Last week you might have been forgiven for thinking there would be something spooky in the weather for Halloween, with several news websites were pushing headlines along the lines of:
So what is ‘blood rain’ and was there any truth in these headlines?
Every year, millions of tonnes of Saharan dust are lifted from the desert surface of the Sahara, and transported by the atmosphere. Mostly, this dust makes its way westwards, across the Atlantic. However, some of it can be transported northwards towards Europe. This is quite a common affair for our southern European neighbours such as Spain, Italy and Greece, but dust outbreaks rarely make it as far north as the UK – on average only around 2-3 times a year, when you might notice a light coating of reddish dust on your car. On its journey through the atmosphere, dust eventually either settles naturally due to gravity (dry deposition), or it encounters cloud and rain, which cause it to be ‘rained out’ (wet deposition). If dust mixes with rain, it can frequently change the colour of raindrops giving them a reddish hue, due to the high amounts of iron in Saharan soils, thus creating the phrase, ‘blood rain.’ Sometimes it can be noticed on the ground or cars after rain has fallen, leaving a reddish residue after the water has evaporated.
Last week, the articles getting excited about this phenomenon associated it with warm air over the UK originating from the Sahara. Indeed, going back to Sunday 21st October, there was a low pressure centred over eastern Spain, with associated southerly winds over the Mediterranean bringing warm air and dust from the Sahara to Europe, as shown by the surface pressure chart below.
An overpass of the CALIPSO satellite, with its onboard lidar, provides us with a vertical cross-section of the atmosphere over the French-German border on that day. The image below shows a layer of dust from the surface up to around 4km at latitudes of around 45-50 degrees north, shown by the yellow colouring here, confirming that there was dust in the air over Europe.
Accordingly, the ECMWF were forecasting a plume of dust to be transported from the Sahara over Europe for Monday 22nd October, part of which reached as far north as Kent and Suffolk in the UK:
For the part of this dust plume which made it as far as the UK, forecasts were for it to be deposited by rainfall during Monday and Tuesday (22nd and 23rd October) last week. There was a warm front over the UK on Tuesday, and Reading did receive a few millimetres of rainfall, but I didn’t see any sign of ‘blood rain’, and nothing seemed to be mentioned further by the press. It’s quite possible that the dust didn’t make it quite as far as the UK, or perhaps more likely that I wasn’t inclined to stand outside in the wet looking for redness in the rain. Did anybody else see any blood rain? The dust event over the UK certainly seemed to be a weak one, if it did indeed happen – for example, the Met Office describe a much larger event that affected the UK in June this year here.
Either way, I think the statement ‘blood rain for Halloween’ is a bit optimistic more than a week in advance of Halloween. As it is, quite a different weather phenomenon has stolen the Halloween show on the other side of the Atlantic…