist at Mahatma Gandhi University in Kottayam, concluded samples left over from the rains did not contain dust and instead had "a clear biological appearance".  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 BC. The 12th Century writer Geoffrey of Monmouth, who made popular the legends of King Arthur, also referred to it and the 12th Century historian, William of Newburgh.  In earlier times it was believed the rain was actually blood and it was considered a bad omen. Often it was used in texts and literature to predict bad events.  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.

BBC News